KR20210010849A - Apparatus comprising electrostatic clamps and methods of operating the apparatus - Google Patents

Abstract

An apparatus is provided that includes an electrostatic clamp for clamping a component and a mechanism for generating free charges adjacent the electrostatic clamp. The mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic clamp during a transition from a first energized state of the electrostatic clamp to a second energized state of the electrostatic clamp.

Description

Apparatus comprising electrostatic clamps and methods of operating the apparatus

Cross-reference to related applications

This application claims the priority of EP Application No. 18166955.7, filed on April 12, 2018 and incorporated herein by reference in its entirety.

Technical field

The present invention relates to an apparatus comprising an electrostatic clamp and a method of operation thereof. More particularly, but not exclusively, the apparatus may include a lithographic tool, and the electrostatic clamp is configured to clamp a component such as a patterning device during lithographic patterning.

A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern of a patterning device (eg, a mask or reticle) onto a layer of radiation-sensitive material (resist) provided on a substrate.

In order to project a pattern onto a substrate, the lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features that can be formed in the substrate. A lithographic apparatus having a wavelength in the range of 4 to 20 nm, for example 6.7 nm or 13.5 nm using extreme ultraviolet (EUV) radiation, is used for lithography using radiation having a wavelength of 193 nm, for example It can be used to form features on the substrate that are smaller than the device.

A lithographic apparatus may typically use a high voltage electrostatic clamp to clamp the patterning device, for example during a patterning operation. Electrostatic clamps and patterning devices are often maintained in a low pressure, hydrogen-rich environment. This environment is non-conductive. Thus, it will be appreciated that charge may accumulate on dielectric or ungrounded surfaces. Charge can accumulate on dielectric or ungrounded surfaces, for example by contacting a component during operation (eg, by clamping a mask) or by particle collisions during gas flow.

It will also be appreciated that EUV radiation can render a hydrogen rich environment conductive due to the creation of an EUV-induced hydrogen plasma. Free charges generated in the EUV induced hydrogen plasma can be attracted (or repelled by the electric field) by the electric field generated by the electrostatic clamp. On the other hand, in the absence of the EUV induced plasma or in a remote or well screened region from the EUV induced plasma, charges may accumulate on the dielectric or ungrounded surfaces and may remain after all electric fields have been removed.

In addition to the accumulation of charges, a very strong electrostatic field (eg, in the range of about 1-100 kV/cm) can be generated between portions of the electrostatic clamp and other system components. In particular, the high voltage applied to the electrodes of the electrostatic clamps causes nearby conductors (eg, conductive coatings that may be present on the surfaces of the mask) to be polarized. Thus, a strong electrostatic field occurs, especially in sharp features (eg, edges of conductive mask coatings). The voltage applied to the electrodes of the electrostatic clamp can be frequently switched in polarity to avoid breakdown in the insulation of the electrostatic clamp. During this transition, the electrostatic field in the area around the clamps can change rapidly.

It is an object of the present invention to prevent or alleviate one or more problems associated with the accumulation of electric charges in a lithographic apparatus and/or generation and conversion of an electrostatic field in a lithographic apparatus.

According to a first aspect of the invention, an apparatus is provided comprising an electrostatic clamp for clamping a component and a mechanism for generating free charges adjacent the electrostatic clamp. The mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic clamp during a transition from a first energization state of the electrostatic clamp to a second energization state of the electrostatic clamp.

By providing free charges around the electrostatic clamp (eg, via an EUV induced H ₂ plasma) during repolarization, a relatively conductive medium is provided near the clamp and any clamped patterning device. As such, as the clamp is repolarized, abundant free charges are generated by any external electric field generated by the clamp, in particular the regions of the clamp extending beyond the clamped patterning device (e.g., leads and contacts). This effectively screens for any external electric field generated by the clamp, thereby reducing the likelihood of particles being emitted from the surfaces of these regions of the clamp.

In use, it will be appreciated that when the component is clamped by the electrostatic clamp, the component will mask portions of the electrostatic clamp from generated free charges. Thus, the mechanism for generating free charges can be configured to generate free charges adjacent to the electrostatic clamp, but such charges are prevented from reaching the areas of the clamp that are typically covered directly by the component being clamped in use. do. That is, in use, the generated free charges provide screening to regions of the clamp that do not contribute to the clamping force. In fact, it will be appreciated that during clamping the clamped component will be clamped by the electric field generated between the clamp and the component, and that free charges present in the vicinity of the clamp and the clamped component do not interfere with this clamping effect. Rather, free charges typically do not extend into the area between the clamp and the clamped components (eg, having a maximum spacing of about 10 μm in place or in direct contact elsewhere).

The electrostatic clamp can include a clamping area configured to clamp the component. When a component is clamped, a clamping electric field may be generated between the clamping region and the component.

The electrostatic clamp may further include a non-clamping region. When the component is clamped by the clamping region, a secondary electric field may be generated around the non-clamping region.

For example, in the absence of a conductive medium grounded around the non-clamping area, a secondary electric field may be generated between the non-clamping area and one or more of the portion of the device and/or the portion of the clamped component.

The clamp may include a first region configured to support the patterning device and a second region not configured to support the patterning device. The first region may include one or more clamping electrodes. The second region may include one or more secondary electrodes. Each of the secondary electrodes may correspond to each of the clamping electrodes. The second region may include a plurality of non-contiguous sub-regions. For example, the second region may include protrusions extending from both sides of the first region.

Each of the (clamping and secondary) electrodes may be coated with a dielectric material. The dielectric material may have a thickness of about 100 μm.

The first region may include a clamping region. The second region may include a non-clamping region. Of course, it will be appreciated that in some embodiments, the clamped component may have a size substantially similar to the size of the first region. However, in an alternative embodiment, when clamped, a part of the first region is covered by the clamped component, while the other part of the first region is not covered by the clamped component, so that the clamped component is the first region. It can have a smaller size. In this configuration, the uncovered portion of the first region can be considered to include the non-clamping region.

The device may include a mechanism for generating free charges adjacent to the non-clamping region. A mechanism for generating free charges may be configured to generate free charges adjacent to the non-clamping region during a transition from the first energized state of the electrostatic clamp to the second energized state of the electrostatic clamp.

The electrostatic clamp includes at least one electrode, and a clamping voltage is applied to at least one electrode such that a clamping electric field is generated between the clamping region and the component when the component is clamped by the electrostatic clamp.

The first region may include a plurality of clamping electrodes. In the first energized state of the electrostatic clamp, a first clamping voltage may be applied to a first clamping electrode of the plurality of clamping electrodes, and a second clamping voltage may be applied to a second clamping electrode of the plurality of clamping electrodes. have. The first and second clamping voltages may have opposite polarities.

The device may further comprise a voltage source. The voltage source can be configured to supply the clamping voltage. The clamping voltage may be, for example, a voltage of about ±1 to 10 kV. The clamping voltage may be, for example, a voltage of about ±2 kV.

The electrostatic clamp may further include at least one contact configured to provide an electrical connection to the at least one electrode. The mechanism for generating free charges may be configured to generate free charges adjacent the at least one contact point during the transition from the first energized state of the electrostatic clamp to the second energized state of the electrostatic clamp.

In the first energized state, a voltage having a first polarity may be applied to at least one electrode. In the second energized state, a voltage having a second polarity opposite to the first polarity may be applied to at least one electrode.

The voltage source may be configured to supply the voltage having the first polarity and/or the voltage having the second polarity.

The electrostatic clamp may include at least two electrodes. In the first energized state, a voltage having a first polarity may be applied to a first electrode among the electrodes, and a voltage having a second polarity may be applied to a second electrode among the electrodes. In the second energized state, a voltage having a second polarity may be applied to the first electrode among the electrodes, and a voltage having the first polarity may be applied to the second electrode among the electrodes.

The electrostatic clamp may further include at least two secondary electrodes. In the first energized state, the voltage having a first polarity may be applied to a first electrode of the secondary electrodes, and the voltage having a second polarity may be applied to a second electrode of the secondary electrodes. In the second energized state, the voltage having a second polarity may be applied to the first electrode of the secondary electrodes, and the voltage having the first polarity may be applied to the second electrode of the secondary electrodes. have.

The first electrode among the electrodes may be electrically connected to the first electrode among the secondary electrodes. The second electrode among the electrodes may be electrically connected to the second electrode among the secondary electrodes.

The clamp may be configured such that in each of the first and second energized states, the component can be clamped by the electrostatic clamp.

A mechanism for generating free charges adjacent to the electrostatic clamp may include a gas source and an ionizing radiation source configured to ionize the gas provided by the gas source.

The ionizing radiation source may include a source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate may also be provided. The lithographic apparatus may comprise an apparatus according to the first aspect of the invention. The patterning device may include the component to be clamped.

The lithographic apparatus may further comprise an illumination system configured to condition the radiation beam. An electrostatic clamp can be configured to clamp the patterning device. The patterning device can impart a pattern to its cross-section of the radiation beam to form a patterned radiation beam. The lithographic apparatus may further include a substrate table configured to hold a substrate. The lithographic apparatus may further include a projection system configured to project the patterned beam of radiation onto the substrate.

The lithographic apparatus may be configured to perform a plurality of imaging exposures-a radiation beam is incident on a patterning device during the exposure and a patterned radiation beam is projected onto a substrate during the exposure. An electrostatic clamp can be configured to clamp the patterning device during the imaging exposure. Between successive exposures of the plurality of imaging exposures, the electrostatic clamp may be configured to transition from the first energized state to the second energized state.

Each imaging exposure may include exposure of a plurality of dies on the wafer. The clamp may be repolarized during the period between exposure of the first wafer and the second wafer.

A lithographic system comprising the lithographic apparatus may also be provided.

The lithographic system may further comprise a radiation source configured to generate the radiation beam. The mechanism for generating free charges may include a secondary ionizing radiation source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

The lithographic system may further comprise a radiation source configured to generate the radiation beam, and the mechanism for generating free charges comprises the radiation source. The radiation source may be an EUV source.

The lithographic system may also be configured to perform at least one non-imaging exposure-a beam of radiation is incident on the patterning device during the exposure and no radiation is projected onto the substrate during the exposure; The non-imaging exposure is performed between successive exposures among the plurality of imaging exposures.

During a series of imaging exposures, the lithographic apparatus may also be configured to perform non-imaging exposures between each successive imaging exposure.

The transition from the first energized state of the electrostatic clamp to the second energized state of the electrostatic clamp may be performed during the non-imaging exposure.

By providing a beam of radiation to the patterning device during non-imaging exposure, it is possible to provide a source of free charges by the plasma that will be created by ionization of gas molecules (eg, hydrogen) present around the patterning device. Plasma is generated both in areas that are directly illuminated by the radiation beam and in adjacent areas (eg, by diffusion and secondary electrons). In this way, a single beam of radiation (eg, a beam of EUV radiation) can be used for imaging purposes and to provide a source of free charges during repolarization of the electrostatic clamp.

The lithographic system can be controlled such that the amount of radiation incident on the patterning device is greater during each imaging exposure than during the non-imaging exposure.

It will be appreciated that the amount of radiation incident on the patterning device can be controlled in a number of ways. For example, the amount of radiation can be controlled by changing the intensity of radiation generated by the radiation source. Alternatively or additionally, the amount of radiation can be controlled by changing the spatial extent of the radiation beam incident on the patterning device (eg, using a shutter or masking blade). Alternatively or additionally, the amount of radiation can be controlled by changing the number or frequency of pulses of radiation generated by the radiation source.

For example, the radiation dose required to provide sufficient free charges to provide useful screening (during a predetermined period of time) may be less than the radiation dose required to perform the imaging exposure.

The amount of radiation incident on the patterning device can be gradually increased from a non-imaging exposure to an imaging exposure of one of the plurality of imaging exposures.

In the above-described embodiment, the shutter or masking blade or masking blade configuration can thus be used to control the amount of free charges generated around the electrostatic clamp, for example by partially blocking the radiation beam.

It is worth mentioning that the portion of the radiation beam impinging on the shutter or masking blade configuration can also cause the generation of free charges. As such, an alternative way to generate free charges adjacent to the electrostatic clamp is to provide a beam of radiation to a different surface than the patterning device, for example the surface of a shutter or masking blade. As described above, the shutter or masking blade configuration can be used to block the radiation beam from reaching the patterning device or to control the spatial expansion of the radiation beam incident on the patterning device. Application of a radiation beam or a portion thereof to the shutter or mask blade(s) may be performed with gas molecules (e.g., hydrogen) present or provided in, around, or near the radiation beam and the shutter or mask blade configuration. The interaction of can also lead to the generation of free charges. Free charges are generated both in areas that are directly illuminated by the radiation beam and in adjacent areas (eg, by diffusion and secondary electrons). Since the shutter or mask blade configuration is typically relatively close to the patterning device and thus the electrostatic clamp, free charges generated by irradiating the shutter or mask blade configuration can also cause free charges adjacent to the electrostatic clamp. So the generation of free charges by irradiating the shutter or mask blade configuration can also be considered a mechanism for generating free charges adjacent to the electrostatic clamp.

Regarding this method of generating free charges adjacent to the electrostatic clamp, it is pointed out that if the shutter or mask blade configuration is completely blocked during the generation of free charges, the risk of unwanted irradiation reaching the substrate can be avoided. can do.

According to a second aspect of the invention there is provided a method of operating an apparatus, the apparatus comprising an electrostatic clamp and a mechanism for generating free charges adjacent the electrostatic clamp. The method includes: controlling the electrostatic clamp to have a first energized state; Controlling the electrostatic clamp to have a second energized state; And during the transition from the first energized state to the second energized state, controlling a mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp.

The method may further include providing a component adjacent to the electrostatic clamp. When the electrostatic clamp is in either or both of the first and/or second energized states, the component may be clamped by the electrostatic clamp.

Controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp and/or component includes providing a gas adjacent to the electrostatic clamp and/or component, and the electrostatic clamp and/or component such that the gas is ionized. And controlling the ionizing radiation source to provide ionizing radiation adjacent to.

Of course, it will be appreciated that any of the features described above in combination with the apparatus of the first aspect of the invention may be combined with features of the method of the second aspect of the invention.

According to a third aspect of the invention, there is provided an illumination system configured to condition a radiation beam; A support structure configured to support a patterning device, the patterning device being capable of imparting a pattern to its cross section of the radiation beam to form a patterned radiation beam, the support structure comprising an electrostatic clamp configured to clamp the patterning device ; A substrate table configured to hold a substrate; And a projection system configured to project a patterned beam of radiation onto a substrate. The lithographic apparatus is configured to perform a pre-imaging exposure-a radiation beam is incident on the patterning device during the exposure and no radiation is projected onto the substrate during the exposure. The amount of radiation incident on the patterning device is gradually increased during exposure prior to imaging. The lithographic apparatus is configured to perform imaging exposure in which a radiation beam patterned by a patterning device is projected onto a substrate.

A gradual or smooth ramp up of the EUV power will gradually increase the conductivity of the patterning device and the area around the electrostatic clamp. This gradual increase in the conductivity of the medium does not cause the electrostatic field to collapse in an abrupt manner, but rather allows charge to leak to various surfaces along the existing electric field lines. This process allows all surfaces that were charged as a result of previous clamp polarization conditions to be neutralized. Likewise, all charged particles on the surfaces of the clamp or patterning device can thus be neutralized. This gradual increase in EUV power can greatly reduce the change in electrostatic discharge that occurs, and thus can slow down the generation rate of particles (often caused by a discharge event).

The imaging exposure may include a burst, the burst including a plurality of radiation pulses. Each of the pulses may comprise a substantially constant radiation dose. It will be appreciated that the radiation beam may be pulsed during imaging exposure, so the instantaneous radiation intensity is not always uniform. However, the pulse rate during imaging exposure may be high enough that the radiation incident on the patterning device and the patterned substrate are substantially uniform within a reference frame comprising tens, hundreds, or thousands of pulses. In addition, the pulse rate is high enough so that any plasma generated by EUV radiation is likely to last longer than the gap between adjacent pulses, so once established, the equilibrium plasma density is reached and held by the pulses in progress. It will be.

The pre-imaging exposure may precede immediately before the imaging exposure.

The exposure before imaging may include a burst, and the burst includes a plurality of radiation pulses.

Each pulse of the imaging exposure and/or the pre-imaging exposure may have, for example, a duration of about 100 ns and a pulse pitch of about 20 ns (ie, a pulse frequency of about 50 kHz).

The pulse rate during pre-imaging exposure may be high enough that the hydrogen plasma generated (directly or indirectly) by the incident EUV photons of each pulse of the pre-imaging exposure lasts longer than the gap between adjacent pulses. The burst may include a plurality of mini-bursts, and each mini-burst includes a plurality (eg, up to 10) of radiation pulses.

A gradual increase in the amount of radiation during exposure prior to the imaging may be configured to be provided over a plurality of the radiation pulses.

In this way, since the amount of radiation incident on the patterning device is gradually increased during exposure before imaging, the plasma density can be gradually increased, so that a desired gradual increase in the conductivity of the area around the patterning device can be achieved. The incremental increase can be configured to be provided across a plurality of said mini bursts.

The gradual increase in radiation during the pre-imaging exposure may be configured to provide over at least 1000 radiation pulses.

In a system with a pulse frequency of about 50 kHz, 1000 radiation pulses are delivered in about 20 ms. The gradual increase in radiation during the pre-imaging exposure can be configured to provide over a maximum of about 50,000 radiation pulses (ie, a maximum of about 1 second). Preferably, the gradual increase in radiation during the exposure prior to imaging may be configured to be provided over a maximum of about 10,000 radiation pulses (ie, maximum about 0.2 seconds).

There is a trade-off between the time it takes to increase the radiation intensity to the maximum intensity required for imaging (if too long, it can lower wafer throughput) and the achievement of the benefits of reducing the likelihood of discharge and/or particle generation or emission. Will understand.

The gradual increase in radiation during exposure before the imaging may be configured to be provided substantially linearly over a plurality of the radiation pulses.

The radiation intensity may increase by a predetermined amount for each of a plurality of successive radiation pulses, so that the radiation intensity incident on the patterning device gradually increases in a substantially linear manner.

During a first portion of the pre-imaging exposure with a predetermined duration, the radiation beam can be controlled to deliver a first dose of radiation to the patterning device, the first dose being delivered to the patterning device during the first portion of the imaging exposure. Less than about 10% of the imaging dosage of the radiation being subjected to, and the first portion of the exposure prior to imaging has the predetermined duration.

It will be appreciated that the actual duration of the first portion of the exposure before imaging and the first portion of the imaging exposure is not critical. Rather, the device provides the dose of radiation delivered during a predetermined period (e.g., 200 μs) during the first portion of the exposure prior to imaging, and the dose of radiation delivered during a corresponding predetermined period (e.g., 200 μs). It is controlled to be less.

The first dose may include a non-zero dose. The first dose may comprise about 5% of the imaging dose of radiation delivered to the patterning device during the first portion of the imaging exposure.

The first portion of the exposure before imaging may include a plurality of pulses. Thus, the first dose may include the total dose delivered by the plurality of pulses within the first portion of the exposure prior to imaging. The first portion of the imaging exposure may also include a plurality of pulses. Thus, the imaging dose of radiation delivered to the patterning device during the first portion of the imaging exposure may include the total dose delivered by the plurality of pulses within the first portion of the imaging exposure.

Prior to the first portion of the exposure prior to imaging, the radiation intensity of the radiation delivered to the patterning device by the radiation beam may be substantially zero. Thus, the first portion of the exposure prior to imaging can constitute a jump from about 0% of the imaging dose to about 5% of the imaging dose.

The amount of radiation incident on the patterning device may be gradually increased from the first portion of the exposure before imaging to the imaging exposure.

During a second portion of the pre-imaging exposure with the predetermined duration-the second portion follows the first portion-the radiation beam may be controlled to deliver a second dose of radiation to the patterning device, the second The dose is greater than the first dose and less than the imaging dose of radiation delivered to the patterning device during the first portion of the imaging exposure.

At least 1000 radiation pulses may be delivered between the initiation of the first portion of the pre-imaging exposure and the initiation of the imaging exposure.

In this way, the radiation dose can be gradually increased from the first portion of the pre-imaging exposure to the imaging exposure over a period spanning at least 1000 radiation pulses.

The lithographic apparatus may also be configured to perform a post-imaging exposure-the radiation beam is incident on the patterning device during the exposure and no radiation is projected onto the substrate during the exposure-and radiation incident on the patterning device. The amount of is gradually decreased during exposure after imaging, and the post-imaging exposure is followed by the imaging exposure.

The lithographic apparatus may also be configured to perform a first imaging exposure and a second imaging exposure. The pre-imaging exposure may be preceded immediately before the second imaging exposure. Between the first imaging exposure and the pre-imaging exposure, the lithographic apparatus may also be configured to perform a non-imaging exposure-a radiation beam is incident on the patterning device during the exposure and no radiation is projected onto the substrate during the exposure. .

During a first portion of the non-imaging exposure having the predetermined duration, a beam of radiation may be controlled to deliver a third dose of radiation to the patterning device, the third dose being less than about 10% of the imaging dose of radiation. Includes. The apparatus may be configured to transition an electrostatic clamp from a first energized state to a second energized state during the non-imaging exposure.

In other words, the clamp can be repolarized during non-imaging exposure between imaging exposures.

The lithographic apparatus may also be configured to transition the electrostatic clamp from the first energized state to the second energized state during the exposure before the imaging.

That is, the clamp can be repolarized during the increase in EUV power prior to the imaging exposure. This allows a low level of radiation (and thus a relatively low plasma density) to be provided during the repolarization of the clamp, and the maximum radiation intensity is provided only during the imaging exposure. Low levels of radiation may be sufficient to mitigate some of the adverse effects associated with repolarization of the clamp (eg particle emission) and may reduce the load on the radiation source. More generally, the application of low levels of radiation can also be advantageously applied during any voltage change applied to the clamp. As an example, the application of a low level of radiation can be advantageously applied when an object is loaded into or unloaded from the clamp.

During a series of imaging exposures, the lithographic apparatus may also be configured to perform non-imaging exposures between each successive imaging exposure.

It will be appreciated that the amount of radiation incident on the patterning device can be controlled in a number of ways. For example, the amount of radiation can be controlled by changing the intensity of radiation generated by the radiation source. Alternatively or additionally, the amount of radiation can be controlled by changing the spatial extent of the radiation beam incident on the patterning device (eg, using a shutter or masking blade). Alternatively or additionally, the amount of radiation can be controlled by changing the number or frequency of pulses of radiation generated by the radiation source.

For example, the radiation dose required to provide sufficient free charges to provide useful screening may be less than the radiation dose required to perform the imaging exposure.

A lithographic system comprising the lithographic apparatus of the third aspect of the present invention may also be provided. The lithographic system may further comprise a radiation source configured to generate the radiation beam. The radiation source may be an EUV source.

According to a fourth aspect of the present invention, a method of operating a lithographic apparatus is provided. A lithographic apparatus comprises: an illumination system configured to condition a radiation beam; A support structure configured to support a patterning device, the patterning device being capable of imparting a pattern in its cross section of the radiation beam to form a patterned radiation beam, comprising: a support structure including an electrostatic clamp configured to clamp the patterning device; A substrate table configured to hold a substrate; And a projection system configured to project the patterned beam of radiation onto the substrate. The method is a step of causing a lithographic apparatus to perform exposure before imaging-a radiation beam is incident on the patterning device during the exposure and no radiation is projected onto the substrate during the exposure-wherein the amount of radiation incident on the patterning device is determined before imaging. And the step, which is gradually increased during exposure. The method further includes causing the lithographic apparatus to perform an imaging exposure in which the radiation beam patterned by the patterning device is projected onto the substrate.

Of course, it will be appreciated that any of the features described above in combination with the apparatus of the third aspect of the invention may be combined with the features of the method of the fourth aspect of the invention.

According to a fifth aspect of the invention there is provided an apparatus comprising an electrostatic clamp for clamping a component and a mechanism for generating free charges adjacent the electrostatic clamp, the apparatus comprising: a first configuration in which the component is clamped by the electrostatic clamp; And the component has a second configuration spaced from the electrostatic clamp. The device is configured to be in the first configuration at a first point in time and in the second configuration at a second time after the first point in time. The mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic clamp and/or component at a third time point between the first time point and the second time point.

In order to avoid adverse effects associated with voltage amplification due to changes in capacitance associated with an increase in the spacing between various electrically isolated system components, freedom from the apparatus during handling operations, for example during removal of the patterning device from the lithographic apparatus. Charges can be used. In particular, as the clamped component is removed from the clamp, the capacitance therebetween decreases (inversely proportional to the spacing). The free charges generated can be used to move the charge to reduce the effect of voltage amplification, thereby lowering the likelihood of discharge (e.g., through hydrogen decomposition when the Paschen limit is exceeded). have.

The mechanism for generating free charges may be configured to generate free charges adjacent to the electrostatic clamp and/or component to prevent a potential difference between the electrostatic clamp and the component from exceeding a predetermined threshold.

It will be appreciated that both the timing and extent of free charge generation are somewhat flexible. In particular, if the potential difference between the electrostatic clamp and the component exceeds the threshold (e.g., the breakdown voltage of hydrogen according to Paschen's law) due to the change in capacitance associated with the increase in the gap between the electrostatic clamp and the component Can occur. However, it will also be understood that the minimum discharge voltage is a function of both distance and gas pressure. In this way, the voltage threshold is different for each configuration.

The predetermined threshold is determined based on the pressure in the device. This pressure may be the pressure of hydrogen.

A third point in time may be selected to prevent a potential difference between the electrostatic clamp and the component from exceeding the predetermined threshold.

Free charges may be generated at a time selected to provide a charge to reduce (or limit) this potential difference before the potential difference between the electrostatic clamp and the component exceeds a threshold value (ie, a third time point).

The predetermined threshold can be about 250 V or less, for example about 130 V.

The device may be configured to be in the second configuration at the third point in time.

In particular, free charges can be generated immediately after the clamping voltage is removed and the component begins to separate from the clamp. In this way, the generated free charges can easily reach the clamp and the surfaces of the clamped component.

At the third point in time, the minimum spacing between the surface of the clamp and the surface of the component may be greater than about 10 μm.

The surface of the clamp may be the surface of the clamp that contacts the component when the component is clamped. It will be appreciated that the clamp may comprise a generally planar surface provided with protrusions (which may be referred to as burls). The protrusions may allow the spacing between the generally planar surface of the clamp and the clamped surface of the component to exceed a minimum value (eg 10 μm) even during clamping. However, it will be appreciated that during clamping, the surface of the protrusions comes into contact with the clamped component, so that during clamping the minimum distance between the surface of the clamp and the surface of the component is zero.

At the third point in time, the minimum spacing between the surface of the clamp and the surface of the component may be about 100 μm or more.

At the third point in time, the minimum spacing between the surface of the clamp and the surface of the component may be less than the predetermined spacing. The predetermined interval may be about 200 μm.

The mechanism for generating free charges can be configured to generate free charges adjacent to the electrostatic clamp and/or component when the device is configured to be in the first configuration.

The device may be configured to be in the first configuration at the third point in time.

In use, it will be appreciated that when the component is clamped by the electrostatic clamp, the component will shield portions of the electrostatic clamp from generated free charges. Thus, the mechanism for generating free charges can be configured to generate free charges adjacent to the electrostatic clamp, but such charges are prevented from reaching the areas of the clamp that are typically covered directly by the component being clamped in use. do. However, as the component separates from the clamp (i.e. after the clamping voltage is removed), free charges can diffuse to the clamp and the surfaces of the clamped component, thus providing compensation for the voltage amplification effect. I can understand. Thus, free charges generated adjacent to the clamping device and/or component when the component is clamped can provide an effective reduction in voltage amplification.

The electrostatic clamp can include a clamping area configured to clamp the component. When a component is clamped, a clamping electric field may be generated between the clamping region and the component.

The electrostatic clamp may include at least one electrode. When a component is clamped by an electrostatic clamp, a clamping voltage may be applied to at least one electrode such that a clamping electric field is generated between the clamping region and the component.

The device may further comprise a voltage source.

A mechanism for generating free charges adjacent to the electrostatic clamp may include a gas source and an ionizing radiation source configured to ionize the gas provided by the gas source.

The ionizing radiation source may comprise a source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

The apparatus may further include a component exchange assembly configured to remove the component from the electrostatic clamp.

The component exchange assembly can be configured to control the spacing between the component and the electrostatic clamp.

A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate may also be provided. A lithographic apparatus may comprise an apparatus according to the fifth aspect of the invention. The patterning device may include the component to be clamped.

The lithographic apparatus may further comprise: an illumination system configured to condition the radiation beam. An electrostatic clamp can be configured to clamp the patterning device. The patterning device can impart a pattern to its cross-section of the radiation beam to form a patterned radiation beam. A lithographic apparatus includes a substrate table configured to hold a substrate; And a projection system configured to project the patterned radiation beam onto the substrate.

A lithographic system comprising a lithographic apparatus may also be provided. The lithographic system may further comprise a radiation source configured to generate the radiation beam.

The mechanism for generating free charges may include a secondary ionizing radiation source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

The mechanism for generating free charges may include the radiation source. The radiation source may be an EUV source.

According to a sixth aspect of the present invention, a method of operating an apparatus is provided. The device includes an electrostatic clamp and a mechanism for generating free charges adjacent the electrostatic clamp. The method comprises: providing a component adjacent to the electrostatic clamp; Controlling the electrostatic clamp so that the component has a first configuration clamped by the electrostatic clamp at a first point in time; Controlling the electrostatic clamp so that the component has a second configuration spaced apart from the electrostatic clamp at a second time point after the first time point; And controlling a mechanism for generating free charges to generate free charges adjacent to the component and/or an electrostatic clamp at a third time point between the first time point and the second time point.

Of course, it will be appreciated that any of the features described above in combination with the apparatus of the fifth aspect of the invention may be combined with the features of the method of the sixth aspect of the invention.

According to a seventh aspect of the invention, an apparatus is provided comprising an electrostatic clamp for clamping a component and a mechanism for generating free charges adjacent the electrostatic clamp. The device comprises a first configuration in which a voltage having a first polarity is applied to at least one clamp electrode and the component is not clamped by an electrostatic clamp; And a second configuration in which a voltage having a second polarity opposite to the first polarity is applied to at least one clamp electrode. The device is configured to be in the first configuration at a first time point and in the second configuration at a second time point after the first time point. The mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the first configuration and not to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the second configuration.

By providing free charges adjacent to the electrostatic clamp when the clamp is polarized in the first energized state, particles trapped in the clamp surface can be charged. After that, when the polarization is reversed and free charges are no longer present, charged particles can be released from the clamp surface by electrostatic repulsion. This cleaning process of an electrostatic clamp can avoid or at least reduce the adverse effects associated with particles trapped on the surface of the clamp.

The device may be a device for cleaning the electrostatic clamp.

A component may be provided adjacent to the electrostatic clamp during the transition from the first configuration to the second configuration.

By providing the component adjacent to the electrostatic clamp during the transition, particles emitted by the clamp due to changes in polarization can be trapped by the component rather than contaminating other surfaces of the device. The component may be referred to as a sacrificial component or a cleaning component. The component can include a patterning device. The component may have a size substantially equal to the size of the component intended to be clamped by an electrostatic clamp during a lithographic operation.

When the clamp is in the second configuration the component can be clamped by means of an electrostatic clamp. That is, the electrostatic clamp can be configured to clamp the component when the clamp is in the second configuration.

The device may have a third configuration in which no voltage is applied to at least one clamp electrode and the component is not clamped by an electrostatic clamp. The device may be configured to be in the third configuration at a third time point after the second time point.

After the particles are released by application of reverse polarization (during the second configuration), the clamp can be returned to the neutral configuration (ie, no voltage is applied to the clamp electrode(s)).

The mechanism for generating free charges may be configured to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the third configuration.

After particles are released by application of reverse polarization (during the second configuration), the clamp can be returned to a neutral configuration (i.e., no voltage is applied to the clamp electrode(s)), and all residual charge on the clamp surface is It can be removed by providing free charges adjacent to.

In the third configuration, no components are provided adjacent to the electrostatic clamp. That is, the device can be controlled so that no components are provided adjacent to the electrostatic clamp in the third configuration.

The apparatus may further comprise a component exchange assembly configured to support a component adjacent to the electrostatic clamp.

The component exchange assembly can be configured to control the spacing between the component and the electrostatic clamp.

The component exchange assembly may be configured to provide the component adjacent to the electrostatic clamp between the first and second time points.

The component exchange assembly may be configured to remove the component from a location adjacent to the electrostatic clamp between the second and third time points.

The electrostatic clamp can include a clamping area configured to clamp the component. When a component is clamped, a clamping electric field may be generated between the clamping region and the component.

When the clamping voltage is applied to at least one electrode, the clamping electric field may be generated. The device may further comprise a voltage source.

A mechanism for generating free charges adjacent to the electrostatic clamp may include a gas source and an ionizing radiation source configured to ionize the gas provided by the gas source.

The ionizing radiation source includes a source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate may also be provided. A lithographic apparatus may comprise an apparatus according to the seventh aspect of the invention. An electrostatic clamp can be configured to clamp the patterning device during a lithographic operation.

The lithographic apparatus may further comprise an illumination system configured to condition the radiation beam. An electrostatic clamp may be configured to clamp the patterning device, and the patterning device may impart a pattern to its cross-section of the radiation beam to form a patterned radiation beam. A lithographic apparatus includes a substrate table configured to hold a substrate; And a projection system configured to project the patterned radiation beam onto the substrate.

A lithographic system comprising a lithographic apparatus may also be provided. The lithographic system may further comprise a radiation source configured to generate the radiation beam.

The mechanism for generating free charges may include a secondary ionizing radiation source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source.

The mechanism for generating free charges may include the radiation source. The radiation source may be an EUV source.

According to an eighth aspect of the invention, a method of operating a device is provided, the device comprising an electrostatic clamp, the electrostatic clamp comprising at least one clamp electrode, and a mechanism for generating free charges adjacent the electrostatic clamp. Include. The method comprises: controlling the electrostatic clamp such that a voltage having a first polarity is applied to at least one clamp electrode at a first time point and the component has a first configuration that is not clamped by the electrostatic clamp; Controlling the electrostatic clamp to have a second configuration in which a voltage having a second polarity opposite to the first polarity is applied to at least one clamp electrode at a second point after the first point of time; Controlling a mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp when the clamp is in the first configuration; And controlling the mechanism for generating free charges so as not to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the second configuration.

The method may further include providing a component adjacent the electrostatic clamp when the electrostatic clamp is in the second configuration.

When the clamp is in the second configuration, the component can be clamped by means of an electrostatic clamp.

The method may further include controlling the device to have a third configuration in which a voltage is not applied to at least one clamp electrode at a third time point after the second time point.

The method may further include removing the component from a location adjacent to the electrostatic clamp between the second and third time points.

The method may further include controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the third configuration.

Of course, it will be appreciated that any of the features described above in combination with the apparatus of the seventh aspect of the invention may be combined with the features of the method of the eighth aspect of the invention.

According to a ninth aspect of the invention, there is provided a method of operating a device, the device comprising an electrostatic clamp having a first electrode, the method comprising: a) providing a component adjacent the electrostatic clamp; b) controlling the electrostatic clamp to provide a first clamping voltage to the first electrode such that the component is clamped by the electrostatic clamp; c) measuring the voltage associated with the portion of the component; And d) determining an adjustment for the first clamping voltage based on the measured voltage.

The electrostatic clamp includes a first electrode to which a first clamping voltage is supplied. The first clamping voltage generates an electric field between the clamp and the component, allowing the component to be clamped to the clamping area of the clamp at its rear surface. The component also includes a front side disposed opposite the rear side. The component can accumulate charge through the electrostatic attraction of particles (i.e. electrons or ions), especially on the back side adjacent to the clamp, so it can be at a different voltage compared to other parts of the system. . By measuring the voltage associated with a portion of the component (eg, the rear of the component), an adjustment to the first clamping voltage can be determined such that the voltage associated with the portion of the component is zero. It will be appreciated that the adjustment may be determined in terms of an additional voltage that is added or subtracted to the first clamping voltage, or as a percentage change in the first clamping voltage, or in any other suitable aspect that is apparent to the skilled person. By determining and applying an adjustment to the first clamping voltage, a virtual grounding of the component to the peripheral system can be achieved. As a result, the electrostatic attraction of charges (and charged particles) on the component can be reduced or eliminated. The component may be a patterning device (eg, a reticle) clamped by an electrostatic clamp. However, it will be appreciated that the component may be another component that is clamped by a clamp.

The method comprises: b1) exposing the component to radiation while the component is clamped by an electrostatic clamp; b2) controlling the electrostatic clamp so that the component is released from the electrostatic clamp; And b3) removing the component from the vicinity of the electrostatic clamp.

In this way, voltage measurements can be carried out in an offline process. In this embodiment, the component is clamped by an electrostatic clamp and exposed to radiation. During exposure, charge may be deposited on the back side of the component (ie, the side of the component clamped to the clamp). When the clamping voltage is removed to release the component, the accumulated charge remains on the surface of the component. The component can then be removed from the vicinity of the clamp to measure the associated voltage. The adjustment for the first clamping voltage can then be determined based on the measured voltage.

The method includes: e) adjusting the first clamping voltage according to the determined adjustment; And f) repeating steps a) to c) to verify the adjustment.

Once the adjustment for the first clamping voltage has been determined, steps a) to c) can be repeated to verify that this adjustment successfully reduces the voltage associated with the component to 0 V. If it is determined that the voltage associated with some of the components is still present, further adjustment and verification steps can be performed. As a result, since no net charge remains in the virtually grounded component after exposure, problems related to voltage amplification during unloading of the previously clamped component can be alleviated.

The method may further include, after repeating steps a) to c), determining an additional adjustment for the first clamping voltage based on the measured voltage. The measurement and adjustment process can be repeated multiple times. The measurement and adjustment process may be repeated until the voltage associated with a portion of the component meets a predetermined criterion (eg, is within a predetermined tolerance of 0 V).

The repetition of steps a) to c) can be performed using other components. In particular, in some implementations, the verification step may be performed using the same components used for the initial measurement. However, in other implementations, it may be desirable to use other components that were not used in the initial measurement.

The method includes: after determining an adjustment for the first clamping voltage, adjusting the first clamping voltage according to the determined adjustment; And measuring a voltage associated with the portion of the component.

As an alternative to the offline measurement process, the measurement can be carried out online, ie while the component is clamped. In an implementation of this method, the adjustment is determined immediately after the measuring step without the component being released and removed from the clamp. It may be desirable to perform adjustment and subsequent measurement steps while the component remains clamped in place. As such, processing time can be shortened because there is no need to remove the component from the clamp to measure the associated voltage. Also, the method can be repeated regularly. Alternatively or additionally, the method may be implemented with a continuous feedback loop in which the voltage associated with the component is monitored and the clamping voltage(s) is automatically adjusted as a result.

The electrostatic clamp may further include a second electrode, and the method may further include: determining an adjustment for the second clamping voltage supplied to the second electrode based on the measured voltage.

The first and second clamping voltages may have different values. In particular, the first and second clamping voltages may have different polarities. For example, the first clamping voltage may be approximately +1 to 10 kV, and the second clamping voltage may be approximately -1 to 10 kV. In particular, the first clamping voltage may be approximately +2 kV, and the second clamping voltage may be approximately -2 kV. Of course, it will be appreciated that there may be differences in the absolute values of the first and second clamping voltages. Further, it will be appreciated that the determined adjustment to the first and/or second clamping voltage may be an adjustment to the first clamping voltage, or the second clamping voltage, or both the first and second clamping voltages. Further, the determined adjustment may be, for example, an adjustment of a difference between the first and second clamping voltages or an adjustment to an average value of the first and second clamping voltages.

According to a tenth aspect of the present invention, a system for virtually grounding a component is provided, comprising an electrostatic clamp configured to clamp the component, the electrostatic comprising a first electrode configured to receive a first clamping voltage. ; A voltage monitor configured to measure a voltage associated with a portion of the component; And a calculation unit configured to determine an adjustment for the first clamping voltage based on the measured voltage.

The system may further include a support assembly configured to support the component, the support assembly including a voltage monitor. The support assembly can be, for example, an exchange assembly configured to carry the component towards or away from the clamp.

The electrostatic clamp may include a second electrode configured to receive a second clamping voltage, and the calculating unit may also be configured to determine an adjustment to the second clamping voltage based on the measured voltage.

The first and second clamping voltages may have different values.

The voltage monitor may be an electrostatic voltmeter.

Of course, it will be appreciated that any of the features described above in combination with the method of the ninth aspect of the present invention may be combined with the features of the system of the tenth aspect of the present invention.

In addition, it will be appreciated that any of the features described above in combination with any of the first to tenth aspects of the invention may be combined with the features described in the context of different of the above aspects.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings.
1 shows a lithographic system comprising a lithographic apparatus and a radiation source.
2A-2C illustrate an electrostatic clamp used in the lithographic apparatus shown in FIG. 1 in cross-sectional, top and cross-sectional views, respectively.
3 shows a simulation of the plasma density around the electrostatic clamp of FIG. 2 during exposure to EUV radiation.
4A and 4B each show an exposure sequence and a clamp polarization sequence of a lithographic apparatus of the prior art.
5A and 5B each show an exposure sequence and a clamp polarization sequence of a lithographic apparatus according to an embodiment of the present invention.
6A and 6B each show an alternative exposure sequence and a clamp polarization sequence of a lithographic apparatus according to an embodiment of the present invention.
7A and 7B respectively show another alternative exposure sequence and clamp polarization sequence of a lithographic apparatus according to an embodiment of the present invention.
8A-8C, respectively, in cross-sectional, top and cross-sectional views, illustrate an electrostatic clamp used in a lithographic apparatus according to an alternative embodiment of the present invention.
9A and 9B illustrate an electrostatic clamp and a patterning device used in a lithographic apparatus according to embodiments of the present invention during a patterning device unloading process.
10 shows a sequence of movement of a patterning device and an electrostatic clamp used in a lithographic apparatus according to embodiments of the present invention during a patterning device unloading process.
11 shows an equivalent circuit model of an electrostatic clamp and patterning device used in a lithographic apparatus according to embodiments of the present invention.
12A-12C show simulated characteristics of the equivalent circuit of FIG. 9 during the movement sequence of FIG. 10.
13 shows an equivalent circuit model of an electrostatic clamp and patterning device used in a lithographic apparatus according to embodiments of the present invention.
14 shows an electrostatic clamp and patterning device used in a lithographic apparatus according to embodiments of the present invention.
15A-15E show a cleaning process of an electrostatic clamp used in a lithographic apparatus according to embodiments of the present invention.
16 shows a flow chart of a method of providing virtual grounding of a patterning device according to an embodiment.
17 shows a flow chart of an alternative method of providing a virtual ground of a patterning device according to another embodiment.
18 shows an electrostatic clamp and patterning device with a support assembly comprising a voltage monitor according to embodiments of the present invention.

1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA is a substrate configured to support an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate W. Includes table WT.

The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident on the patterning device MA. Accordingly, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The facet field mirror device 10 and the facet pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to or instead of the facet field mirror device 10 and the facet pupil mirror device 11.

After being conditioned in this way, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B'is generated. The projection system PS is configured to project the patterned EUV radiation beam B'onto the substrate W. To this end, the projection system PS may comprise a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B'onto the substrate W held by the substrate table WT. have. The projection system PS can apply a reduction factor to the patterned EUV radiation beam B', thereby forming an image with features that are smaller than the corresponding features on the patterning device MA. I can. For example, a reduction factor of 4 or 8 may be applied. The projection system PS is shown in FIG. 1 as having only two mirrors 13, 14, but the projection system PS may include a different number of mirrors (e.g., 6 or 8 mirrors). have.

The substrate W may include a previously formed pattern. In such a case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B'with the pattern previously formed on the substrate W.

A small amount of gas (eg, hydrogen) at a relative vacuum, ie much lower than atmospheric pressure, may be provided to the radiation source SO, the illumination system IL, and/or the projection system PS.

The radiation source (SO) is a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL), or any other capable of generating EUV radiation. Other sources of radiation.

2A shows a cross section of the support structure MT in more detail. The cross section is in the x plane extending vertically in the z direction and horizontally in the y direction in the orientation shown. The support structure MT includes an electrostatic clamp 100 configured to clamp the patterning device MA during a lithographic operation. The clamp 100 includes a clamp body 102 and clamp electrodes 104A to 104D disposed in the clamp body 102. Electrodes 104A-104D are separated from the generally planar clamping surface of clamp 100 by a dielectric coating. Burls 106 protrude from the clamping surface of the clamp body 102 and serve to separate the clamped patterning device MA from the clamp body 102. The burls 106 may have a height of, for example, about 10 μm, and may collectively cover about 1% of the surface of the clamp 100. It will be appreciated that for simplicity, many features of clamp 100 (eg, wiring, additional electrodes) have been omitted.

The patterning device (MA) is a substrate 120 that may be formed of a material typically having an ultra-low coefficient of thermal expansion (e.g., ULE ^® manufactured by Corning or Zerodur ^® manufactured by Schott AG). Includes. The substrate 120 is generally planar and has first and second planar surfaces 122 and 124 opposed to each other. In use (for example, as shown in Fig. 1), the first surface 122 is configured to reflect the radiation beam B to impart a pattern to the beam B. In particular, the area of the first surface 122 may be patterned so that the radiation beam B is patterned. A conductive coating 126 is provided on the patterned area of the first surface.

In order to allow the electrostatic clamp 100 to clamp the patterning device MA, the second surface 124 is typically provided with a conductive coating 128 that covers a majority of the second surface 124.

It will be appreciated that the electrostatic clamp 100 may use a voltage of the order of several kV to clamp the patterning device MA. For example, the clamp 100 may be a bipolar electrostatic clamp, wherein the first subset 102A, 102C of the electrodes 104A to 104D is about +1, ... , Is connected to a voltage supply (not shown) of 10 kV (eg, +2 kV), and the second subset 102B, 102D of electrodes 104A to 104D is about -1, ... , Is connected to a voltage supply of 10 kV (eg -2 kV). Thus, a high electric field is established between the clamp 100 and the patterning device MA, so that the patterning device MA can be pulled toward the clamp 100. In particular, charges are induced in the region of the conductive coating 128 adjacent to the electrodes 104A to 104D having an opposite sign from the applied voltage, and opposite charges at various positions across the clamp 100 and the patterning device MA. An attractive force is formed between them. The area of the clamp 100 configured to support the patterning device MA may be referred to as a support area. Further, when the clamp is operated to clamp the patterning device MA, the region of the clamp configured to generate a clamping force may be referred to as a clamping region.

Masking blades 140 and 142 are provided adjacent to the patterning device MA. The masking blades 140, 142 are configured to selectively mask the patterning device MA from the radiation beam B during the exposure sequence. In particular, the masking blades 140 and 142 may be moved in the y direction (ie, left and right in Fig. 2A) to scan the radiation beam B across the surface of the patterning device MA during exposure. In addition, the masking blades 140 and 142 may be moved toward each other and away from each other in the y direction to provide different levels of masking for the patterning device MA. For example, the masking blades 140 and 142 may be closed to cover the entire patterning device MA, or may be partially closed to allow radiation to pass through a narrow slit.

In general, it will be appreciated that the masking blades 140, 142 can be used to adjust the total dose of radiation incident on the patterning device MA. The blades 140 and 142 may be spaced, for example, about 5 to 10 mm (eg, 10 mm) from the patterning device in the z direction.

2B shows the clamp 100 in a plan view (compared to the cross section shown in FIG. 2A). The plan view is in the z plane extending vertically in the x direction and horizontally in the y direction in the orientation shown. The cross section of FIG. 2A is taken along line A-A' shown in FIG. 2B.

Each of the electrodes 104A to 104D has a rectangular shape and is disposed to be substantially parallel to each other. In this arrangement, each of the four electrodes shown spans the width of the clamped patterning device MA in the x direction, and each of the electrodes covers about 1/4 of the length of the patterning device MA in the y direction. In FIG. 2B, the position of the patterning device MA is indicated by a broken line. The masking blades 140 and 142 are outlined by dotted lines.

The electrostatic clamp 100 may further include regions 108 and 110 protruding from the upper and lower sides of the electrostatic clamp 100 in the orientation shown in FIG. 2B (but not visible in the cross-sectional view shown in FIG. 2A ). ). The protrusions 108 and 110 are generally in the plane of the electrostatic clamp 100 and thus do not protrude further above or below the body 102 of the electrostatic clamp as shown in FIG. 2A. The protrusions 108 and 110 may be referred to as clamp'ears'.

The protrusion 108 includes secondary electrodes 114A and 114B. The protrusions 110 include secondary electrodes 114C and 114D. Each of the electrodes 104A to 104D has corresponding secondary electrodes 114A to 114D. Each of the electrodes 104A to 104D and the corresponding secondary electrodes 114A to 114D are electrically connected to each other. That is, the electrode 104A is made such as being electrically connected to 114A (but these connections are not shown). However, as can be seen in FIG. 2B, the protrusions 108 and 110 extend beyond the circumference of the clamped patterning device MA, thus contributing to the clamping force between the electrostatic clamp 100 and the patterning device MA. I never do that. The protrusions 108 and 110 can thus be referred to as the non-clamping area of the clamp.

The secondary electrodes 114A-114D provide a convenient way to connect the voltage supply to the primary electrodes 104A-104D. It will be appreciated that it is desirable to maintain the flatness of the clamp 100 in order to improve the imaging performance. However, depending on the situation, providing an external electrical connection to the clamp electrodes 104A-104D may result in distortion of the clamp flatness. Thus, by providing an internal connection between the electrodes in the clamp body (as described above), the secondary electrodes 114A to 114D without disturbing the critical areas of the clamp 100 that support the patterning device MA. External connection (not shown) may be made. This can improve the overall flatness of the patterning device MA during the imaging operation. The external connection may include leads (not shown) connected to the contacts provided by the secondary electrodes 114A to 114D. Leads can be connected to voltage sources (not shown) configured to supply clamping voltages as needed.

2C shows a side view of the clamp 100 in a cross section in the y plane. This exemplary view extends vertically in the x direction and horizontally in the z direction. The cross section was taken along line B-B' shown in Fig. 2B. Thus, only electrode 104B and secondary electrodes 114B and 114C are visible. It can be seen that the protrusions 108 and 110 extend further in the x direction than the patterning device MA.

Additional masking blades 144 and 146 are provided adjacent to the patterning device MA. The masking blades 144, 146 are configured to selectively mask the patterning device MA from the radiation beam B during the exposure sequence. In particular, the masking blades 144 and 146 may be moved in the x direction to control the width of the radiation beam B. This control can be used, for example, to make the lithographic apparatus suitable for different sized die exposures, and to adjust the total dose of radiation incident on the patterning device MA as well as the blades 140 and 142. The blades 144 and 146 may be spaced, for example, about 5 to 10 mm (eg, 6 mm) from the patterning device in the z direction.

It will be appreciated that in typical use the electrodes 104A-104D are screened by an overlying patterning device MA. However, the secondary electrodes 114A to 114D are not screened in the same way. This is because the patterning device MA does not extend over the secondary electrodes 114A-114D (as best seen in FIGS. 2B and 2C). Of course, it will be appreciated that the secondary electrodes 114A-114D are insulated from the clamp surface in the same manner as the primary electrodes 104A-104D.

During use, the polarity of the voltage applied to the electrodes 104A to 104D is periodically switched. Switching may occur, for example, between each wafer exposure (eg, after about 100 individual die exposures) or at longer intervals (eg, after 10 or 50 wafer exposures respectively). This conversion reduces the likelihood that the insulation around and between the electrodes will be destroyed during long-term use. This conversion of the polarity of the voltage applied to the electrode may be referred to as repolarizatio. Each of the polarization states may be referred to as an energization state of the clamp. For example, in the first energized state, a positive voltage may be applied to the electrode 104A, and in the second energized state, a negative voltage may be applied to the electrode 104A. Alternatively, the energized state of the clamp may generally be referred to as a configuration of the clamp. The clamp configuration may include an energized state and optionally additional configuration details (eg whether the component is clamped by the device).

The result of the lack of screening of the secondary electrodes 114A to 114D by the patterning device MA is between electrodes of adjacent opposite polarity among the secondary clamp electrodes (e.g., between electrodes 114A to 114B). E), or that an external electric field can be established between the secondary electrodes and other nearby components. For example, between the secondary electrodes 114A to 114D and the conductive coatings 126 and 128 on both sides of the patterning device MA, or between the secondary electrodes 114A to 114D and the masking blades 140, 142, 144, 146) can be established. These field lines are schematically shown in Figs. 2b and 2c.

Of course, it will be appreciated that during normal operation of clamp 100, an electric field will be established between clamp 100 and the surfaces of patterning device MA. In addition, due to the close separation between the various charged surfaces (including other components in the lithographic apparatus, such as masking blades 140, 142, 144, 146), electrostatic discharge may occur. I can. That is, electrostatic discharge may occur between arbitrary charged surfaces, and the probability of discharge increases as the electric field strength increases. Electrostatic discharge can generate particles from the surfaces and may also release particles that have previously adhered to the surfaces in the lithographic apparatus. It will be appreciated that the release of such particles is undesirable in a lithographic apparatus, as the particles can land on critical areas of the lithographic apparatus and possibly lead to patterning defects in the processed substrates.

In addition, a Lichtenberg pattern known to be associated with electrostatic discharge damage was observed in a test performed with a Cr-coated test patterning device. Similarly, Cr particles (eg, molten and reconstituted nano-spheres) with morphology exhibiting production through high energy processes were also observed in these tests. Thus, it will be appreciated that electrostatic discharge can generate unwanted particles.

Of course, it will be appreciated that lowering the clamp voltage can lead to a lowering of the electrostatic field, and thus a reduction of defects on the patterning devices as a result of electrostatic discharge. This reduction was observed in systems where the patterning devices had a Mo-containing front coating. That is, it has been proven that the reduction of the clamp voltage significantly reduces the number of defects caused by Mo particles on the reticle. However, this reduction in clamp voltage may not be practical in some situations, which may lead to a reduction in clamping force.

Depending on the situation, it is possible to reduce the number of defects observed in the reticle by improving the shielding around the clamp electrodes. Such shielding can greatly reduce Mo and Cr defects. However, it will be appreciated that the protrusions 108 and 110 shown in FIGS. 2B and 2C are not shielded in normal use.

Electrostatic discharge can also be a problem with pellicles. Ultra-thin pellicle films can be used to prevent movement of particles between various areas of the lithographic apparatus. However, it will be appreciated that an ultra-thin pellicle film (which may, for example, contain a metal layer having a thickness of a few nanometers), can overheat and burst if significant current flows through the metal layer. Of course, it will be appreciated that electrostatic discharge can cause significant current to flow through such a film. As such, electrostatic discharge also poses a danger to the pellicle.

Also, as mentioned above, the electrostatic clamps are regularly repolarized. Each repolarization causes the sign of the electric field around any unshielded electrodes to change. When charged particles are attracted to the corresponding electrode during the first polarization state (eg, negative voltage application), the charged particles may be repelled when the polarization state is changed. Further, when the clamp is in the first polarization state (eg, when a negative voltage is applied to the electrode), particles on the clamp surface may be charged. However, these charged particles can be repelled when the polarization state is changed. That is, a strong electrostatic force can overcome the attraction and release trapped particles, which can potentially cause more particles to enter the patterning devices or other system components.

More specifically, the high electric field generated by the electrostatic clamps strongly attracts free charges. Free charges refer to charges that are not bound to a physical substrate and can move freely along an electric field line (positive charge-for example, ion, or negative charge-for example, electron). In addition, abundant free charges are generated during EUV exposure. For example, electrons can also be generated by photo-emission and also from EUV induced plasma, which is typically generated in the presence of hydrogen gas (which is often present in lithographic tools). Cations can also be generated in EUV plasma. So, during lithographic exposure, unshielded clamp ears 108, 110 are more likely to attract free charges, resulting in collapse of the free space electric field (i.e. compensated by the free charges). , Which means that the electric field is confined to the inner parts of the clamp (ie between the clamp electrodes and the now charged clamp surfaces). The high charge density at the resulting clamp surface is more likely to transfer the charge to the trapped particles, and the particles are thus attracted more strongly by the electrostatic field.

Next, when the clamp polarity is reversed (EUV plasma no longer exists), the trapped charged particles now have a charge of the same sign as the electrode, which generates a strong electrical repulsion in the direction away from the surface.

For example, each of the secondary electrodes (that is, each of the electrodes 114A to 114D) may be estimated to have a capacitance in the range of about 10 pF to about 500 pF (eg, about 100 pF). The capacitance of the electrode can be calculated based on the (known) area of each electrode, the (known) dielectric constant of the clamp dielectric, and the (known) dielectric thickness.

Assuming a clamping voltage of 2 kV is used, the charge at each ear is 200 nC (Q = CV), which corresponds to a surface charge density of about 720 μC/m ² . This charge density results in an electric field strength E of about 4.10 ⁷ V/m (according to E = σ/2ε ₀ , and assuming that the electric field radiates in both directions, the electric field strength is divided by 2 accordingly).

The plasma generation process will now be discussed in detail. It is understood that EUV photons in beam B ionize hydrogen molecules, generating H2 ⁺ ions and free electrons. In the example using 13.5 nm of EUV radiation, each photon may have an energy of about 92 eV, with the ionization energy of molecular hydrogen being about 15 eV. Thus, the generated free electrons can have sufficient energy (eg, >75 eV) and range to create a secondary plasma relatively far away from the initial ionization event. In addition, electrons released in this manner (ie, having an energy of about 75 eV) can ionize 1, 2, or even 3 additional hydrogen molecules. So, even when the primary plasma is generated only at the place where the EUV photons are incident (generally not including the clamp ears 108 and 110), the secondary plasma can be generated near the clamp ears 108 and 110. have. As described above, the masking blades 140 to 146 are spaced apart from the patterning device MA by about 5 to 10 mm. So, any plasma reaching the clamp ears 108, 110 must diffuse through the slit formed between the blades 140-146 and the patterning device MA, or be generated there as a secondary plasma.

3 shows the modeled plasma density in the area of the patterning device during EUV exposure. The horizontal axis represents the distance R (in cm) from the center of the patterning device MA, while the vertical axis represents the distance z (in cm) from the clamp surface. This model assumes an EUV source with an output of 40 W and a hydrogen pressure of 5 Pa. In the modeled environment, the patterning device MA is shown blocking the radiation path, and the masking blades 140, 142 are opened about 10 mm so that EUV radiation can reach the central portion of the patterning device MA. have.

It can be seen that the plasma density reaches about 10 ⁸ ions/cm ³ in the vicinity of the central region of the patterning device MA. However, this density is about 10 ⁷ ions/cm ³ and from the center of the patterning device MA where it is about 1.5 cm from the center of the patterning device MA below the masking blades (which EUUV photons do not reach directly). At about 4 cm, it decreases to about 10 ⁶ ions/cm ³ .

According to the above calculation and assuming a particle diameter of about 100 nm, and receiving about 10 to 100 charges in a plasma environment with a density of 10 ^{-6 to} 10 ^-7 ions/cm ³ , after repolarization of the clamp ( In the absence), the resulting repulsive static power for the charged surface particles is about 10 ^{-8 to} 10 ^-7 N. The attractive Van de Waals and other adhesion forces are expected to be about 10 ^{-9 to} 10 ^-8 N.

As such, according to the above assumptions and approximate calculations, particles trapped in the clamp surface can thus be subjected to a strong repulsive force, which in general can overcome weaker adhesion. Any particles emitted in this manner will be accelerated along the electric field line and may land on the masking blades 140-146, the patterning device (MA), or other system components.

In addition, due to the generation of hydrogen plasma (as shown in FIG. 3), a large and rapid increase in free charge density immediately after the start of EUV exposure and correspondence of the conductivity of the medium around the clamp 100 and the clamped patterning device MA This increase can lead to high transient currents flowing between system components.

It will be appreciated that during clamping, an electric field may be established both within the clamp dielectric and between the clamp surfaces and the clamped patterning device or other system components (such other system components may be several centimeters away from the clamp surface). . However, a sudden increase in conductivity in the area around the clamp due to the creation of a hydrogen plasma can disrupt any established electric field extending beyond the clamp surface (e.g. between the clamp and patterning device or other system components). , This is because the conductive plasma cannot support the electric field. This rapidly increases the field strength in the clamp dielectric, possibly exceeding the field emission threshold. In addition, if field emission occurs, high current can cause heating of the clamp electrode. Such heating can reduce the field emission threshold and thus lead to an increase in current. This in turn causes the electrode to be heated more, and the field emission threshold may be further reduced. So, in this way, a sudden increase in free charge density immediately after the start of EUV exposure can lead to high transient currents flowing between system components.

Depending on the situation, such a sudden change in conductivity may induce field emission preferentially from the sharp features of the patterning device MA clamped in the presence of such high fields. For example, it will be appreciated that the coating 128 may have sharp edges with a radius of curvature of about 100 nm, for example. Likewise, field emission can also occur from the surfaces of the clamp.

It will also be appreciated that the thickness of the dielectric coating, which is substantially uniform in thickness throughout the planar surface of the clamp electrode, can be reduced at the edges of the electrodes. Thus, the electric field strength at the edges of the electrode can be increased, thereby increasing the risk of field emission in this region.

The degree to which a particular component or feature is susceptible to field emission can vary depending on the nature of the material and/or surface. In addition, the degree to which the electrostatic discharge damages the surfaces will also depend on the material properties. For example, a relatively low conductivity coating (eg, a hardened metal such as CrN or TaN) that can be used as a coating of patterning devices may be susceptible to damage by electrostatic discharge.

Referring now to FIG. 4, the mode of operation of the lithographic apparatus shown in FIGS. 1 and 2A-2C is now described in more detail. 4A shows the EUV dose received at the patterning device. In particular, the magnitude of EUV dose is schematically shown on the vertical axis and time is shown on the horizontal axis.

As mentioned above, the voltage applied to the electrodes 104A to 104D of the electrostatic clamp 100 is regularly inverted to preserve the integrity of the electrical insulation. The voltage applied to one of these electrodes is schematically shown in FIG. 4B, which shows voltage on the vertical axis and time on the horizontal axis. In particular, the voltage rises from ₀ at time t ₀ to clamping voltage +V _C at time t ₁ . The voltage is then held at +V _C during the exposure cycle until time t ₂ , after which the voltage is reduced to zero at time t ₃ and then to -V _C at time t ₄ . After that, the voltage is held at -V _C during another exposure cycle. At time t ₅ , the voltage then increases to zero at time t ₆ . After that, this cycle of positive and negative energization of the clamp electrodes continues, and a stable energization period is provided between each repolarization event.

Lithographic exposure is performed during a stable clamping period. However, it will be appreciated that exposure may occur during positive or negative polarization of the illustrated electrode. In addition, it will be understood that different electrodes will follow different energization cycles. For example, for each positive (+) to negative (-) transition shown for electrode 104A, electrode 104B may follow a reverse transition. In addition, the different electrodes can be repolarized at different times (to ensure that a certain clamping force is always applied to the patterning device MA).

Referring back to FIG. 4A, during the periods T _A , T _B , T _C , and T _D , the EUV beam is mainly used for imaging. It should be noted that each of the periods T _A , T _B , T _C , and T _D corresponds to a period in which the clamp polarization does not change. However, just before the period T _A , there is a period T _A ′ in which EUV energy is incident on the patterning device MA. Similarly, immediately after the period T _A , there is also a short period T _A "in which EUV energy is incident on the patterning device MA. The period T _A may be referred to as an exposure burst. Periods T _A ′ and T _A May be referred to as a pre-exposure burst and a post-exposure burst, respectively. The exposure bursts T _A through T _{D correspond to those} in which the EUV beam is used for imaging (ie, exposure of the substrate) or measurement (eg, alignment, mirror setting, etc.). Thus, the intensity of the radiation reaching the patterning device MA (and provided to the wafer stage) during this period is important. However, during pre-exposure and post-exposure bursts T _A ′, T _A ”, etc., EUV power is still received by the patterning device but not used for imaging. So, it reaches the patterning device MA (and is provided to the wafer stage). ) The intensity of the radiation is less important and can vary.

In the example shown in Fig. 4, both pre-exposure and post-exposure bursts T _A ′ and T _A ″ are also included within a stable clamping period in which the clamp polarization does not change (ie, periods T _A ′, TA, and T _A It should be noted that "is between t ₁ and t ₂ ). That is, the entire EUV pulse is included between clamp repolarization events.

Further, it will be appreciated that the illustrated illumination and polarization sequences are schematic and not inclusive of all events. For example, even during a seemingly continuous EUV power period, pulses may be applied to the EUV source, for example there may be a selective blackout between imaging of different dies or during wafer swaps.

More specifically, each of the bursts T _A ′, TA, and T _A ″ may include a number of individual EUV pulses, and may also include periods in which no pulse is provided. For example, exposure burst T _A May correspond to an entire wafer exposure (including, for example, about 100 die exposures), during which period there may be a total of about 10 ⁶ pulses (eg, 10 ⁴ pulses per die exposure). Between each die exposure, there may be a wafer positioning period in which no EUV beam is present This period may last for about 30 to 40 ms In addition, before starting imaging, various metrology operations may be performed. , During that period, the beam B enters the patterning device MA and is provided to the substrate table WT, but does not enter the substrate itself (substrate table WT is moved so that the substrate W is not placed in the beam path. Can).

Transient effects in the source SO during pre-exposure and post-exposure bursts T _A ′ and T _A ″ (eg, to ensure that the radiation intensity received by the wafer during exposure burst T _A is as uniform as possible) This can be monitored and controlled. Also, the pre-exposure and post-exposure bursts T _A ′ and T _A ″ may be used to perform calibration, alignment, or metrology operations other than wafer exposure. However, regardless of its use and its effect on the substrate, each of the bursts T _A ′, TA, and T _A ″ entails EUV radiation incident on the patterning device MA.

Pre-exposure burst T _A 'to (after that is, a patterning device, and the appropriate clamping) time t immediately after the _one to start according to the EUV power is rapidly increased, the conductivity of the surrounding electrostatic clamp environment is a non (comprising a low-density hydrogen) It will be appreciated that it changes rapidly from a conductive environment to a high conductivity environment (including EUV induced hydrogen plasma). As mentioned above, this rapid change in conductivity can lead to electrostatic discharge. In addition, during clamp repolarization events, particles trapped in clamp protrusions 108 and 110 (which may have been charged in the plasma environment during bursts T _A ', TA, and T _A ″) are clamped by sudden changes in the electric field. It can be released from the surface.

5 shows a modified lighting sequence according to an embodiment of the invention. FIG. 5B shows a polarization sequence of the electrostatic clamp corresponding to that described above with reference to FIG. 4B as a whole. However, FIG. 5A shows a modified lighting sequence compared to that described above with reference to FIG. 4A.

In particular, during the pre-exposure period T _A ', the EUV power is gradually increased instead of suddenly switching from the OFF state to the ON state. That is, while the clamp is already polarized (or repolarized), the EUV power incident on the patterning device MA is gradually increased. Similarly, during the post-exposure period T _A ", the EUV power may gradually decrease instead of suddenly switching from the ON state to the OFF state. During the pre-exposure period T _A ′ and the post-exposure period T _A ″, Note that EUV power is applied to the patterning device but is not used for imaging. Within the meaning of the present invention, this may be referred to as a non-imaging exposure of the patterning device. So, this non-imaging exposure refers to exposure in which the EUV radiation beam is incident on the patterning device but the radiation is not projected onto the substrate; The non-imaging exposure may be performed between successive exposures among the plurality of imaging exposures, for example.

In an embodiment of the present invention, this non-imaging exposure is applied during polarization or repolarization of the clamp.

For example, a soft ramp up of EUV power applied during non-imaging exposure gradually increases the conductivity of the area around the patterning device MA and the electrostatic clamp 100. This gradual increase in the conductivity of the medium does not cause the electrostatic field to collapse in an abrupt manner, but rather allows charge to leak to various surfaces along the existing electric field lines. This process allows all surfaces that were charged as a result of previous clamp polarization conditions to be compensated. Likewise, all charged particles on the surfaces of the clamp or patterning device can also be compensated.

The smooth rise is expected to extend over multiple EUV pulses. For example, a smooth rise may take less than 1000 pulses or less than about 20 milliseconds (ms). Desirably, a smooth rise may take less than 10,000 pulses or less than about 200 milliseconds (ms). In general, the rising period becomes small compared to the exposure time of the wafer. Wafer exposure can last for about 30 seconds, for example. Compared to a wafer exposure of about 30 seconds, a rise period of less than about 1 second (or about 50,000 pulses) can be considered a smooth rise or a gradual increase.

A smooth ascent, such as the one described above, can be beneficial in one or more of the following situations:

a. After initial startup of the EUV device;

b. After loading a new patterning device MA; And

c. After inversion (repolarization) of the polarity of the electrostatic clamp electrode.

This smooth ascent can be done in several different ways. For example, the rising can be done by gradually increasing the pulse energy of the source. Alternatively or additionally, a smooth rise can be implemented by gradually increasing the number of pulses of each of a plurality of mini-bursts that together contribute to the total radiation energy (or dose) delivered to the wafer. Each mini burst may contain, for example, about 10 pulses, each pulse having a duration of about 100 ns and delivered at a frequency of about 50 kHz. When an LPP radiation source is used, the number of EUV pulses generated can be controlled by controlling the operation of the applied laser. In such an LPP radiation source, EUV pulses are generated by irradiating one or more laser beams to a fuel target, such as a tin (Sn) target. The amount of EUV radiation generated can be controlled, for example, by controlling the number of fuel targets to be irradiated. The applied pulsed laser beam can be controlled to irradiate only one out of two fuel targets or one out of three fuel targets, for example, thereby reducing the amount of EUV radiation to 50% or 33%. Alternatively or additionally, a smooth rise may be implemented by inserting one or more of the masking blades 140, 142, 144, 146 and slowly retracting. Of course, alternative mechanisms can also be implemented for this smooth rise in EUV energy.

A smooth ascent may start with, for example, a small jump (eg, about 5-10% of the total imaging power) and then gradually increase from this level to 100% of the imaging power at the start of the imaging exposure. Alternatively, the radiation power can increase linearly from 0% to 100%. During a gradual increase in radiation power, there may be several small steps, which results in a substantially linear overall increase. For example, the power can be increased in 5 steps of 10%, for example. This increase will still lead to a gradual change in the conductivity of the medium around the clamp and patterning device.

In general, it will be appreciated that providing a gradual change in conductivity reduces the risk of discharge associated with a sudden increase in conductivity.

In some cases, the EUV energy may be ramped down in a manner similar to that shown in FIG. 4A (ie, symmetrically). However, in other embodiments, EUV energy may be abruptly interrupted as is conventional in the prior art.

As shown in FIG. 5, repolarization of the clamp electrode may occur during EUV rise or fall. Alternatively, repolarization of the clamp electrodes may occur during the EUV OFF period. It can also be pointed out that the EUV can be turned ON during voltage build-down, i.e. reducing the applied voltage to zero, as is done before unloading the object, for example. The period t ₁₁ -t ₁₂ shown in FIG. 5B may indicate this voltage reduction. As mentioned, it may be advantageous to apply at least a low level of radiation even during this period.

Figure 6 shows another embodiment modified from the conventional EUV power energization. Figure 6b shows a clamp polarization sequence similar to that described above with reference to Figure 4b. However, in FIG. 6A, it can be seen that there is a constant EUV illumination of the patterning device MA between each of the EUV exposure pulses T. That is, EUV radiation is provided to the patterning device MA even during the period in which the electrostatic clamp is repolarized. This means that during the period between exposure pulses, the EUV energy output by the source SO is maintained, and the masking blades 140-146 are controlled so that at least a portion of the EUV radiation can reach the patterning device MA. It costs. As mentioned above, applying the power to the patterning device without using EUV power for imaging is referred to as non-imaging exposure within the meaning of the present invention. Such non-imaging exposure can lead to the generation of EUV induced plasma.

Patterning in which a relatively conductive medium (i.e. EUV induced hydrogen plasma) is clamped by providing EUV illumination to the patterning device (MA) during repolarization (i.e., transition from the first energized state of the clamp to the second energized state). It will be appreciated that it is always provided in the vicinity of the device MA and the electrostatic clamp 100. As such, as the clamp 100 is repolarized, the free space charge provided by the EUV-induced hydrogen plasma (which exists around the clamp protrusions 108 and 110 as shown in FIG. 3) rapidly regenerates the charge. To be distributed. The abundant free charge effectively screens the electric field generated by the secondary electrodes 114A-114D and lowers the likelihood that particles will be emitted by the surface. Free charge also serves to lower the electric field of dangerously high intensity. This will reduce the high transient current between the dielectric surfaces and field-concentrating features of the edges of the patterning device coatings via the EUV induced plasma. That is, by always providing a conductive medium, it is possible to reduce or completely eliminate the risk of discharge and high transient current associated with the sudden onset of EUV energy at the beginning of each exposure pulse. It can also be pointed out that EUV can be turned ON during voltage reduction, i.e. while reducing the applied voltage to zero, as is done before unloading the object, for example. The period t ₁₁ -t ₁₂ shown in FIG. 6B may indicate this voltage reduction. As mentioned, it may be advantageous to apply at least a low level of radiation even during this period. As shown in Fig.7, in another operating mode, there is a gradual rising RU before each exposure burst T pulse, there is a gradual decrease RD after each exposure pulse, and between each EUV burst T there is a low EUV power applied to the reticle ( The EUV power (Fig. 7A) is modulated during repolarization (Fig. 7B) so that there is a low EUV power) L. That is, instead of completely removing the EUV power between exposure pulses, the EUV power is gradually reduced and maintained at a low level during repolarization. This configuration provides the advantage that the medium around the electrostatic clamp is always kept in a conductive state while reducing the load on the EUV source compared to the configurations described above with respect to FIG. 6, and accordingly, it is observed when the EUV switch is suddenly turned on. It minimizes the effects of high transient currents that can be caused and allows charge compensation to occur during each repolarization event. It can also be pointed out that EUV can be turned ON during voltage reduction, i.e. while reducing the applied voltage to zero, as is done before unloading the object, for example. The period t ₁₁ -t ₁₂ shown in FIG. 7B may indicate this voltage reduction. As mentioned, it may be advantageous to apply at least a low level of radiation even during this period.

In each of the configurations described above with reference to FIGS. 5, 6, and 7, the performance of the lithographic apparatus can be improved by reducing the defects caused by the electrostatic discharge event and the emission of charged particles without changing any hardware. I can understand that there is. In particular, to lower the high free space electric field that appears during and after a repolarization event, existing features of the EUV illumination system are used in a new way.

It will be appreciated that in each of the above-described embodiments, an EUV induced plasma needs to be generated that provides a source of free charges in the vicinity of the patterning device MA, in particular in the vicinity of the clamp protrusions 108 and 110. However, it will also be appreciated that in order to be effective in screening the electric field around the electrodes 114A-114D, a sufficient free charge density is required. Of course, the density of the charge generated at any particular location will depend on the specific features of the system, including EUV intensity, gas density, and system geometry. In addition, the amount of free charge required also varies depending on management parameters (eg, electric field strength, electrode geometry).

Considering the amount of charge expected to accumulate on the dielectric surface adjacent to the clamp secondary electrodes 114A to 114D (as discussed in more detail above), in order to properly prevent the establishment of an external electric field during a repolarization event It will be appreciated that the charge must be supplied at least at a minimum rate.

For example, clamp repolarization can be expected to occur with about 200 ms or about 10 ⁴ EUV pulses. Also, the clamp repolarization slew rate may be, for example, about 16 kV/s. At this rate, the clamp voltage changes at a rate of about 0.3 V for each EUV pulse. Also, as discussed above, the number of charges required to screen the clamp ear with a clamping voltage of 2 kV may be about 10 ¹² (200 nC ≒ 1.25 x 10 ¹² charges).

Both the primary plasma ions and the secondary remote plasma ions (caused by electrons removed during ionization) contribute to the charge present in the clamp ear (as shown in Figure 3), and the masking blades ( 140 to 146) are fully open (therefore allowing the entire patterning device to be exposed to the radiation beam B), 10 ⁴ EUV pulses substantially prevent the establishment of an external electric field during a repolarization event. It is believed to provide enough free charges to Assuming a source power of 40 W in this situation, an ion density of about 10 ⁷ /cm ³ can be expected in the clamp ear.

However, if the flux of free charge is reduced, a larger number of EUV pulses may be required to provide sufficient re-balancing charge. For example, if the primary plasma cannot diffuse to the clamp ear locations, as a result only the secondary plasma contributes to free charges at the clamp ear locations, then about 10 ⁶ /cm ³ ions in the clamp ear Density can be expected. This ion density may not provide enough free charges to fully compensate for the repolarization electric field.

If it is determined that this is the case, it may be possible to reduce the rate at which repolarization occurs. For example, if the repolarization rate is reduced to a fifth (to 3.2 kV/s), the clamp repolarization lasts for about 5 x 10 ⁴ EUV pulses, so significantly increased freedom to compensate for the changing electric field. It provides a supply of charge. Of course, it will be appreciated that such adjustments can lead to a decrease in productivity. However, as discussed above, reducing the clamp's conversion rate to a fifth is expected to reduce less than 0.5% of the overall productivity.

Of course, it is common to see that the amount of free charge required will depend on the clamping voltage and geometry, and that the proportion of free charge generated that can provide the screening effect will also depend on many factors (some discussed above). Technicians will understand. However, one of ordinary skill in the art has various parameters as needed (e.g. EUV intensity, EUV pulse duration, number of EUV pulses, H ₂ gas pressure, clamp voltage, switching rate, masking) to ensure that the clamp ear is properly screened. Blade position, etc.). For example, experimental studies can be conducted to establish the level of free charge required and/or provided in a particular configuration. Alternatively, charge density modeling may be performed to determine which level of charge density is experienced at the clamp ear locations.

In the embodiments described with reference to FIGS. 6 to 8, EUV-induced plasma is generated by so-called non-imaging exposure of the patterning device, that is, exposure to which a radiation beam is incident on the patterning device during that period, but no radiation is projected onto the substrate. Become; The non-imaging exposure is performed between successive exposures among the plurality of imaging exposures.

By providing a beam of radiation to the patterning device during non-imaging exposure, it is possible to provide a source of free charges by the plasma that will be created by ionization of gas molecules (eg, hydrogen) present around the patterning device. Plasma is generated both in areas that are directly illuminated by the radiation beam and in adjacent areas (eg, by diffusion and secondary electrons). In this way, a single beam of radiation (eg, a beam of EUV radiation) can be used for imaging purposes and to provide a source of free charges during repolarization of the electrostatic clamp.

In an alternative embodiment, EUV power can be applied as a source of free charges by impinging EUV power on a surface near the patterning device. As such, an alternative way of generating free charges adjacent to the electrostatic clamp is to provide a beam of radiation to a different surface than the patterning device, for example the surface of a shutter or masking blade. As described above, the shutter or masking blade configuration can be used to block the radiation beam from reaching the patterning device or to control the spatial expansion of the radiation beam incident on the patterning device. Application of a radiation beam or a portion thereof to the shutter or mask blade(s) may be performed with gas molecules (e.g., hydrogen) present or provided in, around, or near the radiation beam and the shutter or mask blade configuration. The interaction of can also lead to the generation of free charges. Free charges are generated both in areas that are directly illuminated by the radiation beam and in adjacent areas (eg, by diffusion and secondary electrons). Since the shutter or mask blade configuration is typically relatively close to the patterning device and thus the electrostatic clamp, free charges generated by irradiating the shutter or mask blade configuration can also cause free charges adjacent to the electrostatic clamp. So the generation of free charges by irradiating the shutter or mask blade configuration can also be considered a mechanism for generating free charges adjacent to the electrostatic clamp.

In some embodiments, a secondary ionization source may be provided to allow plasma to be generated near the electrostatic clamp by means other than the EUV source SO. This configuration can reduce the overall output load of the EUV source SO. It will be appreciated that the above-described embodiments may impose additional requirements on the EUV source SO by requiring an additional EUV output than is required for imaging. Also, in some embodiments, it may not be possible for an EUV source to continuously generate power (as shown in FIG. 6A). Likewise, it may not be possible or desirable for an EUV source to provide any EUV pulse energy in the range of 0-100% of the nominal output power while ensuring clean collector operation and pulse energy stability.

As such, it may be desirable in some embodiments to provide an alternative mechanism for the creation of regions with increased gas conductivity than the primary EUV source.

For example, the source may be provided in proximity to the electrostatic clamp 100 and the clamped patterning device MA. The source may preferably be placed near the clamp protrusions 108 and 110. Multiple sources may be used. The source may be, for example, a soft X-ray source or a VUV light source capable of operating at pressures less than 1 bar in a clean environment. The source may comprise a low power ionizer having a power of about 0.1 to 1 W. One such suitable device may be a VUV ionizer L12542 manufactured by Hamamatsu Photonics K.K. of Hamamatsu City, Shizuoka, Japan. In some embodiments, the source may comprise a radiating source or an electron beam source.

8A to 8C (sub-figures and parts shown generally correspond to those shown in FIGS. 2A to 2C), the electrostatic clamp 100 and the patterning device MA A source S is provided adjacently. In an embodiment, the source S may be energized, for example, during the repolarization of the electrostatic clamp 100, whereby the hydrogen gas in the region of the clamp 100 and the patterning device MA is ionized to generate a hydrogen plasma. Guaranteed to produce. As discussed above, providing free charge during repolarization effectively screens the electric field generated by electrostatic clamp 100 (especially around secondary electrodes 114A-114D).

For example, as shown in Figs. 8A to 8C (showing the time at which the source S emits VUV radiation), plasma P is generated as a result of the emitted VUV radiation. Plasma (P) allows a free charge cloud to be provided throughout the clamp and patterning device environment. In this way, an electric field may be established between each of the electrodes 104A to 104D and adjacent regions of the clamped patterning device MA, but the unscreened electrodes 114A to 114D attract a charge cloud from the plasma. Screening charges (QA, QB, QC and QD) are formed. Each of the screening charges QA to QD has a sign opposite to that of the corresponding secondary electrodes 114A to 114D. As a result, no significant undesired electric field is established around the electrostatic clamp 100, thus reducing the effect of the electric fields F1, F2, and F3 shown in Figs. 2B and 2C.

Thus, instead of EUV radiation generated by the source (SO) to provide free space charges to screen for unwanted electric fields, the source (S) can be used, so (a mobile charge that can compensate for any free space electric field). (By providing mobile charges) effectively disrupts the free space electric field. This screening causes the electric field generated by the clamp 100 to be substantially constrained into the inside of the insulator surrounding the electrostatic clamp electrodes 114A to 114D.

It will be appreciated that a secondary ionization source (S) may be provided in a specific vicinity of the protrusions (108, 110) of the clamp (100) to provide local free charges to the areas where it is most needed. In this way, the EUV energy generated by the source SO is provided to the patterning device during repolarization, a secondary source to provide an alternative to the illumination scheme described above with reference to FIGS. 6 and 7. It will be appreciated that (S) can be used.

In general, EUV source SO and source S (which may include, for example, a soft X-ray source or VUV ionizer) can each be considered examples of ionizing radiation sources. In addition, these sources can be considered a mechanism for generating free charges in combination with a source of hydrogen (or other) gas. That is, a hydrogen plasma containing both cations and free electrons can be regarded as a cloud of free charge. Also, these free charges include both positive and negative free charges. This allows free charges to compensate and screen for electric fields of both polarities. It will be appreciated that an electric field of both polarities is experienced at different electrodes in each clamp polarization state and at each of the electrodes during different polarization states.

Further, as briefly described above, a considerable voltage may be established between the clamp 100 and the patterning device MA upon removal of the patterning device MA from the clamp 100. The process of removing the patterning device MA is now described in more detail with reference to FIG. 9.

9A shows the clamp 100 to which the patterning device MA is clamped. The clamped patterning device MA is shown spaced apart from the exchange assembly 150. The exchange assembly includes an exchange device 152 that supports the support structure 154. The support structure 154 includes a small number of protrusions 156 extending from the surface of the support structure 154 toward the patterning device MA. The protrusions 156 may have a height of, for example, about 200 μm. For example, the exchange assembly 150 is to move the patterning device MA towards and away from the clamp 100 so that the patterning device MA can be replaced or cleaned with replacement patterning devices. Is composed. The exchange device 152 can include a robotic arm that moves relative to the rest of the lithographic apparatus to move the patterning device MA to and from a load lock (not shown). The support structure 154, in particular the protrusions 156, supports the patterning device MA during transport. It can be seen from FIG. 9A that the protrusions 156 have an area smaller than the total area of the patterning device MA. As such, when the patterning device MA is supported by the support structure 154, there is only a small contact area. In the configuration shown in Fig. 9A, there is a small gap g between the electrostatic clamp 100 and the patterning device MA. This gap g may be, for example, about 10 μm (corresponding to the height of the burls 106 provided on the surface of the electrostatic 100).

The surface of the patterning device MA is shown spaced apart from the surface of the support structure 154 by a gap b. In the configuration shown in Fig. 9A, the gap b exceeds 200 μm (the height of the protrusions 156). As such, when the patterning device MA is spaced apart from the top surface of the protrusions 156, there should be a gap of at least 200 μm between the planar surfaces of the patterning device MA and the support structure 154.

Conversely, in the configuration shown in FIG. 9B (including the same components shown in FIG. 9A ), the patterning device MA is shown lying on top of the protrusions 156. Thus, the gap b is 200 μm and the gap g (i.e., the gap between the electrostatic clamp 100 and the patterning device MA) is greater than 10 μm, which means that the burls 106 (or actually any part of the clamp 100 It means that there is no physical contact between) and the patterning device MA.

10 shows a time sequence illustrating the change of gaps b and g through removal of the patterning device MA. At time t20, the gap g is initially 10 μm (corresponding to the height of the burls 106, which means that there is contact between the upper end of the burls 106 and the surface of the patterning device MA). The gap b far exceeds about 900 μm at time t20, so there is no contact between the patterning device MA and any part of the exchange assembly 150.

At time t21, the exchange assembly 150 approaches the patterning device MA as can be seen by the reduction in distance b. This movement is stopped at time t22, at which time the exchange assembly 150 is configured to hover under the patterning device MA, so that the height of about 900 μm (i.e., 200 μm of the protrusions 156) + A gap of 700 µm). Then at time t23 the electrostatic clamp 100 and the clamped patterning device MA are gradually lowered toward the exchange device 150. This movement continues slowly until near t24, where the distance b decreases to about 200 μm. That is, the electrostatic clamp 100 and the clamped patterning device MA are (as shown in FIGS. 8A and 8B) until the lower surface of the patterning device MA contacts the upper surface of the protrusions 156. Descends. This configuration is held until time t25. During this period, the voltage supplied to the clamp electrodes 104A to 104D and 114A to 114D is removed, so that the electrostatic clamp 100 no longer clamps the patterning device MA.

At time t25 the electrostatic clamp is moved upward away from the patterning device MA (which is now fully supported by the protrusions 156 of the exchange assembly 150), resulting in an increase in the gap g. This movement continues until near time t26 when the gap g increases to about 700 μm. At this time, the exchange assembly 150 supports the patterning device MA and is held under the electrostatic clamp 100. At time t27, the exchange assembly 150 moves the supported patterning device MA away from the electrostatic clamp 100 so that the gap g increases to a greater distance from 700 μm. This movement continues until time t28 (then the exchange assembly 150 can be moved to the patterning device exchange area, such as a load lock).

It will be appreciated that capacitance exists between several of the system components described above. As shown in FIG. 11, the system can be modeled as multiple capacitances are placed in series. In particular, the capacitance between the exchange device 150 and the lower surface (in the orientation shown in Figs. 9A and 9B) of the patterning device MA can be regarded as a variable capacitance C _b that changes as a function of the gap b. The capacitance C _b can be calculated as follows:

는 자유 공간의 유전율이고,here:

Is the permittivity of free space,

A is the area of the patterning device (MA),

b is the spacing between the surfaces of b.

The capacitance of the patterning device MA itself can be regarded as a fixed capacitance C _r , which can be calculated as follows:

은 기판(120)의 비유전율(relative permittivity)이고,here:

Is the relative permittivity of the substrate 120,

r is the distance between the front and rear surfaces of the patterning device MA.

The gap between the top surface of the patterning device MA and the electrostatic clamp 100 can be regarded as a variable capacitance C _g which can be calculated as follows:

Here, b is a distance between the rear surface of the patterning device MA and the clamp 100.

Finally, a clamp capacitance C _d is provided between the electrodes of the clamp 100 (having a voltage of 0 V on average) and the surface of the electrostatic clamp. This capacitance C _d is a fixed value and can be calculated as follows:

는 클램프 유전체의 비유전율이고,here:

Is the relative dielectric constant of the clamp dielectric,

d is the gap between the clamp electrodes 104A to 104D and the clamp surface.

)과 유사하다고 가정한다. 유사하게, 각각의 커패시턴스의 면적은 A와 같다고 가정하고, 버얼들(106) 및 돌출부들(156)의 영향은 무시된다. 하지만, 설명되는 모델은 커패시턴스 및 전하 분포의 일반적인 추세를 예시하는 역할을 한다. 필요한 경우에 보다 정확한 모델링이 수행될 수 있음을 통상의 기술자는 이해할 수 있을 것이다.Of course, it will be appreciated that the above equations represent a simplification of the actual capacitance, and ignore various parasitic and additional components. Also, the permittivity of a low-pressure hydrogen gas environment is the permittivity of free space (

). Similarly, assuming that the area of each capacitance is equal to A, the influence of burls 106 and protrusions 156 is neglected. However, the model described serves to illustrate the general trend of capacitance and charge distribution. It will be appreciated by those skilled in the art that more accurate modeling can be performed if necessary.

11, the capacitance _{C d, C g, C r} , C _b and are arranged in series. Also, the capacitances C _d and C _r are fixed, while the capacitances C _g and C _b are variable. Thus, in a closed system where no electric charge is allowed to enter the system and for a given initial charge state, the distance g between the clamp 100 and the patterning device MA and the distance between the patterning device MA and the exchange device 150 It will be understood that fluctuations in b will change the variable capacitances Cg and Cb. In addition, such a change in capacitance causes the voltage across various capacitances to change drastically according to the change in the interval.

In particular, the Q = CV relationship must always be maintained for each capacitance (assuming no charge is injected). Accordingly, if the capacitance C changes and the amount of charge Q included in the capacitance remains the same, the voltage V must change in inverse proportion to the change in capacitance C. This can lead to significant voltage amplification.

Of course, it will be appreciated that charge can be injected into the various nodes between the capacitors depending on the situation. In particular, the charge injected into the clamp surface can be modeled as the charge Q _s . The charge injected into the back side of the patterning device can be modeled as the charge Q _b . The charge injected into the front surface of the patterning device can be modeled as the charge Q _f . Charge sources Q _s , Q _b , and Q _f are shown in FIG. 11.

12A to 12C show changes in various parameters of the model of the clamp environment during an unload sequence. In particular, FIG. 12A shows the change in spacings b and g during a series of movements similar to those described above with reference to FIG. 10 when the patterning device MA is removed from the clamp 100. The x-axis represents time in seconds, while the y-axis represents distance in meters (in log scale). First, the distance b decreases from about 100 mm to about 1 mm at a time of 3 seconds, after which the distance b is about 200 μm (i.e., contact between the protrusions 156 and the patterning device MA) at a time of about 6 seconds. Decrease further. Next, the distance g increases from about 10 μm at a time of 8 seconds to about 700 μm at a time of 9 seconds, and then increases again at a time of about 11 seconds.

As described above with reference to FIG. 11, the capacitance associated with various system components also varies with intervals b and g. The varying capacitances associated with these various movements are shown in FIG. 12B, where the x-axis represents time in seconds and the y-axis (also in logarithmic scale) of each of the capacitors C _d , C _g , C _r , and C represents _b . As expected, it can be seen that the capacitances C _d and C _r do not change throughout the period. However, it can be seen that the capacitance C _b increases by approximately three orders of magnitude (10 ³ ) between the time of 2 seconds and the time of 6 seconds (corresponding to the movement of the exchange assembly 150 toward the patterning device (MA)). have. Conversely, from a time of 8 seconds to a time of 12 seconds (corresponding to the capacitance between the patterning device (MA) and the electrostatic clamp (100)), the capacitance C _g is reduced by approximately 4 orders of magnitude: 10 ⁴ in two steps. do. In particular, the capacitance C _g first decreases from about 20 nF to about 300 pF at a time of 9 seconds, and then again to about 20 pF at a time of 12 seconds.

12C shows the voltages at various points within the equivalent circuit shown in FIG. 11 during the stripping sequence discussed above. In particular, the dielectric surface of the clamp voltage Vd (equal and also the voltage across the capacitor C _d) of shows that the only change to a minimum in the sequence.

The voltage V _{FS on} the front surface of the patterning device MA can be understood as the sum of the voltages V _d , V _g , and V _r (which are voltages across the capacitors C _d , C _g , and C _r , respectively). Voltage V _FS is also equal to voltage V _b across capacitor C _b . It can be seen that V _FS rises from a relatively small amount to about 500 V in a time of 11 seconds. This corresponds to a time when the gap g between the patterning device MA and the electrostatic clamp 100 rises significantly.

However, the largest change in voltage occurs at the rear surface of the patterning device MA expressed as the sum of the voltages V _d and V _g (ie, the sum of the voltages across the capacitors C _d and C _g ). It can be seen that this voltage increases to exceed 1000 V during an initial small interval between the patterning device MA and the electrostatic clamp 100 (ie, an interval of about 8 seconds to about 700 μm). However, then there is a much larger rise with a voltage exceeding about 4300 V at a time of about 11 seconds, which corresponds to a significant increase in the distance g from the patterning device MA to the electrostatic clamp 100.

It should be noted that the modeled voltage change is based on the assumption that some charge will be injected from the electrostatic clamp 100 to the rear surface of the patterning device MA. This modeled charge injection is shown in FIG. 13, which is largely similar to the equivalent circuit described above with reference to FIG. 11 and includes a single charge source Q modeled to inject approximately 500 nC of charge in a time of about 1 second. This charge injection can also be seen in the inset of FIG. 11C showing voltages between 1 and 8 seconds on a vertically enlarged scale. In particular, it can be seen that the injection of electric charges of 500 nC at a time of 1 second causes the voltages V _BS and V _FS to increase abruptly to about 80 V.

In practice, charges are not accurately injected in a time of 1 second, and charges may be accumulated on the rear surface of the patterning device MA during clamping. Further, while the patterning device is clamped, field emission may occur from sharp features (eg trapped particles) on the clamp surface. This can cause the patterning device to be (often negatively) charged.

Assuming that no more charges are introduced into the system, the evolution of the voltage according to the above-described capacitance change can be modeled using the equivalent circuit model shown in FIGS. 11 and 13. The high induced voltage shown in FIG. 12C is the lower Paschen of hydrogen in the vicinity of the electrostatic clamp 100 and the patterning device MA (e.g., the voltage at the surface of the patterning device MA is about 250 V). limit) significantly increases the risk of discharge due to decomposition of hydrogen gas.

Thus, another problem associated with electrostatic discharge in a lithographic apparatus is during the unloading of the patterning devices after clamping. As described above, the electric charge can be trapped in the dielectric surfaces of the clamp 100. In addition, the residual charge may remain in the clamped patterning device MA once it is released. As the patterning device MA from which the clamping is released moves away from the clamp surface, an increase in the distance between the clamp surface and the patterning device surface may lead to a reduction in capacitance and an amplification of a voltage. In other words, given a proportional relationship between charge and voltage in a closed system (i.e., Q = CV), when the capacitance changes (inversely proportional to the spacing between parallel plates), the reduction in capacitance is proportional to that of the voltage. Causes an increase. So, as the patterning device MA and the clamp 100 are separated, it is possible that the voltage of the patterning device rises enough to cause electrical breakdown of the hydrogen gas. This discharge can lead to particle generation, which can lead to subsequent defects.

However, it is recognized that the influence of the changing capacitance can be reduced to some extent by introducing free charges during the unloading process. For example, a separate ionization source (S) or indeed an EUV source (SO) can be used to generate a hydrogen plasma, which provides free charges (as detailed above) and provides various dielectric components (and gaps). ) To allow the established electric field to be relaxed during the stripping process.

The provision of free charges can cause a significant reduction in the voltage established between the various system components. That is, the established electric field generated from the high voltage shown in Fig. 12C can be compensated by the introduction of additional free charges. These charges can be considered to be provided to the surfaces of the patterning device and clamp by charge sources Q _s , Q _b , and Q _f as shown in FIG. 11. These charge sources are effectively provided by a hydrogen plasma, which provides sufficient charge to each of the nodes in the clamp 100 and patterning device (MA) equivalent circuit so that they remain neutral. That is, the free charges in the plasma are driven by these electric fields when the electric fields start to be established, causing these electric fields to collapse.

In this way, potential problems associated with a significant voltage established across the patterning device MA upon removal from the electrostatic clamp 10 can be mitigated or avoided entirely. As mentioned above, it will be appreciated that this effect is not binary, and if insufficient charge is provided, some (degraded) electric field may still be established. However, it will be appreciated that even a reduction in voltage amplification (not complete avoidance) can be beneficial, especially if the voltage is therefore always kept below the Paschen lower limit of hydrogen (which is about 250 V).

In addition, free charges may be provided at various times during separation of the clamp 100 and the patterning device MA. Indeed, it will be appreciated that when the patterning device MA is clamped, it may be difficult for free charges to penetrate between adjacent surfaces. So, there may be an effective minimum spacing in which free charges are optimally provided. This spacing depends on the depth of penetration required (i.e. the distance through which free charges must penetrate between the clamp and the separating surfaces of the patterning device to provide effective charge neutralization).

The minimum spacing can also be a function of voltage (as described above, this voltage increases as the spacing increases). For example, when the voltage between the separation surfaces is small (eg, 0), charges do not penetrate deep into the small gaps due to random diffusion and surface recombination. However, if there is a significant voltage between the separating surfaces, the (neutral) environment around the clamp 100 and patterning device MA and between the surfaces of the clamp 100 and patterning device MA with increased voltage An electric field will also be created. This electric field can act to attract free charges of the appropriate sign to the volume between the separating surfaces. The effect of this electric field is generally stronger than the random diffusion of free charges, resulting in the charges penetrating deeper into the volume than if the electric field was not present. In the case of a penetration depth of the order of 1 mm, the minimum gap allowing electric charges to penetrate into the clamp region may be, for example, on the order of 100 μm.

It will be appreciated that there may be a distance between the physical edge of the patterning device MA (or the chamfered edge of the patterning device) and the starting point of the conductive coating 128. This distance may be about 1 mm. As such, a plasma penetration depth of less than 1 mm provides free charges only to the non-conductive surface of the patterning device MA and not the conductive coating 128. The conductive coating 128 can function as a reservoir of charges that can be released locally and in a very short time during discharge. As such, it is beneficial to provide free charges to the conductive coating 128 during separation to neutralize any charge imbalance.

It will also be appreciated that it can be beneficial to provide free charges before the voltage difference between the components being separated exceeds a dangerous level (eg, 250 V). So, there may be an effective maximum spacing (separation) in which free charges are optimally provided. The maximum separation distance for which the charge must be provided will depend on a number of factors.

For example, the initial spacing between the parallel surfaces of the clamp 100 and the patterning device MA is one of these factors. This initial spacing may be of the order of 10 μm (corresponding to the height of the burls 106, which means that there is direct contact between the upper end of the burls 106 and the surface of the patterning device MA). It will be appreciated that the capacitance changes in inverse proportion to the spacing of the parallel surfaces of the clamp 100 and the patterning device MA (not the spacing between the initially zero burls and the patterning device MA). Therefore, the interval corresponding to the dangerous voltage level varies depending on the initial interval level. That is, between the parallel surfaces of the clamp 100 and the patterning device MA when the burls contact the patterning device (for example, a'gap' of 10 μm) and when the patterning device moves away from the clamp. The ratio of the spacing of is substantially equal to the inverse of the ratio of capacitances in these two configurations.

In addition, the spacing corresponding to the dangerous voltage level will also depend on the voltage imbalance between the clamp and the clamped portions of the patterning device. For example, it will be appreciated that clamping is typically achieved by a plurality of electrodes to which an reciprocal voltage is applied. These voltages effectively balance each other, resulting in the patterning device being held at a nominal total voltage of zero (even while a clamping voltage of ±1 to 10 kV is applied to each clamp electrode). However, it will also be appreciated that variations can occur in the capacitance per electrode or between the correct clamping voltage per electrode. Either of these factors can result in a deviation from voltage neutrality across the patterning device. Alternatively or additionally, there may be charge transfer, for example by micro-discharge from charged particles present between the burls of the clamp. In addition, various alternative charge transfer mechanisms may allow the net charge and associated voltage to remain in the patterning device once the clamping voltage has been removed from the clamp 100.

In an embodiment, assume that the voltage imbalance is less than about 10 V (or about 0.5% of the clamping voltage of 2 kV) and that the voltage between the components exceeds 200 V (to minimize the risk of discharge). If so, voltage amplification of less than 20 times can be allowed. This requires that compensating free charges must be provided before the spacing (between parallel surfaces and not burl tips) increases by 20 times. In this example, if the nominal spacing during clamping is 10 μm, a spacing of about 200 μm can be considered as the effective maximum spacing at which free charges are optimally provided.

Of course, it will be appreciated that the effective maximum and minimum spacing at which free charges are provided will depend on many properties and may vary between different device configurations and operating conditions. More generally, free charges are deposited at a time selected to provide charge to reduce (or limit) this potential difference before the potential difference between the electrostatic clamp and the previously clamped (and subsequently released) component exceeds the threshold. It will be understood that it can occur. In an embodiment, free charges are provided from before the interval starts to increase to the point after the minimum interval is exceeded. In an alternative embodiment, free charges are provided from before the interval exceeds the maximum interval to the point after the maximum interval is exceeded. In an alternative embodiment, free charges are provided from before the interval reaches the minimum interval until after the maximum interval is exceeded.

In addition to the use of the free charges associated with the hydrogen plasma as the charge source described above, in an alternative embodiment, the free charges may be used to enable cleaning of the electrostatic clamp 100.

It will be appreciated that, for example, particles may deposit on the surface of the electrostatic clamp 100 between the burls 106. These particles can be detrimental to the clamp's performance. For example, these particles may promote charge transfer between the insulator of the electrostatic clamp 100 and the surfaces of the clamped patterning device MA. This charge transfer may cause extra sticking of the electrostatic clamp 100 even when the clamping voltage is removed from the electrodes. For example, particles can be deposited on the insulator, thus allowing charge to remain on the clamp surface. This trapped charge may cause a corresponding mirror charge to be induced in the polarized coating of the clamped patterning device MA.

Any particles trapped on the surface of the electrostatic clamp 100 can cause field emission from the tips of these particles (which may have sharp features), as a result (assuming that a negative bias voltage is supplied to the corresponding electrode adjacent to the particle). Bottom) it will be appreciated that the flow of electrons is emitted towards the positively polarized coating 128 of the patterning device MA. Alternatively, the charge can be carried by the positive or negative ions of hydrogen (or, in practice, any other gas supplied to the gap between the patterning device and the electrostatic clamp 100). It will be appreciated that tunneling ionization can occur in an electric field that can have an intensity in excess of 100 MV/m. The ions produced in this way can then be attracted to the polarized coating 128 of the patterning device MA.

It is also noted that any particles present on the surface of the conductive coating 128 of the patterning device MA are likely to migrate to the insulating surface of the electrostatic clamp 100. In particular, it will be understood that particles adhere more strongly to insulating surfaces than to conductive surfaces in the presence of an alternating electric field.

14 shows in more detail some of the interactions between the particles and the charged surfaces of the electrostatic clamp 100 and the patterning device MA. In particular, FIG. 14 shows a cross section of a portion of the electrostatic clamp 100 and a portion of the patterning device MA. Several different particle related events are shown in more detail in different parts of the interface of the clamp/patterning device. In the illustrated embodiment, electrode 104A is negatively biased while electrode 104B is positively biased. The electric field lines are indicated by the solid arrow F.

In the first region X, the particles X1 are initially associated with the surface of the patterning device MA. The particles X1 are pressed to move toward the body of the electrostatic clamp 100 by an electrostatic field generated between the electrode 104A and the surface 208 of the patterning device MA. It will be appreciated that particle X1 is initially positively charged and is thus attracted towards negatively biased electrode 104A.

Similarly, another particle Y1 (this time negatively charged) is shown in region Y. The particles Y1 are initially associated with the surface 128 of the patterning device MA, and are initially negatively charged. Under the influence of the positive voltage at the electrode 104B, the particles Y1 are pressed toward the insulating surface of the electrostatic clamp 100.

In the region W, it can be seen that the field emission from the sharp feature on the particle W1 causes electrons to be emitted to the conductive surface 128 of the patterning device MA. This process may allow the region of the positive charge W2 to remain on the clamp surface, and may also cause an attraction force to be generated between the charged particles and the electrode.

In region V, particles V1 are located on the insulating surface of electrostatic clamp 100. Tunneling ionization may occur in the particles V1, and as a result, negative ions are generated. In particular, electrons can tunnel from the clamp surface through the particle's potential barrier under the electric field created by the polarization of the electrode 104A. This process allows negative ions to be generated and a positive charge (V2) to remain on the clamp surface.

In another region Z, the particles Z1 associated with the clamp 100 surface may undergo tunneling ionization (e.g., by electrons tunneling from the particle to the clamp surface), thereby generating positive ions, Allows the negative charge (Z2) to remain on the clamp surface. Such cations are attracted to the patterning device MA under the influence of the electric field established between the electrode 104B and the patterning device MA. Thus, cations (and associated charges) can be transferred to the patterning device.

It will be appreciated that the various scenarios described above with reference to FIG. 14 illustrate different situations that may occur at the interface between the patterning device MA and the electrostatic clamp 100 and are not exhaustive. However, each of these scenarios (and other scenarios not illustrated) can be mitigated to some extent by the use of a free charge source. For example, an ionization source (e.g., VUV source (S) or EUV source (SO)) may be used as a source of mobile charge to compensate for some of the accumulated charges associated with the various scenarios described above. In addition, charge compensation may be used to clean particles trapped from the electrostatic clamp.

The process by which the electrostatic clamp can be cleaned will now be described with reference to FIGS. 15A-15E. In Fig. 15A, a part of the electrostatic clamp 100 is shown again. Only two electrodes 104A and 104B are shown in this example. However, it will be appreciated that the electrostatic clamp 100 may correspond to that described above with reference to FIG. 3. In the initial state shown in Fig. 15A, the electrostatic clamp 100 is not biased. Particles P1 and P2 are trapped on the surface of the clamp 100. The particle P1 is adjacent to the electrode 104A, while the particle P2 is adjacent to the electrode 104B. An ionization source (S) is provided adjacent to the clamp 100 and is initially in a non-conductive state so that VUV is not emitted and hydrogen plasma is not generated.

Referring now to FIG. 15B showing the first step of the cleaning process of the electrostatic clamp 100, the source S is energized to generate the plasma P. The plasma P extends across the surface of the electrostatic clamp 100 and has an intensity that decreases as the distance from the source increases. At the same time, electrodes 104A and 104B are energized so that electrode 104A is negatively biased and electrode 104B is positively biased.

Free charges of the plasma P together with the bias applied to the clamp 100 cause the charges to be attracted to the surfaces of the clamp 100 adjacent to each of the electrodes 104A and 104B. A positive charge Q1 is formed adjacent to the electrode 104A, and a negative charge Q2 is formed adjacent to the electrode 104B. The trapped particles (P1, P2) are charged by charges (Q1, Q2) attracted to the clamp surface.

The voltage applied to the electrodes 104A and 104B during the process described above with reference to FIG. 15B may be lower than a typical clamping voltage. For example, the clamping voltage may be about 1 to 5 kV, while the voltage applied to the electrodes during the cleaning operation may be, for example, about 0.1 to 2 kV.

15C shows an additional step in the cleaning process of the electrostatic clamp 100. A cleaning reticle 160 is provided adjacent to the electrostatic clamp 100. The cleaning reticle 160 includes a body 162 coated with a conductive layer 164 and finally an insulating layer 166. The insulating layer may be formed of an insulating material such as Kapton (Capton), for example. In order to clamp the cleaning reticle 160 to the electrostatic clamp 100, a zero voltage may be applied to the electrodes 104A and 104B. In order to release the particles P1 and P2, a voltage may be briefly applied to the electrodes 104A and 104B as shown in FIG. 15C. However, note that the bias voltage supplied in the configuration shown in Fig. 15C is inverted with respect to the voltage shown in Fig. 15B. That is, a positive voltage is supplied to the electrode 104A, while a negative voltage is supplied to the electrode 104B. The source S is no longer energized.

After these new electric fields are established, any charge previously present on the surface of the clamp 100 is repelled by the reverse bias voltage applied to the electrodes 104A and 104B. This causes the particles P1 and P2 to be repelled from the clamp surface and deposited on the surface of the cleaning reticle 160. That is, the charges accumulated in the particles P1 and P2 during the charging process of FIG. 15B are used to move the particles in the step shown in FIG. 15C. The charges Q1 and Q2 are also repelled by the voltage applied to the electrodes 104A and 104B. However, in the absence of a conductive medium to move, charges Q1 and Q2 may remain on the surface of the clamp 100.

Referring now to FIG. 15D, the cleaning reticle 160 is removed from the electrostatic clamp 100 together with the particles P1 and P2 deposited on the surface of the cleaning reticle 160. In order to remove the cleaning reticle 160, that is, to release the clamp, the voltage shown in Fig. 15B may be applied, that is, the electrodes 104A and the electrodes 104A and so that the electrode 104A is negatively biased and the electrode 104B is positively biased. 104B) is energized. Subsequently, as shown in FIG. 15D, the voltage applied to the electrodes 104A and 104B is removed. Note that trapped surface charges Q1 and Q2 may still remain on the surface of the electrostatic clamp 100.

Turning now to Figure 15e, in the final step of the cleaning process, any residual charge present on the surface of the clamp 100 is once again an ionization field by the source S to provide the plasma P. Can be eliminated by generating. When plasma P is provided in the absence of a bias applied to the electrodes 104A and 104B, residual charges Q1 and Q2 on the surface of the electrostatic clamp 100 can be neutralized by the free space charges of the plasma. have. That is, all residual charges on the insulator of the clamp 100 may be removed.

The cleaning process described above may be performed with an EUV source SO which is also used for lithographic exposure. However, instead of the primary EUV source SO in the process described above with reference to FIGS. 14A to 14E, the use of a separate ionization source S causes generation of mobile charges (eg, hydrogen plasma). This can be particularly beneficial because it allows it to be separated from the operation of the EUV source SO. In addition, the use of a separate source S enables compensating mobile charges to occur in regions where EUV is not available or in a volume isolated from the EUV source. This allows charge compensation to be performed at pressures lower than typical pressures (e.g., pressures between about 0.0001 and 1 Pa) for the EUV source (SO) (which can operate at, for example, a pressure of about 1-10 Pa). Make it possible. As such, the use of a secondary ionization source can be essentially cleaner (based on the availability of higher vacuum levels). In addition, the power of the secondary ionization source can be easily adjusted and optimized for cleaning performance.

It will also be appreciated that separating the cleaning of the electrostatic clamp from the operation of the EUV source SO allows the cleaning process to be carried out in a smaller volume of the lithographic apparatus and not in the projection system. This operation does not require emptying the entire volume of the source (SO), illumination system (IL), and projection system (PS), and only the area where the clamp is being cleaned needs to be kept at a lower level of vacuum. Contributes to the ability to work in the environment.

The above-described cleaning process includes several steps. However, you will understand that not all of these steps are essential. For example, the cleaning reticle can be omitted. When the reticle is not present, any trapped charges can be repelled by reversing the polarity of the clamp electrodes. This process can cause particles to be thrown off into the environment around the clamp. As such, this process can preferably be carried out in an environment other than an operating lithographic apparatus. This cleaning can be carried out in a special cleaning tool.

Further, the cleaning reticle described above is described as comprising a metal surface layer covered with a thin dielectric layer facing the clamp. However, both metal and dielectric layers are optional. However, it will be appreciated that the use of a dielectric layer facing the clamp can provide maximum particle adhesion to any particles incident on the cleaning reticle. In particular, these charged particles are attracted to the dielectric surface by coulomb attraction and retain their charge (due to the dielectric surface), resulting in induction of mirror charges on the metal surface under the dielectric layer.

In the preceding description, various embodiments have been described that can reduce the degree to which a high free space electric field in a lithographic apparatus (or related tool) can cause problems using an EUV induced plasma, or plasma generated by a secondary ionization source. . This free space electric field can be problematic during EUV power up and repolarization of electrostatic clamps. However, by using the free charges generated at an appropriate time as described above, the adverse effects associated with these events can be mitigated or avoided.

In addition, the gradual increase in EUV power provided to electrostatic clamps and patterning devices provides a sudden influx of charge carriers that can lead to high transients and related problems (e.g., discharge, particle generation). Instead, it can be used to enable a gradual collapse of any free space electric field. This effect (ie, a gradual increase in EUV power in the clamp) can be caused by the gradual movement of the masking blades to block the EUV from colliding with the patterning device MA. This control can be used to regulate the supply of free charges (ie, ions or electrons) to the clamp during polarization of the clamp or during repolarization of the clamp. Again, this allows the free space electric field not to change abruptly, but to be removed or reduced in a smoothly controlled manner.

In addition, the EUV induced plasma or plasma generated by the secondary ionizer can be used in patterning device handling procedures (e.g., patterning device handling procedures) to avoid adverse effects associated with voltage amplification due to changes in capacitance associated with increased spacing between various isolated system components. For example, it can be used during patterning device removal).

In addition, the use of free space charges has proven to provide a mechanism to clean the electrostatic clamp to remove trapped particles from the surface of the clamp. This process can be performed in combination with a specially designed sacrificial cleaning reticle.

It will be appreciated that the foregoing embodiments include a number of important advantages. Moreover, in some embodiments the above-described advantages can be obtained without the need to modify the structure of an existing lithographic system. That is, in some embodiments, an existing EUV source can be used in a new way to provide energy at times within an exposure cycle different from the conventional case. Such a configuration can be implemented in an existing device only by changing control procedures without requiring extensive hardware modification. In addition, the nature of the modification to the above-described operation protocol can be implemented without significantly affecting the throughput of the existing device. As such, changes in operating procedures can be performed between existing exposure cycles without significantly affecting the cycle length.

Further, the above-described cleaning mechanism for cleaning the electrostatic clamp can avoid or at least reduce the adverse effects associated with the particles getting trapped on the surface of the clamp. That is, trapped particles (which can also cause trapped charges) can cause unpredictable or drifting clamping forces when the particles are present between the clamped surfaces.

In addition, the clamp cleaning sequence described above can be used to clean the clamp while the clamp is not being used in an active lithographic apparatus. That is, the use of an external or secondary plasma source can enable the clamp to be effectively cleaned without disturbing the normal operation of the lithographic apparatus. In addition, it also allows clamp cleaning to be performed while the lithographic apparatus is offline for other purposes (e.g. routine maintenance) given that it does not depend on the EUV source being used to support the cleaning process. do.

As discussed above, the electrostatic clamp includes an electrode to which a clamping voltage is supplied to generate an electric field that allows the patterning device (ie, reticle) to be clamped. The electrostatic clamp may include a pair of electrodes: that is, one positive electrode and one negative electrode. The clamping voltage supplied to the electrodes may be about ±1 to 10 kV, for example ±2 kV. The pairing of the positive and negative electrodes means that the voltage at the surfaces of the patterning device is nearly zero, that is, the surfaces are maintained at a voltage approximately intermediate the voltages of the positive and negative electrodes. However, tolerances associated with various factors may mean that the voltage on the surface of the patterning device is not actually 0 V compared to the rest of the system. Instead, there may be a voltage imbalance that can lead to an accumulation of charge on the patterning device. It will be appreciated that the patterning device may more generally be referred to as a component.

The electrodes are coated with a material with an ultra-low coefficient of thermal expansion (eg, ULE® manufactured by Corning) having a thickness of about 100 μm. As noted above, the clamp may comprise a generally planar surface provided with protrusions (which may be referred to as burls). The protrusions can allow the spacing between the generally planar surface of the clamp and the clamped surface of the patterning device to exceed a minimum value (eg 10 μm) even during clamping. However, it will be appreciated that during clamping the surface of the protrusions comes into contact with the clamped patterning device, so that during clamping the minimum distance between the surface of the clamp and the surface of the patterning device is zero. The spacing between the generally planar surface of the clamp and the clamped surface of the patterning device determines the capacitive coupling of the electrodes to the patterning device. As a result of the output of the high voltage amplifiers to the electrodes or the tolerance of the individual electrode capacitance (e.g. due to variations in the thickness of the coating material in the micrometer range, e.g. ULE), the surface potential of the patterning device is from zero to maximum. It can deviate up to about 10 V. As an example, a deviation within the tolerance of 0.1% of the 2 kV high voltage amplifier output for the first clamping electrode may cause a maximum of 4 V to be present on the surface of the patterning device. Similarly, a deviation of ±1 μm, ie 1%, of the thickness (100 μm) of the coating material on the electrode(s) can lead to even more significant imbalances.

One problem with the patterning device being non-zero voltage is that charged particles can be attracted to the surface of the patterning device, which can lead to imaging defects. Further, non-zero voltage across the patterning device during EUV exposure can cause charge to accumulate on the back side of the patterning device. For example, during the EUV-ON period, plasma (eg, hydrogen plasma) may be generated by the interaction of EUV radiation with gases present in the scanner. Plasma contains free charges (ions) that can move to the surfaces of the patterning device, in particular to the rear, as a result of the electric field generated by the clamp. That is, when the surfaces of the patterning device are at a non-zero voltage during EUV exposure, the potential field established between the grounded parts of the system and the patterning device having a non-zero voltage is the free charge generated in the plasma. It flows towards the patterning device (or away from the patterning device), resulting in a (positively or negatively) charge. Subsequently, when the clamping voltage is removed (and when the plasma is no longer present), residual charge accumulated on the surface(s) of the patterning device may remain, and thus with the patterning device (e.g., connected to ground). It creates a potential difference between different parts of the system (such as parts of the system).

Another problem with the patterning device being non-zero voltage is that, as discussed above, during the unloading of the patterning device, any charge remaining on the surface of the patterning device moves away from the clamp (and thus the capacitance Decreases), induces a voltage that is greatly amplified there. This can lead to a discharge between the back side of the patterning device (ie, the face facing the clamp) and nearby grounded parts of the system. In addition, particles present on the front surface of the patterning device may be discharged from the surface at high speed, causing damage to the system interior. Accordingly, it is desirable to provide a method to alleviate one or more of the problems mentioned above.

To cope with the problems of electrostatic attraction of particles to the patterning device (reticle) and/or voltage amplification and discharge during unloading, the patterning device is grounded (i.e., the net voltage associated with the patterning device is 0 V). Earth) is suggested. In particular, it may be desirable to provide a “virtual grounding” of the patterning device. In other words, instead of providing a physical ground connection to the patterning device, it may be desirable to balance the voltage induced across the patterning device such that a total of 0 V is produced by adjusting the individual voltages of the electrodes in the electrostatic clamp. In this way, no additional hardware is required and there is no risk of losing the physical ground connection.

In a first method of providing a virtual ground, a patterning device is provided adjacent to the electrostatic clamp in step 200, as shown in FIG. 16, using, for example, the exchange assembly described with reference to FIGS. 9A and 9B. . Next, in step 202, the electrostatic clamp is controlled such that the patterning device is clamped to the electrostatic clamp. In particular, a first clamping voltage may be supplied to the first electrode of the clamp, thereby inducing an electric field that acts to clamp the patterning device to the clamp.

The patterning device is exposed to EUV radiation in step 204, and is then released and removed from the vicinity of the electrostatic clamp in step 206. As described above, charge may accumulate on the rear surface of the patterning device during clamping and exposure. Also, as specifically described with reference to FIGS. 10 to 13, as the patterning device is moved away from the clamp, the voltage associated with the surface of the patterning device, for example the back surface of the patterning device, may increase significantly. This voltage is measured in step 208. It will be appreciated that the voltage associated with other parts of the patterning device can be measured instead. For example, the voltage associated with the front surface of the patterning device may be measured. In particular, as discussed above with reference to FIG. 12C, the voltage at the front side may be less than the voltage at the rear side. For example, due to the internal capacitance of the patterning device, the voltage on the front side may be about 5 times less than the voltage on the back side.

Based on the measured voltage, in step 210 an adjustment to the first clamping voltage is determined. It will be appreciated that any measured offset voltage can be used to calculate the regulation voltage. If the determined adjustment is not 0, the first clamping voltage may be adjusted in step 212 according to the determined adjustment. For example, the output of the high voltage amplifier connected to the first electrode may be adjusted. It will be appreciated that the adjustment may be determined in terms of the voltage being added or subtracted to the electrode voltage, a percentage change to the current voltage, a change in the gain of the high voltage power amplifier, or any other suitable aspect that is apparent to the skilled person. In this way, it is possible to ensure that the voltage induced to the patterning device is balanced to 0 V.

It will be appreciated that an electrostatic clamp may comprise two or more electrodes, in particular a clamp may comprise from 2 to n electrodes. In this case, step 210 also includes determining an adjustment for the second through n-th clamping voltage, and step 212 includes adjusting any or all of the second through n-th clamping voltages according to the determined adjustment. I can. For example, if it is determined that the reticle surface is to be maintained at a voltage of +4 V during clamping, 4 V may be subtracted from the clamping voltage of each of the positive and negative clamping electrodes. Alternatively, as mentioned above, adjust only the positive (or negative) clamping electrodes of the magnitude and direction configured to bring the voltage on the reticle surface to approach 0 V, which will be a substantially intermediate voltage between the potentials of the clamping electrodes. This may be done.

To verify that the correct adjustment has been made, the method can be repeated. It will be appreciated that the same patterning device may be used during the verification method. Alternatively, other patterning devices may be used. This verification process may be repeated until a satisfactory result is determined (eg, until the voltage induced to the patterning device during clamping is substantially zero or below a predetermined threshold).

In some implementations, the measurement of the voltage is performed while the patterning device is clamped by a clamp. An exemplary flowchart of this implementation is schematically shown in FIG. 17. This method can be performed using the system shown in FIG. 18.

The system shown in FIG. 18 corresponds generally to the system illustrated in FIG. 9A and discussed in detail above. In particular, the system of FIG. 18 includes a clamp 100 to which a patterning device MA is clamped. In this particular configuration, clamp 100 includes four electrodes 104A, 104B, 104C, and 104D. The clamped patterning device MA is shown spaced apart from the exchange assembly 150. The patterning device MA includes a front face 126 facing towards the exchange assembly 150 and a rear face 128 adjacent to the clamp 100. The exchange assembly includes an exchange device 152 that supports the support structure 154. For example, the exchange assembly 150 is to move the patterning device MA towards and away from the clamp 100 so that the patterning device MA can be replaced or cleaned with replacement patterning devices. Is composed.

Referring back to FIG. 17, in step 300 a patterning device is provided adjacent to the clamp. Again, this can be achieved using the exchange assembly 150. In a subsequent step 302, the clamp is controlled to clamp the patterning device. In particular, a first clamping voltage may be supplied to the first electrode of the clamp, thereby inducing an electric field that acts to clamp the patterning device to the clamp. The system includes a voltage monitor 180 configured to measure a voltage associated with a portion of the patterning device. In some implementations of this method, the measured voltage associated with a portion of the patterning device is a front side voltage. In some implementations, the voltage monitor 180 may be part of the exchange assembly 150 that contacts the front surface of the patterning device as shown in FIG. 18. However, voltage monitor 180 may be located in any suitable location within the system. In some implementations, voltage monitor 180 may be an electrostatic voltmeter. The electrostatic voltmeter can be arranged to measure a voltage associated with the front surface of the patterning device without contacting the front surface of the patterning device, thereby preventing charge from being transferred to (or from) the patterning device during measurement. A similar measuring device (ie, using an electrostatic voltmeter) can be used to measure the voltage on the front or back side of the patterning device during the off-line method described above. Of course, it will be understood that what is shown in FIG. 18 is only a schematic illustration. In particular, the illustrated connection from the voltage monitor 180 to the front surface of the patterning device serves to illustrate that although the voltage on the front surface of the patterning device is being measured, it does not necessarily mean a physical connection from the voltage monitor 180 to the patterning device. It will be understood that they do. As discussed above, voltage monitor 180 may be an electrostatic voltmeter arranged to measure a voltage associated with the front face of the patterning device without physical contact.

In contrast to the off-line method described above, in this implementation a voltage measurement is made during step 304 while the patterning device is clamped by a clamp. Based on the measured voltage, an adjustment to be made to the first clamping voltage is determined in step 306. If the determined adjustment is not zero, in step 308 the first clamping voltage is adjusted. Following adjustment step 308, the method returns to the step of measuring the voltage associated with a portion of the patterning device. The voltage is measured and the adjustment is determined. In this way, it can be verified whether an appropriate adjustment has been made to the first clamping voltage. It will be appreciated that if the determined adjustment is zero, the method may end after step 306. Alternatively, additional voltage measurements may be performed at appropriate intervals to ensure that the virtual ground is maintained in an accurate state. Further measurements can be performed regularly between EUV exposures, for example, by providing an exchange assembly adjacent to the patterning device without removing the patterning device from the clamp.

If the electrostatic clamp has two or more electrodes, for example 2 to n electrodes, each having its own clamping voltage, step 306 is based on the measured voltage, and the second, ... It will be appreciated that it may also include determining an adjustment to the n th clamping voltage. Likewise, step 308 is a second, ... And adjusting any one or all of the n-th clamping voltages.

It will also be appreciated that steps 300, 302, 304, and 306 correspond to steps 200, 202, 208, and 210 of the method shown in FIG. 16, respectively. Since the method shown in FIG. 17 is performed in real time, there is no need to expose the patterning device to EUV exposure to “fix” charge in place on the patterning device for measurement, unlike in the offline method shown in FIG. 16. However, of course, it will be appreciated that the method of FIG. 17 can include EUV exposure as an optional step. In this case, if the voltage monitor 180 had formed part of the exchange assembly 150, the exchange assembly 150 would have to include an opening to allow EUV radiation to reach the clamped patterning device. Alternatively, if the voltage monitor 180 had formed part of another system component, the exchange assembly could have been moved away from the patterning device so that EUV radiation could reach the patterning device without being disturbed by the exchange assembly. It will also be appreciated that free charges generated by alternative means other than EUV exposure may be used to provide charge to the clamped patterning device (eg, a secondary ionization source).

While specific reference may be made herein to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other uses as well. Other possible uses include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, and the like.

In this specification, specific reference may be made to embodiments of the present invention in the context of a lithographic apparatus, but embodiments of the present invention may also be used in other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterning device). These devices may generally be referred to as lithography tools. These lithography tools can use vacuum conditions or ambient (non-vacuum) conditions. Indeed, embodiments of the present invention may form part of any device in which an electrostatic clamp is used.

In the above, specific reference may be made to the use of embodiments of the present invention in the context of optical lithography, but as far as the context permits, the present invention is not limited to optical lithography and may be used in other applications, such as imprint lithography. I will understand.

Where context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented with instructions stored in a machine-readable medium that can be read and executed by one or more processors. The machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (eg, a computing device). For example, machine-readable media may include read only memory (ROM); Random access memory (RAM); Magnetic storage media; Optical storage media; Flash memory device; Electrical, optical, acoustic, or other forms of radio signals (eg, carrier waves, infrared signals, digital signals, etc.), and others. Additionally, firmware, software, routines, and instructions may be described herein as performing specific operations. However, this description is for convenience only, that such operations actually arise from computing devices, processors, controllers, or other devices executing firmware, software, routines, instructions, etc., and that It should be understood that actuators or other devices can be made to interact with the physical world.

While specific embodiments of the present invention have been described above, it will be understood that the present invention may be implemented differently from those described. The above description is for illustrative purposes only, not limitation. Thus, it will be apparent to those skilled in the art that modifications can be made to the present invention as described without departing from the scope of the claims set forth below.

Claims (89)

An apparatus comprising an electrostatic clamp for clamping a component and a mechanism for generating free charges adjacent the electrostatic clamp,
The mechanism for generating free charges is configured to generate free charges adjacent the electrostatic clamp during a transition from a first energization state of the electrostatic clamp to a second energized state of the electrostatic clamp,
Device. The method of claim 1,
The electrostatic clamp includes a clamping region configured to clamp the component,
When a component is clamped, a clamping electric field is generated between the clamping region and the component,
Device. The method of claim 2,
The electrostatic clamp further comprises a non-clamping region,
When a component is clamped by the clamping region, a secondary electric field is generated around the non-clamping region,
Device. The method according to claim 2 or 3,
The electrostatic clamp includes at least one electrode, and a clamping voltage is applied to the at least one electrode such that the clamping electric field is generated between the clamping region and the component when the component is clamped by the electrostatic clamp.
Device. The method of claim 4,
The electrostatic clamp further comprises at least one contact configured to provide an electrical connection to the at least one electrode, and the mechanism for generating free charges is the first of the electrostatic clamp from the first energized state of the electrostatic clamp. 2 configured to generate free charges adjacent to the at least one contact point during the transition to an energized state,
Device. The method according to claim 4 or 5,
In the first energized state, a voltage having a first polarity is applied to the at least one electrode, and in the second energized state, a voltage having a second polarity opposite to the first polarity is the at least one Applied to the electrode of,
Device. The method of claim 6,
The electrostatic clamp includes at least two electrodes,
In the first energized state, a voltage having the first polarity is applied to a first electrode among the electrodes, and a voltage having the second polarity is applied to a second electrode of the electrodes,
In the second energized state, the voltage having the second polarity is applied to the first electrode among the electrodes, and the voltage having the first polarity is applied to the second electrode of the electrodes,
Device. The method according to any one of claims 1 to 7,
The clamp is configured such that in each of the first and second energized states, a component can be clamped by the electrostatic clamp,
Device. The method according to any one of claims 1 to 8,
The mechanism for generating free charges adjacent the electrostatic clamp comprises a gas source and an ionizing radiation source configured to ionize a gas provided by the gas source.
Device. The method of claim 9,
The ionizing radiation source comprises a source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source,
Device. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, comprising:
The lithographic apparatus comprises an apparatus according to any one of claims 1 to 10, wherein the patterning device comprises the component to be clamped.
Lithographic apparatus. The method of claim 11,
An illumination system configured to condition a radiation beam, wherein the electrostatic clamp is configured to clamp the patterning device, the patterning device being capable of imparting a pattern to its cross-section of the radiation beam to form a patterned radiation beam;
A substrate table configured to hold a substrate; And
A projection system configured to project the patterned beam of radiation onto the substrate:
The lithographic apparatus further comprising. The method of claim 12,
The lithographic apparatus is configured to perform a plurality of imaging exposures, wherein the radiation beam is incident on the patterning device during the exposure and the patterned radiation beam is projected onto the substrate during the exposure, the The electrostatic clamp is configured to clamp the patterning device during the imaging exposure;
Between successive exposures of the plurality of imaging exposures, the electrostatic clamp is configured to transition from the first energized state to the second energized state,
Lithographic apparatus. A lithographic system comprising the lithographic apparatus of claim 13, comprising:
The lithographic system further comprises a radiation source configured to generate the radiation beam, and the mechanism for generating free charges is 2 selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radiation source. Comprising a primary ionizing radiation source,
Lithography system. A lithographic system comprising the lithographic apparatus of claim 13, comprising:
The lithographic system further comprises a radiation source configured to generate the radiation beam, and the mechanism for generating free charges comprises the radiation source.
Lithography system. The method of claim 15,
The lithographic system is further configured to perform at least one non-imaging exposure-the radiation beam is incident on the patterning device during the exposure and no radiation is projected onto the substrate during the exposure; The non-imaging exposure is performed between successive exposures of the plurality of imaging exposures,
Lithography system. The method of claim 16,
The transition from the first energized state of the electrostatic clamp to the second energized state of the electrostatic clamp is performed during the non-imaging exposure,
Lithography system. The method of claim 17,
The lithographic system is controlled such that the amount of radiation incident on the patterning device is greater during each imaging exposure than during the non-imaging exposure,
Lithography system. The method of claim 18,
The amount of radiation incident on the patterning device is gradually increased from non-imaging exposure to one of the plurality of imaging exposures,
Lithography system. A method of operating a device, the device comprising an electrostatic clamp and a mechanism for generating free charges adjacent the electrostatic clamp,
Controlling the electrostatic clamp to have a first energized state;
Controlling the electrostatic clamp to have a second energized state; And
During the transition from the first energized state to the second energized state, controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp:
Containing, method. As a lithographic apparatus,
An illumination system configured to condition the radiation beam;
A support structure configured to support a patterning device, the patterning device being capable of imparting a pattern to its cross-section of the radiation beam to form a patterned radiation beam, the support structure comprising an electrostatic clamp configured to clamp the patterning device. The support structure;
A substrate table configured to hold a substrate; And
A projection system configured to project the patterned beam of radiation onto the substrate:
Including,
The lithographic apparatus is configured to perform a pre-imaging exposure-the radiation beam is incident on the patterning device during the exposure and no radiation is projected onto the substrate during the exposure, and incident on the patterning device. The amount of the radiation to be increased gradually during the exposure before the imaging,
The lithographic apparatus is configured to perform imaging exposure in which the radiation beam patterned by the patterning device is projected onto the substrate,
Lithographic apparatus. The method of claim 21,
The pre-imaging exposure includes a burst, and the burst includes a plurality of radiation pulses,
Lithographic apparatus. The method of claim 22,
Wherein the gradual increase in the amount of radiation during exposure prior to imaging is configured to be provided over a plurality of pulses of radiation,
Lithographic apparatus. The method of claim 23,
Wherein the gradual increase in radiation during exposure prior to imaging is configured to be provided over at least 1000 radiation pulses.
Lithographic apparatus. The method according to any one of claims 22 to 24,
Wherein the gradual increase in radiation during exposure prior to imaging is configured to be provided substantially linearly across a plurality of radiation pulses,
Lithographic apparatus. The method according to any one of claims 21 to 25,
During a first portion of the pre-imaging exposure having a predetermined duration, the radiation beam is controlled to deliver a first dose of radiation to the patterning device, and the first dose is the patterning during the first portion of the imaging exposure. Comprising less than about 10% of the imaging dosage of radiation delivered to the device, wherein the first portion of the pre-imaging exposure has the predetermined duration
Lithographic apparatus. The method of claim 26,
The amount of radiation incident on the patterning device is gradually increased from the first portion of the pre-imaging exposure to the imaging exposure,
Lithographic apparatus. The method of claim 26 or 27,
At least 1000 radiation pulses are delivered between the initiation of the first portion of the pre-imaging exposure and the initiation of the imaging exposure,
Lithographic apparatus. The method according to any one of claims 21 to 28,
The lithographic apparatus is further configured to perform a first imaging exposure and a second imaging exposure,
The exposure before the imaging is preceded immediately before the second imaging exposure,
Between the first imaging exposure and the pre-imaging exposure, the lithographic apparatus is further configured to perform a non-imaging exposure-the radiation beam is incident on the patterning device during the exposure and no radiation is projected onto the substrate during the exposure. Consisting of,
Lithographic apparatus. The method of claim 29, which is dependent on claim 26,
During a first portion of the non-imaging exposure having the predetermined duration, the radiation beam is controlled to deliver a third dose of radiation to the patterning device, and the third dose is about 10% of the imaging dose of the radiation. Less than, wherein the device is configured to transition the electrostatic clamp from a first energized state to a second energized state during the non-imaging exposure,
Lithographic apparatus. The method according to any one of claims 21 to 29,
The lithographic apparatus, further configured to transition the electrostatic clamp from a first energized state to a second energized state during the pre-imaging exposure. A lithographic system comprising the lithographic apparatus of any one of claims 21 to 31, comprising:
A lithographic system, further comprising a radiation source configured to generate the radiation beam. A method of operating a lithographic apparatus, comprising:
The lithographic apparatus:
An illumination system configured to condition the radiation beam;
A support structure configured to support a patterning device, the patterning device being capable of imparting a pattern to its cross-section of the radiation beam to form a patterned radiation beam, the support structure comprising an electrostatic clamp configured to clamp the patterning device. The support structure;
A substrate table configured to hold a substrate; And
A projection system configured to project the patterned beam of radiation onto the substrate:
Including,
The method is:
Causing the lithographic apparatus to perform pre-imaging exposure-the radiation beam is incident on the patterning device during the exposure and no radiation is projected onto the substrate during the exposure, the amount of radiation incident on the patterning device The step, which is gradually increased during the exposure before the imaging; And
Causing the lithographic apparatus to perform imaging exposure in which the radiation beam patterned by the patterning device is projected onto the substrate:
A method of operating a lithographic apparatus comprising a. An apparatus comprising an electrostatic clamp for clamping a component and a mechanism for generating free charges adjacent the electrostatic clamp, the apparatus comprising: a first configuration in which the component is clamped by the electrostatic clamp; And the component has a second configuration spaced apart from the electrostatic clamp,
The device is configured to be in the first configuration at a first point in time and in the second configuration at a second time after the first point in time,
The mechanism for generating free charges is configured to generate free charges adjacent to the electrostatic clamp and/or component at a third time between the first and second time points,
Device. The method of claim 34,
The mechanism for generating free charges is configured to generate free charges adjacent the electrostatic clamp and/or the component to prevent a potential difference between the electrostatic clamp and the component from exceeding a predetermined threshold value,
Device. The method of claim 35,
The predetermined threshold is determined based on the pressure in the device,
Device. The method of claim 36,
The third time point is selected to prevent a potential difference between the electrostatic clamp and the component from exceeding the predetermined threshold value,
Device. The method according to any one of claims 35 to 37,
The predetermined threshold is about 250 V (volts),
Device. The method according to any one of claims 34 to 38,
The device is configured to be in the second configuration at the third time point,
Device. The method according to any one of claims 34 to 39,
At the third point in time, the minimum distance between the surface of the clamp and the surface of the component is greater than about 10 μm,
Device. The method according to any one of claims 34 to 40,
At the third point in time, the minimum distance between the surface of the clamp and the surface of the component is less than a predetermined distance,
Device. The method of claim 41,
The predetermined spacing is about 200 μm,
Device. The method according to any one of claims 34 to 38,
The mechanism for generating free charges is configured to generate free charges adjacent the electrostatic clamp and/or component when the device is configured to be in the first configuration,
Device. The method of claim 43,
The device is configured to be in the first configuration at the third time point,
Device. The method according to any one of claims 34 to 44,
The electrostatic clamp includes a clamping region configured to clamp the component, wherein a clamping electric field is generated between the clamping region and the component when the component is clamped.
Device. The method according to any one of claims 34 to 45,
The electrostatic clamp includes at least one electrode, and a clamping voltage is applied to the at least one electrode such that the clamping electric field is generated between the clamping region and the component when the component is clamped by the electrostatic clamp.
Device. The method according to any one of claims 34 to 46,
The mechanism for generating free charges adjacent the electrostatic clamp comprises a gas source and an ionizing radiation source configured to ionize a gas provided by the gas source.
Device. The method of claim 47,
The ionizing radiation source comprises a source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source,
Device. The method according to any one of claims 34 to 48,
Further comprising a component exchange assembly configured to remove the component from the electrostatic clamp,
Device. The method of claim 49,
The component exchange assembly is configured to control a spacing between the component and the electrostatic clamp,
Device. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, comprising:
The lithographic apparatus comprises an apparatus according to any one of claims 34 to 50, wherein the patterning device comprises the component to be clamped.
Lithographic apparatus. The method of claim 51,
An illumination system configured to condition a radiation beam, wherein the electrostatic clamp is configured to clamp the patterning device, the patterning device being capable of imparting a pattern to its cross-section of the radiation beam to form a patterned radiation beam;
A substrate table configured to hold a substrate; And
A projection system configured to project the patterned beam of radiation onto the substrate:
The lithographic apparatus further comprising. A lithographic system comprising the lithographic apparatus of claim 52, comprising:
The lithographic system further comprises a radiation source configured to generate the radiation beam, and the mechanism for generating free charges is 2 selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radiation source. Comprising a primary ionizing radiation source,
Lithography system. A lithographic system comprising the lithographic apparatus of claim 52, comprising:
The lithographic system further comprises a radiation source configured to generate the radiation beam, and the mechanism for generating free charges comprises the radiation source.
Lithography system. A method of operating a device, the device comprising an electrostatic clamp and a mechanism for generating free charges adjacent the electrostatic clamp,
Providing a component adjacent the electrostatic clamp;
Controlling the electrostatic clamp so that the component has a first configuration clamped by the electrostatic clamp at a first point in time;
Controlling the electrostatic clamp so that the component has a second configuration spaced apart from the electrostatic clamp at a second time point after the first time point; And
Controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp and/or component at a third time point between the first time point and the second time point:
Containing, method. A device comprising an electrostatic clamp for clamping a component and a mechanism for generating free charges adjacent the electrostatic clamp, the device wherein a voltage having a first polarity is applied to at least one clamp electrode and the component is applied by the electrostatic clamp. A first configuration not clamped; And a second configuration in which a voltage having a second polarity opposite to the first polarity is applied to the at least one clamp electrode;
The device is configured to be in the first configuration at a first time point and in the second configuration at a second time point after the first time point,
The mechanism for generating free charges generates free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the first configuration, and free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the second configuration. Configured not to occur,
Device. The method of claim 56,
A component is provided adjacent the electrostatic clamp during the transition from the first configuration to the second configuration,
Device. The method of claim 57,
When the clamp is in the second configuration, the component is clamped by the electrostatic clamp,
Device. The method according to any one of claims 56 to 58,
The device has a third configuration in which no voltage is applied to the at least one clamp electrode and the component is not clamped by the electrostatic clamp; The device is configured to be in the third configuration at a third time point after the second time point,
Device. The method of claim 59,
The mechanism for generating free charges is configured to generate free charges adjacent the electrostatic clamp when the electrostatic clamp is in the third configuration,
Device. The method of claim 59 or 60,
In the third configuration, no component is provided adjacent to the electrostatic clamp,
Device. The method according to any one of claims 56 to 61,
And a component exchange assembly configured to support a component adjacent the electrostatic clamp. The method of claim 62,
The component exchange assembly is configured to control a spacing between the component and the electrostatic clamp,
Device. The method of claim 63,
The component exchange assembly is configured to provide the component adjacent the electrostatic clamp between the first and second time points,
Device. The method of claim 63 or 64, which is dependent on claim 59,
The component exchange assembly is configured to remove the component from adjacent to the electrostatic clamp between the second and third time points,
Device. The method according to any one of claims 56 to 65,
The electrostatic clamp includes a clamping region configured to clamp a component, wherein a clamping electric field is generated between the clamping region and the component when the component is clamped.
Device. The method according to any one of claims 56 to 66,
The mechanism for generating free charges adjacent the electrostatic clamp comprises a gas source and an ionizing radiation source configured to ionize a gas provided by the gas source.
Device. The method of claim 67,
The ionizing radiation source comprises a source selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radioactive source,
Device. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, comprising:
The lithographic apparatus comprises an apparatus according to any one of claims 56 to 68, wherein the electrostatic clamp is configured to clamp the patterning device during a lithographic operation.
Lithographic apparatus. The method of claim 69,
An illumination system configured to condition a radiation beam, wherein the electrostatic clamp is configured to clamp the patterning device, the patterning device being capable of imparting a pattern to its cross-section of the radiation beam to form a patterned radiation beam;
A substrate table configured to hold a substrate; And
A projection system configured to project the patterned beam of radiation onto the substrate:
The lithographic apparatus further comprising. A lithographic system comprising the lithographic apparatus of claim 70, comprising:
The lithographic system further comprises a radiation source configured to generate the radiation beam, and the mechanism for generating free charges is 2 selected from the group consisting of an EUV source, a VUV source, a soft X-ray source, and a radiation source. Comprising a primary ionizing radiation source,
Lithography system. A lithographic system comprising the lithographic apparatus of claim 70, comprising:
The lithographic system further comprises a radiation source configured to generate the radiation beam, and the mechanism for generating free charges comprises the radiation source.
Lithography system. A method of operating a device, the device comprising an electrostatic clamp comprising at least one clamp electrode and a mechanism for generating free charges adjacent the electrostatic clamp,
Controlling the electrostatic clamp to have a first configuration in which a voltage having a first polarity is applied to the at least one clamp electrode at a first point in time and a component is not clamped by the electrostatic clamp;
Controlling the electrostatic clamp to have a second configuration in which a voltage having a second polarity opposite to the first polarity is applied to the at least one clamp electrode at a second time point after the first time point;
Controlling the mechanism for generating free charges to generate free charges adjacent the electrostatic clamp when the clamp is in the first configuration; And
Controlling the mechanism for generating free charges so as not to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the second configuration:
Containing, method. The method of claim 73,
When the electrostatic clamp is in the second configuration, providing a component adjacent the electrostatic clamp. The method of claim 73 or 74,
The method further comprising controlling the device to have a third configuration in which no voltage is applied to the at least one clamp electrode at a third time point after the second time point. The method of claims 75 and 74,
And removing the component from adjacent to the electrostatic clamp between the second and third time points. The method of claim 75 or 76,
And controlling the mechanism for generating free charges to generate free charges adjacent to the electrostatic clamp when the electrostatic clamp is in the third configuration. A method of operating a device, the device comprising an electrostatic clamp having a first electrode,
a) providing a component adjacent the electrostatic clamp;
b) controlling the electrostatic clamp to provide a first clamping voltage to the first electrode such that the component is clamped by the electrostatic clamp;
c) measuring a voltage associated with a portion of the component; And
d) determining an adjustment for the first clamping voltage based on the measured voltage:
Containing, method. The method of claim 78,
b1) exposing the component to radiation while the component is clamped by the electrostatic clamp;
b2) controlling the electrostatic clamp such that the component is released from the electrostatic clamp; And
b3) removing the component from the vicinity of the electrostatic clamp:
The method further comprising. The method of claim 79,
e) adjusting the first clamping voltage according to the determined adjustment; And
f) repeating steps a) to c) to verify the adjustment:
The method further comprising. The method of claim 80,
The method, wherein the repetition of steps a) to c) is performed using other components. The method of claim 78,
After determining the adjustment for the first clamping voltage, adjusting the first clamping voltage according to the determined adjustment; And
Measuring the voltage associated with a portion of the component:
The method further comprising. The method according to any one of claims 78 to 82,
The electrostatic clamp further includes a second electrode,
The method is:
Determining an adjustment to a second clamping voltage supplied to the second electrode based on the measured voltage:
The method further comprising. The method of claim 83,
The first and second clamping voltages have different values,
Way. As a system for virtually grounding components,
An electrostatic clamp configured to clamp the component, the electrostatic comprising a first electrode configured to receive a first clamping voltage;
A voltage monitor configured to measure a voltage associated with a portion of the component; And
A calculation unit configured to determine an adjustment to the first clamping voltage based on the measured voltage:
Containing, system. The method of claim 85,
The system further comprises a support assembly configured to support the component, the support assembly comprising the voltage monitor,
system. The method of claim 85 or 86,
The electrostatic clamp comprises a second electrode configured to receive a second clamping voltage, and the calculation unit is further configured to determine an adjustment to the second clamping voltage based on the measured voltage,
system. The method of claim 87,
The first and second clamping voltages have different values,
system. The method according to any one of claims 85 to 88,
The voltage monitor is an electrostatic voltmeter,
system.

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