CN113795912A

CN113795912A - Apparatus and method including an electrostatic chuck

Info

Publication number: CN113795912A
Application number: CN202080034454.4A
Authority: CN
Inventors: A·M·雅库尼恩; R·范德威克; M·范卡朋; M·A·范德科霍夫; A·拉塞
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV; ASML Netherlands BV
Priority date: 2019-05-10
Filing date: 2020-05-07
Publication date: 2021-12-14
Also published as: WO2020229302A1; TW202101659A; EP3966634A1; KR20220006536A

Abstract

An apparatus, comprising: an electrostatic chuck for clamping a component; and a mechanism for generating a free charge adjacent the electrostatic chuck. The electrostatic chuck includes an electrode or a plurality of electrodes. The device is configured to: operating in a first mode in which the or each electrode is set at an electrical potential such that a clamping electric field is generated between the electrostatic clamp and the component to clamp the component; operating in a second mode in which the or each potential of the electrode is set such that clamping of the component is released; and operating in a third mode in which the or each potential of the electrodes is set such that the flux of free charge generated by the mechanism to a surface of the component adjacent the electrostatic clamp is increased compared to operating in the first or second modes.

Description

Apparatus and method including an electrostatic chuck

Cross Reference to Related Applications

The present application claims priority to EP 19173683.4 filed on day 5/10 2019, EP 19178628.4 filed on day 6/2029, and EP 19186258.0 filed on day 7/15 2019, the entire contents of these european applications being incorporated herein by reference.

Technical Field

The present invention relates to an apparatus comprising an electrostatic clamp, and a method of operating the apparatus. More particularly, but not exclusively, the apparatus may comprise a lithographic apparatus, the electrostatic clamp being configured to clamp a component, such as a patterning device, during lithographic patterning.

Background

A lithographic apparatus is a machine that is configured to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). The lithographic apparatus may, for example, project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on the substrate.

To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. A lithographic apparatus using Extreme Ultraviolet (EUV) radiation having a wavelength in the range 4nm to 20nm (e.g. 6.7nm or 13.5nm) may be used to form smaller features on a substrate than a lithographic apparatus using radiation having a wavelength of 193nm, for example.

Lithographic apparatus can typically use a high voltage electrostatic clamp to hold the patterning device, for example, during a patterning operation. The electrostatic chuck and patterning device 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 surfaces or ungrounded surfaces. For example, during operation, charge may accumulate on dielectric or ungrounded surfaces through touching components (e.g., mask clamping) or through particle collisions during gas flow.

It is also understood that EUV radiation may cause the hydrogen rich environment to become conductive due to the production of EUV-initiated hydrogen plasma. Free charges generated within the EUV initiated hydrogen plasma may be attracted to (or repelled by) the electric field generated by the electrostatic chuck. On the other hand, in the absence of EUV-induced plasma, or in a region that is a distance away from or well shielded from any EUV-induced plasma, charge may accumulate on the dielectric surface or ungrounded surface, and may remain present after any electric field has been removed.

In addition to the accumulation of charge, very strong electrostatic fields (e.g., on the order of-1 kV/cm to 100kV/cm) can also be generated between components of the electrostatic chuck and other system components. In particular, a high voltage applied to the electrodes of the electrostatic chuck causes nearby conductors (e.g., conductive coatings that may be present on the surface of the mask) to be polarized. Thus, a strong electrostatic field is generated, especially at sharp features (e.g., edges of the conductive mask coating).

Disclosure of Invention

According to a first aspect of the invention, there is provided an apparatus comprising: an electrostatic chuck for clamping a component; and a mechanism for generating free charge adjacent the electrostatic chuck: wherein the electrostatic chuck comprises an electrode or a plurality of electrodes, wherein the apparatus is configured to: operating in a first mode in which the or each electrode is set at an electrical potential such that a clamping electric field is generated between the electrostatic clamp and the component to clamp the component; operating in a second mode in which the or each potential of the electrode is set such that clamping of the component is released; and operating in a third mode in which the or each potential of the electrodes is set such that the flux of free charge generated by the mechanism to a surface of the component adjacent the electrostatic clamp is increased compared to operating in the first or second modes.

This may enable acceleration of residual charge neutralization during reticle unloading. Thus, compensation of residual charge on the reticle by EUV-induced plasma during reticle unloading/loading actions may be enhanced. This may avoid electrical breakdown that may cause damage to the surface of the accompanying reticle. In addition, throughput neutral reticle grounding may be achieved.

The electrostatic clamp may comprise the plurality of electrodes, and wherein in the third mode the apparatus may be configured such that the potential of the edge electrode closest to the edge of the electrostatic clamp is set to positive. This may provide an additional negative bias to the reticle second surface (i.e. the surface adjacent to the clamp), which promotes the positive ion flux towards the reticle MA.

In the third mode, the device may be configured such that the or each potential of the electrode is set such that the or an average potential of the electrode is negative. This may capacitively induce a negative potential on the reticle that attracts positive ions toward the second surface of the reticle.

In the third mode, the apparatus may be configured such that the potentials of the plurality of electrodes are set such that an average potential of the plurality of electrodes is substantially 0V. This means that the capacitively induced potential on the reticle, in particular on the second surface of the reticle, remains unchanged.

The electrostatic clamp may comprise the plurality of electrodes, and wherein in the third mode, the apparatus may be configured such that the potential of an edge electrode closest to an edge of the electrostatic clamp is set to be negative, and the potentials of the remainder of the plurality of electrodes are set such that the average potential of the plurality of electrodes has a smaller negative value than the potential of the edge electrode.

In the third mode, the apparatus may be configured such that the or each potential of the electrodes is set such that the surface of the component adjacent the electrostatic clamp has a positive potential before the component moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp. This means that reticle residual charge neutralization at unloading can be achieved by electrons (rather than by positive ions), and thus much faster.

In the third mode, the potential of the electrode or the average potential of the electrode may be set to a predetermined negative value so that the surface of the component has substantially the same potential as the potential of the electrode or the average potential of the electrode during the movement of the component from being nipped by the electrostatic chuck to being spaced apart from the electrostatic chuck. This may prevent charging of the reticle during exposure.

In the third mode, the potential of the electrode or the average potential of the electrode may be set to the predetermined negative value so that the member has substantially zero charge after exposure of the member. This may mean that the potential difference between the second surface of the reticle and the clamping surface of the clamp does not increase relatively significantly during unloading.

In the third mode, the or each potential of the or each electrode may be set for at least one of the following times: a time period before the component moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp; a portion or all of the time it takes to move the component from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp; and a portion or all of the time it takes to move the component from spaced apart from the electrostatic clamp to clamped by the electrostatic clamp.

In the third mode, the potential of the electrode or the average potential of the electrode may be set for at least a portion of the time or the entire time that the mechanism generates free charge.

The mechanism for generating free charge adjacent the electrostatic chuck may comprise: a gas source; and an ionizing radiation source configured to ionize a gas provided by the gas source.

The ionizing radiation source may include at least one of an EUV source, a VUV source, a soft x-ray source, and a radiation source.

The electrostatic clamp may comprise a further electrode or a plurality of further electrodes, wherein the further electrode or the plurality of further electrodes may be positioned at least partially around a volume extending in a direction of the electrostatic clamp away from a surface of the component adjacent the electrostatic clamp, wherein the apparatus may be configured such that the potential of the or each further electrode is set such that the flux of free charge generated by the mechanism to the surface of the component adjacent the electrostatic clamp is reduced.

The device may be configured to: measuring the charge or current passed to the or each electrode from the voltage supply means using at least one charge or current measuring means; calculating the capacitance of the or each electrode using the measured charge or current to the or each electrode; and determining the potential of the surface of the component adjacent the electrostatic clamp using the calculated capacitance of the or each electrode.

The capacitance of the or each electrode may be calculated using at least one measuring device. The apparatus may comprise at least one measuring device. The apparatus may comprise at least one charge measuring device. The apparatus may comprise at least one current measuring device.

According to a second aspect of the invention, there is provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an illumination system configured to condition a radiation beam and an apparatus as described above, wherein the illumination system is configured to project the radiation beam onto the patterning device, and wherein the patterning device comprises a component to be clamped, wherein the lithographic apparatus comprises an apparatus as described above.

According to a third aspect of the invention there is provided a method of operating an apparatus, the apparatus comprising: an electrostatic chuck; and a mechanism for generating a free charge adjacent to the electrostatic chuck, the electrostatic chuck comprising an electrode or a plurality of electrodes, the method comprising: disposing a member adjacent to the electrostatic chuck; controlling the mechanism for generating free charge to generate free charge adjacent the electrostatic chuck; operating the apparatus in a first mode in which the or each electrode is set at an electrical potential such that a clamping electric field is generated between the electrostatic clamp and the component to clamp the component; operating the apparatus in a second mode in which the or each potential of the electrodes is set such that clamping of the component is released; and operating the apparatus in a third mode in which the or each potential of the electrodes is set such that the flux of free charge to a surface of the component adjacent the electrostatic clamp is increased compared to operating in the first or second mode.

The electrostatic chuck may include a plurality of electrodes, and the method may further include: in the third mode, the potential of the edge electrode closest to the edge of the electrostatic chuck is set to be positive.

The method may further comprise: in the third mode, the or each potential of the electrodes is set such that the or an average potential of the electrodes is negative.

The method may further comprise: in the third mode, the potentials of the plurality of electrodes are set so that an average potential of the plurality of electrodes is substantially 0V.

The electrostatic clamp further comprises a further electrode or a plurality of further electrodes positioned at least partially around a volume extending in a direction of the electrostatic clamp away from a surface of the component adjacent the electrostatic clamp. The method may further comprise: the or each potential of the or each further electrode is arranged such that the flux of free charge to the surface of the component adjacent the electrostatic clamp is reduced.

The method may further comprise: measuring the charge or current passed to the or each electrode from the voltage supply means using at least one charge or current measuring means; calculating the capacitance of the or each electrode using the measured charge or current to the or each electrode; and determining the potential of the surface of the component adjacent the electrostatic clamp using the calculated capacitance of the or each electrode.

According to a fourth aspect of the invention, there is provided an apparatus comprising: an electrostatic chuck for clamping a component; and a mechanism for generating free charge adjacent the electrostatic chuck: wherein the electrostatic clamp comprises an electrode or a plurality of electrodes, wherein the electrode or the plurality of electrodes is/are positioned at least partially around a volume extending in a direction of the electrostatic clamp away from a surface of the component adjacent the electrostatic clamp, wherein the apparatus is configured such that the or each electrode's potential is set such that the flux of free charge generated by the mechanism to the surface of the component adjacent the electrostatic clamp is reduced.

This may have the advantage that charging of the reticle may be prevented or at least reduced. This may avoid electrical breakdown (e.g., during reticle unloading) that may cause damage to the accompanying reticle surface.

The electrode or electrodes may be located on one side of the component.

The electrode or electrodes may extend all the way around the volume.

The or each potential of the or each electrode may be set to negative.

The or each potential of the or each electrode may be set to positive.

The or each electrode may be in electrical contact with the surface of the component adjacent the electrostatic clamp.

At least one edge or edges of the surface of the or each electrode adjacent the component may be rounded.

The or each electrode may be rounded at regions corresponding to corner portions of the component.

The or each potential of the electrode may be set to last for at least one of: the mechanism generates at least a portion of or all of the free charge and a time period before the component moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp.

According to a fifth aspect of the invention, there is provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an illumination system configured to condition a radiation beam and an apparatus as described above, wherein the illumination system is configured to project the radiation beam onto the patterning device, and wherein the patterning device comprises a component to be clamped, wherein the lithographic apparatus comprises an apparatus as described above.

According to a sixth aspect of the invention there is provided a method of operating an apparatus, the apparatus comprising: an electrostatic chuck; and a mechanism for generating a free charge adjacent the electrostatic chuck, the electrostatic chuck comprising an electrode or a plurality of electrodes positioned at least partially around a volume extending in a direction of the electrostatic chuck away from a surface of the component adjacent the electrostatic chuck, the method comprising: disposing a member adjacent to the electrostatic chuck; controlling the mechanism for generating free charge to generate free charge adjacent the electrostatic chuck; the or each potential of the or each electrode is arranged such that the flux of free charge to the surface of the component adjacent the electrostatic clamp is reduced.

The method may further comprise setting the or each potential of the or each electrode to negative.

The method may further comprise setting the or each potential of the or each electrode to positive.

The method may further comprise controlling the potential of the surface of the component adjacent the electrostatic clamp via an electrical connection between the or each electrode and the surface of the component adjacent the electrostatic clamp.

According to a seventh aspect of the present invention, there is provided an apparatus comprising: an electrostatic chuck for clamping the component; and a mechanism for generating free charge adjacent the electrostatic chuck: wherein the electrostatic chuck comprises an electrode or a plurality of electrodes, wherein the apparatus is configured to: measuring the charge or current passed to the or each electrode from the voltage supply means using at least one charge or current measuring means; calculating the capacitance of the or each electrode using the measured charge or current to the or each electrode; and determining the potential of the surface of the component adjacent the electrostatic clamp using the calculated capacitance of the or each electrode.

This may have the advantage of providing reliable control of the potential of the surface of the component adjacent the electrostatic clamp. This may have the advantage that charging of the reticle may be prevented or at least reduced. This may avoid electrical breakdown (e.g., during reticle unloading) that may cause damage to the accompanying reticle surface.

The apparatus may be configured such that the potential of the or each electrode of the plurality of electrodes is set such that the potential of the surface of the component adjacent the electrostatic clamp is substantially a predetermined value.

The predetermined value of the potential of the surface of the component adjacent the electrostatic clamp may be at least one of positive, negative, and substantially zero.

The device may be configured to measure a ratio of the capacitances of the plurality of electrodes.

The device may be configured to set the potential of at least one electrode of the plurality of electrodes based on the ratio of the capacitances of the plurality of electrodes.

The apparatus may be configured to set the potential of at least one of the plurality of electrodes based on a variance of the capacitance of the or each electrode.

The apparatus may be configured to change the potential of the plurality of electrodes by a predetermined amount in a stepwise manner and to measure the charge or current passed to the or each electrode from the voltage supply means using the at least one charge or current measuring means after each potential change for determining the individual capacitances of the plurality of electrodes.

The potential of the or each electrode may be set to last for at least one of: prior to at least a portion or all of the time that the mechanism generates free charge; and a time period before the component moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp.

According to an eighth aspect of the invention, there is provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an illumination system configured to condition a radiation beam and an apparatus as described above, wherein the illumination system is configured to project the radiation beam onto the patterning device, and wherein the patterning device comprises a component to be clamped, wherein the lithographic apparatus comprises an apparatus as described above.

According to a ninth aspect of the present invention there is provided a method of operating apparatus, the apparatus comprising: an electrostatic chuck; and a mechanism for generating a free charge adjacent to the electrostatic chuck, the electrostatic chuck comprising an electrode or a plurality of electrodes, the method comprising: disposing a member adjacent to the electrostatic chuck; controlling the mechanism for generating free charge to generate free charge adjacent the electrostatic chuck; measuring the charge or current passed to the or each electrode from the voltage supply means using at least one charge or current measuring means; calculating the capacitance of the or each electrode using the measured charge or current to the or each electrode; and determining the potential of the surface of the component adjacent the electrostatic clamp using the calculated capacitance of the or each electrode.

The method may further comprise setting the potential of the or each electrode of the plurality of electrodes such that the potential of the surface of the component adjacent the electrostatic clamp is substantially a predetermined value.

The method may further comprise setting the potential of the or each electrode of the plurality of electrodes such that the predetermined value of the potential of the surface of the component adjacent the electrostatic clamp is at least one of positive, negative and substantially zero.

The method may further include setting the potential of at least one of the plurality of electrodes based on a ratio of the capacitances of the plurality of electrodes.

The method may further comprise setting the potential of at least one of the plurality of electrodes based on a variance of the capacitance of the or each electrode.

According to a tenth aspect of the invention, there is provided a computer program comprising computer readable instructions configured to cause a processor to perform the method as described above. This has the advantage that no additional hardware is required.

According to an eleventh aspect of the invention, there is provided a computer readable medium carrying a computer program as described above.

According to a twelfth aspect of the present invention there is provided a computer apparatus for operating an apparatus, comprising: a memory storing processor-readable instructions; and a processor arranged to read and execute instructions stored in the memory; wherein the processor readable instructions comprise instructions arranged to control the computer to perform a method as described above.

Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which:

figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;

figures 2a to 2c depict an electrostatic clamp and patterning device for use within a lithographic apparatus according to an embodiment of the invention;

3 a-3 c depict an electrostatic clamp and a patterning device for use within a lithographic apparatus according to an embodiment of the invention;

4 a-4 c depict an electrostatic clamp and a patterning device for use within a lithographic apparatus according to an embodiment of the invention;

FIG. 5 depicts an electrostatic clamp and a patterning device for use in a lithographic apparatus according to an embodiment of the invention;

FIG. 6 depicts a plan view of an electrostatic clamp and patterning device used within a lithographic apparatus according to the embodiment of FIG. 5;

FIG. 6a depicts an electrostatic clamp and a patterning device for use in a lithographic apparatus according to an embodiment of the invention;

FIG. 7 depicts a plan view of an electrostatic clamp and patterning device for use in a lithographic apparatus according to an embodiment of the invention;

FIG. 8 depicts a plan view of an electrostatic clamp and patterning device used within a lithographic apparatus according to an embodiment of the invention;

FIG. 9 depicts a plan view of an electrostatic clamp and patterning device for use in a lithographic apparatus according to an embodiment of the invention;

fig. 10 depicts a schematic circuit diagram of an electrostatic chuck and patterning device for use within a lithographic apparatus according to an embodiment of the invention.

Detailed Description

Fig. 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 and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask or reticle), a projection system PS, and a substrate table WT configured to support a substrate W.

The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. In addition, the illumination system IL may comprise a facet field mirror device 10 and a facet 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 comprise other mirrors or devices in addition to the facet field mirror device 10 and the facet pupil mirror device 11, or instead of the facet field mirror device 10 and the facet pupil mirror device 11.

After being so adjusted, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned beam B' of EUV radiation is produced. The projection system PS is configured to project a patterned beam B' of EUV radiation onto the substrate W. For this purpose, the projection system PS may comprise a plurality of

mirrors

13, 14 configured to project the patterned beam B' of EUV radiation onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thus forming an image having smaller features than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated in fig. 1 as having only two

mirrors

13, 14, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

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

A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure substantially lower than atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL and/or in the projection system PS.

The radiation source SO may be a Laser Produced Plasma (LPP) source, a Discharge Produced Plasma (DPP) source, a Free Electron Laser (FEL) or any other radiation source capable of producing EUV radiation.

Fig. 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 y-direction may be considered as a scanning direction of the lithographic apparatus and the x-direction may be considered as perpendicular to the scanning direction. The support structure MT comprises an electrostatic chuck 100 configured to hold the patterning device MA during lithographic operations. The clamp 100 includes a generally planar clamping surface 102, and clamp electrodes a-D disposed within a clamp body. The electrodes 104A to 104D are separated from the clamping surface 102 of the clamp 100 by a dielectric coating. Knobs (not shown) may protrude from the clamping surface 102 and serve to separate the clamped patterning device MA from the clamping surface 102. The burls may, for example, have a height of about 10 μm and may collectively cover about 1% of the surface of the clip 100. It should be appreciated that many features of the clip 100 (e.g., wiring, additional electrodes) are omitted for simplicity.

The holder 100 may be considered part of the lithographic apparatus LA, or may be considered part of the apparatus forming part of or being separate from the lithographic apparatus LA. The lithographic apparatus LA or the apparatus may comprise a mechanism for generating a free charge.

The patterning device MA is substantially planar and has first and second

planar surfaces

122, 124 opposite one another. In use (e.g. as shown in fig. 1), the first surface 122 is configured to reflect a beam of radiation B and cause a pattern to be imparted to the beam B. In particular, a region of the first surface 122 may be patterned so as to cause the radiation beam B to become patterned. The patterned region of the first surface 122 has a conductive coating. The first surface 122 may be referred to as the front side or front side of the patterning device MA. I.e. the reticle front side is the surface of the patterning device MA facing away from the electrostatic clamp 100.

To enable the electrostatic clamp 100 to clamp the patterning device MA, the second surface 124 is provided with a conductive coating that generally covers most of the second surface 124. The second surface 124 may be referred to as the patterning device MA backside. That is, the back surface of the patterning device MA is the surface of the patterning device MA facing the electrostatic chuck 100. In other words, the patterning device MA backside is the surface of the patterning device MA adjacent to the electrostatic clamp 100.

A base plate 126 facing the first surface 122 of the patterning device MA is located on the opposite side of the electrostatic chuck 100. The backplane 126 is part of an exchange for transferring reticles MA to and from the lithographic apparatus LA. The bottom plate 126 is grounded and may be at a potential of approximately 0V.

It should be understood that the electrostatic clamp 100 may use voltages of the order of several kV in order to clamp the patterning device MA. For example, the chuck 100 may be a bipolar electrostatic chuck, wherein a

first subset

104A, 104C of the electrodes 104A-104D is connected to one or more voltage supplies (not shown) of about +1 … … 10kV (e.g., +2kV), and a

second subset

104B, 104D of the electrodes 104A-104D is connected to one or more voltage supplies of about-1 … … 10kV (e.g., -2 kV). In this way, a high electric field may be established between the fixture 100 and the patterning device MA, resulting in the patterning device MA being attracted to the fixture 100. In particular, charges are induced in regions of the conductive coating of the second surface 124 adjacent to the electrodes 104A-104D, the charges in the regions having opposite sign to the applied voltage, and attractive forces are established between the opposite charges at various locations across the fixture 100 and the patterning device MA. The region of the jig 100 configured to support the patterning device MA may be referred to as a support region. Further, when the gripper 100 is operated to grip the patterning device MA, a region of the gripper 100 configured to generate a gripping force may be referred to as a gripping region.

The electrostatic clamp 100 may be operated in a first mode in which the electrodes 104A to 104D are set to a plurality of potentials such that a clamping electric field is generated between the electrostatic clamp 100 and the reticle MA to clamp the reticle MA. In such a first mode, the electrodes 104A to 104D may be balanced, i.e. the average potential of the electrodes 104A to 104D may be about 0V. The electrostatic clamp 100 may be operated in a second mode in which the electrodes 104A-104D are set to a plurality of potentials such that no or relatively small clamping electric fields are generated between the electrostatic clamp 100 and the reticle MA. For example, a minimum of 300V is typically required to overcome the gravitational force of the reticle MA. Therefore, below 300V there will be no clamping. Such a value may vary depending on the clamping surface quality, and thus it may be 100V or 200V, etc. Thus, in the second mode, the potentials of the electrodes 104A to 104D are set so that the clamping of the reticle MA is released. In the second mode, the electrodes 104A to 104D may also be balanced, i.e. the average potential of the electrodes 104A to 104D may be about 0V. The first and second modes of operating the clamp 100 may be considered normal operation.

Each of the electrodes 104A to 104D has a rectangular shape, and is arranged so as to be substantially parallel to each other. In such an arrangement, the four electrodes illustrated each span the width of the sandwiched patterning device MA in the x-direction and each cover about one quarter of the length of the patterning device MA in the y-direction. It is understood that in other embodiments, a different number of electrodes may be used, such as 1, 2, 3, 5, 6, 7, 8, or more.

During normal operation of the fixture 100, an electric field will be established between the surface of the fixture 100 and the surface of the patterning device MA. Furthermore, electrostatic discharges may occur due to the close separation/separation between the various charged surfaces, including other components within the lithographic apparatus, such as, for example, a shutter blade. That is, electrostatic discharge can occur between any charged surfaces with the possibility that the discharge increases as the electric field strength increases. Electrostatic discharge can damage the components. Electrostatic discharge may generate particles from a surface and may also release particles that were previously attached to a surface within the lithographic apparatus. It will be appreciated that such release of particles is undesirable in a lithographic apparatus, as particles may fall onto critical areas of the apparatus, possibly leading to patterning defects in the processed substrate.

The high electric field generated by the electrostatic chuck strongly attracts any free charges. By free charge is meant a charge (positive charge, e.g., ions, or negative charge, e.g., electrons) that is not bound to a solid substrate, but is free to move according to electric field lines. Furthermore, sufficient free charge is generated during EUV exposure. For example, electrons may be generated from light emission and also from an EUV-induced plasma, which is typically generated in the presence of hydrogen (which is often present in lithography tools). Positive ions can also be generated within the EUV plasma.

The plasma generation process will now be discussed in more detail. It will be appreciated that EUV photons within the beam B will ionize hydrogen molecules, producing H2+ ions and free electrons. In an example using 13.5nm EUV radiation, each photon may have an energy of about 92eV, with the ionization energy of molecular hydrogen being about 15 eV. Thus, the free electrons generated may have sufficient energy (e.g., > 75eV) and range to generate a secondary plasma relatively far from the initial ionization event. In addition, the electrons released in this way (i.e. having an energy of about 75eV) can ionize one, two or even three further hydrogen molecules. Thus, even if the primary plasma is generated only at the incidence of EUV photons, the secondary plasma can be generated in the vicinity of the jig.

In an embodiment, it will be appreciated that it is desirable to generate an EUV-initiated plasma which provides a source of free charge in the vicinity of the patterning device MA.

In some embodiments, a secondary ionization source may be provided, thereby allowing plasma to be generated in the vicinity of the electrostatic clamp by means other than an EUV source SO. This arrangement may reduce the overall output load of the EUV source SO. It is to be understood that the embodiments described above may place additional demands on the EUV source SO by requiring additional EUV output compared to that required for imaging. Additionally, in some embodiments, the EUV source may not be able to continuously generate power. Similarly, it is not possible and/or desirable for an EUV source to provide any EUV pulse energy in the range of 0% to 100% of nominal output power, while also ensuring clean collector operation and pulse energy stability.

As such, in some embodiments, it may be preferable to provide an alternative mechanism for producing a region of increased gas conductivity compared to the primary EUV source.

For example, a source may be disposed proximate to the electrostatic clamp 100 and the clamped patterning device MA. Multiple sources may be used. For example, the source may be a soft x-ray source or a VUV light source capable of operating at pressures below 1bar in a clean environment. The source may comprise a low power ionizer having a power of about 0.1W to 1W. In some embodiments, the source may comprise a radiation source or an electron beam source.

Generally, the EUV source SO and the source (which may for example comprise a soft x-ray source, or a VUV ionizer) may each be considered as examples of ionizing radiation sources. Additionally, such a source in combination with a source of hydrogen (or other) gas may be considered a mechanism for generating free charge. That is, a hydrogen plasma containing both positive ions and free electrons may be considered a free charge cloud. In addition, such free charges include both positive and negative free charges.

A substantial voltage may be established between the gripper 100 and the patterning device MA after the patterning device MA is removed from the gripper 100.

It will be appreciated that there is a capacitance between several components in the lithographic apparatus LA. In particular, the capacitance between the clamping surface 102 and the second surface 124 of the patterning device MA may be considered as a variable capacitance, which varies depending on the gap between the clamping surface 102 and the second surface 124. Similarly, the capacitance between the base plate 126 and the first surface 122 of the patterning device MA may be considered to be a variable capacitance, which varies depending on the gap between the base plate 126 and the first surface 122.

It will be appreciated that in a closed system, any change in the degree of separation between the gripper 100 and the patterning device MA, and any change in the degree of separation between the patterning device MA and the base plate 126, will result in a corresponding change in variable capacitance for a given initial charge state, without charge being able to enter or exit the system. Furthermore, such a change in capacitance will also result in the voltage across the capacitance possibly changing significantly according to the change in separation.

In particular, the relationship Q ═ CV must always be maintained for each capacitance (assuming no charge is injected). Therefore, if the capacitance C is changed and the amount of charge Q contained in the capacitance is maintained the same, the voltage V must be changed in inverse proportion to the changed capacitance C. This may result in significant voltage amplification. The most significant change in voltage occurs at the backside of the patterning device MA, i.e. between the clamping surface 102 and the second surface 124 of the patterning device MA.

The resulting high voltage should be understood to significantly increase the risk of discharge due to dissociation/breakdown of hydrogen gas in the vicinity of the patterning device MA and the electrostatic chuck 100 (e.g., due to the voltage at the surface of the patterning device MA exceeding the lowest Paschen (Paschen) limit for hydrogen, which is about 250V).

Thus, there is an opportunity for electrostatic discharge within the lithographic apparatus LA during unloading of the patterning device MA after clamping. Charge may become trapped at the dielectric surface of the clip 100. Furthermore, residual charge may remain on the clamped patterning device MA once it has been released. As the undamped patterning device MA is moved away from the clamp surface, the increased degree of separation between the clamp surface and the patterning device surface may result in a decrease in capacitance and a voltage amplification. That is, given the proportional relationship between charge and voltage in a closed system (i.e., Q-C.V), when the capacitance changes (inversely proportional to the degree of separation between the parallel plates), any decrease in capacitance will result in a proportional increase in voltage. Thus, as the patterning device MA is separated from the chuck 100, the voltage of the patterning device may rise sufficiently to cause electrical breakdown of the hydrogen gas to occur. Such discharges may cause damage to the patterning device MA, the electrostatic clamp 100 and/or particle generation, which may lead to subsequent defects.

The effect of varying capacitance can be mitigated to some extent by introducing free charge during the unloading process. For example, a separate ionization source, or indeed the EUV source SO, may be used to generate a hydrogen plasma that provides free charge (as described in detail above) and allows the field established across the various dielectric components (and gaps) to relax, i.e., relax, during the removal process.

Providing free charge may result in a significant reduction in the voltage established between the various system components. That is, the established electric field generated by the high voltage can be compensated by introducing additional free charge. These charge sources are effectively provided by the hydrogen plasma. Free charges within the plasma are driven by any electric field as it begins to be established, and cause those fields to collapse/collapse.

In this way, potential problems associated with establishing a significant voltage across the patterning device MA upon removal from the electrostatic chuck 100 may be alleviated or completely avoided. As mentioned above, it will be appreciated that this effect is not binary and that some (reduced strength) field may still be established if insufficient charge is provided. However, it should be understood that even a reduction in voltage amplification (rather than complete avoidance) may be beneficial, particularly if the voltage is thus maintained consistently below the lowest paschen limit of hydrogen (about 250V).

Furthermore, free charge may be provided at various times during separation of patterning device MA from the chuck 100. Indeed, it will be appreciated that when the patterning device MA is clamped, free charges may be difficult to penetrate between adjacent surfaces. Thus, there may be an effective minimum degree of separation for optimally providing free charge.

Reticles (patterning devices) may suffer irreversible damage due to residual charges on the front and back surfaces of the reticle that accumulate during EUV radiation exposure and reticle transport (e.g., loading and unloading of reticles). As mentioned, such residual charge may cause electrical breakdown during reticle unloading, resulting in an overall loss of the reticle. The reticle may be ejected from the lithographic apparatus LA where the reticle potential is about 600V, which corresponds to a very large negative residual charge of about 50 nC.

Due to EUV radiation, the reticle acquires a charge, producing fast electrons and charging the floating reticle surface to a small potential of about-10V. Alternatively, residual charge on the reticle backside may be caused by triboelectric charging (i.e., friction between the reticle surface and clamp burls made of different materials).

As mentioned, during reticle unloading, the capacitance of the reticle-clamp system decreases, which results in an increase of the potential on the reticle backside to about-600V. In some cases, this may result in electrical breakdown occurring, resulting in incidental reticle surface damage.

Residual charge on the reticle BS also causes a high field to occur between the reticle front side and the baseplate, resulting in particle jumping from the baseplate to the reticle FS during reticle transport in the scanner.

Reticle grounding without the use of additional hardware can be achieved by utilizing EUV-induced plasma as a source of charge that can reduce reticle potential. However, providing charge carriers, e.g., 50nC, to the reticle backside surface is limited by the amount of plasma generated by EUV radiation. Furthermore, plasma density is reduced due to the complex geometry of the environment around the reticle and the constraints of the hardware associated with reticle unloading. Therefore, achieving reticle grounding (i.e. soft grounding) using EUV-induced plasma without additional assistance may have a throughput penalty, i.e. an increase in the time it takes for a substrate W to pass through the lithographic apparatus LA, which is undesirable.

Figure 2a shows the polarity and the general relative magnitude of the potential for the four electrodes 104A to 104D of the electrostatic clamp 100 operating in the third mode. That is, in one embodiment, the

electrodes

104A and 104D have a positive potential (+), and the

electrodes

104B and 104C have a negative potential (-, -), wherein the magnitude of the electrode 104B is greater than the magnitude of the other electrodes. This means that the average potential of all the electrodes 104A to 104D is negative.

The patterning device MA has an edge 128 (or end) which, in such embodiments, is closest to the EUV radiation beam B and hence to the EUV-induced plasma. The electrode 104A may be referred to as an edge electrode. In such an embodiment, the edge electrode 104A has a positive potential, as mentioned above.

The electrodes 104 to 104D are set to these potentials before the EUV radiation is turned on to generate the plasma. In the absence of plasma, the reticle MA will float and since the average potential of all the electrodes 104A to 104D is negative and the bottom plate 126 is on the other side of the reticle MA, there will be a capacitively induced negative potential on the reticle MA. For example, the base plate 126 may be at approximately 0V, the clamp 100 may be at approximately-1000V, and thus the reticle MA may be at approximately-900V due to capacitive induced current. Since the reticle MA second surface 124 is closer to the fixture 100 with a negative potential, the second surface 124 will have more negative values than the reticle MA first surface 122. In other embodiments, the backplane need not be in place, and in other embodiments, the backplane may be swapped with different components. In other embodiments, the electrodes may be set to these potentials when the EUV radiation has been switched on, i.e. during plasma generation.

This electrode arrangement will provide an increased positive ion flux to the second surface 124 of the patterning device MA, while at the same time suppressing the positive ion flux to the clamping surface 102 (and other clamp surfaces) of the clamp 100. The configuration of the electrodes 104A-104D provides a positive near field at the reticle edge 128 and an additional negative bias to the reticle MA second surface 124 (i.e., the surface adjacent to the chuck 100), which promotes a positive ion flux towards the reticle MA. This is because the electrode 104A at the edge 128 is positive and therefore pushes positive ions away from the chuck 100 towards the reticle MA, and also because the capacitively induced potential on the reticle MA from the overall average negative potential of the electrodes 104A-140D attracts positive ions towards the second surface 124 of the reticle MA. Since the second surface 124 has more negative values than the first surface 122, the flux of positive ions will be more attracted to the second surface 124.

It should be appreciated that operation of the electrostatic chuck 100 in the third mode means that the electrodes 104A-104D have a potential that increases the positive ion flux to the reticle MA when compared to normal operation of the electrostatic chuck 100 (i.e., operation in the first mode or the second mode). Previously, the clamp would not have been set to have the overall average negative or positive potential of the electrodes, and therefore the flux of free charge (electrons or ions) to the reticle would not have increased significantly when EUV radiation is turned on. Furthermore, it should be appreciated that the third mode of operation of the electrostatic clamp 100 may include operating such that the electrostatic clamp 100 clamps the reticle MA and/or operating such that the electrostatic clamp does not clamp the reticle MA.

Fig. 2b and 2c show the electrostatic clamp 100 operating in a third mode, with a similar electrode arrangement with a positive edge electrode 104A and the average potential of the electrodes 104A to 104D being negative. However, in fig. 2B, electrode 104C has a positive potential and

electrodes

104B and 104D have a negative potential. The magnitude of the potential of electrode 104B is still greater than the magnitude of the potentials of the other electrodes. In fig. 2C, electrode 104B has a positive potential and

electrodes

104C and 104D have a negative potential, with the magnitude of the potential of electrode 104C being greater than the magnitude of the potentials of the other electrodes.

These arrangements of the electrodes 104A-104D, and more particularly, the particular arrangement of voltages applied to the electrodes 104A-104D, enable accelerated residual charge neutralization during reticle MA unloading. The acceleration is achieved by using the electrodes 104A-104D as an additional E-field source to provide a higher plasma flux towards the reticle MA surface. That is, the electrostatic clamp 100 electrodes 104A to 104D are set at voltages in such a way that a net positive charge from the plasma will be attracted to the reticle MA (primary) back surface 124 and the (secondary) front surface 122.

Thus, compensation of residual charge on the reticle MA by EUV-induced plasma during reticle MA unloading/loading actions may be enhanced.

Other advantages may be: no hardware changes are required, which saves commodity costs and shortens development time. Embodiments may be implemented on any lithographic apparatus LA. Furthermore, embodiments may be customized for specific and exceptional reticle MA conditions (such as trimmed backside coating reticle MA usage). Additionally, a contactless grounding scheme may increase the lifetime of the fixture 100/reticle MA.

Fig. 3 a-3 c illustrate further embodiments of the electrostatic clamp 100 operating in a third mode, wherein a common relative magnitude of polarity and potential of the four electrodes 104A-104D is identified.

In the embodiment of fig. 3a, each of the electrodes 104A to 104D has a negative potential, wherein the magnitude of the potential of the electrodes 104B to 104D is greater than the magnitude of the potential of the electrode 104A (edge electrode). This means that the average potential of all the electrodes 104A to 104D is still negative. The edge electrode 104A is negative, but the average potential of all electrodes has an even more negative value than the edge electrode 104A. This electrode arrangement provides increased flux to the reticle MA second surface 124 while suppressing flux to the jig surface 102.

Thus, a similar approach may be applied to achieve a < 0 (negative) capacitively induced potential on the reticle MA by applying a net additional negative bias to the electrodes 104B-104D and combinations thereof. The configuration of the electrodes 104A to 104D provides a near field at the reticle edge 128, which reticle edge 128 has a less negative value than the rest of the reticle MA second surface 124, and provides an additional negative bias to the reticle MA second surface 124 (i.e. the surface adjacent to the clamp 100), which promotes a positive ion flux towards the reticle MA. This can also be achieved by an imbalance of the potential on the positive electrode.

Fig. 3b and 3c show the electrostatic clamp 100 operating in a third mode with a similar electrode arrangement with a negative edge electrode 104A and with the average potential of the electrodes 104A to 104D having more negative values. However, in fig. 3B,

electrodes

104B and 104D have positive potentials and the potential of electrode 104C is negative and has a much larger magnitude of potential than the other electrodes. The embodiment of fig. 3C is the same as the embodiment of fig. 3B, except that the potentials of

electrodes

104B and 104C have been exchanged.

When the clamping of the reticle MA is released and while the reticle MA is still clamped by the clamp 100, an unbalanced/unbalanced electrode potential may be applied before the EUV radiation beam B is "on". Thus, embodiments enable grounding of the reticle MA while it is still on the electrostatic clamp 100, which enables grounding of the reticle MA before starting the unclamping action and thus minimizes the risk of damage to the reticle MA due to discharges. The residual charge of the reticle MA may be brought to zero while the reticle MA is still close to the clamp 100, or even still physically connected to the clamp 100. Thus, the risk of damage to the reticle MA caused by the discharge when the gap between the reticle MA and the clamp 100 becomes too large during unloading is significantly minimized (in the presence of a fixed charge, the voltage increases when the capacitance is reduced by increasing the gap).

Fig. 4A-4 c illustrate further embodiments of the electrostatic clamp 100 operating in a third mode, wherein the polarity and the general relative magnitude of the potential of the four electrodes 104A-104D are identified.

In the embodiment of fig. 4A, the edge electrode 104A is positive, as in fig. 2a to 2c, but the other electrodes 104B to 104D have a potential and a magnitude of the potential such that the average potential of all electrodes 104A to 104D is approximately 0V. More particularly, the

electrodes

104A and 104D have a positive potential (+) and the

electrodes

104B and 104C have a negative potential (-), wherein the magnitude of the potential of each of the electrodes 104A-104D is substantially the same.

Thus, the capacitively induced potential on the reticle MA (in particular on the second surface 124) remains unchanged. The edge electrode 104A, which in this case has a positive potential, protects the clamp 100 from attracting positive ions and thus increasing the positive ion flux to the reticle MA second surface 124, as well as protecting clamp burls (not shown in the figure) from sputtering. This helps maintain a relatively long life for the clip functionality. Even though in such an embodiment the capacitively induced potential on the reticle MA is approximately 0V, the positive ion flux to the reticle MA increases when positive ions are directed away from the clamp 100 by the positive edge electrode 104A.

The configuration of the electrodes 104A-104D provides a positive near field at the reticle edge 128, thereby boosting the positive ion flux towards the reticle MA while maintaining an additional zero bias to the reticle MA second surface 124.

Fig. 4b and 4c show the electrostatic clamp 100 operating in a third mode, with a similar electrode arrangement with a positive edge electrode 104A and an average potential of the electrodes 104A to 104D of substantially 0V. However, in fig. 4B,

electrodes

104B and 104D have a negative potential and electrode 104C is positive. The magnitude of the potential of each of the electrodes 104A to 104D is substantially the same as in fig. 4A. The embodiment of fig. 4C is the same as the embodiment of fig. 4B, except that the potentials of

electrodes

104B and 104C have been exchanged.

The embodiment of fig. 4 a-4 c addresses the plasma flux (positive ions) creating condition primarily towards the reticle MA second surface 124 while suppressing the positive ion flux to the jig 100. Which primarily helps to achieve the goal of minimizing damage to the clamping surface 102 (and other clamp surfaces).

It should be appreciated that the exact configuration of the electrodes 104A-104D in fig. 2 a-4 c described above is merely exemplary and in other embodiments, they may have different polarities and magnitudes so long as they provide the advantages described. For example, in fig. 4C, the polarities of the

electrodes

104B, 104C may be exchanged and the magnitudes of the potentials of these

electrodes

104B, 104C may both be increased to be substantially greater compared to the

electrodes

104A, 104D, so long as the magnitudes substantially match. In such a case, there will still be an edge electrode 104A with a positive potential and the overall average potential of the electrode is approximately 0V.

Modeling of the plasma flux to the reticle MA may show a positive ion flux to the reticle MA second surface 124, enabling charge compensation within only a fraction of 1 second (e.g., about 0.1 s). This enables a throughput and implementation of a soft grounding of the reticle MA to be obtained.

Embodiments may result in a higher neutralization of about 10 times the residual charge on the reticle MA second surface 124, enabling throughput neutralization implementations of soft reticle grounding to be achieved at unloading (and at loading) of the reticle MA.

Another embodiment involves setting the electrostatic clamp 100 to operate in the third mode such that the electrodes 104A to 104D have an average negative potential before the patterning device (reticle) MA is exposed (i.e. when the radiation beam B is incident on the patterning device MA, so as to provide a reflected patterned beam B' of EUV radiation onto the substrate). For example, the electrodes 104A to 104D may be provided to have the same potential as in fig. 2a to 2c or fig. 3a to 3c, or another configuration in which the average potential of the electrodes 104A to 104D is negative.

This method is intended to: inducing a positive charge on the reticle MA (particularly the reticle MA second surface 124) causes the neutralization of the reticle MA residual charge upon unloading to be achieved by electrons (rather than positive ions as in the previous embodiment). This takes advantage of the higher electron mobility compared to the mobility of the ions, and therefore neutralization is achieved much faster (several orders of magnitude faster, i.e. neutralization can be achieved only within 0.1s instead of 10 s). This method may also enable fast neutralization of the backside-trimmed reticle MA. This is because the neutralization of the backside trimmed reticle is slow due to the removal of about 1mm of the metal coating on the reticle backside at the edge. The ion flux to such a trimmed backside coating needs to pass through a narrow slit-i.e., the space/gap between the chuck and the reticle backside. The chance that ions can penetrate into such a slit is relatively very low, which is not a problem for electrons. For example, a coating retraction of 2mm will result in backside neutralization by ions for an infinite time, but will still be completed by electrons for only a few seconds.

For example, to induce a positive charge on the reticle MA during exposure, the clamp electrodes 104A-104D are set to provide a negative reticle MA offset potential of about-1 … … -100V prior to exposure. Once exposure is stopped, i.e. once the EUV radiation beam B is no longer incident on the patterning device MA, the reticle MA will have a positive charge (instead of a negative charge as described above). To achieve this, one or more negative electrodes are set to a higher potential than the positive electrode. For example, two positive electrodes are set to positive +1kV and two negative electrodes are set to negative-1.1 kV. Due to this, electrons will be repelled from the reticle MA and positive ions will be attracted to the reticle MA, thus placing a positive charge on the reticle MA after exposure.

The residual charge of the reticle MA may be brought to zero while the reticle MA is still close to the clamp 100, or even still physically connected to the clamp 100. Thus, the risk of damage to the reticle MA due to electrical discharges when the gap between the reticle MA and the clamp 100 becomes too large during unloading is significantly minimized.

The above-described condition, i.e., the average potential of the electrodes 104A to 104D being set to negative, may be maintained for the entire duration of the exposure, or for only a portion of the time before the reticle MA unloading action. In some embodiments, the electrodes may be set to a certain state (e.g., configuration and/or specific potential) for only a portion of the time of exposure. The electrodes do not have to be in the same configuration for the entire duration of the exposure. It may be necessary to set the electrodes to an equilibrium state (i.e., zero on average) to ensure that the reticle becomes neutral.

Another embodiment is directed to preventing charging of the reticle MA during exposure. Also, such embodiments involve setting the electrostatic clamp 100 in the third mode such that the electrodes 104A to 104D have an average negative potential before the patterning device (reticle) MA is exposed (i.e. when the radiation beam B is incident on the patterning device MA to provide a reflected patterned beam B' of EUV radiation onto the substrate). However, in such an embodiment, the potentials of the electrodes 104A to 104D are set to provide a negative bias in a potential of a specific value. Such specific values may be calibrated to specific reticle MA and exposure conditions, such as EUV dose. This particular value can be measured from a previous exposure and then fed forward. For example, the particular value may be set such that it provides a potential of-2V on the reticle for one reticle and-10V for the other reticle MA. Setting the reticle MA to a calibrated value (i.e., -2V) may mean that the potential of the reticle MA will not generally increase or decrease significantly throughout the exposure since the transfer of charge caused by electrons and ions will be cancelled/balanced. Thus, at the end of an exposure, the reticle MA will have the same or similar charge (e.g., -2V) as the previous exposure. In other embodiments, the particular value may be set such that it provides a potential on the reticle in the range of 0V to-20V.

The particular value of the negative potential of the electrodes 104A to 104D may be selected such that, after exposure, the reticle MA is substantially uncharged, i.e. exhibits substantially zero charge. This means that during unloading (i.e. when the reticle MA is moved away from the clamp 100), the potential difference between the second surface 124 of the reticle MA and the clamping surface 102 of the clamp 100 will not increase significantly, as seen when the reticle MA remains charged after exposure. When the reticle MA is still close to the clamp 100, or even still physically connected to the clamp 100, the reticle MA residual charge is close to zero, which significantly minimizes the risk of reticle MA damage due to electrical discharges when the gap between the reticle MA and the clamp 100 becomes too large during unloading. In other embodiments, the particular value may be selected such that the potential on the reticle MA matches the potential on the clamp 100, and thus the potential difference between the second surface 124 of the reticle MA and the clamping surface 102 of the clamp 100 will not relatively increase.

The above-described condition, i.e. the average potential of the electrodes 104A to 104D being set to a certain negative value, may be maintained for the entire duration of the exposure, or for only a portion of the time before the reticle MA unloading action.

It should be appreciated that embodiments may be implemented by changing software procedures without changing hardware. This means that the lithographic apparatus LA in the art can be implemented relatively quickly and with little impact on production. Further, embodiments may be reversible and flexible. They can be used as temporary mitigation strategies (i.e., switched on and off when needed or tuned).

Since embodiments may not require additional hardware, commodity costs may be saved compared to other methods of implementing grounding of the reticle (e.g., during unloading). Embodiments may be applied directly to all EUV lithographic apparatus LA and may result in improved reliability and usability of the lithographic apparatus LA. Additionally, embodiments may be implemented with throughput neutralization reticle grounding. The examples may result in higher yields.

Figure 5 illustrates an embodiment of an electrostatic clamp 200 in which the polarity of the four electrodes 204A-204D is identified. The components of the electrostatic clamp 200 are similar to the components of the electrostatic clamp 100 of the previous embodiment and similar components will be provided with similar reference numerals incremented by 100.

In the embodiment of fig. 5, the electrodes 204A-204D are the same as the electrodes 104A-104D of fig. 4A. Thus, the capacitively induced potential on the reticle MA (in particular on the second surface 224) remains unchanged (i.e. substantially 0V in this embodiment). However, this is merely an example, and the electrodes 204A to 204D may have different polarities and magnitudes, such as those shown in the previous embodiments. In any case, the electrodes 204A-204D provide clamping of the patterning device (reticle) MA.

In the embodiment of fig. 5, there is an additional (or fifth) electrode 204E. The further electrode 204E does not participate in clamping the reticle MA. The further electrode 204E is located in the same plane as the electrodes 204A to 204D. The further electrode 204E is located in a different plane than the reticle MA. In such an embodiment, the further electrode 204E is above the reticle MA, as shown in fig. 5.

The further electrode 204E is positioned at least partially around an (imaginary) volume 230, which volume 230 extends from the second surface 224 (backside) (i.e. z-direction) of the reticle MA in the direction of the electrostatic clamp 200. In other words, the further electrode 204E is positioned around the space/gap above the second surface 224 of the reticle MA. The second surface 224 may be referred to as a surface of the reticle MA adjacent to the electrostatic chuck 200.

Volume 230 is shown in phantom extending from an edge (or end) 228 of the reticle MA and an opposite end of the reticle MA. It should be appreciated that the right hand side of fig. 5 does not show the entire reticle MA and that the volume 230 may be considered to extend on both sides to the edges of the reticle MA. In such embodiments, the edge 228 is closest to the EUV radiation beam B and thus closest to the EUV-induced plasma.

Figure 6 shows the further electrode 204E and patterning device MA from above (i.e. in plan view) -the electrodes 204A to 204D and the intermediate portion of the electrostatic clamp 200 are not shown for clarity. The further electrodes 204E are shown as extending all the way around (i.e. completely) the volume 230. An additional electrode 204E may be coated on the dielectric of the electrostatic clamp 200. In practice, the area around the reticle MA is coated with the electrode 204E. The further electrode 204E may be a thin metal coating.

The further electrode 204E may be made of any suitable electrically conductive material. For example, a material that is compatible with plasma and does not provide any problems. As an example, chromium nitride may be used as a material for the further electrode 204E.

The reticle may collect charge. Unloading a charged reticle results in an increase in reticle voltage. This may lead to electrical discharge and particle generation or damage.

The voltage on the further electrode 204E may be controlled. For example, in such an embodiment, the potential on the further electrode 204E is set to be negative. This sets up an electric field around the reticle MA. This means that electrons (i.e. free charges) generated by the mechanism may be repelled away from the second surface 224 of the reticle MA. This means that the number of electrons reaching the reticle MA is reduced. Thus, charging of the reticle may be prevented or at least reduced. This may avoid electrical breakdown (e.g., during reticle unloading) that may cause damage to the accompanying reticle surface. The potential of the further electrode 204E may not be comparable in magnitude to the electrodes 204A to 204D, e.g. it may be much smaller.

It should be appreciated that in other embodiments, the additional electrode 204E may be set to positive. This sets up an electric field around the reticle MA. This means that positive ions (i.e. free charges) generated by the mechanism may be repelled away from the second surface 224 of the reticle MA. This means that the number of positive ions reaching the reticle MA will be reduced. Thus, reticle charging will be prevented or at least reduced. This may avoid electrical breakdown (e.g., during reticle unloading) that may cause damage to the accompanying reticle surface.

More generally, the device may be configured such that the further electrode 204E is arranged such that the flux of free charge to the second surface 224 generated by the mechanism is reduced. The flux of such free charges may be considered to be reduced when compared to the case where no electrode is present around the reticle, or when the further electrode has a potential of 0 volts.

It will be appreciated that any size of the additional electrodes 204E will provide some benefits. However, the larger the electrode (e.g., in the y-direction), the more charged particles are repelled, since larger electrodes can produce larger electric fields.

It will be appreciated that any negative (or positive) set of voltages for the additional electrodes 204E will provide some benefits. However, the larger the voltage, the more repulsive the charged particles, since a larger voltage may generate a larger electric field. For example, the voltage may be 10 or 20 or 50 volts.

The surface of the further electrode 204E may be flat. However, in other embodiments, the surface may not be flat. For example, in an embodiment, the further electrodes 204E may be rounded at areas corresponding to corner portions of the reticle MA. This may increase the electric field. More generally, at least one or more edges of the further electrodes 204E (i.e. the edges of the surface adjacent to the reticle MA) may be rounded.

In an embodiment, the further electrode 204E may be in electrical contact with the reticle MA (e.g. the second surface 224 of the reticle MA). This may allow to directly control the potential of the second surface 224 of the reticle MA. This may allow more repulsion of charged particles (e.g., electrons). The electrical contact may be achieved by one or more burls. The one or more burls may be located within the volume 230.

In some embodiments, an electrical conductor (e.g., an existing ground lead) may be used as the additional electrode 204E. In such a case, the electrical supply device may be connected to a ground conductor and a voltage may be provided to the ground conductor.

The potential of the further electrode 204E may be set to last at least part of or all of the time the mechanism generates free charge (e.g. during exposure of the reticle MA). The potential of the further electrode 204E may be set to continue for a period of time before the reticle MA moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp (e.g. just before clamping of the reticle MA is released).

Figure 6a illustrates another embodiment of an electrostatic clamp 200. This embodiment is the same as the embodiment of fig. 5, except that an additional electrode 204E is located on the other side of the clamping surface 202 and comprises a plurality of walls. That is, the wall may be considered to be a sidewall electrode 204F extending upwardly (i.e., in the z-direction) from the further electrode 204E to partially surround the electrostatic chuck 200, and a wall electrode 204G extending downwardly (i.e., in the opposite direction from the sidewall electrode 204F) from the further electrode 204E to partially surround the implemented reticle MA. The wall may be a metal plate. It is understood that in some embodiments, both the sidewall electrode 204F and the wall electrode 204G need not be present. Further, in some embodiments, one or both of the sidewall electrode 204F and the wall electrode 204G may be included in the electrode 204E or in place of the additional electrode 204E. In other embodiments, the further electrode 204E may also be located in a position as shown in fig. 5, and the sidewall electrode 204F and/or the wall electrode 204G may be in a position as shown in fig. 6 a.

In some embodiments, the further electrode 204E may also be set to a voltage that will deplete the free charge in the volume around MA. This may have the advantage of preventing free charges from reaching the reticle MA (and charging the reticle MA). This may only be possible if the further electrode 204E is not coated with an insulating surface. This may be possible, for example, if the coating has a sidewall electrode 204F and a wall electrode 204G as in fig. 6 a.

Figure 7 illustrates another embodiment of the electrostatic clamp 200. This embodiment is the same as the embodiment of fig. 6, except that there is a space (gap) 232 between the volume 230 and the further electrode 204E. That is, the additional electrodes need not be precisely around the volume 230.

Figure 8 illustrates another embodiment of the electrostatic clamp 200. This embodiment is identical to the embodiment of fig. 6, except that the further electrodes 204E are located on only one side of the reticle MA. Thus, the further electrode 204E may be considered to at least partially enclose the volume 230.

In such an embodiment, the further electrode 204E is located adjacent to an edge 228 closest to the EUV radiation beam B and thus closest to the EUV-induced plasma. This side corresponds to the main direction of the charged particles, and it may therefore be preferred to have the further electrode 204E on this side when compared to having the further electrode 204E on only one of the other sides. It is to be appreciated that in other embodiments, the further electrodes 204E may extend more or less in the x-direction. It should be appreciated that in other embodiments, the additional electrode 204E may be on a different side than the edge 228.

Figure 9 illustrates another embodiment of an electrostatic clamp 200. This embodiment is the same as the embodiment of fig. 6, except that there are four additional electrodes 204E. I.e. further electrodes 204E are located on each side of the reticle MA. In such an embodiment, there is a gap between the additional electrodes 204E. Thus, the four further electrodes 204E may be considered to at least partially enclose the volume 230. In other embodiments, the plurality of electrodes may be considered to completely surround the volume 230 in the absence or substantial absence of gaps between the additional electrodes 204E. It will be appreciated that in embodiments there may be more or less than four further electrodes, for example 2, 3, 5 or 6, etc. For example, if there are two further electrodes, they may be located on two different sides, e.g. adjacent or opposite sides, or they may be located on the same side.

In embodiments, the further electrodes 204E may be operated independently or in pairs or in any other group configuration. For example, one of the further electrodes 204E may be positively charged, another of the further electrodes 204E may be negatively charged and the further electrodes 204E may be arranged such that the flux of free charge to the second surface 224 generated by the mechanism is reduced.

With reference to fig. 4c, another embodiment is now described. In the embodiment of fig. 4c, the average potential of all electrodes 104A to 104D is approximately 0V. More particularly, the

electrodes

104A and 104B have a positive potential (+) and the

electrodes

104C and 104D have a negative potential (-), wherein the magnitude of the potential of each of the electrodes 104A-104D is substantially the same. It will be appreciated that the arrangement of the electrodes 104A to 104D in fig. 4c is merely an example and other arrangements may be used, such as in fig. 2a, 3a or 4A. It should be appreciated that the following embodiments may exist: wherein the or each potential of the electrodes is not set such that the flux of free charge generated by the mechanism to a surface of the component adjacent the electrostatic clamp is increased.

In the embodiment of fig. 4c, the reticle (patterning device) MA is electrostatically clamped by two pairs of electrodes 104A to 104D, wherein an electrically conductive second surface 124 (backside) of the reticle MA serves as a counter or counter electrode. The capacitance of each electrode 104A to 104D in a pair of electrodes is approximately equal. As a result, the potential of the second surface 124 remains close to ground. Thus, the capacitively induced potential on the reticle MA (in particular on the second surface 124) remains unchanged.

As mentioned previously, reticle front and back side charging may lead to defectivity. Specifically, during unclamping, the distance d between the reticle backside and the electrode increases. This reduces all electrode-reticle capacitance C (. di-elect cons.1/d) and thus increases the potential V ═ Q/C, where Q is the amount of charge on the reticle. The charge on the back or front side of the reticle remains constant until an electrical discharge occurs, which may lead to defectivity and/or pellicle cracking.

One way of charging the reticle is due to EUV-induced plasma. In the most likely scenario, high energy (photo) electrons reach the reticle backside and induce negative charges. In such a process, the reticle backside potential will become increasingly negative. When the reticle backside starts at ground potential, charging is a fast "ramp up" process, requiring more and more energetic electrons to overcome the increased negative potential. The process may saturate at approximately-10V.

As mentioned, since the capacitance of each of the electrodes 104A to 104D in a pair of electrodes is approximately equal, the potential of the second surface 124 remains close to ground. However, even if the average potential of all electrodes 104A to 104D is approximately 0V, there may be a capacitively induced potential on the second surface 124 of the reticle MA. This may be due to a small imbalance of the electrostatic chuck 100 capacitance (more particularly, the individual electrodes 104A-104D capacitance). This imbalance may induce a positive (or negative) potential in the second surface 124 of the reticle MA.

In some examples, the clamping surface 102 of the electrostatic clamp 100 is almost completely flat, but control of reticle-electrode spacing is not precise. That is, the spacing between the electrodes 104A-104D and the second surface 124 of the reticle MA may be slightly different for some or all of the electrodes 104A-104D. This may, for example, be because the clamping surface 102 is tilted with respect to the second surface 124 of the reticle MA. As a result, each clip 100 will have a slightly different electrode capacitance. In addition, the electrode capacitance will likely vary depending on which reticle MA is clamped.

A positive residual potential on the backside of the reticle may accelerate (negative) reticle backside charging and may cause larger potentials (and discharges) during unloading. Furthermore, the lack of control over the charging of the reticle backside may make controlled unbalanced clamping induced negative reticle backside potentials (no charge) infeasible.

There is a problem in achieving reliable control of reticle backside potential at least below about 10V. This level of control is needed to prevent discharges during reticle unloading. The problem is not the control of the (high) voltage applied to the reticle chuck, but the uncertainty of the chuck electrode-reticle backside capacitance.

Fig. 10 shows a schematic circuit diagram of the backside (second surface 124) of the reticle (patterning device) MA, the electrodes 104A to 104D, and the high voltage supply. The capacitance of each of the electrodes 104A-104D (in combination with the reticle backside) is depicted as C1-C4, respectively. Each of the electrodes 104A-104D is powered by a high voltage supply, having a voltage depicted as V1-V4, respectively.

A plurality of charge measurement devices 300A-300D are provided, one for each electrode 104A-104D. The charge measuring devices 300A to 300D measure the charges from the voltage supply devices to the electrodes 104A to 104D.

It should be appreciated that this is only an embodiment and that in other embodiments, there may be different electronic device arrangements. For example, there may be only two electrodes (e.g.,

electrodes

104A and 104C) or there may be a single charge measurement device configured to measure the charge of each of the electrodes.

The charge measurement devices 300A-300D are used to measure the chuck-reticle capacitance. The measurement is relatively less simple, i.e. relatively less straightforward, because the second surface 124 of the reticle MA has no contact, i.e. it is floating.

At the level of the high voltage power amplifier, it may not be possible to directly measure the capacitance of the individual electrodes 104A to 104D. For example, when changing V1 by step dV1, the change in charge to electrode 104A (i.e., dQ1) is:

dQ1＝dV1*(1/C1+1/(C2+C3+C4))^-1。

that is, the series capacitance of C1 and C2+ C3+ C4 was measured. However, with charge measurement devices 300A-300D (one for each high voltage source of the fixture 100), the capacitance or ratio of capacitances can be determined.

When, for example, the potential of the electrode 104A changes by an amount dV1, the potential of the second surface 124 of the reticle MA will change by a substantially unknown amount dVb. This in turn causes a change in the charge to the electrodes 104A to 104D:

dQ1＝C1*(dV_b–dV1)

dQ2＝C2*dVb

dQ3＝C3*dVb

dQ4＝C4*dVb

(dQ1+ dQ2+ dQ3+ dQ4) ═ 0 (due to the absence of net charge to the second surface 124 of the reticle MA).

The measured charge is given by the following equation:

Q_n＝(V_n-V_b)·C_n

wherein n is 1 … … 4, C_nIs the unknown capacitance, Q, of the electrodes 104A-104D_nIs the electric charge, V, measured by the charge measuring devices 300A to 300D_bIs the potential of the second surface (backside) 124 of the reticle MA, and V_nIs the potential applied across the electrodes 104A to 104D.

In an embodiment, by applying a first set of sufficiently high potentials V_n,1E.g. V_n,1＝(-1)ⁿφ₁To clamp the reticle MA and the reticle MA is flattened against the burls. This yields four equations and five unknowns (C)_1-4And V_b,1). Next, the reticle MA potential is changed, that is, the potentials of the electrodes 104A to 104D are changed, for example, to V_n,2＝(-1)ⁿφ₂. This results in eight equations and six unknowns (C)_1-4And V_b,1,V_b,2). Having more equations than unknowns would allow for C_n、V_b,1And V_b,2Solving the equation. Thus, the capacitances of the electrodes 104A to 104D are determined, with the first group of high potentials V_n,1Of the second surface 124 (V)_b,1) And has a second group of high potentials V_n,2Of the second surface 124 (V)_b,2)。

Now that the potential V of the second surface has been determined_b,2Then the now known C can be used with this equation_nTo adjust the potential of the electrodes 104A-104D to achieve a desired backside potential (V)_b)：

V_b＝sum(V_n·C_n)/sum(C_n)

For example, when zero backside potential is desired, then V may be selected_n＝φ₂·C₁/C_n。

It may be preferred to have the potential of one or more of the electrodes 104A to 104D set such that the potential of the second surface 124 is negative before or at least relatively soon after the mechanism generating free charges is switched on (e.g. the reticle MA is exposed to EUV radiation). This potential can be maintained for the entire time that the mechanism generates free charge. This may minimize the number of negative charges that are attracted to the second surface 124. However, in embodiments, the potential may be set such that the potential of the second surface 124 is negative during part of the EUV exposure and/or during a period of time before the reticle MA moves from being clamped by the electrostatic clamp 100 to being spaced apart from the electrostatic clamp 100.

It may be preferred that the potential of one or more of the electrodes 104A-104D is set such that the potential of the second surface 124 is substantially zero prior to the time that the mechanism generates free charge (e.g., prior to exposure of the reticle MA). This may minimize the number of negative charges that are attracted to the second surface 124. This may be because if the potential of the electrode is set only a certain time after EUV exposure has started, the second surface 124 may already have a negative charge due to the fast moving negative electrons, which may then not be able to be reduced before releasing the clamping of the clamp 100.

It will be appreciated that the described measurement and potential setting procedure may be used in conjunction with, for example, the use of an EUV-induced plasma to remove charge from the reticle backside. It will be appreciated that the measurement and potential setting procedure may be used with the method of increasing the flux of free charge generated by an EUV source described above.

Variations on the above are possible. E.g. from zero to phi₁First potential step of from₁To phi₂Will be substantially larger than the second step of where | phi₁-φ₂I is usually smaller than | φ₁10% of l. As a result, the charge measurement devices 300A-300D may need to have a high dynamic range. It may be beneficial to first clamp the reticle MA at a high potential and then to vary the clamp potential taking into account the change in charge:

ΔQ_n＝(ΔV_n-ΔV_b)·C_n

in a similar manner, C can be determined_nAnd thus can be obtained by applying two sets of potential steps av of similar magnitude_n,1And Δ V_n,2To determine the potential of the second surface 124 of the reticle MA.

In an example, the electrode arrangement of the clamp 100 may be considered similar to the arrangement shown in, for example, fig. 2 a. It will be appreciated that this is merely an example and that other arrangements of electrodes may be used.

In addition to being able to keep the potential of the second surface 124 of the reticle MA at substantially zero volts (i.e. at ground), the potential of the second surface 124 may also be kept at (approximately) a certain negative (or positive) predetermined value. That is, the potential of the second surface 124 of the reticle MA may be maintained at an approximately controlled potential. In other words, the potential of one or more of the electrodes 104A to 104D may be set such that the potential of the second surface 124 is approximately a predetermined value. It may be preferred to set the potential of one or more electrodes 104A to 104D such that the potential of the second surface 124 is negative such that energetic (photo) electrons are repelled from (or at least not attracted to) the second surface 124 of the reticle MA.

Using the one or more charges measured by one or more charge measurement devices 300A-300D, and capacitances C1-C4 of electrodes 104A-104D, the potential of the second surface 124 of the reticle MA may be determined. Determining the potential may be considered, for example, measuring, calculating, or setting the potential of the second surface 124.

The capacitance of the electrodes 104A-104B may vary only within a certain percentage (e.g., +/-10%). An unbalanced electrode 104A of-100V may result in a-25V back side potential for a nominal clamp capacitance. Then, correcting the measured capacitance of the electrode, a potential of-25V +/-10% may be applied to the second surface 124, for example, to mitigate electron charging.

Since the capacitance can vary only +/-10%, only the backside potential intended to be > -10V (using a nominal clamp capacitance) may be sufficient to ensure that a negative potential is present on the second surface 124. In the above example, the-25V backside potential may be intended to ensure that there is some leeway to ensure that the second surface 124 must eventually become a negative value to mitigate electron charging. More generally, the potential of at least one of the electrodes 104A-104D may be set based on a variance of the capacitance of one or more of the electrodes 104A-104D.

In other embodiments, the electrodes may be changed to have more positive values (such as +100V), which may result in, for example, +25V backside potential. This may be useful if it is desired to mitigate positive ionic charging of the second surface 124.

In addition to determining the capacitances C1-C4 as described above, other methods may determine the individual electrodes 104A-104D capacitances C1-C4. This may be done by applying voltage steps to the different electrodes 104A to 104D and then measuring the charge transfer. In other words, the potentials of the electrodes 104A to 104D are changed by a predetermined amount in a stepwise manner, and the electric charges from the voltage supply means to the electrodes 104A to 104D are measured using the electric charge measuring means 300A to 300D, respectively, after each potential change.

This results in an (over) constrained set of equations that can be solved for the individual capacitances C1-C4. The four voltages V1, V2, V3, V4 may be varied in steps and along with the measured charge from each of the charge measurement devices 300A-300D, there is sufficient information to determine C1-C4 individually. In such an embodiment, the charge to each electrode 104A-104D may be measured. However, it should be understood that this is only an example and that a wide range of variations with respect to the measurements are possible.

As an example, at least two voltage steps may be applied, e.g., dV1 and dV 2. This in turn yields 8 equations and 6 unknowns (C)_1-4,dV_b1,dV_b2). This set can be solved. The absolute values of C1 through C4 may then be obtained. This allows setting the second surface 124 of the reticle MA to 0V by observing, for example, V4/V3 ═ -C3/C4 and V2/V1 ═ -C1/C2. Thus, in an embodiment, the potential of the electrodes 104A-104D may be based on a ratio of the capacitances of the electrodes 104A-104D. Alternatively, instead of setting the second surface 124 of the reticle MA to 0V, any arbitrary potential may be set.

Alternatively, in other embodiments, two electrodes may be floated and a single charge measurement may be used to determine the series capacitance of the other two electrodes. For example, the capacitances of C1-C2 (C) may be measured in series₁₂) And then C can be similarly measured₁₃、C₁₄、C₂₃、C₂₄And C₃₄. Again, using 6 unique combinations provides an overconstrained set of equations that can be used to derive the individual capacitances C1-C4. It should be appreciated that many variations are possible and may be optimized in conjunction with hardware development.

The above derivation ignores stray capacitances, such as cable-to-ground capacitances of 50 to 100 pF/m. In actual implementation, these capacitances should be included and corrected. This is possible by, for example, applying a potential to the reticle chuck without the reticle. In that case, the clamp-to-reticle capacitance is virtually zero and stray capacitance can be measured.

While the foregoing generally relates to one or more charge measurement devices 300A-300D that measure the amount of charge Q to the electrodes 104A-104D, it should be appreciated that in other embodiments, other measurement devices may be used. For example, in embodiments, one or more for measuring current to the electrodes may also be used instead of, or in addition to, charge measurement devices 300A-300DA plurality of current measuring devices. In an embodiment, an oscillating electrode potential V may be applied_n＝V_n0+V_aSin (Ω t). The AC portion of the current entering the electrostatic clamp may then be measured. Thus, instead of measuring Q or Δ Q, then dQ/dt (═ I) may be measured.

It should be appreciated that using the one or more current measurement devices to determine the potential of the second surface 124 of the reticle MA (by calculating the capacitance of one or more electrodes 104A-104D using the measured current to one or more of the electrodes 104A-104D) may function in a similar manner to using charge measurement devices 300A-300D. As mentioned in the above examples regarding charge measurement, Q vs V or dQ vs dV were measured. However, dQ/dt ═ I to dV/dt can also be measured. Equivalence will be understood due to: q ═ C ═ V, dQ ═ C ═ dV, dQ/dt ═ I ═ C ═ dV/dt.

It should be appreciated that the calculations, etc. may be performed in the device and/or in a separate system (e.g., a computer device).

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications 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, etc.

Although specific reference may be made herein to embodiments of the invention in the context of lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a 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 be generally referred to as lithographic tools. Such a lithography tool may use vacuum conditions or ambient (non-vacuum) conditions.

Although the foregoing may make specific reference to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications (e.g. imprint lithography), and is not limited to optical lithography, where the context allows.

Embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof, as the context allows. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include Read Only Memory (ROM); random Access Memory (RAM); a magnetic storage medium; an optical storage medium; a flash memory device; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Additionally, firmware, software, routines, instructions may be described herein as performing certain actions. It should be appreciated, however, that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing firmware, software, routines, instructions, etc. and that such operations, when executed, may cause actuators or other devices to interact with the physical world.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative, and not restrictive. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. An apparatus, comprising: an electrostatic chuck for clamping a component; and a mechanism for generating free charge adjacent the electrostatic chuck:

wherein the electrostatic chuck comprises an electrode or a plurality of electrodes,

wherein the device is configured to:

operating in a first mode in which the or each electrode is set at an electrical potential such that a clamping electric field is generated between the electrostatic clamp and the component to clamp the component,

operating in a second mode in which the or each potential of the electrode is set such that clamping of the component is released, an

Operating in a third mode in which the or each potential of the electrodes is set such that the flux of free charge generated by the mechanism to a surface of the component adjacent the electrostatic clamp is increased compared to operating in the first or second modes.

2. The apparatus of claim 1, wherein the electrostatic chuck comprises the plurality of electrodes, and wherein in the third mode, the apparatus is configured such that an edge electrode closest to an edge of the electrostatic chuck is set to a positive potential.

3. A device according to claim 1 or 2, wherein in the third mode the device is configured such that the or each potential of the electrode is set such that the or an average potential of the electrode or electrodes is negative.

4. The apparatus of claim 2, wherein in the third mode, the apparatus is configured such that the potentials of the plurality of electrodes are set such that an average potential of the plurality of electrodes is substantially 0V.

5. The apparatus of claim 1, wherein the electrostatic chuck comprises the plurality of electrodes, and wherein in the third mode, the apparatus is configured such that an edge electrode closest to an edge of the electrostatic chuck is set to a negative potential, and the potentials of the remainder of the plurality of electrodes are set such that the average potential of the plurality of electrodes has a smaller negative value than the potential of the edge electrode.

6. The apparatus of claim 3, wherein in the third mode the apparatus is configured such that the potential of the or each electrode is set such that the surface of the component adjacent the electrostatic clamp has a positive potential before the component moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp.

7. The apparatus of claim 3, wherein in the third mode, the potential of the electrode or the average potential of the electrode is set to a predetermined negative value such that the surface of the component has a potential substantially the same as the potential of the electrode or the average potential of the electrode during movement of the component from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp.

8. The apparatus according to claim 3 or 7, wherein in the third mode, the potential of the electrode or the average potential of the electrode is set to the predetermined negative value so that the member has substantially zero charge after exposure of the member.

9. Apparatus according to any preceding claim, wherein in the third mode the or each potential of the electrode is set for at least one of the following times: a time period before the component moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp; a portion or all of the time it takes to move the component from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp; and a portion or all of the time it takes to move the component from spaced apart from the electrostatic clamp to clamped by the electrostatic clamp.

10. The apparatus of any preceding claim, wherein in the third mode the potential of the electrode or the average potential of the electrode is set for at least a portion of the time or the entire time that the mechanism generates free charge.

11. The apparatus of any preceding claim, wherein the mechanism for generating free charge adjacent the electrostatic clamp comprises: a gas source; and an ionizing radiation source configured to ionize a gas provided by the gas source.

12. The apparatus of claim 11, wherein the ionizing radiation source comprises at least one of an EUV source, a VUV source, a soft x-ray source, and a radiation source.

13. Apparatus according to any preceding claim, wherein the electrostatic clamp comprises a further electrode or a plurality of further electrodes, wherein the further electrode or the plurality of further electrodes are positioned at least partially around a volume extending in a direction of the electrostatic clamp away from a surface of the component adjacent the electrostatic clamp, wherein the apparatus is configured such that the potential of the or each further electrode is set such that the flux of free charge generated by the mechanism to the surface of the component adjacent the electrostatic clamp is reduced.

14. The apparatus of any preceding claim, wherein the apparatus is configured to: measuring the charge or current passed to the or each electrode from the voltage supply means using at least one charge or current measuring means; calculating the capacitance of the or each electrode using the measured charge or current to the or each electrode; and determining the potential of the surface of the component adjacent the electrostatic clamp using the calculated capacitance of the or each electrode.

15. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an illumination system configured to condition a radiation beam and an apparatus according to any one of the preceding claims, wherein the illumination system is configured to project the radiation beam onto the patterning device, and wherein the patterning device comprises a component to be clamped, wherein the lithographic apparatus comprises an apparatus according to any one of the preceding claims.

16. A method of operating a device, the device comprising: an electrostatic chuck; and a mechanism for generating a free charge adjacent to the electrostatic chuck, the electrostatic chuck comprising an electrode or a plurality of electrodes, the method comprising:

disposing a member adjacent to the electrostatic chuck;

controlling the mechanism for generating free charge to generate free charge adjacent the electrostatic chuck,

operating the apparatus in a first mode in which the or each electrode is set at an electrical potential such that a clamping electric field is generated between the electrostatic clamp and the component to clamp the component,

operating the apparatus in a second mode in which the or each potential of the electrodes is set such that clamping of the component is released, an

Operating the apparatus in a third mode in which the or each potential of the electrodes is set such that the flux of free charge to a surface of the component adjacent the electrostatic clamp is increased compared to operating in the first or second modes.

17. The method of claim 16, wherein the electrostatic chuck comprises a plurality of electrodes, the method further comprising: in the third mode, the potential of the edge electrode closest to the edge of the electrostatic chuck is set to be positive.

18. The method of claim 17, further comprising: in the third mode, the or each potential of the electrodes is set such that the or an average potential of the electrodes is negative.

19. The method of claim 17, further comprising: in the third mode, the potentials of the plurality of electrodes are set so that an average potential of the plurality of electrodes is substantially 0V.

20. The method of any of claims 16 to 19, wherein the electrostatic clamp comprises a further electrode or a plurality of further electrodes positioned at least partially around a volume extending in a direction of the electrostatic clamp away from a surface of the component adjacent the electrostatic clamp, the method further comprising:

the or each potential of the or each further electrode is arranged such that the flux of free charge to the surface of the component adjacent the electrostatic clamp is reduced.

21. The method of any of claims 16 to 20, further comprising: measuring the charge or current passed to the or each electrode from the voltage supply means using at least one charge or current measuring means; calculating the capacitance of the or each electrode using the measured charge or current to the or each electrode; and determining the potential of the surface of the component adjacent the electrostatic clamp using the calculated capacitance of the or each electrode.

22. A computer program comprising computer readable instructions configured to cause a processor to perform the method of any of claims 16 to 21.

23. A computer readable medium carrying a computer program according to claim 22.

24. A computer device for operating a device, comprising:

a memory storing processor-readable instructions;

and a processor arranged to read and execute instructions stored in the memory;

wherein the processor readable instructions comprise instructions arranged to control the computer to perform the method of any of claims 16 to 21.

25. An apparatus, comprising: an electrostatic chuck for clamping a component; and a mechanism for generating free charge adjacent the electrostatic chuck:

wherein the electrode or electrodes are positioned at least partially around a volume extending in a direction of the electrostatic clamp away from a surface of the component adjacent the electrostatic clamp,

wherein the apparatus is configured such that the or each electrode's potential is set such that the flux of free charge generated by the mechanism to the surface of the component adjacent the electrostatic clamp is reduced.

26. The apparatus of claim 25, wherein the electrode or electrodes are located on one side of the component.

27. The apparatus of claim 25 or 26, wherein the electrode or electrodes extend all the way around the volume.

28. Apparatus according to any of claims 25 to 27, wherein the or each potential of the electrodes is set to negative.

29. Apparatus according to any of claims 25 to 27, wherein the or each potential of the electrodes is set to be positive.

30. Apparatus according to any of claims 25 to 29, wherein the or each electrode is in electrical contact with the surface of the component adjacent the electrostatic clamp.

31. Apparatus according to any one of claims 25 to 30, wherein at least one edge or edges of the surface of the or each electrode adjacent the component are rounded.

32. Apparatus according to claim 31, wherein the or each electrode is rounded at regions corresponding to corner portions of the component.

33. Apparatus according to any of claims 25 to 32, wherein the or each potential of the electrodes is set to last for at least one of: the mechanism generates at least a portion of or all of the free charge and a time period before the component moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp.

34. The apparatus of any of claims 25 to 33, wherein the mechanism for generating free charge adjacent the electrostatic clamp comprises: a gas source; and an ionizing radiation source configured to ionize a gas provided by the gas source.

35. The apparatus of claim 34, wherein the ionizing radiation source comprises at least one of an EUV source, a VUV source, a soft x-ray source, and a radiation source.

36. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an illumination system configured to condition a radiation beam and an apparatus according to any of claims 25 to 35, wherein the illumination system is configured to project the radiation beam onto the patterning device, and wherein the patterning device comprises a component to be clamped, wherein the lithographic apparatus comprises an apparatus according to any of claims 25 to 35.

37. A method of operating a device, the device comprising: an electrostatic chuck; and a mechanism for generating a free charge adjacent the electrostatic chuck, the electrostatic chuck comprising an electrode or a plurality of electrodes positioned at least partially around a volume extending in a direction of the electrostatic chuck away from a surface of the component adjacent the electrostatic chuck, the method comprising:

disposing a member adjacent to the electrostatic chuck;

the or each potential of the or each electrode is arranged such that the flux of free charge to the surface of the component adjacent the electrostatic clamp is reduced.

38. A method according to claim 37, further comprising setting the or each potential of the or each electrode to negative.

39. A method according to claim 37, further comprising setting the or each potential of the or each electrode to positive.

40. A method according to any one of claims 37 to 39, further comprising controlling the potential of the surface of the component adjacent the electrostatic clamp via an electrical connection between the or each electrode and the surface of the component adjacent the electrostatic clamp.

41. A computer program comprising computer readable instructions configured to cause a processor to perform the method of any of claims 37 to 40.

42. A computer readable medium carrying a computer program according to claim 41.

43. A computer device for operating a device, comprising:

a memory storing processor-readable instructions;

and a processor arranged to read and execute instructions stored in the memory;

wherein the processor readable instructions comprise instructions arranged to control the computer to perform the method of any of claims 37 to 40.

44. An apparatus comprising an electrostatic clamp for clamping a component; and a mechanism for generating free charge adjacent the electrostatic chuck:

wherein the device is configured to:

measuring the charge or current passed to the or each electrode from the voltage supply means using at least one charge or current measuring means;

calculating the capacitance of the or each electrode using the measured charge or current to the or each electrode; and

determining the potential of the surface of the component adjacent the electrostatic clamp using the calculated capacitance of the or each electrode.

45. The apparatus of claim 44, wherein the apparatus is configured such that the potential of the or each electrode of the plurality of electrodes is set such that the potential of the surface of the component adjacent the electrostatic clamp is substantially a predetermined value.

46. The apparatus of claim 44 or 45, wherein the predetermined value of the potential of the surface of the component adjacent to the electrostatic chuck is at least one of positive, negative, and substantially zero.

47. The device of any one of claims 44-46, wherein the device is configured to measure a ratio of the capacitances of the plurality of electrodes.

48. The device of any one of claims 44-47, wherein the device is configured to set the potential of at least one of the plurality of electrodes based on the ratio of the capacitances of the plurality of electrodes.

49. Apparatus according to any of claims 44 to 48, wherein the apparatus is configured to set the potential of at least one of the plurality of electrodes based on a variance of the capacitance of the or each electrode.

50. Apparatus according to any of claims 44 to 49, wherein the apparatus is configured to change the potential of the plurality of electrodes by a predetermined amount in a stepwise manner and to measure the charge or current passed to the or each electrode from the voltage supply means using the at least one charge or current measuring means after each potential change for determining the individual capacitance of the plurality of electrodes.

51. Apparatus according to any of claims 44 to 50, wherein the potential of the or each electrode is set to last for at least one of: prior to at least a portion or all of the time that the mechanism generates free charge; and a time period before the component moves from being clamped by the electrostatic clamp to being spaced apart from the electrostatic clamp.

52. The apparatus of any one of claims 44 to 51, wherein the mechanism for generating free charge adjacent the electrostatic clamp comprises: a gas source; and an ionizing radiation source configured to ionize a gas provided by the gas source.

53. The apparatus of any one of claims 44 to 52, wherein the ionizing radiation source comprises at least one of an EUV source, a VUV source, a soft x-ray source, and a radiation source.

54. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an illumination system configured to condition a radiation beam and an apparatus according to any of claims 44 to 53, wherein the illumination system is configured to project the radiation beam onto the patterning device, and wherein the patterning device comprises a component to be clamped, wherein the lithographic apparatus comprises an apparatus according to any of claims 44 to 53.

55. A method of operating a device, the device comprising: an electrostatic chuck; and a mechanism for generating a free charge adjacent to the electrostatic chuck, the electrostatic chuck comprising an electrode or a plurality of electrodes, the method comprising:

disposing a member adjacent to the electrostatic chuck;

measuring the charge or current passed to the or each electrode from the voltage supply means using at least one charge or current measuring means,

calculating the capacitance of the or each electrode using the measured charge or current to the or each electrode, and

56. The method of claim 55 further comprising setting the potential of the or each electrode of the plurality of electrodes such that the potential of the surface of the component adjacent the electrostatic clamp is substantially a predetermined value.

57. The method of claim 55 or 56, further comprising setting the potential of the or each electrode of the plurality of electrodes such that the predetermined value of the potential of the surface of the component adjacent the electrostatic clamp is at least one of positive, negative and substantially zero.

58. The method of any one of claims 55 to 57, further comprising setting the potential of at least one of the plurality of electrodes based on a ratio of the capacitances of the plurality of electrodes.

59. A method according to any of claims 55 to 58, further comprising setting the potential of at least one of the plurality of electrodes based on a variance of the capacitance of the or each electrode.

60. A computer program comprising computer readable instructions configured to cause a processor to perform the method of any of claims 55 to 59.

61. A computer readable medium carrying a computer program according to claim 60.

62. A computer device for operating a device, comprising:

a memory storing processor-readable instructions;

and a processor arranged to read and execute instructions stored in the memory;

wherein the processor readable instructions comprise instructions arranged to control the computer to perform the method of any of claims 55 to 59.