TW202234139A - A method and apparatus for thermally deforming an optical element - Google Patents

Abstract

A method of thermally deforming an optical element, the optical element comprising a material whose coefficient of thermal expansion is temperature dependent, the method comprising: transferring heat to or from the optical element to establish a temperature set point for the optical element such that the coefficient of thermal expansion has a non-zero value, and heating the optical element to produce a thermal deformation of the optical element.

Description

Method and apparatus for thermally deforming optical elements

The present invention relates to methods and apparatus for thermally deforming optical elements. More particularly, methods and apparatus relate to thermally deforming optical elements to correct for aberrations.

A lithography apparatus is a machine constructed to apply a desired pattern onto a substrate. Lithographic devices can be used, for example, in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus can project a pattern at a patterned device (eg, a reticle) onto a layer of radiation-sensitive material (resist) provided on a substrate.

In order to project the pattern on the substrate, a lithography device may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Lithographic devices that use extreme ultraviolet (EUV) radiation with wavelengths in the range of 4 nm to 20 nm (eg, 6.7 nm or 13.5 nm) can be used in comparison to lithographic devices that use radiation with, for example, a wavelength of 193 nm. for the formation of smaller features on the substrate.

The projection system used to image the pattern from the patterned device onto the substrate can induce some aberrations in the wavefront of the projected image.

During projection of the pattern onto the substrate, the projection system will heat up and this will drift the imaging properties of the projection system. In EUV lithography, this phenomenon is called mirror heating.

Although mirrors in projection systems are optimized for EUV radiation reflection, a significant portion of EUV (and out-of-band) energy is absorbed into the mirrors and converted to thermal energy. This heating causes thermal stress in the material of the mirror, resulting in deformation of the mirror. These deformations ultimately result in aberrations in the projection system, resulting in imaging errors. Additionally, direct or indirect heating can cause thermal stress in materials such as lenses, substrate holders, patterned devices (ie, reticle or reticle), or other components of patterned device holders.

It is an object of the present invention to provide a method of correcting aberrations for avoiding or mitigating one or more of the problems associated with the prior art.

According to a first aspect of the present invention, there is provided a method of thermally deforming an optical element, the optical element comprising a material whose thermal expansion coefficient is temperature-dependent, the method comprising: transferring heat to or from the optical element to establish a temperature set point for the optical element such that the coefficient of thermal expansion has a non-zero value, and The optical element is heated to produce thermal deformation of one of the optical elements.

The method according to the first aspect of the present invention may have the advantage of introducing a desired thermal deformation into the optical element, which may be used to at least partially correct for aberrations, eg caused by other optical elements used in the lithography apparatus LA aberration.

In one embodiment, the temperature set point of the optical element is established such that the coefficient of thermal expansion has a predetermined non-zero value.

In one embodiment, thermal deformation may be applied, for example, in one or more of a plurality of regions of the optical element.

In one embodiment, a heat exchanger is applied to transfer heat to or from the optical element. This heat exchanger may, for example, comprise cooling fins. The temperature of the optical element can be set by transferring heat to or from the optical element. The temperature of the optical element as set may be associated with, for example, a temperature set point for controlling a heat exchanger. The temperature set point can be used, for example, to control the temperature of the conditioning fluid applied in the heat exchanger.

In one embodiment, heating the optical element includes selectively heating the optical element.

In one embodiment, a heater is applied to selectively heat the optical element.

In one embodiment, selectively heating the optical element includes selectively heating one or more of the plurality of regions of the optical element to generate heat at the one or more of the plurality of regions of the optical element deformed.

In one embodiment, the sign of the coefficient of thermal expansion is the same for most or all of the material of one or more of the plurality of regions of the optical element over most or all of the heating activation range of the heater.

In one embodiment, the coefficient of thermal expansion is substantially zero at the zero-crossing temperature and/or has a local minimum as a function of temperature.

In one embodiment, transferring heat to or from the optical element is accomplished using a heat exchanger that is at least partially integrated into the optical element. This heat exchanger can thus cool or heat the optical element so that the optical element has the desired coefficient of thermal expansion.

In one embodiment, the heat exchanger includes a channel integrated in the optical element configured to receive a fluid for exchanging heat with the optical element. In one embodiment, the fluid applied in the heat exchanger may be, for example, a liquid such as water.

In one embodiment, the present invention provides a method of thermally deforming an optical element comprising a material having a coefficient of thermal expansion of zero at a zero crossing temperature, the method comprising: using a heat sink to transfer heat from the optical element to create a thermal One or more temperature set points for one or more of the plurality of zones such that the sign of the coefficient of thermal expansion is within most or all of the heater's heating activation range for one or more of the plurality of zones of the optical element Most or all of the materials are the same, and a heater is used to selectively heat one or more of the plurality of zones of the optical element to one or more of the plurality of zones of the optical element within the heating activation range thermal deformation occurs.

This may have the advantage of introducing corresponding thermal deformations to the optical elements during use of the lithography apparatus LA to correct for aberrations caused, for example, by other optical elements of the lithography apparatus LA.

The method may further include establishing one or more temperature set points such that a change in heat from the heater over most or all of the heater's heating activation range causes one or more of the plurality of zones of the optical element to be in- Variation in thermal deformation in the range of 10 nm to +10 nm.

One or more temperature set points may be established such that changes in heat from the heater result in relative comparison of thermal deformation of one or more of the plurality of zones of the optical element over most or all of the heater's heating activation range Big change.

One or more temperature set points may be established such that the relationship between thermal load and deformation is sufficiently sensitive to provide a relatively good range of thermal manipulation. Larger deviations between the temperature set point and the zero-crossing temperature can result in larger sensitivity.

The one or more temperature set points may be established such that there is a substantially linear dependence between the temperature within most or all of the heater's heating activation range and the thermal deformation of one or more of the plurality of zones of the optical element. Thermal deformation can be viewed as deformation perpendicular to the optical surface of the optical element.

For optical element manipulation, the Rennick actuation step size between substrates can be approximately 0.01 nm.

The method may further include establishing one or more temperature set points such that the coefficient of thermal expansion has a value that is substantially similar for the plurality of regions of the optical element over most or all of the heater's heating activation range.

The method may further include establishing one or more temperature set points within a range of 1 to 8°C from the zero crossing temperature.

The zero-crossing temperature of the material may include the local zero-crossing temperature of one or more of the plurality of regions of the optical element or the average zero-crossing temperature of the optical element.

One or more temperature set points may be greater than the zero-crossing temperature of the material or one or more temperature set points may be less than the zero-crossing temperature of the material.

The optical element may comprise an optical surface. The plurality of regions of the optical element can be defined between the optical surface of the optical element and the heat sink. The optical surface may include a plurality of local sections corresponding to a plurality of regions.

Changes in the zero-crossing temperature of the material on the side of the heat sink opposite the optical surface of the optical element may be considered less relevant than in the plurality of regions.

Thermal interference of the optical element on the side of the heat sink opposite the optical surface of the optical element can be considered negligible and can be disregarded.

The method may further include determining a relationship between the heat provided from the heater to one or more of the plurality of regions of the optical element and the resulting thermal deformation of the corresponding one or more localized regions of the optical surface of the optical element.

The method may further comprise calibrating the heater using the corresponding optical metrology obtained by the optical metrology device.

The method may further include calibrating the heater using corresponding wavefront aberration measurements obtained by the wavefront sensor.

The method may further include using a calibrated heater during exposure of the optical element to EUV radiation to manipulate the wavefront during operation of the lithography device.

The method may further include using a calibrated relationship between heat from the heater and the resulting thermal deformation of one or more of the plurality of regions of the optical element to introduce corresponding thermal deformation to the optical element via the heater during use of the lithographic device The optical surface of the component. This can be used to correct for aberrations caused, for example, by other optical elements of the lithography device.

The method may further include determining one or more temperature set points based on the provided or determined zero-crossing temperature.

The method may further comprise characterizing, by modeling, the distribution of zero-crossing temperatures of the optical surface of the optical element.

The method may further include setting the temperature of the cooling fluid of the heat sink to establish one or more temperature set points for the optical element.

The temperature of the cooling fluid may be higher and the one or more temperature set points may be less than the zero crossing temperature of the material or the temperature of the cooling fluid may be lower and the one or more temperature set points may be greater than the zero crossing temperature of the material. That is, the temperature of the cooling fluid and the one or more temperature set points may be on different sides with respect to the zero-crossing temperature of the material.

The temperature of the cooling fluid can be substantially the same as the zero-crossing temperature of the material and the one or more temperature set points can be less than or greater than the zero-crossing temperature of the material.

The method may further include heating one or more localized sections of the optical surface of the optical element corresponding to one or more of the plurality of regions of the optical element.

The coefficient of thermal expansion can vary with the temperature of the optical element and the optical element temperature can vary with the temperature of the cooling fluid. One or more temperature set points may be established by setting the temperature of the cooling fluid sufficiently far from the zero-crossing temperature of the material of the optical element.

The temperature of the cooling fluid may be set in an example range of +/- 0.5 C, +/- 1 C, or +/- 2 C from the zero-crossing temperature Tzc due to variations in the zero-crossing temperature Tzc.

The temperature of the cooling fluid may be equal to the mean zero crossing temperature. This situation may not be preferred since it may require additional power when compared to the situation where the temperature of the cooling fluid is set away from the zero crossing temperature Tzc.

The plurality of regions of the optical element can be defined between the optical surface of the optical element and at least one channel in the optical element for cooling fluid.

The heater may include a zone heater having a plurality of zones configured to correspondingly heat one or more of the plurality of zones of the optical element.

Each zone may have a heating actuation range per zone. The heating activation range of one of the plurality of zones may be the same or may be different from the other of the plurality of zones. The range of actuation for a given partition can be mapped to some range of wavefront aberrations at the substrate level.

The method may further include calibrating the plurality of zones of the zone heater using corresponding optical measurements obtained by the optical measurement device. The optical measurement may be a wavefront aberration measurement obtained by a wavefront sensor.

The method may further include sequentially modifying the power for the plurality of zones of the zone heater and making corresponding optical measurements. That is, the power is changed one by one for the plurality of zones of the zone heater. This can determine how the wavefront is changed by the temperature change of the corresponding region of the optical element.

The relationship between the power to the zones of the zone heaters and the corresponding optical measurements can be used for thermal manipulation of the wavefront.

According to a second aspect of the present invention, a device is provided, the device comprising: at least one optical element comprising a material whose coefficient of thermal expansion is temperature dependent, a heat exchanger configured to transfer heat to or from the optical element to establish a temperature set point for the optical element such that the coefficient of thermal expansion has a non-zero value, a heater configured to heat the optical element to produce a thermal deformation of the optical element.

The device according to the invention can advantageously be used to apply the method of the invention.

The device according to the second aspect of the invention may have the advantage of enabling the introduction of desired thermal deformations into an optical element, which may be used to at least partially correct aberrations, such as by other applied in the lithography device LA Aberrations caused by optical components.

In one embodiment, the temperature set point of the optical element is established such that the coefficient of thermal expansion has a predetermined non-zero value.

In one embodiment, the heater is configured to selectively heat the optical element.

In one embodiment, the heater is configured to selectively heat the optical element by selectively heating one or more of the plurality of zones of the optical element to the one or more of the plurality of zones of the optical element Thermal deformation occurs at one or more locations.

In one embodiment, the heater of the device can be configured to heat one or more of the plurality of zones of the optical element within the heating activation range of the heater.

In one embodiment, the sign of the coefficient of thermal expansion is the same for most or all of the material of one or more of the plurality of regions of the optical element over most or all of the heating activation range of the heater.

In one embodiment, the coefficient of thermal expansion is substantially zero at the zero-crossing temperature and/or has a local minimum as a function of temperature.

In one embodiment, the heat exchanger of the device is at least partially integrated into the optical element. This heat exchanger can thus cool or heat the optical element so that the optical element has the desired coefficient of thermal expansion.

In one embodiment, the heat exchanger includes a channel integrated in the optical element configured to receive a fluid for exchanging heat with the optical element. In one embodiment, the fluid applied in the heat exchanger may be, for example, a liquid such as water.

In one embodiment, an apparatus is provided comprising: at least one optical element comprising a material having a coefficient of thermal expansion of zero at a zero crossing temperature; a heat sink configured to remove from the optical element Transfer heat to establish one or more temperature set points for one or more of the plurality of zones of the optical element such that the sign of the coefficient of thermal expansion is for most or all of a heating activation range of a heater for Most or all of the material of one or more of the plurality of zones of the optical element is the same, the heater is configured to selectively heat the plurality of zones of the optical element within the heating activation range one or more to produce a thermal deformation at one or more of the plurality of regions of the optical element.

The one or more temperature set points can be established such that a change in heat from the heater causes one of the plurality of zones of the optical element or Multiple changes in one of the thermal deformations in the range of -10 to +10 nm. The optical element may comprise an optical surface. The plurality of regions may be defined between the optical surface and the heat sink. The optical surface may include a plurality of local sections corresponding to a plurality of regions. A heater may be configured to heat one or more localized sections of the plurality of localized sections of the optical surface corresponding to one or more of the plurality of regions of the optical element.

The heater may include a source of electromagnetic waves that produces thermal deformation of one or more of the plurality of zones of the optical element.

The heat sink may contain a cooling fluid for passage through at least one of the channels in the optical element.

The plurality of regions of the optical element can be defined between the optical surface of the optical element and at least one channel in the optical element for cooling fluid.

The heater may include a zone heater having a plurality of zones configured to correspondingly heat one or more of the plurality of zones of the optical element.

The optical element may be a mirror.

According to a third aspect of the present invention, there is provided a lithography apparatus including a projection system configured to project a beam of radiation to project a pattern from a patterned device onto a substrate, wherein the lithography apparatus A device as described above is included.

The lithography device can be an EUV lithography device and the projection system can include a mirror.

FIG. 1 shows a lithography system including a radiation source SO and a lithography device LA. The radiation source SO is configured to generate the EUV radiation beam B and supply the EUV radiation beam B to the lithography device LA. The lithography apparatus LA includes an illumination system IL, a support structure MT configured to support a patterned device MA (eg, a 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 the EUV radiation beam B is incident on the patterning device MA. Additionally, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11 . The faceted field mirror device 10 and the faceted pupil mirror device 11 together provide a beam B of EUV radiation having a desired cross-sectional shape and a desired intensity distribution. In addition to or in lieu of the faceted field mirror device 10 and the faceted pupil mirror device 11, the illumination system IL may include other mirrors or devices.

After so conditioning, the EUV radiation beam B interacts with the patterned device MA. Due to this interaction, a patterned EUV radiation beam B' is produced. Projection system PS is configured to project patterned EUV radiation beam B' onto substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. Projection system PS may apply a reduction factor to patterned EUV radiation beam B', thus forming an image with features smaller than corresponding features on patterned device MA. For example, a reduction factor of 4 or 8 may be applied. Although projection system PS is illustrated as having only two mirrors 13, 14 in FIG. 1, projection system PS may include a different number of mirrors (eg, six or eight mirrors).

The substrate W may include previously formed patterns. In this case, the lithography device LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure well below 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 generated plasma (LPP) source, a discharge generated plasma (DPP) source, a free electron laser (FEL) or any other radiation source capable of generating EUV radiation.

Thermal deformation can occur in the mirrors in the projection system PS of the EUV lithography apparatus LA. In the meaning of the present invention, thermal deformation of an optical element refers to the deformation of the optical element due to a thermal load applied to the element. The thermal load can be, for example, the application of a radiation beam to the optical element, whereby the energy of the radiation beam is at least partially absorbed by the optical element. This thermal load can cause a temperature rise, eg a local temperature rise of the optical element, which causes deformation of the optical element, eg expansion. More generally, thermal deformation can occur in components in EUV lithography apparatus LA. It should be understood that although the following description is generally directed to one or more mirrors in the projection system PS in EUV lithography apparatus LA, the described methods are also applicable to other components in EUV lithography apparatus LA and such as DUV lithography apparatuses other components in other lithography devices. For example, a component can be an optical element, a mirror, a projection system mirror, an illumination system mirror, a lens, a projection system lens, an illumination system lens.

To reduce thermal deformations that cause aberrations in the projection system PS, the mirror material can be optimized for minimal deformation, eg, using ultra-low expansion (ULE) materials. In general, a material with a coefficient of thermal expansion CTE, which is temperature dependent, can have a CTE of zero or near zero at a particular design temperature. Alternatively or additionally, a material with a temperature-dependent CTE may have a CTE that has a minimum value in a particular operating range, eg, other than zero. Materials with zero or near-zero CTE can exhibit a quadratic expansion relationship with temperature that has near-zero-expansion properties around their design temperature, which is referred to as the zero-crossing temperature (Tzc or ZCT). The mirror top portion (the portion of the mirror close to the side of the mirror on which the radiation beam B' is incident) can be kept as close to this Tzc as possible to minimize distortion. In particular, the mirror surface or mirror top portion is designed and shaped in a specific way to ensure proper patterning or exposure procedures. Due to the energy of the radiation beam absorbed by the mirror (optical element in general), local deformation of the mirror surface or top portion may occur. Due to the application of stronger or more aggressive thermal loads (higher power, more extreme lighting conditions) it is not possible to keep the full mirror top section around this sweet spot, i.e. at or near Tzc or ZCT temperature. The mirror material can be made of other materials with relatively low or very low coefficients of thermal expansion (CTE), such as ZERODUR or cordierite. In general, the coefficient of thermal expansion (CTE) can be considered to describe how the size of an object changes with the temperature of the object. As an example, an object of length L will experience a linear expansion ΔL due to a change in temperature ΔT according to: ΔL = L·α·ΔT, where α = linear coefficient of thermal expansion. In the present invention, the coefficient of thermal expansion is used to designate the thermal deformation of the optical element, in particular the deformation of the surface of the optical element, due to the temperature fluctuations that occur.

Thermal deformation of optical elements such as mirrors is generally very sensitive to localized (usually non-uniform) irradiation (thermal loading) and (localized) zero-crossing temperature (Tzc). The specific irradiation provides a uniform temperature across the material surface of the optical element, which results in a well-corrected curvature of the optical surface, ie, the strain in the mirror material. On the other hand, non-uniform irradiation (different specific irradiations) can provide isolated regions of relatively elevated temperature that result in the primary uncorrectable thermal deformation of the optical surface, ie the strain in the mirror material.

A nonlinear thermal expansion curve can be shown for the zero crossing temperature Tzc (ZCT). For each curve, there may be a valley at Tzc, and thus, the curve may be relatively flat around Tzc. This means that at or near Tzc, there is relatively little thermal expansion.

The thermal deformation may be larger for the example Tzc compared to the other example Tzc because the specular environment may be maintained at a (reference) temperature (eg, about 22°C). Therefore, thermal deformation can be extremely sensitive to a specific Tzc.

Manufacturing tolerances have a great influence on the spatial variation of Tzc. This can result in manufacturing tolerances for the average Tzc. In other words, a material designed to have a specific Tzc (ie, the temperature at which thermal expansion is zero or has a minimum value) will have a variation in that Tzc across the material due to manufacturing tolerances, ie the local Tzc may traverse the material or be determined by Optical elements made of this material change.

The spatial variation of Tzc can be measured, for example, on a mirror surface. There may be a temperature change in °C (or in Kelvin). Spatial variations in the Tzc of typically several Kelvins can exist in the mirror. In some projection systems PS of some lithography apparatuses LA, some mirror surfaces may have specific variations, while other mirror surfaces may have different variations. The spatial variation of Tzc can be calculated by, for example, measuring the coefficient of linear thermal expansion (CTE) of the material by using ultrasound. The CTE or CLTE of a material of length L is 1/L×dL/dT.

This non-uniform Tzc has a great influence on the performance of the projection system PS. This effect can be shown by comparing the optical key performance indicators for uniform Tzc and non-uniform Tzc.

In EUV lithography apparatus LA, incoming electromagnetic (EM) waves (ie, EUV radiation) interact with the mirror surface, with some of the radiation being reflected and some being absorbed. Absorbed radiation is dissipated within the mirror, which causes the mirror to heat up. The thermal system is conducted within the mirror and thus the temperature of the mirror varies, eg increases, over time. Temperature changes cause thermal deformation of the mirror surface, and thermal deformation causes wavefront aberrations. Such wavefront aberrations may need to be corrected in the lithography device LA.

With increasing EUV radiation source power, specular heating becomes an increasing problem in EUV lithography apparatus LA. In particular, the mirror surfaces of projection optics (eg projection system PS) can heat up excessively, deform and cause aberrations that cannot be adequately corrected.

In embodiments of the present invention, methods of thermally deforming an optical element comprising a material whose coefficient of thermal expansion is temperature dependent are provided to at least partially mitigate the effects of such deformation or aberrations. According to the present invention, the method may comprise the following steps: - transferring heat to or from the optical element to establish a temperature set point for the optical element such that the coefficient of thermal expansion has a non-zero value, and - heating the optical element to produce thermal deformation of one of the optical elements.

In one embodiment, the temperature set point is established such that the coefficient of thermal expansion has a predetermined non-zero value.

In one embodiment, heating includes selectively heating.

In one embodiment, selectively heating an optical element may include heating one or more of the plurality of regions of the optical element to induce thermal deformation at the one or more of the plurality of regions of the optical element.

To heat the optical element (eg, one or more of the regions of the optical element), a heater may be applied. The heater may, for example, comprise a source of electromagnetic waves that produces thermal deformation of, for example, one or more of the plurality of zones of the optical element.

Typically, optical elements as used, for example, in EUV lithography devices or the like will be made of materials having a temperature-dependent coefficient of thermal expansion. In particular, a material can be selected or designed to have a thermal expansion coefficient that has zero crossings, is close to zero in a particular temperature range, or has a temperature-dependent minimum value.

Typically, optical elements are designed in such a way that, during normal use or operation, the temperature of the optical element is close to the zero crossing, or in a temperature range in which the coefficient of thermal expansion is close to zero or has a minimum value. Hereby, we aim to operate the optical element in a state in which temperature changes (eg local temperature changes) of the mirror surface cause only limited deformation of the optical element (eg the reflective surface of the optical element).

According to the invention, heat is transferred to or from the optical element in order to set a temperature of the optical element at which the thermal expansion coefficient has a non-zero value, eg a predetermined non-zero value. Thus, the present invention employs a different approach, rather than operating the optical element at a temperature at which deformation is minimized, to intentionally select a temperature set point where the optical element is sensitive to thermal deformation. Establishing this temperature of the optical element can be achieved, for example, using a heat exchanger, such as a heat exchanger that is at least partially integrated in the optical element. In particular, the optical element may, for example, be provided with one or more channels configured to receive a conditioning fluid for thermally conditioning the optical element. More details on thermal conditioning of optical elements are provided below.

When this temperature set point is established, this can be achieved, for example, by heating or cooling the optical element to a temperature one or more degrees Celsius away from the ZCT, which can be achieved by selectively heating the optical element to establish a specific desired deformation of the optical element. In an embodiment, such selective heating may include selectively heating one or more of a plurality of zones of the optical element. This selective heating can be achieved using heaters, as will be described in detail below. A specific desired thermal deformation of the optical element can be established by selectively heating one or more zones of the optical element, for example using a heat exchanger, and establishing a temperature of the optical element at which the coefficient of thermal expansion has a non-zero value.

The present invention thus provides a specific combination of heating/cooling of the optical element such that the desired sensitivity with respect to thermal deformation is established and used to produce a specific desired thermal deformation of the optical element, eg selected to at least partially offset or compensate for the application therein Aberrations occurring in the optical system of the optical element (eg the projection system in the lithography apparatus LA).

The present invention may also be embodied in a device configured to perform the aforementioned method according to the present invention.

In one embodiment, the apparatus may include: at least one optical element comprising a material whose coefficient of thermal expansion is temperature dependent, a heat exchanger configured to transfer heat to or from the optical element to establish a temperature set point for the optical element such that the coefficient of thermal expansion has a non-zero value, and a heater configured to heat the optical element to produce a thermal deformation of the optical element.

The mirror surface (eg in the projection system PS) may be exposed to EUV radiation during operation of the EUV lithography device LA. The (pre)heater can establish a constant thermal load to the mirrors 13, 14 irrespective of the presence and spatial distribution (illumination pattern) of EUV radiation.

Typically, EUV radiation is incident at different locations on the mirror surfaces 13, 14, so that there is a spatially non-uniform thermal load. The EUV radiation may, for example, use a bipolar illumination mode so that certain parts of the mirrors 13, 14 are not hit by the EUV radiation. Additionally, EUV radiation may be turned on at some times and turned off at other times. A pre-heater can be used to heat the mirrors 13, 14 so that the spatial heat load distribution at the mirrors stabilizes over time. One particular space heat load distribution may be uniform and another space heat load distribution may be non-uniform. Some pre-heaters may attempt to maintain the average temperature of the mirror's optical footprint (ie, the portion of the mirror's optical surface that receives and reflects EUV radiation in use) at some predefined value. In the absence of either EUV radiation, the temperature will be uniform inside the coverage area, which requires spatially non-uniform radiation due to boundary conditions. This mirror preheating attempts to control the average temperature of the optical footprint of the mirrors 13, 14 to minimize aberrations. The pre-heater can be an IR heater so that the radiation will not affect the imaging on the substrate W. A heater 16 (eg, a preheater) for the mirror 14 is shown in FIG. 1 . It will be appreciated that there may be a plurality of heaters and that there may be one or more heaters for each mirror or more generally each optical element.

Such use of heaters or preheaters is therefore intended to avoid minimising the combined heat load caused by the heater and EUV radiation by applying a specific heat load (such as a specific heat load distribution, such as mirror 13 ). , 14) deformation) to deform the mirror surfaces 13, 14.

FIG. 2 depicts a schematic view of the mirror surface 14 of the projection system PS of the lithography apparatus LA with the heater 16 . The heater 16 may have a relatively high spatial resolution, for example, the heater may be a barcode scanner or a zoned heater with a relatively large number of zones 16A. For example, a zone heater may have a range of, for example, 10 to 200 zones or 20 to 100 zones. The upper limit of the partition can be motivated from the degrees of freedom (orders of magnitude) of the illuminator IL. Zones 16A of heater 16 allow different portions of mirror 14 to be heated with different amounts of heat and/or at different times. To achieve this, partition 16A may be provided with different power levels over time. That is, the power used for one or more of partitions 16A may be modified to provide different levels of heat to different portions of mirror 14, as depicted by the downward facing arrows of different lengths in FIG. Partitions 16A may be of the same physical size or may be of different sizes. Partition 16A is shown as a box to illustrate an array-like configuration, which is 2D in space due to irradiation. However, it should be appreciated that in other embodiments, the partitions may have any suitable shape.

Heater 16 includes a source of electromagnetic waves (eg, IR) to heat mirror 14 . It will be appreciated that in other embodiments, the heater may heat the mirror in other ways, for example the heater may be embedded within the mirror, and more particularly a grid of resistive wires may be present in the mirror.

Mirror 14 has an optical surface 14A configured to reflect EUV radiation. Optical surface 14A may include a reflective (multi-layer) coating.

The mirror 14 has a temperature set point, eg (T _{sp , sh} ), where "sp" is the set point and "sh" is zone heating. More specifically, there are one or more temperature set points for one or more of the plurality of zones of mirror surface 14 . For example, location A corresponding to one zone of mirror 14 may have a different temperature set point than position B corresponding to another zone of mirror 14 . Heater 16 heats one or more localized sections of optical surface 14A of mirror 14 to heat regions of mirror 14 . As an example, if there are 100 partial segments of the optical surface of a pupil ^- like mirror, then for a mirror with a diameter of about 80 mm of optical footprint, it may have partial segments of the optical surface that are about 50 mm in size. For larger mirrors, this size can be about 500 mm ² .

Mirror 14 includes water cooling. At least one channel (not shown) is provided in mirror 14 for passing cooling water 18 or more generally cooling fluid through mirror 14 . Cooling water 18 may be input to mirror 14 at a temperature _{Tw , in} = _Tmf , and may exit from mirror 14 at a temperature _{Tw , out} greater than _Tmf after heat extraction from mirror 14. _Tmf is the ambient temperature (eg, about 22 ^° C) of the mirror 14 made of ULE material. More generally, the cooling water 18 can be viewed as a heat sink configured to transfer heat from the optical element (mirror 14). Cooling water 18 heats (convects) and transports heat away from mirror 14 . In other embodiments, it will be appreciated that the heat sinks may take different forms.

Cooling water 18 may, for example, reduce steady-state deformation due to reduced steady-state temperature difference (ΔT _ss ) and also reduce temperature intra-wafer drift due to Tzc reoptimization and reduced thermal penetration. Steady-state deformation can be viewed as deformation of the mirror surface during normal use whereby EUV-induced thermal loads are applied to the mirror surface. This steady-state deformation can be caused, for example, by a temperature difference of the mirror surfaces (for example, referred to as a steady-state temperature difference). The steady-state temperature difference (ΔT _ss ) varies as a function of position (x,y) and in the uncooled case as a function of EUV-induced thermal load and boundary conditions. The EUV-induced thermal load is a function of: EUV radiation source power, lithography device LA duty cycle, reticle properties (pattern density, reflectivity), illumination pupil, EUV absorption coefficient of the mirror, and the edge of the mirror The position of the optical path (and the coupling between the mirror and the environment): ΔT _ss,mx = max(ΔT _ss (x,y|Q _euv , pupil)).

As mentioned previously, the zero-crossing temperature (Tzc) varies spatially due to material inhomogeneity. The change in Tzc of the portion below the coolant (ie, the cooling water 18 ) can be considered as not compared to the change in Tzc of the portion between the coolant (ie, the cooling water 18 ) and the optical surface 14A of the mirror 14 . too relevant. The coolant design is assumed such that thermal interference of the bulk below the coolant is negligible. This means that the zero-crossing temperature (Tzc) change in the other part of the mirror 14 is less critical than the zero-crossing temperature change in the top part (ie, between the coolant and the coating of the optical surface 14A). The zero-crossing temperature variation of the top part can be calibrated on the lithography device LA. The change in Tzc within the section between coolant and coating means in principle the dissimilar actuator sensitivities between any two positions (eg position A and position B). This requires calibration. It should be noted that actuator sensitivity in this regard refers to the sensitivity for thermal actuation of a specific portion or region of a mirror or optical element, and thermal actuation refers to applying a thermal load (eg, thermal load) to create or obtain deformation. Thus, a portion of the mirror with a Tzc at or near the actual temperature will have a low actuator sensitivity and the portion with a Tzc away from the actual temperature of a portion of the mirror will have a high actuator sensitivity.

Previously, heaters have been used to heat the mirror so that the mirror is heated uniformly both spatially and over time to keep the thermal load constant. However, according to the present invention, the heater is configured to (selectively) heat the optical element to produce a specific thermal deformation of the optical element. A heater such as heater 16 as shown may be used for this purpose. That is, in accordance with the present invention, additional functionality has been added to heater 16 to intentionally induce a desired thermal deformation of mirror 14 by means of which aberrations caused by other mirrors or modules can be corrected. This can be viewed as thermal manipulation, activation or actuation of mirror 14 by heater 16, ie heater 16 can be viewed as a thermal manipulator or actuator. In one embodiment, a heater, such as heater 16, is configured to selectively heat one or more of the plurality of zones of the optical element within a heating activation range, such as in the plurality of zones of the optical element A specific thermal deformation of the optical element occurs at the one or more of these. The heating activation range can be viewed as a specified range of heat that the heater 16 can provide to the mirror 14 (and generally to the optical element). The heating activation range may be related to the minimum and maximum power limits of the heater 16, which may be set. The heaters may have different heating activation ranges for different zones 16A of the heaters 16 . Implementations with similar ranges for each partition 16A may be selected. However, some partitions 16A can be designed to have less power, for example because the local EUV loading is orders of magnitude smaller compared to other spots on the mirror, or because that spot is irrelevant from the aberration manipulator perspective. The heating activation range can be expressed, for example, as the available power density, eg in W/mm2, which can be applied by the heater to different regions of the optical element.

The partial sections of the optical surface 14A of the mirror surface 14 correspond to a plurality of regions of the mirror surface 14 . Heater 16 heats one or more local sections of optical surface 14A of mirror 14 corresponding to the plurality of sections of mirror 14 .

Regions of mirror 14 may be defined between optical surface 14A of mirror 14 and the coolant (or more precisely, passages for cooling water 18). This is because the change in the zero-crossing temperature of the material on the side of the cooling water 18 opposite the optical surface 14A of the mirror 14 can be considered less relevant than in the regions of the mirror 14 . Thermal interference of the mirror surface 14 on the side of the cooling water 18 opposite the optical surface 14A of the mirror surface 14 can be considered negligible and can be disregarded.

As can be seen in FIG. 2 , the steady-state temperature difference ΔT _ss (x,y|..), which varies according to the position (x,y) (ie, the position across the optical surface 14A of the mirror 14 ), is derived from the cooling water 18 position obtained. That is, the steady-state temperature of mirror 14 is taken from a depth z corresponding to the location of cooling (eg, 5 mm from the optical surface of mirror 14).

Cooling water 18 is configured to transfer heat from mirror 14 (to ambient) to establish a temperature set point for mirror 14 . More specifically, one or more temperature set points for one or more of the plurality of zones of the optical element can be established. The one or more temperature setpoints may be set away from the zero-crossing temperature (Tzc) of the material of mirror 14 such that the sign of the coefficient of thermal expansion (CTE) is within most or all of the heating activation range of heater 16 for mirror 14 The plurality of regions are the same. The cooling water 18 acts as a heat sink. The zero-crossing temperature of the material is the local zero-crossing temperature of each of the plurality of regions of mirror surface 14 . That is, all temperature set points are defined at equal offsets from the local zero-crossing temperature. This can provide similar nanometer power sensitivity for each partition 16A. In other embodiments, the zero-crossing temperature of the material can be considered as the average zero-crossing temperature of the mirror surface 14 (ie, the average of all the regions of the mirror surface 14).

In one embodiment of the invention, the temperature of the optical element is set to a specific temperature set point, eg using a heat exchanger, resulting in the optical element or at least a substantial portion thereof having a (predetermined) non-zero coefficient of thermal expansion. In one embodiment, a cooling water configuration as schematically shown in Figure 2 may be used to set the temperature set point. In general, however, in accordance with the present invention, a heat exchanger as applied may be used to cool the optical element or to heat the optical element in order to set the temperature of the optical element to a point where the material of the optical element has the desired predetermined non-zero coefficient of thermal expansion. temperature. In one embodiment, such a heat exchanger may include one or more channels configured to receive a conditioning fluid (ie, a fluid for thermally conditioning the optical element). In one embodiment, multiple distinct channels may be used whereby each channel may be provided with a different conditioning fluid, eg at a different temperature. In one embodiment, the heat exchanger comprises, for example, a grid of resistive wires integrated in the optical element. By applying one or more suitable voltages to the grid, a specific desired thermal load can be created in the optical element, resulting in a desired temperature or temperature distribution of the optical element. Thereby, one or more temperature set points can be implemented in the optical element using the method or apparatus according to the invention. In one embodiment of the invention, the heat exchanger as used is at least partially integrated into the optical element. Examples of such configurations may include the use of a grid of channels or resistive filaments in the optical element.

In an embodiment, one or more temperature set points as applied using the present invention in an optical element such as mirror 14 may be established in the range of 1 to 8°C from the zero crossing temperature Tzc. A uniform ULE material may have a +/- 1C tolerance, so it may be desirable to set the temperature set point at approximately 1°C from the zero crossing temperature Tzc. As another example, the temperature set point may be set at approximately 4°C from the zero crossing temperature Tzc. Thereby, we establish that the at least sign of the thermal expansion coefficient of substantially the entire optical element, or at least the thermally conditioned part, is the same.

Figure 3 shows a curve illustrating this. The graph shows the localized specular optical surface 14A temperature (x-axis) relative to the deformation (y-axis) of the optical surface 14A perpendicular to the mirror 14 . As can be seen from the lowest temperature plotted, the deformation initially decreases with increasing temperature (i.e., has a negative thermal expansion coefficient) before reaching a minimum value (i.e., at temperature Tzc, where the thermal expansion coefficient is zero), And then the deformation increases with gradually increasing temperature (ie, has a positive coefficient of thermal expansion). The zero crossing temperature (Tzc), where the ULE material has nearly zero expansion properties, coincides with the minimum point. The main (bold) line M can be considered to be for the mean zero crossing temperature Tzc. The dashed lines U, L in the curve corresponding to the main line M of the curve show the upper U and lower L limits due to the change in the zero-crossing temperature of the material (δT _zc (T)). The variation of the zero-crossing temperature Tzc in the material is intrinsic and deterministic. It can be seen that the temperature set points T _{sp , sh} (q _sh ) of the mirror 14 and the cooling or conditioning temperature T _{w , in} are both on the same side (ie, on the same side of the parabola) with respect to the zero crossing temperature Tzc. In this case, both the temperature set points T _{sp , sh} (q _sh ) and the cooling temperature T _{w , in} of the mirror surface 14 are greater than the zero crossing temperature Tzc. It will be appreciated that in other embodiments, the temperature set points T _{sp , sh} (q _sh ) and the cooling temperature Tw _{, in} of the mirror surface 14 may both be less than the zero-crossing temperature _Tzc , ie, both are different from those shown in FIG. 3 . Zero-crossing temperature Tzc on one side and the other side. By transferring heat to or from the mirror 14 (optical elements in general), we can set the temperature of the mirror 14 to a (desired) value according to the present invention, ie, whereby the mirror has a (predetermined) non-zero value. The value of the coefficient of thermal expansion.

As will be appreciated, the cooling temperature _{Tw , in} of the mirror as mentioned above can be considered as an example of adjusting the temperature at which the optical element is located, eg by means of the aforementioned heat exchanger. Therefore, the cooling temperature _{Tw , in} can also be referred to as the adjustment temperature _{Tw , in} of the optical element.

In Figure 3, the coolant or conditioning temperature Tw _{, in} is slightly higher than the zero-crossing temperature _Tzc as shown. This setting may be better for the coolant temperature T _{w , in} on the other side of the zero crossing temperature Tzc than the temperature set point T _{sp , sh} (q _sh ). This may be because it ensures that the minimum point of thermal expansion (ie, at Tzc, where the thermal expansion coefficient is zero) will not reach and thus the sign of the thermal expansion coefficient of the region of mirror 14 will be different from that of other regions of mirror 14.

The higher flow rate of the cooling water 18 means more disturbance to the mirror surface 14 and it introduces aberrations. Therefore, in practice, it is best to minimize the temperature set point T _{sp , sh} (q _sh ) as much as possible so that there is sufficient start-up range (to implement the necessary thermal manipulation). Once a certain section 16A of the heater 16 is turned off, it means that the heating activation range has been exceeded for that particular section of the optical coverage area.

There is a section of the curve in which the slope of the line M indicates a linear or quasi-linear relationship between deformation and temperature (see straight line S depicting this). That is, there is an approximately linear dependence between the temperature within most or all of the heating activation range (ΔT _sh ) of the (zone) heater 16 and the thermal deformation of the zone of mirror surface 14 . The vertical dashed line shows the range of the heating activation range (ΔT _sh ) of the (zone) heater 16 . The maximum temperature excursion ΔT _ss,mx between the coolant (ie, cooling water 18 ) and the front side (ie, optical surface 14A) is determined by the highest local power density among the relevant EUV loads for a given source power. The upper limit of the manipulator control range ΔT _sh is determined by the maximum actuation range required to correct worst-case conditions for EUV heating or no heating induced aberrations.

The temperature set point (T _sp,sh (q _sh )) is established in the substantially linear dependence section of the curve, where q is the heat flux from zone heater 16 in watts per square meter. It can be seen that the temperature set points T _{sp , sh} (q _sh ) are located sufficiently far from the zero-crossing temperature (Tzc) such that the thermal expansion coefficient is positive for the entire heating activation range of the zone heater (heater 16 ). It should be appreciated that in other embodiments, this may not be the case for all of the heating activation range ΔT _sh and may only be the case for a majority of the heating activation range.

It can be seen that the temperature set point T _{sp , sh} (q _sh ) is located sufficiently far from the zero-crossing temperature (Tzc) such that the thermal expansion coefficient is positive for a complete change δT _zc (T) of the zero-crossing temperature of the material. Therefore, even if the zone 16A of zone heater 16 is set at the lowest power and the spatial variation of the zero-crossing temperature Tzc due to a particular zone of mirror 14 is at its highest possible value, the minimum point of the coefficient of thermal expansion will not be reached (also That is, Tzc) and thus the sign of the coefficient of thermal expansion of the region of mirror 14 will not be different from other regions of mirror 14 . It should be understood that if the mirror temperature corresponding to a particular region of mirror 14 is such that Tzc is reached, and is exceeded, then the sign of the coefficient of thermal expansion will be different when compared to regions where Tzc is not reached and not exceeded, and The deformation will be the opposite for that region.

FIG. 4 depicts another curve showing localized specular optical surface 14A temperature (x-axis) relative to the deformation of optical surface 14A perpendicular to mirror 14 (y-axis). Except that the temperature set points T _{sp , sh} (q _sh ) and the cooling temperature Tw _, in of the mirror surface 14 are on different sides (ie, on different sides of the parabola) with respect to the zero-crossing temperature _{Tzc in} the graph of FIG. 4 , The curve of FIG. 4 is similar to the curve of FIG. 3 . In this case, the temperature set point T _{sp , sh} (q _sh ) is greater than the zero crossing temperature Tzc and the cooling temperature T _{w , in} of the mirror surface 14 is less than the zero crossing temperature Tzc. More specifically, 0 on the x-axis corresponds to the temperature _{Tw , in} of the cooling water 18 at the entrance of the mirror 14 . _{Tw , in} is preferably close to or equal to the temperature ( _Tmf ) of the mirror surface 14 (made of ULE material).

In this embodiment, the temperature set points T _{sp , sh} (q _sh ) are still far away from the zero-crossing temperature Tzc of the material of the mirror 14 such that the coefficient of thermal expansion (CTE) has a sign over a large portion of the heating activation range of the heater 16 or Most or all of the material for the regions of mirror surface 14 is the same throughout. In an embodiment, the few materials of the regions of mirror surface 14 may not have the same sign of coefficient of thermal expansion (CTE). This may be the portion of the material closest to the cooling water 18 .

In an embodiment, the temperature of the cooling water may be equal to the provided mean zero-crossing temperature Tzc, with one or more temperature set points sufficiently far from the mean zero-crossing temperature Tzc (eg, about +4C) such that the optical surface of the mirror and the cooling water Most of the materials in between have the same signed coefficient of thermal expansion (CTE). In this case, the portion of the material closest to the cooling water may not have the same sign thermal expansion coefficient. This situation may not be preferred since it may require additional power when compared to the situation where the temperature of the cooling fluid is set away from the zero crossing temperature Tzc.

In general, one or more temperature set points for one or more of the plurality of zones of the optical element are established away from the zero-crossing temperature Tzc of the material of the mirror 14 such that the sign of the coefficient of thermal expansion (CTE) is at the heater 16 Most or all of the heating activation range is the same for these zones of mirror 14 . In some embodiments, one or more temperature set points may be set such that the coefficient of thermal expansion has a value that is substantially similar for the region of mirror 14 over most or all of the heating activation range of heater 16 . That is, the thermal expansion coefficients of the regions of the mirror surfaces 14 may have comparable values--deformed to comparable magnitudes between the regions of the mirror surfaces 14 .

The steady state temperature set point T _{sp , sh} (q _sh ) during calibration is established at or near the middle of the heating activation range of the heater 16 . A temperature set point T _{sp , sh} (q _sh ) for the mirror surface 14 is established using the cooling water 18 that transfers heat from the mirror surface 14 . The temperature of the cooling water 18 ( _{Tw , in} ) may be set at a specific amount to establish the desired temperature set point T _{sp , sh} (q _sh ) for the mirror surface 14 . The coefficient of thermal expansion (CTE) is a function of the temperature of the mirror surface 14 and the temperature of the mirror surface 14 is a function of the temperature of the cooling water 18 . The temperature set point T _{sp , sh} (q _sh ) can be established by setting the temperature (Tw _{, in} ) of the cooling water 18 sufficiently far from the zero crossing temperature ( _Tzc ) of the material of the mirror surface 14 . The temperature of the cooling water 18 may be set in an example range of +/- 0.5 C, +/- 1 C, or +/- 2 C away from the zero-crossing temperature Tzc due to variations in the zero-crossing temperature Tzc.

If cooling water 18 is not used to limit the temperature of mirror 14, over time (ie, due to exposure to EUV radiation and/or heating by heater 16), the temperature of mirror 14 may increase so much that it exceeds the temperature of mirror 14 by The range of thermal manipulation, ie the temperature, of the heater 16 will drift.

The temperature set points T _{sp , sh} (q _sh ) are established such that the change in heat from the heater 16 causes the mirror 14 to shrink over the entire heating activation range of the heater 16 when compared to the thermal deformation around Tzc. The relatively large change in thermal deformation of the zone. Relatively large changes in thermal deformation can be in the range of -10 to +10 nm. For optical element manipulation, the Rennick actuation step size between substrates can be approximately 0.01 nm. The temperature set point can be considered to be located sufficiently far from the zero crossing temperature Tzc that the slope of line M in this section of the curve is relatively steep when compared to the slope of line M in this section of the curve around Tzc.

The temperature set points T _{sp , sh} (q _sh ) are established such that the relationship between thermal load and deformation is sensitive enough to provide a relatively good range of thermal manipulation. Larger deviations between the temperature set point and the zero-crossing temperature can result in larger sensitivity. That is, a temperature set point further away from the zero-crossing temperature (away from the lower part of line M of the curve) means that for the same temperature increase, the deformation will be larger. This is advantageous because it means that a relatively large amount of thermal manipulation can be achieved for a relatively low power increase to zone 16A of zone heater 16 .

The amount of thermal deformation of mirror 14 for a particular power to zone 16A may depend on the depth of cooling and the size of zone 16A. For example, with a cooling depth of 10 mm and a partition size of 10 mm x 10 mm, a sensitivity of 0.5 nm/W can be found for ULE materials. Nanoscale distance reflects surface topography deformation.

The temperature set point T _{sp , sh} (q _sh ) may be based on the zero-crossing temperature provided or determined by the material of the mirror 14 . That is, the zero-crossing temperature has been measured, estimated or modeled. The method may include characterizing the distribution of the zero-crossing temperature Tzc of the optical surface 14A of the mirror 14 by modeling. That is, the local zero-crossing temperature Tzc of the material of mirror 14 can be determined individually and then used as a starting point to establish a temperature set point.

Once the desired temperature set point T _{sp , sh} (q _sh ) has been reached, the additional degrees of freedom of heater 16 must be calibrated. For each zone of heater 16, the corresponding wavefront error must be measured using a wavefront sensor (not shown). Wavefront sensors can perform wavefront aberration measurements that take into account multiple degrees of freedom, such as the number of mirrors, the position and orientation of mirrors, and not only mechanical behavior. Therefore, calibration is required to determine how thermal manipulation by heater 16 affects the wavefront error.

More generally, heater 16 may be calibrated using corresponding optical measurements obtained by optical measurement devices, such as wavefront sensors. The plurality of zones 16A of zone heater 16 may be calibrated using corresponding optical measurements obtained by optical measurement devices, such as wavefront aberration measurements obtained by a wavefront sensor.

The relationship between the heat provided from heater 16 to one or more of the plurality of zones of mirror 14 and the resulting thermal deformation of the corresponding one or more local zones of optical surface 14A of mirror 14 needs to be determined.

The method may include sequentially modifying the power for each of the plurality of zones 16A of zone heater 16 and making corresponding optical measurements. That is, the power is changed one by one for each of the plurality of zones 16A of zone heater 16 . This creates a sensitivity curve, ie, how the deformation varies with the varying power of the zone 16A (or activation area). Taking optical measurements allows a determination of how the wavefront is changed by changes in the temperature of the corresponding region of the mirror 14 . The relationship between the power to zone 16A of zone heater 16 and the corresponding optical measurements can be used for thermal manipulation of the wavefront of mirror 14 .

In general, it can be mentioned that the relationship between the application of a specific thermal load to a specific partition or part of an optical element and the deformation of the optical element will depend on a number of parameters. In other words, in addition to the actual thermal load as applied, the deformation of the optical element or a portion thereof will also be a function of various parameters. In order to achieve accurate thermal actuation of optical elements, eg to correct or compensate for aberrations, these parameters may need to be taken into account. An example of this parameter worth mentioning is the actual CTE or CTE distribution of the optical element. As mentioned, due to manufacturing tolerances, the CTE of the material of the optical element may not be homogeneous, but may vary across the optical element. This parameter can be taken into account by applying a specific thermal load to the optical element, eg using a heater as described above, and observing the corresponding deformation. It can be pointed out that one may wish to determine the actual CTE or CTE distribution for different thermal conditions of the optical element. In particular, the CTE or CTE distribution can be determined, for example, depending on the temperature of the optical element. Thus, for different temperatures of the optical element, the CTE or CTE distribution can be determined, for example, as part of a calibration or initialization procedure. As a practical example, the CTE or CTE distribution of an optical element can be determined for different temperature set points used to control a heat exchanger for transferring heat to or from the optical element. Due to the use of materials with a temperature-dependent CTE, the effect of applying a particular thermal load will therefore depend on the actual temperature of the optical element.

It will be appreciated that in other embodiments, the calibration may be performed differently.

In an embodiment, a check may be performed to verify that modifying one or more of the power of zones 16A of heater 16 to heat one or more zones of mirror 14 may not substantially change (ie, heat and deform) Other areas of the mirror 14 that are specifically heated. If this were the case, it would be difficult or impossible to obtain the necessary thermal actuation to correct for aberrations and the like. May need to have separate partitions. If the crosstalk of adjacent or adjacent partitions is high, it means that the responses of adjacent or adjacent partitions also need to be considered when actuating a given partition. This means that the crosstalk etc. will need to be calibrated.

During batch production (exposure) (ie, during operation of lithography apparatus LA), the calibrated relationship can then be used to manipulate the waves of mirror 14 (or mirrors) by inducing corresponding thermal deformations via heater 16 forward. A calibrated relationship between the heat from heater 16 and the resulting thermal deformation of one or more of the plurality of regions of mirror 14 may be used to introduce corresponding thermal deformation to mirror 14 via heater 16 during use of lithography device LA. Optical surface 14A. This can be used to correct for example aberrations caused by other optical elements of the lithography device LA. As an example, power provided to zone 16A of zone heater 16 in the range 5 W to 10 W may result in a 1 nm change in the coefficient of the Rennick wavefront.

FIG. 5 depicts a flowchart 100 of a method of thermally deforming the mirror surface 14 . That is, one or more regions of mirror 14 are heated to thermally manipulate mirror 14, eg, to correct for wavefront aberrations.

In step 102 , the temperature set point T _{sp , sh} (q _sh ) is determined based on the provided or determined zero-crossing temperature of the material of the mirror 14 . An alternative strategy could be to define the temperature Twin of the cooling water 18 to be slightly above the zero crossing temperature Tzc, then calculate the required temperature set point (depending on source power, illumination pupil, etc.), and add a thermal range for manipulation-- This is for the example shown in the graph of FIG. 3 . In general, step 102 may include transferring heat to or from the mirror or optical element in order to adjust the mirror to a temperature corresponding to a temperature set point, whereby the mirror has a (predetermined) non-zero coefficient of thermal expansion. In one embodiment, the temperature set point(s) are selected away from the zero-crossing temperature (Tzcs) such that the coefficient of thermal expansion (CTE) has a sign within the heating activation range of heater 16 (eg, most or all of the heating activation range). ) are the same for the plurality of regions of the mirror surface 14. The CTE may have a value that is substantially similar for the plurality of zones of mirror surface 14 within the heating activation range of heater 16 (eg, most or all of the heating activation range).

In step 104 , the temperature of the cooling water 18 is set to establish a temperature set point for the mirror surface 14 . The temperature set point can be far away from the Tzcs so that the relationship between thermal load and deformation has sufficient sensitivity in the heating activation range.

In step 106 , one or more of the plurality of zones of the mirror surface 14 are heated using a heater 16 having a heating activation range to thermally deform the mirror surface 14 .

In step 108, the heater 16 is calibrated using the corresponding wavefront aberration measurements obtained by the wavefront sensor.

In step 110, calibrated heater 16 is used during EUV radiation exposure of mirror surface 14 to manipulate the wavefront during operation of lithography device LA.

In the present invention, a method for establishing thermal deformation of an optical element is proposed. In the present invention, the temperature-dependent thermal expansion coefficient is assumed to exist for the optical element. Figure 6 schematically illustrates the thermal expansion coefficients of various materials. Curve A in Figure 6 illustrates a material having a coefficient of thermal expansion (CTE) that is substantially independent of temperature T, ie the CTE of such a material does not vary with temperature T. Curve B in Figure 6 illustrates a material with a coefficient of thermal expansion (CTE) that is temperature dependent. In detail, the material has a CTE with a minimum value at temperature T1.

Curve C in Figure 6 illustrates a material with a coefficient of thermal expansion (CTE) that is also temperature dependent. In detail, the material illustrated by curve C has a minimum CTE at temperature T2 and a CTE equal to zero at temperature T3. Both the materials illustrated by curves B and C can be advantageously used in the present invention. 3 and 4, we can for example consider that the temperature T _ZC in Fig. 3 or 4 will correspond to the temperature T3 in Fig. 6; the deformation illustrated in Figs. The CTE of curve C was obtained for the material case.

According to the invention, heat or thermal load is transferred to or from an optical element (eg an optical element made of a material having a CTE according to curve B or C), thereby bringing the optical element to a (desired) temperature , with an associated temperature-dependent CTE. According to the invention, the temperature of the material of the optical element as obtained by applying heat or thermal load is such that the material behaves according to a (predetermined) non-zero CTE. The heat or thermal load as applied (eg using the heat exchanger described above) can thus enable control of the CTE of the material of the optical element. Thereby, we control the sensitivity of the optical element to thermal actuation, such as that performed by a heater such as a barcode scanner or heater or zone heater. The present invention is thus provided in a versatile and flexible manner to address aberrations occurring in optical systems, such as projection systems of lithographic devices. In cases where only minor aberrations need to be compensated or corrected, it may be advantageous to set the operating temperature of the optical element to a value whereby the sensitivity of the optical element to thermal actuation is relatively low, and when substantial aberrations need to be compensated or corrected When calibrating, it may be advantageous to set the operating temperature of the optical element to a value whereby the sensitivity of the optical element to thermal actuation is relatively high. In both cases, the applied heater can be used over a large portion of its hot start range, thus achieving a good resolution of the applied thermal load.

Although specific reference may be made herein to the use of lithography devices in IC fabrication, it should be understood that the lithography devices described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

Although specific reference is made herein to embodiments of the invention in the context of lithography devices, embodiments of the invention may be used in other devices. Embodiments of the invention may form part of a reticle inspection device, a metrology device, or any device that measures or processes an object such as a wafer (or other substrate) or a reticle (or other patterned device). Such devices may generally be referred to as lithography tools. This lithography tool can use vacuum conditions or ambient (non-vacuum) conditions.

Although the above may have made specific reference to the use of embodiments of the present invention in the context of optical lithography, it should be understood that the present invention is not limited to optical lithography, and may be used in other applications ( such as imprint lithography).

Embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof, as the context allows. Embodiments of the invention can also be implemented as instructions stored on a machine-readable medium, which can 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 (eg, a computing device). By way of example, machine-readable media may include: read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustic, or other forms of Propagated signals (eg, 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 understood, however, that these descriptions are for convenience only and that such actions are in fact caused by a computing device, processor, controller, or other device executing firmware, software, routines, instructions, and the like. And doing so enables actuators or other devices to interact with the physical world.

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

10: Faceted Field Mirror Device 11: Faceted Pupil Mirror Device 13: Mirror 14: Mirror 14A: Optical Surface 16: Heater 16A: Partition 18: Cooling Water 100: Flowchart 102: Step 104: Step 106: Step 108: Step 110: Step A: Position/Curve B: Position/EUV Radiation Beam/Curve B': Patterned EUV Radiation Beam C: Curve CTE: Coefficient of Thermal Expansion IL: Illumination System L: Dotted Line LA: Lithography Device M : main line MA: patterned device MT: support structure PS: projection system S: straight line SO: radiation source T1: temperature T2: temperature T3: temperature T _mf : mirror temperature T _sp,sh : temperature set point T _sp,sh (q _sh ): temperature set point _Tw,in : cooling temperature/coolant or conditioning temperature _Tw,out : temperature T _zc : zero-crossing temperature ΔT _sh : heating start range ΔT _ss,mx : steady-state temperature difference ΔT _ss,mx (x,y|..): maximum temperature excursion δT _zc (T): change in zero-crossing temperature of material U: dashed line W: substrate WT: substrate stage

Embodiments of the invention will now be described by way of example with reference to the accompanying schematic drawings in which: - Figure 1 depicts a lithography system comprising a lithography device and a radiation source; - FIG. 2 depicts a schematic diagram of a mirror and a heater of a lithography apparatus according to an embodiment of the present invention. - Figure 3 depicts a plot of local specular optical surface temperature versus deformation of an optical surface perpendicular to the mirror, according to an embodiment of the invention. - Figure 4 depicts a graph of local specular optical surface temperature versus deformation of the optical surface perpendicular to the mirror, according to an embodiment of the invention. - Figure 5 depicts a flow chart of the method of thermally deforming the mirror surface. - Figure 6 depicts the CTE curves of different materials as a function of temperature.

14: Mirror

14A: Optical Surface

16: Heater

16A: Partition

18: Cooling water

A: Location

B: Location

T _mf : mirror temperature

T _sp,sh : temperature set point

T _w,in : cooling temperature

T _w,out : temperature

△T _ss,mx (x,y|..): steady-state temperature difference

Claims (35)

A method of thermally deforming an optical element comprising a material whose coefficient of thermal expansion is temperature dependent, the method comprising: transferring heat to or from the optical element to establish a temperature set point for the optical element such that the coefficient of thermal expansion has a non-zero value, and The optical element is heated to produce thermal deformation of one of the optical elements. The method of claim 1, wherein a heat exchanger is applied to transfer heat to or from the optical element, and wherein a heater is applied to heat the optical element. The method of claim 1 or 2, wherein heating the optical element comprises selectively heating the optical element. 3. The method of claim 3, wherein selectively heating the optical element comprises selectively heating one or more of the plurality of regions of the optical element such that the one or more of the plurality of regions of the optical element where the thermal deformation occurs. The method of claim 4, wherein a sign of the coefficient of thermal expansion for the material of the one or more of the plurality of zones of the optical element over most or all of a heating activation range of the heater Most or all are the same. The method of claim 1 or 2, wherein the coefficient of thermal expansion is substantially zero at a zero-crossing temperature and/or has a local minimum as a function of temperature. The method of claim 2, wherein the heat exchanger is at least partially integrated into the optical element. The method of claim 7, wherein the heat exchanger includes a channel integrated in the optical element, the channel configured to receive a fluid for exchanging heat with the optical element. The method of claim 4, further comprising establishing the temperature set point such that a change in the heat from the heater results in the plurality of the optical elements over most or all of the heating activation range of the heater The one or more of the regions vary in one of the thermal deformations in a range of -10 nm to +10 nm. The method of claim 1 or 2, further comprising establishing the temperature set point such that the coefficient of thermal expansion has a value for a plurality of the optical elements over most or all of a heating activation range of the heater regions are substantially similar. The method of claim 1 or 2, further comprising establishing the temperature set point within a range of 1 to 8°C from a zero crossing temperature of the material. The method of claim 4, wherein a zero-crossing temperature of the material comprises a local zero-crossing temperature for the one or more of the plurality of regions of the optical element or an average zero-crossing temperature for the optical element. The method of claim 1 or 2, wherein the temperature set point is greater than a zero-crossing temperature of the material or the temperature set-point is less than the zero-crossing temperature of the material. The method of claim 4, wherein the optical element includes an optical surface and wherein a plurality of regions of the optical element are defined between the optical surface and the heat exchanger, and wherein the optical surface includes a region corresponding to the plurality of regions a plurality of local sections. The method of claim 14, further comprising determining the corresponding one or more localities of the heat provided by the heater to the one or more of the plurality of zones of the optical element and the optical surface of the optical element The relationship between the resulting thermal deformation of the segment. The method of claim 2, further comprising calibrating the heater using corresponding optical measurements obtained by an optical measurement device. The method of claim 16, further comprising calibrating the heater using corresponding wavefront aberration measurements obtained by a wavefront sensor. The method of claim 16, further comprising using the calibrated heater during EUV radiation exposure of the optical element to manipulate the wavefront during operation of a lithography device. The method of claim 1 or 2, further comprising determining the temperature set point based on the provided or determined zero-crossing temperature. The method of claim 2, further comprising setting a temperature of a conditioning fluid of the heat exchanger to establish the temperature set point for the optical element. The method of claim 14, further comprising heating one or more local sections of the optical surface of the optical element corresponding to the one or more of the plurality of regions of the optical element. The method of claim 2, wherein the heater comprises a zone heater having a plurality of zones configured to correspondingly heat the one or more of the plurality of zones of the optical element. An apparatus comprising: at least one optical element comprising a material whose coefficient of thermal expansion is temperature dependent, a heat exchanger configured to transfer heat to or from the optical element to establish a temperature set point for the optical element such that the coefficient of thermal expansion has a non-zero value, a heater configured to heat the optical element to produce a thermal deformation of the optical element. 23. The device of claim 23, wherein the heater is configured to selectively heat the optical element by selectively heating one or more of the plurality of zones of the optical element to heat the optical element at the plurality of zones of the optical element The thermal deformation occurs at the one or more of the regions. The device of claim 24, wherein a sign of the coefficient of thermal expansion for the material of the one or more of the plurality of zones of the optical element over most or all of a heating activation range of the heater Most or all are the same. 23. The device of any one of claims 23 to 25, wherein the coefficient of thermal expansion is substantially zero at a zero-crossing temperature and/or has a local minimum as a function of temperature. The apparatus of any one of claims 24 to 25, wherein the temperature set point is established such that a change in heat from the heater causes the optical element within most or all of the heating activation range of the heater The one or more of the plurality of regions varies in one of the thermal deformations in a range of -10 to +10 nm. 25. The device of any one of claims 24 to 25, wherein the optical element comprises an optical surface and wherein a plurality of regions are defined between the optical surface and the heat exchanger, and wherein the optical surface comprises a plurality of regions corresponding to the plurality of a plurality of local sections of a zone, and wherein the heater is configured to heat one or more of the plurality of local sections of the optical surface corresponding to the one or more of the plurality of zones of the optical element a local section. The apparatus of any one of claims 24 to 25, wherein the heater includes a source of electromagnetic waves that produces the thermal deformation of the one or more of the plurality of zones of the optical element. 25. The apparatus of any one of claims 23 to 25, wherein the heat exchanger contains a conditioning fluid for passage through at least one channel in the optical element. The device of claim 30, wherein the plurality of regions of the optical element are defined between the optical surface of the optical element and the at least one channel in the optical element for the cooling fluid. The apparatus of any of claims 24-25, wherein the heater comprises a zone heater having a plurality of zones configured to correspondingly heat the one or more of the plurality of zones of the optical element. The device of any one of claims 23 to 25, wherein the optical element is a mirror surface. A lithography device comprising a projection system configured to project a beam of radiation to project a pattern from a patterned device onto a substrate, wherein the lithography device comprises any one of claims 23 to 33 device. The lithography device of claim 34, wherein the lithography device is an EUV lithography device and the projection system includes a mirror surface.

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