CN116368436A

CN116368436A - Method and apparatus for thermally deforming optical element

Info

Publication number: CN116368436A
Application number: CN202180073012.5A
Authority: CN
Inventors: M·A·范登布林克
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2020-11-11
Filing date: 2021-10-29
Publication date: 2023-06-30
Also published as: EP4244676A1; TW202234139A; WO2022101039A1; KR20230098590A

Abstract

A method of thermally deforming an optical element, the optical element comprising a material having a coefficient of thermal expansion that 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 thermal deformation of the optical element.

Description

Method and apparatus for thermally deforming optical element

Cross Reference to Related Applications

The present application claims priority from european application 20206889.6 filed 11/2020 and european application 21154172.7 filed 29/2021, and the entire contents of these european applications are incorporated herein by reference.

Technical Field

The present invention relates to a method and apparatus for thermally deforming an optical element. More particularly, the method and apparatus relate to thermally deforming an optical element to correct aberrations.

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). For example, a lithographic apparatus may project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a 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 of 4nm to 20nm (e.g., 6.7nm or 13.5 nm) may be used to form smaller features on a substrate than a lithographic apparatus using radiation having a wavelength of 193nm, for example.

A projection system used to image the pattern from a patterning device onto the substrate may cause some aberrations in the wavefront of the projected image.

During projection of the pattern onto the substrate, the projection system will become heated and this will drift the imaging properties of the projection system. In EUV lithography, this phenomenon is referred to as mirror heating.

While the mirrors in the projection system are optimized for EUV radiation reflection, a substantial portion of the EUV (and out-of-band) energy is absorbed into the mirrors and converted to heat. This heating causes thermal stresses in the material of the mirror, resulting in deformation of the mirror. These deformations ultimately cause aberrations in the projection system, resulting in imaging errors. In addition, heating, either directly or indirectly, can cause thermal stresses in the material of other components such as the lens, substrate holder, patterning device (i.e., reticle or mask), or patterning device holder.

It is an object of the present invention to provide a method of correcting aberrations that avoids or alleviates one or more of the problems associated with the prior art.

Disclosure of Invention

According to a first aspect of the present invention, there is provided a method of thermally deforming an optical element comprising a material having a coefficient of thermal expansion dependent on temperature, 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, an

Heating the optical element to produce thermal deformation of the optical element.

The method according to the first aspect of the invention may have the advantage of introducing a required thermal deformation into the optical element, which may be used to at least partially correct aberrations, such as those caused by other optical elements applied in the lithographic apparatus LA.

In an 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 embodiments, thermal deformation may be applied, for example, in one or more of the plurality of zones of the optical element.

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

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

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

In an embodiment, selectively heating the optical element includes selectively heating one or more of a plurality of regions of the optical element to generate the thermal deformation at the one or more of the plurality of regions of the optical element.

In an 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-enabled range of the heater.

In an embodiment, the coefficient of thermal expansion is substantially zero at the zero crossing temperature and/or the coefficient of thermal expansion has a local minimum that varies with temperature.

In an embodiment, the transfer of heat to or from the optical element is achieved using a heat exchanger at least partially integrated in the optical element. Such a heat exchanger can thus cool or heat the optical element so that the optical element has the desired coefficient of thermal expansion.

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

In an embodiment, the present invention provides a method of thermally deforming an optical element comprising a material having a coefficient of thermal expansion that is zero at a zero crossing temperature, the method comprising: using a heat sink to transfer heat from the optical element to establish one or more temperature set points for one or more of the plurality of regions of the optical element such that the sign of the coefficient of thermal expansion is the same for most or all of the material of the one or more of the plurality of regions of the optical element over most or all of the heating-enabling range of the heater; and selectively heating one or more of the plurality of zones of the optical element within the heating-enabled range using the heater to generate thermal deformation at one or more of the plurality of zones of the optical element.

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

The method may further include establishing the one or more temperature set points such that a change in heat from the heater results in a change in the thermal deformation of one or more of the plurality of regions of the optical element in a range of-10 nm to +10nm over a majority or all of the heating-enabled range of the heater.

The one or more temperature setpoints may be established such that a change in heat from the heater results in a relatively large change in the thermal deformation of one or more of the plurality of zones of the optical element over a majority or all of the heating-enabled range of the heater.

The one or more temperature set points may be established such that the relationship between thermal load and deformation has sufficient sensitivity to provide a relatively good range of thermal manipulation. A larger deviation between the temperature set point and the zero crossing temperature may result in a greater sensitivity.

The one or more temperature set points may be established such that there is an approximately linear correlation between the internal temperature and the thermal deformation of one or more of the plurality of zones of the optical element for most or all of the heating-enabling range of the heater. The thermal deformation may be regarded as a deformation perpendicular to the optical surface of the optical element.

For optical element manipulation, the zernike actuation step size between substrates may be about 0.01nm.

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

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

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

The one or more temperature set points may be greater than the zero crossing temperature of the material or the 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 may be defined between the optical surface of the optical element and the heat sink. The optical surface may include a plurality of localized segments corresponding to the plurality of regions.

The change in zero crossing temperature of the material of the heat sink on the side opposite the optical surface of the optical element may be considered less relevant than in the plurality of zones.

The thermal disturbance of the optical element on the side of the heat sink opposite the optical surface of the optical element may be considered negligible and may not be considered.

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

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

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

The method may further comprise using a calibrated heater during EUV radiation exposure of the optical element to manipulate the wavefront during operation of the lithographic apparatus.

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

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

The method may further comprise characterizing a distribution of the zero crossing temperature of the optical surface of the optical element by modeling.

The method may further include setting a temperature of a cooling fluid of the heat sink to establish the one or more temperature set points of 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 relative to the zero crossing temperature of the material.

The temperature of the cooling fluid may be substantially the same as the zero crossing temperature of the material and the one or more temperature set points may 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 the one or more of the plurality of regions of the optical element.

The coefficient of thermal expansion may vary with the temperature of the optical element, and the optical element temperature may vary with the cooling fluid temperature. The 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 within an exemplary range of +/-0.5C, +/-1C, or +/-2C, which deviates from the zero crossing temperature Tzc, taking into account the zero crossing temperature Tzc variation.

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

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

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

Each zone may have a heating actuation range per zone. The heating enablement scope of one of the plurality of partitions may be the same or may be different from another of the plurality of partitions. The actuation range of a given zone may map to certain ranges of wavefront aberration at the substrate level.

The method may further include calibrating the plurality of zones of the zone heater using corresponding optical measurements obtained by an 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 and making corresponding optical measurements for the plurality of zones of the zone heater. That is, the power is changed for the plurality of zones of the zone heater one by one. This may determine how to change the wavefront by a change in temperature of a corresponding region of the optical element.

The relationship between the power reaching the zones of the zone heater and the corresponding optical measurements may be used for thermal manipulation of the wavefront.

According to a second aspect of the present invention there is provided an apparatus comprising:

at least one optical element comprising a material having a temperature dependent coefficient of thermal expansion,

a heat exchanger arranged 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 arranged to heat the optical element to produce thermal deformation of the optical element.

The device according to the invention may advantageously be applied to apply the method according to the invention.

The apparatus according to the second aspect of the invention may have the advantage of enabling a desired thermal deformation to be introduced into the optical element, which may be used to at least partially correct aberrations, for example those caused by other optical elements applied in the lithographic apparatus LA.

In an embodiment, the heater is arranged to selectively heat the optical element.

In an embodiment, the heater is arranged to selectively heat the optical element by selectively heating one or more of the plurality of regions of the optical element to produce the thermal deformation at the one or more of the plurality of regions of the optical element.

In an embodiment, a heater of the device may be configured to heat one or more of the plurality of zones of the optical element within a heating-enabled range of the heater.

In an embodiment, the heat exchanger of the device is at least partially integrated in the optical element. Such a heat exchanger may thus cool or heat the optical element so that the optical element has a desired coefficient of thermal expansion.

In an embodiment, there is provided an apparatus comprising: at least one optical element comprising a material having a coefficient of thermal expansion that is zero at a zero crossing temperature; a heat sink arranged to transfer heat from the optical element to establish one or more temperature set points for one or more of the plurality of regions of the optical element such that 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 a heating-enabled range of a heater arranged to: selectively heating one or more of the plurality of regions of the optical element within the heating-enabled range to generate thermal deformations at one or more of the plurality of regions of the optical element.

The one or more temperature setpoints may be established such that a change in heat from the heater results in a change in the thermal deformation of one or more of the plurality of zones of the optical element in the range of-10 nm to +10nm over most or all of the heating-enabled range of the heater. 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 localized segments corresponding to the plurality of regions. The heater may be arranged to heat one or more partial sections of the plurality of partial sections of the optical surface corresponding to one or more of the plurality of regions of the optical element.

The heater may include an electromagnetic wave source that generates thermal deformation of one or more of the plurality of regions of the optical element.

The heat sink may include a cooling fluid for passing through at least one channel in the optical element.

The heater may comprise a zone heater having a plurality of zones arranged 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 invention, there is provided a lithographic apparatus comprising a projection system configured to project a beam of radiation to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an apparatus as described above.

The lithographic apparatus may be an EUV lithographic apparatus and the projection system may comprise a mirror.

Drawings

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

FIG. 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;

FIG. 2 depicts a schematic view of a mirror and a heater of the lithographic apparatus according to an embodiment of the invention.

Fig. 3 depicts a plot of local mirror optical surface temperature versus deformation perpendicular to the optical surface of the mirror, according to an embodiment of the invention.

Fig. 4 depicts a plot of local mirror optical surface temperature versus deformation perpendicular to the optical surface of the mirror, according to an embodiment of the invention.

Figure 5 depicts a flow chart of a method of thermally deforming a mirror.

Figure 6 depicts CTE curves as a function of temperature for different materials.

Detailed Description

FIG. 1 depicts a lithographic system including 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 includes an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate 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, illumination system IL may include 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 an EUV radiation beam B having a desired cross-sectional shape and a desired intensity distribution. In addition to or in lieu of facet field mirror device 10 and facet pupil mirror device 11, illumination system IL may include other mirrors or devices.

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

mirrors

13, 14 configured to project the patterned EUV radiation beam B' onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B' and thus form an image having features smaller than the 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 with 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 the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure substantially 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 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.

Thermal deformations may occur in mirrors in the projection system PS of the EUV lithographic apparatus LA. Thermal deformation of an optical element in the sense of the present invention refers to deformation of the element due to a thermal load applied to the optical element. The thermal load may for example be 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. Such thermal loading may cause a temperature rise, for example a local temperature rise of the optical element, which causes a deformation, for example an expansion, of the optical element. More generally, thermal deformations may occur in components in the EUV lithographic apparatus LA. It should be appreciated that while the following description generally relates to one or more mirrors of the projection system PS in the EUV lithographic apparatus LA, the described method is also applicable to other components in the EUV lithographic apparatus LA as well as other components in other lithographic apparatuses, such as DUV lithographic apparatuses. For example, the component may 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 causing aberrations in the projection system PS, the mirror material may be optimized, for example, using Ultra Low Expansion (ULE) materials to obtain minimal deformation. In general, a material having a coefficient of thermal expansion CTE (which is temperature dependent) may have a CTE of zero or near zero at a particular design temperature. Alternatively or additionally, the material having a temperature dependent CTE may have a CTE with a minimum value in a specific operating range, e.g. different from zero. Materials with zero or near zero CTE may exhibit a secondary expansion or secondary expansion relationship with temperature that has nearly zero expansion properties around their design temperature, referred to as zero crossing temperature or zero-crossing temperature (zero-crossing temperature) (Tzc or ZCT). The mirror top portion (i.e. the portion of the mirror that is close to the side of the mirror that is incident by the radiation beam B') may be kept as close to such Tzc as possible to minimize distortion. In particular, the mirror surface or mirror top portion is designed and shaped in a specific way in order to ensure a proper patterning or exposure process. Local deformations of the mirror surface or top portion may occur due to the energy of the radiation beam absorbed by the mirror (typically, the optical element). Due to the application of a stronger or more intense thermal load (higher power, more extreme irradiation profile or distribution), it is not possible to keep the complete mirror top portion around this sweet spot or sweet spot, i.e. at or near the temperature at Tzc or ZCT. The mirror material may be made of other materials having a relatively low or very low Coefficient of Thermal Expansion (CTE), such as zero or cordierite. In general, it is contemplated that the Coefficient of Thermal Expansion (CTE) may be used to describe how the size of an object changes with changes in the temperature of the object. As an example, an object having a length L will experience a linear expansion Δt according to the following due to a change in temperature Δl:

Δl=l·α·Δt, where α=the linear coefficient of thermal expansion.

In the present invention, the thermal expansion coefficient is used to designate thermal deformation of an optical element due to the occurrence of temperature variation, particularly deformation of the surface of the optical element.

Thermal deformations of optical elements such as mirrors are often very sensitive to local (often non-uniform) irradiation (thermal load) and (local) zero crossing temperature (Tzc). The specific irradiation may provide a uniform temperature across the entire material surface of the optical element, which results in a well-correctable bending of the optical surface (i.e. strain in the mirror material). On the other hand, non-uniform illumination (different specific illumination) may provide isolated areas of relatively increased temperature, which creates a primarily uncorrectable thermal deformation of the optical surface (i.e., strain in the mirror material).

A non-linear thermal expansion curve may be shown for the zero crossing temperature Tzc (ZCT). For each curve, there may be a trough at Tzc, and thus, the curve may be relatively gentle around Tzc. This means that there is relatively little thermal expansion at or near Tzc.

Thermal distortion may be greater for example Tzc than for another example Tzc because the mirror environment may be maintained at a certain (reference) temperature (e.g., about 22 ℃). Thus, the thermal deformation may be very sensitive to a specific Tzc.

Manufacturing tolerances have a large impact on the spatial variation of Tzc. This may result in manufacturing tolerances for the average Tzc. In other words, a material designed to have a specific Tzc (i.e. a temperature where thermal expansion is zero or has a minimum value) will have variations of the Tzc across the material due to manufacturing tolerances, i.e. the local Tzc may vary across the material or an optical element made of the material.

The spatial variation of Tzc may be measured, for example on a mirror. There may be a temperature change in degrees celsius (or in kelvin). There may be spatial variations of Tzc in the mirror, typically several kelvin. In some projection systems PS of some lithographic apparatus LA, some mirrors may have particular variations, while other mirrors may have different variations. The spatial variation of Tzc may be calculated by measuring the linear Coefficient of Thermal Expansion (CTE) of the material, for example by using ultrasound. The CTE or CLTE of the material of length L is 1/L X dL/dT.

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

In an EUV lithographic apparatus LA, an incident Electromagnetic (EM) wave (i.e., EUV radiation) interacts with a mirror, wherein some of the radiation is reflected and some is absorbed. The absorbed radiation is dissipated within the mirror, which results in heating of the mirror. Heat is conducted within the mirror and thus the mirror temperature changes, e.g. increases, over time. The temperature change causes thermal deformation of the mirror, which leads to wavefront aberrations. These wavefront aberrations may need to be corrected in the lithographic apparatus LA.

With increasing EUV radiation source power, heating of the mirrors becomes a problem of increasing in the EUV lithographic apparatus LA. In particular, the mirrors of the projection optics (e.g., projection system PS) may heat up too much, deform, and cause aberrations that cannot be adequately corrected.

In an embodiment of the invention, a method of thermally deforming an optical element comprising a material having a thermal expansion coefficient dependent on temperature is provided to at least partially mitigate the effects of such deformation or aberration. According to the invention, the method may comprise the steps of:

-transferring heat to or from the optical element to establish a temperature set point of 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.

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

In an embodiment, the heating comprises selectively heating.

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

To heat the optical element (e.g., one or more of the plurality of zones of the optical element), a heater may be applied. The heater may for example comprise an electromagnetic wave source generating the thermal deformation of one or more of the zones of the optical element, for example.

In general, an optical element as applied, for example, in EUV lithographic apparatus and the like will be made of a material having a temperature-dependent coefficient of thermal expansion. In particular, materials having zero crossings, near zero in a particular temperature range, or having a minimum coefficient of thermal expansion as a function of temperature may be selected or designed.

Typically, the optical element is designed such that during normal use or operation the temperature of the optical element is close to the zero crossing, or in a temperature range where the coefficient of thermal expansion is close to zero or has a minimum value. Thereby, it is intended to operate the optical element in a state in which a temperature change of the mirror (e.g. a local temperature change) only causes a limited deformation of the optical element (e.g. a reflecting surface of the optical element).

According to the invention, heat is transferred to or from the optical element in order to set the temperature of the optical element at which the coefficient of thermal expansion has a non-zero value, for example a predetermined non-zero value. As such, the present invention applies a different approach whereby the temperature set point at which the optical element is sensitive to thermal deformation is intentionally selected, rather than operating the optical element at a temperature at which deformation is minimized. Establishing such a temperature of the optical element may be achieved, for example, using a heat exchanger, e.g. a heat exchanger 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. Further details regarding the thermal conditioning of the optical element are provided below.

When such a temperature set point is established, this may be achieved, for example, by heating or cooling the optical element to a temperature that deviates from ZCT by one or more degrees celsius, and a specific desired deformation of the optical element may be established by selectively heating the optical element. In embodiments, such selective heating may include selectively heating one or more of the plurality of regions of the optical element. Such selective heating may be achieved using a heater, as will be described in detail below. By means of selectively heating one or more regions of the optical element and establishing a temperature of the optical element, in which case the thermal expansion coefficient has a non-zero value, for example using a heat exchanger, a specific desired thermal deformation of the optical element can be established.

The invention thus provides a specific combination of heating/cooling of the optical element such that a desired sensitivity with respect to thermal deformations is established and used to produce a specific desired thermal deformations of the optical element, e.g. selected to at least partially counteract or compensate for aberrations occurring in an optical system in which the optical element is applied, e.g. a projection system in a lithographic apparatus LA.

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

In an embodiment, such an apparatus may include:

at least one optical element comprising a material having a thermal expansion coefficient dependent on temperature,

a heat exchanger arranged 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, an

During operation of the EUV lithographic apparatus LA, a mirror (e.g. in the projection system PS) may be exposed to EUV radiation. The (pre) heater may establish a constant thermal load to the

mirrors

13, 14 irrespective of the presence and spatial distribution (illumination mode) of the EUV radiation.

Typically, EUV radiation is incident on different locations on the

mirrors

13, 14 such that there is a spatially non-uniform thermal load. EUV radiation may for example use a dipole illumination mode, such that certain portions of the

mirrors

13, 14 are not hit by EUV radiation. In addition, the EUV radiation may be on at some times and off at other times. The pre-heater may be used to heat the

mirrors

13, 14 so that the spatial heat load distribution at the mirror surface is stable over time. One particular spatial heat load distribution may be uniform and another spatial heat load distribution may be non-uniform. Some pre-heaters may attempt to maintain the average temperature of the optical footprint of the mirror (i.e. the portion of the optical surface of the mirror that receives and reflects EUV radiation in use) at some predefined value. In the absence of any EUV radiation, the temperature would be uniform within the footprint which requires spatially non-uniform illumination in view of boundary conditions. Such mirror preheating attempts to control the average temperature of the optical footprint of the

mirrors

13, 14 to minimize aberrations. The pre-heater may be an IR heater so that the radiation will not affect the imaging on the substrate W. A heater 16 (e.g., a preheater) for the mirror 14 is shown in fig. 1. It will be appreciated that there may be multiple heaters and that there may be one or more heaters for each mirror or more generally each optical element.

Such use of a heater or pre-heater is thus intended to avoid deformation of the

mirrors

13, 14 by applying a specific thermal load (e.g. a specific thermal load distribution such that the combined thermal load caused by the heater and EUV radiation minimizes deformation of the mirrors (e.g. mirrors 13, 14)).

FIG. 2 depicts a schematic view of a mirror 14 of the projection system PS of the lithographic apparatus LA provided with the heater 16. The heater 16 may have a relatively high spatial resolution, for example, the heater may be a bar code scanner or a segmented heater having a relatively large number of segments 16A. For example, the zone heater may have a range of, for example, 10 to 200 zones, or 20 to 100 zones. The upper limit of the zone may be stimulated from the degree of freedom (order of magnitude) of the illuminator IL. The sections 16A of the heater 16 allow different portions of the 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 for one or more of the partitions 16A may be modified to provide different levels of heat to different portions of the mirror 14, as depicted by the different lengths of downward facing arrows in fig. 2. The partitions 16A may have the same physical size or may be different sizes. The partitions 16A are shown as boxes arranged in a pictorial array, as the illumination is 2D in space. However, it should be appreciated that in other embodiments, the partitions may have any suitable shape.

The heater 16 includes a source of electromagnetic waves (e.g., IR) for heating the 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, more particularly there may be a grid of resistive wires in the mirror.

The mirror 14 has an optical surface 14A configured to reflect the EUV radiation. The optical surface 14A may include a reflective (multilayer) coating.

The mirror 14 has a temperature set point, e.g. (T) _sp,sh ) Where "sp" is the set point and "sh" is the zone heating. More specifically, there are one or more temperature set points for one or more of the multiple zones of the mirror 14. For example, a location a corresponding to one region of the mirror 14 may have a different temperature set point than a location B corresponding to another region of the mirror 14. The heater 16 may heat one or more localized sections of the optical surface 14A of the mirror 14 to heat multiple regions of the mirror 14. As an example, if there are 100 partial sections of the optical surface of a pupil-like mirror, it may have a size of the optical surface of about 50mm for a mirror with an optical footprint of about 80mm diameter ² Is a local segment of the same. For larger mirrors, this size may be about 500mm ² 。

Reflection ofThe mirror 14 includes water cooling. At least one passage (not shown) is provided in the mirror 14 for passing cooling water 18 (or more generally, cooling fluid) through the mirror 14. The cooling water 18 may be at a temperature T _w,in ＝T _mf Is input to the mirror 14 and may be at a temperature T after heat is extracted from the mirror 14 _w,out Greater than T _mf Is withdrawn from the mirror 14. T (T) _mf Is the ambient temperature (e.g., about 22 c) of the mirror 14 made of ULE material. More generally, the cooling water 18 may be regarded as a heat sink arranged to transfer heat from the optical element (mirror 14). The cooling water 18 heats (convects) and transports heat away from the mirror 14. In other embodiments, it will be appreciated that the heat sink may take different forms.

The cooling water 18 may be cooled by a reduced steady state temperature difference (Δt) _ss ) While for example reducing steady state distortion and also reducing intra-wafer temperature drift due to Tzc re-optimization and reduced heat penetration (thermal penetration), i.e., heat penetration. The steady state deformation may be regarded as a deformation of the mirror during normal use, whereby EUV-induced thermal loads are applied to the mirror. Such steady state deformations may be caused, for example, by temperature differences in the mirrors (e.g., referred to as steady state temperature differences). Steady state temperature difference (delta T) _ss ) As a function of position (x, y) and without cooling as a function of EUV induced thermal load and boundary conditions. EUV induced thermal load as a function of: EUV radiation source power, lithographic apparatus LA duty cycle, mask properties (pattern density, reflectivity), illumination pupil, EUV absorption coefficient of the mirror, and position of the mirror along the optical path (and coupling between the mirror and the environment): delta T _ss,mx ＝max(ΔT _ss (x,y|Q _euv Pupil).

As previously mentioned, the zero crossing temperature (Tzc) varies spatially due to material inhomogeneities. The change in Tzc of the portion below the coolant (i.e., cooling water 18) may be considered less relevant than the change in Tzc of the portion between the coolant (i.e., cooling water 18) and the optical surface 14A of the mirror 14. The coolant design is assumed such that thermal disturbances or disturbances of the volume or body (bulk) under the coolant are negligible. This means that the zero crossing temperature (Tzc) variation within the portion of the mirror 14 is less critical than the zero crossing temperature variation of the top portion (i.e. between the coolant and the coating of the optical surface 14A). The zero crossing temperature variation of the top part may be calibrated on the lithographic apparatus LA. The variation of Tzc in the portion between the coolant and the coating in principle implies a different actuator sensitivity between any two sites (e.g. site a and site B). This requires calibration. It should be noted that actuator sensitivity in this respect refers to the sensitivity for thermal actuation of a specific portion or zone of the mirror or optical element, and thermal actuation refers to the application of a thermal load (e.g. a heating load) to produce or obtain a deformation. As such, the portion having Tzc at or near the actual temperature of a portion of the mirror will have a low actuator sensitivity, while the portion having Tzc that is far from the actual temperature of a portion of the mirror will have a high actuator sensitivity.

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

A localized section of the optical surface 14A of the mirror 14 corresponds to multiple regions of the mirror 14. The heater 16 heats one or more localized sections of the optical surface 14A of the mirror 14 corresponding to the plurality of regions of the mirror 14.

The multiple zones of the mirror 14 may be defined between the optical surface 14A of the mirror 14 and a coolant (or more precisely, a passage for the cooling water 18). This is because the change in 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 multiple regions of the mirror 14. Thermal disturbances or thermal disturbances of the mirror 14 on the side of the cooling water 18 opposite the optical surface 14A of the mirror 14 may be considered negligible and may not be considered.

As can be seen in fig. 2, the steady state temperature difference Δt is obtained from the location of the cooling water 18 _ss (x, y) is varied according to position (x, y), i.e. the position across the optical surface 14A of the mirror 14. Namely, from the principal and the subordinateThe steady state temperature of the mirror 14 is obtained by a depth z corresponding to the cooled spot (e.g. 5mm from the optical surface of the mirror 14).

The cooling water 18 is arranged to transfer heat from the mirror 14 (to the environment) to establish, i.e. establish, a temperature set point of the mirror 14. More specifically, one or more temperature set points for one or more of the plurality of zones of the optical element may be established. The one or more temperature set points may be set to zero crossing temperatures (Tzc) of materials remote from the mirror 14 such that the sign of the Coefficient of Thermal Expansion (CTE) is the same for multiple regions of the mirror 14 throughout most or all of the heating-enabled range of the heater 16. 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 the mirror 14. That is, all temperature set points are defined at equal offsets from the local zero crossing temperature. This may provide a similar nano-power sensitivity for each partition 16A. In other embodiments, the zero crossing temperature of a material may be considered the average zero crossing temperature of the mirror 14 (i.e., the average of all of the multiple regions of the mirror 14).

In an embodiment of the invention, the temperature of the optical element is set to a specific temperature set point, for example using a heat exchanger, resulting in the optical element or at least a substantial part thereof having a (predetermined) non-zero thermal expansion coefficient. In an embodiment, a cooling water arrangement as schematically shown in fig. 2 may be used for making such an arrangement. However, in general, according to the invention, the heat exchanger as applied may be used to cool or heat the optical element in order to set the temperature of the optical element to a temperature at which the material of the optical element has a desired predetermined non-zero thermal expansion coefficient. In embodiments, such a heat exchanger may include one or more channels configured to receive conditioning fluid (i.e., fluid for thermally conditioning the optical element). In an embodiment, a plurality of different channels may be applied, whereby each channel may be provided with a regulating fluid which is different, for example at different temperatures. In an embodiment, the heat exchanger comprises a grid of resistive wires, for example, integrated in the optical element. By applying one or more suitable voltages to the grid, a specific desired thermal load can be generated in the optical element, resulting in a desired temperature or temperature profile of the optical element. Thus, one or more temperature set points may be implemented in the optical element using the method or apparatus according to the present invention. In an embodiment of the invention, the heat exchanger as applied is at least partially integrated in the optical element. Examples of such arrangements may include the use of a grid of channels or resistive wires in the optical element.

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

Fig. 3 shows a curve illustrating this situation. The graph shows the local mirror optical surface 14A temperature (x-axis) relative to deformation (y-axis) perpendicular to the optical surface 14A of the mirror 14. As can be seen from the lowest temperature plotted, the deformation initially decreases with increasing temperature (i.e., has a negative coefficient of thermal expansion) before reaching a minimum value (i.e., at temperature Tzc, where the coefficient of thermal expansion is zero), and then the deformation increases with increasing temperature (i.e., has a positive coefficient of thermal expansion). The zero crossing temperature (Tzc), where the ULE material has the property of almost zero expansion, coincides with the minimum point. The main (bold) line M may be regarded as being for the average zero crossing temperature Tzc. A broken line U in the graph corresponding to the main line M of the graph, L shows the upper and lower limits U and L of the variation δtzc (T) due to the zero crossing temperature of the material. The change in zero crossing temperature Tzc in the material is intrinsic, i.e. inherent, and deterministic. It can be seen that the temperature set point T of the mirror 14 _sp,sh (q _sh ) Cooling or regulating temperature T _w,in Both are located on the same side (i.e., on the same side of the parabolic line) with respect to the zero crossing temperature Tzc. In such a case, the temperature set point T of the mirror 14 _sp,sh (q _sh ) And a cooling temperature T _w,in Both greater than the zero crossing temperature Tzc. It will be appreciated that in other embodiments, the temperature set point T of the mirror 14 _sp,sh (q _sh ) And a cooling temperature T _w,in Both may be smaller than the zero crossing temperature Tzc, i.e. both on the other side of the implemented zero crossing temperature Tzc than the one shown in fig. 3. By transferring heat to the mirror 14 (typically the optical element) or from the mirror 14, the temperature of the mirror 14 can be set to a (desired) value according to the invention, i.e. whereby the mirror has a (predetermined) value of a non-zero thermal expansion coefficient.

As will be appreciated, the cooling temperature T of the mirror as mentioned above _w,in Can be regarded as an example of adjusting the temperature of the optical element, for example by the aforementioned heat exchanger. Thus, the cooling temperature T _w,in The temperature T of the optical element can also be referred to as the temperature T _w,in 。

In FIG. 3, the coolant or conditioning temperature T _w,in Slightly above said zero crossing temperature Tzc as shown. Such a setting can be made at a temperature different from the temperature set point T _sp,sh (q _sh ) The coolant temperature T on the other side of the zero crossing temperature Tzc of (c) _w,in More preferably. This may be because this may ensure that the minimum point of thermal expansion (i.e., at Tzc, where the coefficient of thermal expansion is zero) will not be reached and thus the sign of the coefficient of thermal expansion of the region of the mirror 14 will be different from the other regions of the mirror 14.

The higher flow rate of the cooling water 18 means that the reflection is directedThe mirror 14 is more noisy and this introduces aberrations. In practice, therefore, it may be desirable to minimize the temperature set point T as much as possible _sp,sh (q _sh ) So that there is sufficient range of activation (to perform the necessary thermal manipulation). Once a certain zone 16A of the heater 16 is disconnected, it means that the heating-enabling range has been exceeded for the particular section of the optical footprint.

In a section of the graph, the slope of line M indicates a linear or quasi-linear relationship between the deformation and the temperature (refer to line S illustrating this case). That is, in the heating enabling range (Δt) of the (partitioned) heater 16 _sh ) There is an approximate linear correlation between most or all of the internal temperature and the thermal deformation of the region of the mirror 14. The vertical dashed line shows the heating enabling range (Δt) of the (zone) heater 16 _sh ) Is not limited in terms of the range of (a). Determining a maximum temperature offset Δt between the coolant (i.e. cooling water 18) and the front side (i.e. the optical surface 14A) by the highest local power density among the relevant EUV loads given the source power _ss,mx . Determining the manipulator control range DeltaT by the maximum actuation range required for correcting the worst of EUV heating induced or non-heating induced aberrations _sh Upper limit of (2).

Said temperature set point (T _sp,sh (q _sh ) In an approximately linear correlation section of the graph, where q is the heat flux in watts per square meter from the zone heater 16. It can be seen that the temperature set point T _sp,sh (q _sh ) Is positioned sufficiently far from the zero crossing temperature (Tzc) such that the coefficient of thermal expansion is positive for the full heating activation range of the zone heater (heater 16). It should be appreciated that in other embodiments, this enables the range ΔT for all heating _sh This may not be the case and may only be the case for most of the heating enabled range.

It can be seen that the temperature set point T _sp,sh (q _sh ) Is positioned sufficiently far from the zero crossing temperature (Tzc) so that for materialsIs a complete change δT of the zero crossing temperature of (1) _zc (T) said coefficient of thermal expansion being positive. Thus, even if the zone 16A of the zone heater 16 is set at the lowest power and the zero crossing temperature Tzc is at its highest possible value due to the spatial variation of a particular zone of the mirror 14, the minimum point of thermal expansion coefficient (i.e., tzc) will not be reached and thus the sign of the thermal expansion coefficient of the zone of the mirror 14 will not be different from the other zones of the mirror 14. It will be appreciated that if the mirror temperature corresponding to a particular region of the mirror 14 is such that Tzc is reached and exceeded, the sign of the coefficient of thermal expansion will not be the same and the deformation will be opposite for that region when compared to the region in which Tzc is not reached and exceeded.

Fig. 4 depicts another curve showing the local mirror optical surface 14A temperature (x-axis) relative to deformation (y-axis) perpendicular to the optical surface 14A of the mirror 14. Except for the temperature set point T in the curve of fig. 4 _sp,sh (q _sh ) And the cooling temperature T of the reflecting mirror 14 _w,in The graph of fig. 4 is similar to the graph of fig. 3 except that it is located on a different side (i.e., on a different side of the parabolic curve) with respect to the zero crossing temperature Tzc. In such a case, the temperature set point T _sp,sh (q _sh ) Greater than the zero crossing temperature Tzc and the cooling temperature T of said mirror 14 _w,in Less than the zero crossing temperature Tzc. More specifically, 0 on the x-axis corresponds to the temperature T of the cooling water 18 at the entrance of the mirror 14 _w,in 。T _w,in Preferably close to or equal to the temperature (T) of the mirror 14 (made of ULE material) _mf )。

In such an embodiment, the temperature set point T _sp,sh (q _sh ) The zero crossing temperature Tzc, still away from the material of the mirror 14, is such that the sign of the Coefficient of Thermal Expansion (CTE) is the same for most or all of the material of the region of the mirror 14 for most or all of the heating-enabled range of the heater 16. In an embodiment, a minority of the material of the region of the mirror 14 may not have the same sign of Coefficient of Thermal Expansion (CTE). This may beSo as to 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 average zero crossing temperature Tzc, with one or more temperature set points sufficiently distant from the average zero crossing temperature Tzc (e.g., about +4c) such that a majority of the material between the optical surface of the mirror and the cooling water has the same sign of Coefficient of Thermal Expansion (CTE). In such a case, the portion of the material closest to the cooling water may not have the same sign of thermal expansion coefficient. This situation may not be preferable because it may require additional power when compared to the case where the temperature of the cooling fluid is set away from the zero crossing temperature Tzc.

Typically, the one or more temperature set points for the one or more of the plurality of regions of the optical element are established, i.e., 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 the same for the plurality of regions of the mirror 14 over most or all of the heating-enabled range of the heater 16. In some embodiments, the one or more temperature set points may be set such that the coefficient of thermal expansion has a substantially similar value for the plurality of regions of the mirror 14 throughout most or all of the heating-enabled range of the heater 16. That is, the coefficients of thermal expansion of the multiple regions of the mirror 14 may have comparable values, i.e., comparable values-the deformations are comparable, i.e., comparable dimensions, between the multiple regions of the mirror 14.

Steady state temperature set point T during calibration _sp,sh (q _sh ) Is established, i.e., established, at or near the middle of the heating-enabling range of the heater 16. The temperature set point T of the mirror 14 is established using the cooling water 18 transferring heat from the mirror 14 _sp,sh (q _sh ). The temperature (T) of the cooling water 18 _w,in ) Can be set at a specific amount to establish the desired temperature set point T for the mirror 14 immediately _sp,sh (q _sh ). The Coefficient of Thermal Expansion (CTE) is a function of the temperature of the mirror 14 and the mirror 14 temperature is a function of the temperature of the cooling water 18. Can be achieved by setting the temperature (T _w,in ) The zero crossing temperature (Tzc) of the material disposed sufficiently far from the mirror 14 to establish, i.e., establish, the temperature set point T _sp,sh (q _sh ). The temperature of the cooling water 18 may be set within an exemplary range of +/-0.5C, +/-1C, or +/-2C away from the zero crossing temperature Tzc, taking into account the zero crossing temperature Tzc variation.

If the cooling water 18 is not used to limit the temperature of the mirror 14, over time (i.e. due to exposure to EUV radiation and/or heating by the heater 16), the temperature of the mirror 14 may increase too much, such that it is outside the range thermally operated by the heater 16, i.e. the temperature will drift.

The temperature set point T _sp,sh (q _sh ) Is established such that a change in heat from the heater 16 results in a relatively large change in thermal deformation of the region of the mirror 14 over the entire heating-enabled range of the heater 16 when compared to thermal deformation around Tzc. The relatively large variation in thermal deformation may be in the range of-10 nm to +10 nm. For optical element manipulation, the zernike actuation step size between substrates may be approximately 0.01nm. The temperature set point may be considered to be located sufficiently far from the zero crossing temperature Tzc such that the slope of line M in such a section of the curve is relatively steep when compared to the slope of line M in a section of the curve around Tzc.

The temperature set point T _sp,sh (q _sh ) Is established such that the relationship between thermal load and deformation is sufficiently sensitive to provide a relatively good range of thermal manipulation. A larger deviation between the temperature set point and the zero crossing temperature may result in a greater sensitivity. That is, the temperature set point being farther from the zero crossing temperature (i.e., away from the lower portion of the curve's line M) means that the deformation will be greater for the same temperature increase. This is thatAdvantageously, this is because it means that a relatively large amount of thermal management can be achieved for a relatively low power increase to the zone 16A of the zone heater 16.

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

The temperature set point T _sp,sh (q _sh ) May be based on the provided or determined zero crossing temperature of the material of the mirror 14. That is, the zero crossing temperature has been measured, estimated or modeled. The method may include characterizing a 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 the mirror 14 may be determined separately and then used as a starting point to establish, i.e., establish, the temperature set point.

Once the desired temperature set point T has been reached _sp,sh (q _sh ) The additional degrees of freedom of the heater 16 must be calibrated. For each zone of the heater 16, a wavefront sensor (not shown) must be utilized to measure the corresponding wavefront error. The wavefront sensor can make wavefront aberration measurements that take into account various degrees of freedom, such as the number of mirrors, the position and orientation of the mirrors, and not only mechanical aspects. Thus, calibration is required to determine how thermal manipulation by the heater 16 affects the wavefront error.

More generally, the heater 16 may be calibrated using corresponding optical measurements obtained by an optical measurement device (e.g., a wavefront sensor). The plurality of zones 16A of the zone heater 16 may be calibrated using corresponding optical measurements obtained by the optical measurement device (e.g., wavefront aberration measurements obtained by the wavefront sensor).

It is desirable to determine a relationship between heat provided from the heater 16 to one or more of the plurality of regions of the mirror 14 and the resulting thermal deformation of the corresponding one or more localized sections of the optical surface 14A of the mirror 14.

The method may include sequentially modifying the power sum for each of the plurality of zones 16A of the zone heater 16 to make a corresponding optical measurement. That is, the power is changed for each of the plurality of zones 16A of the zone heater 16 one by one. This establishes a sensitivity curve, i.e. how the deformation varies with the varying power of the zone 16A (or active zone). Making the optical measurements allows determining how the wavefront changes by changing the temperature of the corresponding region of the mirror 14. The relationship between the power reaching the zone 16A of the zone heater 16 and the corresponding optical measurement may be used for thermal manipulation of the wavefront of the mirror 14.

In general, it may be mentioned that the relation between the application of a specific thermal load to a specific section or portion of an optical element and the deformation of said optical element will depend on a number of parameters. In other words, the deformation of the optical element or a part thereof will also be a function of various parameters in addition to the actual thermal load as applied. In order to achieve an accurate thermal actuation of the optical element, for example in order to correct or compensate for aberrations, it may be necessary to take into account the parameters. Examples of such parameters worth mentioning are the actual CTE or CTE profile of the optical element. As mentioned, the CTE of the material of the optical element may not be homogeneous due to manufacturing tolerances, but may vary across the entire optical element. By applying a specific thermal load to the optical element, for example using a heater as described above, and observing the corresponding deformations, the parameters can be taken into account. It may be noted that it may be desirable to determine the actual CTE or CTE profile of the optical element for different thermal conditions. In particular, the CTE or CTE profile may be determined, for example, from the temperature of the optical element. As such, the CTE or CTE profile may be determined as part of, for example, a calibration or initialization process for different temperatures of the optical element. As a practical example, the CTE or CTE profile of the optical element may be determined for different temperature setpoints that are used to control a heat exchanger for transferring heat to or from the optical element. Since a material with a temperature dependent, i.e. temperature dependent CTE is used, the effect of applying a specific thermal load will thus 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 made to verify that modifying one or more of the powers of the zones 16A of the heater 16 to heat one or more zones of the mirror 14 may not substantially alter (i.e., heat and deform) other zones of the mirror 14 that are not specifically heated. If this is the case, it will be difficult or impossible to get the necessary thermal actuation to correct aberrations etc. It may be desirable to have separate partitions. If the crosstalk of adjacent or neighboring partitions is large, this means that the response of the adjacent or neighboring partitions also needs to be considered when actuating a given partition. This means that such crosstalk etc. will need to be calibrated.

During mass production (exposure) (i.e. during operation of the lithographic apparatus LA), the calibrated relationship may then be used to manipulate the wavefront of the mirror 14 (or mirrors) by inducing a corresponding thermal deformation via the heater 16. The calibrated relationship between the heat from the heater 16 and the resulting thermal deformation of one or more of the plurality of regions of the mirror 14 may be used to introduce a corresponding thermal deformation to the optical surface 14A of the mirror 14 via the heater 16 during use of the lithographic apparatus LA. This may be used to correct for aberrations caused, for example, by other optical elements of the lithographic apparatus LA. As an example, a power provided to the zone 16A of the zone heater 16 in the range 5W to 10W may result in a 1nm variation of the coefficients of the zernike wavefront.

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

In step 102, a temperature set point T is determined based on the provided or determined zero crossing temperature of the material of the mirror 14 _sp,sh (q _sh ). An alternative strategy may be to cool the temperature T of the water 18 _win Defined slightly above the zero crossing temperature Tzc, then calculates the required temperature set point (depending on the source power, illumination pupil, etc.), and adds 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 an embodiment, one or more temperature set points are selected to be away from a zero crossing temperature (Tzc) such that a sign of a Coefficient of Thermal Expansion (CTE) is the same for the plurality of regions of the mirror 14 within the heating-enabling range of the heater 16 (e.g., most or all of the heating-enabling range). CTE may have a substantially similar value for the plurality of regions of the mirror 14 within the heating-enabled range of the heater 16 (e.g., most or all of the heating-enabled range).

In step 104, the temperature of the cooling water 18 is set to establish a temperature set point for the mirror 14. The temperature set point may be remote from Tzc such that the relationship between thermal load and deformation has sufficient sensitivity over the heating-up enabled range.

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

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

In step 110, a calibrated heater 16 is used during EUV radiation exposure of the mirror 14 to manipulate the wavefront during operation of the lithographic apparatus LA.

Herein, a method for establishing thermal deformation of an optical element is proposed. In this context, it is assumed that there is a temperature-dependent, i.e. temperature-dependent, coefficient of thermal expansion for the optical element. Fig. 6 schematically illustrates the coefficients of thermal expansion of various materials. Curve a in fig. 6 illustrates a material having a Coefficient of Thermal Expansion (CTE) that is substantially independent of, i.e., substantially independent of, temperature T (i.e., the CTE of such material does not vary with temperature T). Curve B in fig. 6 illustrates a material having a temperature dependent Coefficient of Thermal Expansion (CTE). In particular, the material has a CTE that has a minimum at temperature T1.

Curve C in fig. 6 illustrates a material having a Coefficient of Thermal Expansion (CTE) that is also temperature dependent. In particular, the material illustrated by curve C has a minimum CTE at temperature T2 and a CTE equal to zero at temperature T3. Both materials illustrated by curves B and C can be advantageously applied to the present invention. Referring to fig. 3 and 4, the temperature T of fig. 3 or 4 may then be considered, for example _ZC Will correspond to temperature T3 in fig. 6; the deformations as illustrated in fig. 3 and 4 can be obtained, for example, in the case of using a material with CTE according to curve C of fig. 6.

According to the invention, heat or thermal load is transferred to or from an optical element (e.g. made of a material having a CTE according to curve B or C), thereby bringing the temperature of 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 a thermal load, is such that the material behaves according to a (predetermined) non-zero CTE, i.e. the behaviour of the material corresponds to the (predetermined) non-zero CTE. The heat or thermal load as applied (e.g. using the heat exchanger described above) can thus enable control of the CTE of the material of the optical element. Thereby, the sensitivity of the optical element to thermal actuation, such as that performed by a heater (e.g. a bar code scanner or a heater or a zone heater), is controlled. The invention is thus provided in a versatile and flexible way to address aberrations occurring in an optical system, such as a projection system of a lithographic apparatus. In case only small 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; when considerable 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 high. In both cases, the applied heater can be used over a large part of its thermal activation range, thus achieving a good solution for the applied thermal load.

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 fabrication 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 a lithographic apparatus, embodiments of the invention may be used in other apparatuses. Embodiments of the invention may form part or portion of a mask inspection apparatus, metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning apparatus). These devices may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

While the foregoing may have specifically referred to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography, and may be used in other applications, for example imprint lithography, where the context allows.

Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. 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.); etc. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. And doing so may allow the actuator or other device 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. Accordingly, it will be apparent to those 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. A method of thermally deforming an optical element, the optical element comprising a material having a coefficient of thermal expansion that is temperature dependent, the method comprising:

2. 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.

3. The method of claim 1 or 2, wherein heating the optical element comprises selectively heating the optical element.

4. The method of claim 3, wherein selectively heating the optical element comprises selectively heating one or more of a plurality of regions of the optical element to produce the thermal deformation at the one or more of the plurality of regions of the optical element.

5. The method of claim 4, wherein the sign of the coefficient of thermal expansion is the same for most or all of the material of the one or more of the plurality of regions of the optical element over most or all of a heating-enabled range of the heater.

6. A method according to any one of the preceding claims, wherein the coefficient of thermal expansion is substantially zero at zero crossing temperature and/or the coefficient of thermal expansion has a local minimum that varies with temperature.

7. A method according to any one of the preceding claims when dependent on claim 2, wherein the heat exchanger is at least partially integrated in the optical element.

8. The method of claim 7, wherein the heat exchanger comprises a channel integrated in the optical element, the channel configured to receive a fluid for exchanging heat with the optical element.

9. The method of claim 4, further comprising: the temperature set point is established such that a change in the heat from the heater causes a change in the thermal deformation of the one or more of the plurality of regions of the optical element in a range of-10 nm to +10nm over most or all of the heating-enabled range of the heater.

10. The method of any of the preceding claims, further comprising: the temperature set point is established such that the coefficient of thermal expansion has substantially similar values for the plurality of zones of the optical element over most or all of the heating-enabling range of the heater.

11. The method of any of the preceding claims, further comprising establishing the temperature set point within a range of 1 ℃ to 8 ℃ from the zero crossing temperature of the material.

12. The method of any of the preceding claims when dependent on claim 4, wherein the zero crossing temperature of the material comprises a local zero crossing temperature of the one or more of the plurality of regions of the optical element or an average zero crossing temperature of the optical element.

13. The method of any of the preceding claims, wherein the temperature set point is greater than the zero crossing temperature of the material or the temperature set point is less than the zero crossing temperature of the material.

14. The method of any of the preceding claims when dependent on claim 4, wherein the optical element comprises 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 comprises a plurality of partial sections corresponding to the plurality of regions.

15. The method of claim 14, further comprising determining a relationship between heat provided from the heater to the one or more of the plurality of regions of the optical element and a resulting thermal deformation of the corresponding one or more partial sections of the optical surface of the optical element.

16. The method of any one of the preceding claims when dependent on claim 2, further comprising calibrating the heater using corresponding optical measurements obtained by an optical measurement device.

17. The method of claim 16, further comprising calibrating the heater using corresponding wavefront aberration measurements obtained by a wavefront sensor.

18. The method of claim 16 or 17, further comprising using a calibrated heater during EUV radiation exposure of the optical element to manipulate a wavefront during operation of a lithographic apparatus.

19. The method of any preceding claim, further comprising determining the temperature set point based on a provided or determined zero crossing temperature.

20. The method of any one of the preceding claims when dependent on claim 2, further comprising setting a temperature of a conditioning fluid of the heat exchanger to establish the temperature set point of the optical element.

21. The method of any one of claims 14 or 15, further comprising heating one or more localized sections of the optical surface of the optical element corresponding to the one or more of the plurality of regions of the optical element.

22. A method as claimed in any preceding claim when dependent on claim 2, wherein the heater comprises a zone heater having a plurality of zones arranged to correspondingly heat the one or more of the plurality of zones of the optical element.

23. An apparatus, comprising:

24. The apparatus of claim 23, wherein the heater is arranged to selectively heat the optical element by selectively heating one or more of the plurality of regions of the optical element to produce the thermal deformation at the one or more of the plurality of regions of the optical element.

25. The apparatus of claim 24, wherein the sign of the coefficient of thermal expansion is the same for most or all of the material of the one or more of the plurality of regions of the optical element over most or all of a heating-enabled range of the heater.

26. The apparatus of any one of claims 23 to 25, wherein the coefficient of thermal expansion is substantially zero at zero crossing temperature and/or the coefficient of thermal expansion has a local minimum that varies with temperature.

27. The apparatus of any of claims 24 to 26, wherein the temperature set point is established such that, within a majority or all of the heating-enabled range of the heater, a change in the heat from the heater results in a change in the thermal deformation of the one or more of the plurality of regions of the optical element in a range of-10 nm to +10 nm.

28. The apparatus of any of claims 24 to 27, wherein the optical element comprises an optical surface and wherein a plurality of zones are defined between the optical surface and the heat exchanger, and wherein the optical surface comprises a plurality of partial sections corresponding to the plurality of zones, and wherein the heater is arranged to heat one or more partial sections of the plurality of partial sections of the optical surface corresponding to the one or more of the plurality of zones of the optical element.

29. The apparatus of any one of claims 24 to 28, wherein the heater comprises an electromagnetic wave source that generates the thermal deformation of the one or more of the plurality of regions of the optical element.

30. The apparatus of any one of claims 23 to 29, wherein the heat exchanger comprises a conditioning fluid for passing through at least one channel in the optical element.

31. The apparatus 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.

32. The apparatus of any of claims 24 to 31, wherein the heater comprises a zone heater having a plurality of zones arranged to correspondingly heat the one or more of the plurality of zones of the optical element.

33. The apparatus of any one of claims 23 to 32, wherein the optical element is a mirror.

34. A lithographic apparatus comprising a projection system configured to project a beam of radiation to project a pattern from a patterning device onto a substrate, wherein the lithographic apparatus comprises an apparatus according to any one of claims 23 to 33.

35. The lithographic apparatus of claim 34, wherein the lithographic apparatus is an EUV lithographic apparatus and the projection system comprises a mirror.