DE102022131353A1 - Method for heating an optical element and optical system - Google Patents

Abstract

The invention relates to a method for heating an optical element in an optical system, in particular in a microlithographic projection exposure system, and to an optical system. In a method according to the invention, a heating power is introduced into the optical element (200) using a thermal manipulator (210) to produce a thermally induced deformation, wherein this heating power is adjusted with regard to a target state of the optical element before starting operation of the optical system in which useful light strikes the optical element, in which first optical aberration is at least partially compensated, and wherein after starting operation of the optical system, the heating power is regulated to the target state depending on the heat load of the useful light striking the optical element.

Description

BACKGROUND OF THE INVENTION

Field of the invention

The invention relates to a method for heating an optical element in an optical system, in particular in a microlithographic projection exposure system, and to an optical system.

State of the art

Microlithography is used to produce microstructured components, such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure system, which has an illumination device and a projection lens. The image of a mask (= reticle) illuminated by the illumination device is projected by the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens in order to transfer the mask structure onto the light-sensitive coating of the substrate.

In projection lenses designed for the EUV range, i.e. at wavelengths of e.g. about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process due to the lack of availability of suitable light-transmitting refractive materials.

A problem that occurs in practice is that, as a result of manufacturing variations in the mirrors and the finite precision of the assembly and adjustment processes to be carried out when assembling the projection exposure system, unavoidable optical aberrations result due to existing deviations from the ideal optical design. In order to correct such optical aberrations (also known as "cold aberrations"), one possible approach - in addition to the targeted actuation of the respective mirrors in their rigid body degrees of freedom - is to apply a suitable heating profile to the mirrors using a heating arrangement (e.g. coupling in infrared radiation) in order to correct the optical cold aberrations via the thermally induced deformation achieved in this way.

Another problem that occurs in practice is that the EUV mirrors experience heating (also known as "mirror heating") and associated thermal expansion or deformation as a result of absorption of the radiation emitted by the EUV light source, among other things, which in turn can impair the imaging properties of the optical system. In order to avoid surface deformations caused by heat input into an EUV mirror and the associated optical aberrations, it is known, among other things, to use a material with ultra-low thermal expansion ("ultra-low expansion material") as the mirror substrate material, e.g. a titanium silicate glass sold under the name ULE™ by Corning Inc., and to set the so-called zero crossing temperature (ZCT = "zero crossing temperature") in an area close to the optical effective surface. At this zero-crossing temperature, which for example is around ϑ = 30°C for ULE™, the thermal expansion coefficient has a zero crossing in its temperature dependence, in the vicinity of which there is no or only a negligible dependence of the thermal expansion of the mirror substrate material on temperature variations. Furthermore, by using a heating arrangement (e.g. based on infrared radiation) in phases of comparatively low absorption of EUV useful radiation, active mirror heating can take place, whereby this active mirror heating is reduced accordingly as the absorption of the EUV useful radiation increases.

In practice, the further problem arises that the approaches described above for correcting “cold aberrations” on the one hand and for avoiding deformations induced during operation of the optical system by exposure to EUV or useful light in the respective mirror (ie “mirror heating”) on the other hand contain opposing or contradictory requirements, as - as shown in the schematic diagram by 3a indicated - the favorable or favored temperature ranges or target values for the two approaches differ from one another. For example, to avoid optical aberrations thermally induced by EUV radiation during operation of the optical system, a temperature window in the range of the above-mentioned zero crossing temperature (= ZCT) is desirable, in which the thermally induced deformations that occur are as insensitive as possible with regard to local temperature variations at different mirror positions. In contrast, to achieve the desired corrective effect with regard to the "cold aberrations", the respective mirror must be heated to a temperature range in which the mirror's deformation is sufficiently sensitive to additional heat radiation, which then in turn increases the aberrations induced during operation by exposure to EUV light or the influence of the above-mentioned "mirror heating".

Even when the heating power introduced into the mirror by a thermal manipulator is set to a target state in which, in addition to the cold aberrations to be compensated, the heat load acting on the mirror due to incident useful light during operation of the optical system is already taken into account and in which - as in 3b As already mentioned, the heating power generated by the thermal manipulator is initially optimized with regard to both the cold aberrations and the aspect of "mirror heating", a further problem arises that, as a result of the fundamental departure from the above-mentioned zero crossing temperature (ZCT), the temperature increase associated with the EUV load during operation inevitably leads to transient aberrations or aberrations that change over time from the start of operation (i.e. starting time), which impairs the performance of the optical system or the projection exposure system.

The state of the art is only given as an example EN 10 2019 219 289 A1 referred to.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for heating an optical element, in particular in a microlithographic projection exposure system, as well as an optical system, which enable an effective avoidance of surface deformations caused by heat input into the optical element and associated optical aberrations.

This object is achieved by the method or the optical system according to the features of the independent patent claims.

• - wobei in das optische Element zur Erzeugung einer thermisch induzierten Deformation eine Heizleistung unter Verwendung eines thermischen Manipulators eingebracht wird;
• - wobei diese Heizleistung vor Aufnahme eines Betriebs des optischen Systems, in welchem Nutzlicht auf das optische Element trifft, im Hinblick auf einen Sollzustand des optischen Elements eingestellt wird, in welchem eine erste optische Aberration wenigstens teilweise kompensiert wird; und
• - wobei nach Aufnahme des Betriebs des optischen Systems eine Regelung der Heizleistung auf den Sollzustand in Abhängigkeit von der Wärmelast des auf das optische Element treffenden Nutzlichts erfolgt.

According to one aspect, the invention relates to a method for heating an optical element in an optical system, in particular a microlithographic projection exposure system,

• - wherein a heating power is introduced into the optical element to produce a thermally induced deformation using a thermal manipulator;
• - wherein this heating power is adjusted before starting operation of the optical system in which useful light strikes the optical element with respect to a desired state of the optical element in which a first optical aberration is at least partially compensated; and
• - whereby, after the optical system has started operating, the heating power is regulated to the desired state depending on the heat load of the useful light striking the optical element.

The first optical aberration can in particular be at least partially due to manufacturing or adjustment. However, the invention is not limited to this, so that the first optical aberration can also be caused by other sources of error.

In embodiments, the invention is initially based on the approach of including both aspects of “correction of cold aberrations” and “compensation of the influence of mirror heating” when determining a suitable set of target values for the heating power introduced into the optical element by a thermal manipulator, in view of the basic problem explained at the beginning of opposing or contradictory requirements for the temperature range favored for correcting cold aberrations on the one hand and the temperature range favored for compensating for the influence of “mirror heating” on the other. This is done by taking into account the corresponding effect of the heating power on the optical aberration generated during the actual operation of the optical element or the optical system having this element as a result of incident useful light (in particular by means of “co-optimization”) when setting the heating power, in addition to correcting the above-mentioned “cold aberrations”.

The invention is not further restricted with regard to the specific design of the thermal manipulator, i.e. the manner in which heating power is introduced into the optical element. For example, the introduction of heating power can be carried out in a known manner via infrared (IR) radiators, whereby individual sectors can also be exposed to IR radiation by setting appropriate heating profiles. For this purpose, for example, a EN 10 2019 219 289 A1 described heating arrangement can be used. Alternatively, the heating power can also be introduced via electrodes that can be charged with electrical voltage and are arranged on the optical element or mirror to be heated.

The wording “set of target values” is intended to express that the thermal manipulator can also specifically heat individual sectors by setting appropriate heating profiles with IR radiation (e.g. with the EN 10 2019 219 289 A1 described heating arrangement). In this respect, the “set of target values” for the heating power of the thermal manipulator may include a value for each of the sectors (like a vector with a plurality of components).

The first optical aberration (e.g. due to manufacturing or adjustment) can be caused by the optical element itself or elsewhere in the optical system (e.g. by another optical element).

Based on the above-mentioned approach, the invention is now based on the further concept of not keeping the heating power of the thermal manipulator, which as described above is set at the start of operation in particular with regard to the correction of cold aberrations, constant over time during operation of the optical system, but rather adjusting it depending on the heat load of the useful light striking the optical element.

In embodiments, the concept according to the invention differs in particular from conventional preheating approaches in that during operation of the optical system, a homogeneous or constant temperature distribution within the optical element or mirror is not sought, but rather - taking existing cold aberrations into account - a constant wavefront effect is maintained, which in turn can be achieved by generating a correspondingly inhomogeneous heating profile within the mirror and, if necessary, also by maintaining this inhomogeneous heating profile under payload by means of readjusting the heating power during operation of the optical system.

The inventive readjustment of the heating power of the thermal manipulator can be carried out in different ways according to the invention, as described below. In this respect, the invention includes, on the one hand, embodiments with a temperature-based control concept, which in turn can be based on the determination of an average temperature on the optical effective surface or alternatively also based on a temperature distribution on the optical effective surface (in which different temperature values for different sectors on the optical effective surface are possibly used as a basis). Furthermore, the invention also includes embodiments in which the inventive readjustment is carried out on the basis of a wavefront effect of the optical element or the associated optical system (estimated using at least one wavefront sensor and/or based on a model).

According to one embodiment, the optical system has at least one further mirror that can be actuated in a plurality of degrees of freedom, in particular a plurality of further mirrors that can each be actuated in a plurality of degrees of freedom, wherein the entirety of these degrees of freedom is used for the at least partial compensation of the first optical aberration.

According to one embodiment, the heating power is adjusted before the optical system starts operating, taking into account the respective effect of the heating power on a second optical aberration that is caused by useful light hitting the optical element during subsequent operation of the optical system. In other words, before the optical system starts operating, the heating power is adjusted, as already mentioned, in the sense of co-optimization both with regard to the cold aberrations and with regard to the "mirror heating" that occurs during subsequent operation due to the EUV load.

According to one embodiment, the target state is defined by a thermal state of the optical element.

According to one embodiment, the desired state is defined by a wavefront provided by the optical system in an image plane.

According to one embodiment, the heating power is controlled in such a way that the average temperature of the optical element remains constant up to a maximum deviation of 0.5 K, in particular of a maximum of 0.2 K. In this case, the invention includes in particular the further concept of ensuring during control that the average temperature of the optical element remains essentially constant over the entire period from an initial state - i.e. the state before exposure to (e.g. EUV) useful light - to the state during exposure to (e.g. EUV) useful light during operation of the optical system, in order to avoid or at least reduce "overshoots". This condition of maintaining the average temperature of the optical element over time can be incorporated as a secondary condition in the control of the heating power. Furthermore, different lighting settings can also be taken into account here. In other words, for each lighting setting, the corresponding set of target values for the heating power of the thermal manipulator can be selected when controlling the heating power in such a way that the average temperature of the optical element is maintained over the period from the initial state or the state before exposure to (e.g. EUV) useful light during operation of the optical system to the state during exposure to (e.g. EUV) useful light during operation of the optical system.

According to one embodiment, the heating power is regulated after the optical system has started operating on the basis of at least one temperature measured using at least one temperature measuring device. The temperature measuring device can be designed in any suitable manner, e.g. as a temperature sensor or thermal imaging camera.

According to one embodiment, the heating power is controlled after the optical system has started operating on the basis of at least one average temperature at the optical effective surface of the optical element estimated using at least one temperature measuring device.

According to a further embodiment, the heating power is controlled after the optical system has started operating on the basis of a temperature distribution on the optical effective surface of the optical element estimated using one or more temperature measuring devices.

According to one embodiment, the temperature distribution on the optical effective surface of the optical element is estimated from measurement signals supplied by the temperature measuring devices using an observer based on a model.

According to one embodiment, the heating power is controlled after the optical system has started operating on the basis of an estimate of the wavefront effect of the optical system, in particular on the basis of a feed-forward model.

According to one embodiment, this estimation of the wavefront effect of the optical element is carried out using at least one wavefront sensor.

According to one embodiment, the wavefront effect of the optical element is estimated on the basis of setpoint values for the heating power set by the thermal manipulator.

According to one embodiment, the estimation of the wavefront effect of the optical element is based on a combination of wavefront and temperature measurements.

According to one embodiment, information about the reticle used, the lighting setting used and/or information from an intensity measurement is used in the feed-forward model. The information about the reticle used can in particular relate to information about the optical far field generated during operation of the optical system by diffraction of the (e.g. EUV) useful light on the structures of the reticle, which can be determined by optical forward simulation. The information from an intensity measurement can in particular relate to the intensity of the radiation of the thermal manipulator (e.g. infrared laser) used to generate the heating power.

According to one embodiment, the control of the heating power is carried out transiently on the basis of the feed-forward model. In particular, the temperature distribution in the optical element suitable for setting the target state or the desired wavefront can be set transiently or re-determined at different times during operation of the optical system.

According to one embodiment, this transient control is carried out on the basis of the feed-forward model as model predictive control to take into account a change in the reticle used and/or the illumination setting used. In other words, a change in the reticle used and/or the illumination setting used can be anticipated in order to avoid the occurrence of overshoots after this change.

According to one embodiment, a combination of a plurality of mirrors is used in the control, wherein this plurality comprises at least one mirror in which a heating device generates a heating profile complementary to a temperature distribution caused by useful light impinging on this mirror, and at least one mirror which is actively deformed for wavefront manipulation.

According to one embodiment, the optical element is a mirror.

According to one embodiment, the optical element is designed for an operating wavelength of less than 400 nm, in particular less than 250 nm, further in particular less than 200 nm.

According to one embodiment, the optical element is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm.

• - wenigstens einem optischen Element,
• - einem thermischen Manipulator zum Heizen dieses optischen Elements, und
• - einer Regelungseinheit zur Regelung der durch den thermischen Manipulator in das optische Element eingebrachten Heizleistung,
• - wobei diese Regelung auf Basis eines Sollzustandes, in welchem eine erste optische Aberration wenigstens teilweise kompensiert wird, und in Abhängigkeit von der Wärmelast des auf das optische Element treffenden Nutzlichts erfolgt.

The invention further relates to an optical system, in particular a microlithographic projection exposure system, with

• - at least one optical element,
• - a thermal manipulator for heating this optical element, and
• - a control unit for controlling the heating power introduced into the optical element by the thermal manipulator,
• - wherein this control is carried out on the basis of a target state in which a first optical aberration is at least partially compensated and as a function of the heat load of the useful light incident on the optical element.

According to one embodiment, the first optical aberration is at least partially due to manufacturing or adjustment.

According to one embodiment, the optical system is configured to carry out a method with the features described above. For advantages and further preferred embodiments of the optical system, reference is made to the above-mentioned statements in connection with the method according to the invention.

Further embodiments of the invention can be found in the description and the subclaims.

The invention is explained in more detail below with reference to embodiments shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

• 1 eine schematische Darstellung des möglichen Aufbaus einer für den Betrieb im EUV ausgelegten mikrolithographischen Projektionsbelichtungsanlage;
• 2 eine schematische Darstellung zur Erläuterung einer möglichen Realisierung eines erfindungsgemäßen Verfahrens; und
• 3a-3b Diagramme zur Erläuterung eines der vorliegenden Erfindung zugrundeliegenden, prinzipiellen Problems.

Show it:

• 1 a schematic representation of the possible structure of a microlithographic projection exposure system designed for operation in the EUV;
• 2 a schematic representation to explain a possible implementation of a method according to the invention; and
• 3a-3b Diagrams to explain a fundamental problem underlying the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

1 first shows a schematic representation of a projection exposure system 1 designed for operation in the EUV, in which the invention can be implemented, for example.

According to 1 the projection exposure system 1 has an illumination device 2 and a projection lens 10. The illumination device 2 serves to illuminate an object field 5 in an object plane 6 with radiation from a radiation source 3 via an illumination optics 4. A reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be displaced via a reticle displacement drive 9, in particular in a scanning direction. In 1 For explanation purposes, a Cartesian xyz coordinate system is shown. The x-direction runs perpendicular to the drawing plane. The y-direction runs horizontally and the z-direction runs vertically. The scanning direction runs in 1 along the y-direction. The z-direction is perpendicular to the object plane 6.

The projection lens 10 is used to image the object field 5 into an image field 11 in an image plane 12. A structure on the reticle 7 is imaged onto a light-sensitive layer of a wafer 13 arranged in the area of the image field 11 in the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be displaced via a wafer displacement drive 15, in particular along the y-direction. The displacement of the reticle 7 on the one hand via the reticle displacement drive 9 and the wafer 13 on the other hand via the wafer displacement drive 15 can be synchronized with each other.

The radiation source 3 is an EUV radiation source. The radiation source 3 emits in particular EUV radiation, which is also referred to below as useful radiation or illumination radiation. The useful radiation has in particular a wavelength in the range between 5 nm and 30 nm. The radiation source 3 can be, for example, a plasma source, a synchrotron-based radiation source or a free-electron laser (“free-electron laser”, FEL). The illumination radiation 16 emanating from the radiation source 3 is bundled by a collector 17 and propagated through an intermediate focus in an intermediate focus plane 18 into the illumination optics 4. The illumination optics 4 have a deflection mirror 19 and, downstream of this in the beam path, a first facet mirror 20 (with schematically indicated facets 21) and a second facet mirror 22 (with schematically indicated facets 23).

The projection lens 10 has a plurality of mirrors Mi (i= 1, 2, ...), which are numbered according to their arrangement in the beam path of the projection exposure system 1. In the 1 In the example shown, the projection lens 10 has six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or another number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation 16. The projection lens 10 is a double obscured optic. The projection lens 10 has a numerical aperture on the image side, which can be, for example, greater than 0.3, and in particular also greater than 0.5, and further in particular greater than 0.6.

During operation of the microlithographic projection exposure system 1, the electromagnetic radiation striking the optical effective surface of the mirrors is partially absorbed and, as explained above, leads to heating and the associated thermal expansion or deformation, which in turn can result in an impairment of the imaging properties of the optical system. As described above, a thermal manipulator in the form of a heating arrangement can now be used to actively heat the mirrors in phases of comparatively low absorption of EUV useful radiation, with this active mirror heating being reduced accordingly as the absorption of the EUV useful radiation increases.

In 1 a heating arrangement is merely shown schematically and designated with "25", whereby this heating arrangement 25 serves in the example to introduce a heating power into the mirror M3. The invention is not further restricted with regard to the manner in which the heating power is introduced or the design of the heating arrangement used for this purpose. Merely as an example, the heating power can be introduced in a known manner via infrared radiators or via electrodes that can be charged with electrical voltage and are arranged on the optical element or mirror to be heated.

Furthermore, the invention is not further restricted with regard to the number of optical elements or mirrors to be heated, so that the control according to the invention can be applied to the heating of only a single optical element or also to the heating of a plurality of optical elements.

The embodiments described below have in common that the introduction of a heating power into an optical element or a mirror is not only carried out with (initial) adjustment to a target state, taking into account both optical aberrations to be compensated for as a result of manufacturing or adjustment errors and, if applicable, taking into account the effects of the heating power on a second optical aberration caused by "mirror heating" in subsequent operation, but that in addition - after the optical system has started operating or the optical element has been exposed to useful light - the heating power is also regulated depending on the heat load of the useful light (i.e. the EUV radiation) striking the optical element.

In particular, the invention takes into account the fact that in a projection exposure system during the microlithographic exposure process, in addition to the heating power introduced into a mirror via IR radiation, useful light in the form of EUV radiation also acts on the mirror surface, with the result that the mirror temperature at the optical effective surface increases locally and depending on the selected illumination setting compared to the temperature profile existing without EUV radiation, as already shown by 3b was explained.

To take this effect into account, different control concepts according to the invention are described below, with reference first to 2 temperature-based control concepts are explained. For this purpose, one or more temperature measuring devices 203a, 203b (e.g. temperature sensors) are arranged in the relevant optical element or mirror 200 in a manner known per se in access channels 202 located within the mirror substrate 201, wherein an estimate of the average temperature or also a temperature distribution on the optical effective surface of the mirror 200 is made via these temperature measuring devices 203a, 203b.

In 2 Furthermore, a thermal manipulator in the form of an infrared heating arrangement is designated with “210” and the EUV radiation (= useful light) acting on the mirror 200 during operation is designated with “220”. Furthermore, several sectors in the area of the optical effective surface of the mirror are designated with “211” to “214”, wherein in embodiments, if a sufficient number of temperature measuring devices 203a, 203b are present and, if necessary, also using an observer 230 and based on a model, an estimate of a local temperature distribution on the optical effective surface of the mirror 200 can be made.

In a first embodiment of a temperature-based control concept according to the present invention, the control of the heating power supplied to the mirror 200 after the start of operation via the thermal manipulator 210 can be carried out on the basis of the estimated average temperature at the optical effective surface (ie initially without a spatially resolved determination of the temperature distribution over several sectors). This approach is based on the consideration that, in a simplified view, the EUV radiation 220 absorbed by the mirror 200 leads to an increase in the average mirror temperature: $$\overline{T_{\text{total}}}=\overline{T_{\text{IR}}+T_{\text{EUV}}}$$

In the following, it is now assumed that the thermal manipulator 210 is designed as a “sector heater” in that it specifically directs the introduction of heating power into the mirror 200 into different sectors (e.g. “211” to “214” according to 2 ) is possible. For example, a EN 10 2019 219 289 A1 The heating arrangement described can be used.

erfolgen.According to the first embodiment, after estimating the average temperature on the optical effective surface of the mirror 200, the heating power can be regulated in such a way that this average temperature is kept constant during operation and the effect of EUV radiation 220 on the mirror 200. Alternatively, this can be done in such a way that the thermal manipulator 210 or IR radiant heater enables a corresponding homogeneous heating of the optical effective surface, which can then be regulated down accordingly in the course of the increase in the average mirror temperature associated with the EUV radiation 220. Alternatively, heating power can also be subtracted from the individual IR radiant heaters designed as sector heaters in such a way that an overall reduction in the average temperature on the optical effective surface is achieved while maintaining the inhomogeneous heating profile that is introduced via the thermal manipulator 210 or IR radiant heater. This can be done, for example, via a global scaling factor S according to $$\overline{T_{S\cdot {\mathrm{<figure-callout\; id="1"\; label="P"\; filenames="DE102022131353A1\_0011.png"\; state="\{\{state\}\}">P</figure-callout>}}_1,\dots S\cdot P_N}+T_{\text{eu}}}=\overline{T_{\text{init}}}$$

take place.

Alternatively, a linear combination of heating powers applied to each sector can be determined, which produces a temperature increase that is as homogeneous as possible and can then be used for the control according to the invention. Instead of a global scaling factor, other suitable mathematical approaches can also be used to reduce the average temperature at the optical effective surface.

In order to implement the temperature-based control concept described above based on the average temperature at the optical effective surface, a sufficiently large offset of the average temperature at the optical effective surface is preferably already maintained when the heating power of the thermal manipulator 210 is initially set (by co-optimizing the corresponding merit function both with regard to cold aberrations and with regard to "mirror heating"). In other words, the (average) temperature at the optical effective surface in each sector that initially results according to the corresponding "co-optimized" setpoint value for the heating power should generally be greater than the maximum temperature caused by EUV radiation 220 in this sector. In individual cases, it may be useful to heat with less than the maximum temperature caused by EUV radiation 220, particularly depending on the specific expansion behavior of the optical element (position of the ZCT).

As already mentioned, in further embodiments, the temperature-based control concept can be expanded in that the temperature for the individual sectors 211, 212, 213, 214 on the optical effective surface is kept constant over time compared to the respective initial value (i.e. the temperature value set when operation begins via the thermal manipulator 210). This not only keeps the average temperature of the mirror 200 constant over time, but also, to a first approximation, the spatial temperature distribution desired for correcting the cold aberrations is kept constant. Since the number of temperature measuring devices 203a, 203b,... that can be integrated into the mirror 200 is typically limited due to boundary conditions in the optical system and sometimes N > K applies (where N denotes the number of sectors 211, 212,... and K the number of temperature measuring devices 203a, 203b,...), in embodiments of the invention, an observer 230 according to 2 used to estimate the temperature distribution in the individual sectors 211, 212,... based on the temperature measurement values and a process model. The estimation quality that can be achieved here depends on both the quality of the process model and the knowledge of the input signals. To improve the estimation quality, the EUV load (as a significant heat load) can also be fed to the observer 230.

In further embodiments, the inventive control of the heating power coupled into the optical element or mirror 200 during operation via the thermal manipulator 210 can also be carried out on a wavefront basis, ie the heating profile set via the thermal manipulator 210 - again designed in particular as a sector heater - is optimized directly with regard to the wavefront generated by the mirror 200 or the associated optical system. The respective detection or estimation of the current wavefront effect can be carried out using a wavefront sensor (eg arranged in the area of the wafer stage) and preferably with additional use of a feed-forward model. Since the aim of this wavefront-based approach is not to determine the initial If the temperature distribution (ie the temperature distribution set at the start of operation or at the beginning of exposure of the mirror 200 to useful light or EUV radiation 220) is not kept constant in accordance with the setpoint value for the heating power set in particular to correct the cold aberrations, but the aim is to directly keep the wavefront effect of the mirror constant, significant deviations from the initial temperature distribution can arise.

der Position und Orientierung der optischen Elemente $\overrightarrow{x},$

eine weitere Wellenfrontwirkung $f\left(\overrightarrow{h}\right)$

des thermischen Manipulators bzw. Sektorheizers, sowie eine über diesen thermischen Manipulator eingestellte (Heizleistungs-)Amplitude $\overrightarrow{h}$

über eine Gewichtungs-Metrik D ein skalarer Wert zugeordnet wird. Die wellenfrontbasierte Optimierung entspricht dann einer Minimierung dieser Merit-Funktion gemäß $$\overrightarrow{x^{\ast }},\overrightarrow{h^{\ast }}=\underset{\overrightarrow{x},\overrightarrow{h}}{\text{argmin}}D\left(S+l\left(\overrightarrow{x}\right)+f\left(\overrightarrow{h}\right)\right).$$

The corresponding wavefront-based optimization can be done by minimizing a merit function, where for a given disturbance 5, a wavefront effect $l\left(\overrightarrow{x}\right)$

the position and orientation of the optical elements $\overrightarrow{x},$

another wavefront effect $e\left(\overrightarrow{H}\right)$

of the thermal manipulator or sector heater, as well as a (heating power) amplitude set via this thermal manipulator $\overrightarrow{H}$

a scalar value is assigned via a weighting metric D. The wavefront-based optimization then corresponds to a minimization of this merit function according to $$\overrightarrow{x^{\ast }},\overrightarrow{H^{\ast }}=\underset{\overrightarrow{x},\overrightarrow{H}}{\text{argmin}}D\left(S+l\left(\overrightarrow{x}\right)+e\left(\overrightarrow{H}\right)\right).$$

und der durch „Mirror Heating“ infolge EUV-Last induzierten Aberrationen $Z{\left(\overrightarrow{h},t\right)}_{FF,MH}$

erweitert: $$\overrightarrow{x^{\ast }}\left(t\right),\overrightarrow{h^{\ast }}\left(t\right)=\underset{\overrightarrow{x},\overrightarrow{h}}{\text{argmin}}D\left(S+l\left(\overrightarrow{x}\right)+f\left(\overrightarrow{h}\right)+Z{\left(\overrightarrow{h},t\right)}_{FF,MH}\right)$$

According to the invention, this merit function is now used for transient, wavefront-based “co-optimization” by predicting the wavefront effect introduced by the thermal manipulator or sector heater $e\left(\overrightarrow{H}\right)$

and the aberrations induced by mirror heating due to EUV load $Z{\left(\overrightarrow{H},t\right)}_{FF,MH}$

expanded: $$\overrightarrow{x^{\ast }}\left(t\right),\overrightarrow{H^{\ast }}\left(t\right)=\underset{\overrightarrow{x},\overrightarrow{H}}{\text{argmin}}D\left(S+l\left(\overrightarrow{x}\right)+e\left(\overrightarrow{H}\right)+Z{\left(\overrightarrow{H},t\right)}_{FF,MH}\right)$$

Disturbance 5 can be determined in particular via an initial wavefront measurement and repeatedly updated using further wavefront measurements. Alternatively, disturbance 5 can also be determined using simulations. The delay in the heating and cooling behavior of the sector heater can be taken into account in the optimization. A feed-forward model can be used to predict the respective wavefront effect between the respective wavefront measurements. Furthermore, the feed-forward model can also be used for model-based predictive control in order to prepare the heating power introduced via the thermal manipulator or sector heater for the new EUV load in the event of an upcoming setting change.

Although the invention has been described with reference to specific embodiments, numerous variations and alternative embodiments will become apparent to those skilled in the art, e.g. by combining and/or exchanging features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are included in the present invention, and the scope of the invention is limited only in accordance with the appended claims and their equivalents.

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• DE 102019219289 A1 [0008, 0014, 0015, 0059]

Claims (25)

Method for heating an optical element in an optical system, in particular a microlithographic projection exposure system, - wherein a heating power is introduced into the optical element (200) to produce a thermally induced deformation using a thermal manipulator (210); - wherein this heating power is adjusted before starting operation of the optical system in which useful light strikes the optical element (200) with regard to a target state of the optical element in which a first optical aberration is at least partially compensated; and - wherein after starting operation of the optical system, the heating power is regulated to the target state depending on the heat load of the useful light (220) striking the optical element. Procedure according to Claim 1 , characterized in that the first optical aberration is at least partially due to manufacturing or adjustment. Procedure according to Claim 1 or 2 , characterized in that the optical system has at least one further mirror which can be actuated in a plurality of degrees of freedom, in particular a plurality of further mirrors which can each be actuated in a plurality of degrees of freedom, the entirety of these degrees of freedom being used for the at least partial compensation of the first optical aberration. Method according to one of the Claims 1 until 3 , characterized in that the heating power is adjusted before the optical system starts operating, taking into account the respective effect of the heating power on a second optical aberration caused by useful light striking the optical element (200) during subsequent operation of the optical system. Method according to claim one of the Claims 1 until 4 , characterized in that the desired state is defined by a thermal state of the optical element (200). Method according to claim one of the Claims 1 until 4 , characterized in that the desired state is defined by a wavefront provided by the optical system in an image plane. Method according to one of the preceding claims, characterized in that the control of the heating power is carried out in such a way that the average temperature of the optical element (200) remains constant up to a maximum deviation of 0.5 K, in particular of a maximum of 0.2 K. Method according to one of the preceding claims, characterized in that the control of the heating power after commencement of operation of the optical system takes place on the basis of at least one temperature measured using at least one temperature measuring device (203a, 203b). Method according to one of the preceding claims, characterized in that the control of the heating power after commencement of operation of the optical system is carried out on the basis of at least one average temperature at the optical active surface of the optical element (200) estimated using at least one temperature measuring device (203a, 203b). Method according to one of the preceding claims, characterized in that the control of the heating power after commencement of operation of the optical system takes place on the basis of a temperature distribution on the optical effective surface of the optical element (200) estimated using one or more temperature measuring devices (203a, 203b). Procedure according to Claim 10 , characterized in that the temperature distribution on the optical effective surface of the optical element (200) is estimated from measurement signals supplied by the temperature measuring devices (203a, 203b) using an observer (230) based on a model. Method according to one of the preceding claims, characterized in that the control of the heating power after commencement of operation of the optical system is carried out on the basis of an estimate of the wavefront effect of the optical system, in particular on the basis of a feed-forward model. Procedure according to Claim 12 , characterized in that this estimation of the wavefront effect of the optical element (200) is carried out using at least one wavefront sensor. Procedure according to Claim 12 or 13 , characterized in that this estimation of the wavefront effect of the optical element (200) is carried out on the basis of target values for the heating power set by the thermal manipulator. Method according to one of the Claims 12 until 14 , characterized in that this estimation of the wavefront effect of the optical element (200) is based on a combination of wavefront and temperature measurements. Method according to one of the Claims 12 until 15 , characterized in that information about the reticle used, the illumination setting used and/or information from an intensity measurement is used in the feed-forward model. Method according to one of the Claims 12 until 16 , characterized in that the control of the heating power is carried out transiently on the basis of the feed-forward model. Procedure according to Claim 17 , characterized in that this transient control is carried out on the basis of the feed-forward model as a model predictive control to take into account a change in the reticle used and/or the illumination setting used. Method according to one of the preceding claims, characterized in that a combination of a plurality of mirrors is used in the control, this plurality comprising at least one mirror in which a heating profile complementary to a temperature distribution caused by useful light impinging on this mirror is generated via a heating device, and at least one mirror which is actively deformed for wavefront manipulation. Method according to one of the preceding claims, characterized in that the optical element (200) is a mirror. Method according to one of the preceding claims, characterized in that the optical element (200) is designed for an operating wavelength of less than 400 nm, in particular less than 250 nm, further in particular less than 200 nm. Method according to one of the preceding claims, characterized in that the optical element (200) is designed for an operating wavelength of less than 30 nm, in particular less than 15 nm. Optical system, in particular a microlithographic projection exposure system, with • at least one optical element (200); • a thermal manipulator (210) for heating this optical element (200); and • a control unit for controlling the heating power introduced into the optical element (200) by the thermal manipulator (210); • wherein this control is carried out on the basis of a target state in which a first optical aberration is at least partially compensated, and depending on the heat load of the useful light striking the optical element (200). Optical system according to Claim 23 , characterized in that the first optical aberration is at least partially due to manufacturing or adjustment. Optical system according to Claim 23 or 24 , characterized in that it is configured to carry out a method according to one of the Claims 1 until 22 to carry out.

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