The present invention relates to a drive device for driving an actuator of an opti- cal system, to an optical system comprising such a drive device, and to a lithogra- phy apparatus comprising such an optical system.

The content of the priority apphcation DE 10 2020 205 279.4 is incorporated by reference in its entirety.

Microlithography apparatuses are known which have actuatable optical ele- ments, such as, for example, microlens element arrays or micromirror arrays. Mi- crolithography is used for producing microstructured components, such as, for ex- ample, integrated circuits. The microhthography process is performed using a li- thography apparatus, which has an illumination system and a projection system. The image of a mask (reticle) illuminated by means of the illumination system is in this case projected by means of the projection system onto a substrate, for ex- ample a silicon wafer, which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate. The imaging of the mask on the substrate can be improved by means of actuatable optical elements. By way of example, wavefront aberrations during exposure, which result in mag- nified and/or unsharp imagings, can be compensated for.

Such correction by means of the optical element requires detection of the wave- front and signal processing in order to determine a respective position of an opti- cal element which enables the wavefront to be corrected as desired. In the last step, it is necessary to amplify the drive signal for a respective optical element and to output it to the actuator of the optical element. By way of example, a PMN actuator (PMN; lead magnesium niobate) can be used as actuator. A PMN actuator enables distance positioning in the sub -micrometre range or sub-nanometre range. In this case, the actuator, having actuator ele- ments stacked one on top of another, experiences a force that causes a specific linear expansion as a result of a DC voltage being applied. The position set by way of the DC voltage (DC; Direct Current) can be adversely influenced by exter- nal electromechanical crosstalk at the fundamentally arising resonance points of the actuator driven by the DC voltage. Owing to this electromechanical crosstalk, precise positioning is no longer able to be set in a stable manner. In this case, the mechanical resonances are all the greater the higher the applied DC voltage.

Said resonance points may also change in the long term, for example as a result of temperature drift or as a result of adhesive drift if the mechanical linking of the adhesive material changes, or as a result of hysteresis or ageing. An imped- ance measurement would be helpful in this context.

However, conventional impedance measuring devices are often too cost-intensive and furthermore do not have an inline capability, that is to say that they are reg- ularly not able to be used in a lithography apparatus. Furthermore, integrated impedance measuring bridges, which are usuahy designed for excessively high impedance values, prove not to be suitable for the present application in a lithog- raphy apparatus since the impedance value range of interest here encompasses a plurality of orders of magnitude and the range of interest is only a fraction of the total range.

Against this background, it is an object of the present invention to improve the driving of an actuator of an optical system.

In accordance with a first aspect, a drive device for driving a capacitive actuator of an optical system is proposed. The drive device comprises: a drive unit for applying a drive voltage to the actuator for setting a specific position of the driven actuator, wherein the drive unit and the actuator are cou- pled via a first node, a source controlled by an excitation signal and coupled to the first node, for feeding a time-dependent AC current signal into the first node in such a way that a specific AC voltage arises at the actuator as a result of the superposition of the drive voltage and an AC voltage corresponding to a product of the AC current sig- nal and the impedance of the actuator, a filter unit connected to the output of the actuator and serving for filtering an output signal of the actuator, and a determining unit coupled to an output of the filter unit and configured to determine an impedance behaviour of the actuator depending on the filtered out- put signal and to output at its output the drive signal for driving the source.

The actuator is, in particular, a PMN actuator (PMN; lead magnesium niobate) or a PZT actuator (PZT; lead zirconate titanate). The actuator is configured, in particular, to actuate an optical element of the optical system. Examples of such an optical element include lens elements, mirrors and adaptive mirrors.

The present drive device enables a fast determination, with an inline capability, of the impedance behaviour of the actuator, in particular an impedance measure- ment of the actuator installed in the lithography apparatus.

On the basis of the determined impedance behaviour of the actuator, suitable remedies or countermeasures, in particular an active inline calibration or inline damping, can also be implemented by means of the drive signal and a corre- sponding AC current signal fed in at the first node.

In accordance with one embodiment, the source comprises a signal generator con- trolled by the excitation signal, and a current or voltage source controlled by an output signal of the controlled signal generator for outputting the time-depend- ent AC current signal.

In accordance with a further embodiment, the determining unit is configured to determine the transfer function of a section between the output of the signal gen- erator and the output of the filter unit, wherein the section comprises the con- trolled current or voltage source, the first node, the actuator and the filter unit to determine an inverse of the determined transfer function, and to generate the drive signal using the calculated inverse.

The inverse can also be referred to as an inverse transfer function. As an alterna- tive to the inverse transfer function, it is also possible to use some other transfor- mation of the transfer function which can particularly cause the resonance points of the actuator in order to be able to determine them optimally.

In accordance with a further embodiment, the transfer function is a frequency- dependent signal transfer function of the drive signal embodied as a time-depend- ent excitation voltage and of the filtered output signal embodied as a complex ex- citation response voltage.

In accordance with a further embodiment, the filter unit is embodied as a high- pass filter for providing a high-pass-filtered output signal.

Preferably, the determining unit can then determine the impedance behaviour of the actuator depending on the high-pass-filtered output signal.

In accordance with a further embodiment, a peak-to-peak detector connected downstream of the high-pass filter and an output stage connected downstream of the peak-to-peak detector for providing at least one narrowband partial output sig- nal are provided, wherein the determining unit is configured to carry out a broadband determination of the impedance behaviour of the actuator on the basis of the high-pass-filtered output signal and/or to carry out a narrowband determi- nation of the impedance behaviour of the actuator on the basis of the at least one narrowband partial output signal.

An inverse Fourier transformation, in particular, is used in the broadband deter- mination of the impedance behaviour of the actuator. For this reason, the broad- band determination of the impedance behaviour by the determining unit can also be referred to as an IFT mode (IFT; Inverse Fourier Transformation). Narrow fre- quency bands can be examined or scanned very accurately in the narrowband de- termination of the impedance behaviour. For this reason, the narrowband deter- mination of the impedance behaviour by the determining unit can also be referred to as a scanning mode.

In accordance with a further embodiment, the determining unit is configured to generate the drive signal depending on the determined impedance behaviour of the actuator in such a way that the specific AC voltage arising at the actuator has an amplitude that is constant over the frequency.

The specific AC voltage having the amplitude that is constant over the frequency has the advantage that very precise positioning of the optical element to be actu- ated is possible.

In accordance with a further embodiment, the drive device is configured to control, by open-loop control, the specific AC voltage arising at the actuator to the ampli- tude that is constant over the frequency using the drive signal.

In accordance with a further embodiment, the drive device is configured to control, by closed-loop control, the specific AC voltage arising at the actuator to the ampli- tude that is constant over the frequency using the drive signal. Depending on the application, the specific AC voltage can be controlled to the am- plitude that is constant over the frequency by open-loop or closed-loop control. Suit- able active inline damping measures are made possible as a result.

In accordance with a further embodiment, the drive unit comprises a DC voltage source. Furthermore, an input resistance is coupled between the DC voltage source and the first node.

In accordance with a further embodiment, the source comprises a controllable volt- age or current source. Moreover, a coupling capacitance is connected between the voltage or current source and the first node.

In accordance with a further embodiment, the drive device is configured to drive a plurality N of capacitive actuators of the optical system. In this case, an optical element of the optical system is assigned to each actuator. By way of example, N = 100.

In accordance with a further embodiment, the respective actuator is assigned a respective drive unit for applying a drive voltage to the actuator for setting a spe- cific position of the driven actuator and a respective filter unit connected to the output of the actuator and serving for filtering an output signal of the actuator. In this case, the determining unit is coupled to the output of the respective filter unit and is configured to determine the impedance behaviour of the respective actuator depending on the respective filtered output signal and to output at its output the drive signal for the respective actuator.

The determining unit can in particular select the actuator to be measured, for ex- ample by way of a number of switches that can be driven by the determining unit. In particular, the determining unit is configured to drive said switches, to drive the drive units, to calculate the excitation signals and to sample the outputs of the filter units. The determining unit is embodied as an SPU (Signal Processing Unit), for example.

In this case, the excitation signal is preferably an excitation signal calculated by means of the inverse Fourier transformation (IFT stimulus). In this case, the IFT stimulus is preferably calculated from a predefined excitation frequency profile, wherein a suitably chosen excitation profile can particularly increase the sensitiv- ity of the impedance measurement, for example by way of a frequency response chosen deliberately to be flat in the vicinity of a resonant frequency.

In accordance with a further embodiment, the respective first node is connectable to the source by means of a respective controllable switch. In this case, the deter- mining unit is configured, for the purpose of determining the impedance behaviour of a specific actuator of the plurality of actuators, to drive the drive unit assigned to the specific actuator and the switch assigned to the specific actuator.

The respective unit, for example the determining unit, can be implemented in terms of hardware technology and/or else in terms of software technology or as a combination of the two. In the case of an implementation in terms of hardware technology, the respective unit can be embodied as a device or as part of a device, for example as a computer or as a microprocessor. In the case of an implementation in terms of software technology, the respective unit or a part of the unit can be embodied as a computer program product, as a function, as a routine, as an inde- pendent process, as part of a program code and/or as an executable object.

In accordance with a second aspect, an optical system comprising a number of ac- tuatable optical elements is proposed. Each of the actuatable optical elements of the number is assigned an actuator and each actuator is assigned a drive device for driving the actuator in accordance with the first aspect or one of the embodi- ments of the first aspect.

The optical system comprises, in particular, a micromirror array and/or a micro- lens element array having a multiplicity of optical elements that are actuatable independently of one another.

In embodiments, groups of actuators can be defined, wherein all actuators of a group are assigned the same drive device.

In accordance with a third aspect, a lithography apparatus comprising an optical system in accordance with the second aspect is proposed.

A lithography apparatus comprises an illumination system and an imaging sys- tem, for example. The illumination system comprises, in particular, a light source and a beam-shaping optical unit. The imaging system comprises, in particular, an imaging optical unit for imaging the mask onto the substrate.

The optical system can be used in the illumination system, in the beam-shaping optical unit, and also in the imaging system. In preferred embodiments, the optical system is embodied as a microlens element array or a micromirror array and serves for example for wavefront correction in the imaging system.

The lithography apparatus is for example an EUV lithography apparatus, the working light of which is in a wavelength range of 0.1 nm to 30 nm, or a DUV lithography apparatus, the working light of which is in a wavelength range of 30 nm to 250 nm.

Preferably, the lithography apparatus additionally comprises a measuring system configured for detecting a wavefront and configured for outputting a correction signal for correcting the wavefront by means of the optical system. The correction signal can serve in particular as the input signal for the drive device.

“A(n); one” in the present case should not necessarily be understood as restrictive to exactly one element. Rather, a plurality of elements, such as, for example, two, three or more, can also be provided. Any other numeral used here, too, should not be understood to the effect that there is a restriction to exactly the stated number of elements. Rather, numerical deviations upwards and downwards are possible, unless indicated to the contrary.

Further possible implementations of the invention also comprise not explicitly mentioned combinations of features or embodiments that are described above or below with respect to the exemplary embodiments. In this case, a person skilled in the art will also add individual aspects as improvements or supplementations to the respective basic form of the invention.

Further advantageous configurations and aspects of the invention are the subject matter of the dependent claims and also of the exemplary embodiments of the in- vention described below. In the text that follows, the invention is explained in more detail on the basis of preferred embodiments with reference to the accompa- nying figures.

Fig. 1 shows a schematic block diagram of a first embodiment of a drive device for driving a capacitive actuator of an optical system;

Fig. 2 shows an equivalent circuit diagram to a first approximation of an actuator according to Fig. 1,

Fig. 3 shows a diagram for illustrating the control-voltage-dependent, electrome- chanical resonances of an actuator according to Fig. 1, Fig. 4 shows a schematic block diagram of a second embodiment of a drive device for driving capacitive actuators of an optical system;

Fig. 5 shows a diagram for illustrating a measurement voltage profile of the fil- tered output signal of an actuator according to Fig. 4;

Fig. 6 shows a diagram for illustrating a determined transfer function and the in- verse thereof for generating the excitation signal according to Fig. 4;

Fig. 7 shows a diagram for illustrating an exemplary embodiment of a suitable phase profile for generating the excitation signal according to Fig. 4;

Fig. 8A shows a diagram for illustrating an exemplary embodiment of a suitable excitation signal and the AC component of the resulting response signal;

Fig. 8B shows a diagram for illustrating a further exemplary embodiment of a suitable excitation signal;

Fig. 9A shows a diagram for illustrating the theoretical impedance behaviour to be measured of an actuator according to Fig. 4 for a predefined drive voltage;

Fig. 9B shows a first diagram for illustrating an experimentally measured imped- ance behaviour of an actuator according to Fig. 4 for predefined drive voltages;

Fig. 9C shows a second diagram for illustrating the experimentally measured im- pedance behaviour of the actuator according to Fig. 4 for predefined drive volt- ages;

Fig. 10 shows a schematic block diagram of an embodiment of an optical system; Fig. 11A shows a schematic view of an embodiment of an EUV lithography appa- ratus and

Fig. 11B shows a schematic view of an embodiment of a DUV lithography appa- ratus.

Identical elements or elements having an identical function have been provided with the same reference signs in the figures, unless indicated to the contrary. It should also be noted that the illustrations in the figures are not necessarily true to scale.

Fig. 1 shows a schematic block diagram of a first embodiment of a drive device 100 for driving a capacitive actuator 200 of an optical system 300. Examples of an optical system 300 are shown in Figs. 10, 11A and 11B.

The actuator 200 can be, for example, a PMN actuator (PMN; lead magnesium niobate) or a PZT actuator (PZT; lead zirconate titanate). The actuator 200 is suitable for actuating an optical element 310, such as, for example, a lens ele- ment, a mirror or an adaptive mirror.

The first embodiment of the drive device 100 in accordance with Fig. 1 is also ex- plained below with reference to Figs. 2 and 3 and the further figures. In this case, Fig. 2 shows an equivalent circuit diagram of the actuator 200 according to Fig. 1, in particular in the region of a resonance point, and Fig. 3 shows a diagram for illustrating the mechanical resonances of the actuator 200 according to Fig. 1.

The drive device 100 in Fig. 1 comprises a drive unit 110 for applying a drive voltage U _DC to the actuator 200 for setting a specific position of the driven actua- tor 200. The drive unit 110 and the actuator 200 are coupled via a first node K1. In the example in Fig. 1, the drive unit 110 is a DC voltage source and the drive voltage U _DC is a DC voltage. In this case, an input resistance R _IN is connected be- tween the DC voltage source 110 and the first node K1.

In accordance with the equivalent circuit diagram in Fig. 2, the actuator 200 can be regarded as a DC -voltage-independent RLC series circuit comprising Rs, Cs and Ls, wherein a further RLC series circuit comprising R _x , C _x and L _x is con- nected in parallel with the reactive components Cs and Ls. In accordance with the equivalent circuit diagram in Fig. 2, L _s and L _x shall be lossless, ideal induct- ances in the simplified actuator equivalent circuit diagram in Fig. 2.

The text hereinafter demonstrates that the dynamic range of the impedance measurement and the measurement speed can be particularly increased by means of the measurement architecture in Fig. 1 (and in Fig. 4) and a corre- spondingly chosen excitation signal s(t) (see Fig. 8A, for example).

The influence of the drive voltage U _DC on the resonance points (see Fig. 3) will firstly be considered in a decoupled way, proceeding from the impedance behav- iour IV to be measured (see Fig. 9) of the actuator 200. As a result of introducing the purely resistive influence - to be considered firstly in a virtual fashion - of the series resistance R _s in accordance with Fig. 2 and the associated resonance mani- festation, by means of the likewise virtual volt age -depen dent series damping re- sistance R _x it is possible to design the measurement architecture in accordance with Fig. 1, which can be used to carry out suitable impedance measurements of the actuator 200, for example in accordance with Fig. 9.

The drive device 100 in Fig. 1 furthermore has a source 120 controlled by an exci- tation signal s(t) and coupled to the first node K1, for feeding a time-dependent AC current signal l(t) into the first node K1 in such a way that a specific AC volt- age arises at the actuator 200 as a result of the superposition of the drive voltage U _DC and an AC voltage corresponding to a product of the AC current signal l(t) and the impedance Z of the actuator 200.

The source 120 in Fig. 1 comprises a signal generator 121 controlled by the exci- tation signal s(t), and a current or voltage source 122 controlled by the output signal AS of the controlled signal generator 121 for outputting the time-depend- ent AC current signal l(t). A coupling capacitance C _IN for coupling to the first node K1 is connected between the current or voltage source 122 and the first node K1.

The drive device 100 in Fig. 1 further has a filter unit 130 connected to the out- put of the actuator 200 and serving for filtering the output signal A of the actua- tor 200.

A determining unit 140 is coupled to the output of the filter unit 130 and is con- figured to determine an impedance behaviour IV of the actuator 200 depending on the filtered output signal r(t) and to output at its output the excitation signal s(t) for driving the source 120.

In particular, the determining unit 140 is configured to determine the transfer function H (see Fig. 6) of a section between the output of the signal generator 121 and the output of the filter unit 130, wherein the section comprises the controlled current or voltage source 122, the couphng capacitance C _IN , the first node K1, the actuator 200 and the filter unit 130. Furthermore, the determining unit 140 is configured to determine an inverse I of the determined transfer function H (see Fig. 6). In this case, the determining unit 140 is preferably configured to generate the excitation signal s(t) using the calculated inverse I.

Fig. 4 shows a schematic block diagram of a second embodiment of a drive device 100 for driving capacitive actuators 200, 205 of an optical system 300. The drive device 100 in Fig. 4 is suitable for driving a plurality N of capacitive ac- tuators 200, 205 of the optical system 300, wherein an optical element 310 of the optical system is assigned to each actuator 200, 205 of the plurality N.

In Fig. 4, the reference sign 200 denotes the actuator i that is currently to be measured, wherein the reference sign 205 denotes the further (N-1) actuators that are currently not being measured with regard to their impedance behaviour. The actuator 200 that is respectively to be measured is selected by means of the switch S. A drive unit 110, a filter unit 130, a peak-to-peak detector 150 and an output stage 160 are provided for the actuator 200 to be measured.

Analogously to the reference sign 205 denoting the group of (N-1) actuators, the reference sign 115 denotes a group of drive units for said group 205 of (N-1) actu- ators. Correspondingly, the reference sign 135 denotes a group of filter units for said (N-1) actuators, the reference sign 155 denotes a group of peak-to-peak de- tectors for said (N-1) actuators 205, and the reference sign 165 denotes a group of output stages for said group of (N-1) actuators.

Therefore, the respective actuator 200, 205 is assigned a respective drive unit 110, 115 for applying a drive voltage U _DC to the actuator 200, 205 for setting a specific position of the driven actuator 200, 205 and a respective filter unit 130, 135 connected to the output of the actuator 200, 205 and serving for filtering the output signal of the actuator 200, 205.

The determining unit 140 in Fig. 4 is coupled to the output of the respective filter unit 130, 135. As already explained above, the impedance behaviour IV of the actuator i having the reference sign 200 is currently being measured in the case of the switch posi- tion of the switch S in Fig. 4.

In the example in Fig. 4, the filter unit 130 is embodied as a high-pass filter for providing a high-pass-filtered output signal r ₁ (t). A peak-to-peak detector 150 and an output stage 160 connected downstream of the peak-to-peak detector 150 for providing at least one narrowband partial output signal r ₂ (t) are connected downstream of the high-pass filter 130. In this case, the determining unit 140 is configured to carry out a broadband determination of the impedance behaviour IV of the actuator 200 on the basis of the high-pass-filtered output signal r ₁ (t) and/or to carry out a narrowband determination of the impedance behaviour IV of the actuator 200 on the basis of the at least one narrowband partial output signal r ₂ (t). In this case, the determining unit 140 samples the outputs of the high-pass filter 130 in order to obtain the signal r ₁ (t) and the output of the output stage 160 in order to obtain the signal r ₂ (t). Furthermore, the determining unit 140 controls the switches S, wherein the switches S assigned to the N-1 actuators 205 are not depicted in Fig. 4 for illustration reasons, and calculates the respective excitation signal s(t) for the source 120.

The determining unit 140 is embodied as an SPU (Signal Processing Unit), for ex- ample. To summarize, the determining unit 140 controls the switches S and the drive voltages U _DC , calculates the excitation signals s(t) for the source 120 and samples the outputs of the high-pass filter 130 and of the output stage 160.

Overall, in the case of the drive device 100 in Fig. 4 for fast inline measurement of the impedance behaviour IV of the individual actuator 200, 205 the following measures are implemented: 1. Dedicated inline impedance measurement with applied DC+AC voltage comprising drive voltage U _DC and the AC voltage l(t) * Z. Application measure- ment frequencies of the order of magnitude of Hz to 100 kHz are covered here.

2. A broadband, low-noise controllable current or voltage source 122 is used as part of the source 120.

3. The measurement signal (amplitude response and phase response) of the current or voltage profile arising at the actuator impedance is coupled out at the outputs 130 and 160.

4. The measurement of the voltage dropped across the actuator 200 can be carried out either in a narrowband fashion by way of the signal r ₂ (t) (also re- ferred to as scanning mode, e.g. by means of the sinusoidal excitation signal) or else in a broadband fashion by means of the signal r ₁ (t) (also referred to as IFT mode, e.g. by means of inverse Fourier transformation).

Referring to the equivalent circuit diagram in Fig. 2, the decoupling of the imped- ance alteration of the purely resistive influence - to be considered firstly in a vir- tual fashion - of the series resistance R _s (= R _S,0 in the zero voltage state) and the associated resonance manifestation is achieved by means of the likewise virtual control-voltage-dependent series damping resistance R _x in that the actuator 200, as already explained above, is regarded as a DC-voltage-independent RLC series circuit (R _s , C _s and L _s ) (cf. Fig. 2) and the further RLC series circuit comprising R _x , C _x and L _x is connected in parallel with the reactive components C _s and L _s .

Variation of R _x thus simulates the DC voltage influence to a first approximation; if the resistance R _x decreases (with higher U _DC ), the resonance is amplified; by contrast, if the resistance R _x increases (with lower U _DC ), a lower manifestation of the resonance results. In this respect, Fig. 5 shows a diagram for illustrating a measurement voltage profile of the filtered output signal r ₂ (t) of the actuator 200 according to Fig. 2.

The following can equivalently be formulated: An increase in the drive voltage U _DC of the actuator 200 which results in a slight variation of the real R _s accord- ingly causes a slight decrease in R _x which in turn results in a strong manifesta- tion of the resonances in the actuator 200. In this respect, Fig. 5 shows a peak value of the measurement voltage profile in the scanning mode: Amplitude meas- urement value in the settled state for actuator impedances without resonance (variation of the series resistance R _s ) in the case of a resonance (variation of the series damping resistance R _x ) measured at a chosen measurement frequency.

For a suitable design of the coupling capacitance C _IN and of the input resistances R _IN , the following may hold true: C _IN > > C _S and R _IN > > R _S

One example in this respect: C _IN ≥ 10 * C _S and R _IN ≥ N * R _s for a number of N ac- tuators, as shown in Fig. 4, in order to ensure a sufficient couphng of the excita- tion signal (sinusoidal or calculated by IFT).

A fast broadband image of the actuator resonances can be obtained by means of fast Fourier calculation (FFT; Fast Fourier Transformation) of the output signal r ₁ (t). The maximum measurement frequency can be of the order of magnitude of MHz, preferably approximately 100 kHz or less. In this case, the excitation signal s(t) is a sinusoidal signal or preferably an excitation signal calculated by means of inverse Fourier transformation. The IFT stimulus is preferably calculated from a predefined excitation frequency profile. In this case, by means of a suitably cho- sen profile, the sensitivity of the impedance measurement can be particularly in- creased, for example by way of a frequency response chosen deliberately to be flat in the vicinity of a resonant frequency (cf. Fig. 3). A flat profile of the output r ₂ (t) can be achieved e.g. by the excitation signal s(t) having an inverse excitation fre- quency profile with respect to the actuator impedance.

As explained above, the determining unit 140 in Fig. 4 is able to determine the impedance behaviour IV of the actuator 200 inline and rapidly, in particular in real time. On the basis thereof, it is possible to implement active damping measures against the electromechanical disturbances (see Fig. 3) in the actuator resonances that arise as a result of the drive voltage U _DC .

In this case, the determining unit 140 is configured in particular to generate the excitation signal s(t) depending on the determined impedance behaviour IV of the actuator 200 in such a way that the AC voltage arising at the actuator 200 has an amplitude that is constant over the frequency. In this respect, Fig. 9A shows a di- agram for illustrating the theoretical impedance behaviour to be measured of the actuator 200 according to Fig. 4 for a predefined drive voltage U _DC . In detail, in Fig. 9A, the curve 901 shows the impedance of the actuator 200, the curve 902 shows the real resistance of the actuator and the curve 903 shows the capaci- tance (relative to a reference capacitance).

Thus, in order to carry out a faster measurement of all actuator resonances in the frequency range of interest, the impedance measurement architecture in accord- ance with Fig. 4 can be used. In this case, in particular, a broadband excitation signal s(t) for each actuator of interest, here for example the actuator 200, is cal- culated by means of inverse Fourier transformation proceeding from an excita- tion frequency profile and is transferred to a sufficiently fast digital-to-analogue converter, accommodated in the signal generator 121, for example. By means of a fast Fourier transformation of the output signal r ₁ (t), for each ac- tuator with an apphed drive voltage U _DC in the frequency range of interest all resonance points are determined in real time:

An initial transfer function H of the section between the output of the signal gen- erator 121 and the output of the high-pass filter 130 is determined, in particular with a drive voltage U _DC of 0 V or for R _s = R _s,0 .

The inverse transfer function or inverse I (see Fig. 6) of the initial transfer func- tion H is then calculated.

The excitation signal s(t) can then be calculated from the inverse Fourier trans- formation of the inverse transfer function I and a suitable phase profile Φ(f). In this respect, Fig. 7 shows an exemplary embodiment of a suitable phase profile Φ(f) for calculating the IFT excitation signal s(t).

In this respect, Fig. 8A shows a diagram for illustrating an exemplary embodi- ment of a suitable excitation signal s(t) and the AC component I(t)*Z(jΩ) of the resulting response signal r(t).

Moreover, Fig. 8B illustrates a diagram for illustrating a further exemplary em- bodiment of a suitable excitation signal s(t). Fig. 8B shows a different excitation signal s(t) by comparison with Fig. 8A. The excitation signal s(t) in Fig. 8B is based on a different phase function compared with the excitation signal s(t) in Fig. 8A. Comparison of Figs. 8A and 8B shows that the energy in Fig. 8B is more distributed over time than in Fig. 8A. The excitation signal s(t) in Fig. 8B can also be regarded as a pseudo-noise signal and can also be used permanently dur- ing operation of the lithography apparatus. Alternatively, it is also possible to use, instead of the inverse transfer function I with respect to the transfer function H, some other transformation of H that can particularly cause the resonance points of the actuator 200. In this case, the phase profile Φ(f) can have any desired profile. It is also possible to use different frequency ranges for groups or subgroups of actuators 200, 205, both for excita- tion and for detection. Frequency- division multiplexing operation can be used for this purpose.

In this respect, Fig. 9B and Fig. 9C show diagrams for illustrating the experi- mentally measured impedance behaviour of the actuator 200 according to Fig. 4. As shown in Fig. 9A, the falling curve 901 represents a theoretical actuator im- pedance to be measured wherein the fall extends over three orders of magnitude (from 10 kΩ down to 10 Ω in this exemplary embodiment). As can be gathered from the box showing an enlarged view in Fig. 9A, three very small resonance points of the actuator 200 can be seen for a drive voltage.

In this context, the impedance behaviour of an actuator 200 according to Fig. 4 was measured in an experiment for predefined drive voltages.

The measured impedance behaviour of the actuator 200 exhibits three resonance points RS1, RS2 and RS3 in Fig. 9B and in Fig. 9C. The first resonance point RS1 and the third resonance point RS3 arise for one and the same drive voltage, which can also be referred to as actuator DC voltage and is 75 volts, for example. The second resonance point RS2 at f/f _REF = 3.1 arises for three different drive volt- ages or actuator DC voltages of, for example, 50 volts, 75 volts and 100 volts. The box showing an enlarged view in Fig. 9B shows that three different peaks P1 (for a drive voltage of 100 volts), P2 (for a drive voltage of 75 volts) and P3 (for a drive voltage of 50 volts) are able to be differentiated at the resonance point RS2. The same is shown by the box showing the enlarged view in Fig. 9C and this empha- sizes that at the first resonance point RS1 one peak is caused by a single actuator DC voltage and at the second resonance point RS2 three peaks P1, P2 and P3 are caused by three actuator DC voltages.

Overall, the experiment in accordance with Fig. 9B and in accordance with Fig. 9C shows how the impedance measurement values for an IFT excitation with a length of just under 50 milliseconds, for example, can be correctly mapped.

An example of a computation algorithm for the excitation signal s(t) is presented below:

U _Stim (t) := _s (t) shall be the output voltage of the high-pass filter 130 - also the in- put voltage for the source 120;

U _out (jω) shall be the complex excitation response voltage at the output of the high-pass filter 130; H _o (jω) shall be the frequency-dependent signal transfer function of U _Stim (jω) with respect to the output signal of the high-pass filter 130;

Z _Act shall be the actuator impedance;

Z _o shall be a fixed, selected reference resistance (e.g. 10Ω or 50Ω); a shall be a constant transfer factor and

U _{Stim, o} shall be a voltage amplitude set by the SPU 140. s(t) := U _Stim (t) shall be the excitation signal. The following equations hold true in this exemplary embodiment:

(1), and where

(2) there follows:

(3); and where

(4); This results in an, as shown in Fig. 9A, almost flat profile of the frequency re- sponse apart from the resonance points of the actuator 200 that are to be particu- larly emphasized.

Fig. 10 shows a schematic block diagram of an embodiment of an optical system 300 comprising a plurahty of actuatable optical elements 310. The optical system 300 is embodied here as a micromirror array, wherein the optical elements 310 are micromirrors. Each micromirror 310 is actuatable by means of an assigned actuator 200. By way of example, a respective micromirror 310 can be tilted about two axes and/or displaced in one, two, or three spatial axes by means of the assigned actuator 200. The reference signs only of the topmost row of these ele- ments are depicted, for reasons of clarity.

The optical system 300 comprises a correction unit 320 configured for generating an input signal E for each of the micromirrors 310. By way of example, the opti- cal system 300 is configured for correcting a wavefront of light in a lithography apparatus 600A, 600B (see Figs. 6A, 6B), wherein the correction unit 320, for ex- ample, depending on a measured shape of the wavefront and a target shape of the wavefront, determines a target position of each of the micromirrors 310 and outputs a corresponding input signal E.

The respective input signal E is fed to a drive device 100 assigned to a respective actuator 200. The drive device 100 drives the respective actuator 200 for example with a filtered, amplified modulation signal fPWM. The drive device 100 has been described with reference to Figs. 1 to 9, in particular. A position of the respective micromirror 310 is thus set.

Fig. 11A shows a schematic view of an EUV lithography apparatus 600A com- prising a beam-shaping and illumination system 602 and a projection system 604. In this case, EUV stands for “extreme ultraviolet” and denotes a wavelength of the working light of between 0.1 nm and 30 nm. The beam-shaping and illumi- nation system 602 and the projection system 604 are respectively provided in a vacuum housing (not shown), wherein each vacuum housing is evacuated with the aid of an evacuation device (not shown). The vacuum housings are sur- rounded by a machine room (not shown), in which drive devices for mechanically moving or setting optical elements are provided. Moreover, electrical controllers and the like can also be provided in this machine room.

The EUV lithography apparatus 600A comprises an EUV light source 606A. A plasma source (or a synchrotron), which emits radiation 608A in the EUV range (extreme ultraviolet range), that is to say for example in the wavelength range of 5 nm to 20 nm, can for example be provided as the EUV light source 606A. In the beam-shaping and illumination system 602, the EUV radiation 608A is focused and the desired operating wavelength is filtered out from the EUV radiation 608A. The EUV radiation 608A generated by the EUV light source 606A has a relatively low transmissivity through air, for which reason the beam-guiding spaces in the beam-shaping and illumination system 602 and in the projection system 604 are evacuated.

The beam-shaping and illumination system 602 illustrated in Fig. 11A has five mirrors 610, 612, 614, 616, 618. After passing through the beam-shaping and il- lumination system 602, the EUV radiation 608A is guided onto a photomask (ret- icle) 620. The photomask 620 is likewise embodied as a reflective optical element and can be arranged outside the systems 602, 604. Furthermore, the EUV radia- tion 608A can be directed onto the photomask 620 by means of a mirror 622. The photomask 620 has a structure which is imaged onto a wafer 624 or the hke in a reduced fashion by means of the projection system 604.

The projection system 604 (also referred to as a projection lens) has five mirrors Ml to M5 for imaging the photomask 620 onto the wafer 624. In this case, individual mirrors Ml to M5 of the projection system 604 can be arranged sym- metrically in relation to an optical axis 626 of the projection system 604. It should be noted that the number of mirrors Ml to M6 of the EUV lithography ap- paratus 600 A is not restricted to the number represented. A greater or lesser number of mirrors Ml to M5 can also be provided. Furthermore, the mirrors Ml to M5 are generally curved at their front side for beam shaping.

Furthermore, the projection system 604 comprises an optical system 300 having a plurality of actuatable optical elements 310, for example the micromirror array described with reference to Fig. 10. The optical system 300 is configured in par- ticular for correcting dynamic imaging aberrations. The projection system 604 comprising the optical system 300 can be referred to as an adaptive optical unit. A resolution of the lithography apparatus 600A can thereby be increased. By way of example, depending on measured values of the wavefront of the projection light, a correction unit 320 generates an input signal E, which can comprise an individual signal in particular for a respective micromirror 310. The input signal E is converted into an amplified, filtered modulation signal fPWM by the drive device 100 for a respective optical element 310, and output to the respective actu- ator 200 for actuating the optical element 310. The respective actuator 200 corre- spondingly actuates the assigned micromirror 310.

Fig. 11B shows a schematic view of a DUV lithography apparatus 600B, which comprises a beam-shaping and illumination system 602 and a projection system 604. In this case, DUV stands for “deep ultraviolet” and denotes a wavelength of the working light of between 30 nm and 250 nm. As has already been described with reference to Fig. 11A, the beam-shaping and illumination system 602 and the projection system 604 can be arranged in a vacuum housing and/or be sur- rounded by a machine room with corresponding drive devices. The DUV lithography apparatus 600B has a DUV light source 606B. By way of example, an ArF excimer laser that emits radiation 608B in the DUV range at 193 nm, for example, can be provided as the DUV light source 606B.

The beam-shaping and illumination system 602 illustrated in Fig. 11B guides the DUV radiation 608B onto a photomask 620. The photomask 620 is embodied as a transmissive optical element and can be arranged outside the systems 602, 604. The photomask 620 has a structure which is imaged onto a wafer 624 or the like in a reduced fashion by means of the projection system 604.

The projection system 604 has a plurahty of lens elements 628 and/or mirrors 630 for imaging the photomask 620 onto the wafer 624. In this case, individual lens elements 628 and/or mirrors 630 of the projection system 604 can be ar- ranged symmetrically in relation to an optical axis 626 of the projection system 604. It should be noted that the number of lens elements 628 and mirrors 630 of the DUV hthography apparatus 600B is not restricted to the number repre- sented. A greater or lesser number of lens elements 628 and/or mirrors 630 can also be provided. Furthermore, the mirrors 630 are generally curved at their front side for beam shaping.

Furthermore, the projection system 604 comprises an optical system 300 having a plurahty of actuatable optical elements 310, for example a microlens element array, which can be constructed in particular according to the micromirror array described with reference to Fig. 10, wherein microlens elements are used instead of the micromirrors. The optical system 300 is configured in particular for cor- recting dynamic imaging aberrations. The projection system 604 comprising the optical system 300 can be referred to as an adaptive optical unit. A resolution of the lithography apparatus 600B can thereby be increased. In order to improve the imaging performance, in the present case an input signal E is predefined from outside. The input signal E comprises, in particular, an individual signal for each of the microlens elements 310 of the optical system 300. The drive device 100 converts the signal contained in the input signal E for a respective microlens element 310 into an amplified, filtered modulation signal fPWM and outputs the latter to the respective actuator 200. The respective actuator 200 correspondingly actuates the assigned microlens element 310.

An air gap between the last lens element 628 and the wafer 624 may be replaced by a liquid medium 632 which has a refractive index of > 1. The liquid medium 632 may be for example high-purity water. Such a construction is also referred to as immersion lithography and has an increased photolithographic resolution. The medium 632 can also be referred to as an immersion liquid.

Although the present invention has been described on the basis of exemplary em- bodiments, it is modifiable in diverse ways.

LIST OF REFERENCE SIGNS

100 Drive device

110 Drive unit

115 Group of drive units

120 Source

121 Signal generator

122 Controlled current or voltage source

130 Filter unit

135 Group of filter units

140 Determining unit

150 Peak to-peak detector

155 Group of peak-to-peak detectors

160 Output stage

165 Group of output stages

200 Actuator

205 Group of N-1 actuators

300 Optical system

310 Optical element

320 Correction unit

600A EUV lithography apparatus

600B DUV lithography apparatus

602 Beam-shaping and illumination system

604 Projection system

606A EUV light source

606B DUV hght source

608A EUV radiation

608B DUV radiation

610 Mirror

612 Mirror 614 Mirror

616 Mirror

618 Mirror

620 Photomask

622 Mirror

624 Wafer

626 Optical axis

628 Lens element

630 Mirror

632 Medium

901 Curve

902 Curve

903 Curve A Output signal aPWM Amplified signal

AS Output signal of the controlled signal generator

ClN Coupling capacitance

Cs Capacitance

Cx Capacitance

E Input signal f Frequency fPWM Filtered signal f _REF Reference frequency

H Transfer function

I Inverse of the transfer function

Kt) AC current signal

IV Impedance behaviour

K1 First node

K2 Second node

K3 Third node L _s Inductance

L _x Inductance Ml Mirror M2 Mirror M3 Mirror M4 Mirror M5 Mirror P1 Peak P2 Peak P3 Peak r(t) Filtered output signal r ₁ (t) High-pass-filtered output signal r ₂ (t) Narrowband output signal RIN Input resistance R _s Resistance R _s,0 Resistance in the zero voltage state RS1 Resonance point RS2 Resonance point RS3 Resonance point R _x Resistance

S Switch s(t) Drive signal

U _DC Drive voltage

U _DC ,1 Drive voltage

U _DC ,2 Drive voltage

U _DC ,3 Drive voltage

Z Impedance of the actuator