WO2022200203A1

WO2022200203A1 - Optical assembly and method for the production thereof, method for deforming an optical element, and projection exposure apparatus

Info

Publication number: WO2022200203A1
Application number: PCT/EP2022/057120
Authority: WO
Inventors: Erik Loopstra; Matthias Manger; Alexander Vogler; Markus Raab; Johannes Lippert; Sascha Bleidistel; Axel Witt; Andreas Raba
Original assignee: Carl Zeiss Smt Gmbh
Priority date: 2021-03-22
Filing date: 2022-03-18
Publication date: 2022-09-29
Also published as: TW202405505A; JP2024511089A; DE102021202769A1; TW202246841A; TWI821951B

Abstract

The invention relates to an optical assembly (1) comprising an optical element (2) with an optically active front side (3) and a back side (4) facing away from the front side (3). The optical assembly (1) comprises a plurality of electrostatic actuators (5) arranged distributed along the back side (4) of the optical element (2). Each actuator (5) comprises an electrode pair of two spaced apart electrodes (6, 7), each actuator (5) being configured and mechanically coupled to the back side (4) of the optical element (2) so that an electrostatic force which is generated by means of an electrical control voltage (U_1...n) between the electrodes (6, 7) of the electrode pair and which serves to deform the optical element (2) is transferred to the optical element (2). Provision is made for a dielectric (8) to be arranged between the electrodes (6, 7) of the electrode pair.

Description

Optical assembly and method for the production thereof, method for deforming an optical element, and projection exposure apparatus

This application claims priority to German Patent Application No. DE 10 2021 202 769.5 filed March 22, 2021 , incorporated by reference herein in their entirety to form a part of the present disclosure.

The invention relates to an optical assembly comprising an optical element with an optically active front side and a back side facing away from the front side and a plurality of electrostatic actuators arranged distributed along the back side of the optical element.

The invention further relates to a method for deforming an optical element by means of a plurality of electrostatic actuators and to a computer program product having program code means to carry out such a method.

The invention also relates to a method for producing an optical assembly which comprises an optical element with an optically active front side and a back side facing away from the front side and a plurality of electrostatic actuators.

The invention moreover relates to a microlithographic projection exposure apparatus comprising an illumination system, which has a radiation source, an illumination optical unit and a projection optical unit.

Projection exposure apparatuses or lithography apparatuses are used to produce integrated circuits with high precision. Here, the light of a radiation source is steered to a wafer to be exposed by way of optical elements such as mirrors and/or lens elements. The arrangement, position and shape of the optical elements decisively contribute to the quality of the exposure in this case.

On account of the advancing miniaturization of semiconductor circuits, there are ever more stringent demands put on both the resolution and the accuracy of projection exposure apparatuses. Correspondingly stringent demands are put, in particular, on the optical elements thereof and the actuation of the optical elements.

To increase the imaging accuracy of a projection exposure apparatus, experience has taught the targeted deformation of optical elements by way of actuatable components in order to correct imaging aberrations within the projection exposure apparatus. As a rule, ferroelectric actuators are fastened to the optical elements to this end, said actuators facilitating actuation on the basis of the piezoelectric or electrostrictive principle. By driving each individual actuator, it is finally possible to set targeted profiles of the optical element, for example of a mirror, and consequently correct the optical system.

Known actuation systems frequently require closed-loop operation, and hence a constant detection of the actually implemented deformation, in order to be able to be operated sufficiently accurately. However, the use of a sensor with a sufficiently high measurement accuracy is not always possible, especially within lithography apparatuses. For this reason, the known actuation systems often have feedforward operation. To this end, comprehensive system modelling is required for a sufficient accuracy and this requires a deep system understanding of the influence of the individual actuator on the optical element. All disturbances not taken into account and all non-ideal instances have a direct effect on the performance obtainable by the actuation system.

In this case, the change in temperature can represent one of the most significant disturbance variables. By way of example, the temperature of an optical element within a projection exposure apparatus can vary in a range between 20°C and 40°C. Further problems in system modelling relate to intrinsic nonreproducibilities of the actuators, for example creep, hysteresis, thermal hysteresis, variation in the behaviour and/or variations in the coefficients of thermal expansion.

To facilitate highly accurate positioning or deformation of the optical element, it is consequently necessary to also model and calibrate the electrostrictive and thermal hysteresis, and the actuator drift, in addition to the temperature calibration. This is only even possible with a very significant measurement outlay and very comprehensive measurement equipment.

The problem of the material-intrinsic non-reproducibilities of the actuator (for example hysteresis and creep) and the significant temperature dependence can be circumvented by virtue of switching to actuators on an electrostatic basis. The dynamic effect of electrostatic actuators is based on the force of an electric field acting between two adjacent electrodes. By way of example, the use of electrostatic actuators for deforming mirrors is proposed in the generic US 7,692,838 B2.

However, a problem with the deformation of optical elements by means of electrostatic actuators is the comparatively restricted dynamic effect in comparison with conventional actuators, for example in comparison with ferroelectric actuators. To increase the dynamic effect, US 7,692,838 B2, for example, proposes the use of complex nanolaminate films.

DE 102016209847 A1 also considers the use of electrostatic actuators for deforming mirrors. To increase the dynamic effect, the use of comb-like electrodes with meshing comb teeth is proposed.

The methods for increasing the dynamic effect of electrostatic actuators for the deformation of mirrors proposed in US 7,692,838 B2 and DE 10 2016 209 847 A1 are however comparatively complex. Further, it was surprisingly found that the force of the electrostatic actuators themselves is generally insufficient even in the case of the measures proposed in the aforementioned documents, in particular it does not suffice to deform optical elements in projection exposure apparatuses. In view of the known prior art, the object of the present invention consists in the provision of an optical assembly which facilitates a defamation of an optical element with high precision and with a high dynamic effect.

The present invention is also based on the object of providing a method for deforming an optical element, the method facilitating a deformation of an optical element with high precision and a high dynamic effect. Finally, it is also an object of the invention to provide an advantageous computer program product for carrying out the aforementioned method.

Moreover, it is an object of the invention to provide a method for producing an optical assembly, by means of which an improved optical assembly for deforming an optical element is able to be produced.

Finally, it is also an object of the invention to provide a microlithographic projection exposure apparatus which comprises at least one optical assembly with an optical element which is deformable by at least one electrostatic actuator for the purposes of correcting imaging aberrations with high precision.

The object is achieved for the optical assembly, the deformation method, the computer program product, the method for producing the optical assembly, and the projection exposure apparatus by means of the respective independent claims. The dependent claims and the features described below concern advantageous embodiments and variants of the invention.

An optical assembly is provided, comprising an optical element with an optically active front side and a back side facing away from the front side.

The back side of the optical element might optionally also have an optically active form, for example if the optical element is in the form of a lens element. However, the back side is preferably not optically active or is at least not used to influence the beam path from a radiation source.

The optically active front side of the optical element is preferably in the form of the mirror surface, in particular for reflecting or influencing the beam path of DUV ("deep ultraviolet") or EUV ("extreme ultraviolet") radiation.

The back side of the optical element can extend in plane parallel fashion or at least substantially in plane parallel fashion with respect to the front side, in particular in the non-deformed basic state of the optical element. However, this is not mandatory within the scope of the invention. The front side and/or the back side may also have an arched form, in particular be concave or convex.

According to the invention, the optical assembly comprises a plurality of electrostatic actuators arranged distributed along the back side of the optical element. The electrostatic actuators each have an electrode pair of two spaced apart electrodes. Electrodes preferably have a flat form, but may also have an arched, step-like and/or comb-like structure. Planar electrodes are preferably provided, for example in the style of a slab, a film or a vapour-deposited layer. By way of example, the thickness of an individual electrode can be 0.01 pm to 500 pm, preferably 0.01 pm to 100 pm, very preferably 0.01 pm to 10 pm.

The two electrodes of a common electrode pair may each be formed in one piece, but optionally else have a multi-part embodiment. Thus, to the extent that reference is made to "an" electrode within the scope of the present description, this may in principle relate to a single electrode or to a grouping of a plurality of individual electrodes that together form the electrode. Accordingly, a single actuator is not precluded from having more than two individual electrodes. Further, an individual actuator may also comprise a plurality of electrode pairs, each made from two spaced apart electrodes. This is not necessarily important within the scope of the invention; for simplification purposes, the invention is essentially described below on the basis of actuators which comprise exactly one electrode pair of exactly two spaced apart electrodes. However, this should not be understood to be restrictive.

According to the invention, each actuator is configured and mechanically coupled to the back side of the optical element so that an electrostatic force which is generated by means of an electrical control voltage between the electrodes of the electrode pair and which serves to deform the optical element is transferred to the optical element.

Where applicable, the optical assembly can have exactly one actuator for the targeted deformation of the optical element. However, the optical assembly preferably has a plurality of actuators; particularly preferably, the optical assembly comprises at least two actuators, at least ten actuators, at least 50 actuators, at least 100 actuators, at least 500 actuators, at least 1000 actuators, at least 5000 actuators or at least 10 000 actuators for the targeted deformation of the optical element. However, particularly preferably, the optical assembly has significantly more actuators.

The actuators are preferably in each case spaced apart equidistantly from the immediately adjacent actuators or distributed uniformly over the back side of the optical element.

As a result of the actuators, the optical element can be elastically deformable or at least substantially reversibly deformable. The deformation of the optical element is preferably implemented without hysteresis.

The "deformation" should be understood to mean, in particular, a deformation of the material of the optical element, as a result of which, for example, a sectional change in length of the material of the optical element or sectional surface deformation of the optical element can be caused. As a result of using electrostatic actuators, it is possible to reduce or completely circumvent the problems in the positioning accuracy caused by the intrinsic actuator non-reproducibilities since the dynamic effect is only determined by the force of the electric field between the electrodes of the electric pair.

The smallest possible distance between the electrodes of the electrode pair is preferably provided since an electrostatic drive or the electrostatic actuator can develop the greatest dynamic effect for small plate spacings. However, the maximum possible electrostatic force may be restricted by the breakdown voltage, above which a statically occurring ionization leads to an ionization avalanche between the two electrodes. It is well known that the breakdown voltage can be described by Paschen's law and can be determined accordingly for the respective actuator.

According to the invention, a dielectric is arranged between the electrodes of the electrode pair.

A solid or liquid dielectric is preferably provided although a gaseous dielectric can also be provided (but preferably not air or hydrogen).

In a development of the invention, provision can be made in particular for the dielectric to have a dielectric strength greater than the dielectric strength of air.

As a result of using a dielectric (in particular a solid or liquid dielectric), it is possible to increase the dielectric strength of the actuator and, in the case of the corresponding high permittivity, it is moreover possible to increase the force of the actuator. By way of example, the inventors have recognized that the maximum force between electrodes of the electrode pair when using air as a dielectric would only be 3 V/pm in the case of an electrode area of 100 mm², and consequently only approximately 0.004 N. As a result of the proposed use of a dielectric between the electrodes of the electrode pair that is preferably not air, this force can be sufficiently increased to facilitate a deformation of an optical element in practice.

Provision can be made for the dielectric to completely fill the space between the electrodes of the electrode pair. However, provision can also be made for the dielectric to only partly fill the space between the electrodes of the electrode pair, for example to be arranged centrally between the electrodes of the electrode pair and be spaced apart from one or both electrodes. Provision can also be made for one or both electrodes of the electrode pair to be covered by the dielectric, for example be coated therewith, in particular in the style of an insulation layer in order to increase the dielectric strength of the actuator.

Preferably, provision is made for a dielectric with a permittivity greater than 1 .0 (or greater than in vacuo), preferably with a permittivity greater than air, in particular a permittivity greater than 2, particularly preferably with a permittivity greater than 5, very particularly preferably with a permittivity greater than 10, even more preferably with a permittivity greater than 50, for example also with a permittivity of greater than 100 or more. Particularly when a solid dielectric is provided, the dielectric is preferably compressible in order to facilitate a variation in the distance between the electrodes of the electrode pair when the actuator is in operation.

In an advantageous development of the invention, provision can be made for one of the electrodes of the electrode pair to be designed as a control electrode, the control voltage being able to be applied thereto. The other electrode is preferably designed as a reference electrode, a reference potential being able to be applied together thereto.

The reference electrodes are preferably embodied as earth electrodes. The reference potential is preferably an earth potential.

In particular, provision can be made for the reference potential being able to be applied together to the reference electrode of the actuator and to a reference electrode of at least one of the further actuators, preferably of all actuators.

Contacting actuators can be significantly simplified by combining the reference electrodes or by way of a common contacting of all reference electrodes.

According to a development the invention, provision can be made for one of the electrodes of the electrode pair to be mechanically coupled to the back side of the optical element and for the other electrode to be mechanically coupled to a reference body that is spaced apart from the back side of the optical element.

Preferably, the control electrode is mechanically coupled to the reference body and the reference electrode is mechanically coupled to the back side of the optical element. This may ease the accessibility of the control electrodes for driving purposes or for the application of the control voltage.

In particular, the reference body can be part of the optical body, a mount for the optical element, a fastening frame for the optical element (for example, a fastening frame of an optical unit or a test bench) or a housing part for the optical element. As a rule, the reference body is statically coupled to a surrounding component.

Preferably, the electrode is cohesively connected to the optical element, in particular to the back side of the optical element. In principle, the electrode can be connected to the optical element in any desired way, for example in force fit or interlocking fashion. In particular a cohesive connection, for example by adhesively bonding the electrode to the optical element or else by vapour depositing the electrode on the optical element, was found to be particularly suitable, however. An integral embodiment of the electrode with the optical element can also be provided, for example using an additive manufacturing technique. The same fastening techniques can be provided for fastening the other electrode to the reference body.

According to a development of the invention, provision can be made forthe electrode coupled to the optical element to be fastened directly to the optical element. This allows deformations to be introduced into the optical element in particularly targeted fashion. However, this does not allow the introduction of global modes into the optical element even though this may be envisaged for various applications.

For this reason, provision can be made in an alternative development of the invention for the electrode coupled to the optical element to be indirectly connected to the optical element via an intermediate element that is spaced apart from the back side of the optical element.

The fastening techniques already described within the scope of fastening the electrode to the optical element may be provided for fastening the electrode to the intermediate element and/or for connecting the intermediate element to the optical element.

The intermediate element can consequently serve as a balancing plate and thus allow the introduction of global modes into the optical element, or the implementation of a more "long-wave" deformation of the optical element. The deformation of the actuators can consequently initially have a direct effect on the intermediate element and subsequently be transmitted to the optical element via the intermediate element.

Depending on application, a person skilled in the art might or might not provide an intermediate element and might moreover vary the thickness and elasticity or material properties of the intermediate element where required.

In a development of the invention, provision can be made for the intermediate element to be fastened to the optical element by spacer elements and/or spacer struts that are arranged distributed along the back side of the optical element.

The fastening techniques already described within the scope of fastening the electrode to the optical element may be provided for fastening the spacer elements/spacer struts to the optical element and/or to the intermediate element.

The influence of the actuators on the optical element via the intermediate element can be optimally set by way of the use of spacer elements and/or spacer struts, for example by varying the geometry of the spacer elements or spacer struts and/or material properties of the spacer elements or spacer struts.

By way of example, provision can be made for the individual spacer elements and/or spacer struts to be arranged in the middle or centrally (in respect of the electrode surfaces) relative to the electrodes of a respective actuator. In an advantageous development of the invention, provision can be made for the reference body to be fastened to the optical element or on the intermediate element via bearing units and/or bearing struts that are arranged distributed along the back side of the optical element.

The fastening techniques already described within the scope of fastening the electrode to the optical element may be provided for fastening the bearing units/bearing struts to the optical element and/or to the intermediate element and/or to the reference body.

Preferably, the electrodes of a respective actuator are arranged between individual bearing units and/or bearing struts. An advantageous force distribution can be facilitated in this way.

In a development of the invention, provision can be made for the spacer units or spacer struts to be offset from the bearing units or bearing struts along the back side of the optical element.

In this way, there can be a deformation of the optical element over a broad range without there being restrictions on account of the optical element being borne on the reference body.

In an advantageous development of the invention, provision can be made for the electrode coupled to the optical element to extend parallel to the back side of the optical element.

However, in principle, an arrangement of the electrode relative to the back side of the optical element that deviates from a parallel extent may also be provided. However, a parallel extent was found to be particularly suitable and can also be implemented comparatively easily from a technical point of view.

In an advantageous development of the invention, provision can be made for the electrode coupled to the optical element (preferably the control electrode) to be arranged on a side wall of a cutout extending into the optical element from the back side.

Optionally, additional provision can be made for the electrode coupled to the reference body (preferably the reference electrode) to be arranged on a projection extending into the cutout of the optical element from the reference body. Optionally, the electrode coupled to the reference body may also form the projection itself.

Preferably, the electrodes of the electrode pair are arranged orthogonal or at least substantially orthogonal with respect to the back side and/or front side of the optical element in their basic state when extending into the cutout or through the cutout.

In particular, the cutouts can be slots with a small width, for example slots with a width of a few micrometres. In the described way, it is possible to generate forces within the cutout and hence a deformation within the optical element when the control voltage is applied, and this in turn may lead to a deformation of the optically active front side of the optical element. In this way, it is also possible to introduce global deformations into the optical element.

In an advantageous development of the invention, provision can be made for both electrodes of the electrode pair to be mechanically coupled to the optical element and to this end be arranged on opposite side walls of a cutout extending into the optical element from the back side.

In particular, the two electrodes can be vapour deposited on or adhesively bonded to the two opposite side walls.

In a development of the invention, provision can be made for the dielectric to be a liquid medium.

The liquid medium can preferably be distilled water or formamide. However, in principle any desired dielectric media may be provided, in particular those with a high dielectric strength and/or high permittivity.

The use of a liquid medium as a dielectric was found to be particularly suitable.

When combining different dielectrics, in particular different liquid media, it should be observed that the overall breakdown voltage may be determined by the dielectric layer in which the respective critical potential difference or field strength is reached first.

Should a gaseous dielectric be provided, it may be sulfur hexafluoride (SF6), for example.

A high vacuum may also be advantageously suitable.

According to the development of the invention, provision can be made for a fluidic connection to be formed between the electrode pairs of adjacent actuators in order to facilitate an exchange of the liquid medium between the actuators.

In the case of incompressible liquid media, this can ensure that these can flow out of the gap between the electrodes in the case of an actuation, in order to facilitate the movement of the actuator.

In particular, the fluidic connection between the actuators can be cutouts, bores or the like in the bearing units and/or bearing struts, which preferably extend between the individual actuators. According to the development, provision can be made for the optical assembly to have at least one balancing container fluidically connected to one of the actuators, to a plurality of the actuators or to all of the actuators in order to facilitate an expansion of the liquid medium into the balancing container.

In particular, the balancing container can be balancing bellows, which are able to elastically stretch depending on the amount of fluid medium received.

The balancing container can ensure, inter alia, that there is no deformation of the optical element as a result of thermal expansion of the dielectric. The balancing container, in particular the balancing bellows, is able to receive excess liquid when there is thermal expansion of the liquid medium or when the gap between the electrodes of a common electrode pair reduces on account of the actuation.

To prevent cross talk between the actuators, it may be advantageous to provide a separate balancing container for each actuator or for each individual group of actuators.

According to a development, provision can moreover be made for the optical assembly to have at least one inflow and at least one outflow for the liquid medium, these being fluidically connected to the actuators in such a way that the liquid medium is able to rinse the electrode pairs of the actuators.

Additionally, a turbo machine, for example a pump, may optionally be provided to facilitate the passage of the liquid medium.

In particular, the inflow can be connected to an external liquid supply. In this case, a balancing container may optionally be dispensed with.

In an advantageous development of the invention, provision can be made forthe electrodes of the electrode pair to be arranged extending parallel to one another.

However, provision may optionally also be made for the electrode of the electrode pair to be arranged at an angle to one another. However, this is not preferred.

In an advantageous development of the invention, provision can be made forthe actuators to be distributed in a grid along the back side of the optical element.

Different deformation profiles may be able to be set as a result of a grid-like distribution of the actuators. Preferably, the actuators are arranged distributed between the above-described bearing units and/or bearing struts. By way of example, the actuators may be arranged in segments of quadrilaterals. Moreover, there is the option of an arrangement in individual longitudinal slots in only one spatial direction. An arrangement in the form of triangles or any other grid-like arrangement may also be provided.

According to a development of the invention, the optical assembly may comprise a control device for generating control voltages for the actuators for the purposes of deforming the optical element in a targeted manner.

The control device may be in the form of a microprocessor. Instead of a microprocessor, any further device for implementing the control device may be provided, for example one or more arrangements of discrete electrical components on a printed circuit board, a programmable logic controller (PLC), an application- specific integrated circuit (ASIC) or any other programmable circuit, for example a field programmable gate array (FPGA), a programmable logic array (PLA) and/or a commercially available computer.

According to a development of the invention, provision can be made for the control device to be configured to use the actuators as sensor units by way of an excitation with the control voltage in a frequency range unimportant to the deformation operation, in order to register an actual state of the deformation.

Optionally, the actuators can consequently also be used as sensor units by virtue of the current actual state of the deformation being deduced by way of the electric behaviour of the two electrodes of the common electrode pair. The actuation with the control voltage for the operation as a sensor unit can be implemented in the radiofrequency range, for example of the order of a few megahertz. Since the driving during the use as actuators is generally implemented in a bandwidth of the order of a few hundred hertz, there are generally no mutual disturbances in this case, and so actuator and sensor operation can be implemented in parallel.

The invention also relates to a method for deforming an optical element by means of a plurality of electrostatic actuators, each with an electrode pair of two spaced apart electrodes. A control voltage is applied between the electrodes of the electrode pair in order to generate an electrostatic force that is transferred from the actuator to the optical element for the purposes of deforming the optical element. Moreover, provision is made for a dielectric to be arranged between the electrodes of the electrode pair. Preferably, a solid or liquid dielectric is provided. However, a gaseous dielectric or a high vacuum may also be provided.

The use of the aforementioned electrostatic actuators may be particularly advantageous since, in the case of electrostatic actuators, the force or extension is not generated by the properties of the material itself, as is otherwise conventional in the case of piezoelectric or electrostrictive actuators, but is based on the attractive forces of electric charges. The forces arising during the actuation are therefore much more predictable and conducive to modelling. A high precision system can be provided by the proposed use of electrostatic actuators for deforming the optical element since the electrostatic actuators have only very small non-reproducibilities and their dynamic effect may be significantly increased on account of the proposed use of a dielectric between the electrodes of the electrode pair.

A tensile or compressive force can optionally be generated between the electrodes involved, by applying the control voltage or the reference potential to the electrodes. The generation of a tensile force is preferably provided.

The invention moreover relates to a computer program product having program code means for carrying out a method for deforming an optical element according to the explanations given above and below when the program is executed on a control device, in particular the control device of the above-described optical assembly.

In particular, the invention is suitable for use within the projection exposure apparatus yet to be mentioned below or, in general, for use in lithography optics. However, in principle the invention may be suitable for any desired application in which optical elements should be deformed, in particular also for applications in aerospace and astronomy, and for military applications.

The invention also relates to a method for producing an optical assembly which comprises an optical element with an optically active front side and a back side facing away from the front side and a plurality of electrostatic actuators, each with an electrode pair of two spaced apart electrodes. The actuators are mechanically coupled to the back side of the optical element so that an electrostatic force which is generated by means of an electrical control voltage between the electrodes of the electrode pair and which serves to deform the optical element is able to be transferred to the optical element. Moreover, provision is made for a dielectric to be introduced between the electrodes of the electrode pair. Preferably, a solid or liquid dielectric is provided. However, a gaseous dielectric or a high vacuum may also be provided.

The techniques described in DE 102016209847 A1 for establishing mechanical connections between the optical element, the reference body, the intermediate element, the spacer elements/spacer struts, the bearing units/bearing struts and/or the electrodes can advantageously be used within the scope of the presently claimed invention. The disclosure of DE 10 2016 209 847 A1 is completely integrated into the present description by way of this reference.

In particular, provision can be made for the individual elements of the optical component to be connected to one another byway of silicate or direct bonding, or alternative by way of an adhesive connection, a solder connection or similar, in particular in relation to the connection between the bearing units/bearing struts and the optical element and/or the reference body. The application of the electrode on the optical element, the reference body and/orthe intermediate element can be implemented by vapour depositing an electrically conductive layer, for example. Other techniques for applying the electrodes may also be provided. In principle, any cohesive, interlocking and/or force-fit connecting techniques may be possible.

The invention also relates to a microlithographic projection exposure apparatus comprising an illumination system, which has a radiation source, an illumination optical unit and a projection optical unit. The illumination optical unit and/orthe projection optical unit comprises at least one optical assembly as per the explanations given above and below.

The invention is particularly suitable for correcting imaging aberrations of the projection exposure apparatus by deformation of the optical element of the optical assembly.

The invention is suitable, inter alia, for use with a microlithographic DUV projection exposure apparatus but particularly suitable for use with an EUV projection exposure apparatus. A possible use of the invention also relates to immersion lithography.

Features described in conjunction with one of the subjects of the invention, specifically given by the optical assembly, the method for deforming an optical element, the computer program product, the method for producing an optical assembly and the projection exposure apparatus, are also advantageously implementable for the other subjects of the invention. Likewise, advantages specified in conjunction with one of the subjects of the invention can also be understood in relation to the other subjects of the invention.

Additionally, reference is made to the fact that terms such as “comprising”, “having” or “with” do not exclude other features or steps. Furthermore, terms such as "a(n)" or "the" which indicate single steps or features do not preclude a plurality of features or steps - and vice versa.

However, in a puristic embodiment of the invention, provision can also be made for the features introduced in the invention using the terms “comprising”, “having” or “with” to be listed in exhaustive fashion. Accordingly, one or more lists of features can be considered complete within the scope of the invention, for example when respectively considered for each claim. By way of example, the invention can consist exclusively of the features specified in Claim 1.

Labels such as “first” or “second”, etc. are predominantly used for reasons of distinguishability between respective apparatus and method features and are not necessarily intended to indicate that features require one another or are related to one another.

Further, it is emphasized that the values and parameters described presently also include deviations or variations from the respectively specified value or parameter of ±10% or less, preferably ±5% or less, further preferably ±1% or less, and very particularly preferably ±0.1% or less, provided these deviations are not precluded when implementing the invention in practice. The specification of ranges by a start and end value also comprises all the values and fractions included by the respectively specified range, in particular the start and end value and a respective mean value.

The invention also relates to an optical assembly independent of Claim 1 , comprising an optical element and at least one electrostatic actuator with at least two spaced apart electrodes, with the actuators being configured and mechanically coupled to the optical element so that an electrostatic force generated between the electrodes of the actuator is transferred to the optical element for the purposes of deforming and/or aligning and/or positioning the optical element. The further features of claim 1 and the dependent claims, and the features described in the present description, relate to advantageous embodiments and variants of this optical assembly.

Exemplary embodiments of the invention are described in greater detail below with reference to the drawing.

The figures in each case show preferred exemplary embodiments in which individual features of the present invention are illustrated in combination with one another. Features of an exemplary embodiment are also able to be implemented independently of the other features of the same exemplary embodiment, and may readily be combined accordingly by a person skilled in the art to form further expedient combinations and sub-combinations with features of other exemplary embodiments.

In the figures, functionally identical elements are provided with the same reference signs.

Schematically:

Fig. 1 shows a meridional section of an EUV projection exposure apparatus;

Fig. 2 shows a DUV projection exposure apparatus;

Fig. 3 shows a lateral sectional representation of an optical assembly having an optical element, a reference body and a plurality of electrostatic actuators with a solid dielectric arranged between the reference body and the optical element;

Fig. 4 shows an excerpt of a sectional illustration along the cut line IV in Figure 3 for representing the grid-like arrangement of the actuators along the back side of the optical element;

Fig. 5 shows a lateral sectional representation of an optical assembly according to a further exemplary embodiment with an inflow and an outflow for a liquid dielectric and a fluidic connection between the actuators; Fig. 6 shows a lateral sectional representation of an optical assembly according to a further exemplary embodiment with a balancing container for a liquid dielectric and a fluidic connection between the actuators;

Fig. 7 shows a lateral sectional representation of an optical assembly according to a further exemplary embodiment with a plurality of balancing containers for a liquid dielectric and a fluidic connection between the actuators;

Fig. 8 shows a lateral sectional representation of an optical assembly according to a further exemplary embodiment with a balancing container for a liquid dielectric, a fluidic connection between the actuators and an intermediate element arranged between the actuators and the optical element;

Fig. 9 shows a lateral representation of an optical assembly according to a further exemplary embodiment with electrodes arranged within a cutout in the back side of the optical element, in a non-deflected state of the actuators;

Fig. 10 shows the optical assembly of Figure 9 in a deflected state of the actuators;

Fig. 11 shows an example of a grid arrangement of the actuators of an optical assembly according to Figure 9;

Fig. 12 shows an example of a further grid arrangement of the actuators of an optical assembly according to Figure 9; and

Fig. 13 shows a lateral representation of an optical assembly according to a further exemplary embodiment with electrodes arranged within a cutout in the back side of the optical element.

With reference to Figure 1 , the essential components of a microlithographic EUV projection exposure apparatus 100 are initially described below in exemplary fashion. The description of the basic structure of the EUV projection exposure apparatus 100 and of the components thereof should not be interpreted restrictively here.

An illumination system 101 of the EUV projection exposure apparatus 100 comprises, besides a radiation source 102, also an illumination optical unit 103 for the illumination of an object field 104 in an object plane 105. Here, a reticle 106 arranged in the object field 104 is exposed. The reticle 106 is held by a reticle holder 107. The reticle holder 107 is displaceable by way of a reticle displacement drive 108, in particular in a scanning direction. In Figure 1 , a Cartesian xyz-coordinate system is plotted to aid the explanation. The x-direction runs perpendicularly into the plane of the drawing. The y-direction runs horizontally and the z-direction runs vertically. In Figure 1 , the scanning direction runs along the y-direction. The z-direction runs perpendicular to the object plane 105.

The EUV projection exposure apparatus 100 comprises a projection optical unit 109. The projection optical unit 109 serves for imaging the object field 104 into an image field 110 in an image plane 111. The image plane 111 runs parallel to the object plane 105. Alternatively, an angle between the object plane 105 and the image plane 111 that differs from 0° is also possible.

A structure on the reticle 106 is imaged onto a light-sensitive layer of a wafer 112 arranged in the region of the image field 110 in the image plane 111. The wafer 112 is held by a wafer holder 113. The wafer holder 113 is displaceable by way of a wafer displacement drive 114, in particular along the y-direction. The displacement on the one hand of the reticle 106 by way of the reticle displacement drive 108 and on the other hand of the wafer 112 by way of the wafer displacement drive 114 can take place in such a way as to be synchronized with one another.

The radiation source 102 is an EUV radiation source. The radiation source 102 emits, in particular, EUV radiation 115, which is also referred to below as used radiation or illumination radiation. In particular, the used radiation 115 has a wavelength in the range between 5 nm and 30 nm. The radiation source 102 can be a plasma source, for example an LPP source (“laser produced plasma”) or a GDPP source (“gas discharged produced plasma”). It can also be a synchrotron-based radiation source. The radiation source 102 can be a free electron laser (FEL).

The illumination radiation 115 emerging from the radiation source 102 is focused by a collector 116. The collector 116 can be a collector with one or more ellipsoidal and/or hyperboloidal reflection surfaces. The at least one reflection surface of the collector 116 can be impinged with the illumination radiation 115 with grazing incidence (Gl), i.e., with angles of incidence greater than 45°, or with normal incidence (Nl), i.e., with angles of incidence less than 45°. The collector 116 can be structured and/or coated, firstly, for optimizing its reflectivity for the used radiation 115 and, secondly, for suppressing extraneous light.

Downstream of the collector 116, the illumination radiation 115 propagates through an intermediate focus in an intermediate focal plane 117. The intermediate focal plane 117 can represent a separation between a radiation source module, having the radiation source 102 and the collector 116, and the illumination optical unit 103.

The illumination optical unit 103 comprises a deflection mirror 118 and, arranged downstream thereof in the beam path, a first facet mirror 119. The deflection mirror 118 can be a plane deflection mirror or, alternatively, a mirror with a beam-influencing effect that goes beyond the purely deflecting effect. Alternatively or in addition, the deflection mirror 118 can be in the form of a spectral filter which separates a used light wavelength of the illumination radiation 115 from extraneous light with a wavelength deviating therefrom. If the first facet mirror 119 is arranged in a plane of the illumination optical unit 103 that is optically conjugate to the object plane 105 as a field plane, it is also referred to as a field facet mirror. The first facet mirror 119 comprises a multiplicity of individual first facets 120, which are also referred to below as field facets. Only a few of these facets 120 are illustrated in Figure 1 in exemplary fashion.

The first facets 120 can be in the form of macroscopic facets, in particular as rectangular facets or as facets with an arcuate peripheral contour or a peripheral contour of part of a circle. The first facets 120 may be in the form of plane facets or alternatively as convexly or concavely curved facets.

As known for example from DE 102008009600 A1 , the first facets 120 themselves may also be composed in each case of a multiplicity of individual mirrors, in particular a multiplicity of micromirrors. The first facet mirror 119 can in particular be formed as a microelectromechanical system (MEMS system). For details, reference is made to DE 10 2008 009 600 A1 .

Between the collector 116 and the deflection mirror 118, the illumination radiation 115 travels horizontally, that is to say along the y-direction.

In the beam path of the illumination optical unit 103, a second facet mirror 121 is arranged downstream of the first facet mirror 119. If the second facet mirror 121 is arranged in a pupil plane of the illumination optical unit 103, it is also referred to as a pupil facet mirror. The second facet mirror 121 can also be arranged at a distance from a pupil plane of the illumination optical unit 103. In this case, the combination of the first facet mirror 119 and the second facet mirror 121 is also referred to as a specular reflector. Specular reflectors are known from US 2006/0132747 A1 , EP 1 614 008 B1 and US 6,573,978.

The second facet mirror 121 comprises a plurality of second facets 122. In the case of a pupil facet mirror, the second facets 122 are also referred to as pupil facets.

The second facets 122 can likewise be macroscopic facets, which can for example have a round, rectangular or else hexagonal periphery, or alternatively be facets made up of micromirrors. In this regard, reference is likewise made to DE 10 2008 009 600 A1 .

The second facets 122 can have plane or alternatively convexly or concavely curved reflection surfaces.

The illumination optical unit 103 consequently forms a doubly faceted system. This basic principle is also referred to as fly's eye integrator.

It can be advantageous to arrange the second facet mirror 121 not exactly in a plane that is optically conjugate to a pupil plane of the projection optical unit 109. With the aid of the second facet mirror 121 , the individual first facets 120 are imaged into the object field 104. The second facet mirror 121 is the last beam-shaping mirror or else, in fact, the last mirror for the illumination radiation 115 in the beam path upstream of the object field 104.

In a further embodiment, not shown, of the illumination optical unit 103, a transfer optical unit contributing in particular to the imaging of the first facets 120 into the object field 104 can be arranged in the beam path between the second facet mirror 121 and the object field 104. The transfer optical unit can have exactly one mirror or else alternatively two or more mirrors, which are arranged one behind the other in the beam path of the illumination optical unit 103. In particular, the transfer optical unit can comprise one or two mirrors for normal incidence (Nl mirror, “normal incidence” mirror) and/or one or two mirrors for grazing incidence (Gl mirror, “grazing incidence” mirror).

In the embodiment shown in Figure 1 , the illumination optical unit 103 comprises exactly three mirrors downstream of the collector 116, specifically the deflection mirror 118, the field facet mirror 119 and the pupil facet mirror 121 .

The deflection mirror 118 can also be dispensed with in a further embodiment of the illumination optical unit 103, and so the illumination optical unit 103 can then have exactly two mirrors downstream of the collector 116, specifically the first facet mirror 119 and the second facet mirror 121 .

As a rule, the imaging of the first facets 120 into the object plane 105 by means of the second facets 122 or using the second facets 122 and a transfer optical unit is only approximate imaging.

The projection optical unit 109 comprises a plurality of mirrors Mi, which are numbered in accordance with their arrangement in the beam path of the EUV projection exposure apparatus 100.

In the example illustrated in Figure 1 , the projection optical unit 109 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve or any other number of mirrors Mi are similarly possible. The penultimate mirror M5 and the last mirror M6 each have a through opening for the illumination radiation 115. The projection optical unit 109 is a double-obscured optical unit. The projection optical unit 109 has an image-side numerical aperture which is greater than 0.5 and which can also be greater than 0.6 and, for example, be 0.7 or 0.75.

Reflection surfaces of the mirrors Mi can be embodied as free-form surfaces without an axis of rotational symmetry. Alternatively, the reflection surfaces of the mirrors Mi can be designed as aspherical surfaces with exactly one axis of rotational symmetry of the reflection surface shape. Just like the mirrors of the illumination optical unit 103, the mirrors Mi can have highly reflective coatings for the illumination radiation 115. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon. The projection optical unit 109 has a large object-image offset in the y-direction between a y-coordinate of a centre of the object field 104 and a y-coordinate of the centre of the image field 110. In the y-direction, this object-image offset can be of approximately the same magnitude as a z-distance between the object plane 105 and the image plane 111.

In particular, the projection optical unit 109 can have an anamorphic form. In particular, it has different imaging scales bc, py in the x- and y-directions. The two imaging scales bc, py of the projection optical unit 109 are preferably (px, py) = (+/-0.25, +/-0.125). A positive imaging scale b means imaging without image inversion. A negative sign for the imaging scale b means imaging with image inversion.

The projection optical unit 109 consequently leads to a reduction in size with a ratio of 4:1 in the x-direction, that is to say in a direction perpendicular to the scanning direction.

The projection optical unit 109 leads to a reduction in size of 8:1 in the y-direction, that is to say in the scanning direction.

Other imaging scales are similarly possible. Imaging scales with the same sign and the same absolute value in the x-direction and y-direction are also possible, for example with absolute values of 0.125 or of 0.25.

The number of intermediate image planes in the x-direction and in the y-direction in the beam path between the object field 104 and the image field 110 can be the same or, depending on the embodiment of the projection optical unit 109, can differ. Examples of projection optical units with different numbers of such intermediate images in the x- and y-directions are known from US 2018/0074303 A1 .

In each case one of the pupil facets 122 is assigned to exactly one of the field facets 120 for forming in each case an illumination channel for illuminating the object field 104. In particular, this can yield illumination according to the Kohler principle. The far field is decomposed into a multiplicity of object fields 104 with the aid of the field facets 120. The field facets 120 produce a plurality of images of the intermediate focus on the pupil facets 122 respectively assigned thereto.

By way of respectively assigned pupil facets 122, the field facets 120 are imaged on the reticle 106 in a manner superposed on one another for the purposes of illuminating the object field 104. The illumination of the object field 104 is in particular as homogeneous as possible. It preferably has a uniformity error of less than 2%. Field uniformity can be attained by overlaying different illumination channels.

The illumination of the entrance pupil of the projection optical unit 109 can be defined geometrically by way of an arrangement of the pupil facets. The intensity distribution in the entrance pupil of the projection optical unit 109 can be set by selecting the illumination channels, in particular the subset of the pupil facets which guide light. This intensity distribution is also referred to as illumination setting. A likewise preferred pupil uniformity in the region of sections of an illumination pupil of the illumination optical unit 103 which are illuminated in a defined manner can be achieved by a redistribution of the illumination channels.

Further aspects and details of the illumination of the object field 104 and in particular of the entrance pupil of the projection optical unit 109 are described below.

In particular, the projection optical unit 109 can have a homocentric entrance pupil. The latter can be accessible. It can also be inaccessible.

The entrance pupil of the projection optical unit 109 cannot be exactly illuminated using the pupil facet mirror 121 on a regular basis. In the case of imaging of the projection optical unit 109 which telecentrically images the centre of the pupil facet mirror 121 onto the wafer 112, the aperture rays often do not intersect at a single point. However, it is possible to find an area in which the distance of the aperture rays determined in pairs becomes minimal. This area represents the entrance pupil or an area in real space that is conjugate thereto. In particular, this area has a finite curvature.

It may be the case that the projection optical unit 109 has different positions of the entrance pupil for the tangential beam path and for the sagittal beam path. In this case, an imaging element, in particular an optical component of the transfer optical unit, should be provided between the second facet mirror 121 and the reticle 106. With the aid of this optical component, it is possible to take account of the different relative positions of the tangential entrance pupil and the sagittal entrance pupil.

In the arrangement of the components of the illumination optical unit 103 illustrated in Figure 1 , the pupil facet mirror 121 is arranged in an area conjugate to the entrance pupil of the projection optical unit 109. The field facet mirror 119 is arranged in tilted fashion with respect to the object plane 105. The first facet mirror 119 is arranged in tilted fashion with respect to an arrangement plane defined by the deflection mirror 118.

The first facet mirror 119 is arranged in tilted fashion with respect to an arrangement plane defined by the second facet mirror 121 .

Figure 2 illustrates an exemplary DUV projection exposure apparatus 200. The DUV projection exposure apparatus 200 comprises an illumination system 201 , a device known as a reticle stage 202 for receiving and exactly positioning a reticle 203, by which the later structures on a wafer 204 are determined, a wafer holder 205 for holding, moving and exactly positioning the wafer 204 and an imaging device, specifically a projection optical unit 206, with a plurality of optical elements, in particular lens elements 207, which are held by way of mounts 208 in a lens housing 209 of the projection optical unit 206. As an alternative or in addition to the lens elements 207 illustrated, provision can be made of various refractive, diffractive and/or reflective optical elements, inter alia, also mirrors, prisms, terminating plates and the like.

The basic functional principle of the DUV projection exposure apparatus 200 makes provision for the structures introduced into the reticle 203 to be imaged onto the wafer 204.

The illumination system 201 provides a projection beam 210 in the form of electromagnetic radiation, which is required for the imaging of the reticle 203 on the wafer 204. A laser, a plasma source or the like can be used as the source of this radiation. The radiation is shaped in the illumination system 201 by means of optical elements such that the projection beam 210 has the desired properties with regard to diameter, polarization, shape of the wavefront and the like when it is incident on the reticle 203.

An image of the reticle 203 is generated by means of the projection beam 210 and transferred from the projection optical unit 206 onto the wafer 204 in an appropriately reduced form. In this case, the reticle 203 and the wafer 204 can be moved synchronously, so that regions of the reticle 203 are imaged onto corresponding regions of the wafer 204 virtually continuously during a so-called scanning process.

An air gap between the last lens element 207 and the wafer 204 can optionally be replaced by a liquid medium which has a refractive index of greater than 1.0. The liquid medium can be high-purity water, for example. Such a setup is also referred to as immersion lithography and has an increased photolithographic resolution.

The use of the invention is not restricted to use in projection exposure apparatuses 100, 200, in particular also not with the described structure. The invention and the following exemplary embodiments should also not be understood as being restricted to a specific design. The following figures merely show the invention by way of example and highly schematically.

A targeted deformation of the optical elements 118, 119, 120, 121 , 122, Mi, 207 of a projection exposure apparatus, for example of the projection exposure apparatuses 100, 200, might be suitable for correcting imaging aberrations thereof, in particular. This is the starting point for the invention.

Figures 3 to 13 show various exemplary embodiments of optical assemblies 1 according to the invention in exemplary and very schematic fashion. The optical assemblies 1 facilitate a targeted deformation of optical elements 2, in particular for correcting imaging aberrations in a projection exposure apparatus 100, 200. The optical elements 2 to be deformed may be arranged, in particular, within the illumination optical unit 103 and/or the projection optical unit 109, 206 of the projection exposure apparatus 100, 200.

In the exemplary embodiments, the optical element 2 is illustrated as a mirror in exemplary fashion; however, this should not be construed as restrictive. The optical element 2 has an optically active front side 3 and a back side 4 facing away from the front side 3. When incident on the optical element 2, the illumination radiation 115 or the projection beam 210 is influenced in defined fashion by the optically active front side 3, in particular in order to guide the beam path.

Initially, a first exemplary embodiment of the proposed optical assembly 1 is intended to be described on the basis of Figures 3 and 4.

The optical assembly 1 comprises a plurality of electrostatic actuators 5 arranged distributed along the back side 4 of the optical element 2. In particular, the actuators 5 may be arranged distributed in grid-like fashion along the back side 4 of the optical element 2, as can be recognized particularly well on the basis of Figure

4. Each of the actuators 5 has an electrode pair of two spaced apart electrodes 6, 7.

Preferably, the electrodes 6, 7 of the common electrode pair are arranged extending parallel to one another (in their non-deflected state), as shown in all exemplary embodiments. However, in principle, a tilted arrangement relative to one another may also be provided.

Each actuator 5 is configured and mechanically coupled to the back side 4 of the optical element 2 so that an electrostatic force which is generated by means of an electrical control voltage Ui _n between the electrodes 6, 7 of the electrode pair and which serves to deform the optical element 2 is transferred to the optical element 2. For elucidation purposes, Figure 3 and subsequent Figures 5 to 7 in each case show one of the actuators 5 in a deflected state, and hence show the optical element 2 in a partially deformed state.

A solid or liquid dielectric 8 is arranged between the electrodes 6, 7 of the electrode pair for the purposes of a sufficiently high dynamic effect and an increase in the dielectric strength between the electrodes 6, 7. Optionally, a gaseous dielectric 8 may also be provided, but preferably a gaseous dielectric 8 that deviates from air. In the exemplary embodiment of Figure 3, a solid medium is provided as a dielectric 8, which is masked in Figure 4 for illustrative reasons. By contrast, a liquid medium is provided in the subsequent exemplary embodiments.

Preferably, the solid medium according to the exemplary embodiment of Figures 3 and 4 is deformable in order to facilitate a reduction in the distance between the electrodes 6, 7 in the deflected state of the actuator

5. Alternatively, however, provision can also be made for the dielectric 8 to not completely fill the interstice between the electrodes 6, 7. By way of example, provision can be made for the dielectric 8 to be arranged on only one of the two electrodes 6, 7 or for the electrodes 6, 7 to be coated with an insulation layer of the dielectric 8. In this way, the dielectric strength and the force of the actuator 5 can be increased.

In exemplary embodiments, provision is made for one of the electrodes of the electrode pairto be designed as a control electrode 6, the control voltage Ui _n being applied thereto. The other electrode is in the form of reference electrode 7, a common reference potential GND being able to be applied together thereto and to the further reference electrodes 7 of the other actuators 5.

In the exemplary embodiments of Figures 3 to 8 and 13, one of the electrodes 6, 7, preferably the reference electrode 7, as illustrated, is mechanically coupled to the back side 4 of the optical element 2. The other electrode 6, 7, preferably the control electrode 6, as illustrated, is mechanically coupled to a reference body 9 that is spaced apart from the back side 4 of the optical element 2.

By way of example, the reference body 9 can be a mount for the optical element 2, a fastening frame for the optical element 2 or even the optical element 2 itself.

In the exemplary embodiments of Figures 3 to 7, the reference body 9 is fastened to the optical element 2 by way of longitudinal struts 10 (cf., in particular, Figure 4 as well) which are arranged distributed along the back side 4 of the optical element 2. The individual actuators 5 are arranged between the individual bearing struts 10. In the exemplary embodiments of Figures 3 to 8, the actuators 5 are aligned parallel to the back side 4 of the optical element 2.

A control device 11 , shown using dashed lines in Figure 3, may be provided for generating the control voltage Ui _n for the actuators 5. The control device 11 may be configured to deform the optical elements 2 in a targeted fashion.

However, the control device 11 may also be configured to use the actuators 5 as sensor units by way of an excitement with the control voltage Ui _n in a frequency range unimportant to the deformation operation. In this way it is possible to capture an actual state of the deformation of the optical element 2, for example in a higher frequency band. The method for deforming the optical element 2, described above and below, can be carried out as a computer program product having program code means on the control device 11 .

Provision is preferably made for the dielectric 8 to be a liquid medium, in particular distilled water or formamide. A corresponding embodiment is shown in Figure 5, for example. To facilitate an exchange of liquid medium between the actuators 5, a respective fluidic connection is established between the electrode pairs of adjacent actuators 5 in the exemplary embodiments of Figures 5 to 13. In the exemplary embodiments of Figures 5 to 8, the fluidic connection between the actuators 5 is realized by a respective bore 12 through the bearing struts 10 arranged between the actuators 5. An inflow 13 and an outflow 14 for the liquid medium may be provided, which are fluidically connected to the actuators 5 in such a way that the electrode pairs of the actuators 5 are rinsed by the liquid medium, as indicated in Figure 5. To this end, additionally optionally, provision can be made for a turbomachine, for instance a pump (not shown).

However, a closed system is provided in a particularly preferred embodiment, as shown in the following exemplary embodiments. In this case, provision can preferably be made for the provision of an expansion possibility for the liquid medium, in particular in order to facilitate a thermally caused expansion and/or a displacement of the dielectric 8 in the case of the deflected actuators 5. In this respect, provision can be made for a balancing container 15, for example, which is fluidically connected to one of the actuators 5 in order to facilitate an expansion of the liquid medium (via the network of fluidic connections) into the balancing container 15. Balancing bellows are indicated in the figures in exemplary fashion. To facilitate a decoupling of the actuators 5 from one another, it is also possible to provide a plurality of balancing containers 15, for example a balancing container 15 for each actuator 5 as indicated in Figure 7.

In the exemplary embodiments shown in Figures 3 to 7, one electrode 6, 7 of the electrode pair, the respective reference electrode 7 in the present case, is in each case fastened directly to the optical element 2. This can facilitate a local or short-wavelength deformation of the optical element 2, as indicated in the figures. However, to the extent that a global deformation or a longer wavelength deformation is desired, provision can be made for the electrode 6, 7 coupled to the optical element 2 (in particular the reference electrode 7) to be indirectly connected to the optical element 2 via an intermediate element 16 that is spaced apart from the back side 4 of the optical element 2. This is shown on the basis of Figure 8.

In this case, the reference body 9 can be fastened to the intermediate element 16 via the bearing struts 10, the intermediate element 16 itself being fastened to the optical element 2 via spacer struts 17 arranged distributed along the back side 4 of the optical element 2. The bearing struts 10 and the spacer struts 17 can be arranged offset along the backside 4 of the optical element 2, the spacer struts 17 being arranged as centrally as possible below the actuator surface or the electrode surface at the optical element 2. In this way, the optical element 2 can be influenced particularly advantageously.

As can be recognized on the basis of the exemplary deflection in Figure 8, the intermediate element 16 can bring about a certain amount of decoupling of the individual actuators 5 from the optical element 2, as a result of which a global deformation or a long-wavelength deformation of the optical element 2 is rendered possible. This may be advantageous for some applications.

An alternative embodiment of the optical assembly 1 is shown in Figures 9 to 12. Figure 9 shows the optical assembly 1 in a non-deflected state of the actuators 5 and Figure 10 shows said optical assembly with a deflection of the actuators 5 that is purely exemplary and only to be understood to be schematic.

Figures 11 and 12 show possible grid arrangements regarding the distribution of the actuators 5 along the back side 4 of the optical element 2. The variants shown in Figures 9 to 12 are particularly suitable for a rectangularly distributed arrangement (cf. Figure 11) or for a line-like arrangement (cf. Figure 12).

The dielectric 8 has been masked in Figures 9 to 13 to make the illustration simpler. A liquid medium is once again preferably provided. In the exemplary embodiments of Figures 9 to 13, the fluidic connection between the actuators 5 is provided by an appropriate spacing of the reference body 9 and a channel 18 formed as a result. A common balancing container 15 is provided purely by way of example. As emerges on the basis of Figures 9 and 10, both electrodes 6, 7 of the electrode pair may also be mechanically coupled to the optical element 2. To this end, the electrode 6, 7 may be arranged in particular on opposing side walls of a cutout 19, for example a slot, from the backside 4 of the optical element 2. The side walls can be coated (fully or partly) with a conductive medium, for example. When an appropriate electrical control voltage Ui _n is applied to the electrode pairs, a tensile and/or compressive force can be introduced directly into the optical element 2, causing a deformation on the optically active front side 3 of the optical element 2. In a manner similar to the exemplary embodiment in Figure 8 with the intermediate element 16, comparatively long wavelength or global deformations may also be achievable in this way.

Figure 13 shows a further exemplary embodiment of an optical assembly 1 . Once again, provision is made for an electrode 7 to be coupled to the optical element 2 and, to this end, be arranged on a side wall of a cutout 19 extending into the optical element 2 from the back side 4. By contrast, the electrode 6 coupled to the reference body 9 is arranged on a projection 20 extending into the cutout 19 of the optical element 2 from the reference body 9 (or forms the projection 20 itself). This also advantageously allows forces to be directly introduced into the optical element 2, in a manner similar to what was described on the basis of the exemplary embodiment of Figures 9 and 10. Bores 12 may be provided in the projection 20 to provide the fluidic connection, in addition to a channel 18 between the actuators 5.

Optionally, the force of an individual actuator 5 can be further increased in the exemplary embodiment of Figure 13, as illustrated, if two electrode pairs are provided instead of one electrode pair, the respective control electrodes 6 being electrically connected to one another, being able to have the same control voltage Ui _n applied thereto and in each case being aligned to one of the side walls of the cutout 19. In this way, the effective electrode area can be doubled.

Naturally, it is also possible to provide a structure in which the reference electrodes 7 protrude into the cutout 19 from the reference body 9.

Finally, it should be mentioned that the exemplary embodiments shown are always combinable with one another as a matter of principle, provided this is not precluded from a technical point of view. Further, even further embodiments of the actuators 5 and alignments/arrangements of the electrodes 6, 7 of the electrode pair may be provided. In particular, the geometric relationships and scales shown are purely schematic and exemplary and should not be construed as true to scale.

A further option for increasing the electrostatic force may also lie in teething of the electrodes 6, 7 of a common electrode pair and/or a division of the individual electrodes 6, 7 among a plurality of individual electrodes.

Claims

Claims:

1. Optical assembly (1), comprising an optical element (2) with an optically active front side (3) and a back side (4) facing away from the front side (3) and a plurality of electrostatic actuators (5) arranged distributed along the back side (4) of the optical element (2), each actuator comprising an electrode pair of two spaced apart electrodes (6, 7), each actuator (5) being configured and mechanically coupled to the back side (4) of the optical element (2) so that an electrostatic force which is generated by means of an electrical control voltage (Ui _n) between the electrodes (6, 7) of the electrode pair and which serves to deform the optical element (2) is transferred to the optical element (2), characterized in that a dielectric (8) is arranged between the electrodes (6, 7) of the electrode pair.

2. Optical assembly (1) according to Claim 1 , characterized in that the dielectric (8) is a solid or liquid dielectric (8).

3. Optical assembly (1) according to Claim 1 or 2, characterized in that the dielectric (8) has a permittivity greater than the permittivity in vacuum, preferably a permittivity greater than the permittivity of air.

4. Optical assembly (1) according to any one of Claims 1 to 3, characterized in that the dielectric (8) has a dielectric strength greater than the dielectric strength of air.

5. Optical assembly (1) according to any one of Claims 1 to 4, characterized in that one of the electrodes of the electrode pair is designed as a control electrode (6), the control voltage (Ui _n) being able to be applied thereto, and the other electrode is designed as a reference electrode (7), a common reference potential (GND) being able to be applied together thereto and to the reference electrode (7) of at least one of the further actuators (5).

6. Optical assembly (1) according to any one of Claims 1 to 5, characterized in that one of the electrodes (6, 7) of the electrode pair is mechanically coupled to the back side (4) of the optical element (2) and the other electrode (7, 6) is mechanically coupled to a reference body (9) that is spaced apart from the back side (4) of the optical element (2).

7. Optical assembly (1) according to Claim 6, characterized in that the electrode (6, 7) coupled to the optical element (2) is fastened directly to the optical element (2).

8. Optical assembly (1) according to Claim 6, characterized in that the electrode (6, 7) coupled to the optical element (2) is indirectly connected to the optical element (2) via an intermediate element (16) that is spaced apart from the back side (4) of the optical element (2).

9. Optical assembly (1) according to Claim 8, characterized in that the intermediate element (16) is fastened to the optical element (2) by spacer elements and/or spacer struts (17) that are arranged distributed along the back side (4) of the optical element (2).

10. Optical assembly (1) according to any one of Claims 6 to 9, characterized in that the reference body (9) is fastened to the optical element (2) or on the intermediate element (16) via bearing units and/or bearing struts (10) that are arranged distributed along the back side (4) of the optical element (2).

11 . Optical assembly (1) according to Claim 9 and 10, characterized in that the spacer units or spacer struts (17) are offset from the bearing units or bearing struts (10) along the back side (4) of the optical element (2).

12. Optical assembly (1) according to any one of Claims 6 to 11 , characterized in that the electrode (6, 7) coupled to the optical element (2) extends parallel to the back side (4) of the optical element (2).

13. Optical assembly (1) according to any one of Claims 6 to 11 , characterized in that the electrode (6, 7) coupled to the optical element (2) is arranged on a side wall of a cutout (19) extending into the optical element (2) from the back side (4), the electrode (7, 6) coupled to the reference body (9) being arranged on a projection (20) extending into the cutout (19) of the optical element (2) from the reference body (9).

14. Optical assembly (1) according to any one of Claims 1 to 5, characterized in that both electrodes (6, 7) of the electrode pair are mechanically coupled to the optical element (2) and are to this end arranged on opposite side walls of a cutout (19) extending into the optical element (2) from the back side (4).

15. Optical assembly (1) according to any one of Claims 1 to 14, characterized in that the dielectric (8) is a liquid medium, preferably distilled water or formamide.

16. Optical assembly (1) according to Claim 15, characterized in that a fluidic connection is formed between the electrode pairs of adjacent actuators (5) in order to facilitate an exchange of the liquid medium between the actuators (5).

17. Optical assembly (1) according to Claim 15 or 16, characterized by at least one balancing container (15) fluidically connected to one of the actuators (5), to a plurality of the actuators (5) orto all of the actuators (5) in order to facilitate an expansion of the liquid medium into the balancing container (15).

18. Optical assembly (1) according to any one of Claims 15 to 17, characterized by at least one inflow (13) and at least one outflow (14) for the liquid medium, these being fluidically connected to the actuators (5) in such a way that the liquid medium is able to rinse the electrode pairs of the actuators (5).

19. Optical assembly (1) according to any one of Claims 1 to 18, characterized in that the electrodes (6, 7) of the electrode pair are arranged extending parallel to one another.

20. Optical assembly (1) according to any one of Claims 1 to 19, characterized in that the actuators (5) are distributed in a grid along the back side (4) of the optical element (2).

21 . Optical assembly (1) according to any one of Claims 1 to 20, characterized by a control device (11) for generating control voltages (Ui _n) for the actuators (5) for the purposes of deforming the optical element (2) in a targeted manner.

22. Optical assembly (1) according to Claim 21 , characterized in that the control device (11) is configured to use the actuators as sensor units by way of an excitation with the control voltage (Ui _n) in a frequency range unimportant to the deformation operation, in order to register an actual state of the deformation.

23. Method for deforming an optical element (2) by means of a plurality of electrostatic actuators (5), each with an electrode pair of two spaced apart electrodes (6, 7), a control voltage (Ui _n) being applied between the electrodes (6, 7) of the electrode pair according to the method in order to generate an electrostatic force which is transferred from the actuator (5) to the optical element (2) for the purposes of deforming the optical element (2), characterized in that a dielectric (8) is arranged between the electrodes (6, 7) of the electrode pair.

24. Computer program product having program code means for carrying out a method for deforming an optical element (2) according to Claim 23 when the program is executed on a control device (11).

25. Method for producing an optical assembly (1) comprising an optical element (2) with an optically active front side (3) and a back side (4) facing away from the front side (3) and a plurality of electrostatic actuators (5), each actuator comprising an electrode pair of two spaced apart electrodes (6, 7), the actuators (5) are mechanically coupled to the back side (4) of the optical element (2) according to the invention so that an electrostatic force which is generated by means of an electrical control voltage (Ui _n) between the electrodes (6, 7) of the electrode pair and which serves to deform the optical element (2) is able to be transferred to the optical element (2), characterized in that a dielectric (8) is introduced between the electrodes (6, 7) of the electrode pair.

26. Microlithographic projection exposure apparatus (100, 200) comprising an illumination system (101 , 201), which comprises a radiation source (102), an illumination optical unit (103) and a projection optical unit (109, 206), wherein the illumination optical unit (103) and/or the projection optical unit (109, 206) comprises at least one optical assembly according to any one of Claims 1 to 22.