Optical systems and lithographic apparatus

Abstract

The optical system (200) includes at least one mirror (210) having a mirror body (212) and a mirror surface (214), and at least one actuator device (220) coupled to the mirror body (212) for deforming the mirror surface (214). The actuator device (220) includes at least one electrostrictive actuator element (222) for generating a mechanical stress in the mirror body (212) for deforming the mirror surface (214) in response to an electrical driving voltage (VS), and at least one electrostrictive sensor element (224) for outputting a sensor signal (SS) in response to deformation of the sensor element (224). The at least one sensor element (224) is positioned directly adjacent to the actuator element (222) and / or configured to at least partially transmit the mechanical stress generated by the actuator element (222) to the mirror body (212).

Description

[Technical Field]

[0001] The present invention relates to an optical system and a lithographic apparatus comprising such an optical system.

[0002] The content of the priority application German Patent Application No. 10 2020 201 724.7 is incorporated in its entirety by reference. [Background technology]

[0003] Microlithography is used to manufacture finely structured components, such as integrated circuits. The microlithography process is carried out using a lithography apparatus having an illumination system and a projection system. In this case, an image of a mask (reticle), illuminated using the illumination system, is projected using the projection system onto a substrate, e.g., a silicon wafer, coated with a photosensitive layer (photoresist) and positioned in the image plane of the projection system, so as to transfer the mask structure into the photosensitive coating of the substrate.

[0004] Mirrors are known that can deform their mirror surface in a targeted manner using appropriately positioned actuators. This allows for the correction or compensation of optical imaging errors, also known as aberrations. Aberrations can have different causes. Temperature fluctuations, in particular, can lead to mechanical stresses that can distort the optical element holder and / or the optical element itself. As a result, the optical properties of the optical element can change. Such mirrors are used, for example, in astronomy, where the concept of adaptive optics is well established. In this case, the mirror surface is scanned with a measuring beam to determine deviations from the ideal shape. Therefore, it is possible to build a closed-loop control circuit that drives the corresponding actuator based on the deviations identified, thereby approximating the mirror surface to its ideal shape.

[0005] Mirrors, the surface of which can be deformed by actuators, are also used in lithography systems. However, the installation space of known lithography systems is very small, since many functional units, such as active and passive components, must be arranged in a small volume. Therefore, it is not practical to arrange an additional optical measurement system in the lithography system in order to scan the mirror surface as described above. Therefore, feedback control of the actuators has not been possible to date, or can only be implemented with economically unacceptable effort. Summary of the Invention [Problem to be solved by the invention]

[0006] Against this background, it is an object of the present invention to make available an improved optical system. [Means for solving the problem]

[0007] According to a first aspect, an optical system is proposed having at least one mirror. The mirror comprises a mirror body and a mirror surface. At least one actuator device is coupled to the mirror body to deform the mirror surface. The actuator device includes at least one electrostrictive actuator element that generates a mechanical stress in the mirror body to deform the mirror surface in response to an electrical driving voltage, and at least one electrostrictive sensor element that outputs a sensor signal in response to deformation of the sensor element. The at least one sensor element is arranged directly adjacent to the actuator element, and / or arranged on a side of the mirror body opposite the mirror surface and separated from the mirror body by at least the actuator element, and / or arranged so as to be configured to at least partially transmit the mechanical stress generated by the actuator element to the mirror body.

[0008] This optical system has the advantage of being able to detect the effects in the mirror body caused by the actuator elements without directly scanning the mirror surface. Advantageously, this is possible in this case within a physical domain, mechanically or electrically, i.e., within the coupling between an externally applied electric field and the polarization of the deformation-dependent material, i.e., without the actuator and sensor being based on different physical mechanisms of action. It can be said that the detection and actuation of the deformation are carried out with the help of the same mechanism of action, for example electromechanically via the piezoelectric effect. This allows the effects that occur to be uniformly described and handled. This keeps the complexity of the optical system low. The actuator elements and sensor elements are of the same material class. This also reduces the number of different process steps required to manufacture the optical system. Furthermore, for this reason, the optical system is very robust, relatively inexpensive to manufacture, and prone to failures.

[0009] The optical system is designed in particular as a projection optical unit for a lithography apparatus. Furthermore, the optical system can be part of the beam shaping and illumination system of the lithography apparatus. The optical system can also be used in other technical fields where high-precision beam guidance is necessary or desired, such as astronomy, scientific instrumentation, and military optics. Compared to known adaptive optics units, the optical system is particularly advantageous when a robust system is required, installation space is limited, and / or cost plays a key role.

[0010] The optical system includes at least one mirror. In addition to the mirror, the optical system can also comprise further mirrors, lens elements, gratings, diaphragms, filters, cavities, etc. However, the optical system can also consist of a mirror with an actuator device. Furthermore, the optical system can comprise multiple mirrors, each with an actuator device. Further optically active elements of the optical system can comprise additional or other actuators, such as electric heaters, coolers, etc.

[0011] The mirror surface of at least one mirror can have any geometric shape, such as planar, convex, or concave, or can be partially varied. The mirror can be positioned at different positions in the beam path within the optical system, particularly in the pupil or hatch region. Depending on the position in the beam path, various imaging errors or aberrations are corrected by deformation of the mirror surface. Deformation is understood to mean, in particular, deviation from the basic shape of the mirror surface. For example, the mirror assumes its basic shape when it is placed without stress in a holder and orientation intended for its operation. In this case, the basic shape may deviate from the ideal shape, for example, due to the mirror's own weight. However, deformation of the mirror surface restores the ideal shape. Preferably, deformation of the mirror surface is understood to mean deviation from the shape of the mirror surface when there is no charge, current, or voltage on the actuator element.

[0012] The actuator device comprises at least an actuator element and a sensor element. Preferably, the actuator device further comprises a drive unit for driving the actuator element with a drive voltage. The drive voltage is predetermined, for example by a control computer, depending on the mechanical stress to be achieved. The mechanical stress directly determines the deformation of the mirror surface. The functional relationship between the mechanical stress and the deformation depends, for example, on the material parameters of the mirror body and on boundary conditions such as geometric factors. Alternatively, the control computer can determine the mechanical stress to be achieved. The drive unit itself determines the required drive voltage for this.

[0013] The actuator device is coupled to the mirror body of the mirror such that the mirror surface is deformable in response to an electrical drive voltage on the actuator element.

[0014] The drive unit comprises, for example, a voltage source or a current source. Depending on the application, the actuator element can be operated with closed-loop controlled voltage or with closed-loop controlled charge. In particular, in dynamic applications where the position of the actuator element is changed frequently, driving with closed-loop controlled charge is advantageous.

[0015] The actuator and sensor elements comprise electrostrictive materials, which are understood to include, for example, piezoelectric materials. Electrostrictive materials have the property that, due to the interaction between an external electric field and dipoles present in the material, mechanical stresses or forces can be generated in the material, which manifest as deformations of the material. Conversely, deformation of the material leads to a change in the polarization of the material, which can be measured and from which conclusions can be drawn about the amplitude of the deformation.

[0016] It is pointed out that the terms force, mechanical stress, or deformation can be used interchangeably.

[0017] Examples of electrostrictive materials are ceramic compounds containing the elements lead magnesium niobate (PMN) or lead zirconate titanate (PZT). These are preferably alloyed with platinum to improve their mechanical properties. Platinum reduces the dielectric properties, especially the maximum polarization. The alloys therefore have a material composition that can be characterized by an alloying factor x: PMN x -P t1- x or PZT x P t1-x is.

[0018] In a preferred embodiment of the optical system, the electrostrictive material has a Curie temperature in the range of 0-40° C. At the Curie temperature, a phase transition occurs from a first crystalline structure predominating below the Curie temperature to a second crystalline structure predominating above the Curie temperature.

[0019] The actuator element preferably comprises an active area, e.g. a layer made of electrostrictive material. This area is arranged between two electrodes, an anode and a cathode. By applying a drive voltage between the anode and the cathode, an electric field is generated in the active area, which leads to a mechanical stress.

[0020] The actuator element is mechanically coupled to the mirror body, so that the mechanical stress of the actuator element is transferred to the mirror body, which manifests itself as a corresponding deformation of the mirror body and thus the mirror surface. The degree of deformation achieved with a particular force depends in this case on the strength of the material of the mirror body.

[0021] The sensor element preferably also comprises an active area, e.g. a layer structure, made of an electrostrictive material arranged between two electrodes. However, the sensor element and the actuator element may differ, e.g., in their geometric dimensions and / or material composition.

[0022] The sensor element preferably comprises an assigned measuring unit which determines the polarization of the active area of ​​the sensor element by means of a measuring AC voltage applied between the cathode and anode of the sensor element. The sensor signal output by the sensor element contains, in particular, the polarization determined by the measuring unit. The deformation of the active area of ​​the sensor element can be derived therefrom. In particular, the measuring AC voltage has an amplitude which does not lead to any significant mechanical stress on the active area. The measuring unit can constitute an independent unit of the actuator device. The measuring unit and the sensor element are assigned to each other. In the following description, the term sensor element refers to the entire sensor element or to only the active area of ​​the sensor element.

[0023] The sensor element is mechanically coupled to the actuator element and / or the mirror body such that it is deformed when the actuator element is deformed and / or when the mirror body is deformed, e.g. by mechanical stress applied by the actuator element.

[0024] In a preferred embodiment, the sensor element is arranged directly adjacent to the actuator element. Directly adjacent is understood to mean, for example, that there are no intermediate layers or additional materials between the actuator element and the sensor element. It can also be said that the sensor element and the actuator element are in contact.

[0025] In a further preferred embodiment, the sensor element is arranged on the side of the mirror body opposite the mirror surface, separated from the mirror body by at least the actuator element. Preferably, the sensor element is arranged directly on the back side of the actuator element. The back side of the actuator element is the side pointing away from the mirror body. It is also possible for there to be additional layers between the actuator element and the sensor element.

[0026] In a further preferred embodiment, the sensor element is arranged to at least partially transmit the mechanical stress generated by the actuator element to the mirror body. The sensor element can also be said to transmit force from the actuator element to the mirror body or to form an operational connection between the actuator element and the mirror body. For example, the sensor element can be arranged between the actuator element and the mirror body. Alternatively, the sensor element can be embedded in a matrix adjacent to the actuator element. In this case, the mechanical connection between the sensor element, the actuator element, and the matrix material is such that the mechanical stress propagates at least substantially uninterruptedly at the transition between the actuator element, the matrix, and the sensor element. In this case, the matrix including the sensor element and the actuator element can also be said to appear substantially homogeneous when viewed from the outside.

[0027] This arrangement ensures a strong mechanical connection between the sensor element, the mirror body and the actuator element, which is why a high measurement accuracy can be achieved.

[0028] The mechanical connection between the actuator element, the sensor element and the mirror body is effected in particular by a form-locking interlocking connection, preferably by a material-locking cohesive connection, such as an adhesive bond.

[0029] Preferably, the calibration measurement is performed before the optical system is put into operation. In this case, the mirror surface is illuminated with measurement light and the actuator device deforms the mirror surface. The measurement light can determine the deformation of the mirror surface achieved at a specific drive voltage. A corresponding correlation can therefore be established. Additionally, if the sensor signal is simultaneously detected, a correlation between the deformation of the mirror surface and the sensor signal can also be established. The sensor element is preferably only subjected to voltages with small absolute values. Therefore, no hysteresis occurs. Therefore, the dielectric properties of the sensor element remain constant. In contrast, the properties of the actuator element may change during operation. Therefore, precise control based on a correlation between the drive voltage and the deformation achieved in the calibration measurement is almost impossible. Using the sensor signal, it can be confirmed during operation whether the actual deformation achieved corresponds to the desired deformation.

[0030] According to one embodiment of the optical system, the optical system comprises a closed-loop control unit that controls the drive voltage for the actuator element in response to the sensor signal output by the sensor element so that a predetermined mechanical stress is achieved on the mirror body.

[0031] The closed-loop control unit is advantageously configured to control the drive voltage depending on the sensor signal output by the sensor element. This can be understood to mean that the closed-loop control unit comprises, for example, an evaluation unit that evaluates the output sensor signal and derives the achieved mechanical stress therefrom. Alternatively, this step can be performed by a control computer. The closed-loop control unit can be, for example, combined with the drive unit.

[0032] The closed-loop control unit can be implemented in the form of hardware and / or software. When implemented in the form of hardware, the closed-loop control unit can be designed as, for example, a computer or a microprocessor. When implemented in the form of software, the closed-loop control unit can be designed as a computer program product, a function, a routine, a part of program code, or an executable object.

[0033] Thus, by using an actuator device, the mirror surface can be precisely deformed with actuator elements that are subject to closed-loop control. By locating the sensor element directly on the actuator element and / or behind the actuator element and / or in the direct motion or force path from the actuator element to the mirror body, high precision of the closed-loop control is ensured.

[0034] According to a further embodiment of the optical system, the at least one actuator element and the at least one sensor element are manufactured monolithically.

[0035] This means, in particular, that the actuator element and the sensor element are made from materials of the same material class and that the same manufacturing technology is used to manufacture both elements. Furthermore, monolithic means that the actuator and the sensor are based on the same operating mechanism. Preferably, the two elements are manufactured together, i.e., in the same process. This has the advantage that the exact same technology is used for the actuator element and the sensor element. This keeps the complexity of the optical system low.

[0036] In particular, an actuator-sensor arrangement in which actuation is, for example, electrical, but measurement is optical, is not monolithic. Similarly, a system in which actuation is electrical, using a magnetic field, but measurement is electrical, for example by capacitance measurement, is also not monolithic.

[0037] According to a further embodiment of the optical system, the at least one actuator element and the at least one sensor element are integrated into a layer arranged on the mirror on the side of the mirror body opposite to the mirror surface.

[0038] This embodiment has the advantage that it is not necessary to arrange and mount individual actuator elements and individual sensor elements on the mirror body, but rather a layer is produced in which the actuator elements and sensor elements are integrated. The layer is then attached entirely to the rear side of the mirror, i.e., the side of the mirror body opposite the mirror surface. The layer preferably covers the entire rear side of the mirror. Alternatively, multiple two-dimensional planar elements can be arranged side by side on the rear side of the mirror from this layer. By "integrating at least one actuator element and at least one sensor element into the layer," it is understood that, in particular, the active area of ​​the actuator element (the area where mechanical stress occurs) and the sensor element (the area where its polarization is detected) are integrated into the layer. Other units, such as a measurement unit of the sensor element, are not integrated into the layer.

[0039] The layer preferably consists entirely of electrostrictive material. The active areas, i.e. the actuator sub-areas and the sensor sub-areas, are formed by the placement of electrodes. The remainder of the material can be referred to as the matrix material or passive material.

[0040] According to a further embodiment of the optical system, the at least one sensor element is at least partially arranged between the at least one actuator element and the mirror body in a direction along a surface normal to the mirror surface.

[0041] It can also be said that the actuator element and the sensor element are partially or completely overlapped. This arrangement allows the sensor element to be positioned closer to the mirror body, and therefore closer to the mirror surface, than the actuator element. This is why the deformation or mechanical stress derived from the sensor signal of the sensor element has a very high correlation with the mechanical stress of the mirror body, and therefore the deformation of the mirror surface. This makes closed-loop control more accurate.

[0042] According to a further embodiment of the optical system, the at least one actuator element and the at least one sensor element each comprise at least one layer made of electrostrictive material.

[0043] Here, a layer is preferably understood to mean a shape having an aspect ratio of thickness to extent, i.e., length or width, of at least 1:5, preferably at least 1:10, preferably at least 1:100, preferably up to 1:1000, assuming that the length and width are approximately comparable. For example, the layer may have a thickness between 10 μm and 500 μm, a length and width each of 0.5-5 cm, or may be circular with a diameter in the range of 0.5-5 cm.

[0044] The layer has the advantage that the effect achieved, i.e., the mechanical stress or deformation achieved at a specific driving voltage in the plane of the layer, is greater by a factor of the aspect ratio than the effect occurring in the direction perpendicular to it. Expansion in one direction due to the electrostrictive effect is always accompanied by contraction in a second direction different from the first direction due to the incompressibility of the material.

[0045] Very thin layers have the further advantage that high deformations or forces can be achieved with low driving voltages.

[0046] According to a further embodiment of the optical system, at least one actuator element comprises a plurality of layers of electrostrictive material, each layer of the plurality of layers having an assigned cathode and an assigned anode and being drivable with a respective drive voltage.

[0047] This multilayer structure has the advantage that the force or mechanical stress achievable by the actuator element can be significantly increased as the sum of the forces achieved by the individual layers compared to a single layer, thus allowing for larger deformations and thus a larger correction region in which aberrations can be effectively corrected.

[0048] In an optical system embodiment, a cathode enclosed between two layers forms a common cathode for adjacent layers, and an anode enclosed between two layers forms a common anode for adjacent layers.

[0049] In this case, the electric field has opposite directions in adjacent layers. An advantage of this embodiment is that, for example, no insulating separation layer is required between the two cathodes or anodes, and the amount of cathode or anode material is also minimized, thereby maximizing the proportion of active material in the actuator element.

[0050] In this embodiment, the electric field has opposite directions in adjacent layers, and therefore, advantageously, an electrostrictive material is chosen whose mechanical strain is proportional to the square of the polarization, since the opposite direction of the electric field does not affect the direction of the force.

[0051] According to a further embodiment of the optical system, the at least one actuator element and the at least one sensor element form a layer stack comprising at least two layers.

[0052] In this case, the sensor layer is attached directly to the actuator layer, for example, with a common electrode therebetween. This allows for a particularly compact design. Furthermore, the mechanical coupling between the sensor element and the actuator element is maximized. The sensor element deforms in the same way as the actuator element. This is why the drive voltage of the actuator element can be subject to very precise closed-loop control.

[0053] According to a further embodiment of the optical system, the actuator device comprises at least two sensor elements, the electrostrictive materials of which have different material compositions, and each of the at least two sensor elements is configured to output a sensor signal.

[0054] This embodiment is advantageous because, for example, temperature fluctuations can affect the polarization of the active material and lead to measurement errors. Temperature fluctuations have different effects on layers with different compositions. Therefore, the temperature effect can be removed in the calculation. This increases reliability and measurement accuracy. In this case, since the layer stack consisting of the two sensor elements has only a small thickness, it can be assumed, for example, that both sensor elements have the same temperature.

[0055] According to a further embodiment of the optical system, a determination unit is provided which is configured to determine the temperature in the mirror body in response to the sensor signals output by the at least two sensor elements.

[0056] By comparing two sensor signals from sensor elements with different compositions, both mechanical stress and temperature can be determined by corresponding evaluation based on a physical model that describes the underlying physics. This is particularly advantageous when optimal cooling of the required areas of the mirror body is not possible or is only possible with great effort. Mechanical stresses in the mirror body caused by temperature fluctuations or local temperature differences can be taken into account when determining the current shape of the mirror surface and / or can be compensated for by an actuator device. Advantageously, this is possible without any additional systems.

[0057] The temperature that can be ascertained in this case is that which relates to the temperature of the mirror body in the region of the two sensor elements and is therefore in particular the local temperature.

[0058] According to a further embodiment of the optical system, a measurement unit is provided for applying a measurement AC voltage to the at least two sensor elements to generate a sensor signal, the frequency of the measurement AC voltage being different for the different sensor elements.

[0059] This has the advantage that crosstalk between different conductor tracks to the sensor elements does not lead to measurement errors during evaluation. Frequency division multiplexing can also be mentioned. The measurement unit can in particular be part of a closed-loop control unit or an actuator device, but can also form an independent unit.

[0060] According to a further embodiment of the optical system, the actuator device comprises a plurality of M actuator elements and a plurality of N sensor elements, M and N being integers, and the actuator elements and sensor elements being arranged alternately.

[0061] Preferably, N>M, in particular N=M+1. Therefore, in each case, one sensor element is located on the outside. For example, N=2 and M=1, and two sensor elements form a sandwich structure together with the actuator element. Here, a two-dimensional deformation function can be advantageously determined. Therefore, the actual deformation at a distance from the actuator element can be determined or predicted more accurately. This improves the accuracy of the closed-loop control. It can also be said that a more precise closed-loop control is possible due to the layered determination.

[0062] According to a further embodiment of the optical system, the actuator device comprises an assignment unit configured to assign a value of a sensor signal output from the at least one sensor element to an achieved deformation of the mirror surface based on a calibration measurement, and the closed-loop control unit is configured to control the drive voltage depending on the assigned value and the predetermined deformation of the mirror surface.

[0063] In one embodiment, the allocation unit includes a look-up table (LUT) in which the sensor signal values ​​are assigned to the resulting deformation of the mirror surface. This embodiment is particularly simple and does not require high computational power. For intermediate values, i.e., sensor signal values ​​not stored in the LUT, the value for the deformation can be determined from the two closest values, for example by linear interpolation.

[0064] According to a further embodiment of the optical system, a plurality of actuator devices are arranged on the at least one mirror, each of the plurality of actuator devices being individually controllable.

[0065] Preferably, the mirror is covered, for example, entirely with actuator devices. By appropriately driving the actuator devices, the mirror surface can be freely deformed in a variety of ways. Each actuator device is advantageously provided with a closed-loop control unit. This allows each actuator device to be closed-loop controlled independently of the other actuator devices. In particular, in the case of adjacent actuator devices, mechanical stresses propagate throughout the mirror body, so that a deformation at the position of a first actuator device can lead to a slight deformation at the position of a second actuator device. This deformation is also detected by the sensor elements of the second actuator device. If the deformation is in any case undesirable, it is also possible to suppress the deformation by appropriately closed-loop controlling the actuator elements.

[0066] According to a second aspect, there is proposed a lithographic apparatus comprising an optical system according to one of the first aspect or embodiments.

[0067] This lithographic apparatus has the advantage that imaging errors can be corrected or compensated for if a closed-loop control circuit is present that controls each actuator element, thereby allowing for much more accurate correction of aberrations than would be possible without such a closed-loop control circuit. In particular, in lithographic apparatuses, the installation space is so small that conventional solutions for adaptive optics cannot be used, since the optical units used operate, for example, in a vacuum.

[0068] The optical system preferably constitutes or is part of a beam forming and illumination system or a projection system of a lithographic apparatus.

[0069] According to a third aspect, the use of a mirror coupled to an actuator device in an optical system is proposed. The mirror comprises a mirror body and a mirror surface. The actuator device includes at least one electrostrictive actuator element that generates a mechanical stress in the mirror body to deform the mirror surface in response to an electrical driving voltage, and at least one electrostrictive sensor element that outputs a sensor signal in response to deformation of the sensor element. The at least one sensor element is arranged directly adjacent to the actuator element, and / or is arranged on a side of the mirror body opposite the mirror surface and separated from the mirror body by at least the actuator element, and / or is arranged so as to be configured to at least partially transmit the mechanical stress generated by the actuator element to the mirror body. The actuator device is coupled to the mirror body such that the mirror surface is deformed in response to the electrical driving voltage.

[0070] According to a fourth aspect, a method for operating an optical system is proposed. The optical system includes a mirror having a mirror body and a mirror surface, and an actuator device coupled to the mirror body for deforming the mirror surface. In a first step, an electrostrictive actuator element of the actuator device is driven with an electrical drive voltage, which generates a mechanical stress in the mirror body and deforms the mirror surface. In a second step, at least one electrostrictive sensor element of the actuator device is used to detect a sensor signal in response to the deformation of the sensor element. The at least one sensor element is arranged directly adjacent to the actuator element, and / or is arranged on the side of the mirror body opposite the mirror surface and separated from the mirror body by at least the actuator element, and / or is arranged so as to at least partially transmit the mechanical stress generated by the actuator element to the mirror body.

[0071] In embodiments, the optical systems described above and below are configured to perform or operate in accordance with the aforementioned methods.

[0072] According to a further aspect, a method for detecting an achieved deformation of a mirror surface of an optical system according to one of the first aspect or embodiments is proposed. In a first step, at least one actuator element is driven with a drive voltage corresponding to a predetermined deformation of the mirror surface. For example, a control computer determines an ideal shape of the mirror surface that deviates from the actual target shape of the mirror surface in the current state of the optical system, and accordingly determines a required drive voltage for the actuator element and outputs this drive voltage to the actuator element. In a second step, at least one sensor element outputs a sensor signal. This is done, in particular, by applying a measurement AC voltage to the active area of ​​the sensor element. From the complex impedance of the sensor element, the dielectric susceptibility and thus the polarization of the active area can be inferred. In a third step, the deformation of the sensor element is determined in response to the output sensor signal. Therefrom, the achieved deformation of the mirror surface is determined. The polarization of the active area of ​​the sensor element indicates the deformation present in the sensor element. From this, the deformation of the mirror surface can be determined using a mechanical model of the mirror. Preferably, the deformation of the mirror surface corresponding to the deformation of the sensor element is previously determined in a calibration measurement and stored in a look-up table (LUT).

[0073] In an advantageous embodiment of the method, the drive voltage is controlled in a closed-loop manner so that a predetermined deformation of the mirror surface is achieved.

[0074] The embodiments and features described for the optical system are correspondingly applicable to the proposed method.

[0075] As used herein, the indefinite article and number "A(n)" should not be understood as necessarily limiting to exactly one element. Rather, there can be a plurality of elements, e.g., two, three, or more. Other numbers used herein should not be understood as implying a strict limitation to the number of elements recited. Rather, upward and downward numerical deviations are possible unless otherwise indicated.

[0076] Further possible implementations of the present invention also include combinations not explicitly mentioned of the features or embodiments described above or below with respect to the exemplary embodiments, in which case a person skilled in the art can also add individual aspects as improvements or supplements to each basic form of the invention.

[0077] Further advantageous configurations and aspects of the invention are the subject of the dependent claims and also of the examples of the invention described below. The invention is explained in more detail below by means of preferred embodiments with reference to the accompanying drawings. [Brief explanation of the drawings]

[0078] [Figure 1] 1 is a schematic diagram of a first exemplary embodiment of an optical system. [Figure 2] 1 is a schematic diagram of a first exemplary embodiment of an arrangement of mirrors with actuator devices; [Figure 3] 10 is a schematic diagram of a second exemplary embodiment of an arrangement of mirrors with actuator devices; [Figure 4] 10 is a schematic diagram of a third exemplary embodiment of an arrangement of mirrors with actuator devices; [Figure 5] 10 is a schematic diagram of a fourth exemplary embodiment of an arrangement of mirrors with actuator devices; [Figure 6] FIG. 10 is a schematic diagram of a fifth exemplary embodiment of an arrangement of mirrors with actuator devices. [Figure 7] FIG. 10 is a schematic diagram of a sixth exemplary embodiment of an arrangement of mirrors with actuator devices. [Figure 8] FIG. 10 is a schematic diagram of a seventh exemplary embodiment of an arrangement of mirrors with actuator devices. [Figure 9] 1 is a schematic diagram of an exemplary embodiment of the structure of an actuator element; [Figure 10] 1 is a schematic diagram of an exemplary embodiment of driving multiple actuator elements and sensor elements in an actuator device; [Figure 11]1 is a graph of three behaviors of physical variables as a function of driving voltage. [Figure 12] 1 is a graph showing multiple curves of dielectric susceptibility as a function of driving voltage. [Figure 13] FIG. 2 is a schematic block diagram of an exemplary embodiment of a closed-loop control circuit. [Figure 14A] 1 is a schematic diagram of an exemplary embodiment of an EUV lithography apparatus; [Figure 14B] 1 is a schematic diagram of an exemplary embodiment of a DUV lithography apparatus; [Figure 15] 1 is a schematic block diagram of an exemplary embodiment of a method for correcting aberrations in an optical system; DETAILED DESCRIPTION OF THE INVENTION

[0079] Unless otherwise noted, identical or functionally identical elements are designated by the same reference numerals in the figures. It should also be noted that the figures are not necessarily drawn to scale.

[0080] 1 shows a schematic diagram of a first embodiment of an optical system 200. The optical system 200 here comprises a light source LS, a lens element 128 that collimates light incident from the light source LS, two mirrors 110, 210, a further lens element 128, and a wafer 124 or objective slide onto which the light from the further lens element 128 is focused. For example, the optical system 200 is an illumination system of a microscope or lithography apparatus 100A, 100B (see FIGS. 14A, 14B).

[0081] The second mirror 210 of the optical system 200 consists of a mirror body 212. A mirror surface 214 is arranged on the front side of the mirror body 212. An actuator device 220 is arranged on the back side of the mirror body 212. The actuator device 220 is configured to introduce mechanical stress into the mirror body 212, thereby deforming the mirror surface 214. Without limiting generality, only one actuator device 220 each is shown herein and in the following figures. However, it will be appreciated that multiple such actuator devices 220 can be arranged on the mirror 210 to achieve a targeted deformation of the mirror surface 214 with high spatial resolution and / or to achieve an overall deformation of the mirror 210.

[0082] Due to the fact that the mirror surface 214 of the mirror 210 can be deformed, aberrations, i.e. imaging errors, can be compensated for. In this case, the imaging errors depend, inter alia, on the operating state of the optical system 200 and / or further optical systems coupled to the optical system 200. For example, spatial and / or temporal temperature differences and / or temperature fluctuations can be compensated for by the actuator device 220. This can also be referred to as an adaptive optics system 200, whose state is subject to closed-loop control or kept constant relative to a reference state. Advantageously, the actuator device 220 comprises a closed-loop control circuit that adjusts the actuator element 222 (see Figures 2 to 8, 10, 13, 14A or 14B).

[0083] The exact mode of operation of the actuator device 220 will be explained in more detail with reference to the following figures.

[0084] 2-7 each show a schematic diagram of an exemplary embodiment of an arrangement of a mirror 210 with an actuator device 220. The exemplary embodiments differ in the particular arrangement of the actuator device 220 and the mirror 210. Each of the exemplary embodiments can be used in an optical system 200 such as that shown in FIGS. 1, 14A, or 14B.

[0085] 2 shows an actuator device 220 arranged between the mirror body 212 and the mirror carrier 216. The mirror carrier 216 forms a mechanical fixing point, i.e. is rigid and fixed. The actuator device 220 comprises an actuator element 222 and a sensor element 224. In this case, the sensor element 224 is in mechanical contact with the mirror body 212. The actuator element 222 is supported on the mirror carrier 216. Furthermore, a drive unit 226 is present, which provides a drive voltage Vs for driving the actuator element 222. In this embodiment, the drive unit 226 also detects a sensor signal Ss output by the sensor element 224. Preferably, the drive unit 226 is designed as a closed-loop control unit or comprises a control unit which adjusts the drive voltage Vs depending on the sensor signal Ss.

[0086] In this arrangement, the actuation direction 223 is parallel to the surface normal of the mirror surface 214. The actuator elements 222 extend or expand in the actuation direction 223 when a drive voltage Vs is applied. Because the mirror carrier 216 is fixed, the extension of the actuator elements 222 results in a bulging or displacement of the mirror surface 214 at the position of the actuator device 220. Here, the mirror body 212 can be supported on the mirror carrier 216 at one or more points by fixed connecting elements (not shown). Such fixed connecting elements fix, for example, the spacing between the mirror body 212 and the mirror carrier 216 at the locations where they are arranged.

[0087] 3 shows an alternative embodiment of the actuator device 220 arranged between the mirror body 212 and the mirror carrier 216. In this case, the sensor element 224 is arranged laterally directly on the actuator element 222. In this arrangement, the sensor element 224 accurately follows any deformation, in particular extension or compression, of the actuator element 222 along the actuation direction 223. The actual deformation of the actuator element 222 can therefore be determined very accurately from the sensor signal SS.

[0088] 4 to 7 each show a different integration form of the actuator device 220. In this case, in contrast to FIGS. 2 and 3, a lateral actuation direction 223 is used, i.e., in the plane of the mirror body 212. In FIGS. 4 and 5, the actuator device 220 is glued to the rear side of the mirror body 212 or is firmly connected to it in a similar way. In this case, the sensor element 224 is arranged between the actuator element 222 and the mirror body 212 (FIG. 4) or behind the actuator element 222 (FIG. 5). In the arrangement shown in FIG. 4, the sensor element 224 transfers any mechanical stress generated by the actuator element 222 to the mirror body 212. In particular, the sensor element 224 undergoes the same deformation as the actuator element 222 and the region of the mirror body 212 that is in direct contact with the sensor element 224. The lateral actuation 223, which can expand or contract relative to the actuator element 222, generates mechanical stresses in the mirror body 212, which results in a corresponding deformation of the mirror surface 214.

[0089] In the exemplary embodiments of Figures 6 and 7, the actuator elements 222 and the sensor elements 224 are embedded in a matrix MX. The matrix MX is made, in particular, of a material of the same material class as the actuator elements 222. The sensor elements 224 are preferably made of an electrostrictive ceramic material. Advantageously, the actuator elements 222, the sensor elements 224 and the matrix MX form a substantially homogeneous material layer 221. This material layer 221 is fixed two-dimensionally and planarly, preferably over its entire surface, to the rear side of the mirror body 212. The active areas of the actuator elements 222 and the sensor elements 224 are defined by the arrangement of the electrodes in the matrix MX. In these embodiments, the mechanical coupling of the actuator elements 222 with the mirror body 212 is particularly strong. This has an advantageous effect on the maximum achievable deformation of the mirror surface 214.

[0090] The exemplary embodiments shown in Figures 2-7 can also be combined with one another as desired.

[0091] FIG. 8 shows a schematic diagram of a fourth exemplary embodiment of an arrangement of a mirror 210 with actuator devices 220. The basic arrangement corresponds to the arrangement shown in FIG. 7. The actuator device 220 here has two sensor elements 224 sandwiched around an actuator element 222. Advantageously, the two sensor elements 224 have different chemical compositions. As a result, they have different dependencies on temperature and deformation. Therefore, from the two sensor signals SS, it is possible to determine not only the deformation but also the temperature of each sensor element 224. This is done by a determination unit 230 specially designed for this purpose.

[0092] FIG. 9 shows a schematic diagram of an exemplary embodiment of the structure of an actuator element 222, here consisting of a plurality of individual layers L1-Ln. An electrode A1 is arranged on the top layer L1. Here, the electrode A1 acts, for example, as an anode. Between the top layer L1 and the layer L2 arranged adjacently below it, a further electrode K1, here acting as a cathode, is arranged. The anode A1 and the cathode K1 can be said to surround the top layer L1 in a sandwich-like manner. A drive voltage VS (see FIGS. 2-8 or 10-13) applied between the anode A1 and the cathode K1 generates an electric field in the layer L1. The roles of the anode A1 and the cathode K1 can also be reversed in embodiments, in which case the direction of the electric field is reversed.

[0093] Between the second layer L2 from the top and the third layer L3 from the top, electrode A2 is again placed. Electrode A2 acts as an anode. Here, it can be seen that cathode K1 forms a common cathode for adjacent layers L1 and L2. Anode A2 also forms a common anode for adjacent layers L2 and L3.

[0094] In this embodiment, this layering continues until the desired number of layers is reached. Below the lowest layer Ln, a terminal electrode Kn is placed. The terminal electrode Kn acts as a cathode in this embodiment. In this alternating structure with common electrodes A1-An, K1-Kn, electrostrictive materials, in particular, have a mechanical strain proportional to the square of the polarization. Therefore, layers L1-Ln deform in the same direction despite different electric field directions.

[0095] In other embodiments where electrostrictive materials are used in which the mechanical strain is proportional to the polarization, the electrodes A1-An, K1-Kn should be positioned and driven so that the electric fields in all layers L1-Ln point in the same direction, since otherwise the layers L1-Ln will act in opposition to each other.

[0096] The thickness of each of the layers L1 to Ln can be different. Preferably, the thicknesses of the different layers L1 to Ln are substantially the same and are selected from the range of 10 μm to 500 μm. In this case, the material composition can be selected to be different for each of the layers L1 to Ln, for example, to achieve different electrostrictive properties.

[0097] In a further embodiment, the sensor element 224 has a layered structure, as shown in Fig. 9 for the actuator element 222. In this case, in particular, the use of different chemical compositions for the different layers L1-Ln has the advantage that a comparison of the sensor signals SS of the individual layers L1-Ln can be used to derive and determine not only the mechanical stress but also further influencing variables, in particular the temperature of the layers L1-Ln.

[0098] 10 shows a schematic diagram of an exemplary embodiment of an actuator device 220 comprising a plurality of actuator elements 222, a plurality of sensor elements 224, and a drive unit 226. In this case, three sensor elements 224 and two actuator elements 222 are provided, forming a stack. Here, the actuator element 222 comprises, for example, three layers L1 to L3, each made of an electrostrictive material.

[0099] Two of the sensor elements 224 sandwich the entire stack. The third sensor element 224 divides the stack in the middle. In this case, the passive areas are each located between the sensor element 224 and the actuator element 222. These can also be omitted in embodiments in which the electrode located between the sensor element 224 and the actuator element 222 constitutes a common electrode. In this case, the common electrode is preferably grounded, as the sensor elements advantageously operate without a bias voltage.

[0100] The drive unit 226 here comprises a voltage source under closed-loop control for providing a drive voltage Vs for the actuator elements 222, and three measurement units configured to generate a sensor signal Ss for each sensor element 224 using a measurement AC voltage Vm. The measurement AC voltages Vm in this case preferably have different frequencies to avoid mutual interference. In further embodiments, the actuator elements 222 can also be operated with different drive voltages Vs.

[0101] FIG. 11 shows three diagrams of the behavior of physical variables for an exemplary electrostrictive layer: mechanical stress σ, the gradient ∂e / ∂VS of the deflection e achieved with respect to the driving voltage VS, and the dielectric constant χ as a function of the driving voltage. This exemplary electrostrictive layer can be used in an actuator element 222 (see FIGS. 2-10 or 13) or a sensor element 224 (see FIGS. 2-8, 10 or 13). The variables σ, e, and χ are shown in arbitrary units. The unit of volts is chosen as a scale for the driving voltage VS. The electrostrictive material is, in particular, of the composition PMN. x P t1- x. The curves shown relate to the mechanically free state of the layer, i.e., without mechanical prestress and without the layer being embedded in a rigid material system.

[0102] The figure above shows the mechanical stress σ (unit: N / m 2or Pa). When the driving voltage Vs=0, the layer does not generate any mechanical stress σ. Similarly, it can be seen that a small driving voltage Vs results in only a negligible mechanical stress σ. When the driving voltage Vs is negligible, the generated mechanical stress σ is, for example, proportional to the square of the driving voltage Vs.

[0103] The central diagram shows the slope of the deflection e achieved in relation to the drive voltage Vs as a function of the drive voltage Vs.

[0104] The bottom diagram shows the dielectric constant χ as a function of the driving voltage Vs. It can be seen that the dielectric constant χ has a maximum value at driving voltage Vs=0.

[0105] As shown in FIG. 12, the permittivity χ at a drive voltage Vs = 0 also has a very high sensitivity to mechanical stress. In FIG. 12, several curves of the permittivity χ as a function of the drive voltage Vs are shown in one diagram. The five illustrated curves 1 to 5 differ in this case in the deformation of the layer. For example, curve 3 corresponds to a mechanically free or unstressed state. Curves 1 and 2 correspond, for example, to layer extension in the ppm range, e.g., 10 ppm for curve 1 and 5 ppm for curve 2. Curves 4 and 5 correspond, for example, to layer compression in the ppm range, e.g., 5 ppm for curve 4 and 10 ppm for curve 5. Due to this sensitivity maximum of the permittivity χ, this physical variable is particularly well suited as a measurement variable for determining layer deformation, particularly in the sensor element 224. The permittivity χ can be determined in this case by impedance measurements using a measurement AC voltage V.

[0106] 13 shows a schematic block diagram of an exemplary embodiment of a closed-loop control circuit in the actuator device 220. In this case, the drive unit 226 is designed as a closed-loop control unit. The closed-loop control unit 226 receives from the outside, for example via a control computer (not shown), predetermined mechanical target stresses σ , which the actuator device 220 should apply or generate by means of the actuator elements 222.s The closed-loop control unit 226 then drives the actuator element 222 with a drive voltage Vs, which is preferably coupled to a target mechanical stress σ, which is known, for example from measuring the properties of the actuator element 222. s However, due to hysteresis or various environmental influences, the actuator element 222 may not be able to achieve the mechanical target stress σ s There is a possibility that the measurement may be slightly off.

[0107] The sensor element 224 is deformed by the actuator element 222. The degree of deformation can be ascertained as described above. This is represented in this case by the sensor signal SS. The sensor signal SS is characteristic, in particular, of the deformation of the sensor element 224 and thus of the mechanical stress σ achieved in the sensor element 224. Due to the direct coupling of the sensor element 224 to the actuator element 222, the mechanical stress σ in the sensor element 224 substantially corresponds to the mechanical stress σ in the actuator element 222. The sensor signal SS is therefore suitable as a closed-loop control signal. The closed-loop control unit 226 therefore readjusts the drive voltage VS of the actuator element 222 based on the sensor signal SS.

[0108] In this case, the closed-loop control cycle may last for example in the range of 1 ms to 100 ms, corresponding to a closed-loop control frequency between 10 Hz and 1 kHz.

[0109] FIG. 14A shows a schematic diagram of an EUV lithography apparatus 100A. The EUV lithography apparatus 100A includes a beam forming and illumination system 102 and an optical system 200, here designed as a projection system. In this case, EUV stands for "extreme ultraviolet" and refers to a working light wavelength between 0.1 nm and 30 nm. The beam forming and illumination system 102 and the projection system 200 are each provided in a vacuum housing (not shown). Each vacuum housing is evacuated by an exhaust system (not shown). The vacuum housings are enclosed in a machine room (not shown). The machine room contains a drive device for mechanically moving or setting optical elements. Furthermore, an electrical control unit may be provided in this machine room.

[0110] The EUV lithography apparatus 100A includes an EUV light source 106A. The EUV light source 106A may be, for example, a plasma source (or a synchrotron) that emits radiation 108A in the EUV range (extreme ultraviolet range) in the wavelength range of 5 nm to 20 nm. In the beam forming and illumination system 102, the EUV radiation 108A is focused and a desired operating wavelength is filtered from the EUV radiation 108A. The EUV radiation 108A generated by the EUV light source 106A has a relatively low transmittance in air. For this purpose, the beam forming and illumination system 102 and the beam guide spaces in the projection system 200 are evacuated.

[0111] 14A includes five mirrors 110, 112, 114, 116, and 118. After passing through beam forming and illumination system 102, EUV radiation beam 108A is directed to photomask (reticle) 120. Photomask 120 may also be designed as a reflective optical element and may be located external to systems 102, 104. Furthermore, EUV radiation beam 108A may be directed to photomask 120 using mirror 122. Photomask 120 has a structure that is imaged, in a reduced form, onto wafer 124 or the like using projection system 200.

[0112] The projection system 200 (also referred to as a projection lens) includes five mirrors M1-M5 that image the photomask 120 onto the wafer 124. In this case, the individual mirrors M1-M5 of the projection system 200 may be arranged symmetrically with respect to the optical axis 126 of the projection system 200. It should be noted that the number of mirrors M1-M5 in the EUV lithography apparatus 100A is not limited to the number shown; more or fewer mirrors may be provided. Furthermore, the mirrors M1-M5 typically have curved front surfaces to shape the beam.

[0113] The projection system 200 further comprises a further mirror 210. Arranged on its rear side are a number of actuator devices 220, each of which can be designed as shown in FIGS. 2-8, 10, or 13. Each actuator device 220 includes an assigned actuator element 222 and an assigned sensor element 224. A drive unit 226 is configured to drive the actuator element 222 with a drive voltage VS and apply a measurement AC voltage VM to the sensor element 224 to generate a respective sensor signal SS. For clarity, only one drive unit 226 is shown here, which drives all of the actuator elements 222 and the sensor elements 224. The front side of the mirror 210 can be deformed by target-driven actuation of the actuator devices 220. This allows for correcting optical aberrations and improving the resolution of the lithography process.

[0114] In an advantageous embodiment, the actuator elements 222 are actuated in a closed-loop manner, in that the actual deformation as desired is detected by the sensor elements 224 and the drive voltage Vs can be adjusted accordingly by evaluating the sensor signal Ss. This closed-loop control is performed individually for each actuator device 220. For clarity, only one actuator device 220, only one drive voltage Vs, one measured AC voltage Vm and one sensor signal Ss are shown.

[0115] The projection system 200 or the beam forming and illumination system 102 may also comprise a further mirror 210 with an assigned actuator device 220 .

[0116] FIG. 14B shows a schematic diagram of a DUV lithography apparatus 100B. The DUV lithography apparatus 100B includes a beam-forming and illumination system 102 and an optical system 200, here designed as a projection system. In this case, DUV stands for "deep ultraviolet" and refers to a working light wavelength between 30 nm and 250 nm. As already explained with reference to FIG. 14A, the beam-forming and illumination system 102 and the projection system 200 can be arranged in a vacuum housing and / or enclosed by a machine room with corresponding drive devices.

[0117] The DUV lithography apparatus 100B includes a DUV light source 106B, which may be, for example, an ArF excimer laser that emits radiation 108B in the DUV region at 193 nm.

[0118] Beam forming and illumination system 102, shown in Figure 14B, directs DUV radiation beam 108B onto photomask 120. Photomask 120 may be formed as a transmissive optical element and located external to systems 102, 104. Photomask 120 has structures that are imaged onto, for example, wafer 124 using projection system 200.

[0119] The projection system 200 includes a plurality of lens elements 128 and / or mirrors 130 that image the photomask 120 onto the wafer 124. In this case, the individual lens elements 128 and / or mirrors 130 of the projection system 104 may be arranged symmetrically with respect to the optical axis 126 of the projection system 200. It should be noted that the number of lens elements 128 and mirrors 130 in the DUV lithography apparatus 100B is not limited to the number shown. More or fewer lens elements 128 and / or mirrors 130 may be provided. Furthermore, the mirrors 130 typically have a curved front surface to shape the beam.

[0120] The gap between the final lens element 128 and the wafer 124 can be replaced with a liquid medium 132 having a refractive index >1. The liquid medium 132 can be, for example, high-purity water. Such a structure is also called immersion lithography and has high photolithographic resolution. The medium 132 can also be called an immersion liquid.

[0121] The projection system 200 further comprises a further mirror 210, behind which an actuator device 220 is arranged. The actuator device can be designed as shown in FIGS. 2 to 8, 10, or 13. Without limiting generality, only one actuator device 220 is shown here. However, it goes without saying that preferably there are multiple actuator devices 220, each of which can be individually controlled with open-loop and / or closed-loop control. The actuator device 220 comprises an assigned actuator element 222 and an assigned sensor element 224. A drive unit 226 is configured to drive the actuator element 222 with a drive voltage VS and to apply a measurement AC voltage VM to the sensor element 224 in order to generate a respective sensor signal SS.

[0122] Also shown in FIG. 14B is a predetermined mechanical target stress σ to be achieved by the actuator element 222. s It has been shown that the mechanical target stress σ is specified externally. s is determined by a control computer, for example, based on a target deformation to be achieved of mirror 210. By targeting the front side of mirror 210, optical aberrations can be corrected to increase the resolution of the lithography process.

[0123] In an advantageous embodiment, the actuator elements 222 are actuated in a closed-loop manner, in that the actual deformation is detected by the sensor elements and the drive voltage Vs can be adjusted accordingly by evaluating the sensor signal Ss. This closed-loop control is performed individually for each actuator device 220. For clarity, only one actuator device 220, only one drive voltage Vs, one measured AC voltage Vm and one sensor signal are shown.

[0124] The projection system 200 or the beam forming and illumination system 102 may also comprise a further mirror 210 with an assigned actuator device 220 .

[0125] FIG. 15 shows a schematic block diagram of an exemplary embodiment of a method for correcting aberrations in optical system 200, e.g., optical system 200 of FIG. 1, by targeted deformation of mirror surface 214 (see FIGS. 1-8) of mirror 210 (see FIGS. 1-8, or 14A, 14B) of optical system 200.

[0126] In a first step S1, at least one actuator element 222 (see FIGS. 2-8 or 13) is driven with a drive voltage Vs (see FIGS. 2-8 or 10-13) that corresponds to a specific deformation of the mirror surface 214 (see FIGS. 2-8 or 10-13). As a result, a mechanical stress is generated in the actuator element 222. This mechanical stress is transmitted to the mirror body 212 (see FIGS. 1-8) of the mirror 210, resulting in a local deformation of the mirror surface 214.

[0127] In a second step S2, a sensor signal SS (see Fig. 2 to 8 or 13) is output by at least one sensor element 224 (see Fig. 2 to 8 or 13). For this purpose, a measuring AC voltage VM is applied to the sensor element 224, as described above.

[0128] In a third step S3, the deformation of the sensor element 224 is ascertained according to the detected sensor signal SS, from which it can be determined that a deformation of the mirror surface 214 has been achieved.

[0129] In an optional fourth step S4, the drive voltage Vs is closed-loop controlled so as to achieve a predetermined deformation of the mirror surface 214. For this purpose, for example, the predetermined deformation is compared with the achieved deformation, which indicates whether the drive voltage Vs needs to be higher or lower to achieve the predetermined deformation.

[0130] Although the present invention has been described with reference to exemplary embodiments, it can be modified in many ways. In particular, many of the physical variables described are interchangeable with other variables. Thus, instead of mechanical stress, one can refer to force or mechanical strain or deformation. Furthermore, the sensor signal can be said to depend on the permittivity, impedance, polarization, capacitance, etc., which are all interchangeable if the respective material parameters are known.

[0131] In particular, the present invention provides a number of possible arrangements of sensor elements relative to actuator elements in an actuator device, where different positions may be advantageous depending on the specific application. Furthermore, by combining different arrangements in one actuator element, detection accuracy can be increased.

[0132] Furthermore, the closed-loop control circuit for controlling the drive voltage can be implemented or realized in a variety of ways without limiting the present invention to any particular method. [Explanation of symbols]

[0133] 1 Function curve 2 Function curves 3 Function curves 4 Function curves 5 Function curves 100A EUV lithography equipment 100B DUV lithography equipment 102 Beam forming and lighting system 106A EUV light source 106B DUV light source 108A EUV radiation beam 108B DUV radiation beam 110 Mirror 112 Mirror 114 Mirror 116 Mirror 118 Mirror 120 Photomask 122 Mirror 124 wafers 126 Optical axis 128 lens elements 130 Mirror 132 Medium 200 Optical system 210 Mirror 212 Mirror body 214 Mirror Surface 216 Mirror carrier 220 Actuator Device 221 layers 222 Actuator element 223 Operating direction 224 Sensor Element 226 Drive Unit 230 Confirmation Unit A1 Anode A2 anode A3 Anode An anode K1 cathode K2 cathode K3 cathode Kn cathode L1 layer L2 layer L3 layer Ln layer LS light source MX Matrix M1 mirror M2 mirror M3 mirror M4 mirror M5 mirror S1 Method Steps S2 Method Steps S3 Method Steps S4 Method Steps SS sensor signal VM measurement AC voltage VS driving voltage χ dielectric constant σ mechanical stress σ s Mechanical stress

Claims

1. An optical system (200) comprising at least one mirror (210) having a mirror body (212) including a front side and a rear side facing opposite the front side, and a mirror surface (214) disposed on the front side of the mirror body, and at least one actuator device (220) coupled to the mirror body (212) for deforming the mirror surface (214), the actuator device (220) comprising: at least one electrostrictive actuator element (222) disposed on the rear side of the mirror body (212) for generating a mechanical stress in the mirror body (212) to deform the mirror surface (214) in response to an electrical driving voltage (V); at least one electrostrictive sensor element (224), which outputs a sensor signal (SS) in response to deformation of the electrostrictive sensor element (224), and which is arranged directly adjacent to the electrostrictive actuator element (222) without being in direct contact with the mirror body, and / or which is arranged on the rear side of the mirror body (212) and separated from the mirror body (212) by at least the electrostrictive actuator element (222), and / or which is arranged so as to be configured to at least partially transmit the mechanical stress generated by the electrostrictive actuator element (222) to the mirror body (212); the actuator device (220) is coupled to the mirror body (212) such that the mirror surface (214) is deformable in response to the electrical drive voltage (Vs) of the electrostrictive actuator element (222); the electrostrictive sensor element is deformed in accordance with the deformation of the electrostrictive actuator element in the actuation direction; Optical system (200).

2. 2. The optical system according to claim 1, further comprising a closed-loop control unit that controls the drive voltage (VS) for the electrostrictive actuator element (222) in response to the sensor signal (SS) output by the electrostrictive sensor element (224) so ​​that a predetermined mechanical stress is achieved in the mirror body (212).

3. 3. The optical system according to claim 1 or 2, wherein the at least one electrostrictive actuator element (222) and the at least one electrostrictive sensor element (224) are monolithically manufactured.

4. The optical system according to any one of claims 1 to 3, wherein the at least one electrostrictive actuator element (222) and the at least one electrostrictive sensor element (224) are integrated into a layer (221) arranged on the mirror (210) at the rear side of the mirror body (212).

5. 5. The optical system according to claim 1, wherein the at least one electrostrictive sensor element (224) is at least partially arranged between the at least one electrostrictive actuator element (222) and the mirror body (212) in a direction along a surface normal to the mirror surface (214).

6. 6. The optical system according to claim 1, wherein the at least one electrostrictive actuator element (222) and the at least one electrostrictive sensor element (224) each comprise at least one layer (L1, ..., Ln) made of an electrostrictive material.

7. 7. The optical system according to claim 6, wherein the at least one electrostrictive actuator element (222) comprises a plurality of layers (L1, ..., Ln) made of an electrostrictive material, each layer (L1, ..., Ln) of the plurality of layers comprising an assigned cathode (K1 to Kn) and an assigned anode (A1 to An), and capable of being driven by a respective driving voltage (VS).

8. 8. The optical system according to claim 6 or 7, wherein the at least one electrostrictive actuator element (222) and the at least one electrostrictive sensor element (224) form a layer stack (SL) comprising at least two layers (L1, ..., Ln).

9. The optical system according to any one of claims 1 to 8, wherein the actuator device (220) comprises at least two electrostrictive sensor elements (224), the electrostrictive materials of which have different material compositions, and each of the at least two electrostrictive sensor elements (224) is configured to output a sensor signal (SS).

10. 10. The optical system according to claim 9, further comprising a confirmation unit (230) for confirming the temperature within the mirror body (212) in response to the sensor signals (SS) output by the at least two electrostrictive sensor elements (224).

11. 11. The optical system according to claim 9 or 10, wherein a measurement unit is provided for applying a measurement AC voltage (VM) to the at least two electrostrictive sensor elements (224) to generate the sensor signal (SS), and the frequency of the measurement AC voltage (VM) is different for different electrostrictive sensor elements (224).

12. 12. The optical system according to claim 1, wherein the actuator device (220) comprises a plurality of M electrostrictive actuator elements (222) and a plurality of N electrostrictive sensor elements (224), where N and M are integers, and the electrostrictive actuator elements (222) and the electrostrictive sensor elements (224) are arranged alternately.

13. 3. The optical system of claim 2, wherein the actuator device (220) includes an assignment unit configured to assign a value of a sensor signal (SS) output from the at least one electrostrictive sensor element (224) to an achieved deformation of the mirror surface (214) based on a calibration measurement, and the closed-loop control unit is configured to control the drive voltage (VS) in response to the assigned value and a predetermined deformation of the mirror surface (214).

14. 14. The optical system according to claim 1, wherein a plurality of actuator devices (220) are arranged on the at least one mirror (210), each of the plurality of actuator devices (220) being individually controllable.

15. A lithographic apparatus (110A, 100B) comprising an optical system (200) according to any one of claims 1 to 13.

16. 1. Use in an optical system of a mirror (210) coupled to an actuator device (220), the mirror (210) comprising a mirror body (212) including a front side and a rear side facing away from the front side, and a mirror surface (214) disposed on the front side of the mirror body, the actuator device (220) comprising: at least one electrostrictive actuator element (222) disposed on the rear side of the mirror body (212) for generating a mechanical stress in the mirror body (212) to deform the mirror surface (214) in response to an electrical driving voltage (V); at least one electrostrictive sensor element (224), which outputs a sensor signal (SS) in response to deformation of the electrostrictive sensor element (224), and which is arranged directly adjacent to the electrostrictive actuator element (222) without being in direct contact with the mirror body, and / or which is arranged on the rear side of the mirror body (212) and separated from the mirror body (212) by at least the electrostrictive actuator element (222), and / or which is arranged so as to be configured to at least partially transmit the mechanical stress generated by the electrostrictive actuator element (222) to the mirror body (212); the actuator device (220) is coupled to the mirror body (212) such that the mirror surface (214) is deformed in response to the electrical drive voltage (Vs); The electrostrictive sensor element is deformed in accordance with the deformation of the electrostrictive actuator element in the actuation direction.

17. A method of operating an optical system (200), the optical system (200) comprising: a mirror (210) having a mirror body (212) including a front side and a rear side facing away from the front side, and a mirror surface (214) disposed on the front side of the mirror body; and an actuator device (220) coupled to the mirror body (212) for deforming the mirror surface (214), wherein at least one electrostrictive actuator element (222) is disposed on the rear side of the mirror body (212). driving the electrostrictive actuator element (222) of the actuator device (220) with an electrical driving voltage (VS), thereby generating a mechanical stress in the mirror body (212) and deforming the mirror surface (214); detecting a sensor signal (SS) in response to a deformation of the electrostrictive sensor element (224) using at least one electrostrictive sensor element (224) of the actuator device (220), wherein the at least one electrostrictive sensor element (224) is arranged directly adjacent to the electrostrictive actuator element (222) without being in direct contact with the mirror body, and / or is arranged at the rear side of the mirror body (212) and separated from the mirror body (212) by at least the electrostrictive actuator element (222), and / or is arranged so as to be configured to at least partially transmit the mechanical stress generated by the electrostrictive actuator element (222) to the mirror body (212), and the electrostrictive sensor element is deformed following the deformation along the actuation direction of the electrostrictive actuator element; and determining a deformation of said mirror surface (214) in response to said detected sensor signal (SS).

Images