Optical system comprising multi-mirror array, and operational control method

Abstract

The invention relates to an optical system, in particular an illumination system for an EUV apparatus, comprising at least one multi-mirror array (MMA) having a carrier structure (TS) and a multiplicity of individually tiltable mirror units (MU) arranged next to one another on the carrier structure in a grid arrangement. A vibration cleaning system for cleaning at least one selected region of the multi-mirror array (MMA) so as to remove contamination particles (PART1, PART2) adhering to a mirror element and / or caught in a gap is provided, wherein the multi-mirror array comprises at least one electromechanical vibration generator (VIB1, VIB2) which is controllable by means of signals from a control unit and which is configured to generate cleaning-effective vibrations of at least one mirror element in reaction to signals from the control unit during a vibration cleaning operation.

Description

[0001] Optical system with multiple mirror arrangement and operating control method

[0002] The following disclosure is based on the German patent application with file number 10 2024 127 052.7, which was filed on September 19, 2024. The disclosure content of this patent application is made clear by reference to the content of the present application.

[0003] SCOPE OF APPLICATION AND STATE OF THE ART

[0004] The invention relates to an optical system with at least one multiple mirror arrangement according to the preamble of claim 1, and to an operating control method for controlling the operation of an optical system comprising at least one multiple mirror arrangement. A preferred application lies in the field of EUV systems, i.e., systems that use operating wavelengths from the extreme ultraviolet (EUV) range. The optical system can, for example, be an illumination system for an EUV system.

[0005] Microlithographic projection exposure processes are predominantly used today for the production of semiconductor devices and other finely structured components. These processes employ masks (reticles) or other pattern-generating devices that carry or form the pattern of the structure to be imaged, for example, a line pattern of a layer in a semiconductor device. The pattern is illuminated by an illumination system that transforms the radiation from a primary radiation source into pattern-directed illumination radiation, characterized by specific illumination parameters, which strikes the pattern within an illumination field of defined shape and size. The radiation, modified by the pattern, passes through a projection lens, which projects the pattern onto the substrate to be exposed, which is coated with a radiation-sensitive layer.

[0006] Many current microlithographic projection exposure methods and systems utilize radiation from the relatively short-wavelength part of the ultraviolet (UV) range, particularly radiation with wavelengths below 260 nm. These include systems with operating wavelengths from the deep ultraviolet (DU) range, such as approximately 248 nm or 193 nm, which can be used to generate many medium-critical structures. The wavelength range below approximately 200 nm down to the EUV range is sometimes also referred to as the vacuum ultraviolet (VUV) range. To enable the production of increasingly finer structures, optical systems have been developed in recent years that achieve high resolution primarily through the short wavelength of the electromagnetic radiation used, specifically from the extreme ultraviolet (EUV) range, particularly with operating wavelengths between 5 nm and 30 nm.EUV radiation cannot be focused or guided using refractive optical elements because the short wavelengths are absorbed by known optical materials that are transparent at longer wavelengths. Therefore, mirror systems are used in EUV systems, for example, in EUV lithography.

[0007] Typically, different illumination modes (so-called illumination settings) are used depending on the type of structures to be imaged. These modes are characterized by different local intensity distributions of the illumination radiation in a pupil plane of the illumination system. To allow for flexible adjustment of these settings, an illumination system has a pupil-shaping unit for receiving radiation from the primary radiation source and generating a variably adjustable two-dimensional intensity distribution in the pupil area of ​​the illumination system.

[0008] Some concepts envision using at least one controllable multi-mirror array (MMA) in the pupil-forming unit. This array comprises numerous individual mirror elements supported by a common structure, which can be tilted independently to precisely alter the angular distribution of the radiation incident on the array, thus achieving the desired spatial distribution of illumination intensity in the pupil plane. The mirror surfaces are arranged to essentially fill the entire area. A gap, bounded by the mirror substrate's side surfaces, remains between immediately adjacent mirror elements to ensure collision-free relative movement. Such multi-mirror arrays are often referred to as faceted mirrors, with the reflective front surfaces of the mirror elements forming the facets.

[0009] To precisely adjust the geometric reflection properties of a controllable multi-mirror array, each mirror element typically has an actuator system coupled to it. This system allows for the controlled adjustment of the mirror element's position relative to the supporting structure. The actuator system's movements are controlled by a control unit associated with the multi-mirror array. Under the control of this unit, the orientation of the mirror element's surface can be precisely adjusted from its neutral position. Often, the goal is to progressively reduce the size of the individually adjustable mirror surfaces to achieve high spatial resolution and precise adjustment of the multi-mirror array's geometric-optical reflection properties.For manufacturing, technologies from the field of micro-electro-mechanical systems (MEMS for short) are often used to create drive elements, sensor elements and mechanical elements as MEMS structures in the area between the mirror surface and the support structure.

[0010] The operation of an EUV system using high-energy EUV radiation can lead to degradation phenomena inherent in the EUV system. WO 2017 / 072195A1 (corresponding to DE 10 2015 221 209 A1) describes how, for example, contaminants are generated in an EUV light source where tin droplets are used to generate the EUV radiation. These droplets are converted into a plasma state by a laser beam, causing the tin droplets to partially evaporate and form tin particles. These particles spread within the EUV lithography system and adhere either directly or as a tin layer to the optical surface of optical elements, such as those in the illumination or projection systems, as well as to mechanical or mechatronic components of the EUV lithography system.Tin contamination can also occur through outgassing effects on tin-contaminated components in the EUV lithography system, caused by hydrogen or hydrogen plasma present in the system. To protect the individual mirrors of an MMA produced using MEMS techniques, a protective element is described. This element comprises a membrane supported by a lattice-like frame and formed by multiple membrane segments, each protecting a specific area of ​​the MMA surface, such as a group of adjacent mirror elements, from the contaminated substances. The frame-supported membrane is positioned at a distance from the mirror surfaces.

[0011] DE 10 2016 206 202 A1 describes an optical assembly with a multiple mirror arrangement. To prevent contamination of the mirror elements with tin particles and other contaminants originating from the EUV radiation source, the optical assembly includes a purge device for generating a purge gas stream. This stream passes through a gap between the mirror elements of at least two adjacent individual mirrors, with the purge gas stream directed from a second side of the mirror elements facing the support structure towards the radiation entrance side of the mirror elements. PROBLEM AND SOLUTION

[0012] Against this background, the invention aims to provide an optical system comprising at least a multiple mirror arrangement that remains functional for extended periods when used under operating conditions susceptible to contamination. A further objective is to provide an operational control method for controlling the operation of such an optical system.

[0013] To solve this problem, the invention provides an optical system with the features of claim 1 and an operating control method with the features of claim 7. Advantageous embodiments are specified in the dependent claims. The wording of all claims is made clear by reference to the content of the description.

[0014] According to one formulation, the invention provides an optical system with at least one multiple mirror arrangement. The optical system can, in particular, be an illumination system for an EUV system.

[0015] A multiple-mirror arrangement of the type considered here comprises a support structure and a multitude of mirror units arranged side by side on the support structure in a grid pattern. Each mirror unit has a base element and a mirror element that is individually movable relative to the base element. Each mirror element has a mirror substrate which, on a front surface facing away from the base element, bears a reflective coating to form a mirror surface that reflects the radiation used effectively. Each mirror substrate also has a rear surface facing the base element and side surfaces around its perimeter.The arrangement of the mirror elements is designed in such a way that a relative movement of the mirror elements to each other takes place in the entire specified range of motion of the mirror elements without mutual collision, so that mutual contact of the mirror elements is structurally excluded.

[0016] The mirror surfaces of the multiple mirror arrangement are essentially arranged side by side, filling the entire area. They thus form a total reflective surface that is significantly larger than the individual mirror surfaces. However, the total surface is not completely reflective. Rather, there is a gap between immediately adjacent mirror elements, bounded by the side surfaces of the adjacent mirror substrates. This gap is functionally necessary to allow collision-free relative movement of the adjacent mirror elements against each other. In each mirror unit, functional components of the unit are arranged in a space between the base element and the mirror element.For example, each mirror unit can contain components of a suspension system for the movable mounting of the mirror element on the base element, as well as components of an actuator system for generating movements of the mirror element relative to the base element in response to control signals, within the space between the base element and the mirror substrate. Sensor elements, such as position sensors that detect the orientation of the mirror surfaces relative to the base element, can also be arranged in this space. Control electronics components can be located on the upper surface of the base element facing the mirror element.

[0017] The suspension system mechanically connects the mirror substrate to the base element. The suspension system can, for example, include spring-elastic or flexible sections or components that provide the necessary mobility. The suspension system can also contain joints that connect components or sections of it, for example, in the form of solid-state joints. The actuator system can be a separate system, independent of the suspension system, that provides forces and torques for moving the mirror elements. It is also possible for the suspension system and the actuator system to be integrated, with sections of the suspension system also functioning as parts of the actuator system. Sensor components can also be integrated.

[0018] In this design, all actuators, sensors, and other mechanical elements can be arranged below the mirror surface, i.e., between it and the supporting structure. This allows the proportion of the reflective surface area of ​​the individual mirrors to the total surface area of ​​the multiple mirror arrangement to be very large.

[0019] To achieve higher integration density, technologies from the field of microelectromechanical systems (MEMS) manufacturing are increasingly being used in the production process, for example, to create drive elements (elements of the actuator system), mechanical elements (such as elements of the suspension system), and / or sensor elements or the like. Such MEMS processes are currently based primarily on structuring processes in which, for example, a starting substrate made of silicon or a silicon compound is structured, thereby shaping the required components. As mentioned earlier, operating such an optical system in an EUV facility carries the risk of particulate contamination, i.e., contamination with contaminant particles that may originate, for example, from the light source. Such contamination can impair the function of the multiple mirror array in several ways.Contamination particles, such as tin particles from an EUV radiation source or particles from mechanical parts of the optical system, can enter the gap between the mirror elements and, for example, block mirror movement and / or impair the positioning of the mirrors during operation. This occurs when the contamination particles pass through the gap into the space between the mirrors and interfere with the interaction between actuators and sensors. Contamination particles that land on and adhere to a mirror surface cause an immediate loss of transmission and, depending on the particle material, can accelerate functional degradation, for example, through particle-plasma interaction.

[0020] With regard to these problems, there were already approaches in the prior art to prevent such contamination, for example by using membranes (see DE 10 2015 221 209 A1) or by generating a purge gas flow penetrating the gap to keep the gap clean (see, for example, DE 102016206202 A1).

[0021] The inventors recognized that while existing solutions can certainly reduce the problem of contamination, they can simultaneously lead to functional impairments. For example, solutions using membranes or pellicles can hinder the relative movement between adjacent mirror elements and cause parasitic forces within the mirror assembly.

[0022] The present invention pursues a different approach to reducing problems caused by contamination particles, avoiding the disadvantages of conventional contamination protection systems. According to one formulation of the invention, the optical system is characterized by a vibration cleaning system for cleaning at least a selected area of ​​the multiple mirror arrangement of contamination particles adhering to a mirror element and / or trapped in a gap. For this purpose, the multiple mirror arrangement has at least one electromechanical vibration generator, which can be controlled via signals from a control unit and is configured to generate cleaning-effective vibrations of at least one mirror element during a vibration cleaning operation in response to signals from the control unit.An electromechanical vibration generator is an actuator that can be controlled by the control unit via electrical signals and, in response to these signals, can generate vibrations in the parts of a mirror unit coupled to the actuator. However, not every vibration is suitable for producing the desired cleaning effect. The vibration cleaning system is specifically designed to set at least one of the mirror elements into vibrations effective for cleaning. This means that the vibrations are optimized for the desired cleaning effect in terms of their frequency and amplitude, so that, for example, contaminant particles that are not firmly adhered to the mirror surface can be detached from the mirror surface by the generated vibrations.Effective cleaning vibrations also occur when, for example, a contamination particle is trapped in a gap between immediately adjacent mirror elements and it is possible to loosen the contamination particle by vibrating at least one of the adjacent mirror elements, so that the blockage between the mirror elements is resolved.

[0023] According to another aspect of the invention, an operating control method for controlling the operation of an optical system with at least one multiple mirror arrangement of the type mentioned is provided. The operating control method is characterized by operation in a cleaning mode, wherein, in response to control signals from a control unit of an electromechanical vibration cleaning system, a preferably electromechanical vibration cleaning operation is carried out over a predefinable cleaning period by at least partially freeing at least a selected area of ​​a multiple mirror arrangement from contamination particles adhering to the mirror element and / or trapped in a gap by generating cleaning-effective vibrations of at least one mirror element.

[0024] A multiple mirror arrangement can include at least one actively controllable electromechanical vibration generator of the electromechanical vibration system. During operation, it can be provided that, in cleaning mode, this vibration generator is controlled to produce vibrations in at least one of the mirror elements.

[0025] The vibration generator can, for example, comprise one or more piezoelectric vibration elements. A piezoelectric vibration generator has at least one piezoelectric material that vibrates or oscillates in response to an electrical signal. The piezoelectric material is positioned between a first and a second electrode structure. Depending on the design, the transverse effect, the longitudinal effect, or the shear effect can be utilized.

[0026] An electromechanical vibration generator can, for example, also operate according to an electromagnetic principle.

[0027] In many multi-mirror designs, a mirror element, together with its associated movable components of the suspension system and any other coupled components, forms a vibrating unit characterized by a natural or resonant frequency. The vibration cleaning system is preferably configured to induce a cleaning-effective oscillation in this vibrating unit, with a frequency significantly outside its natural frequency. This prevents the risk of mirror element movement amplifying and potentially leading to destruction. Simultaneously, the vibration frequency can be set so close to the natural frequency that sufficiently strong vibrations can be generated with moderate energy input.

[0028] According to further training, the vibration cleaning system is configured to set this vibrating unit into a cleaning-effective vibration that has a vibration frequency in the range of 50% to 90% of the resonance frequency or in the range of 105% to 170% of the resonance frequency.

[0029] The resonant frequency for a lowest mode in many typical arrangements can be in the range of 100 Hz to 1000 Hz, especially in the range of 250 - 800 Hz.

[0030] The vibration frequencies during a vibration cleaning operation can therefore, for example, be in the range of approximately 50 Hz to 1500 Hz at least in phases, and possibly also below or above this range.

[0031] In particular, the vibration frequency can be tuned to the vibration characteristics of the vibrating unit in such a way that a vibration frequency is generated which lies within the range of a maximum of the frequency-dependent stiffness of the vibrating unit. This utilizes the fact that vibrating units cannot be set into sustained vibration at every frequency, so that at suitable excitation frequencies, the vibration generator can be coupled more or less rigidly to the mirror element to be set into vibration. There are various ways to integrate at least one actively controllable electromechanical vibration generator of the vibration cleansing system into a multi-mirror arrangement.According to a further development, at least one of the mirror units has a vibration generator separate from the actuator system of the mirror unit, which is coupled to the mirror element of the mirror unit in a vibration-transmitting manner and can be controlled to generate vibrations in cleaning operating mode in response to control signals from the control unit of the vibration cleaning system independently of the actuator system.

[0032] Alternatively or additionally, an existing actuator of the mirror unit's actuator system can be used as the vibration generator for the vibration cleaning system. With appropriate design, this actuator can be controlled by the vibration generation system's control unit to induce vibrations in the coupled mirror element that are effective for cleaning. While in "normal" operation this actuator is used to quickly and precisely tilt and hold the coupled mirror element in a desired position, its function during the vibration cleaning operation is to vibrate the coupled mirror element in such a way that any adhering contaminant particles are loosened and, if necessary, removed.

[0033] In some embodiments, at least one electromechanical vibration generator is arranged between the support structure and a base element of a mirror unit. In this case, with the support structure fixed, the entire mirror unit it carries, including its base element and the components mounted on it, can be set into vibration. Alternatively or additionally, it is also possible for at least one electromechanical vibration generator to be arranged between the base unit and a mirror element of a mirror unit. Such a vibration generator can, for example, be integrated into a component of the suspension system or it can be a functional part of the actuator system. In the latter case, at least one actuator of the actuator system would be designed or used as a vibration generator.

[0034] A key objective of the new development is to integrate the vibration cleaning operation into the normal operation of the optical system in such a way as to minimize disruption. In this context, it was recognized, among other things, that it is generally not necessary to vibrate all mirror elements during a vibration cleaning operation. Rather, locally targeted cleaning is usually sufficient. Furthermore, it was observed that the extent of contamination during operation can vary not only spatially but also temporally in a way that is difficult to predict. One way to keep the optical system sufficiently clean despite these unpredictable circumstances could be to interrupt normal operation at predetermined, e.g., regular, intervals for a vibration cleaning operation of all mirror elements.According to further training, a vibration cleaning operation is only initiated when and where there is actually a need.

[0035] Some embodiments are characterized by spatially resolved contamination monitoring for identifying or locating mirror elements or groups of mirror elements whose function is impaired by contaminant particles. Based on signals from the spatially resolved contamination monitoring, at least one electromechanical vibration generator of the electromechanical vibration cleaning system is selectively controlled such that, in cleaning mode, only mirror elements of contaminated mirror units or groups of mirror elements are set into vibrations effective for cleaning. Based on this monitoring, cleaning can thus be limited to potentially quite small sub-areas of the multi-mirror arrangement.

[0036] For spatially resolved contamination monitoring, the vibration cleaning system can include at least one spatially resolved camera system directed at the reflective mirror units. This system can detect which mirror units and / or between which mirror units contamination-related problems occur. Spatially resolved contamination monitoring can also be performed electromechanically using at least one spatially resolved diagnostic system. For example, for each mirror unit, a functional relationship can be recorded between a change in the position of a mirror element (e.g., tilting) and the force or electrical power required to generate the change in position. This data can then be evaluated to identify malfunctioning mirror elements or groups of mirror elements.This approach is based, among other things, on the consideration that if adjacent mirror elements are blocked by a trapped contaminant particle, it may be impossible, or only possible with significantly increased force, to tilt such a blocked mirror element. This inhibition of movement can be used as an indicator of the presence of a blocking contamination. Another possibility is optical diagnosis, in which, for example, pupil measurement is observed during the activation of the multi-mirror arrangement, such as when changing the lighting settings. If local deviations from the desired pupil illumination occur, it can be determined by calculation which mirror unit or units failed to assume the desired tilt position and are responsible for the faulty illumination. These could then be cleaned of contaminants using vibration cleaning.

[0037] According to a training course, the control unit is configured such that, based on spatially resolved contamination monitoring (or spatially resolved diagnostics), a vibration cleaning operation is initiated promptly within a predefined timeframe after the detection of contamination considered critical to function. In other words, it is proposed here to continuously or at least frequently monitor and detect particle contamination of mirrors during operation and to initiate or perform a vibration cleaning operation as soon as a potentially functionally impairing fresh contamination is detected.Timely cleaning can significantly improve the service life of the optical system, as many particulate materials are known to permanently damage the reflective coating of the mirror units after a few days of exposure to EUV radiation and / or plasma. They can also bond so strongly to the mirror surfaces that vibration cleaning is no longer possible. The vibration cleaning system can be configured, for example, to initiate a cleaning operation within less than 10 hours, and in particular within less than half an hour, after the detection of contamination defined as functionally critical.

[0038] By generating relatively high-frequency vibrations with a suitable frequency and amplitude, many initially adhering contaminant particles can be loosened or completely detached. Further developments of the invention offer several possibilities to enhance this cleaning effect and thereby improve cleaning efficiency.

[0039] For example, in cleaning mode, a gap occupied by contaminant particles can be opened or widened by relative movement of the mirror elements forming the gap, either sequentially with or at least partially overlapping with an electromechanical vibration cleaning process. This allows for better removal of dissolved contaminant particles. For instance, the mirror elements forming the gap can be tilted in opposite directions and arranged in a roof-shaped or V-shaped configuration, which facilitates the release of contaminant particles.

[0040] Another measure to improve cleaning efficiency is to generate a vacuum or plasma effective in the area of ​​the affected mirror elements before and / or during a vibration cleaning operation, thereby reducing the adhesive forces acting between the contaminant particles and the mirror element. The remaining adhesion can then be eliminated by the vibration-induced cleaning operation.

[0041] Further options for supporting vibration cleaning can be provided to improve the removal of detached contaminant particles from a mirror element during and / or after a vibration cleaning operation. According to a further development, a rinsing fluid flow is generated at the micromirror assembly, flowing from the intermediate area through the gaps towards the mirror surfaces. This ensures that detached contaminant particles do not enter the intermediate area but are instead transported to non-critical areas.

[0042] Alternatively or additionally, it is also possible to tilt contaminated mirror units and / or the multiple mirror arrangement and / or a component containing the multiple mirror arrangement in such a way that detached contaminant particles are moved downwards by gravity, similar to an inclined plane. This can, for example, cause a detached contaminant particle to roll off the mirror surface towards its edge and then potentially be carried away by a purge gas stream.

[0043] Alternatively or additionally, it is also possible to use electrostatic forces to improve removal. This can be achieved by generating an inhomogeneous electric field in the area of ​​contaminated mirror units, such that the removal process is supported by electrostatic forces.

[0044] Alternatively or additionally, a pretreatment can be performed prior to vibration cleaning. This pretreatment aims to reduce the cohesion between mirror elements and adhering contaminant particles from a starting point, thus enabling the subsequent vibration cleaning operation to be more efficient. For example, the area of ​​the optical system containing the multiple mirror array can be evacuated sufficiently long before the planned vibration cleaning operation begins. This can loosen bonds between mirror elements and contaminant particles and / or prevent existing bonds from strengthening. The vacuum treatment thus reduces the adhesion of contaminant particles.It is also possible to achieve a reduction in adhesion by exposing the multi-mirror assembly to be cleaned to the action of a plasma prior to the start of the vibration cleaning operation, which acts in such a way that the adhesive forces between contamination particles and the mirror unit are reduced.

[0045] To detect contamination that could compromise functionality, a detector sensitive to contamination particles can be installed in the radiation path between an EUV light source and the multiple mirror array. This detector can promptly detect, for example, a sudden increase in contamination particles, such as tin-containing particles, caused by a sudden malfunction of the EUV radiation source, allowing for immediate initiation of a vibration cleaning operation. This largely prevents contamination particles from having sufficient time to form strong bonds with the substrate. Such a contamination particle detector could be positioned, for instance, at an intermediate focus between the collector of the EUV radiation source and the downstream mirror of the illumination system.

[0046] The occurrence of contamination can also be detected by directly observing the multiple mirror array from the reflective side using optical methods. Based on this, the reflectivity of all mirror surfaces can be reconstructed using known algorithms. The contamination particles then become identifiable as dark spots in an otherwise brighter environment due to reflection. It is also possible to measure the contamination by recording the influence of adhering contamination particles on the absorption of EUV radiation and the resulting local temperature increase. This indirect method can also be used to locate areas with particularly high levels of contamination.

[0047] According to further training, additional measures are taken to fully or at least to such an extent that the functionality of mirror units is maintained even in the event of contamination particles, so that the optical system can continue to operate within its specifications. For this purpose, a multi-mirror arrangement incorporates protective shielding elements located on the back side of the mirror substrates, away from the mirror surfaces. These shielding elements protect components of the suspension and actuator systems located in the space between the mirrors, and potentially also components of a sensor system located there, from contamination particles that could penetrate the gap and reach the space between the mirrors.

[0048] BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Further advantages and aspects of the invention will become apparent from the claims and from the description of exemplary embodiments of the invention, which are explained below with reference to the figures.

[0050] Fig. 1 schematically shows optical components of an EUV microlithography projection exposure system with an illumination system that includes one or more EUV multiple mirror arrangements according to an exemplary embodiment;

[0051] Fig. 2 shows a schematic section of an EUV multiple mirror arrangement with integrated piezoelectric vibration generators;

[0052] Fig. 3 shows examples of shielding elements that can shield the sensor-actuator volume behind the mirror units.

[0053] DETAILED DESCRIPTION OF THE EXECUTION EXAMPLES

[0054] The following are examples of multiple mirror configurations, illustrated by their applications in EUV systems in the field of microlithography. A multiple mirror configuration with reflective layers that reflect EUV radiation is referred to here as an EUV multiple mirror configuration.

[0055] The following section describes, with reference to Fig. 1, the essential components of a projection exposure system 1 for microlithography. This description of the basic structure of the projection exposure system 1 and its components is not intended to be restrictive.

[0056] A lighting system 2 of the projection exposure system 1 has, in addition to a radiation source 3, a lighting optic 4 for illuminating an object field 5 in an object plane 6. A reticle 7 arranged in the object field 5 is exposed. The reticle 7 is held by a reticle holder 8. The reticle holder 8 can be moved, particularly in a scanning direction, by means of a reticle displacement drive 9.

[0057] Figure 1 shows a Cartesian xyz coordinate system for illustrative purposes. The x-direction runs perpendicular to the plane of the drawing. The y-direction runs horizontally, and the z-direction runs vertically. In Figure 1, the scan direction runs along the y-direction. The z-direction runs perpendicular to the object plane 6.

[0058] The projection exposure system 1 comprises a projection optic 10. The projection optic 10 serves to image the object field 5 onto an image field 11 in an image plane 12. The image plane 12 is parallel to the object plane 6. Alternatively, an angle other than 0° between the object plane 6 and the image plane 12 is also possible. A structure on the reticulum 7 is imaged onto a photosensitive layer of a wafer 13 located in the image field 11 within the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 can be moved, particularly along the y-direction, via a wafer transfer drive 15. The movement of the reticulum 7 via the reticulum transfer drive 9 and of the wafer 13 via the wafer transfer drive 15 can be synchronized.

[0059] Radiation source 3 is an EUV radiation source. Specifically, radiation source 3 emits EUV radiation 16, which is also referred to below as useful radiation or illumination radiation. The useful radiation has a wavelength in the range between 5 nm and 30 nm. Radiation source 3 can be a plasma source, for example, an LPP (Laser Produced Plasma) or a DPP (Gas Discharged Produced Plasma). It can also be a synchrotron-based radiation source. Radiation source 3 can be a free-electron laser (FEL).

[0060] The illumination radiation 16 emanating from the radiation source 3 is focused by a collector 17. The collector 17 can be a collector with one or more ellipsoidal and / or hyperboloid reflective surfaces. The at least one reflective surface of the collector 11 can be illuminated by the illumination radiation 16 at grazing incidence (Gl), i.e., with angles of incidence greater than 45°, or at normal incidence (NI), i.e., with angles of incidence less than 45°. The collector 11 can be structured and / or coated to optimize its reflectivity for the useful radiation and to suppress stray light. After the collector 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18.The intermediate focus plane 18 can represent a separation between a radiation source module, comprising the radiation source 3 and the collector 17, and the illumination optics 4.

[0061] The illumination optics 4 comprise a deflecting mirror 19 and, downstream in the beam path, a first faceted mirror 20. The deflecting mirror 19 can be a planar deflecting mirror or, alternatively, a mirror with an effect that influences the beam shape beyond the mere deflection effect. Alternatively or additionally, the mirror 19 can be designed as a spectral filter that separates a useful wavelength of the illumination radiation 16 from stray light of a different wavelength. If the first faceted mirror 20 is arranged in a plane of the illumination optics 4 that is optically conjugate to the object plane 6 as the field plane, it is also referred to as a field faceted mirror. The first faceted mirror 20 comprises a plurality of individual first facets 21, which are hereinafter also referred to as field facets. Only a few of these facets 21 are shown in Fig. 1 as examples.

[0062] The first facets 21 can be designed as macroscopic facets, in particular as rectangular facets or as facets with an arcuate or semicircular border contour. The first facets 21 can be designed as planar facets or alternatively as convexly or concavely curved facets.

[0063] As is known, for example, from DE 10 2008 009 600 A1, the first facets 21 can themselves each be composed of a plurality of individual mirrors, in particular a plurality of micromirrors. The first facet mirror 20 can in particular be designed as a microelectromechanical system (MEMS system). For details, reference is made to DE 10 2008 009 600 A1.

[0064] Between the collector 17 and the deflecting mirror 19, the illumination radiation 16 runs horizontally, i.e. along the y-direction.

[0065] In the beam path of the illumination optics 4, a second faceted mirror 22 is arranged downstream of the first faceted mirror 20. If the second faceted mirror 22 is arranged in a pupil plane of the illumination optics 4, it is also referred to as a pupil faceted mirror. The second faceted mirror 22 can also be arranged at a distance from a pupil plane of the illumination optics 4. In this case, the combination of the first faceted mirror 20 and the second faceted mirror 22 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.

[0066] The second faceted mirror 22 comprises a plurality of second facets 23. In the case of a pupil faceted mirror, the second facets 23 are also referred to as pupil facets. The second facets 23 can also be macroscopic facets, which may, for example, have round, rectangular, or hexagonal edges, or alternatively, facets composed of micromirrors. In this regard, reference is also made to DE 10 2008 009 600 A1. The second facets 23 can have planar or, alternatively, convexly or concavely curved reflective surfaces.

[0067] The illumination optics 4 thus form a double-faceted system. This basic principle is also known as a honeycomb condenser (Fly's Eye Integrator).

[0068] It may be advantageous not to arrange the second faceted mirror 22 exactly in a plane which is optically conjugate to a pupil plane of the projection optics 7.

[0069] With the aid of the second faceted mirror 22, the individual first facets 21 are imaged into the object field 5. The second faceted mirror 22 is the last beam-shaping, or indeed the last, mirror for the illumination radiation 16 in the beam path before the object field 5.

[0070] In another embodiment of the illumination optics 4, not shown, a transmission optic can be arranged in the beam path between the second facet mirror 22 and the object field 5, which contributes in particular to imaging the first facets 21 into the object field 5. The transmission optic can have exactly one mirror, or alternatively two or more mirrors, which are arranged one behind the other in the beam path of the illumination optics 4. The transmission optic can in particular comprise one or two mirrors for normal incidence (Nl mirrors, normal incidence mirrors) and / or one or two mirrors for grazing incidence (Gl mirrors, grazing incidence mirrors).

[0071] In the embodiment shown in Fig. 1, the illumination optics 4 has exactly three mirrors after the collector 17, namely the deflecting mirror 19, the field facet mirror 20, and the pupil facet mirror 22. In another embodiment of the illumination optics 4, the deflecting mirror 19 can also be omitted, so that the illumination optics 4 can then have exactly two mirrors after the collector 17, namely the first facet mirror 20 and the second facet mirror 22.

[0072] The mapping of the first facets 21 by means of the second facets 23 or with the second facets 23 and a transmission optic into the object plane 6 is regularly only an approximate mapping.

[0073] The projection optics 10 comprise a plurality of mirrors Mi, which are numbered according to their arrangement in the beam path of the projection exposure system 1.

[0074] In the example shown in Fig. 1, the projection optics 10 comprises six mirrors M1 to M6. Alternatives with four, eight, ten, twelve, or any other number of mirrors Mi are also possible. The penultimate mirror M5 and the last mirror M6 each have a passage for the illumination radiation 16. The projection optics 10 is a double-obscured optic. The projection optics 10 has an image-side numerical aperture greater than 0.5, and which can also be greater than 0.6, for example, 0.7 or 0.75. The reflective surfaces of the mirrors Mi can be designed as freeform surfaces without an axis of rotational symmetry. Alternatively, the reflective surfaces of the mirrors Mi can be designed as aspherical surfaces with exactly one axis of rotational symmetry of the reflective surface shape.The mirrors Mi, like the mirrors of the lighting optics 4, can have highly reflective coatings for the lighting radiation 16. These coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

[0075] The projection optics 10 has a large object-image offset in the y-direction between a y-coordinate of a center of the object field 5 and a y-coordinate of the center of the image field 11. This object-image offset in the y-direction can be approximately as large as a z-distance between the object plane 6 and the image plane 12.

[0076] The projection optics 10 can be designed to be anamorphic. In particular, they exhibit different image scales β. x , ß y in the x and y directions. The two image scales ß x , ß y the projection optics 7 are preferably located at (ß x , ß y) = (+ / - 0.25, / +- 0.125). A positive image scale β indicates a projection without image inversion. A negative sign for the image scale β indicates a projection with image inversion. Projection optic 7 thus results in a reduction ratio of 4:1 in the x-direction, i.e., in the direction perpendicular to the scan direction. Projection optic 10 results in a reduction ratio of 8:1 in the y-direction, i.e., in the scan direction. Other image scales are also possible. Image scales with the same sign and absolute values ​​in the x- and y-directions, for example with absolute values ​​of 0.125 or 0.25, are also possible.

[0077] The number of intermediate image planes in the x- and y-directions in the beam path between the object field 5 and the image field 11 can be the same or, depending on the design of the projection optics 10, different. Examples of projection optics with different numbers of such intermediate images in the x- and y-directions are known from US 2018 / 0074303 A1.

[0078] Each pupil facet 23 is assigned to exactly one of the field facets 21 to form an illumination channel for illuminating the object field 5. This can result, in particular, in illumination according to Köhler's principle. The far field is divided into a multitude of object fields 5 by means of the field facets 21. The field facets 21 generate a plurality of images of the intermediate focus on the pupil facets 23 assigned to each of them.

[0079] The field facets 21 are each superimposed on the reticulum 7 by an associated pupil facet 23 to illuminate the object field 5. The illumination of the object field 5 is particularly homogeneous. It preferably exhibits a uniformity error of less than 2%. Field uniformity can be achieved by superimposing different illumination channels.

[0080] The illumination of the entrance pupil of the projection optics 10 can be geometrically defined by the arrangement of the pupil facets. By selecting the illumination channels, in particular the subset of pupil facets that guide light, the intensity distribution in the entrance pupil of the projection optics 10 can be adjusted. This intensity distribution is also referred to as the illumination setting.

[0081] A similarly preferred pupil uniformity in the area of ​​defined illuminated sections of an illumination pupil of the illumination optics 4 can be achieved by redistributing the illumination channels. Further aspects and details of the illumination of the object field 5, and in particular of the entrance pupil of the projection optics 10, are described below. The projection optics 10 can, in particular, have a homocentric entrance pupil. This can be accessible. It can also be inaccessible.

[0082] The entrance pupil of the projection optics 10 cannot always be illuminated exactly by the pupil facet mirror 22. When the projection optics 10 image the center of the pupil facet mirror 22 telecentrically onto the wafer 13, the aperture rays often do not intersect at a single point. However, a surface can be found where the pairwise determined separation of the aperture rays is minimized. This surface represents the entrance pupil or a surface conjugate to it in real space. In particular, this surface exhibits a finite curvature.

[0083] The projection optics 10 may have different entrance pupil positions for the tangential and sagittal beam paths. In this case, an imaging element, in particular an optical component of the transmission optics, should be provided between the second faceted mirror 22 and the reticle 7. This optical element can accommodate the different positions of the tangential and sagittal entrance pupils.

[0084] In the arrangement of the components of the illumination optics 4 shown in Fig. 1, the pupil facet mirror 22 is arranged in a plane conjugate to the entrance pupil of the projection optics 10. The field facet mirror 20 is arranged tilted relative to the object plane 5. The first facet mirror 20 is arranged tilted relative to an arrangement plane defined by the deflecting mirror 19.

[0085] The first faceted mirror 20 is arranged at an angle to an arrangement plane defined by the second faceted mirror 22.

[0086] The mirror modules defining the illumination beam path of illumination system 1 are surrounded by a low-pressure atmosphere containing small amounts of hydrogen (H2) and housed in an evacuable chamber inside a casing. The chamber communicates with a vacuum pump via a fluid line containing a shut-off valve. The operating pressure in the evacuable chamber is a few Pascals. The partial pressure of hydrogen (H2) can, for example, range from 2 Pa to 20 Pa. All other partial pressures are typically in the ppm range. The first faceted mirror (field faceted mirror) 20 and the second faceted mirror (pupil faceted mirror) 22 are examples of controllable EUV multi-mirror arrays (MMAs).

[0087] Figure 2 illustrates an exemplary design of such an EUV multiple mirror array (MMA). Furthermore, different variants of an electromechanical vibration suppression system are described, each comprising one or more vibration generators integrated into the MMA multiple mirror array.

[0088] A large number of mirror units MU are arranged on a dimensionally stable support structure TS in the form of a base plate, which are arranged next to each other in rows and columns on the support structure TS in a matrix-like two-dimensional grid arrangement.

[0089] Each mirror unit MU has a base element BE mounted on the support structure and a mirror element ME, which is individually movable relative to the base element BE by means of a flexible suspension system SUS. In this example, the mirror elements ME can be individually tilted relative to their respective base element BE in two rotational degrees of freedom. Each mirror element ME has a substantially plate-shaped mirror substrate SUB, which carries a reflective coating REF on its front surface facing away from the base element BE. This coating forms a mirror surface MS that reflects EUV radiation. The reflective coating has a multilayer structure with several pairs of alternating high-refractive-index and relatively low-refractive-index layer material (for example, Mo-Si), possibly with intermediate layers. The front surface, or the corresponding mirror surface MS, can be flat or slightly concave or convex.Curved surfaces can be spherically or aspherically curved.

[0090] The mirror units MU are positioned so close together that the mirror surfaces MS are essentially adjacent to one another, filling the entire area. This means, in particular, that the ratio of the sum of the mirror surfaces of the individual mirror elements ME to the total area of ​​the mirror array covered by mirror units—the so-called integration density—is relatively high, for example, greater than 0.7, 0.8, or 0.9. Complete surface coverage is not possible because a gap SP, bounded by the side surfaces SF of the adjacent mirror substrates SUB, remains between each immediately adjacent mirror element ME. This gap ensures collision-free relative movement of the neighboring mirror elements against each other.The gap widths between facing side surfaces of immediately adjacent mirror substrates can, for example, be on the order of a few tens of micrometers, for example in the range between 20 pm and 100 pm.

[0091] In each mirror unit MU, components of a spring-elastic suspension system SUS are arranged between the base element BE and the mirror element ME. This system structurally connects the mirror element ME and the base element BE, creates a movable bearing for the mirror element relative to the base element with defined degrees of freedom, and provides restoring forces that cause the mirror element to automatically return to a neutral position without tilting in the absence of actuator forces. Additionally, components of an actuator system (not shown in detail) for generating movements of the mirror element relative to the base element in response to control signals from the control device are arranged in the space between the mirror substrate and the base element. These components can be integrated into the structure of the suspension system, for example, in the form of sections that change length and / or are flexible in response to control signals.Furthermore, components not shown, such as capacitive position sensors, are present. These sensors detect the current relative position of the mirror element to the base element and send corresponding signals to the control unit. This ensures control of the individual tilt angles of the mirror substrates relative to the support structure. The tiltable mirror elements can be tilted around the zero position within a displacement range of ± 50 mrad, and in particular ± 100 mrad or more. Adjustment accuracies can be less than 0.2 mrad, and in particular less than 0.1 mrad.

[0092] High integration densities can be achieved, among other things, by manufacturing the mirror assemblies using technologies from the field of microelectromechanical systems (MEMS). In the example shown in Fig. 2, at least the base element BE, the suspension system SUS, the actuator system, and the position sensors are designed as MEMS structures. MEMS structures are currently mostly made of silicon or silicon compounds. DE 10 2015 204 874 A1 (corresponding to US 2017 / 363861 A1) describes EUV multiple mirror assemblies with MEMS structures made of silicon or a silicon compound, which can serve as examples of conventional EUV multiple mirror assemblies and are thus incorporated into the description by reference.

[0093] Figure 2 schematically illustrates some problems that can lead to contamination-related deterioration of the functionality of EUV multiple mirror arrays. Contamination particles PART1, for example zinc particles from the EUV radiation source, and / or particles from mechanical parts in the beam-guiding area, can, for example, enter the gap SP and block the relative mobility of the mirror elements or impair the function of the actuators AKT. Furthermore, particles PART2 can land on the mirror surfaces and thereby cause a transmission loss in the optical system. Additionally, due to particle-plasma interactions, other nearby mirror elements can be affected.In the exemplary embodiment, the consequences of such contamination can be largely avoided by targeted cleaning operations, in which selected mirror elements can be set into vibration in such a way that adhering particles are detached by the vibrations, thereby creating a cleaning effect.

[0094] The electromechanical vibration cleaning system provided for this purpose comprises, in this example, various electromechanical vibration generators that are integrated into the multiple mirror arrangement at suitable locations. A first

[0095] Vibration generator VI B1 in the form of a piezoelectric actuator is integrated into the

[0096] The SUS suspension system is integrated. The enlarged detail view shows a piezoelectric vibration generator with the attached electrodes EL, which are connected via electrical leads to the control unit 190 of the multiple mirror arrangement. When this actuator is excited to vibrate, the mirror element ME it supports also vibrates with strong vibration components primarily in the lateral direction (i.e., essentially parallel to the mirror surface) and with a smaller vibration component perpendicular to it. This causes the mirror element ME to oscillate relative to the base element BE, which itself does not vibrate.

[0097] In the mirror unit shown on the right, an electromechanical actuator VI B2 in the form of a piezoelectric layer is arranged between the support structure TS and the base element BE of the mirror unit. Here, too, the electrodes are connected to the control unit of the vibration generation system. When the vibration generator is excited to vibrate, the entire mirror unit MU, including its base unit BE, the suspension system SUS, and the mirror element ME, vibrates in order to detach, in this example, the particles PART2 adhering to the mirror surface MS.

[0098] The mirror units MU of the exemplary embodiment each form oscillatory units whose vibration characteristics are determined, among other things, by the design of the suspension system, the size and weight of the mirror elements, etc. The design-related natural frequencies of the mirror units can, for example, be in the range of 100 Hz to 1000 Hz for the lowest vibration mode. In order to generate the vibrations with as little energy input as possible and, on the other hand, to avoid a resonance catastrophe and thus the risk of damage to the mirror units, the mirror units are set into vibrations during a vibration cleaning operation by controlling the actuators. These vibrations are at a distance from the natural frequency, but still close enough to be generated with high electrical efficiency.For example, excitation frequencies can be used that are 10% to 20% below the respective resonance frequency of the mirror units.

[0099] The vibration cleaning system is configured so that vibration cleaning operations are selectively performed only on those areas of the multi-mirror array (MMA) where contamination occurs, i.e., locally limited to individual mirror units or a group of adjacent mirror units. Furthermore, the vibration cleaning system is configured to generate vibrations promptly after detecting an operating condition that causes or could cause contamination.

[0100] The vibration cleaning system of the exemplary embodiment comprises several detectors that enable optimal temporal and spatial control of the vibration cleaning process. A first detector, DET1, monitors the beam path of the EUV radiation in the region of the intermediate focus plane 18 and is designed to generate detection signals when contaminant particles, such as tin droplets, migrate from the light source 3 through the intermediate focus area into the subsequent beam path. Using the first detector, DET1, it is thus possible to determine when significant contamination is likely to occur. Vibration cleaning operations are then performed promptly thereafter to prevent contaminant particles from adhering to mirror units or other components.

[0101] Furthermore, each multiple mirror array is assigned a second detector, DET2, which enables spatially resolved contamination monitoring of the assigned multiple mirror array. This allows the detector to detect contamination in a locally limited area of ​​the multiple mirror array. The vibration cleaning operation can then be limited to the contaminated area by selectively vibrating only the mirror elements located in that area. In this example, the first and second detectors, DET1 and DET2, are both optical detectors.There are also embodiments of spatially resolved contamination monitoring that operate differently, for example, with electronic diagnostics of the functional relationship between the forces applied to a mirror unit to generate a tilt and the tilt actually produced. If, for instance, it is determined that the expected degree of tilt is not achieved at a mirror element, this can be interpreted as an indicator of a blockage caused by contaminant particles that have penetrated the gap, and the corresponding mirror units are vibrated to clean them. The removal of blocking contaminant particles that have entered the gap can be supported by deliberately tilting the adjacent mirror elements away from each other, thus opening the gap before the vibration cleaning begins.

[0102] The efficiency of the cleaning process can be enhanced by additional measures if required. For example, the adhesive forces between contaminant particles and the mirror element can be reduced by generating an effective vacuum or plasma in the area of ​​the affected mirror element before and / or during a vibration cleaning operation. In this case, weaker vibrations are sufficient for reliable cleaning. A similar cleaning improvement can be achieved by generating an inhomogeneous electric field in the area of ​​the affected mirror unit.

[0103] In some embodiments, the possibilities for generating vibrations for cleaning purposes are combined with targeted shielding of those components behind the mirror elements whose function could be particularly affected by contamination. In particular, suitable shielding can minimize the risk of contaminant particles penetrating the area of ​​the suspension system (SUS) and the actuators located therein, both during normal operation of the optical system and during the vibration cleaning operation. Schematic Fig. 3 shows some examples of shielding elements (SLD) that can shield the sensor-actuator volume behind the mirror units. The configuration in sub-Fig. 3A offers very good shielding of the sensitive area but can limit the available tilt range of the mirror element. The variants in sub-Fig.3B also offers excellent blocking of the sensitive areas, but compared to the examples in Part A, the inclined positioning of the shielding elements allows for larger tilting ranges. Part C illustrates this in direct comparison. These configurations benefit from the fact that the blocking occurs near the tilting point of the mirror units, thus achieving an optimal compromise between shielding efficiency and the size of the tilting range.

Claims

Patent claims 1. Optical system, in particular lighting system for an EUV system, comprising: at least one multiple mirror arrangement (20, 22, MMA) having a support structure (TS) and a plurality of mirror units (MU) arranged side by side on the support structure in a grid arrangement, wherein: each mirror unit (MU) has a base element (BE) and a mirror element (ME) mounted independently of the base element (BE) and having a mirror surface (MS) on a side facing away from the base element (BE), wherein the mirror surfaces (MS) of the mirror units (MU) are arranged side by side substantially filling the area and a gap (SP) remains between immediately adjacent mirror elements to ensure collision-free relative movement of the adjacent mirror elements.and in each mirror unit (MU) in a space (ZR) between the base element (BE) and the mirror element (ME) components of a suspension system (SUS) for movable mounting of the mirror element (ME) on the base element (BE) as well as components of an actuator system (AKT) for generating movements of the mirror element (ME) relative to the base element (BE) in response to the reception of control signals from a control unit are arranged, characterized by a vibration cleaning system for cleaning at least a selected area of ​​the multiple mirror arrangement (MMA) of contamination particles (PART1, PART2) adhering to a mirror element and / or trapped in a gap, wherein the multiple mirror arrangement comprises at least one electromechanical vibration generator (VI B1 , VI B2) controllable via signals from a control unit, which is configured,to generate cleaning-effective vibrations of at least one mirror element during a vibration cleaning operation in response to signals from the control unit.

2. Optical system according to claim 1, characterized in that the electromechanical vibration generator comprises one or more piezoelectric vibration generators, wherein a piezoelectric vibration generator comprises at least one piezoelectric material which vibrates in accordance with an electrical signal and which is arranged between a first and a second electrode structure.

3. Optical system according to claim 1 or 2, characterized in that a mirror element (ME) together with movable components of a suspension system (SUS) associated with the mirror element (ME) forms a vibrating unit with a resonant frequency and that the vibration cleaning system is configured to set the vibrating unit into a cleaning-effective vibration with a vibration frequency that lies outside the resonant frequency, wherein preferably the resonant frequency for a lowest mode is in the range of 100 Hz to 1000 Hz, in particular in the range of 250 to 800 Hz and / or that the vibration cleaning system is configured to set the vibrating unit into a cleaning-effective vibration having a vibration frequency that lies in the range of 50% to 90% of the resonant frequency or in the range of 105% to 170% of the resonant frequency.

4. Optical system according to one of the preceding claims, characterized in that the vibration generator is mechanically coupled to a mirror element of a mirror unit, wherein at least one of the following conditions applies: at least one electromechanical vibration generator (actuator) is arranged between the support structure (TS) and a base element (BE) of a mirror unit; at least one electromechanical vibration generator is arranged between the base unit and a mirror element of a mirror unit; At least one actuator is integrated into the suspension system of the mirror unit; At least one actuator of the actuator system is designed as a vibration generator.

5. Optical system according to one of the preceding claims, characterized by a detector sensitive to contamination particles, which is arranged and configured on the radiation path between an EUV light source and the multiple mirror arrangement to detect a sudden increase in contamination particles caused by a malfunction in a timely manner and to cause the control unit to initiate a vibration cleaning operation immediately thereafter.

6. Optical system according to one of the preceding claims, characterized by a spatially resolved contamination monitoring system for identifying and / or localizing mirror elements or groups of mirror elements whose function is restricted by contamination particles, wherein, based on signals from a spatially resolved contamination monitoring system, electromechanical vibration generators of the electromechanical vibration cleaning system can be selectively controlled such that, in cleaning mode, only mirror elements affected by contamination are Mirror units are set into cleaning-effective vibrations, wherein preferably the spatially resolving contamination monitoring system comprises at least one spatially resolving camera system.

7. Operational control method for controlling the operation of an optical system comprising at least one multiple mirror arrangement (20, 22, MMA) which has a support structure (TS) and a plurality of mirror units (MU) arranged side by side on the support structure in a two-dimensional arrangement, wherein: each mirror unit (MU) has a base element (BE) and a mirror element (ME) mounted independently of the base element (BE) and having a mirror surface (MS) on a side facing away from the base element (BE), wherein the mirror surfaces (MS) of the mirror units (MU) are arranged side by side substantially filling the area and a gap (SP) remains between immediately adjacent mirror elements to ensure collision-free relative movement of the adjacent mirror elements.and in each mirror unit (MU) in a space (ZR) between the base element (BE) and the mirror element (ME) components of a suspension system (SUS) for movable mounting of the mirror element (ME) on the base element (BE) as well as components of an actuator system (AKT) for generating movements of the mirror element (ME) relative to the base element (BE) in response to the reception of control signals from a control unit are arranged, characterized by operation in a cleaning mode, wherein, in response to control signals from a control unit, a vibration cleaning operation is carried out over a predefinable cleaning period by at least partially freeing at least a selected area of ​​the multiple mirror arrangement from contamination particles adhering to the mirror element and / or trapped in a gap by generating cleaning-effective vibrations of at least one mirror element.

8. Operating control method according to claim 7, characterized in that the multiple mirror arrangement has at least one actively controllable electromechanical vibration generator of the electromechanical vibration cleaning system and that in the cleaning operating mode this vibration generator is controlled to generate vibrations of at least one of the mirror elements and / or that at least one of the mirror units has a vibration generator separate from the actuator system of the mirror unit, which is coupled to the mirror element of the mirror unit in a vibration-transmitting manner and in the cleaning operating mode in response to Control signals from the control unit of the vibration cleaning system are triggered independently of the actuator system to generate vibrations.

9. Operational control method according to claim 7 or 8, characterized in that a mirror unit is set into a cleaning-effective vibration at least phasewise during a vibration cleaning operation, which fulfills one or more of the following conditions: (i) The cleaning-effective vibration has a vibration frequency in the range of 50 kHz to 400 kHz; (ii) The cleaning-effective vibration has a vibration frequency that lies far outside a resonance frequency of a vibrating unit of the mirror unit comprising the mirror element, wherein preferably the resonance frequency for a lowest mode is in the range of 100 Hz to 1000 Hz, in particular in the range of 250 to 800 Hz and / or wherein the vibration frequency is in the range of 50% to 90% of the resonance frequency or in the range of 105% to 170% of the resonance frequency. (iii) The cleaning-effective vibration has a vibration amplitude that is at least 0.2 micrometers at the edge of a mirror surface in at least one direction.

10. Operational control method according to claim 7, 8 or 9, characterized by spatially resolved contamination monitoring for identifying and / or localizing mirror elements or groups of mirror elements whose function is restricted by contamination particles, wherein, based on signals from the spatially resolved contamination monitoring, electromechanical vibration generators of the electromechanical vibration cleaning system are selectively controlled such that, in cleaning operating mode, only mirror elements of mirror units affected by contamination are set into cleaning-effective vibrations.

11. Operational control method according to claim 10, characterized in that the spatially resolved contamination monitoring is carried out optically by means of at least one spatially resolved camera system and / or that the spatially resolved contamination monitoring is carried out electromechanically by means of at least one spatially resolved diagnostic system, wherein preferably for each mirror unit a functional relationship between a change in position of the mirror element and the force or electrical power required to generate the change in position is detected and used to The identification of functionally restricted mirror elements or mirror element groups is evaluated.

12. Operational control method according to one of claims 10 or 11, characterized in that the control unit is configured such that, based on the spatially resolved contamination monitoring, a vibration cleaning operation is initiated promptly after detection of a contamination considered to be functionally critical, preferably within less than 10 hours, in particular within less than half an hour.

13. Operational control method according to one of claims 7 to 12, characterized in that, in order to improve cleaning efficiency in the cleaning operating mode, at least one of the following actions is initiated with a time delay to or at least partially overlapping with an electromechanical vibration cleaning: (A) Opening: Widening a gap occupied by contamination particles by tilting the mirror elements forming the gap in opposite directions into a roof-shaped configuration or a V-configuration; (B) Reducing contamination particle adhesion forces on a mirror element by generating a vacuum or plasma effective in the area of ​​the affected mirror elements before and / or during a vibration cleaning operation.

14. Operational control method according to one of claims 7 to 13, characterized in that, in order to improve the removal of detached contamination particles from a mirror element during and / or after a vibration cleaning operation, at least one of the following actions is initiated: (A) Generating a flushing fluid flow from the intermediate area through the slit towards the mirror surfaces; (B) Tilting affected mirror units and / or the multiple mirror arrangement and / or an assembly containing the multiple mirror arrangement such that detached contamination particles are moved predominantly by gravity in a downward direction; (C) Generating an inhomogeneous electric field in the area of ​​contaminated mirror units to electrostatically assist removal.

15. Operational control method according to one of claims 7 to 14, characterized by a pretreatment preceding the vibration cleaning operation of the Multiple mirror arrangement by creating a vacuum atmosphere at least 10 hours before the start of one of the vibration cleaning operations.

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