Method for producing a MEMS mirror array and MEMS mirror array

Abstract

The invention relates to a method for producing a MEMS mirror array (100), for example, which can be used for lithography, and to a MEMS mirror array (100) produced according to this method. The method has a particular difference in that, before or after at least one of the production steps provided according to the invention, at least a region of the component part forming the MEMS mirror array (100) is provided with a protective layer (400) which protects the underlying material from environmental influences, in particular against hydrogen-induced outgassing.

Description

[0001] The present application claims priority to German patent application 10 2023 203 205.8, filed on April 6, 2023. The content of this German patent application is hereby incorporated by reference into the present application. TECHNICAL FIELD

[0002] The present application relates to a method of producing a MEMS mirror array, which can be used, for example, for lithography, and to a corresponding MEMS mirror array. BACKGROUND

[0003] Lithography is used to manufacture microstructured components, such as, for example, integrated circuits. Projection exposure apparatuses as used herein comprise an illumination system and a projection system. An image of a mask (also referred to as reticle), illuminated by the illumination system, is projected by the projection system in a reduced manner onto a substrate, for example a silicon wafer, which is coated with a light-sensitive layer and arranged in the image plane of the projection system, in order to transfer the mask structure to the light-sensitive coating of the substrate.

[0004] In the illumination system, in particular in projection exposure apparatuses designed for the EUV range (i.e. with an exposure wavelength of 5 nm to 30 nm), two facet mirrors are usually arranged in the beam path between the actual exposure radiation source and the mask to be illuminated, which can homogenize the radiation in a manner corresponding essentially to the principle of a fly’s eye condenser. The facet mirror closer to the exposure radiation source in the beam path is usually a so-called field facet mirror, while the other mirror is a so-called pupil facet mirror.

[0005] In order to be able to produce different intensity and / or angle of incidence distributions during the illumination of the mask, it is known for the facets of at least one of the two facet mirrors, in particular the facets of the field facet mirror, to be formed by one or more electromechanically independently pivotable micro mirrors. This corresponds to the disclosure in, for example, WO 2012 / 130768 A2.

[0006] In order to be able to implement the small size of the individual micro mirrors, it is known for the group of micro mirrors to be realized in the form of a so-called MEMS mirror array, i.e. a mirror array composed of microelectromechanical systems (MEMS).

[0007] In the case of a MEMS mirror array, a plurality of small mirror elements are each movably mounted individually with respect to a common base. For each mirror element, there is at least one actuator which enables adjustment of the mirror element along a respective predetermined degree of freedom. The mirror elements are generally pivotable about two axes which are perpendicular to one another and parallel to the base, and then a sufficient number of actuators are also provided to enable the mirror elements to be pivoted independently of one another about these axes. For the individual mirror elements, sensors can also be provided, so that the position of the mirror elements with respect to the base can be determined, in order to be able to monitor the alignment of the mirrors. DE 10 2015 204 874 A1 describes a particularly advantageous embodiment with respect to a mirror in a MEMS mirror array.

[0008] A method for producing a micro mirror or a MEMS mirror array containing a plurality of such micro mirrors is disclosed in DE 10 2015 220 018 A1, and further details about possible configurations of the micro mirror.

[0009] As explained therein, the MEMS system and in particular the MEMS mirror array is produced in a manner comparable to semiconductors and in particular by comparable methods. Thus, a considerable part of the mirror mechanical structure of the MEMS mirror array is in particular generally composed of silicon (Si), in particular single-crystal silicon or polycrystalline silicon, with established processing methods by means of semiconductor production, so that in principle the MEMS mirror array can be actually produced for the person skilled in the art.

[0010] In the case mentioned in the introduction for lithography purposes, the MEMS mirror array is used in the region of an illumination system, in which in principle at least during the production operation there is a vacuum. However, a small amount of hydrogen gas (so-called purge gas) is generally introduced into this vacuum in order to remove contaminants from specific regions of the illumination system and / or the mirror surface. This hydrogen can interact with the exposure radiation within the illumination system and ionize to form a hydrogen plasma. This applies in particular to EUV exposure radiation with a wavelength of 13.5 nm.

[0011] Hydrogen, in particular if ionized, can react with the silicon of the MEMS mirror array and in particular its structure, and lead to outgassing of for example silicon hydroxide; this process is also referred to as hydrogen-induced outgassing (HIO). The outgassing can cause so-called hydrogen embrittlement and impair the material strength of the filament-like silicon structure. Furthermore, there is a risk of deposition of the outgassed substances on optical surfaces within the illumination system or, if configured within the same vacuum chamber, the projection system, which in turn leads to a deterioration thereof.

[0012] Furthermore, the MEMS mirror array can be attacked by environmental influences other than hydrogen or hydrogen plasma, which can have a disadvantageous effect on the structural integrity and / or the function of the MEMS mirror array or cause substances to be deposited on the optical surfaces which are to be released. SUMMARY

[0013] It is an object of the application to reduce as far as possible problems which can be caused by environmental influences on a MEMS mirror array in a projection exposure apparatus. Particular attention is paid here to avoiding or at least reducing hydrogen-induced outgassing of substances in the case of a MEMS mirror array in a projection exposure apparatus.

[0014] This object is achieved by a method for producing a MEMS mirror array as claimed in claim 1 and by a MEMS mirror array as claimed in claim 10. The dependent claims relate to advantageous developments.

[0015] The application therefore relates to a method for producing a MEMS mirror array for photolithography, the MEMS mirror array having a predetermined number of individual mirrors which are adjustable in at least one degree of freedom, the method comprising the following steps:

[0016] a) providing a mirror wafer comprising a plurality of mirror sections which are separated from one another by release sections, the number of mirror sections corresponding to the number of adjustable mirrors;

[0017] b) providing an actuator wafer comprising a plurality of actuator sections corresponding to the adjustable plurality of mirrors, wherein the actuator sections are spaced apart from one another in accordance with the mirror sections of the mirror wafer and the individual actuator sections have at least one functional structure;

[0018] c) joining the mirror wafer and the actuator wafer together such that the mirror sections and the actuator sections are fixedly connected to one another in each case in a defined region; and

[0019] d) removing at least the release sections of the mirror wafer such that the individual mirror sections are adjustable in each case relative to the respective actuator sections by at least one degree of freedom by using at least one part of the functional structure;

[0020] wherein, before or after at least one of the aforementioned steps, at least a region of the mirror wafer and / or the actuator wafer is provided with a protective layer against environmental influences in order to protect the underlying material from hydrogen-induced outgassing.

[0021] The application further relates to a MEMS mirror array for lithography having a predetermined number of individual mirrors adjustable in at least one degree of freedom, wherein the MEMS mirror array is produced according to the method of the application.

[0022] First, some terms used in the present application will be explained:

[0023] In the case of a "MEMS mirror array for lithography", a predetermined number of individual mirrors is provided which are arranged close to each other in a 2D grid and are individually adjustable in at least one degree of freedom. In this case, at least 4, at least 16, at least 64, at least 250 or at least 1000 mirrors can be provided, preferably the mirrors are arranged in a square or hexagonal grid. An exception is that the mirror array can also contain only one mirror.

[0024] The outer contour of the active reflective surface of each of the individual mirrors can be configured circular or polygonal, i.e. in particular can be triangular, quadrangular or hexagonal. In this context, in the case of a polygonal configuration, the edge length can range between 10 pm and 10 mm, preferably between 100 pm and 4 mm, more preferably between 0.6 mm and 1.5 mm, preferably the length of all edges is identical. All mirrors of the MEMS mirror array can have identically embodied reflective surfaces; however, this is not mandatory.

[0025] The "reflective surface" of a mirror is in principle a surface which reflects light of at least one predefined wavelength, i.e. in particular one or more exposure wavelengths used for lithography, and which, at least in the envisaged state of the mirror during use of the MEMS mirror array, is actually used for deflecting light rays for further use. In particular, due to production, inner regions of the MEMS mirror array can in principle have the required reflective properties, but at no time are actively used for the deflection of light, e.g. for exposure purposes, and thus do not constitute a "reflective surface" in the sense of the present application. The actual reflective surface of a mirror can be embodied in a planar manner. However, the surface can also be curved in a concave or convex manner. Furthermore, the reflective surface of a mirror can also have any other shape.

[0026] A high integration density of the mirrors in the MEMS mirror array is in principle sought. The integration density can be expressed here, for example, as the proportion of the reflective surface of the mirrors formed by the individual mirrors relative to the total surface area of the MEMS mirror array (so-called "fill factor"). The fill factor is preferably 0.5 or more, more preferably 0.75 or more, particularly preferably 0.9 or more.

[0027] A mirror is "adjustable in at least one degree of freedom" if it can be adjusted without influence of other possible degrees of freedom. In particular, rotational degrees of freedom are relevant in this context with respect to the MEMS mirror array. Preferably, the mirror is pivotable about an axis perpendicular to the normal of the mirror's reflecting surface. It is particularly preferred if the mirror is able to pivot independently about two axes arranged perpendicular to each other, the two mentioned axes preferably extending perpendicular to the normal of the reflecting surface in the case of a predetermined zero alignment of the mirror. In particular, it can be preferred if the mirror has a hexagonal shape and / or is arranged in a hexagonal grid, if the mirror is able to pivot about three axes arranged in a common plane and in each case angularly spaced by 60°. The zero alignment of all mirrors of the mirror array is preferably chosen such that all normals of the reflecting surfaces of the individual mirrors are parallel to each other and / or to the normal of the entire surface of the MEMS mirror array.

[0028] A "functional structure" in the area of a mirror segment of the mirror wafer or of an actuator segment of the actuator wafer is a structure which is important during the joining of the mirror wafer and the actuator wafer or for the subsequent functioning of the MEMS mirror. In this regard, the functional structure can be those areas where the mirror wafer and the actuator wafer are actually connected to each other. For this purpose, the areas in question can also optionally have a specific shape which enables or improves the connection or which subsequently enables the movability of the mirror. However, the functional structure can also be a structure which, after completion of the MEMS mirror array, forms, alone or together with other functional structures, for example, an actuator or a sensor which enables or monitors the movement of a predetermined degree of freedom. It can also include structures which enable the electrical connection of the actuator and / or the sensor.

[0029] The present application is based on the concept that negative environmental influences can be reduced, if not completely prevented, by providing a suitable protective layer. In particular, the appropriate selection of the protective layer makes it possible to reduce or completely prevent hydrogen-induced outgassing in the case of a MEMS mirror array. In this case, the present application consists in integrating a protective layer which resists environmental influences into the process for producing a MEMS mirror array, in particular for lithography applications, which results in particularly high requirements for the impermeability and uniformity of the protective layer which resists environmental influences. In order to provide comprehensive protection, the protective layer should also be applied in a planar manner to the surfaces which in principle are at risk of environmental influences, preferably to all surfaces, environmental influences being in particular hydrogen-induced outgassing.

[0030] In the method for producing a MEMS mirror array for microlithography according to the present application, it is provided that both a mirror wafer and an actuator wafer are provided, which are joined together during the course of the method and are subsequently further processed in order to finally establish a functional MEMS mirror array.

[0031] The mirror wafer is a planar structure which usually consists at least partially, if not for the most part, of monocrystalline silicon. This mirror wafer is subdivided into mirror segments which, after the production process, will in each case form a movable mirror with a reflective surface. The number of mirror segments thus corresponds to the number of mirrors of the MEMS mirror array to be produced. When the mirror wafer is provided, the individual mirror segments can be determined on the wafer on the basis of protrusions or recesses on at least one side of the mirror wafer, although this is not mandatory.

[0032] When the mirror wafer is provided, selectively at least some functional structures can already be provided in the area of each mirror segment on the wafer; however, this is not necessary. When the mirror wafer is provided, the functional structures can be identified on the basis of protrusions or recesses. However, the functional structures can also be integrated into a mirror wafer with a planar surface. The functional structures can then be revealed in a subsequent processing step; however, this is not absolutely necessary.

[0033] The provision of the functional structures already when the mirror wafer is provided can provide the advantage that the corresponding functional structures can be provided without any problems even in areas of the mirror segments which can no longer be accessible or are only difficult to access, after the mirror wafer is envisaged to be joined together with the actuator wafer at a later point in time. Of course, this applies equally to the functional structures to be provided on the actuator wafer.

[0034] When the mirror wafer is provided, the individual mirror segments on the mirror wafer are all connected to one another by means of a release segment. By means of the release segment, which is removed in a subsequent method step, the mirror segments are fixedly connected to one another in such a way that a relative movement of the mirror segments with respect to one another is not possible, nor is an individual treatment of the individual mirror segments necessary. The mirror wafer processing steps before the provision of the mirror wafer and the treatment of the mirror wafer in the course of the method according to the application are thus greatly simplified.

[0035] In addition to the mirror wafer, an actuator wafer is also provided, which usually consists at least partially, if not for the most part, of polycrystalline silicon. Alternatively, the actuator wafer can of course also consist of monocrystalline silicon. Similar to the mirror segments on the mirror wafer, actuator segments are provided on the actuator wafer and each likewise contain at least one functional structure. In this case, the actuator segments are likewise spaced apart from one another, wherein the material of the actuator wafer in the area between the actuator segments, in contrast to the release segment of the mirror wafer, is not removed later, even if the dimensions are identical.

[0036] The wafer provided, that is to say the mirror wafer and the actuator wafer, are joined together in a subsequent step. In this case, one of the two wafers is arranged on the other wafer in such a way that the respective mirror segments of the mirror wafer correspond to the respective actuator segments of the actuator wafer and the respective segments are thus fixedly connected to one another in a defined region.

[0037] The two wafers can be aligned with one another for the joining here, with a positional accuracy in the plane of the common contact area of better than 5 μm, preferably better than 2 μm, more preferably better than 1 μm. The mirror segments of the mirror wafer are thus highly accurately aligned with their respective actuator segments and are then connected to the actuator segments.

[0038] In a further step, the release segments of the mirror wafer are then at least completely removed. In particular, the release segments can be removed by etching, wherein, by means of a suitable etch stop layer or an originally internal etch stop layer (hereinafter collectively referred to as etch stop layer), it is necessary to ensure that the specialist structure area which is not required or not (any longer) desired is removed. Corresponding etch stop layers and their application or integration in semiconductor or semi-conductor-like structures are known in principle from the prior art. In particular, if the parts of the structure which are to be removed or which are to be retained consist of silicon, in particular polysilicon, for example silicon dioxide can be used as etch stop layer, it being assumed, of course, that the etching medium used does not remove the silicon dioxide. Even if silicon dioxide is not susceptible to environmental influences, in particular hydrogen-induced outgassing, and remains permanently in the structure produced and can form a relatively small structure surface portion there, compared to (polysilicon) silicon, silicon dioxide is only applied as etch stop layer in principle selectively or in a structured manner for selective etching. The majority of the structure surface produced by the etching process thus still consists of (polysilicon) silicon, which needs to be protected from environmental influences, in particular hydrogen-induced outgassing.

[0039] The outer surface of the mirror segments, which is intended to form the reflective surface at the end of the method, can also be protected from damage, in particular due to etching, during the production method by the application and suitable curing of a photoresist layer. Corresponding and suitable "photoresist layers" are well known from semiconductor production. The photoresist layer should generally be removed again at the appropriate time. As an alternative to a photoresist layer, for example, a layer consisting of silicon dioxide, silicon nitride, aluminium oxide or aluminium can also be provided. Combinations of the layers mentioned in a multilayer structure are also conceivable.

[0040] If the release segments are removed by means of an etching method, it is also possible for function structures which can be hidden and recessed to emerge in the process. Furthermore, it is thus possible, for example, to remove any undesirable partial structures which impede at least one degree of freedom of each individual mirror.

[0041] The removal of at least the release segments makes it possible in principle that the mirror segments, which are then separated from one another, can be adjusted in at least one degree of freedom relative to the respective actuator segment to which they are subsequently connected individually. At least one part of the functional structure already present at the provided actuator wafer and optionally also at the mirror wafer should be used for this adjustment. As explained, the functional structure can be a specific structural configuration, for example, for implementing one or more degrees of freedom of bending. Alternatively, for example, the functional structures on the mirror segments and the actuator segments can cooperate in order thus to form an actuator or sensor which is able to implement or monitor the movement of a predetermined degree of freedom.

[0042] The above-mentioned method steps are only those which are necessary and thus required for the method according to the application for producing a MEMS mirror array. Any further (intermediate) steps can be provided before, between and after the mentioned method steps. For example, additional surface treatment steps, such as cleaning or chemical mechanical polishing, can be provided in order to improve the surface quality for subsequent steps. Furthermore, steps can be carried out in parallel with the mentioned steps or these steps can be integrated into a common method step. An example thereof is the selective removal of unwanted structures in the course of the removal of the release segments.

[0043] However, according to the application, it is provided that at least one of the above-mentioned steps for producing a MEMS mirror array for lithography is preceded or followed by the provision of at least a region of the mirror wafer and / or the actuator wafer with a protective layer for protecting the underlying material from environmental influences. In particular, the protective layer preferably provides protection from hydrogen-induced outgassing, for which purpose only the material of the protective layer needs to be appropriately selected.

[0044] In this case, the protective layer resistant to environmental influences is preferably provided on at least all surfaces of the completed MEMS mirror array which are susceptible to the expected environmental influences and / or hydrogen-induced outgassing, that is to say which are at risk of the function of the MEMS mirror array being impaired or of the release of substances, in particular hydrogen-induced outgassing, in the case of the expected environmental influences. Since all relevant surfaces are provided with a corresponding protective layer resistant to environmental influences, this greatly reduces the risk of negative consequences for the function of the MEMS mirror array and / or the release of unnecessary substances during operation as a result of environmental influences. In addition to the structural elements of the MEMS mirror array, there are also functional elements, such as, for example, the electrode combs of the actuators and / or sensors, which can be protected from environmental influences with the protective layer.

[0045] Depending on the final selection of details of the method for producing a MEMS mirror array for lithography, the protective layer resistant to environmental influences can be applied at different times within the scope of the application.

[0046] In particular, at least one portion of the environmental-influence-resistant protective layer can be applied after at least the release section of the mirror wafer has been removed. At this point, the surfaces which are in principle susceptible to the intended environmental influences and / or hydrogen-induced outgassing are at least partially, if not completely, exposed, and thus, by depositing the environmental-influence-resistant protective layer on all exposed surfaces, the desired protective effect can be achieved comprehensively.

[0047] Alternatively or additionally, the final desired at least portion of the environmental-influence-resistant protective layer can be provided in regions which have been adapted on the mirror wafer and / or on the actuator wafer. In particular, the protective layer can also be integrated into the mirror wafer and / or into the actuator wafer, such that the protective layer is only uncovered in subsequent process steps, in particular by removing material which is arranged thereon.

[0048] If the removal of the material and / or of the release section takes place in an etching process, preferably including the detachment of possible applications or integrations before the mirror wafer and / or the actuator wafer is provided, the protective layer used for at least some possible etching processes is preferably an etch stop layer, in particular for the removal of at least the release section by etching. If the protective layer can be used as an etch stop layer, this ensures that the protective layer is in principle not damaged by etching processes which are provided after its application or integration. At the same time, from a certain point in time, the material which is not needed, for example the release section in regions which are at least partially delimited by the protective layer, can be removed in a targeted manner, just like possible material residues and other contaminants before the previous method steps. If the protective layer itself cannot be used as an etch stop layer, a multilayer structure which comprises the protective layer and an etch stop layer arranged thereon, which consists, for example, of silicon dioxide, can also be provided instead of the protective layer itself.

[0049] Furthermore, it is preferred that, before or after at least one of the aforementioned steps and before or after the application of the protective layer, a reflective coating is applied at least in the region of the individual mirror segments which is intended to be a reflective surface, the reflective coating reflecting light of at least one predetermined wavelength. In general, only a corresponding coating enables the mirror segments to have sufficient reflectivity. In this case, the reflective coating will be arranged in a manner which is adapted to the wavelength which is provided for the lithography, and can also have, for example, a multilayer structure. For example, if the MEMS mirror array is set up for an EUV projection exposure apparatus and must be able to reflect light of a wavelength of 13.5 nm well, a multilayer structure consisting of molybdenum and silicon layers can preferably be provided as the reflective coating, the individual layers each having a thickness of only a few nanometers. This coating is known to reflect EUV exposure radiation sufficiently well. A single-layer reflective coating consisting of other materials can be sufficient for other wavelengths or wavelength ranges.

[0050] If the reflective coating is applied before the removal of at least the release segments of the mirror wafer, the individual mirror segments are still fixed and immovably integrated in the mirror wafer. This simplifies the high-precision, in particular uniform and reproducible, application of the reflective coating, in particular if it is a multilayer reflective coating. However, it is often necessary to provide a layer which is also removed later, for example a photoresist layer or an inorganic layer on the reflective coating, in order to properly protect the reflective coating during subsequent method steps, in particular during the removal of the release segments. In this procedure, it is in principle not important whether the reflective coating is applied before or after the mirror wafer and the actuator wafer are joined together.

[0051] Alternatively, the reflective coating can be applied only after the release segments have been removed. Since at this stage the mirror regions are usually already movable in at least one degree of freedom relative to the respective actuator regions, more challenges arise in the implementation of a high-precision coating. However, it is usually not necessary to provide the additional layer for protecting the reflective coating only temporarily.

[0052] If the reflective coating is applied after a protective layer against environmental influences has been applied, this protective layer can also be configured in the region provided for the reflective coating. If the protective layer against environmental influences is suitable as a basis for the reflective coating in terms of surface quality, hardness and adhesion, the reflective coating can be applied directly on this protective layer. Depending on the reflective coating, it is desirable or necessary here that the reflective coating is electrically linked. If the protective layer against environmental influences is not electrically conductive, suitable vias through the protective layer can be provided for this purpose, which are created at the latest before the application of the reflective coating and can be selectively filled with an electrically conductive material. The vias can be created by plasma etching, ion milling, ion or electron polishing, laser ablation or selective atomic layer deposition etching. As electrically conductive filling material for the vias, silicon or alternatively the material used for the reflective coating can be used, provided that this material is electrically conductive. Alternatively, it is conceivable that a protective layer which is electrically insulating in principle becomes sufficiently electrically conductive by suitable subsequent treatment. For this purpose, for example by irradiation, directional deposition or incandescent introduction of ions can be used to reduce the resistivity of the protective layer preferably to the range of 1 to 1000 MΩ x m. 2 / m.

[0053] Alternatively, the protective layer against environmental influences can first be removed from the region in question before the reflective coating is applied.

[0054] If a reflective coating is applied before a protective layer against environmental effects is applied, the protective layer will extend its influence onto the reflective coating after application. The protective layer may remain on the reflective coating if it transmits sufficiently for the wavelengths of light that the reflective coating intends to reflect. In this case, it is, of course, necessary to ensure that the surface quality of both the reflective and protective layers is good enough, particularly in terms of roughness, to avoid any unwanted optical effects. Alternatively, the protective layer may be at least partially removed from the reflective coating. To facilitate this, a temporary layer may be placed between the reflective coating and the protective layer against environmental effects, and this temporary layer also protects the reflective coating, especially during the separation of the protective layer.

[0055] In the manufacturing process according to the invention, those skilled in the art can, of course, provide intermediate layers between the explicitly mentioned layers and coatings. In particular, these can be temporary intermediate layers, for example, to protect the underlying layer or coating and / or to facilitate the removal of the layer or coating located thereon. If the intermediate layer is intended to be permanently retained in the MEMS mirror array produced according to the invention, it is necessary to ensure that the intermediate layer does not adversely affect the function of the MEMS mirror array and the individual layers (as can be seen above).

[0056] It is self-evident to those skilled in the art that, as needed, the surface should be checked to ensure it meets the required surface quality before applying a layer or coating, and if appropriate, processing steps should be performed to obtain the desired surface quality.

[0057] Finally, layers or coatings can of course be removed in any intermediate step from areas where they are no longer needed.

[0058] Suitable materials for protective layers that resist environmental impacts include, for example, electrically insulating materials such as aluminum oxide (Al₂O₃). x O y ) or titanium dioxide (Ti x O y However, conductive materials or multilayer structures are also possible. In particular, the protective layer against environmental influences can be composed of a reflective coating or at least one layer thereof used in the reflective surface.

[0059] In embodiment variants in which a separation protection can be required, it is also possible to dispense with the joining together of the provided mirror wafer and the provided actuator wafer, but instead to use a single step of providing a combined wafer in place of the corresponding method steps (a) to (c). In each case of any of the above-described configurations, the combined wafer here essentially constitutes the combination of mirror wafer and actuator wafer, as additionally obtained by joining together according to step (c). In the case of a combined wafer, the gap that can remain when joining the mirror wafer and the actuator wafer together is generally only filled with material, however, the gap can again be removed in a subsequent processing step, if required. In principle, the corresponding combined wafer can be produced by known material application and / or material removal methods, which are carried out selectively and / or only locally in each case. Step (d) and the provision of a protective layer for at least regions of the combined wafer to protect the underlying materials from the environment can in each case be carried out analogously to the methods described for the joining together of mirror wafer and actuator wafer.

[0060] For the purpose of elucidation of the MEMS mirror array according to the application, reference is made to the preceding explanations. BRIEF DESCRIPTION OF DRAWINGS

[0061] The application will be explained by way of example on the basis of advantageous embodiments and with reference to the drawings, in which:

[0062] Figure 1 a schematic diagram of a projection exposure apparatus for photolithography, which comprises a MEMS mirror array produced according to the application, is shown;

[0063] Figure 2 a-e show schematic diagrams of a first exemplary embodiment of a method according to the application for producing a MEMS mirror array which can be used in a projection exposure apparatus according to Figure 1 ;

[0064] Figure 3 a-e show schematic diagrams of a second exemplary embodiment of a method according to the application for producing a MEMS mirror array which can be used in a projection exposure apparatus according to Figure 1 ;

[0065] Figure 4 a-e show schematic diagrams of a third exemplary embodiment of a method according to the application for producing a MEMS mirror array which can be used in a projection exposure apparatus according to Figure 1 ;

[0066] Figure 5 a-e show schematic diagrams of a fourth exemplary embodiment of a method according to the application for producing a MEMS mirror array which can be used in a projection exposure apparatus according to Figure 1 ; Detailed Implementation

[0067] Figure 1 A schematic meridional cross-sectional view of a projection exposure apparatus 1 for photolithography is shown. In this case, the projection exposure apparatus 1 includes an illumination system 10 and a projection system 20, the illumination system 10 being developed using a device 100 according to the invention.

[0068] The illumination system 10 is used to illuminate the object field 11 in the object plane or mask master plane 12. For this purpose, the illumination system 10 includes an exposure radiation source 13, which, in the illustrated exemplary embodiment, emits illumination radiation containing at least the light used in the EUV range, i.e., particularly illumination radiation with wavelengths between 5 nm and 30 nm. The exposure radiation source 13 may be a plasma source, such as a laser-generated plasma (LPP) source or a gas discharge-generated plasma (DPP) source. A synchrotron-based radiation source may also be involved. The exposure radiation source 13 may also be a free-electron laser (FEL).

[0069] The illumination radiation emitted from the exposure radiation source 13 is first focused in a concentrator 14. The concentrator 14 may be a concentrator having one or more elliptical and / or hyperboloidal reflective surfaces. At least one reflective surface of the concentrator 14 may be irradiated with illumination radiation having grazing incidence (GI), i.e., an incident angle greater than 45°, or with illumination radiation having perpendicular incidence (NI), i.e., an incident angle less than 45°. The concentrator 14 may be structured and / or coated, primarily to optimize its reflectivity to the radiation used, and secondarily to suppress intrusive light.

[0070] Downstream of the condenser 14, illumination radiation propagates through the intermediate focal point in the intermediate focal plane 15. If the illumination system 10 is constructed with a modular design, in principle, the intermediate focal plane 15 can be used to separate the illumination system 10 (including its structure) into a radiation source module and an illumination optical unit 16, described below, which has an exposure radiation source 13 and a condenser 14. Given this separation, the radiation source module and the illumination optical unit 16 then together form the modularly constructed illumination system 10.

[0071] The illumination optics unit 16 includes a deflector 17. The deflector 17 may be a planar deflector, or alternatively a mirror having beam-affecting effects in addition to the pure deflection effect. Alternatively or additionally, the deflector 17 may be implemented as a spectral filter that separates the wavelength of the illumination radiation used from the wavelength of the incoming light that deviates from it.

[0072] The radiation originating from the exposure radiation source 13 is deflected by the deflection mirror 17 onto the first facet mirror 18. If the first facet mirror 18 here, as in the present case, is arranged in a plane of the illumination optical unit 16 which is optically conjugate with respect to the mask mother version plane 12 as a field plane, the facet mirror is also referred to as a field facet mirror.

[0073] The first facet mirror 18 comprises a plurality of micro mirrors 18', which in each case can be pivoted individually about two axes extending perpendicularly to one another, for controllably forming facets, each of which is preferably provided with an orientation sensor (not shown) for determining the orientation of the micro mirror 18'. The first facet mirror 18 is thus a micro electro mechanical system (MEMS system), as is also described, for example, in DE 10 2008 009 600 A1.

[0074] In the beam path of the illumination optical unit 16, a second facet mirror 19 is arranged downstream of the first facet mirror 18, forming in turn a two-facet system, the basic principle of which is also referred to as fly's eye condenser (fly's eye integrator). If the second facet mirror 19 is arranged, as in the exemplary embodiment shown, in a pupil plane of the illumination optical unit 16, the facet mirror is also referred to as a pupil facet mirror. However, the second facet mirror 19 can also be arranged at a distance from the pupil plane of the illumination optical unit 16, whereby a catadioptric system results from the combination of the first facet mirror 18 and the second facet mirror 19, as is described, for example, in US 2006 / 0132747 A1, EP 1 614 008 B1 and US 6,573,978.

[0075] In principle, the second facet mirror 19 need not be composed of pivotable micro mirrors, but can comprise individual facets formed by one mirror or a manageable number of mirrors which are significantly larger than micro mirrors, and which can be fixed or are tiltable only between two defined end positions. However, as shown, it is also possible in the case of the second facet mirror 19 to provide a micro electro mechanical system having a plurality of micro mirrors 19', which in each case can be pivoted individually about two axes extending perpendicularly to one another, in each case preferably comprising an orientation sensor.

[0076] By means of the second facet mirror 19, the individual facets of the first facet mirror 18 are imaged into the object field 11, which is generally only an approximate imaging. The second facet mirror 19 can be the last beam shaping mirror or indeed the last mirror for the illumination radiation in the beam path upstream of the object field 11.

[0077] Each of the facets of the second facet mirror 19 is assigned to exactly one facet of the first facet mirror 18, respectively, to form an illumination channel for illuminating the object field 11. This can lead to an illumination according to the Kohler principle, among others.

[0078] The facets of the first facet mirror 18 are each imaged by the respective facet of the second facet mirror 19 in an additive manner to one another in order to illuminate the object field 11. In this case, the illumination of the object field 11 is as uniform as possible. Its uniformity error is preferably less than 2%. The field uniformity can be achieved by the superposition of the different illumination channels.

[0079] By selecting the illumination channel used in the end, by appropriately setting the micro mirrors 18’ of the first facet mirror 18, this can be done without any problems, in addition, it is also possible to set the intensity distribution in the entrance pupil of the projection system 20 described below. This intensity distribution is also referred to as the illumination setting. In addition, it can be advantageous here that the second facet mirror 19 is not precisely arranged in a plane that is optically conjugate to the pupil plane of the projection system 20. In particular, the pupil facet mirror 19 can be arranged tilted with respect to the pupil plane of the projection system 20, as described, for example, in DE 10 2017 220 586 A1.

[0080] In the case of the component arrangement of the illumination optical unit 16 as shown in Figure 1 However, in the case of the component arrangement of the illumination optical unit 16 as shown in

[0081] In an alternative embodiment of the illumination optical unit 16 (not shown), a transfer optical unit containing one or more mirrors can also be provided in the beam path between the second facet mirror 19 and the object field 11. The transfer optical unit can in particular comprise one or two normal incidence mirrors (NI mirrors) and / or one or two grazing incidence mirrors (GI mirrors). Using an additional transfer optical unit, in particular different poses of the entrance pupils of the tangential and sagittal beam paths of the projection system 20 described below can be taken into account.

[0082] Alternatively, the deflection mirror 17 shown in Figure 1 may be dispensed with for this purpose, the facet mirrors 18, 19 being appropriately arranged with respect to the radiation source 13 and the condenser 14.

[0083] By means of the projection system 20, the object field 11 in the mask plane 12 can be transferred to an image field 21 in an image plane 22.

[0084] For this purpose, the projection system 20 comprises a plurality of mirrors Mi, which are consecutively numbered according to their arrangement in the beam path of the projection exposure apparatus 1.

[0085] In Figure 1 the example shown, the projection system 20 comprises 6 mirrors M l to M6. Alternatively, there can also be 4, 8, 10, 12 or any other number of mirrors M i . The penultimate mirror M5 and the last mirror M6 each have a passage opening for the illumination radiation, so that the projection system 20 shown is a doubly shadowed optical unit. The image-side numerical aperture of the projection system 20 is greater than 0.3 and can also be greater than 0.6, for example 0.7 or 0.75.

[0086] The reflection surfaces of the mirrors Mi can be in the form of free-form surfaces without an axis of rotational symmetry. However, the reflection surfaces of the mirrors M i may alternatively also be designed as aspherical surfaces with exactly one axis of rotational symmetry of the reflection surface shape. As with the mirrors of the illumination optical unit 16, the mirrors M i may have a high-reflectivity coating for the illumination radiation. These reflection coatings can be designed as multilayer coatings, in particular with alternating layers of molybdenum and silicon.

[0087] The projection system 20 has a large object-image offset in the y-direction between the y-coordinate of the center of the object field 11 and the y-coordinate of the center of the image field 21. This amount of object-image offset in the y-direction can be approximately equal to the amount of the z-distance between the object plane 12 and the image plane 22.

[0088] In particular, the projection system 20 can be designed anamorphic, that is to say, it has different imaging ratios βχ, βγ, in particular in the x-direction and the y-direction. The two imaging ratios βχ, βγ of the projection system 20 are preferably (βχ, βγ) = (+ / - 0.25, + / - 0.125). Here, an imaging ratio β of 0.25 corresponds to a reduction ratio of 4:1, while an imaging ratio β of 0.125 results in a reduction ratio of 8:1. A positive sign of the imaging ratio β means imaging without image reversal; a negative sign means imaging with image reversal.

[0089] It is also possible for other imaging ratios. It is also possible for the imaging ratios βχ, βγ to have the same sign and the same absolute value in the x- and y-directions.

[0090] The number of intermediate image planes in the x-direction and the y-direction in the beam path between the object field 11 and the image field 21 can be the same or different, depending on the embodiment of the projection system 20. Examples of projection systems 20 with a different number of such intermediate images in the x-direction and the y-direction are known from US 2018 / 0074303 A1.

[0091] In particular, the projection system 20 can comprise a concentric entrance pupil. The latter can be accessible. However, it can also be inaccessible.

[0092] A reticle 30 (also referred to as a mask) arranged in the object field 11 is exposed by the illumination system 10 and transferred by the projection system 20 onto the image plane 21. The reticle 30 is held by a reticle holder 31. The reticle holder 31 is movable, in particular in the scanning direction, by a reticle displacement drive 32. In the exemplary embodiment shown, the scanning direction extends along the y direction.

[0093] The structures on the reticle 30 are imaged onto a photosensitive layer of a wafer 35 arranged in the area of the image field 21 in the image plane 22. The wafer 35 is held by a wafer holder 36. The wafer holder 36 is movable, in particular longitudinally with respect to the y direction, by a wafer displacement drive 37. The reticle 30 is moved first by the reticle displacement drive 32 and the wafer 35 is moved second by the wafer displacement drive 37, so that a mutual synchronization can be implemented.

[0094] Figure 1 The projection exposure apparatus 1 shown or its illumination system 10 (the description thereof above essentially reflects the known prior art) is characterized in that the first and / or second facet mirror 18, 19 comprises one or more MEMS mirror arrays 100 produced according to the application. In this case, each MEMS mirror array 100 has a plurality of individual mirrors 101 which are in each case independently adjustable in two rotational degrees of freedom and are arranged in a two-dimensional grid. Each of the facet mirrors 18, 19 can be formed by an individual MEMS mirror array or a plurality of MEMS mirror arrays 100 arranged next to one another.

[0095] Figures 2 to 5 Different exemplary embodiments of a method for producing a mirror array 100 for lithography according to the application are each schematically depicted, in each case with a plurality of subvariants. In this case, each figure shows a partial sectional view through the MEMS mirror array 100 to be produced, i.e. through two mirrors 101 or mirror segments 201, in different method stages.

[0096] This figure shows, using dashed boundary lines and also essentially dashed hatching, layers and coatings which are only provided selectively in certain method steps. This applies to layers and coatings which are applied selectively and also to layers and coatings which are removed again only selectively.

[0097] Figure 2 A first exemplary embodiment of a production method according to the application is shown.

[0098] At the beginning of the method, Figure 2 a) a mirror wafer 200 and an actuator wafer 300 are provided.

[0099] Starting from a planar sandwich wafer (also referred to as "silicon-on-insulator wafer" or "SOI wafer") as a mirror wafer 200 with an inner silicon dioxide layer 202, which has a very smooth surface with a root mean square roughness of less than 0.2 nm, preferably even less than 0.1 nm, and has layers 203, 204 composed of monocrystalline silicon on both sides, functional structures are shaped in each case in defined mirror segments 201 and are arranged in one layer 204. The individual mirror segments 201 are implemented here identically.

[0100] In addition to the mirror body 205 forming the rear reflection surface 102, the mirror segment 201 also contains, for example, in each case as a functional structure, a connection column portion 206 for chaining an actuator wafer 300, a coating 207, which is suitable for fixing to the actuator wafer 300 provided on the underside of the connection column portion. Furthermore, a functional structure 208 is provided in each mirror segment 201, which subsequently forms, together with a corresponding structure 308 on the actuator wafer 300, an actuator 108 for the corresponding mirror 101. In this case, the schematically depicted actuator 108 represents only one of a plurality of possible designs of the actuator 108. In the case of other actuator designs, the functional structure 208 is not required in the area of the mirror segment 201, so that the mirror wafer 200 can also be implemented in this case completely without functional structures 208.

[0101] All structures 205, 206 and 208 are covered by a silicon dioxide layer 209, which is remote from the coating 207. This silicon dioxide layer 209 can remain from the production steps required for establishing the structures 205, 206 and 208, or can be applied deliberately for protecting the surfaces of the structures 205, 206 and 208. The silicon dioxide layer 209 serves as an etch stop layer for an etching of pure silicon processes.

[0102] The structures in the layer 204 are limited here to the mirror segments 201. However, when the mirror wafer 200 is provided, the layer 204 is fixedly connected to the silicon dioxide layer 202 and via the latter to the further silicon layer 203, so that the individual mirror segments 201 are fixed constituent parts of the mirror wafer 200. In the present case, the area between each two mirror segments 201 is designated as a release section 210.

[0103] The actuator wafer 300 is based on a planar silicon layer composed of monocrystalline and / or polycrystalline silicon, on which various functional structures are shaped identically in each case in actuator segments 301. In particular, a functional structure 308 is provided, which subsequently forms, in conjunction with a corresponding structure 208 on the mirror wafer 200, an actuator 108 to enable an adjustment of the mirror 101 in predetermined degrees of freedom, i.e. pivoting.

[0104] Furthermore, a joining structure 302 is also provided, which is covered by a silicon dioxide layer 303, which serves as an etch stop layer for subsequent method steps for producing a flexure 103 from the joining structure 302, which enables adjustment of the mirror 101 in two independent degrees of freedom. A coating 307 suitable for a fixed connection is provided in the region of the joining structure 302, which is provided for the connection to the connection post 206 of the mirror wafer 200. In addition to the precisely this region of the coating 307, the actuator wafer 300 is covered on the side which provides for the connection of the mirror wafer 200 with a silicon dioxide layer 309.

[0105] The mirror wafer 200 and the actuator wafer 300 thus provided are placed on top of one another with a precision of 2 pm, preferably less than 1 pm, such that the coatings 207, 307 contact one another. Depending on the configuration of the coatings 207, 307, these coatings can already be activated by the contact and establish a permanent and fixed connection between the mirror wafer 200 and the actuator wafer 300. Alternatively, the coatings 207, 307 can also be activated separately, for example by heat or plasma.

[0106] The silicon layer 203 of the mirror wafer 200 is then removed, for example by etching using a plasma (for example SF6) or using a chemical (for example XeFE2). In this process or in a separate step, the portion of the joining structure 302 on the actuator wafer 300 which is delimited by the silicon dioxide layer 303 is also removed. In this case, as is known from the prior art, openings can also be formed on the rear side of the actuator wafer 300, in particular in the region of the joining structure 302.

[0107] At least one portion of the silicon layer 203 can also be removed by grinding and polishing, preferably by chemical-mechanical polishing, preferably followed by an etching step, also in order to expose the joining structure 302 on the actuator wafer 300.

[0108] All exposed silicon dioxide layers 202, 209, 303, 309 are then also removed without residues (for example using hydrogen fluoride vapor).

[0109] Figure 2 b shows the result of the aforementioned steps.

[0110] Until this stage, Figure 2 The method shown corresponds to the known prior art and can be carried out directly by the person skilled in the art. The method steps described are also described in, for example, DE 10 2015 220 018 A1, and in addition, possible configurations of the individual structures are also shown in detail, in the present case only schematically.

[0111] In Figure 2In the production state shown in b, in principle, the mirror body 205 is already adjustable with one of the degrees of freedom generated by the actuator 108 formed by the functional structures 208, 308 on the mirror wafer 200 and the actuator wafer 300 with the joining structure 302. However, what is missing is not only the reflective coating 212 which is absolutely necessary for the reflection of EUV radiation in the area of the reflective surface 211 provided, but also a protective layer 400 against environmental influences, which is provided according to the application.

[0112] In a first variant, which is shown in Figure 2 c, d and e, in each case with reference to the left-hand mirror 101 or mirror / actuator segment 201, 301 shown, the reflective coating 212 is applied first Figure 2 c, left side). In the exemplary embodiment of the application here, the reflective coating 212 consists of a plurality of alternating layers of silicon and molybdenum, which are applied with the appropriate layer thicknesses by methods known for this purpose, in order to make the reflective surface 211 reflective for EUV radiation with a wavelength of 13.5 nm. For other wavelengths, the reflective surface 211 should be formed from other appropriate materials and / or layer configurations, if appropriate.

[0113] In this case, the surface in the area of the reflective surface 211 generally has a sufficiently high surface quality for the direct application of the reflective coating 212 thereon, since the silicon dioxide layer 202 with very low surface roughness has just been removed. Alternatively, the reflective surface 211 should be provided with an appropriate surface treatment step, for example chemical mechanical polishing, although this must be carried out before the removal of the release segment 210, and in this case, the surface to be polished should also be exposed before the removal of the release segment 210.

[0114] During the application of the coating 212, regions 312 with reflective properties can also be formed in the area of the actuator wafer 300. However, these regions 312 are not important for the subsequent use of the MEMS mirror array 100 and can remain. If these regions are still important, they can be removed or covered with a non-reflective material in a subsequent process step.

[0115] Optionally, and therefore only shown by dashed lines, a reflective coating protection layer 213 can be provided on the reflective coating 212 and protects the reflective coating 212 from damage in subsequent process steps. In particular, the reflective coating protection layer 213 can also help to remove layers subsequently applied on the reflective coating 212 again. The reflective coating protection layer 213 can be applied, for example, by sputtering, atomic layer deposition or chemical or physical vapor deposition.

[0116] After the application of the reflective coating 212 and the optional reflective coating protection layer 213, at least the surfaces of the atmosphere or vacuum inside the contact projection exposure apparatus 1 of the MEMS mirror array 100 during later use are covered with a protection layer 400 against environmental influences. In this case, the protection layer 400 must have a high uniformity and form an impermeable continuous layer even on difficult-to-access surfaces. To achieve this, the protection layer 400 can be applied, for example, by atomic layer deposition or chemical vapor deposition.

[0117] If the protection layer 400 against environmental influences is intended to prevent hydrogen-induced outgassing, in particular electrically insulating substances such as aluminum oxide (AI x O y ) or titanium oxide (Ti x O y ) are suitable for this purpose. However, electrically conductive substances or multilayer constructions are also possible for the protection layer 400 against environmental influences.

[0118] From Figure 2 d the left side it can be seen directly that the protection layer 400 against environmental influences also extends over the reflective coating 212. If the protection layer 400 here is completely transmissive for the light that can in principle be reflected by the reflective coating 212, i.e. in the present case EUV light having a wavelength of 13.5 nm, the protection layer 400 against environmental influences can remain on the reflective coating 212. In this case, the reflective coating protection layer 213 is usually omitted. In this case, the production method ends with Figure 2 d.

[0119] Alternatively, the protection layer 400 against environmental influences can be removed from the reflective coating 212, for which purpose the reflective coating protection layer 213 can generally prove useful for protecting the reflective coating 212 from damage during the removal of the protection layer 400 against environmental influences. The reflective coating protection layer 213 should ultimately also be removed, thus resulting in the final state shown on the left side in Figure 2 e.

[0120] Instead of the procedure in which the reflective coating 212 is applied first and the protection layer 400 against environmental influences is applied only later, this procedure is explained with reference to the left side in Figure 2 c, d and e, in each case with reference to the mirror 101 or mirror / actuator segment 201, 301 shown on the left side, it is also possible for the sequence to be reversed, which is explained below with reference to the mirror 101 or mirror / actuator segment 201, 301 shown on the right side in each case in Figure 2 c, d and e.

[0121] From Figure 2Starting from the state shown in b, at least the surface of the MEMS mirror array 100 inside the contact projection exposure device 1, which is exposed to the atmosphere or vacuum, is directly covered by a protective layer 400 that resists environmental influences during subsequent use (see [reference]). Figure 2 (c right side). For the required properties and possible application methods of protective layer 400, please refer to the preceding explanation.

[0122] The protective layer 400, which resists environmental influences, is in principle suitable for allowing the reflective coating 212 to be applied directly thereon. For this purpose, the protective layer 400 must have sufficient surface quality and strength, which can be provided directly or ensured by appropriate post-treatment of the protective layer 400 in the area of ​​the subsequent reflective surface 102.

[0123] If the protective layer 400, which resists environmental influences, is not suitable for allowing the application of the reflective coating 212, or if it cannot ensure the necessary electrical connection between the reflective coating and the reflector body 205, the protective layer 400 should be removed from the area of ​​the reflective surface 102. Figure 2 The dashed line diagram on the right side of d, indicating the protective layer 400, points out that this is precisely the reason 102. To facilitate the removal of the protective layer 400 from the area of ​​the reflective surface 102, a temporary layer (not shown) can also be provided below the environmentally resistant protective layer 400. This temporary layer protects the mirror body 205 from damage during separation of the area of ​​the protective layer 400 and, in particular, can help obtain or maintain a high surface quality. In this case, the temporary layer can be applied before or after the removal of the release segment 210.

[0124] Finally, the reflective coating 212 is applied to the area of ​​the reflective surface 102, which can be applied directly to the previously uncovered reflective mirror body 205 or to the protective layer 400 that remains there to resist environmental influences.

[0125] According to the reflective coating 212, it is desirable or necessary to electrically connect this reflective coating to the mirror body 205. If a protective layer 400 resisting environmental influences remains between the reflective coating 212 and the mirror body 205, and if this protective layer is non-conductive, then, for example... Figure 3 As shown on the right side of e, suitable vias 401 are provided through the protective layer 400. These vias can be formed at appropriate times in the manufacturing process, such as by plasma etching, ion milling, ion or electropolishing, laser ablation, or selective atomic layer deposition etching. Alternatively, it is conceivable that, in principle, the conductivity of the insulating protective layer 400 can be made sufficient by appropriate post-processing, such as by introducing ions through radiation, directional deposition, or incandescence, for example by reducing the resistivity to 1 to 1000 MΩ × m. 2 / m.

[0126] Figure 2This illustrates a second exemplary embodiment of the manufacturing method according to the present invention. In this case, the various structural features of the manufactured MEMS mirror array 100 and the various method steps herein are similar to those described above. Figure 3 The methods and steps are as follows. Therefore, the following focus is on... Figure 2 The specific features of the production method shown may also be found in the above-mentioned... Figure 3 The explanation, especially more specific details about the materials and processes used.

[0127] At the beginning of this method ( Figure 2 a) Provides both a mirror chip 200 and an actuator chip 300. Here, the actuator chip 300 and... Figure 2 The same configuration as a is used, therefore refer to the explanation regarding this. Most of the mirror chip 200 also uses a similar configuration. Figure 2 The configuration is as follows, particularly regarding the reflector body 205 and the functional structures 206 and 208 linked to it.

[0128] However, the rear side of the reflector body 205 is not covered and is selectively (and therefore only shown by dashed lines) provided with a reflective coating 212 in this stage.

[0129] Because the reflector wafer 200 lacks a structural support layer 203 composed of silicon (see...) Figure 2 Alternatively, the layer may have been removed, thus providing only a fairly thin material bridge 214 in the release segment 210. If the material bridge 214 cannot ensure sufficient integrity of the mirror body 205, however, selectively, the area adjacent to it, as shown by the dashed line, may be composed of silicon, particularly monocrystalline silicon and / or polycrystalline silicon, thus imparting significantly higher stability to the mirror wafer 200.

[0130] As related Figure 3 As explained, the mirror chip 200 and the actuator chip 300 are joined together with high precision, so that the two chips 200, 300 are fixedly connected to each other by means of the coating 207, 307 provided theretherein.

[0131] If the reflective coating 212 is not already present, it should be provided later after bonding. A photoresist layer 215 should be disposed thereon, and it protects the reflective coating 212 during subsequent processing steps, particularly during possible etching. Alternatively, a reflective coating protection layer 213 may be provided between the reflective coating 212 and the photoresist layer 215, and it can protect the reflective coating 212 during subsequent processing steps, particularly by removing other layers disposed on the reflective coating protection layer 213, and a suitable configuration may also be used as an etching stop layer in subsequent processing steps.

[0132] Subsequently, the material in at least the release section 210 is removed, for example by a vertical etching process and others, without the need to even disturb the silicon (see Figure 3 c). The silicon dioxide layers 209, 309 act as an etch stop for this purpose. These silicon dioxide layers 209, 309 can also be removed subsequently. However, this is not necessary, for which reason the layers in question are shown in Figure 3 c by dashed lines.

[0133] According to a method variant, the photoresist layer 215 and / or the reflective coating protection layer 213 can be removed subsequently as required. However, this is not absolutely necessary in all method variants, for which reason the layers 213, 215 in question are shown in Figure 3 d by dashed lines.

[0134] A protection layer 400 against environmental influences is then applied to all surface regions of the MEMS mirror array 100, which, during subsequent use, is in contact with the atmosphere or vacuum inside the projection exposure apparatus 1 (see Figure 3 d). The protection layer 400 here can be applied on the silicon dioxide layers 209, 309, or, in the case where the latter have been removed previously, directly on the underlying structure, and, depending on the preceding process steps, on the reflective coating 212, the photoresist layer 215 and / or the reflective coating protection layer 213.

[0135] If the protection layer 400 against environmental influences is transmissive for the light to be reflected by the reflective coating 212 and is intended to remain permanently on the latter, the method ends in the state shown in Figure 3 d, wherein in this case the photoresist layer 215 and / or the reflective coating protection layer 213 should generally be removed before the application of the protection layer 400 against environmental influences.

[0136] If it is desired to remove the protection layer 400 against environmental influences in the region of the reflective coating 212, for example because of insufficient light transmission, this is done by appropriate process steps. In this process, layers that can remain, such as the photoresist layer 215 and / or the reflective coating protection layer 213, can also be removed, so that, as shown in Figure 4 e, the reflective surface 102 is formed directly by the reflective coating 212.

[0137] Figure 2 A third exemplary embodiment of a production method for a MEMS mirror array 100 according to the application is shown. In this case, the steps of the method and the materials and structures used are similar to Figure 3 and Figure 4 For this purpose, reference is made in principle to the preceding explanations, and only the special features of the method according to Figure 4 are explained below.

[0138] As shown in Figure 2As shown in Fig. 2a, the mirror wafer 200 and the actuator wafer 300 are in principle configured in a similar way as Figure 2 , in particular Figure 2 in Fig. 2a, even if additional silicon material is provided on the mirror wafer 200, the mirror wafer thereby obtains a flat surface on both sides.

[0139] However, compared to the special features of the embodiment according to Figure 4 , the embodiment according to Fig. 2a is characterized in that the provided silicon dioxide layer 202 is implemented in the present case as an environmental protection layer 400, that is to say, in particular, composed of a material suitable for this purpose. At the same time, this environmental protection layer 400 must also serve as an etch stop layer for subsequent process steps, so that the environmental protection layer 400 and the associated etching processes must be coordinated with one another. Alternatively, a combination of the environmental protection layer 400 and the etch stop layer (for example, composed of silicon dioxide) can also be provided as a multilayer.

[0140] After the mirror wafer 200 and the actuator wafer 300 have been joined together, the coating 207, 307 is firmly bonded to one another, by etching away a layer of single-crystal silicon 203, the silicon dioxide layer 202 forms an etch stop layer (cf. Figure 4 b).

[0141] Subsequently, at least in the region remote from the mirror segments 201, that is to say, in particular, in the release segments 210, the silicon dioxide layer 202 is removed (cf. Figure 4 c left). Alternatively, the silicon dioxide layer 202 can also be completely removed, and an environmental protection layer 400 can be applied in the mirror segments 201 (cf. Figure 4 c right). In this case, the type of environmental protection layer 400 applied in this step can be the same as the type of the internal environmental protection layer 400 in the mirror wafer 200 and the actuator wafer 300. However, it is also possible to apply an environmental protection layer 400 which differs from the material properties of the internal environmental protection layer 400. In this regard, the environmental protection layer 400 to be applied can be, for example, electrically conductive.

[0142] Subsequently, the unwanted silicon is removed from the respective gaps by an etching process. In this case, as previously mentioned, the environmental protection layer 400 serves as an etch stop layer (cf. Figure 4 d).

[0143] If the silicon dioxide layer 202 is still partially retained (cf. Figure 4 d left), it should now be removed in order to subsequently apply the reflective layer 212 to the reflective surface 102 of the mirror 101.

[0144] If the mirror body 105 is completely surrounded by the environmental protection layer 400 (cf. Figure 4The protective layer 400 in the area of ​​the reflective surface 102 (d on the right side), especially if it is composed of an electrically insulating material, may be provided with through holes 401 as needed or its conductivity may be increased by appropriate treatment.

[0145] Finally, the reflective layer 212 can then be applied to the protective layer 400 that resists environmental influences. Figure 5 (e. right side). If the protective layer 400 previously applied to the area of ​​the reflective surface 102 is removed before applying the reflective layer 212, this will produce the following: Figures 2 to 4 The structure shown on the left side of e.

[0146] Figure 4 A fourth exemplary embodiment of the production method according to the present invention is shown. Here, reference is also made to the relevant provisions. Figure 5 Therefore, the following description can focus on the specific features of this fourth exemplary embodiment.

[0147] Given the configuration of the mirror chip 200 and the actuator chip 300, most of them are related to... Figure 5 The configuration is the same. Specifically, the various inner layers set as etch stop layers are directly implemented as protective layers 400 to resist environmental influences. In this case, the protective layer 400 to resist environmental influences can be directly used as an etch stop layer. Alternatively, a combination of protective layer 400 and etch stop layer (e.g., composed of silicon dioxide) can also be provided as a multilayer.

[0148] These areas of the reflector body 205 that subsequently serve as the reflecting surface 102 are either uncovered or have a reflective coating 212 applied (therefore) Figure 5 (A is indicated only by dashed lines). In particular, if a reflective coating 212 is provided, a reflective coating protective layer 213 (also indicated by dashed lines) may be disposed on it.

[0149] To further increase the structural integrity of the mirror chip 200 or to enable better mechanical handling and gripping of the chip, an additional silicon layer 203 may be provided on the side opposite to the connecting post 206.

[0150] After the mirror chip 200 and the actuator chip 300 are joined together ( Figure 5 (b) A reflective coating 212 should be provided in all cases. Depending on the configuration of the mirror wafer 200 during its provision, the reflective coating 212 may also be applied, or the silicon layer 203 may be removed from the existing reflective coating 212. Whether to provide a reflective coating protective layer 213 after removing the silicon layer 203 or to retain the reflective coating protective layer is, in principle, optional, but generally advantageous.

[0151] Only in the reflective mirror section 201, the photoresist layer 215 is applied ( Figure 5c) to the reflective coating 212 or a reflective coating protection layer 213, which can be provided thereon, and to protect the reflective coating 212 within the mirror segment 201 during a subsequent etching process, in which unwanted silicon is removed ​ d).

[0152] Finally, residues of the photoresist layer 215 and possibly of the reflective coating protection layer 213 are also removed, thus resulting in the state shown in Fig. e and the MEMS mirror array 100 produced according to the application. ​ e.

[0153] The foregoing exemplary embodiments of the method for producing a MEMS mirror array 100 according to the application are not comprehensive and exhaustive. In particular, during the process, various additional steps can also be provided, for example for surface treatment, in order to further improve the reliability, accuracy and reflective properties of the MEMS mirror array 100 produced. However, the steps common to the individual exemplary embodiments in principle form the basic framework of the production method according to the application.

Claims

1. A method for producing a MEMS mirror array (100) for use in photolithography, the MEMS mirror array having a predetermined number of individual mirrors (101) adjustable in at least one degree of freedom, the method comprising the following steps: a) providing a mirror wafer (200) comprising a plurality of mirror segments (201) separated from one another by means of release segments (210), the number of said mirror segments corresponding to the number of adjustable mirrors (101); b) providing an actuator wafer (300) comprising a plurality of actuator segments (301) corresponding to the plurality of adjustable mirrors (101), wherein said actuator segments (301) are spaced apart from one another in accordance with said mirror segments (201) of the mirror wafer (200) and individual said actuator segments (301) have at least one functional structure (302, 308); c) joining the mirror wafer (200) and the actuator wafer (300) together such that mirror segments (201) and actuator segments (301) are fixedly connected to one another in each case in a defined region; and d) removing at least said release segments (210) of the mirror wafer (200) such that individual said mirror segments (201) are enabled to be adjusted in each case relative to the respective actuator segment (301) by using at least one degree of freedom of at least one portion of said functional structures (206, 302, 308); wherein at least one of the preceding steps is preceded or followed by at least a region of the mirror wafer (200) and / or the actuator wafer (300) being provided with a protective layer (400) against environmental influences in order to protect the underlying material from hydrogen-induced outgassing.

2. The method of claim 1, wherein, The protective layer (400) against environmental influences is provided on all surfaces of the completed MEMS mirror array (100) which are susceptible to expected hydrogen-induced outgassing.

3. The method of any of the preceding claims, wherein, At least one portion of the protective layer (400) against environmental influences is applied after the removal of said release segments (210).

4. The method of any of the preceding claims, wherein, At least one portion of the protective layer (400) against environmental influences has been provided on or integrated into the mirror wafer (200) and / or the actuator wafer (300) as provided.

5. The method of any of the preceding claims, wherein, The protective layer (400) against environmental influences is an etch stop layer in the case of at least some possible etching processes, in particular when said release segments (210) are removed.

6. The method of any of the preceding claims, wherein, Outer surfaces of said mirror segments (301) form the reflective surface (102) at the end of the method and are protected from damage by the application of suitable layers.

7. The method of any of the preceding claims, wherein, At least one reflective coating (212) is applied in the region provided as the reflective surface (102) before or after at least one of the preceding steps and before or after the application of the protective layer (400) against environmental influences, wherein the reflective coating (212) reflects light of at least one predefined wavelength.

8. The method of claim 7, wherein, Before or after the reflective coating (212) in the region provided for this purpose, at least partially removing the protective layer (400) against environmental influences present or subsequently applied in said region.

9. The method of claim 7 or 8, wherein, Before or after the reflective coating (212) in a predefined region, increasing the electrical conductivity of the protective layer (400) against environmental influences within said region, preferably by introducing vias (401) or reducing the specific resistance.

10. The method of any of the preceding claims, wherein, The protective layer (400) against environmental influences transmits light of at least one predetermined wavelength, preferably light of a wavelength of 13.5 nm.

11. The method of any of the preceding claims, wherein, The mirror (101) is adjustable in two degrees of freedom, preferably in two rotational degrees of freedom extending perpendicular to each other.

12. A MEMS mirror array (100) for use in photolithography, having a predetermined number of individual mirrors (101) adjustable in at least one degree of freedom, characterized in that The MEMS mirror array is produced according to the method of any of the preceding claims.

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