Method for stabilizing adhesive connections in optical assemblies, optical assembly, and projection exposure apparatus for semiconductor lithography

Abstract

The present invention relates to a method for thermally stabilizing an adhesive connection (37) between two components (Mx, 117, 33, 38, 34) of an optical assembly (30.1, 30.2, 30.3), the method comprising the steps of forming an adhesive connection (37) between the components (Mx, 117, 33, 38, 34), curing an adhesive (36), and subjecting the adhesive (36) to a heat treatment to increase the degree of cure and the temperature of the glass transition region of the adhesive (36). The present invention also relates to an optical assembly (30.1, 30.2, 30.3) comprising an optical element (Mx, 117) and a solid-state actuator (38) connected to the optical element (Mx, 117) by an adhesive connection (37), wherein a bias voltage is applied to the solid-state actuator (38) so that the optical element (Mx, 117) is in a zero position.

Description

[Technical Field]

[0001] This application claims priority from German Patent Application Publication No. 10 2022 208 206.0 of August 8, 2022, the content of which is incorporated herein by reference in its entirety.

[0002] The present invention relates to a method for stabilizing adhesive connections in optical assemblies, in particular in projection exposure apparatus for semiconductor lithography.The present invention also relates to an optical assembly and to a projection exposure apparatus for semiconductor lithography. [Background technology]

[0003] Such devices are used in particular for producing extremely fine structures on semiconductor components or other microstructured components. The operating principle of these devices is based on the production of very fine structures, down to the nanometer range, by using a so-called reticle to image, generally in reduced size, the structure on the mask onto the element to be structured, a so-called wafer, which is provided with a photosensitive material. The minimum dimensions of the structures produced depend directly on the wavelength of the light used. Recently, light sources with emission wavelengths in the range of a few nanometers, for example 1 nm to 120 nm, in particular around 13.5 nm, have increasingly been used. The wavelength range described is also referred to as the EUV range.

[0004] In addition to EUV systems, microstructured components are also fabricated using commercially established DUV systems with wavelengths between 100 nm and 400 nm, particularly 193 nm. The introduction of the EUV range, and thus the resulting miniaturization of structures, further increases the demand for optical correction in DUV systems with a wavelength of 193 nm. Furthermore, to increase profitability, throughput also increases with each generation of projection exposure tools, regardless of wavelength. This typically leads to higher thermal loads and therefore thermally induced imaging aberrations. To correct imaging aberrations, manipulators can be used, particularly those that change the position and alignment of optical elements or affect the imaging properties of optical elements, particularly mirrors, by deforming their optically active surfaces. Manipulators typically include actuators and sensors, which are often connected to the optical elements via adhesives. The adhesive has a glass transition region corresponding to the temperature range in which the adhesive's behavior transitions from purely elastic, through at least partial plasticity, to fully plasticity.

[0005] However, temperatures in the glass transition region are periodically reached during the manufacture of the corresponding optical assemblies and their subsequent operation in the device, and if temperatures in the glass transition region are reached during such processes, undesirable effects can occur in the corresponding adhesive connections. Summary of the Invention [Problem to be solved by the invention]

[0006] SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and apparatus that overcomes the above-mentioned drawbacks of the prior art. [Means for solving the problem]

[0007] This object is achieved by a method and an apparatus having the features of the independent claims. The dependent claims relate to advantageous developments and variants of the invention.

[0008] A method according to the present invention for thermally stabilizing an adhesive connection between two components of an optical assembly comprises: forming adhesive connections between the components; allowing the adhesive to cure; subjecting the adhesive to a heat treatment to increase the degree of cure and the temperature of the glass transition region of the adhesive; Includes:

[0009] The measure according to the invention of subjecting an already cured adhesive to a heat treatment and the associated increase in the glass transition temperature have the effect that the adhesive connection exhibits high thermal robustness during further processing of the optical assembly, and in particular does not begin to exhibit mechanical drift, i.e., does not begin to creep, even when subjected to external forces. The heat treatment process begins, in particular at room temperature, after the adhesive has cured and consists in heating the adhesive one or more times above room temperature in a controlled manner. The desired increase in the glass transition temperature of the adhesive in this regard depends on various parameters, in particular the temperature change over time, the maximum temperature, further ambient conditions, and, of course, also on the type of adhesive used.

[0010] During the heat treatment process itself, the heated adhesive is expected to assume a stress-relieved state or undergo plastic deformation of the adhesive under the action of the forces, thus changing the mechanical parameters of the adhesive connection relative to the state before the heat treatment process. This can lead to undesirable effects, in particular a change in the alignment of the two joined components relative to each other or deformation of the components in the area of the adhesive connection after the heat treatment. It has therefore been found to be advantageous if, during the heat treatment of the adhesive, at least one of the components is influenced in such a way as to reduce the introduction of forces by that component into the adhesive.

[0011] Thus, for example, the effect of gravity on one of the components during the heat treatment process and the resulting temporary weakening of the adhesive connection may cause a mechanical drift of these components in the direction of gravity. In this case, the introduction of forces into the adhesive due to the acting gravity can be reduced by providing or otherwise supporting the corresponding component with a holder. The holder can be designed passively or actively to compensate for the different thermal expansion coefficients.

[0012] However, the force may also be a force resulting from a temperature change of at least one of the components.

[0013] Thus, for example, different thermal expansion coefficients of one of the components and the adhesive can cause stresses to be generated along the joining surface and in the adhesive itself due to temperature changes, which are relieved during the heat treatment process, thus having the effect of changing the mechanical condition of the joint relative to its state before the heat treatment process.

[0014] This situation is especially likely whenever one of the components is an optical element and the other of the components is a solid-state actuator, for example an electrostrictive or piezo actuator.

[0015] In this case, temperature changes have an effect in at least two different ways. First, temperature changes lead to different thermal expansions of the adhesive, the optical element, and the actuator, in a known manner, which can cause undesirable stresses during the heat treatment process. Second, as the temperature changes, the actuation behavior of electrostrictive or piezo actuators in particular also changes, resulting in a change in the deflection of the actuator due to the temperature dependence of the electrostrictive or piezoelectric effect, while the applied voltage remains unchanged.

[0016] These issues can be addressed by actuating the solid-state actuators during the heat treatment process with a variable voltage, so that the above effects can be compensated for by proper electrical actuation of the actuators, resulting in a reduced introduction of external forces into the adhesive connection during the heat treatment process.

[0017] There are various options for connecting an actuator to the body of an optical element, in particular a mirror of a projection exposure apparatus for semiconductor lithography. Thus, for example, in a first variant, the actuator can be connected to the mirror in a non-deflected state, i.e., without any applied voltage. In this state, the region of the actuator on the optically effective surface of the mirror deviates from the so-called zero position assumed in conventional operation. Thus, for example, in this case, the region of the actuator can have a concave partial region on the optically effective surface. The zero position of the effective surface of the optical element is reached by applying a default voltage to the actuator, causing the actuator to expand, thereby nullifying the concave region. Local deformation of the optically effective surface near the zero position, i.e., both in the convex and concave directions, can be achieved by applying a voltage to the actuator that exceeds or falls short of the default voltage. In this case, it is advantageous for the thermal treatment process if the voltage at the start of the thermal treatment process corresponds to the voltage of the non-deflected state of the actuator, since the actuator exerts no or only a small force on the adhesive connection.

[0018] However, there is also the option of initially manufacturing the mirror so that it assumes a zero position without the action of the actuator on the optically effective surface. In this case, the actuator can be connected to the mirror in a deflected state, i.e., under the action of a voltage. The desired deformation of the optically effective surface around the zero position can also be achieved by varying the voltage applied to the actuator. For the above variants, the introduction of force by the actuator into the adhesive connection is minimized as long as a specific control voltage is applied to the actuator, i.e., as long as the actuator has already been deflected a certain amount. In this case, it is advantageous if the voltage at the start of the heat treatment process corresponds to the voltage applied to the actuator during adhesive bonding of the actuator to the mirror in a deflected state of the actuator, in particular in a state corresponding to the zero position of the optically effective surface.

[0019] In particular, a laser beam specifically directed at the adhesive connection can be used in the heat treatment process.

[0020] By this measure, only the adhesive connection is heated during the heat treatment process, thereby reducing or avoiding the above-mentioned adverse thermal effects on the surrounding components.

[0021] The optical assembly according to the present invention comprises an optical element and a solid-state actuator connected to the optical element by an adhesive connection, and a bias voltage is applied to the solid-state actuator so that the optical element is in a zero position. In other words, when no voltage is applied, the optical element is in a deformed state in a specific region or in a spatial alignment that deviates from the zero position of the optical element. The zero position of the optical element should be understood to mean the shape or alignment that the optical element would have if it did not perform local correction of the wavefront and only contributed to the imaging of the reticle.

[0022] In an advantageous variant of the invention, the bias voltage is selected so that the local deformations or changes in alignment around the zero position required for the optically corrective effect of the optically active surface can be achieved without changing the polarity of the voltage applied to the solid-state actuators, which avoids polarity changes during operation, making it possible to avoid particularly disadvantageous hysteresis effects.

[0023] The solid state actuator may in particular be an electrostrictive actuator, for example a piezo actuator.

[0024] The bias voltage may be in the range of 30% to 70%, particularly 40% to 60%, of the maximum voltage of the actuator, the maximum voltage corresponding to the voltage at which the maximum displacement of the actuator is reached.

[0025] Assuming that the maximum movement of the actuator reaches a voltage of about 100V, the bias voltage can be selected to be, for example, 30V to 70V, particularly 40V to 60V.

[0026] In an advantageous variant of the invention, the actuation direction of the actuator extends perpendicular to the optically effective surface of the optical element, which makes it particularly easy to set, for example, a desired local deformation of the optically effective surface.

[0027] In a variant of the invention, during application of a bias voltage, the mechanical stress acting on the adhesive connection can be reduced compared to voltages deviating from the bias voltage. This can be achieved, for example, by providing an adhesive connection during application of a bias voltage to the actuator. In this way, in the zero position of the optical element, the mechanical stress in the adhesive connection is also reduced. Given that the zero position is the main state during operation of the associated projection exposure apparatus, and furthermore, only a small deflection of the actuator is required to correct the optically active surface, such a setting can be advantageous, since it reduces the mechanical stress in the adhesive connection as a whole.

[0028] A projection exposure apparatus according to the invention comprises an optical assembly as described above and is characterized by improved imaging properties.

[0029] Exemplary embodiments and variants of the invention are explained in more detail below on the basis of the drawings. [Brief explanation of the drawings]

[0030] [Figure 1] 1 shows a schematic meridian section of a projection exposure apparatus for EUV projection lithography; [Figure 2] 1 shows a schematic meridian section of a projection exposure apparatus for DUV projection lithography; [Figure 3] 1 shows an optical assembly known from the prior art; [Figure 4a] 2 shows yet another optical assembly known from the prior art; [Figure 4b] 2 shows yet another optical assembly known from the prior art; [Figure 5] 1 shows a diagram illustrating the method according to the invention; [Figure 6] 3 shows yet another diagram illustrating the method according to the invention; [Figure 7] 1 shows a flow diagram of a manufacturing method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0031] In the following text, the essential components of a microlithography projection exposure apparatus 1 will be described exemplarily, initially with reference to Figure 1. The description of the basic structure of the projection exposure apparatus 1 and its components is to be understood here as non-limiting.

[0032] In addition to the radiation source 3, an embodiment of the illumination system 2 of the projection exposure apparatus 1 comprises an illumination optical unit 4 which illuminates an object field 5 in an object plane 6. In an alternative embodiment, the light source 3 can also be provided as a module separate from the rest of the illumination system. In this case, the illumination system does not include the light source 3.

[0033] 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 is displaceable, in particular in the scanning direction, by a reticle displacement drive 9.

[0034] For illustrative purposes, Figure 1 shows a Cartesian xyz coordinate system. The x direction extends perpendicular to the plane of the drawing. The y direction extends horizontally and the z direction extends vertically. In Figure 1, the scanning direction extends in the y direction. The z direction extends perpendicular to the object plane 6.

[0035] The projection exposure apparatus 1 comprises a projection optical unit 10. The projection optical unit 10 serves to image the object field 5 into an image field 11 in an image plane 12. The image plane 12 extends 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.

[0036] The structures on the reticle 7 are imaged onto a photosensitive layer of a wafer 13 arranged in the region of the image field 11 of the image plane 12. The wafer 13 is held by a wafer holder 14. The wafer holder 14 is displaceable, in particular in the y direction, by a wafer displacement drive 15. The displacement of the reticle 7 by the reticle displacement drive 9 on the one hand and the displacement of the wafer 13 by the wafer displacement drive 15 on the other hand can be performed synchronously with respect to one another.

[0037] The radiation source 3 is an EUV radiation source. The radiation source 3 in particular emits EUV radiation 16, also referred to in the following as working radiation, illumination radiation or illumination light. In particular, the working radiation has a wavelength in the range of 5 nm to 30 nm. The radiation source 3 may be a plasma source, such as a laser-produced plasma (LPP) source or a gas discharge plasma (GDPP) source. It may also be a synchrotron-based radiation source. The radiation source 3 may be a free electron laser (FEL).

[0038] Illumination radiation 16 leaving radiation source 3 is focused by collector 17. Collector 17 may be a collector with one or more ellipsoidal and / or hyperbolic reflecting surfaces. Illumination radiation 16 may be incident on at least one reflecting surface of collector 17 at grazing incidence (GI), i.e. at an angle of incidence greater than 45° relative to the direction of the normal to the mirror surface, or at normal incidence (NI), i.e. at an angle of incidence smaller than 45°. Collector 17 may be structured and / or coated, firstly to optimize its reflectivity for the radiation used and secondly to suppress extraneous light.

[0039] Downstream of the collector 17, the illumination radiation 16 propagates through an intermediate focus in an intermediate focal plane 18. The intermediate focal plane 18 may be a separation point between the radiation source module, which includes the radiation source 3 and the collector 17, and the illumination optics unit 4.

[0040] The illumination optical unit 4 comprises a deflection mirror 19 and a first facet mirror 20 arranged downstream thereof in the beam path. The deflection mirror 19 can be a plane deflection mirror or a mirror with a beam-influencing effect beyond a pure deflection effect. Alternatively or additionally, the deflection mirror 19 can be in the form of a spectral filter that separates the used optical wavelength of the illumination radiation 16 from extraneous light of wavelengths outside of it. If the first facet mirror 20 is arranged in a plane of the illumination optical unit 4 that is optically conjugate with the object plane 6 as a field plane, this facet mirror is also referred to as a field facet mirror. The first facet mirror 20 comprises a plurality of individual first facets 21, also referred to as field facets in the following. FIG. 1 shows only some of these facets 21 by way of example.

[0041] The first facet 21 may be in the form of a macroscopic facet, in particular a rectangular facet, or a facet with an arcuate or part-circular edge profile. The first facet 21 may be a planar facet or in the form of a convexly or concavely curved facet.

[0042] The first facet 21 itself can also consist of a number of individual mirrors, in particular a number of micromirrors, as is known, for example, from DE 10 2008 009 600. The first facet mirror 20 can in particular be in the form of a microelectromechanical system (MEMS system). See DE 10 2008 009 600 for further details.

[0043] Between the collector 17 and the deflection mirror 19, the illumination radiation 16 travels horizontally, i.e. in the y direction.

[0044] A second facet mirror 22 is arranged downstream of the first facet mirror 20 in the beam path of the illumination optical unit 4. If the second facet mirror 22 is arranged in the pupil plane of the illumination optical unit 4, it is also referred to as a pupil facet mirror. The second facet mirror 22 can also be arranged away from the pupil plane of the illumination optical unit 4. In this case, the combination of the first facet mirror 20 and the second facet mirror 22 is also referred to as a specular reflector. Specular reflectors are known from US Patent Application Publication No. 2006 / 0132747, EP 1 614 008 and US 6,573,978.

[0045] The second facet mirror 22 includes a plurality of second facets 23. In the case of a pupil facet mirror, the second facets 23 are also referred to as pupil facets.

[0046] The second facet 23 may likewise be a macroscopic facet which may have, for example, a circular, rectangular or hexagonal boundary, or may be a facet made up of a micromirror. In this respect, reference is also made to DE 10 2008 009 600 A1.

[0047] The second facet 23 may have a flat reflecting surface or a convexly or concavely curved reflecting surface.

[0048] The illumination optical unit 4 therefore forms a double-faceted system, this basic principle also being called a fly's eye integrator.

[0049] It may be advantageous not to arrange the second facet mirror 22 exactly in a plane that is optically conjugate with the pupil plane of the projection optical unit 10. In particular, the pupil facet mirror 22 may be arranged at an angle with respect to the pupil plane of the projection optical unit 10, as described, for example, in DE 10 2017 220 586 A1.

[0050] The individual first facets 21 are imaged into the object field 5 by means of a second facet mirror 22. The second facet mirror 22 is the last beam-shaping mirror in the beam path upstream of the object field 5 or indeed the final mirror for the illumination radiation 16.

[0051] In a further embodiment, not shown, of the illumination optical unit 4, a transfer optical unit, which in particular contributes to the imaging of the first facet 21 into the object field 5, can be arranged in the beam path between the second facet mirror 22 and the object field 5. The transfer optical unit can have exactly one mirror or two or more mirrors arranged one after the other in the beam path of the illumination optical unit 4. 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).

[0052] In the embodiment shown in FIG. 1, downstream of the collector 17 the illumination optical unit 4 comprises exactly three mirrors, in particular a deflection mirror 19 , a field facet mirror 20 and a pupil facet mirror 22 .

[0053] In yet another embodiment of the illumination optical unit 4, the deflection mirror 19 can also be omitted, so that the illumination optical unit 4 then has exactly two mirrors downstream of the collector 17, specifically a first facet mirror 20 and a second facet mirror 22.

[0054] The imaging of the first facet 21 onto the object plane 6 by the second facet 23 or by means of the second facet 23 and the transfer optical unit is generally only an approximate imaging.

[0055] The projection optical unit 10 comprises a number of mirrors Mi, which are consecutively numbered according to their location in the beam path of the projection exposure apparatus 1 .

[0056] 1, the projection optical unit 10 includes six mirrors M1 to M6. 4, 8, 10, 12 or any other number of mirrors Mi are equally possible. The penultimate mirror M5 and the final mirror M6 each have a passage aperture for the illumination radiation 16. The projection optical unit 10 is a double-shielded optical unit. The projection optical unit 10 has an image-side numerical aperture that is greater than 0.5, may be greater than 0.6, and may be, for example, 0.7 or 0.75.

[0057] The reflective surface of the mirror Mi can be embodied as a freeform surface without an axis of rotational symmetry. Alternatively, the reflective surface of the mirror Mi can be designed as an aspheric surface, with exactly one axis of rotational symmetry of the reflective surface shape. Like the mirrors of the illumination optical unit 4, the mirror Mi can have a coating that is highly reflective with respect to the illumination radiation 16. These coatings can take the form of multilayer coatings, in particular with alternating layers of molybdenum and silicon.

[0058] Projection optical unit 10 has a large object-image offset in the y direction between the y coordinate of the center of object field 5 and the y coordinate of the center of image field 11. This object-image offset in the y direction can be approximately as large as the z distance between object plane 6 and image plane 12.

[0059] In particular, the projection optical unit 10 can have an anamorphic configuration. In particular, it has different imaging scales βx, βy in the x and y directions. The two imaging scales βx, βy of the projection optical unit 10 are preferably (βx, βy)=(+ / -0.25, + / -0.125). A positive imaging scale β means imaging without image inversion. A negative sign of the imaging scale β means imaging with image inversion.

[0060] The projection optical unit 10 results in a size reduction in the x-direction, ie perpendicular to the scanning direction, by a ratio of 4:1.

[0061] The projection optical unit 10 provides a size reduction of 8:1 in the y-direction, ie the scan direction.

[0062] Other imaging scales are possible as well, including imaging scales of the same sign and magnitude in the x and y directions, for example 0.125 or 0.25.

[0063] 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 may be the same or may differ depending on the design of the projection optical unit 10. An example of a projection optical unit with a different number of such intermediate images in the x and y directions is known from US Patent Application Publication No. 2018 / 0074303.

[0064] Each pupil facet 23 is assigned to exactly one of the field facets 21 in order to form a respective illumination channel that illuminates the object field 5. In particular, this makes it possible to obtain illumination according to the Köhler principle. The far field is decomposed into a plurality of object fields 5 by means of the field facets 21. The field facets 21 generate a plurality of images of intermediate foci on the respectively assigned pupil facets 23.

[0065] The field facets 21 are each imaged onto the reticle 7 in an overlapping manner by means of the assigned pupil facets 23 in order to illuminate the object field 5. The illumination of the object field 5 is in particular as uniform as possible. Its uniformity error is preferably less than 2%. By overlapping the different illumination channels, field uniformity can be achieved.

[0066] The illumination of the entrance pupil of the projection optical unit 10 can be geometrically defined by the arrangement of the pupil facets. By selecting the illumination channels, and in particular the subset of pupil facets, that direct light, the intensity distribution at the entrance pupil of the projection optical unit 10 can be set. This intensity distribution is also referred to as the illumination setting.

[0067] A similarly favorable pupil uniformity in the region of a defined illuminated portion of the illumination pupil of the illumination optical unit 4 can be achieved by redistribution of the illumination channels.

[0068] Further aspects and details of the illumination of the object field 5, in particular the entrance pupil of the projection optical unit 10, are explained below.

[0069] The projection optical unit 10 may in particular have a concentric entrance pupil, which may be accessible, or which may be inaccessible.

[0070] The entrance pupil of the projection optical unit 10 generally cannot be illuminated exactly using the pupil facet mirror 22. In the case of imaging of the projection optical unit 10 telecentrically imaging the center of the pupil facet mirror 22 onto the wafer 13, the aperture rays often do not intersect at a single point. However, it is possible to find a surface at which the determined separation between pairs of aperture rays is minimal. This surface represents the entrance pupil or a surface conjugate to it in real space. In particular, this surface exhibits a finite curvature.

[0071] The projection optical unit 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 transfer optical unit, should be provided between the second facet mirror 22 and the reticle 7. This optical element can be used to take into account the different positions of the tangential and sagittal entrance pupils.

[0072] 1 of the components of the illumination optical unit 4, the pupil facet mirror 22 is arranged in a plane conjugate with the entrance pupil of the projection optical unit 10. The field facet mirror 20 is arranged so as to be inclined with respect to the object plane 6. The first facet mirror 20 is arranged so as to be inclined with respect to the arrangement plane defined by the deflection mirror 19.

[0073] The first facet mirror 20 is disposed so as to be inclined with respect to the disposition plane defined by the second facet mirror 22 .

[0074] FIG. 2 shows diagrammatically in meridian section a further projection exposure apparatus 101 for DUV projection lithography in which the invention can also be used.

[0075] The structure and imaging principle of the projection exposure apparatus 101 are identical to those described in connection with Fig. 1. Identical components are designated by reference numerals which are 100 higher than those in Fig. 1, i.e. the reference numerals in Fig. 2 start with 101.

[0076] 1, since the wavelength of the DUV radiation 116 used as the light is large, in the range of 100 nm to 300 nm, in particular about 193 nm, refractive, diffractive and / or reflective optical elements 117, such as lens elements, mirrors, prisms, end plates, etc., can be used for imaging or illumination in the DUV projection exposure apparatus 101. The projection exposure apparatus 101 essentially comprises in this case an illumination system 102, a reticle holder 108 for receiving and precisely positioning a reticle 107 provided with structures and determining the subsequent structures on a wafer 113, a wafer holder 114 for holding, moving and precisely positioning said wafer 113, and a projection lens 110 with a number of optical elements 117, which are held by mounts 118 in a lens housing 119 of the projection lens 110.

[0077] Illumination system 102 provides the DUV radiation 116 required for imaging reticle 107 onto wafer 113. A laser, plasma source, etc. can be used as the source of this radiation 116. The radiation 116 is shaped by optics in illumination system 102 so that upon incidence on reticle 107, the DUV radiation 116 has desired properties with respect to diameter, polarization, wavefront shape, etc.

[0078] Besides the additional use of refractive optical elements 117 such as lens elements, prisms, end plates, etc., the configuration of the downstream projection optical unit 101 with the lens housing 119 is essentially the same as that described in FIG. 1 and therefore will not be described in further detail.

[0079] 3 shows an optical assembly 30.1 known from the prior art, which can be used in one of the projection exposure apparatuses 1, 110 shown in FIGS. 1 and 2. This assembly comprises, as components, a mirror Mx, 117 and a sensor element in the form of a reference mirror 33 for an interferometer (not shown). The mirror Mx, 117 comprises an optically effective surface 32 that reflects the radiation used by the projection exposure apparatus 1, 101 and a side surface 40 adjacent to the optically effective surface. In the embodiment shown in FIG. 3, the reference mirror 33 is connected to the mirror side surface 40 by an adhesive connection 37 with adhesive 36. Heating above the glass transition region of the adhesive 36 in further method steps for producing the optical assembly 30.1 or during operation of the optical assembly 30.1 can lead to plastic deformation of the adhesive 36, which, together with a weight G acting on the reference mirror 33, can displace the reference mirror 33, as shown by the dashed line in FIG. 3.

[0080] To mechanically stabilize the adhesive connection 37 for the subsequent method steps, it is subjected to a heat treatment in accordance with the method according to the invention. The adhesive 36 of the adhesive connection 37 is heated in a controlled manner, for example by a directed laser beam (not shown in Fig. 3), so as to raise the glass transition region of the adhesive 36, i.e., the temperature range in which the adhesive begins to exhibit glass-like mechanical properties. The heat treatment is expediently carried out so that the glass transition region of the adhesive 36 after the heat treatment step is above temperatures such as 40°C that occur in further method steps, or at least exceeds the lower limit of the glass transition region by a few Kelvin.

[0081] An advantage of heating the adhesive 36 with a laser is that the associated localized heating that can occur at the mirror Mx, 117 and the reference mirror 33 introduces less heat, so that changes in the position and alignment of the reference mirror 33 relative to the mirror Mx, 117 as a result of thermal expansion of the components involved are kept to a minimum or can be avoided altogether.

[0082] In order to avoid changes in the position and / or alignment of the reference mirror 33 relative to the mirror Mx, 117 as a result of plastic deformation of the adhesive 36 during the above heat treatment step, the reference mirror 33 is held in a predetermined target position relative to the mirror Mx, 117 by a holder 39, so that it is possible to compensate in particular for the effects of gravity. In case of heating of the entire optical assembly, it may be necessary to readjust the holder 39 to maintain the predetermined target position of the reference mirror 33 during the heat treatment in order to compensate for the different thermal expansions of the reference mirror 33, the mirror Mx, 117 and the adhesive 36.

[0083] FIG. 4a shows an optical assembly 30.2 known from the prior art, which can be used in one of the projection exposure apparatuses shown in FIGS. 1 and 2. It comprises as its components a mirror Mx, 117, an actuator 38, and a backplate 34. The mirror Mx, 117 has an optically active surface 32 and a mirror backside 31 located opposite the optically active surface 32. The actuator 38 is connected by adhesive 36 to the mirror backside 31 on one side and to a contact surface 35 of the backplate 34 facing the mirror backside 31 of the mirror Mx, 117 on the other side. The actuator 38 is supported relative to the backplate 34, and deflection of the actuator 38 causes deformation of the mirror Mx, 117. The actuator 38, which in the embodiment shown in FIG. 4a is in the form of a piezo actuator, can be supplied with a bias voltage by an actuation controller (not shown) during the adhesive bonding process, so that the actuator 38 is adhesively bonded to the mirror backside 31 already in a partially deflected state. The mirror Mx, 117 is in this case in a zero position during adhesive bonding to the actuator 38, which is already partially deflected. In this zero position, the optically effective surface 32 of the mirror Mx, 117 is designed to correspond to a target optically effective surface predetermined by the optical design. From this zero position, in order to compensate for imaging aberrations, the mirror Mx, 117 can be deflected in two directions by exceeding or not reaching the bias voltage applied to the actuator 38 during operation of the projection exposure apparatus 1, 101 shown in Figures 1 and 2. In this case, the actuator 38 does not introduce any stress into the adhesive 36 at the zero position of the optically effective surface 32.

[0084] FIG. 4b shows yet another embodiment of an optical assembly 30.3 known from the prior art, which can be used in one of the projection exposure apparatuses shown in FIGS. 1 and 2. In contrast to the optical assembly 30.2 shown in FIG. 4a, the optical assembly 30.3 shown in FIG. 4b does not have a backplate 34. The actuator 38, also in the form of a piezo-electric actuator 38, is connected to the mirror back side 31 opposite the optically effective surface 32 by adhesive 36 in the form of an adhesive connection 37. The actuator 38 utilizes the effect of the piezoelectric material, i.e., it expands or contracts when deflected in a first strain direction perpendicular thereto. Via the adhesive connection 37, the force generated thereby is transmitted to the mirror back side 31, thereby deforming the mirror Mx, 117 and thus the optically effective surface 32. As already explained with reference to FIG. 4a, the actuator 38 can be adhesively bonded to the mirror back side 31 by applying a bias voltage supplied by an actuation controller, which has the effect of allowing the deformation of the optically effective surface 32 to be similarly set, as already explained with reference to FIG. 4a.

[0085] As an alternative to applying a bias voltage to the actuator 38 during adhesive bonding of the actuator 38 to the mirror backside 31, the deformation of the optically effective surface 32 caused by the deflection of the actuator 38 can be maintained during the formation of the optically effective surface 32.

[0086] In other words, the mirror Mx, 117 is manufactured in such a way that the optically effective surface 32 deviates from the zero position in various regions when the actuator is not present or activated. In that case, the zero position of the optically effective surface 32 is set during operation of the corresponding device by corresponding activation of the actuator 38. In this case, mechanical stresses act on the adhesive connection 37 even during zero setting of the optically effective surface. These mechanical stresses are at least partially relieved during the heat treatment process. This results in a stress ratio before the heat treatment being different from the stress ratio after the heat treatment process.

[0087] 5 illustrates a heat treatment method according to the present invention that can stabilize the adhesive bond 37 of an optical assembly. The glass transition region of the adhesive 36 is raised by a post-adhesive heat treatment process so that the glass transition region begins at a temperature higher than the processing temperature of the subsequent method, or so that the relaxation of mechanical stresses in the adhesive 36 during the subsequent method is reduced to an acceptable level.

[0088] In a first method step 41, the component 38 is connected to the optical element Mx, 117 by means of adhesive 36.

[0089] In a second method step 42, the adhesive 36 is allowed to cure.

[0090] In a third method step 43, the adhesive 36 is subjected to a heat treatment to increase the degree of cure and glass transition region of the adhesive 36.

[0091] As already mentioned, the heat treatment process is problematic in that even if it is started without the actuator 38 exerting any external force on the adhesive connection 37, undesirable forces develop as the temperature increases.

[0092] One of the underlying mechanisms is illustrated by Figure 6, which shows a schematic diagram of the deflection s of the actuator 38 versus the applied voltage U at different temperatures. In the individual curves recorded at different temperatures T1, T2, T3, T4, and T5, it is clear that the relationship between the applied voltage U and the deflection s depends clearly on the temperature T. T1 corresponds to the lowest current ambient temperature, and T5 corresponds to the highest current ambient temperature, and the diagram is normalized to the ratio at temperature T1.

[0093] Figure 6 clearly shows that as the temperature increases the dependence of the actuator expansion s on the voltage U decreases. Figure 6 also clearly shows the effect of thermal expansion of the actuator 38 with increasing temperature.

[0094] According to the invention, the voltage applied to the actuator 38 is adapted based on the respective current temperature during the heat treatment process, according to the exemplary diagram of Fig. 7. The selected relationship between temperature T and voltage U ensures that stresses arising in the adhesive connection 37 as a result of thermal effects, i.e., on the one hand due to the decreasing sensitivity of the actuator to voltage changes as the temperature increases, and on the other hand due to the thermal expansion of the actuator itself, are minimized, and therefore that minimal or no relaxation processes of the adhesive connection 37 occur during the heat treatment process.

[0095] The voltage variation with temperature required for the desired effect can be determined by experiment or simulation, particularly by finite element methods. [Explanation of symbols]

[0096] 1. Projection exposure equipment 2. Lighting system 3 Radiation source 4. Illumination optical unit 5 Object field of view 6 Object plane 7 Reticle 8 Reticle Holder 9 Reticle Displacement Drive 10 Projection optical unit 11 Image field 12 Image plane 13 wafers 14 wafer holder 15 Wafer Displacement Drive 16 EUV radiation 17 Collector 18 Intermediate focal plane 19 Deflecting mirror 20 Faceted Mirror 21 Facets 22 Faceted Mirror 23 Facets 30.1~30.3 Optical Assembly 31 Behind the Mirror 32 Optically effective surface 33 Reference Mirror 34 Backplate 35 Backplate contact surface 36 Adhesive 37 Adhesive Connection 38 Actuator 39 Holder 40 Side 41 Method Steps 42 Method Steps 43 Method Steps 101 Projection exposure equipment 102 Lighting System 107 Reticle 108 Reticle Holder 110 Projection optical unit 113 wafers 114 Wafer holder 116 DUV radiation 117 Optical Elements 118 Mount 119 Lens Housing M1~M6 mirrors

Claims

1. 1. A method for thermally stabilizing an adhesive connection (37) between two components (Mx, 117, 33, 38, 34) of an optical assembly (30.1, 30.2, 30.3), comprising: forming said adhesive connections (37) between said components (Mx, 117, 33, 38, 34); allowing the adhesive (36) to cure; subjecting the adhesive (36) to a heat treatment to increase the degree of cure and the temperature of the glass transition region of the adhesive (36); A method comprising:

2. 10. The method of claim 1, 10. A method comprising: influencing at least one of said components (Mx, 117, 33, 38, 34) during a thermal treatment of said adhesive (36) in such a way that the introduction of forces by said component (Mx, 117, 33, 38, 34) into said adhesive (36) is reduced.

3. 3. The method of claim 2, The method, characterized in that the force is a force caused by a temperature change of the component (Mx, 117, 33, 38, 34).

4. The method according to any one of claims 1 to 3, A method characterized in that one of said components is an optical element (Mx, 117).

5. The device according to any one of claims 1 to 4, The method of claim 1, wherein one of the components is a solid-state actuator (38).

6. 6. The method of claim 5, The method of claim 1, wherein the solid-state actuator (38) is actuated with a variable voltage during the heat treatment process.

7. 7. The method of claim 6, The method of claim 1, wherein the voltage at the start of the heat treatment process corresponds to the voltage in the undeflected state of the actuator (38).

8. 7. The method of claim 6, The method of claim 1, wherein the voltage at the start of the heat treatment process corresponds to the voltage at the deflected state of the actuator (38).

9. The method according to any one of claims 1 to 8, A method characterized in that a laser beam specifically directed at the adhesive connection (37) is used in the heat treatment process.

10. The method according to any one of claims 1 to 9, A method, characterized in that it is carried out on a component of a projection exposure apparatus for semiconductor lithography (1, 101).

11. An optical assembly (30.1, 30.2, 30.3) comprising an optical element (Mx, 117) and a solid-state actuator (38) connected to said optical element (Mx, 117) by an adhesive connection (37), An optical assembly, characterized in that a bias voltage is applied to said solid-state actuator (38) so that said optical element (Mx, 117) is in a zero position.

12. 12. An optical assembly (30.1, 30.2, 30.3) according to claim 11, The bias voltage is selected so that the local deformation required for the optical correction effect of the optically effective surface (32) near the zero position can be achieved without changing the polarity of the voltage applied to the solid-state actuator (38).

13. 13. An optical assembly (30.1, 30.2, 30.3) according to claim 11 or 12, An optical assembly, wherein the solid-state actuator (38) is an electrostrictive actuator.

14. 14. An optical assembly (30.1, 30.2, 30.3) according to claim 13, An optical assembly, characterized in that the solid-state actuator (38) is a piezo actuator.

15. An optical assembly (30.1, 30.2, 30.3) according to any one of claims 11 to 14, An optical assembly, characterized in that the bias voltage is in the range of 30% to 70%, in particular 40% to 60%, of the maximum voltage of the actuator.

16. An optical assembly (30.1, 30.2, 30.3) according to any one of claims 11 to 15, An optical assembly, characterized in that the actuation direction of the actuator (38) extends perpendicular to the optically effective surface (32) of the optical element (Mx, 117).

17. An optical assembly (30.1, 30.2, 30.3) according to any one of claims 11 to 16, An optical assembly, characterized in that during application of said bias voltage, the adhesive connection (37) is subjected to reduced mechanical stress compared to voltages deviating from said bias voltage.

18. A projection exposure apparatus (1, 101) for semiconductor lithography, comprising: A projection exposure apparatus comprising an optical assembly according to any one of claims 11 to 17.

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