Manufacturing methods and measurement methods

Abstract

A method for measuring the surface shape of an optical element in a predetermined measurement environment, wherein the body of the optical element has a substrate and a reflective surface, and wherein at least one cooling channel for receiving coolant is formed in the substrate, the method comprising the steps of: a) recording the cooling channel pressure, b) recording the measurement environment pressure, c) determining an actual pressure difference based on the cooling channel pressure and the measurement environment pressure, d) comparing the actual pressure difference with a predetermined target pressure difference, e) monitoring the deviation between the actual pressure difference and the target pressure difference, wherein if a deviation greater than a predetermined limit is detected, the cooling channel pressure is adjusted such that the deviation is less than or equal to the predetermined limit, and f) if the deviation is less than or equal to the predetermined limit, the surface shape is measured.

Description

Technical Field

[0001] The present invention relates to a method for measuring the surface shape of an optical element in a measurement environment, wherein the body of the optical element has a substrate and a reflective surface, and wherein at least one cooling channel for receiving coolant is formed in the substrate.

[0002] The present invention also relates to a measuring apparatus for measuring the surface shape of an optical element, a method for manufacturing an optical element, and a projection exposure apparatus. Background Technology

[0003] Microlithography is used to fabricate microstructured components such as integrated circuits or LCDs (liquid crystal displays). The microlithography process is performed in a projection exposure apparatus, which includes an illumination device and a projection lens. In this case, an image of a mask (=mask master) illuminated by the illumination device is projected through the projection lens onto a substrate (e.g., a silicon wafer) coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection lens, so as to transfer the mask structure onto the photosensitive coating of the substrate.

[0004] In projection lenses designed for the EUV (Extreme Ultraviolet) range (i.e., at wavelengths of approximately 13 nm or approximately 7 nm), mirrors are used as optical elements in the imaging process due to the lack of suitable refractive materials for light transmission. Typical projection lenses designed for EUV, such as those known from US 2016 / 0085061 A1, can have, for example, an image-side numerical aperture (NA) in a region of NA = 0.55 and image an object field (e.g., in annular shape) onto an image plane or wafer plane. An increase in the image-side numerical aperture (NA) typically accompanies an increase in the required mirror area of ​​the mirrors used in projection exposure equipment. This, in turn, means that not only fabricating, but also inspecting or measuring the surface shape of the optical element is a significant challenge. The deviation between the actual shape of the surface of the optical element to be determined and a predetermined target shape is commonly referred to as the "shape." Interferometry methods are typically used for high-precision shape measurement or measurement of the surface shape of optical elements.

[0005] It is also known that during the operation of an optical system (e.g., a projection exposure apparatus using such optical elements), particularly during EUV operation, the optical elements are heated due to the absorption of the light used, especially EUV light used during operation. This heating of the optical elements specifically leads to problems of thermal deformation, such as expansion, and thus the optical performance of the system using the optical elements no longer corresponds to predetermined specifications.

[0006] To prevent optical elements from heating up, cooling concepts have been developed to dissipate the heat generated by the optical elements during operation. Known cooling concepts particularly involve forming at least one cooling channel for receiving coolant within the solid body or substrate of the optical element. Optical elements having at least one such cooling channel are known from WO 2012 / 126830A1, US 7,591,561 B2, DE 10 2018 208 783 A1, DE 10 2010 034 476A1, WO 2009 / 046955A2, DE 10 2017 221 388 A1, and DE 10 2018 202 687 A1.

[0007] The challenge of optical elements with at least one such cooling channel is the high-precision interferometry measurement of the optical element.

[0008] The problem is that, typically when optical elements are used under typical EUV conditions, especially under vacuum conditions, a pressure differential arises due to ambient pressure and cooling channel pressure, which differs from the pressure differential generated during measurement under the specified conditions or during measurement under conditions deviating from EUV. These different pressure differentials then lead to undesirable deformations, particularly on the surfaces of the optical elements, when transitioning from EUV conditions to measurement conditions deviating from EUV, or vice versa. Consequently, the surface shape measured under these conditions differs from the surface shape formed during EUV operation. The undesirable effect of this deformation is that the optical elements, especially during EUV operation of the optical system, no longer meet or are no longer able to meet predetermined specifications. For example, the wavefront may be undesirably affected by deformation, or stray light may be generated in an undesirable manner, affecting the imaging quality of the projection lens.

[0009] Another problem is that the substrate material and, in particular, the typically air-filled cooling channels have different refractive indices. When measuring the surface shape with measuring light, the different refractive indices cause undesirable back reflections of the measuring light, especially at the interface between the substrate and the cooling channels.

[0010] In view of the above background, an object of the present invention is to provide methods and measuring apparatus by which the aforementioned problems can be solved, particularly enabling reliable and high-precision measurement of the surface shape of an optical element having at least one cooling channel. Another object is to provide a method for manufacturing an optical element that can be reliably and with high precision, and to provide such an optical element.

[0011] This objective is achieved according to the features of the independent claim. Summary of the Invention

[0012] According to the present invention, a method for measuring the surface shape of an optical element is carried out through the following steps: a) recording the cooling channel pressure, b) recording the measurement environment pressure, c) determining the actual pressure difference based on the cooling channel pressure and the measurement environment pressure, d) comparing the actual pressure difference with a predetermined target pressure difference, e) monitoring the deviation between the actual pressure difference and the target pressure difference, wherein if a deviation greater than a predetermined limit value is detected, the cooling channel pressure is adjusted such that the deviation is less than or equal to the predetermined limit value, f) if the deviation is less than or equal to the predetermined limit value, the surface shape is measured. The method according to the present invention has the advantage that the actual pressure difference can be adjusted or regulated to the target pressure difference in a particularly simple manner (especially by adjusting only one parameter, i.e., the cooling channel pressure). Therefore, a target surface shape formed under any predetermined target pressure difference can be generated particularly easily by corresponding adjustment of the actual pressure difference, and then it can be measured. In this example, "measurement environment" means an environment in which a preferably predetermined measurement environment pressure exists and in which the optical element is measured and / or operated.

[0013] According to the development example, the cooling channel pressure is adjusted such that the deviation is less than 10 mbar, particularly less than 1 mbar, and preferably less than 0.5 mbar. The advantage here is that the actual pressure difference is adjusted or regulated to the target pressure difference with exceptional precision. The measured surface shape, or the surface shape formed under the actual pressure difference, therefore corresponds with exceptional precision to the surface shape formed under the target pressure difference. Preferably, the cooling channel pressure is adjusted such that the deviation is equal to zero.

[0014] According to the development example, gaseous or liquid coolant is fed into the cooling channel, where the pressure on the coolant is increased or decreased to adjust the cooling channel pressure. The advantage here is that the cooling channel pressure is adjusted or adjustable in a particularly simple manner by increasing or decreasing the pressure of the coolant itself.

[0015] According to the development example, the pressure on the coolant is increased or decreased by hydraulic or pneumatic means. This has the advantage of making it particularly easy to adjust the pressure in the cooling channels. A controllable hydraulic or pneumatic pump is preferably used to hydraulically or pneumatically increase or decrease the pressure on the coolant. Alternatively, adjustment is made electronically by a controllable electric pump. The increase or decrease in pressure depends particularly on the increase or decrease in the delivery rate of the corresponding pump, for example, by adjusting the speed or delivery rate.

[0016] According to the development example, the coolant flows through the cooling channel at a predetermined flow rate. This predetermined flow rate is preferably at least substantially the same as the flow rate of the coolant through the cooling channel, particularly during the operation of the EUV lithography equipment under EUV conditions. This ensures that measurements related to the flow rate are performed under EUV conditions. Additionally or alternatively, the flow rate is preferably selected based on the geometry or cross-section of the cooling channel. Preferably, the flow rate is selected such that laminar flow is formed in the cooling channel. Therefore, critical pressure losses due to turbulent flow and vibration or oscillation of the resulting optical elements are avoided. Furthermore, the dynamic viscosity of the coolant is preferably selected or set to be at least substantially the same as that of water, particularly at least 0.89 mPa·s and at most 1.52 mPa·s.

[0017] According to the development example, the target pressure difference is determined based on a predetermined target measurement ambient pressure and a predetermined target cooling channel pressure, wherein the target measurement ambient pressure is at least 0.01 mbar and at most 0.20 mbar, and the target cooling channel pressure is at least 200 mbar and at most 10,000 mbar. The advantage here is that when the optical element is used or operated, particularly under EUV conditions, multiple target pressure differences exist or can exist based on a predetermined or selectable target measurement ambient pressure and target cooling channel pressure that are determinable or settable. The surface shape to be measured, or the surface shape formed under the actual pressure difference, therefore corresponds to the surface shape formed, particularly under EUV conditions. Alternatively, the target measurement ambient pressure is preferably at least 0.01 mbar and at most 1,000 mbar. Preferably, the target measurement ambient pressure is 1,000 mbar, and the target cooling channel pressure is at least 1,200 mbar and at most 10,000 mbar. In this example, the measured surface shape, or the surface shape formed under the actual pressure difference, corresponds to the surface shape formed, particularly under atmospheric pressure conditions.

[0018] According to the development example, the target measurement ambient pressure can be predetermined to be at least 0.03 mbar and at most 0.1 mbar, and the target cooling channel pressure to be at least 500 mbar and at most 1000 mbar. The advantage here is that the target pressure differential is determined based on a particularly tightly selected range of target measurement ambient pressure and target cooling channel pressure. Specifically, the target pressure differential selected from this range corresponds to the pressure differential typically present when using optical elements or operating under EUV conditions.

[0019] According to the development example, the target measurement ambient pressure can be predetermined as 0.05 mbar, and the target cooling channel pressure can be predetermined as 500 mbar. The advantage is that a precisely defined target pressure difference is predetermined based on this target measurement ambient pressure and the target cooling channel pressure. In particular, this precisely defined target pressure difference corresponds to the pressure difference typically present when the optical element is used or operated under EUV conditions. The measured surface shape, or the surface shape formed under the actual pressure difference, therefore corresponds particularly accurately to the surface shape formed under EUV conditions.

[0020] The present invention also relates to a measuring apparatus for measuring the surface shape of an optical element, wherein the body of the optical element has a substrate and a reflective surface, and wherein at least one cooling channel for receiving coolant is formed in the substrate, wherein the measuring apparatus comprises: i) a measuring light source; ii) an interferometer, with which at least a portion of the surface of the optical element can be inspected by the interference superposition of a test wave and a reference wave, the test wave being obtained from measuring light generated by the measuring light source and directed onto the optical element; iii) at least one controllable coolant reservoir for storing coolant; and iv) a control device designed to perform the method as described in any one of claims 1 to 8 when used as intended. This provides the advantages already mentioned. Other advantages and preferred features will be apparent from the foregoing description and claims.

[0021] According to the development examples of measuring equipment, the refractive index of the coolant is at least substantially equal to the refractive index of the substrate of the optical element to be inspected. This yields the advantage of particularly accurate measurement or measurability of the surface shape of the optical element, especially minimizing destructive reflections. The appropriate selection of the coolant's refractive index prevents undesirable back reflections of the measurement light, particularly at the interface between the substrate and the cooling channel, caused by the refractive index difference. In particular, interference between the measurement light reflected at reflective surfaces and undesirable back reflections is thus avoided. Furthermore, the cooling channel, particularly the walls of the cooling channel, may optionally have a predetermined roughness to ensure diffuse scattering. Diffuse scattering reduces destructive reflections. Preferably, the dynamic viscosity of the coolant is at least substantially the same as that of water, particularly at least 0.89 mPa·s and at most 1.52 mPa·s.

[0022] According to the development example, the coolant is a solution of an inorganic or organic substance in water. The advantage here is that the refractive index can be set variably, particularly depending on the predetermined concentration of the substance. This substance could be, for example, sugar or potassium iodide.

[0023] According to the development example, the substance forms a homogeneous phase when mixed with water. "Homogeneous" here means that the distribution of the substance in the water is the same at every point. This specifically ensures that the refractive index of the coolant is the same at every point in the cooling channels into which the coolant is fed.

[0024] The present invention also relates to a method for manufacturing an optical element, wherein the body of the optical element has a substrate and a reflective surface, and wherein at least one cooling channel for receiving coolant is formed in the substrate, and wherein said cooling channel is manufactured by a cutting manufacturing process (particularly drilling) and / or by an etching process. The substrate and the reflective surface are particularly integrally formed.

[0025] According to the invention, the reflector has a reflective surface, wherein the substrate and the reflector with the reflective surface are joined together by a bonding process (particularly adhesive bonding). In this case, cooling channels are formed in the substrate, particularly by etching, grinding, and / or milling. According to these methods, the substrate and the reflective surface are not integrally formed. The reflector and the substrate are preferably made of the same material. Alternatively, cooling channels are formed in the reflector with the reflective surface, particularly by grinding, milling, and / or etching. In this case, the substrate is preferably at least partially polished, such that the substrate and the reflector with the reflective surface can be advantageously joined together or are connectable to each other by a bonding process.

[0026] According to the invention, a bonding process is performed such that the reflective surface and the boundary layer formed between the reflective surface and the substrate as a result of the bonding process are at least partially misaligned with each other. "Misaligned" means that a first tangent plane at a predetermined point on the reflective surface and a second tangent plane at a predetermined point on the boundary layer are not parallel to each other. In other words, the deviation between the normal vectors of the first and second tangent planes is greater than zero. The predetermined points on the reflective surface and the predetermined points on the boundary layer are arranged along a straight line, which is aligned parallel to the optical axis of the optical element. The advantage here is that the superposition of the measurement light reflected at the reflective surface and the measurement light reflected at the boundary layer is particularly effectively avoided. Therefore, the reflection angle of the measurement beam incident on the boundary layer will be different from the reflection angle of the measurement beam incident on the reflective surface. This is particularly advantageous if the refractive index of the formed boundary layer is different from the refractive index of the substrate or the mirror having the reflective surface. The reflective surface and the boundary layer can each be flat, i.e., without curvature, or have curvature.

[0027] According to a development example, a layer designed to reflect light with wavelengths of at least 193 nm and at most 633 nm, particularly at least 532 nm and at most 633 nm, is at least partially applied to the reflective surface. This has the advantage of enabling high-precision measurement of optical elements using measurement light, i.e., light with wavelengths of at least 193 nm and at most 633 nm. Furthermore, this layer or measurement layer prevents the measurement beam from reaching or being able to reach the boundary layer formed between the reflective surface and the substrate due to the bonding process. This layer preferably has at least one silicon layer and / or at least one chromium layer. Advantageously, the reflective surface can be processed by ion beam shaping.

[0028] According to an example, the substrate includes a material formed such that it absorbs light of a predetermined wavelength, particularly wavelengths of at least 193 nm and at most 633 nm, particularly at least 532 nm and at most 633 nm. The substrate material is preferably doped with an absorption-enhancing material. Preferably, the substrate and the reflective surface are integrally formed. If the substrate and the mirror having the reflective surface are bonded together, the mirror preferably comprises a material designed to absorb light of a predetermined wavelength.

[0029] Furthermore, the present invention relates to a projection exposure apparatus for semiconductor photolithography, comprising: i) an illumination device; ii) a projection lens; and iii) at least one optical element, the main body of which has a substrate and a reflective surface, wherein at least one cooling channel for receiving coolant is formed in the substrate. The projection exposure apparatus is characterized in that the optical element is manufactured by the method of any one of claims 13 to 15. Attached Figure Description

[0030] The invention will be explained in more detail below with reference to the accompanying drawings. In this respect:

[0031] Figure 1 A schematic diagram of a measuring device according to an exemplary embodiment is shown.

[0032] Figure 2 A schematic diagram of an optical element according to a first exemplary embodiment is shown.

[0033] Figure 3 A schematic diagram of an optical element according to a second exemplary embodiment is shown.

[0034] Figure 4 A schematic diagram of an optical element according to a third exemplary embodiment is shown.

[0035] Figure 5 A flowchart illustrating a method for measuring the surface shape of an optical element is shown, along with...

[0036] Figure 6 A schematic diagram of a projection exposure apparatus designed for operation in EUV is shown. Detailed Implementation

[0037] Figure 1 A schematic diagram of an interferometric measuring device 1 is shown for measuring the surface shape of an optical element 2, particularly a mirror 3, in a microlithography projection exposure apparatus. The measuring device 1 has at least one measuring light source 4 (not shown here), an interferometer 5, and a coolant reservoir 6.

[0038] The measurement light source 4 generates measurement light or measurement light radiation with one or more predetermined wavelengths, such as 193 nm, 532 nm, and / or 633 nm. The measurement light radiation enters the interferometer 5 from the exit surface of the optical waveguide 7 in the form of an input wave 8 having a spherical wavefront.

[0039] Interferometer 5 includes, but is not limited to, the beam splitter 9, a diffractive optical element 10 in the form of a particularly complex coded computer-generated hologram (CGH), three reflective elements 12, 13, and 14, the optical element 2 to be measured, and an interferometer camera 15. Interferometer 5 may optionally include fewer or more components than those described. Therefore, interferometer 5 may include fewer or more than three reflective elements 12, 13, and 14, or it may also include a measurement light source 4.

[0040] The measured light radiation, or input wave 8, passes through beam splitter 9 and is then incident on CGH 11. In a transmission manner, CGH 11 generates a total of four output waves from the input wave 8 according to its complex coding. One of these output waves is incident as a test wave on the surface of the optical element 2 to be measured, and the wavefront of the test wave is adapted to the target shape of the surface of the optical element 2.

[0041] According to this exemplary embodiment, CGH 11 generates three additional output waves from input wave 8 via transmission, each output wave incident on one of reflective elements 12, 13, and 14. In this exemplary embodiment, of these reflective elements 12, 13, and 14, elements 12 and 13 are each designed as plane mirrors, and reflective element 14 is designed as a spherical mirror. An optional shutter is indicated by reference numeral 16. CGH 11 is also used to superimpose a test wave reflected by the optical element 2 to be measured with a reference wave, which is reflected by reflective elements 12, 13, and 14 and incident again as a converging beam on beam splitter 9, and thus reflected in the direction of interferometer camera 15, wherein the test wave and reference wave pass through eyepiece 17. Interferometer camera 15 captures an interferogram generated by the interferometric waves, and an evaluation device (not shown) determines the actual shape or surface shape of optical element 2 from the interferogram.

[0042] In this example, the body 18 of the optical element 2 has a substrate 19 and a reflective surface 20, wherein at least one cooling channel 21 (not shown here) is formed in the substrate 19 for receiving a gaseous or liquid coolant 22. The material of the substrate 19 is, for example, a glass material such as quartz glass or a glass-ceramic material, such as that manufactured by Glaswerke Schott. Or manufactured by Corning (Ultra-low expansion) glass. At a wavelength of 546.1 nm, the refractive index of quartz glass is 1.45. The refractive index of the glass is 1.4828. Its refractive index is 1.5447.

[0043] The measuring device 1 has a coolant reservoir 6 for storing coolant 22. Preferably, the measuring device 1 additionally has a controllable delivery device 23, particularly a pump 24, connected to the coolant reservoir 6 for delivering coolant 22 from the coolant reservoir 6 and thus delivering coolant 22 into the cooling channel 21, and / or for pressurizing coolant 22. The delivery device 23 is preferably a hydraulic pump, a pneumatic pump, or an electric pump.

[0044] In this example, coolant 22 is supplied to the optical element 2, particularly to the cooling channel 21, via a supply line 25 connected to the coolant reservoir 6, and removed from the optical element 2, particularly from the cooling channel 21, via an discharge line 26 connected to the coolant reservoir. Coolant 22 is preferably returned to the coolant reservoir 6 via the discharge line 26 so that it can be transferred again from there. In this example, the supply line 25 and the discharge line 26 form a transfer line 27. Preferably, each of the supply line 25 and the discharge line 26 is connectable to the optical element 2, and is designed for detachable connection. The supply line 25 and / or the discharge line 26 may each be designed as a flexible tube with a predetermined diameter. The supply line 25 and the discharge line 26 are specifically designed such that oscillations or vibrations that may occur during the operation of the measuring device 1, particularly during the transfer of coolant 22 from the coolant reservoir 6, are attenuated or suppressed. Therefore, during the transfer of coolant 22 from coolant reservoir 6, optical element 2 is not affected by oscillation and vibration; in particular, optical element 2 itself is not excited to oscillate and vibrate.

[0045] To dampen vibrations, the supply line 25 is arranged, for example, such that it sags or is not taut: between the optical element 2 and the sensor 37 (particularly the flow sensor), or between the optical element 2 and the pressure recording device 35, or between the optical element 2 and the outlet side 32 of the pressure regulating device 28. If the supply line 25 is intended to be arranged such that it sags, for example, between the optical element 2 and the sensor 37, the length of the supply line 25, particularly the length of the supply line segment 88 between the optical element 2 and the sensor 37, is chosen to be greater than the distance between the optical element 2 and the sensor 37 to ensure sag. To dampen oscillations, the discharge line 26 is arranged, for example, such that it sags or is not taut: between the optical element 2 and the pressure recording device 36, or between the optical element 2 and the pressure regulating device 34.

[0046] Coolant 22 is preferably supplied from coolant reservoir 6, such that a pressure of at least 200 mbar and at most 10,000 mbar or cooling channel pressure is formed in at least one cooling channel 21. The pressure on coolant 22 is increased or decreased to increase or decrease the cooling channel pressure or to pressurize coolant 22. The pressure is preferably adjusted by regulating (i.e., by increasing or decreasing) the conveying rate of conveying device 23 (e.g., by adjusting the conveying speed of pump 24). The predetermined flow rate or volumetric flow rate of coolant 22 through conveying line 27, particularly cooling channel 21, is preferably also set by regulating the conveying rate of conveying device 23.

[0047] To ensure the settableness or predeterminability of the cooling channel pressure formed in the cooling channel 21, the supply line 25 preferably has at least one controllable pressure regulating device 28, particularly a two-way pressure regulating valve 29 or a three-way pressure regulating valve 30. The pressure regulating device 28 has an inlet side 31 assigned to the coolant reservoir 6 and an outlet side 32 assigned to the optical element 2 or the cooling channel 21. The two-way pressure regulating valve 29 and the three-way pressure regulating valve 30 are preferably each designed to convert the inlet side pressure into a predeterminable outlet side pressure. In particular, the three-way pressure regulating valve 30 is preferably designed to open if the predeterminable pressure on the inlet side 31 is exceeded, thus the pressure on the outlet side 32 is lower than the pressure on the inlet side 31. The three-way pressure regulating valve 30 preferably has an overflow outlet 33 connected to the coolant reservoir 6, such that if the predeterminable pressure on the inlet side 31 is exceeded, the overflow outlet 33 opens and coolant 22 can be discharged from the pressure regulating valve 28 and returned to the coolant reservoir 6. Optionally, the discharge line 26 has other controllable pressure regulating device 34. Optionally, the pressure regulating device 28 may be connected to or directly connected to the optical element 2, or the optical element 2 may have the pressure regulating device 28.

[0048] To record the pressure of the cooling channel, particularly the pressure of the coolant 22 in the supply line 25, a pressure recording device 35 (e.g., a pressure sensor or gauge) is preferably arranged between the pressure regulating device 28 and the optical element 2, particularly in the cooling channel 21 or on the inlet side of the cooling channel 21. Alternatively, other pressure recording devices 36 may be provided between the optical element 2 (particularly on the cooling channel 21 or the outlet side of the cooling channel 21) and other pressure regulating devices 34.

[0049] Sensor 37 (particularly a flow sensor) is preferably arranged between pressure regulating device 28 and optical element 2 (particularly cooling channel 21 or the inlet side of cooling channel 21) for recording the flow velocity or volumetric flow rate of coolant 22 in cooling channel 21. Alternatively, the flow velocity is determined by the pressure recorded by pressure recording device 35 and other pressure recording devices 36.

[0050] For temperature control, particularly for cooling or heating the coolant 22, the measuring device 1 optionally includes a temperature control device 83 connected to the coolant reservoir 6. Because the dynamic viscosity of the coolant 22 depends on temperature and pressure, the temperature of the coolant 22 is preferably controlled such that the dynamic viscosity of the coolant 22 corresponds to a predetermined dynamic viscosity, particularly the dynamic viscosity of water, preferably at least 0.891 mPa·s and at most 1.52 mPa·s. Alternatively, the temperature of the coolant 22 is controlled such that the coolant temperature is at least substantially equal to a predetermined temperature, such as the operating temperature of an EUV lithography apparatus. Additionally or alternatively, the pressure on the coolant 22 is adjusted to change the dynamic viscosity. For recording the temperature of the coolant 22, the measuring device 1 or the temperature control device 83 preferably includes a temperature sensor.

[0051] According to this exemplary embodiment, the measuring device 1 is arranged in a housing 40, which surrounds an internal space 38 or a measuring environment 39, particularly a vacuum chamber 41. At least one controllable vacuum generation unit 42 (e.g., a vacuum pump) is assigned to the housing 40 for generating a vacuum in the internal space 38 or the measuring environment 39. The vacuum generation unit 42 is preferably designed to generate a vacuum in the housing 40 with a total pressure or measuring environment pressure of at least 0.01 mbar, particularly less than 0.01 mbar and at most 0.1 mbar. This ensures that the surface shape of the optical element 2 is, or can be, measured at a measuring environment pressure corresponding to the pressure present when the optical element 2 is used in an EUV operating vacuum (particularly when used in conjunction with an EUV lithography apparatus). Alternatively, the surface shape is measured at the pressure predominant at the measurement location, particularly at 1 bar, or at any predetermined measuring environment pressure. The measuring device 1 preferably has a pressure sensor 43 for recording the measuring environment pressure.

[0052] Alternatively, the measuring device 1 is not arranged in the housing 40 or the vacuum chamber 41. In this example, the surface shape is measured under atmospheric pressure, particularly atmospheric pressure. The measurement environment can be the internal space 38 or another measurement environment, particularly one not enclosed by a housing.

[0053] Furthermore, the measuring device 1 has a control device 44. The control device 44 is specifically configured to control, in particular, at least one of the transmission device 23 and / or pressure regulating valves 29, 30, especially pressure regulating valve 29. In this example, but not limited thereto, the control device 44 is connected, in particular via a wired or wireless data cable, to the transmission device 23, to at least one of the pressure regulating devices 28, 34, pressure sensor 43, at least one of the pressure recording devices 35, 36, flow sensor 37, and preferably to the measuring light source 4.

[0054] Preferably, the refractive index of the coolant 22 is at least substantially equal to the refractive index of the substrate 19 or substrate material of the optical element 2 to be measured. This ensures that the optical element 2 can be measured in a particularly advantageous manner because undesirable back reflections at the interface between the substrate 19 and the cooling channel 21 are minimized. The coolant 22 preferably contains an inorganic or organic substance, which is preferably miscible with water, particularly forming a homogeneous phase when mixed with water. Preferably, the coolant 22 has a very low or very high vapor pressure. This particularly ensures that the coolant 22 remains liquid under low pressure or vacuum pressure conditions. If the coolant 22 is carried into a vacuum system, the coolant 22 can therefore be easily or effortlessly removed from the cooling channel 21. The substance or coolant 22 is preferably not a hazardous substance, i.e., it can be safely handled and disposed of in an environmentally friendly manner.

[0055] The substance is, for example, sugar, particularly a solution of 79% by weight sucrose with water. This sucrose solution is preferably prepared by dissolving the sugar in water at at least 70°C and then cooling. This results in a refractive index of 1.483 at a temperature of 20°C, where this refractive index at least substantially corresponds to... The refractive index of the glass. Alternatively, the substance is potassium iodide, wherein a refractive index of at least 1.33 (0 percent potassium iodide) and at most 1.502 (saturated potassium iodide solution) can be determined by a predetermined proportion of potassium iodide in water. Alternatively, coolant 22 is glycerol (refractive index: 1.474).

[0056] Alternatively, coolant 22 is a sodium polytungstate solution, wherein a refractive index of at least 1.33 (0 percent sodium polytungstate) and at most 1.55 (saturated sodium polytungstate solution) is set, depending on a predetermined proportion of sodium polytungstate in the solution. Alternatively, the following oils or organic substances are provided as coolant 22: tetrahydronaphthalene (refractive index: 1.541), methyl salicylate (refractive index: 1.535), or eugenol (refractive index: 1.541). These refractive indices at least substantially correspond to, in particular... The refractive index.

[0057] Since refractive index is a function of wavelength and temperature, the temperature of the measurement environment 39 and the wavelength of the measured light radiation are preferably considered when selecting coolant 22 or its refractive index. If the measurement light source 4 is designed to emit measurement light radiation of different wavelengths, such as 532 nm and 633 nm, a first refractive index and a second refractive index of substrate 19 are determined for the corresponding wavelengths, and an average refractive index is formed therefrom. Based on the formed average, coolant 22 is then selected such that its refractive index is at least substantially the same as the average. Alternatively, the refractive index is calculated based on the Abbe number of the material of substrate 19 and the temperature coefficient of refractive index, defined as dn / dT, where n is the refractive index of substrate 19 and T is the temperature of the measurement environment. Alternatively, the refractive index of substrate 19 is measured, for example, by a refractometer, spectrometer, interferometer, or immersion and ellipsometric method. The aforementioned refractive indices of the corresponding coolant 22 and substrate 19 or substrate material should therefore be understood as examples. The actual refractive index may deviate from the stated refractive index, particularly depending on the wavelength of the measured light radiation and the temperature of the measurement environment 39.

[0058] Alternatively, the coolant is a gaseous coolant, such as nitrogen or dry air, i.e., air with a predetermined relative humidity (e.g., less than 40%). Alternatively, the liquid coolant is water, for example, especially ultrapure water.

[0059] Figure 2 A simplified cross-sectional view of the optical element 2 according to the first embodiment is shown. The body 18 of the optical element 2 has a substrate 19 and a reflective surface 20. At least one cooling channel 21, which is adjacent to each other in this example, is formed in the substrate 19 for receiving coolant 22. The spacing between two adjacent cooling channels 21, 45, 46, 47 is preferably at least 1 mm and at most 12 mm. The adjacent cooling channels 21, 45, 46, 47 are currently arranged in a plane or cooling channel plane 50. The substrate 19 may optionally have at least one other cooling channel plane, which is particularly arranged below the cooling channel plane 50.

[0060] Preferably, at least 20 and at most 200 cooling channels 21 are formed in the substrate 19. Each cooling channel 21 preferably has a rectangular or circular cross-section, wherein the diameter of each cooling channel 21 is at least 0.5 mm and at most 5 mm. Cooling channels 21, 45, 46, and 47 are formed, for example, in a tortuous shape or parallel to each other. The spacing between the cooling channels 21 (particularly the upper cooling channel wall 90 of each cooling channel 21, 45, 46, and 47) and the reflective surface 20 is preferably at least 2 mm and at most 30 mm.

[0061] Preferably, the optical element 2 (particularly the substrate 19) has first and second connection openings 48 and 49, wherein the first connection opening 48 is configured to connect to the supply line 25, and the second connection opening 49 is configured to connect to the discharge line 26, and vice versa. Alternatively or additionally, at least one of the connection openings 48 and 49 is designed for direct connection to the pressure regulating device 28, i.e., without insertion into the supply line 25. In particular, cooling channels 21, 45, 46, and 47, which are designed parallel to or aligned with each other, preferably lead to the first connection opening 48 and the second connection opening 49, respectively. The spacing between the connection openings 48 and 49 is preferably at least 50 mm and at most 1000 mm.

[0062] Preferably, a single or multiple reflective layer 51 is applied to the reflective surface 20. This reflective layer 51 is specifically designed to reflect measurement light radiation with wavelengths, particularly 193 nm, 532 nm, and / or 633 nm. This ensures reliable reflection of the measurement light radiation and thus ensures reliable measurement or measurability of the surface shape of the optical element 2. The single or multiple reflective layer 51 preferably has sputtered chromium and / or silicon, particularly at least one chromium layer and / or one silicon layer. Optionally or additionally, the material of the substrate 19 is formed (particularly doped) such that it absorbs the measurement light radiation.

[0063] Alternatively, no reflective layer 51 is applied to the reflective surface 20.

[0064] Cooling channels 21, 45, 46, and 47 are preferably manufactured by a cutting process (e.g., by drilling) or have already been manufactured. In this example, the substrate 19 and the reflective surface 20 are integrally formed.

[0065] Optical element 2 may optionally have a pressure regulating device 28. The pressure regulating device 28 is preferably arranged directly, for example, inserted into a connecting element such as supply line 25, or arranged directly at one of the connection openings 48, 49.

[0066] Figure 3 An optical element 52 according to the second embodiment is shown in a simplified cross-sectional view. The optical element 52 substantially corresponds to... Figure 2The optical element in the mirror body 55. However, the difference is that the mirror body 55 has reflective surfaces 20 and 54, and the substrate 53 and reflective surfaces 20 and 54 or the substrate 53 and the mirror body 55 having reflective surfaces 20 and 54 are not integrally formed.

[0067] The structures for forming cooling channels 56, 57, 58, and 59 are preferably introduced or have been preferably incorporated into or manufactured, particularly by milling, grinding, and / or etching in the substrate 53. After milling and / or grinding, the substrate 53 or substrate material is preferably etched. The structures of cooling channels 56, 57, 58, and 59 may optionally be produced by laser-based methods, such as laser ablation or selective laser etching. Alternatively, cooling channels 56, 57, 58, and 59 are formed in the mirror body 55 having reflective surfaces 20 and 54, particularly by grinding, milling, and / or etching. In this example, the substrate 53 is preferably at least partially polished, such that the substrate 53 and the mirror body 55 having reflective surfaces 20 and 54 can be particularly advantageously joined or interconnected with each other through a bonding process.

[0068] In this example, the reflective surfaces 20, 54 or the mirror body 55 having the reflective surfaces 20, 54 are connected to the substrate 53 by a bonding process (particularly adhesive bonding). Cooling channels 56, 57, 58, 59 are formed by adhesive bonding, and these cooling channels are particularly closed on all sides, i.e., on all sides. The substrate 53 and the reflective surfaces 20, 54 or the mirror body 55 having the reflective surfaces 20, 54 are preferably made of the same material, particularly the material of the substrate 53.

[0069] As a result of adhesion, a boundary layer 60 can be formed. This boundary layer 60 is shown here. The refractive index of the boundary layer 60 is typically different from that of the substrate material.

[0070] To avoid destructive or undesirable reflections of the measurement light or measurement light radiation at the boundary layer 60, the optical element 52 preferably has a reflective layer 51. Alternatively, the optical element 52 may not have a reflective layer. In this example, the substrate material and / or the mirror 55 are preferably formed (particularly doped) such that they absorb the measurement light radiation to prevent it from penetrating into the substrate 53 and / or the mirror 55 and thus, particularly, into the boundary layer 60. Alternatively, the material of the boundary layer 60 is doped such that it absorbs light of a predetermined wavelength, particularly at least 193 nm and at most 633 nm, particularly at least 532 nm and at most 633 nm. Optionally, the cooling channels, particularly the walls of the cooling channels, have a predetermined roughness to ensure a diffuse scattering effect. The roughness is achieved, in particular, by a corresponding etching process.

[0071] Figure 4A simplified cross-sectional view of other optical elements 61 is shown, wherein the reflective surface 62 or the mirror body 63 having the reflective surface 62 is connected to the substrate 64 by a bonding process, particularly adhesive bonding.

[0072] According to this exemplary embodiment, adhesion occurs such that the reflective surface 62 and the boundary layer 65 formed in the process are at least partially misaligned with each other. "Misaligned" means that the first tangent plane 84 at a predetermined point on the reflective surface 62 (here, point P1) and the second tangent plane 85 at a predetermined point on the boundary layer 65 (here, point P2) are not parallel to each other. In other words, the deviation between the normal vectors of the first tangent plane 84 and the second tangent plane 85 is greater than zero.

[0073] If, as in the example here, the reflective surface 62 and the formed boundary layer 65 are each flat, i.e. without curvature, then “inconsistency” means that the reflective surface 62 or the first tangent plane 84 and the formed boundary layer 65 or the second tangent plane 85 are not parallel to each other.

[0074] For example, if only the boundary layer 65 is flat and the reflective surface 62 is at least partially curved (or vice versa), as shown by the dashed lines here, “inconsistency” means that the boundary layer 65 or the second tangent plane 85 and the reflective surface 62, especially the third tangent plane 86 connected to the curved reflective surface 62 at point P1, are not parallel to each other.

[0075] If both the reflective surface 62 and the boundary layer 65 are at least partially curved, “inconsistency” means that the tangent plane at a predetermined point P2 of the boundary layer and the tangent plane at a predetermined point P1 of the reflective surface are not parallel to each other.

[0076] Predeterminable points P1 and P2 are preferably arranged along a straight line 87, wherein the straight line 87 is aligned with the optical axis 89 of optical elements 2, 52, and 61.

[0077] According to this embodiment, a reflective layer is not provided, but a reflective layer may optionally be provided.

[0078] The substrate 64 and / or the mirror 63 may optionally comprise a material formed such that the material absorbs light of a predetermined wavelength, particularly at least 193 nm and at most 633 nm, particularly at least 532 nm and at most 633 nm. Alternatively, the doped boundary layer 65 may be made of a material that absorbs light of a predetermined wavelength, particularly at least 193 nm and at most 633 nm, particularly at least 532 nm and at most 633 nm.

[0079] Figure 5A flowchart illustrating a method for measuring the surface shape of optical elements 2, 52, 61 in a measurement environment 39 using a measuring device 1, according to an exemplary embodiment, is shown. Preferably, the method can be implemented by a control device 44. For this purpose, the control device 44 preferably has a microprocessor, particularly for executing computer programs whose program code enables the described method to be implemented, and also has RAM and ROM modules, wherein data (e.g., a predetermined target pressure) and programs (such as algorithms) are preferably stored in the ROM module. For simplicity, the method is described with reference only to cooling channel 21, but is not limited thereto.

[0080] In the first step S1, optical elements 2, 52, and 61 are provided.

[0081] In the second step S2, liquid or gaseous coolant 22 is fed into cooling channel 21. This is accomplished in particular by controlling the conveying device 23 connected to coolant reservoir 6. Coolant 22 is selected or has been selected such that the refractive index of coolant 22 is at least substantially equal to the refractive index of substrate 19 of optical elements 2, 52, 61.

[0082] In the third step S3, the pressure of the cooling channel is recorded in particular by the pressure recording device 35.

[0083] In the fourth step S4, the ambient pressure is recorded in particular by pressure sensor 43.

[0084] In step S5, the actual pressure difference Δp IST =p K,IST -p M,IST Based on recorded cooling channel pressure or actual cooling channel pressure p K,IST And the recorded measured environmental pressure or the actual measured environmental pressure p M,IST To determine.

[0085] In step S6, the actual pressure difference Δp IST Pressure difference Δp from the target SOLL A comparison is made. The target pressure difference preferably depends on the predetermined target measurement ambient pressure p. M,SOLL and a predetermined target cooling channel pressure p K,SOL Determine: Δp SOLL =p K,SOLL -p M,SOLLThe target measurement ambient pressure and target cooling channel pressure are selected such that the target measurement ambient pressure is at least 0.01 mbar and at most 0.20 mbar, and the target cooling channel pressure is at least 200 mbar and at most 10,000 mbar. Specifically, the predetermined target measurement ambient pressure is at least 0.03 mbar and at most 0.1 mbar, and the target cooling channel pressure is at least 500 mbar and at most 1,000 mbar. This ensures that the target conditions, i.e., the target cooling channel pressure and the target measurement ambient pressure, at least substantially correspond to EUV conditions, i.e., the pressure conditions typically present during EUV lithography equipment operation in a vacuum. The target cooling channel pressure is selected to be greater than the target measurement ambient pressure, particularly to create overpressure in cooling channel 21, thereby ensuring coolant transport. Preferably, the target measurement ambient pressure and the target cooling channel pressure are selected such that the predetermined target measurement ambient pressure is 0.05 mbar and the predetermined target cooling channel pressure is 500 mbar. The target measurement environmental pressure preferably corresponds to the environmental pressure in an EUV lithography apparatus or a projection exposure apparatus designed for operation in EUV, particularly a projection exposure apparatus designed as a scanner for semiconductor lithography, especially the operating environmental pressure. Therefore, the target measurement environmental pressure is, for example, the target scanner environmental pressure.

[0086] In step S7, the deviation between the actual pressure difference and the target pressure difference is monitored. If the deviation is found to be greater than a predetermined limit value, the cooling channel pressure is adjusted so that the deviation is less than or equal to the predetermined limit value. For example, if the limit value is 10 mbar and a deviation exceeding 10 mbar is detected, the cooling channel pressure is adjusted by controlling at least one of the transmission device 23 and / or pressure regulating valves 28 and 29 so that the deviation is less than or equal to 10 mbar. The deviation is preferably determined based on the following equation:

[0087] Δp SOLL =Δp IST (1)

[0088] In particular, the transformation based on (1) is

[0089] p K,IST = p K,SOLL -p M,SOLL +p M,IST (2).

[0090] Due to the measured ambient pressure or the actual measured ambient pressure p M,ISTPreferably, the readings are continuously recorded, taking into account dynamic pressure fluctuations in the measurement environment 39 when determining deviations and thus when adjusting the actual cooling channel pressure. Alternatively, this limit value is preferably less than or equal to 1 mbar, particularly less than or equal to 0.5 mbar, and particularly 0 bar. The predetermined flow rate or volumetric flow rate of the coolant 22 through the cooling channel 21 is preferably also set by adjusting the conveying rate of the conveying device 23. Optionally, a predetermined flow rate is set, wherein surface shape measurements are taken at two different actual pressure differences, particularly at two different measurement environment pressures, and then the average of the measurement results is formed. Optionally, a temperature control device 83 connected to the coolant reservoir 6 is controlled for temperature control, particularly for cooling or heating the coolant 22, and particularly for setting the predetermined dynamic viscosity of the coolant 22.

[0091] If the deviation is less than or equal to a predetermined limit value, the surface shape is measured in step S8. Specifically, at least one component of the measuring light source 4 and / or interferometer 5 is controlled or activated for this purpose. To ensure that the deviation remains less than a predetermined deviation during measurement, steps S1 to S7 are preferably continued or repeated during the measurement. If a deviation greater than the limit value is detected, the measurement is preferably interrupted and resumed only if the deviation is less than or equal to the limit value.

[0092] The advantage of this method is that the surface shape is measured under an actual pressure difference that at least substantially corresponds to the target pressure difference under EUV conditions. This ensures that the determined measurement result or the determined surface shape of the optical elements 2, 52, 61 corresponds to a surface shape that is formed or can be formed, particularly under EUV conditions. This ensures particularly reliable operation of the optical elements 2, 52, 61 or the projection exposure apparatus having such optical elements 2, 52, 61. Measurements can be performed under any ambient pressure (e.g., atmospheric pressure or vacuum pressure).

[0093] In this example, "EUV conditions" refers to the conditions that typically exist during EUV operation of a projection exposure apparatus or EUV lithography apparatus. These conditions specifically relate to the operating measurement ambient pressure currently defined by the target measurement ambient pressure and the operating cooling channel pressure currently defined by the target cooling channel pressure. Optionally, but not limited to, these conditions also relate to the flow rate of the coolant in the cooling channel, the operating temperature of the EUV lithography apparatus, and / or the wavelength of the EUV light.

[0094] Figure 6A projection exposure apparatus 66, designed for operation in an EUV or EUV lithography system, is shown schematically. It takes the form of an EUV lithography apparatus having at least one optical element 2, 52, 61 manufactured and / or measured as described above. The projection exposure apparatus 66 has an EUV light source 67 for generating EUV radiation with high energy density in the EUV wavelength range below 50 nm, particularly between about 5 nm and about 15 nm. The EUV light source 67 may, for example, take the form of a plasma source or a synchrotron radiation source for generating laser-induced plasma. In the former example, a light-collecting mirror 68, such as... Figure 6 As shown, this is to focus the EUV radiation from the EUV source 67 into the illumination beam 69, thereby further increasing the energy density. The illumination beam 69 is used to illuminate the structured object M through an illumination device 70, which in this example has five reflective optical elements 71 to 75 (mirrors).

[0095] The structured object M can be, for example, a reflective mask or a mask master having reflective and non-reflective or at least less reflective regions for generating at least one structure on the object M.

[0096] The structured object M reflects a portion of the illumination beam 69 and shapes the projection beam path 75, which carries information about the structure of the structured object M, and is radiated into the projection lens 76. This generates an image representation of the structured object M or a corresponding portion thereof on the substrate W. The substrate W (e.g., a wafer) comprises a semiconductor material (e.g., silicon) and is mounted on a mount also known as a wafer stage WS.

[0097] In this example, the projection lens 76 has six reflective optical elements 77 to 82 (mirrors) to generate an image of the structure present at the structured object M on the wafer W. The number of mirrors in the projection lens 76 is typically between four and eight; however, two mirrors may also be used if appropriate.

[0098] The optical elements 2, 52, and 61 used to inspect the surface shape or profile within the scope of this invention can be any mirror of the projection exposure device 66, such as a condenser mirror 68, one of the mirrors 71 to 75 of the illumination device 70, or one of the mirrors 77 to 82 of the projection lens 76. At least one of these mirrors is preferably manufactured and / or measured according to the method described above.

[0099] List of reference numerals

[0100] 1. Measuring equipment

[0101] 2 Optical Components

[0102] 3. Reflector

[0103] 4. Measuring light source

[0104] 5. Interferometer

[0105] 6. Coolant reservoir

[0106] 7 Optical waveguide

[0107] 8 Input Wave

[0108] 9 beam splitters

[0109] 10 Diffractive optical elements

[0110] 8. Computer-Generated Holograms (CGH)

[0111] 12 Reflective elements

[0112] 13 Reflective elements

[0113] 14 Reflective elements

[0114] 15 Interferometer Camera

[0115] 16 shutter speeds

[0116] 17. Eyepiece

[0117] 18 Main Body

[0118] 19 substrate

[0119] 20 Reflective Surface

[0120] 21 Cooling Channels

[0121] 22 Coolant

[0122] 23 Conveying device

[0123] 24 pumps

[0124] 25 Supply Line

[0125] 26 Emission Line

[0126] 27. Conveyor Line

[0127] 28 Pressure regulating device

[0128] 29 Two-way pressure regulating valve

[0129] 30 Three-way pressure regulating valve

[0130] 31. Entrance side

[0131] 32 Export side

[0132] 33 Overflow outlet

[0133] 34 Pressure regulating device

[0134] 35 Pressure recording device

[0135] 36 Pressure recording device

[0136] 37 Sensors

[0137] 38 Interior Space

[0138] 39 Measurement Environment

[0139] 40 Housing

[0140] 41 Vacuum Chamber

[0141] 42 Vacuum Generation Unit

[0142] 43 Pressure Sensor

[0143] 44 Control device

[0144] 45 Cooling Channels

[0145] 46 Cooling Channels

[0146] 47 Cooling Channels

[0147] 48 First connecting opening

[0148] 49 Second connecting opening

[0149] 50 Cooling channel plane

[0150] 51 Cooling channel wall

[0151] 52 Optical Components

[0152] 53 substrate

[0153] 54 Reflective Surface

[0154] 55. Reflecting mirror body

[0155] 56 Cooling Channels

[0156] 57 Cooling Channels

[0157] 58 Cooling Channels

[0158] 59 Cooling Channels

[0159] 60 Boundary Layer

[0160] 61 Optical Components

[0161] 62 Reflective Surface

[0162] 63. Main body of the reflector

[0163] 64 substrates

[0164] 65 Boundary Layer

[0165] 66 Projection Exposure Equipment

[0166] 67 EUV light source

[0167] 68. Concentrator reflector

[0168] 69 Illumination Beams

[0169] 70 lighting fixtures

[0170] 71 Optical Components

[0171] 72 Optical Components

[0172] 73 Optical Components

[0173] 74 Optical Components

[0174] 75 Optical Components

[0175] 76 Projection Lenses

[0176] 77 Optical Components

[0177] 78 Optical Components

[0178] 79 Optical Components

[0179] 80 Optical Components

[0180] 81 Optical Components

[0181] 82 Optical Components

[0182] 83 Temperature control device

[0183] 84 The First All Planes

[0184] 85 Second tangent plane

[0185] 86 Third tangent plane

[0186] 87 Straight Line

[0187] 88 Supply Line Section

[0188] 89 optical axes

Claims

1. A method for measuring the surface shape of optical elements (2, 52, 61) in a predetermined measurement environment (39), wherein, The optical element (2, 52, 61) has a body (18) having a substrate (19, 53, 64) and a reflective surface (20, 54, 62), wherein at least one cooling channel (21, 45, 46, 47, 56, 57, 58, 59) for receiving coolant (22) is formed in the substrate (19, 53, 64). The method includes the following steps: a) Record the pressure in the cooling channel. b) Record the ambient pressure at the measurement site. c) Determine the actual pressure difference based on the cooling channel pressure and the measured ambient pressure. d) Compare the actual pressure difference with a predetermined target pressure difference. e) Monitor the deviation between the actual pressure difference and the target pressure difference, wherein if the deviation is detected to be greater than a predetermined limit value, the cooling channel pressure is adjusted such that the deviation is less than or equal to the predetermined limit value. f) If the deviation is less than or equal to the predetermined limit value, then measure the surface shape.

2. The method according to claim 1, characterized in that, The pressure in the cooling channel is adjusted so that the deviation is less than 10 mbar.

3. The method according to claim 2, characterized in that, The deviation is less than 1 mbar.

4. The method according to claim 2, characterized in that, The deviation is less than 0.5 mbar.

5. The method according to any one of claims 1 to 4, characterized in that, Liquid or gaseous coolant (22) is fed into the cooling channels (21, 45, 46, 47, 56, 57, 58, 59), wherein the pressure on the coolant (22) is increased or decreased to adjust the pressure of the cooling channels.

6. The method according to claim 5, characterized in that, The pressure on the coolant (22) can be increased or decreased hydraulically or pneumatically.

7. The method according to claim 5, characterized in that, The coolant (22) flows through the cooling channels (21, 45, 46, 47, 56, 57, 58, 59) at a predetermined flow rate.

8. The method according to any one of claims 1 to 4, characterized in that, The target pressure difference is determined based on a predetermined target measurement ambient pressure and a predetermined target cooling channel pressure, wherein the target measurement ambient pressure is at least 0.01 mbar and at most 0.20 mbar and the target cooling channel pressure is at least 200 mbar and at most 10,000 mbar.

9. The method according to claim 8, wherein, The predetermined target ambient pressure is at least 0.03 mbar and at most 0.1 mbar, and the target cooling channel pressure is at least 500 mbar and at most 1000 mbar.

10. The method according to claim 8, characterized in that, The predetermined target ambient pressure is 0.05 mbar, and the predetermined target cooling channel pressure is 500 mbar.

11. The method according to claim 5, characterized in that, The coolant (22) is selected such that the refractive index of the coolant (22) is at least substantially equal to the refractive index of the substrate (19) of the optical element (2, 52, 61).

12. A measuring device for inspecting the surface shape of optical elements (2, 52, 61), wherein, The optical element (2, 52, 61) has a body (18) having a substrate (19, 53, 64) and a reflective surface (20, 54, 62), wherein at least one cooling channel (21, 45, 46, 47, 56, 57, 58, 59) for receiving coolant (22) is formed in the substrate (19, 53, 64), and the measuring device has: i) Measuring the light source (4). ii) Interferometer (5), which is used to inspect at least a portion of the surface of the optical element (2, 52, 61) by the interference superposition of a test wave and a reference wave, the test wave being obtained from the measurement light generated by the measurement light source (4) and directed to the optical element (2, 52, 61); iii) at least one controllable coolant reservoir (6) for storing coolant (22), and iv) A control device (44) configured to perform the method as described in any one of claims 1 to 11 when used as intended.

13. The measuring device according to claim 12, characterized in that, The refractive index of the coolant (22) is at least substantially equal to the refractive index of the substrate (19, 53, 64) of the optical element (2, 52, 61) to be inspected.

14. The measuring device according to claim 12 or 13, characterized in that, The coolant (22) is a solution of inorganic or organic substances in water.

15. The measuring device according to claim 14, characterized in that, The substance forms a homogeneous phase when mixed with water.

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