DE102021206850A1 - Process for producing a diffractive optical element and diffractive optical element - Google Patents

Abstract

In a method for producing a diffractive optical element (DOE), in particular a computer-generated hologram, for use in an interferometric measuring device for interferometrically measuring a shape of a surface of a test object, a substrate (SUB) is provided for the diffractive optical element and attached to a surface of the A diffractive structure (DIFF) is produced on the substrate, which has a large number of raised ridges (ST) and depressions (VT) between the ridges. A removable cover layer (DKS) with a predetermined layer thickness is then produced on the diffractive structure (DIFF) by controlled coating of the diffractive structure with a removable cover layer material. After handling the diffractive optical element provided with a cover layer, this is removed again before the diffractive optical element is used as intended, optionally together with impurities that have grown on the cover layer.

Description

FIELD OF APPLICATION AND PRIOR ART

The invention relates to a method for producing a diffractive optical element (DOE) for use in an interferometric measuring device and a diffractive optical element that can be produced using the method.

A preferred area of application is the production of a diffractive optical element in the form of a computer-generated hologram (CGH) for use in an interferometric measuring device for measuring the surface shape of mirrors for use in optical systems for EUV microlithography, i.e. microlithography that uses extreme ultraviolet radiation ( EUV) uses.

Large mirrors are required for the production of optical systems for EUV microlithography. The surface shape of the mirrors is measured with high precision interferometrically during the manufacturing process. In order to achieve high measurement accuracy with the help of an interferogram or to be able to record even the smallest deviations between the desired shape and the actual shape, it is important to generate the smallest possible number of interference fringes.

Mirrors for EUV systems often have free-form surfaces, i.e. surface shapes that deviate significantly from rotationally symmetrical spherical or aspherical surface shapes. This means that an interferometric measurement requires a freely formed input wave in order to achieve an interferogram with a small number of fringes.

Computer-generated holograms can fulfill the function of converting a "simple", e.g. spherical or plane input wave into a free-form wave following the DUT design. The wave front is adapted to the target shape of the surface in such a way that the test wave would strike the target shape at any point and would be reflected back into itself. Depending on the application, several other wavefronts can also be generated from the input wave, e.g. for calibration.

the DE 10 2017 217 369 A1 describes a measuring arrangement in which a diffractive optical element has a complex-coded phase grating with at least three different phase functions to generate at least one calibration wave.

The intensity or the proportion of light that is in each diffraction order depends, among other things, on the so-called filling level, which primarily depends on the width or the critical dimensions (CD, critical dimension) of the diffractive structures.

Critical dimensions for the line widths and line spacings of a diffractive structure can be, for example, of the order of a few tens of nanometers (e.g. down to the range of 100 nm or below). Such structures can be produced, for example, by means of electron beam lithography.

It has been shown that with increasingly smaller structure sizes on the diffractive optical element, it becomes more and more difficult to systematically achieve high measurement accuracies. This is partly attributed to contamination effects, which are difficult to avoid, especially outside of clean rooms.

TASK AND SOLUTION

Against this background, the invention is based on the object of providing a method for producing a diffractive optical element (DOE) with which it is possible to produce DOEs, in particular in the form of CGHs, the use of which enables systematic interferometric measurements with the highest measurement accuracy.

To solve this problem, the invention provides a method with the features of claim 1 and a diffractive optical element with the features of claim 8. Preferred developments are specified in the dependent claims. The wording of all claims is incorporated into the description by reference.

The method is used to produce a diffractive optical element (DOE), which can be a computer-generated hologram (CGH), in particular. A diffractive optical element is an optical element for shaping a light beam. The physical principle is diffraction (also called diffraction) on an optical grating. The diffractive optical element should preferably be suitable for being used in an interferometric measuring device for interferometrically measuring the shape of a surface of a test object. In the method, a substrate for the diffractive optical element is first provided. The substrate can be, for example, a plane-parallel plate made of a transparent optical material with a low coefficient of thermal expansion, such as quartz glass (fused silica). A diffractive structure is produced on a surface of the substrate. The diffractive structure has a large number of raised ridges and depressions between the ridges. The production can take place, for example, in a lithographic process chain, as is also used for the production of phase-shifting lithography masks. First, the substrate is coated with a chromium layer and a photoresist layer, which is then exposed using electron beam lithography. This is followed by a structure transfer into the chromium mask and a structure transfer into the substrate by reactive ion etching. The chrome mask is then removed.

A special feature of the process is that subsequently, i.e. after completion of the diffractive structure, a removable cover layer with a specified layer thickness is produced on the diffractive structure. This process step is carried out by the controlled coating of the diffractive structure with a removable cover layer material. A removable top layer material within the meaning of this application is a coating material suitable for producing a coating, which offers sufficient adhesion to the material of the diffractive structure to be coated in order to form a reliably adhering coating, namely the top layer, but the adhesion is at the same time such moderate is that the cover layer material can be essentially completely removed from the coated diffractive structure by a targeted cleaning procedure without damaging the underlying diffractive structure.

The measure can also be described in such a way that as the last step in production or production, a cover layer is applied to the diffractive structure or the grating formed by it in a controlled manner, with the cover layer being a sacrificial layer in terms of purpose certain phases of further use of the diffractive optical element should remain on the diffractive structure, but is removed again at a suitable point in time, possibly weeks or months later, without damaging the diffractive structure and during actual use of the diffractive optical element, i.e. in particular at an interferometric measurement, is no longer present on the diffractive structure. In this respect, it is a temporary coating with properties that are controlled as precisely as possible.

The diffractive optical element with the cover layer applied to the diffractive structure can be regarded as an intermediate product of the method. This intermediate product can already be an independently marketable product, which is supplied by a manufacturer for further use to an end user who may be located at a different location. The cover layer protects the underlying diffractive structure from direct contamination.

The invention is based, inter alia, on the following findings. As mentioned at the outset, the intensity distribution in the diffraction orders generated by the diffractive structure is determined, among other things, by the degree of filling and thus ultimately by the width of the structures. If there are impurities or contamination on the surfaces of the webs, their effective critical dimension widens. The indentations appear tighter. According to the inventor's observations, even unknown contaminations in the order of magnitude of more than 0.3 nm can cause relatively high measurement inaccuracies. In particular, such uncontrolled contamination can mean that very large mirrors with high demands on the dimensional accuracy can no longer be manufactured or can no longer be manufactured systematically with sufficient quality.

From this it was concluded that knowledge of the contamination and knowledge of the absolute layer thickness of a contamination layer are extremely important. If a removable cover layer is applied to the grating in a controlled manner as the last step in production, this can ensure a homogeneous surface structure of the diffractive optical element, since all free surfaces are then produced in the same way and consist of the same material, namely the cover layer material.

After the desired coating has been applied, any further contamination that may occur, especially from materials that are difficult to clean, is now on the substrate formed by the top layer and no longer directly on the diffractive optical element, which is ultimately used for the measurement. Such contamination can then be removed from the diffractive optical element together with the cover layer before it is used as intended.

Before the diffractive optical element is used as intended, the method thus also includes handling the diffractive optical element with the cover layer applied to the diffractive structure in at least one handling operation. Handling can, for example, include transport from the manufacturer to the end user. Furthermore, measurements can be carried out on or with the diffractive optical element in order to qualify it. Eventually will the removable cover layer, possibly together with contamination that has grown on the cover layer, is removed as residue-free as possible before the diffractive optical element is used as intended. There can be many weeks, months or years between the application and removal of the top coat. These steps are often not performed by the manufacturer, but at least in part by an end user who is not the manufacturer. Manufacturer and end user can be different entities.

With the help of the invention it can be achieved, among other things, that at the time the diffractive optical element is installed in a measuring device, the diffractive optical element is essentially free of contamination, or that any basic contamination that may be present is very low. In this way, the times of use can be extended until further cleaning becomes necessary. In particular, it can be achieved that no contaminants that are difficult to remove or impossible to remove are present on the diffractive structure, which would mean a permanent restriction of the usable area of contamination.

The basic contamination of the diffractive optical element in a "sharp" measurement can correspond to that of a perfectly cleaned substrate - this means that in-situ cleaning (i.e. cleaning of the DOE built into the measuring device) can also be carried out without re-measuring. For in-situ cleaning, a measuring device can have an internal cleaning chamber with means for cleaning the DOE.

This means that highly precise measurements can be carried out over longer periods of use than before.

According to a development, a carbon-containing cover layer material is applied to the diffractive structure to produce the cover layer, the cover layer material preferably consisting essentially of carbon. The use of a carbon-containing top layer material can be advantageous, inter alia, because the top layer material, due to the carbon content, becomes at least partially chemically similar to typical naturally occurring contaminations, which contain residues of oils and fats, for example. In addition, when using a cover layer material consisting more or less entirely of carbon, one can fall back on experience with targeted coating, even with thin layer thicknesses. Finally, it is also possible to fall back on established cleaning processes that already exist specifically for the removal of carbon-containing impurities. A carbon-containing cover layer material within the meaning of this application consists of at least 50 at.% carbon. The designation "essentially of carbon" means a carbon content of at least 90 at.%.

In order on the one hand to meet the requirement of a layer thickness that can be defined as well as possible and on the other hand not to impair the control steps interposed by the cover layer more than necessary, it is preferably provided that the cover layer is applied with a layer thickness in the range from approx. 5 nm to approx. 15 nm. With significantly greater layer thicknesses, the diffraction properties of the diffractive structure provided with a cover layer change too much. With significantly lower layer thicknesses, the desired effect may not be sufficient.

Preferably, the top layer is applied by atomic layer deposition. Atomic layer deposition (ALD) is a process for depositing extremely thin layers up to atomic monolayers on the starting material. It is a highly modified chemical vapor deposition (CVD) process with two or more cyclically performed self-limiting surface reactions. A top layer can be applied chemically very pure, homogeneous and without internal stresses.

In preferred variants of the method, after the production of the diffractive structure, a pre-qualification is carried out, usually outside of a clean room. A final qualification is later carried out in a clean room at a time interval from the pre-qualification. This final qualification usually takes place shortly before the installation of the diffractive optical element (freed from the cover layer) in a measuring device. As part of a qualification, formal proof is provided that the diffractive optical element at the time of the qualification meets all the requirements of a specification or the like specified for this point in time. Fulfills. For example, as part of a qualification, the diffractive structure can be measured tactilely, optically and/or dimensionally in some other way and/or checked optically, for example, with regard to the desired optical effect. It has turned out to be advantageous if the cover layer is applied before the start of the pre-qualification and is removed in a cleaning step before the start of the final qualification. As a result, the cover layer can remain on the diffractive structure during the complete pre-qualification (and afterwards) and can be taken into account in all calculations due to its known layer thickness. the Reini ment step is preferably carried out outside the clean room.

For cleaning the diffractive structure, i. H. for the step of removing the cover layer from the diffractive structure, different methods can be used. To remove the cover layer from the diffractive structure, a cleaning operation is preferably carried out, which comprises at least one wet cleaning step. The cover layer is removed using at least one cleaning liquid. Wet cleaning has proven itself, among other things, for the complete removal of thin, carbonaceous top layers. The cleaning effect can be partly mechanical and partly chemical in nature and reaches even the smallest gaps in order to remove the cover layer material completely and the structured substrate gently from the area of the diffractive structure.

The invention also relates to a measuring method for interferometrically measuring the shape of a surface of a test object. In this case (at least) one diffractive optical element produced according to the method is used.

character list

• 1 zeigt schematisch eine mit einem CGH ausgestattete Messvorrichtung zum interferometrischen Vermessen eines reflektiven optischen Elements mit asphärischer Oberfläche;
• 2 illustriert anhand von 2A bis 2C schematisch den Einfluss von Kontaminationen an der diffraktiven Struktur auf deren Beugungsverhalten;
• 3 zeigt in 3A bis 3C wie eine Kontaminationsschicht gleichmäßig über die Zeit (A-C) aufwächst;
• 4 zeigt in 4A bis 4D, dass CGHs ggf. mit einem Schichtstapel (4A) hergestellt werden und eine Zwischenschicht schwer reinigbare Kontamination anziehen kann (4B), dass die tief geätzten Strukturen andere Oberflächeneigenschaften, z.B. Rauheit, als die polierte Oberseite haben können (4C) und deshalb mehr (4D) oder weniger Kontaminationswachstum erhalten können, was zur Folge haben kann, dass sich die effektive Tiefe ändert (4D);
• 5 zeigt ein Diagramm, welches die zeitliche Entwicklung von kontaminationsabhängigen Werten schematisch darstellt;
• 6 zeigt in 6A eine CGH Struktur und in 6B eine mit ALD aufgebrachte Opferschicht;
• 7 zeigt die in 6B aufgebrachte Opferschicht auf Strukturen wie in 4A oder 4C gezeigt. Für ankommende Kontamination sieht nun jede Oberfläche gleich aus, es bilden sich weniger Stellen mit besonderer Kontamination aus, und mehr gleichmäßige Kontamination;
• 8 zeigt verschiedene Beispiele von schwer reinigbaren Kontamination, die auf der Opferschicht verbleibt, so dass kein direkter Kontakt mit dem CGH besteht;
• 9 zeigt schematisch den Effekt einer Reinigung der Opferschicht und damit verbunden aller aufgewachsenen Kontamination;
• 10 zeigt eine zeitabhängige Kontaminationsdarstellung analog zu 5.

Further advantages and aspects of the invention result from the claims and from the description of exemplary embodiments of the invention, which are explained below with reference to the figures.

• 1 shows schematically a measuring device equipped with a CGH for interferometric measuring of a reflective optical element with an aspherical surface;
• 2 illustrated by 2A until 2C Schematically the influence of contamination on the diffractive structure on its diffraction behavior;
• 3 3A to 3C shows how a contamination layer grows uniformly over time (AC);
• 4 shows in 4A to 4D that CGHs may be produced with a layer stack (4A) and an intermediate layer can attract contamination that is difficult to clean (4B), that the deeply etched structures can have different surface properties, e.g. roughness, than the polished upper side (4C) and therefore may receive more (4D) or less contamination growth, which may cause the effective depth to change (4D);
• 5 shows a diagram which schematically shows the development of contamination-dependent values over time;
• 6 FIG. 6A shows a CGH structure and FIG. 6B shows a sacrificial layer applied with ALD;
• 7 FIG. 6 shows the sacrificial layer deposited in FIG. 6B on structures as shown in FIG. 4A or 4C. For incoming contamination, every surface now looks the same, fewer spots of particular contamination develop, and more uniform contamination;
• 8th shows various examples of difficult-to-clean contamination that remains on the sacrificial layer so that there is no direct contact with the CGH;
• 9 shows schematically the effect of cleaning the sacrificial layer and associated all grown contamination;
• 10 shows a time-dependent contamination plot analogous to 5 .

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of methods according to the invention for the production of a diffractive optical element DOE are presented below using the example of the production of a computer-generated hologram (abbreviation CGH). This is to be used in an interferometric measuring device for interferometrically measuring the shape of a surface of a test object.

1 12 schematically shows a measuring device 100 for interferometric measuring of a test object 110 in the form of an EUV mirror, which has a mirror surface 112 designed as a free-form surface. The computer-generated hologram DOE is used here to convert an incident spherical input wave EW into a test wave PW directed at the mirror surface, the waveform of which is converted in relation to the spherical shape of the input wave in such a way that the test wave PW falls perpendicularly to the target shape of the test object surface 112 at every location and would then be reflected back into itself. If the actual surface shape (actual shape) deviates from the target shape, this is reflected in the generated interferograms and can be evaluated quantitatively.

With this method it is important that the CGH, ie the diffractive optical element DOE, generates the test wave PW exactly as it is predetermined for the measurement. Deviations from this lead to measurement inaccuracies. Based on 2 an example explains how contamination on the diffractive structure can lead to measurement inaccuracies.

The enlarged detail shows a section of the DOE. This has a substrate SUB that is transparent to the measurement radiation. A diffractive structure DIFF, which acts as a phase grating, is formed on a surface of the substrate SUB. The diffractive structure has a multiplicity of raised webs ST and depressions VT between the webs.

the 2A until 2C serve to illustrate problems that can arise from impurities or contamination of the diffractive structure 2A shows in the upper partial figure a plan view of a binary lattice with lattice lines shown in black. The distance between corresponding structure elements on adjacent grid lines is referred to as pitch P1, the line width or structure width as critical dimension CD1. A schematic side view of the diffraction orders generated by this grating is shown underneath. In the intensity diagram shown below, the arrow lengths show how the intensity of the radiation is distributed over the individual diffraction orders with an ideal grating.

In the 2 B Corresponding representations are given one below the other. However, contamination KO has accumulated laterally on the raised structures that form the grid lines, which leads to an effective broadening of the lines (see enlarged detail in Fig 2C ). In the contaminated lattice, the pitch P1, ie the structure spacing, remains unchanged, but the result is an effective critical dimension CD that is larger than the critical dimension CD1 of the clean lattice (CD > CD1). The partial figure below shows the changed intensity distribution in the diffraction orders. The intensity diagram shown below shows that the accumulation of contamination KO results in a change in the intensity distribution, whereby in the example the intensity in the non-diffracted, continuous zeroth diffraction order ("0") compared to the intensities in the first ("+1", "- 1") and second ("+2", "-2") orders of diffraction has decreased.

It is understandable that the change in diffraction properties caused by contamination can lead to measurement inaccuracies, especially if the extent of the contamination is not known. It has been shown that even unknown contamination with a thickness of a little more than 0.3 nm can result in too high a measurement inaccuracy, so that very large EUV mirrors can no longer be tested with the required reliability. The inventor has concluded that the absolute thickness of the contamination and knowing it are extremely important.

Within the framework of preferred processes, a diffractive optical element is subjected to a qualification several times in order to check whether the diffractive optical element has the properties intended for this process step with sufficient accuracy. It has been found to be expedient that after the original manufacture of the diffractive structures, which may have been carried out at a location other than the location of end use, a pre-qualification should take place before the start of use of the diffractive optical element by the end user. This can begin weeks or months after the original generation or manufacture of the diffractive structures and possibly after intermediate transport to the end user. Further qualification steps can then be carried out by the end user as promptly as possible before actual use, in particular a final qualification, which is preferably already carried out in a clean room before installation in a measuring device.

3A until 3C illustrate schematically how a carbonaceous contamination layer KO can build up over time. 3A shows the DOE immediately after generating the diffractive structures DIFF on a transparent substrate SUB. The substrate SUB can be, for example, an initially plane-parallel plate made of a transparent optical material with a low coefficient of thermal expansion, such as quartz glass (fused silica). A diffractive structure DIFF, which acts as a phase grating, is formed on a surface of the substrate. The diffractive structure has a multiplicity of raised webs ST and depressions VT between the webs.

While the free surfaces of the diffractive structure DIFF are still free of contamination immediately after the production of the diffractive structure DIFF ( 3A) , a more or less thick carbonaceous contamination layer KO ( 3B) . During the pre-qualification and after, the contamination layer KO can continue to build up, so it can have an even greater thickness at the beginning of the final qualification ( 3C ). In the course of the qualification of a DOE or a CGH, more and more contamination grows, especially up to the start of the final qualification, since from this point on the CGH is usually only moved in the clean room. The naturally grown contamination layer can be quite loose and porous, it can be chemically inhomogeneous, the layer thickness of the naturally grown contamination can vary locally, typical layer thicknesses can be 1 nm or a few nm, for example.

It has been shown that, due to the materials used and/or due to the surface properties of the diffractive structure, contamination that is difficult or impossible to clean can also occur in the area of the web feet, i.e. at the transition from the raised webs to the gaps. the 4A until 4D schematically illustrate some problem cases. The cross section in 4A shows a diffractive structure DIFF with remnants of an etch stop layer (etch stop layer) ASS in the foot regions of the raised webs ST. In the intermediate depressions VT, the etch stop layer has been removed after the etching step. 4B illustrates how the different materials in the foot areas attract certain contaminants, such as oily or greasy residues, etc., which accumulate in the foot areas and can lead to a widening of the effective width. 4C illustrates that the free surface of the ridges ST is relatively smooth due to the finishing by ion beam irradiation and polishing, while structures with greater surface roughness can arise on the flanks of the ridges ST, at the bottom of the depressions VT and in the transition areas of the inner corners by etching. 4D schematically illustrates the change in the effective height (double arrow) of the diffractive structure due to contamination, especially in the bottom area of the wells.

In order to illustrate one of the problems on which the invention is based, FIG 5 a schematic diagram that shows the time course of contamination-relevant values. The dashed line KA represents the extent of the absolute contamination over time t, the solid line KU represents the contamination uncertainty over time t, ie the uncertainty with regard to the actual extent of the absolute contamination KA. It is understandable that the uncertainty tends to increase when there has been more time for contamination to build up and, accordingly, the absolute extent also tends to increase. Immediately after production (t=0) there is no contamination (KA=0, KU=0).

While a certain amount of contamination is acceptable in the context of a measurement, in practice both quantities are assumed to have maximum permissible values that should not be exceeded. If it is foreseeable that the maximum values will be exceeded, cleaning (reference RE) will be required. With cleaning, the contamination that can be removed by cleaning can be eliminated. However, there are also contaminations that cannot be removed by cleaning and therefore remain permanently on the diffractive optical element. Their level is represented by the line NEK. The point in time MR is the point in time from which the diffractive optical element moves from a normal environment into a clean room environment in which the intended use of the diffractive optical element also takes place, for example in an interferometric measuring device.

As can be seen from the schematic images below the time axis, there is no contamination on the diffractive structure immediately after production (t=0). Over time, the absolute contamination KA and the uncertainty KU about the absolute extent gradually increase. During this time, for example, the diffractive optical element can be transported (transport vehicle symbol) from the manufacturer to the end user. During this period, a pre-qualification QV is also carried out, typically by the end user. When the contamination reaches the maximum allowable level, an RE cleaning is carried out, which reduces the absolute contamination before gradually building up again. From the time the diffractive optical element is taken over into the clean room (from point in time MR), the contamination gradually increases again, albeit at a lower rate than outside the clean room. The uncertainty KU now also increases more slowly. If the contamination within the clean room comes close to the permissible contamination limits again, cleaning is carried out again (RE1). A final qualification QE is carried out in the clean room.

Significant improvements can be achieved using the claimed invention. For this purpose, as the last step in production, ie as soon as possible after producing the diffractive structure DIFF on the substrate, a removable cover layer DKS, preferably made of carbon, is applied to the grating or the diffractive structure DIFF. 6A shows the diffractive optical element DOE immediately after production and in 6B the same diffractive structure on which a carbon top layer with a precisely defined layer thickness was applied by atomic layer deposition (ALD). The layer thickness SD is generally between 5 nm (nanometers) and 15 nm and varies only slightly over the entire surface of the diffractive optical element. The layer thickness is thus essentially constant.

In 7A it can be seen that the cover layer DKS also covers critical areas of different materials, for example at the transition between raised webs ST and adjacent depressions VT. 7B illustrates that the applied top layer DKS can also compensate for differences in roughness in order to achieve the smoothest possible surface of the top layer DKS.

In the 8A , 8B and 8C Various examples of NRV impurities that are difficult or impossible to remove are shown, which, thanks to the cover layer DKS, are not deposited directly on the diffractive structure DIFF, i.e. on the substrate material, but at a distance from it on the cover layer DKS.

9 illustrates the wet cleaning step (wave symbol), which converts the diffractive optical element DOE shown in the top row, provided with a top layer DKS and actually non-removable impurities NRV, into a cleaned and therefore ready-to-use DOE (bottom row). The wet cleaning step is carried out in such a way that both the cover layer DKS and all contaminations KO deposited on it are removed by means of wet cleaning. Cleaning fluids containing solvents, for example, can be used for cleaning, and cleaning can be mechanically supported by introducing ultrasound. After wet cleaning, the original substrate surface with the diffractive structures DIFF is exposed again, but without any contamination deposited on it.

It is preferably handled in such a way that the cover layer DKS is applied immediately after the production of the diffractive structure DIFF and remains on the diffractive optical element DOE during the pre-qualification QV and is taken into account in all calculations. After the pre-qualification QV, before the diffractive optical element is only used in the clean room, this layer DKS with all contamination that has grown on it is removed with the help of a wet cleaning step ( 9 ). The DOE or the CGH is then only moved in a highly clean environment within a clean room and thus experiences significantly less residual contamination.

Preferably, before a "high quality" measurement, the CGH is cleaned and measured again. The pre-qualification is used to determine the condition of the CGH and can determine whether the CGH is manufactured within tolerance. This can be determined on the relatively lower (measurement) performance. A high quality qualification, on the other hand, is used to characterize the CGH for later mathematical corrections of the measurement data created with the CGH. This is carried out with a significantly higher measurement accuracy and spatial resolution.

10 shows a time-dependent contamination plot analogous to 5 . A direct comparison shows how the use of the removable cover layer DKS can reduce the overall contamination problems and result in longer service lives without intermediate cleaning.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102017217369 A1 [0006]

Claims (8)

Method for producing a diffractive optical element (DOE), in particular a computer-generated hologram (CGH), for use in an interferometric measuring device (100) for interferometrically measuring a shape of a surface (112) of a test object (110), comprising the following steps: providing a substrate (SUB) for the diffractive optical element (DOE); producing a diffractive structure (DIFF) on a surface of the substrate, the diffractive structure having a multiplicity of raised ridges (ST) and depressions (VT) between the ridges; characterized by producing a removable cover layer (DKS) with a predetermined layer thickness (SD) on the diffractive structure (DIFF) by controlled coating of the diffractive structure with a removable cover layer material. procedure after claim 1 , characterized by handling of the diffractive optical element (DOE) with the cover layer (DKS) applied to the diffractive structure (DIFF) in at least one handling operation prior to the intended use of the diffractive optical element (DOE) and removal of the removable cover layer ( DKS) optionally together with contaminations (KO) that have grown on the cover layer before the intended use of the diffractive optical element, the handling preferably involving transport from a manufacturer to an end user and/or at least one measurement on or with the cover layer (DKS) covered diffractive optical element (DOE) includes to qualify this. procedure after claim 1 or 2 , characterized in that to produce the cover layer (DKS), a carbon-containing cover layer material is applied to the diffractive structure (DIFF), the cover layer material preferably consisting essentially of carbon. Method according to one of the preceding claims, characterized in that the cover layer (DKS) is applied with a layer thickness (SD) in the range from approximately 5 nm to approximately 15 nm, the layer thickness being essentially constant. Method according to one of the preceding claims, characterized in that the cover layer (DKS) is applied by means of atomic layer deposition (ALD). Method according to one of the preceding claims, characterized in that after the generation of the diffractive structure (DIFF), a pre-qualification (QV), preferably outside a clean room, and at a time interval from the pre-qualification (QV) a final qualification (QE ) is carried out within a clean room before installation in a measuring device (100), the cover layer (DKS) being applied before the start of the pre-qualification and being removed in a cleaning step before the start of the final qualification. Method according to one of the preceding claims, characterized in that a cleaning operation (RE) which comprises at least one wet cleaning step is carried out to remove the cover layer (DKS) from the diffractive structure (DIFF). Measuring method for interferometrically measuring the shape of a surface (112) of a test object (110), comprising the following steps: mounting a diffractive optical element (DOE) in a measuring device (100); providing an input wave (EW) directed onto the diffractive optical element and a reference wave; Generating a test wave (PW) directed towards the test object (110) with a wavefront that is at least partially adapted to a desired shape of the optical surface (112) by diffraction at the diffractive optical element (DOE), superimposing the test wave (PW) after interaction with the test object with the reference wave; Detection of an interferogram generated by the superimposition of the test wave (PW) and the reference wave in a detection plane Evaluation of the interferogram to determine the shape of the surface (112), characterized in that a diffractive optical element (DOE) is used, which according to the method according to is made according to any one of the preceding claims.

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