Manufacturing method of optical imaging system for microlithographic apparatus

Abstract

In a method for manufacturing an optical imaging system of an EUV microlithographic apparatus, a plurality of optical modules are mounted at distributed mounting positions of a force frame, where each optical module carries a mirror. At least one of the optical modules is designed as an exchangeable replacement module having a mirror selected as a correction mirror. The method comprises the following steps: A) performing a shape measurement by a component measurement system to determine a surface shape of a mirror selected as a correction mirror; b) providing a tool module having a tool mirror wherein (i) the tool module has a mounting structure which is compatible with the mounting position of the optical module designed as a replacement module; and (ii) the surface shape of the tool mirror is the same or substantially the same as the surface shape of the correction mirror according to the shape measurement of the component measurement system; c) establishing an auxiliary imaging system by mounting an optical module with a mirror at a distribution mounting position of the force frame, wherein the tool module is mounted at the mounting position of the optical module designed as a replacement module; d) performing system measurement through a system measurement system to determine the imaging quality of the auxiliary imaging system after the optical module is installed at the installation position and rigid body alignment; e) comparing the measured imaging quality with a target imaging quality of the optical imaging system to determine an imaging quality error; f) determining a surface shape change of the correction mirror adapted to reduce the imaging quality error; g) machining the correction mirror, changing the surface shape to a modified surface shape suitable for reducing imaging quality errors; h) removing the tool module and installing an optical module designed as a replacement module wherein the correction mirror has a modified surface shape; i) performing system measurements to determine the imaging quality of the optical imaging system.

Description

[0001] The following disclosure is based on German patent application 102023113819.7, filed on May 25, 2023, which is incorporated by reference into the present application. TECHNICAL FIELD

[0002] The present invention relates to a method for manufacturing an optical imaging system of an EUV microlithography apparatus and to an optical imaging system for an EUV microlithography apparatus.

[0003] A preferred field of application is the manufacture or repair of an optical imaging system designed as a projection lens of an EUV projection exposure apparatus or of an EUV mask inspection apparatus for inspecting masks (reticles) for EUV microlithography. BACKGROUND

[0004] Today, microlithographic projection exposure methods are mainly used for manufacturing semiconductor devices and other fine-structured components, such as lithography masks. Here, a mask (reticle) or other pattern-generating device is used which carries or forms a pattern of structures to be imaged, such as a line pattern of a layer of a semiconductor device. This pattern is located in the region of the object plane of the imaging system between an illumination system and an optical imaging system (often referred to as a projection lens or projection optical unit) in a projection exposure apparatus and is illuminated with illumination radiation shaped by the illumination system. The radiation altered by the pattern travels along an imaging beam path through the imaging system, which images the pattern onto a substrate to be exposed in a reduced scale. The surface of the substrate is arranged in the image plane of the imaging system, which is optically conjugate to the object plane. The substrate is usually coated with a radiation-sensitive layer (resist, photoresist).

[0005] One of the goals of the development of projection exposure apparatuses is to manufacture structures on substrates by means of lithography in ever-smaller dimensions, for example, in order to achieve higher semiconductor device integration densities. One approach is to use electromagnetic radiation of shorter wavelength. For this purpose, optical systems have been developed which use electromagnetic radiation from the extreme ultraviolet range (EUV) having a working wavelength in the range between 5 nanometers (nm) and 30 nm, in particular at 13.5 nm. The imaging systems of EUV microlithography apparatuses use only mirrors to image the structures from the object plane to the image plane, for example, from a reticle to a wafer.

[0006] Many modern EUV projection lenses are designed modularly. The optical modules are mounted at designated mounting positions in a common force frame and each contain a mirror and in each case, in the mounted state, lie at the mounting positions provided for the optical modules, then form the imaging beam path with the optically effective surface of the mirror. To facilitate the assembly, maintenance and, if appropriate, optimization of such imaging systems, the optical module(s) are designed as exchangeable optical modules, that is to say as replacement modules (cf. DE 10 2021 201 162 A1).

[0007] EUV microlithography apparatuses are technically highly complex capital goods whose procurement and operation incur considerable costs for their end users. Such investments are only profitable when the microlithography apparatuses in use on site reliably and permanently meet the technical specifications and can be operated efficiently without major interruptions. SUMMARY

[0008] Against this background, the object of the present invention is based on providing a method for manufacturing an optical imaging system for an EUV microlithography apparatus, and a corresponding optical imaging system and an EUV microlithography apparatus equipped with the system, so that the optical imaging system reliably meets the specifications expected by the manufacturer in use and can be operated highly efficiently with a minimum downtime for maintenance purposes.

[0009] To achieve these objects, the present invention provides a method for manufacturing an optical imaging system having the features as set forth in claim 1. Furthermore, an optical imaging system having the features as set forth in claim 13, and an EUV microlithography apparatus having the features as set forth in claim 14 are provided.

[0010] The method is for manufacturing (or repairing) an optical imaging system for an EUV microlithography apparatus. The optical imaging system contains a plurality of optical modules, each optical module carrying a mirror, the optical modules being arranged along an imaging beam path leading from an object plane to an image plane of the imaging system, the optical modules being mounted at designated mounting positions in a fixed spatial relationship to a force frame. At least one of the optical modules is designed as an exchangeable replacement module, wherein a mirror is selected as a correction mirror.

[0011] For the purposes of the present application, an "optical module" comprises the mirror and further components. The mirror comprises a mirror substrate. One region of the substrate surface is optically precisely processed, thus essentially determining the surface shape of the mirror. In the case of an optical module which has been completed in the course of a manufacturing operation for an EUV apparatus, this region of the substrate surface is coated with a reflective coating which has a reflective effect on EUV radiation, in order to optimize the reflectivity. Substrate fixation components are also attached to the mirror substrate, for example a spider or a substrate-side component of a bearing device for supporting the mirror substrate on or in the spider, and if appropriate components of actuators and / or sensors. If appropriate, the position of the mirror substrate relative to the spider can be set or changed by appropriate setting devices, for example for the purpose of rigid body alignment. Components of the setting devices can be attached to the mirror substrate. The spider can have attachment structures, for example flange-like attachment structures, for attaching the spider to a force frame. The optical module can be designed such that an interface is located between the exchangeable optical module and the force frame, between the spider and the force frame, or within the spider, so that the optical module comprises the mirror substrate and the components of the spider which are coupled to it. The interface can also be located in a region of the spider or in a region of the bearing device, so that the optical module comprises only the mirror substrate and the attachment components of the spider or bearing device, and if appropriate the components of sensors and / or actuators which are fixedly mounted to the substrate, and the components of the spider or bearing device and / or sensors and / or actuators which are fixedly mounted to the force frame are not exchanged during the exchange operation.

[0012] The method is based on the concept according to which at least one optical module is designed as an exchangeable replacement module, wherein the mirror is selected as a correction mirror. Such an optical module is referred to hereinafter as "optical module with correction mirror" or "replacement module with correction mirror". The idea behind this is that it is still possible to utilize the correction mirror to correct any residual aberrations which remain after the system alignment, by changing the optically effective surface of the correction mirror in accordance with the results of the system measurements, so that this change reduces the residual aberration level to the extent that the imaging system meets its specifications. The correction mirror is thus a mirror which is provided or selected as a correction mirror, or if appropriate a mirror whose surface shape needs to be changed in order to correct the residual aberrations which remain after the rigid body alignment. The method comprises a plurality of steps, which are also identified here by capital letters for the sake of simplicity of reference.

[0013] According to step A, the mirror selected as the correction mirror is subjected to a shape measurement in order to determine its surface shape in the optical area of use. The shape measurement is carried out by means of a component measuring system, for example by means of interferometry. For example, a Fizeau interferometer can be used as the component measuring system. An example of such an interferometer is disclosed in WO 2006 / 077145 A2. According to the shape measurement, the surface shape of the correction mirror is known within the measuring accuracy of the component measuring system.

[0014] The substrate surface is machined to be so smooth in the optical area of use that it can strongly reflect light from, for example, the visible spectral range (VIS). For the shape measurement, in particular by means of a measurement light from the VIS range, it is therefore not necessarily necessary for the substrate surface in the optical area of use to have a coating which can reflect EUV radiation. The shape measurement can be carried out on an uncoated substrate or on a coated substrate.

[0015] Furthermore, step B according to the method provides a tool module with a tool mirror. A "tool module" within the meaning of the application is an optical module which is assigned to a particular optical module or to a replacement module with a correction mirror, has a mounting structure which is compatible with the mounting position of the assigned optical module and contains a tool mirror whose surface shape is identical or substantially identical to the surface shape of the correction mirror of the assigned replacement module with a correction mirror according to the shape measurement carried out using the component measuring system.

[0016] The surface shape and the optical effect of the tool mirror can be identical to the surface shape and the optical effect of the assigned correction mirror within the manufacturing tolerances. However, this is not mandatory. The optical effect of the tool mirror can differ significantly from the optical effect of the assigned optical module with a correction mirror. However, any possible differences in the optical effect should be small so that the level of aberration does not become too poor and a meaningful system measurement can still be carried out.

[0017] The mirrors for EUV systems contain a mirror substrate which can consist, for example, of a glass or glass ceramic with a low coefficient of thermal expansion and which has a substrate surface which is optically precisely machined and substantially determines the surface shape of the mirror. In order to optimize the reflectivity of the incident EUV radiation, this substrate surface has a reflective coating which has a reflective effect on EUV radiation, for example a multiple reflection layer (multilayer) consisting of a plurality of individual layers. The tool mirror is also constructed in such a way that the reflection properties, in particular the reflectivity and the influence on the wavefront, are as similar as possible to the reflection properties of the assigned correction mirror.

[0018] The tool mirror can have relaxed specifications, for example with respect to machining, and small mechanical defects (e.g. small scratches, protrusions, cosmetic damage) or life cycle effects. Most importantly, it is important to know the surface shape of the tool mirror as precisely as possible when the system is measured.

[0019] For example, an optical module nominally identical to the specified optical module to be exchanged can be used as a tool module, but after long-term use at a different location, its performance has degraded to the extent that it must be replaced. Thus, even the used optical module can still be used as a "means to an end", more precisely as an aid to the system measurement. This saves resources and limits costs.

[0020] It is also possible to install a mirror with special tolerances as a tool mirror to save new manufacturing time for a new tool mirror. In this sense, the tool mirror only serves as an auxiliary mirror in the optical imaging system, which needs to be realigned in order to carry out a reliable system measurement.

[0021] According to step C, an auxiliary imaging system is established by installing optical modules with mirrors at the relevant mounting positions of the force frame, wherein the specified tool modules are installed in the mounting positions of the optical modules designed as replacement modules (replacement modules with correction mirrors). The auxiliary imaging system is a temporary imaging system, since it differs in structure from the optical imaging system to be manufactured or repaired, in particular the specified tool modules are installed instead of the optical modules equipped with correction mirrors.

[0022] According to step D, a system measurement is then carried out on the assembled auxiliary imaging system in order to determine the imaging quality of the auxiliary imaging system. This system measurement is carried out after the optical modules have been installed and the optical modules are rigidly aligned in their mounting positions. The rigid alignment of the optical modules can be supported by the results of the system measurement in order to ensure that the possibilities of rigid alignment are utilized as much as possible and the imaging performance of the auxiliary imaging system has been optimized in the direction of the ideal target imaging performance. The system measurement is carried out using a system measurement system, for example a wavefront measurement system.

[0023] In step E, the imaging quality measured in step D is compared with a target imaging quality of the optical imaging system, which is derived, for example, from the specifications. The measured imaging quality is compared with the target imaging quality in order to determine an imaging quality error. Based on the comparison, it is then determined in step F in which way a change in the surface shape of the correction mirror(s) can be produced in order to reduce the imaging quality error and, in turn, to bring the auxiliary imaging system into line with the specifications.

[0024] The information obtained in step F is used in step G. In this method step G, the mirror selected as correction mirror is processed in such a way that a change in the surface shape occurs in a direction that is suitable for reducing the modified surface shape in terms of imaging quality errors.

[0025] For this purpose, an uncoated mirror (mirror substrate that has been polished to a high gloss in the optical use region or has been finely machined to a high gloss in another way but has not yet been coated) can be used as correction mirror, the surface shape of which in the optical use region is processed by a suitable shape processing process, for example by ion beam etching, and is changed in turn. A reflective coating designed for EUV radiation is then applied. It is also possible to use a correction mirror whose mirror substrate already has an EUV reflective coating. The surface shape of the correction mirror already having a reflective coating can be mechanically machined, for example in the manner described in the following documents: US 2012 / 212721 A1, US 2014 / 307308 A1, US 2012 / 300184 A1, DE 10 2014 225 197 A1, US 2019 / 018324 A1, DE 10 2021 213 148 A1, DE 10 2015 201 141 A1, US 2016 / 209750 A1 or DE 10 2011 076 014 A1.

[0026] After the surface shape change processing of the mirror selected as correction mirror has been completed, an exchange operation (swapping operation) is carried out in accordance with step H. In this process, the installed tool module is removed from its installation position and the designated optical module, which has the correction mirror with the modified surface shape, is designed as replacement module and is installed in the installation position instead of the tool module. As a result, the auxiliary imaging system becomes the ideal imaging system in terms of structural components, which is also referred to as the final imaging system.

[0027] In accordance with step I, the success of the measurement is verified by performing a system measurement to determine the imaging quality of the optical imaging system for control purposes.

[0028] Typically, after mounting an optical module with a corrector mirror, rigid alignment of the optical module in an imaging system is advantageous for optimizing the imaging quality of the system. In this paper, multiple other system measurements are taken, and based on this, alignment steps for optimizing the spatial position of the optical modules at their respective mounting locations may be useful in an iterative approach. Therefore, the method preferably includes evaluating the results of the system measurements, and if the system measurements indicate that the imaging quality is outside the tolerance range, alignment operations are performed to align the mounted optical modules in their rigid degrees of freedom to improve the imaging quality, and further system measurements are performed, wherein the alignment operations and system measurements are repeated until the system measurements indicate that the imaging quality is within the tolerance range.

[0029] In this method, a tool module with an assigned tool mirror is thus used for at least one optical module, which is designed as an interchangeable replacement module with a correction mirror. The tool module is temporarily mounted within the force frame of the optical imaging system. Further work can then be done on the resulting auxiliary imaging system in preparation for commissioning, while the mirror provided as the correction mirror can be fabricated based on system measurements on the auxiliary imaging system equipped with the tool mirror. This significantly improves time management because many activities required in the process can be performed simultaneously, and, if appropriate, at different locations.

[0030] After system measurements of the auxiliary imaging system determine how the selected corrector mirror must be machined to reduce residual aberration levels, the machining of the selected corrector mirror can begin and be performed (step G) while the auxiliary imaging system is still operational and usable. This is for example, for activities related to commissioning preparation. This reduces the time required to commission the imaging system to be manufactured. This minimizes the downtime (unavailable time) of the EUV microlithography equipment. Even if a machining scheme to change the surface shape of the corrector mirror already exists, the tool mirror can be retained in the auxiliary imaging system for a period of time until the optical module with the machined corrector mirror, designed for replacement, arrives and is ready for installation. For example, this time can be used for testing, or it can already be used for necessary commissioning work that does not require full optical performance. Therefore, the auxiliary imaging system with the installed tool module can operate in auxiliary mode to perform testing and / or prepare for commissioning in a second location.

[0031] Although an optical imaging system may have only a single interchangeable optical module and select a mirror as a correction mirror, it is preferable to provide multiple such optical modules, such as two, three or four such interchangeable modules and corresponding designated tool mirrors.

[0032] Multiple shape measurements and multiple system measurements can be performed at the same location, for example, at the location of the manufacturer of the mirror and / or imaging system, in the same measurement space, or in different measurement spaces at the same manufacturing location.

[0033] However, according to one development, it is stipulated that shape measurements(s) be performed at a first location and system measurements be performed at a second location located far from the first location. For example, the first and second locations may be located in different cities or different regions of different countries or different countries and / or different continents. For example, the distance between the first and second locations may be greater than 100 km and / or greater than 1000 km and / or greater than 10000 km.

[0034] This spatial separation between component measurement (measurement of individual components) and system measurement (measurement of an assembled system containing multiple components) offers numerous technical and economic advantages, taking into account the many uncontrollable sources of error in manufacturing such a highly complex optical imaging system.

[0035] Shape measurement is preferably performed at the manufacturer's location (the first location) of the mirror and / or imaging system, i.e., spatially close to the mirror's manufacturing process. This allows for efficient, low-error interaction between manufacturing and control through shape measurement. Expensive system measurement techniques are not required at the first location.

[0036] The second location can be, for example, the system integrator location for setting up an EUV microlithography apparatus consisting of many other components using an optical imaging system. If appropriate, system measurements can also be performed at the end-user's location, making the second location even a usability location. The system measurements are used to determine the imaging quality of the established auxiliary imaging system. The measurements also record the effects of the usage environment and any effects that may occur during transport between the mirror's manufacturing location and the overall system's usability location.

[0037] System measurements can also be performed at the system integrator's site and exchanged (switchover) at the end-user's site. This allows for the parallelization of mirror manufacturing and the transfer of the entire system to the customer, which also saves time. Therefore, individual method steps can also be distributed across two or more, especially three, different locations.

[0038] This method takes into account the fact that individual components of the imaging system, as well as the entire system composed of many individual components, must meet their respective specifications. The method also considers that system measurements performed at the component manufacturer's site are not necessarily meaningful, as subsequent usage locations may have different environmental conditions than the first location. Therefore, this variant assumes spatial separation between the first and second locations and optimizes the distribution of the required measurement tasks.

[0039] Preferably, a wavefront measurement system is used to perform system measurements. Preferably, spatially resolved wavefront measurements at multiple field points are provided. Preferably, integrated measurement techniques are used to perform system measurements directly within the EUV lithography apparatus. This eliminates the need for a separate system measurement facility. Instead, integrated measurement techniques can be used, which can also be applied to further operations of the EUV lithography apparatus.

[0040] The optical performance of a mirror is particularly determined by its surface shape, also known as "surface pattern." Therefore, the deviation from the target surface shape specified according to the optical design is called the surface pattern error. Each mirror should have a precisely specified surface shape. The mirror surfaces of EUV systems are typically designed as freeform surfaces, meaning their surface shapes differ significantly from spherical or rotationally symmetric aspherical surfaces. Precision manufacturing is extremely complex. This makes accurate shape measurement via a component measurement system even more crucial.

[0041] These shape measurements may be subject to measurement errors. Furthermore, machining errors may occur during the molding process. Moreover, surface shapes can be altered by measures taken during assembly, such as by introducing deformation due to adhesive or threaded connection effects, which can also be termed surface pattern deformation error (SFD error). Additionally, thermal effects affecting surface shapes may occur in fully assembled systems. Positioning errors may also occur, such as differences between the mounting position in the finished system and the mounting position during component measurement.

[0042] This embodiment also takes into account that the type of environment in which such EUV microlithography equipment will operate at the end-user's location is typically unknown until the completed optical imaging system is delivered to the system integrator, customer, or end-user. For example, availability performance may be affected by gravity conditions at the installation site, ground deformation caused by other nearby machinery, etc., to the point that it may not meet the specifications required for manufacturing operations. System measurements at the end-user's location can capture these conditions.

[0043] In the scenario outlined here, an imaging system with a tool mirror mounted to establish basic functionality may not meet the specifications of the manufacturing operation, but it is sufficient, for example, to perform further testing and / or preparation of the imaging system for commissioning at a second location. By operating the imaging system with the mounted tool module in auxiliary mode to perform testing and / or preparation for commissioning at the second location, the downtime of the EUV lithography equipment at the second location can be kept short, and useful measures may also be taken during the time required to complete the work on the selected mirror to change or correct the surface shape. Specifically, it is possible to perform the auxiliary mode of the auxiliary imaging system and the processing of the correction mirror for changing the surface shape simultaneously or concurrently, or at least in stages.

[0044] Once the work is complete and the selected optical module with the corrected surface shape has reached the second position, the tool module can be removed, and the optical module with the selected mirror having the modified surface shape can be reinstalled in the same mounting position.

[0045] Then, for example, when constructing another nominally identical optical imaging system, the tool module can be freely used further and can be used in the same manner.

[0046] According to another development, a particular advantage lies in using the same component measurement system as that used to measure, or also to measure, the selected mirror (correction mirror), which is replaced by the tool mirror, to perform the shape measurement for determining the surface shape of the tool mirror. This procedure takes into account the fact that even component measurement systems cannot always perform measurements without error; therefore, their measurement results may include absolute errors that are difficult to determine or require considerable effort to ascertain. However, if the correction mirror and the designated tool mirror are measured using the same component measurement system, the shape measurement results in both cases include the same absolute error, and therefore the error disappears or becomes ineffective in further steps of the method. In other words, if the only problem of interest is the optical difference effect caused by changes in surface shape, the absolute errors of the component measurements of the two mirrors in the interchanged pair cancel each other out in the formation of optical differences.

[0047] Generally, the measurement systems do not need to be exactly the same, but if they are identical, such as identical designs in pairs, it is sufficient. It is crucial to ensure that the absolute errors between the two surface pattern measurement systems are identical. Therefore, using exactly the same measurement system is advantageous.

[0048] In this approach, it is not absolutely necessary for all mirrors used to construct the optical imaging system to undergo shape measurements to determine their surface shapes. However, according to one development, shape measurements are performed using a component measurement system to determine the surface shape to be performed on each mirror provided for constructing the imaging system. If the surface shape is known within a range of measurement accuracy, more reliable conclusions and predictions can be derived from the system measurement results.

[0049] According to this concept, a tool mirror is used temporarily to establish an auxiliary imaging system, which can already be used for testing purposes, etc., while a replacement module with a calibration mirror, intended for permanent installation, is manufactured to finalize its surface shape and then delivered. After the exchange, the tool mirror can be freely used to construct other optical imaging systems, which require mirrors with existing or modified surface shapes, either as temporary tool mirrors or as "sharp" mirrors ultimately placed in the imaging system for later use. According to one development, the same tool module can thus be used multiple times. Therefore, the tool mirror initially installed in the imaging system can then be reinstalled in another newly built imaging system, for example, as a tool mirror for initial wavefront measurements or as a replacement mirror with a calibrated surface pattern for alignment. This concept is also referred to herein as the "concept of a cyclic tool mirror." Components of the tool module with the tool mirror can be remanufactured, repaired, or interchanged as needed. The number of cycles of the tool mirror can be limited, for example, two, three, four, or five times or more.

[0050] This concept also has advantages in terms of knowledge about the measurement system. According to one development, in addition to the first part measurement, at least one other part measurement is performed on a tool mirror using the same part measurement system, and the results of two or more shape measurements performed on the same tool mirror using the same part measurement system with a time interval are compared to identify possible drift effects at the part measurement system. In this variation, the specific tool mirror can thus serve as a reference element for calibrating the part measurement system.

[0051] The repeated use of the same tool mirror in different imaging systems can also be utilized in the following way: System measurements are performed on each imaging system to determine the imaging quality of the imaging system with one or more mounted tool mirrors, and the results of these system measurements are reconstructed to determine the contribution of the tool mirror's shape error. In other words, by mathematically combining the results of the system measurements on the imaging system, the surface pattern error introduced by the surface shape of the tool mirror can be determined through reconstruction.

[0052] The use of such tool modules with tool mirrors also provides other advantages to the fabrication process of the optical imaging system in EUV microlithography equipment. For example, according to one development, at least one other part measurement can be performed on the tool mirror by means of a part measurement system, and the drift effect at the part measurement system can be determined using the results of two or more shape measurements performed on the same tool mirror by the same part measurement system with a time interval.

[0053] Based on this development, the same tool module is provided for multiple uses in different imaging systems.

[0054] Furthermore, during the installation of the tool module, system measurements are performed on the imaging system to determine the imaging quality of the imaging system under each condition.

[0055] The initially installed tool reflector can then be reinstalled in a newly delivered system for initial wavefront measurements (system measurements) and alignment. Components of the tool module can be remanufactured, repaired, or interchanged as needed. The concept of a reusable tool module with a tool reflector is also referred to herein as a "recycled tool reflector concept."

[0056] The present invention also relates to an optical imaging system for an EUV microlithography apparatus, which is manufactured (initially manufactured) or repaired (after the use phase) by the method, and to an EUV microlithography apparatus equipped with the system. Attached Figure Description

[0057] Further advantages and aspects of the invention will become clearer from the claims and the following description of exemplary embodiments of the invention based on the accompanying drawings, in which:

[0058] Figure 1 Components of an EUV microlithography projection exposure apparatus having a projection lens according to an exemplary embodiment are shown;

[0059] Figures 2A to 2D The different steps of a method for manufacturing an optical imaging system are shown. Detailed Implementation

[0060] Exemplary embodiments of the present invention will be described below based on the manufacture and commissioning of a projection exposure apparatus for EUV microlithography.

[0061] Indicative Figure 1 Components of an EUV microlithography projection exposure apparatus EXP are shown. This apparatus is used to expose a radiation-sensitive substrate W disposed in an image plane IS of the projection lens PO using at least one image of a pattern of a reflective mask M disposed in an area of ​​the object plane OS of the projection lens PO. The projection lens PO is an example of an optical imaging system for EUV microlithography. The projection lens PO images the mask pattern onto the image plane at a reduced scale, in which the substrate W to be exposed, such as a semiconductor wafer, is disposed.

[0062] The projection exposure equipment is operated using radiation from the main radiation source RS. The illumination system ILL receives the radiation from the main radiation source and shapes the illumination radiation guided onto the pattern on the mask M. The projection lens PO is used to image the pattern structure onto the photosensitive substrate W.

[0063] The main radiation source produces radiation in the extreme ultraviolet (EUV) range, particularly radiation with wavelengths between 5 nm and 15 nm. To enable the lighting system and projection lens to operate within this wavelength range, they are constructed from optical elements that reflect EUV radiation.

[0064] The lighting system shapes the radiation from the radiation source and uses it to illuminate the illumination field located in or near the object plane OS of the projection lens PO. The shape and size of the illumination field determine the shape and size of the object field effectively used in the object plane OS. The illumination field is typically a slit shape with a large aspect ratio between its width and height.

[0065] The device RST for holding and manipulating the mask M (mask master) is configured such that the pattern disposed on the mask lies in the object plane OS of the projection lens PO, which is also referred to herein as the mask master plane. In this plane, the mask can be moved in the scanning direction (y direction) perpendicular to the reference axis of the projection lens (parallel to the z direction) for scanner operation with the aid of the scan driver.

[0066] The substrate W to be exposed is held in place by a device WST containing a scanner driver, which moves the substrate synchronously with the mask M in a scanning direction (y-direction) perpendicular to the reference axis. Depending on the design of the projection lens PO, these movements of the mask and the substrate can be parallel to each other or parallel to each other in opposite directions.

[0067] The WST (also known as the “wafer platform”) and RST (also known as the “mask master platform”) are part of a scanner device controlled by a scan control device, which in this embodiment is integrated into the central control unit (CU) of the projection exposure equipment.

[0068] The projection lens PO in this example comprises six mirrors M1 to M6, each with a concave or convex reflective surface. These surfaces can be of free shape. An intermediate image is generated between the object field and the image field. Other configurations are also possible, such as having more or fewer mirrors with or without an intermediate image.

[0069] All optical components of the projection exposure system EXP are housed within a vacuum-evaporizable housing H. The projection exposure apparatus operates under vacuum. EUV projection exposure apparatus is known, for example, from the disclosure DE 10 2021 201 162 A1, the contents of which are incorporated herein by reference.

[0070] The projection exposure apparatus includes a wavefront measurement system (WMS) that operates at EUV operating wavelengths and is designed to measure the wavefront of projected radiation that propagates from a mask through a projection lens to the substrate to be exposed. Preferably, spatially resolved measurements at multiple field points are provided. For example, wavefront measurement systems of the type described in US 7,333,216 B2 or US 6,650,399 B2 are available, the disclosures of which are incorporated herein by reference.

[0071] Reference illustration Figures 2A to 2D This paper explains some characteristics of the method proposed for manufacturing optical imaging systems in the form of EUV projection lenses. Figures 2A to 2D Different method steps are shown, some of which occur in different locations. Figure 2A and 2C The process occurring at the projection lens manufacturer's location (first location, LOC1) is shown. Figure 2B and 2D This relates to the processes and corresponding method steps that occur remotely at the end-user's manufacturing facility location and at a second location (LOC2) of the projection lens.

[0072] In the illustrative example, the projection lens has four reflectors (first reflector M1, second reflector M2, third reflector M3, and fourth reflector M4), which are mounted in appropriate positions on the force frame FF. In a ready-to-use assembled, aligned, and calibrated state ( Figure 2D The projection lens PO meets the specifications for its use in manufacturing operations and can be used to manufacture delicate structural components such as semiconductor chips. Projected radiation for imaging travels from the pattern of the mask master M along... Figure 2B and 2D The projection beam path P, schematically shown, propagates through the reflective surfaces of the first reflector M1, the second reflector M2, the third reflector M3, and the fourth reflector M4 to the surface of the semiconductor wafer W to be structured, generating a mask image on it.

[0073] At the final assembly and usage location (second location, LOC2), the System Measurement System (SMS) can be used to measure the wavefront of the projected radiation traveling from the object plane OS to the image plane IS via spatially resolved wavefront measurement, and compare it with the wavefront required according to the specification. This allows for comparison of the actual imaging quality with the target imaging quality according to the specification.

[0074] The System Measurement System (SMS) is used as part of the assembly and alignment of the projection lens to bring it into a ready-to-use state, i.e., conforming to specifications. In this example, the SMS is part of the projection exposure apparatus and is also used for wavefront measurement during the operation of the projection exposure apparatus. It is used to adapt the imaging characteristics of the projection lens to varying boundary conditions, if appropriate, with the aid of a manipulator.

[0075] The wavefront of the projected radiation P is particularly affected by the surface shape of the optically usable area of ​​the mirror surface. This surface shape is also referred to herein as the "surface pattern." Furthermore, the wavefront quality is largely determined by the accuracy of the spatial position of the reflecting surface within the projection lens. Any deviation from the target position will affect the wavefront progression and the corresponding imaging quality error. This connection is utilized during alignment in the rigid body degrees of freedom.

[0076] Each mirror has a target surface shape specified according to the standard, and ideally, this target surface shape should exist to provide the theoretically best possible imaging performance under perfect alignment. However, in reality, there are deviations from the target surface shape caused by various error sources, which are also known as surface pattern errors.

[0077] In the illustrated method, the surface shape of each reflector used in the process is determined by the manufacturer at the first location (LOC1) using a component measurement system (CMS) as part of the shape measurement. As shown, the CMS is constructed using a Fizeau interferometer. Since such measuring devices are known in themselves, a detailed description is omitted here. Examples, for instance, can be found in WO 2006 / 077145 A2.

[0078] In the exemplary method, the surface shape of each mirror used in the process is determined by shape measurement, preferably using an identical part measurement system (CMS). The advantages of this method will be further explained below. In short, the negative impact of the unavoidable absolute errors of the part measurement techniques used can be eliminated when the only critical issue is the difference in surface patterns—that is, the difference in surface patterns between mirrors. More details will be explained below.

[0079] The projection lens PO is designed to allow some or all of the reflectors to be relatively easily interchangeable, for example, for maintenance and repair purposes. To this end, each reflector is part of an optical module, which can be mounted in a designated location with a fixed spatial relationship to the force frame, and, depending on its design, can also be easily disassembled and interchanged. In this example, all reflectors are integrated into optical modules designed for replacement. In some other embodiments, only a subset of the reflectors can be easily interchanged in this manner.

[0080] In the context of the methods used to manufacture the projection lens, different types of optical modules are used, which are identified by different shaded lines in Figure 2.

[0081] In this example, during the method, the first reflector M1 and the third reflector M3 are installed in place at once, and their positions are changed during alignment in the installation location area if appropriate, but they are not swapped again.

[0082] At least one of the optical modules is configured as an interchangeable replacement module and is designed to have a reflector selected as a correction reflector. These optical modules or reflectors are marked with the reference numeral CM (correction reflector). In this example, the projection lens includes two such correction reflectors: a second reflector M2 and a fourth reflector M4. Their function will be described below.

[0083] In addition, two tool modules are used in the manufacture of the projection lens PO, each tool module having a tool mirror (reference numeral TM). Each interchangeable optical module with a correction mirror CM is assigned exactly one tool mirror TM.

[0084] A tool module with a tool reflector is characterized by having a mounting structure compatible with the mounting position of the relevant optical module. It is designed to allow for the replacement of the module and the inclusion of a correction reflector CM, thus enabling, in principle, installation in the same spatial location at the same mounting position. Another important criterion is that, based on shape measurements using a component measurement system, the surface shape of the tool reflector should be the same as or substantially the same as the surface shape of the assigned reflector CM selected as the correction reflector.

[0085] The tool module with the tool reflector™ can therefore be a design twin of the assigned optical module with the correction reflector CM. However, uniformity in surface shape and reflective coating is technically impossible and unnecessary. It is sufficient if the optical performance of the tool reflector is substantially identical to that of the assigned correction reflector CM; therefore, meaningful system measurements can be performed if the assigned tool module with the assigned tool reflector (rather than the optical module with the correction reflector) is mounted in the correct position.

[0086] Projection lenses with mounted tool reflectors may have compliant optical performance, so in principle the tool reflector can be retained within the fully assembled projection lens.

[0087] However, this correspondence between the tool reflector and the correction reflector is unnecessary, as long as the method is designed to prevent the tool reflector from remaining in the projection lens, but to replace it with an assigned optical module containing the correction reflector before commissioning and the start of manufacturing operations.

[0088] In the illustrated example, Figure 2B This illustrates an intermediate stage of the manufacturing process, where the mounting tools for the second reflector M2 and the fourth reflector TM are shown. In the completed system ( Figure 2D Then, the corresponding optical module with the correction mirror CM is installed in the same mounting position as the second and fourth mirrors.

[0089] For example, the following process can be used for manufacturing. First, all reflectors are measured using a component measurement system (CMS) to determine their surface shape (i.e., surface pattern). It should be noted that the surface shape or surface pattern of the tool reflector TM and the assigned calibration reflector CM should be known as precisely as possible. Using the same component measurement system to measure both the calibration and tool reflectors solves the potential problems caused by the unavoidable absolute errors in surface shape measurement techniques or the CMS, because in this method, only the difference in surface patterns—that is, the difference between the surface pattern of the installed tool reflector and the surface pattern of the replacement reflector to be installed—must be known as precisely as possible. It is generally preferable to use the same component measurement system to measure the surface shape of all reflectors to be installed.

[0090] At the second location LOC2, for example at the end-user's location, the projection lens PO is then assembled, part of which corresponds to the projection lens to be manufactured. However, instead of the reflector set as the correction reflector CM or its optical module, it still includes the corresponding associated tool reflector TM and its optical module. The projection lens has the mounted tool reflector (see...). Figure 2B This can be considered an auxiliary imaging system that has not yet reached the imaging quality required for manufacturing operations within specifications, but the imaging quality should be good enough to begin operation in auxiliary mode, such as for alignment in solid-state degrees of freedom.

[0091] The auxiliary imaging system is then mechanically aligned as much as possible under the control of the system measurement system (SMS) until the reflector achieves its initial, possibly optimal, spatial position in the mounting location. Final system measurements are then performed to determine the optimal alignment of the auxiliary imaging system after mounting the optical modules and rigid body alignment at the mounting location, thus determining the optimal image quality.

[0092] The measured imaging quality is then compared to the target imaging quality required for the manufacturing operation to identify any imaging quality errors. These residual aberrations or remaining imaging quality errors typically cannot be significantly reduced by aligning solid degrees of freedom.

[0093] This is where the correction mirror (CM) comes into play. Based on system measurements and comparisons with target imaging quality, the surface shape that the correction mirror (CM) belonging to the tool mirror should have is determined to minimize the measured wavefront error. In other words, for each installed tool mirror, the differential surface shape or surface shape difference required to achieve the imaging performance specified in the specification is calculated.

[0094] Once this information is available, the mirror selected as the correction mirror CM can be processed to transform its surface shape into a modified shape suitable for reducing imaging errors. In this typically very time-consuming process, the assembled auxiliary imaging system ( Figure 2B You can continue operating, for example, to prepare for further debugging steps or to test other specifications of the projection lens.

[0095] After the surface processing of the correction mirror CM is completed, the correction mirror CM is replaced with the corresponding tool mirror by removing the optical module with the tool mirror from the auxiliary imaging system and installing the replacement module with the corresponding correction mirror in its position through a replacement operation (exchange operation).

[0096] Further systematic measurements are then performed, followed by rigid body alignment to reduce the optical effects of mechanical mounting tolerances. Generally, the image quality is significantly improved. Experience shows that this can be optimized through further (small) alignment steps of the rigid body degrees of freedom, again under the control of the System Measurement System (SMS), until the best possible image quality is achieved.

[0097] Compared to traditional concepts, the procedure presented here as an example offers significant economic advantages in terms of resource utilization and time consumption without sacrificing imaging performance. Furthermore, this concept offers the advantage that the projection lens manufacturer (located here at the first position LOC1) does not need to provide any expensive system measurement techniques, such as, for these purposes, in the form of a spatially resolved wavefront measurement system. System measurements are performed by the end user at the second position LOC2, where the system measurement techniques have already been used in the projection exposure equipment for subsequent manufacturing operations.

[0098] This method can also be described as follows: The end user initially has a largely misaligned imaging system, but the surface shape or surface pattern of the individual mirrors containing the tool mirror™ is known through component measurements. Although the manufacturer does not require the system measurement technology already available to the user, this process provides the possibility of precisely manufacturing one or more correction mirrors CM to reduce residual aberrations remaining after rigid body alignment. This system correction requires mirror replacement. However, this does not result in significant downtime at the user end because the projection lens is already available as an auxiliary imaging system with mounted tool mirrors, used for many tasks prior to commissioning.

[0099] After being used to manufacture projection lenses, tool mirrors can be used for other purposes, such as for the manufacture of projection lenses with nominally identical structures. They can be used herein as tool mirrors or permanently mounted calibration mirrors, provided the quality of the mirror surface (including the reflective coating) is good enough. Therefore, an initially installed tool mirror™ can be reinstalled in a newly delivered system for initial wavefront measurements and alignment. Components of the tool mirror or optical module can be remanufactured, repaired, or replaced as needed.

[0100] Tool mirrors can be used consecutively in multiple processes, such as two, three, four, five, six, or more processes. If the quality is insufficient after use, the tool mirror can be remanufactured and reused.

Claims

1. A method for manufacturing an optical imaging system for an EUV microlithography apparatus, wherein the optical imaging system has a plurality of optical modules, each optical module carrying a mirror, the optical modules being mounted along an imaging beam path guided from an object plane to an image plane of the imaging system, the optical modules being mounted in a distributed mounting position on a force frame, wherein at least one of the optical modules is designed as an interchangeable replacement module having a mirror selected as a correction mirror, the method comprising the following steps: A) The surface shape of the mirror selected as the correction mirror is determined by shape measurement using a component measurement system; B) Provide a tool module with a tool reflector, wherein (i) The tool module has a mounting structure that is compatible with the mounting location of the optical module designed for module replacement; as well as (ii) According to the shape measurement using the component measurement system, the surface shape of the tool reflector is the same as or substantially the same as the surface shape of the correction reflector; C) An auxiliary imaging system is established by installing an optical module with a reflector at the designated mounting location of the force frame, wherein the tool module is installed at the mounting location of the optical module designed for module replacement. D) Perform system measurements using a system measurement system to determine the imaging quality of the auxiliary imaging system after the optical module is installed at the mounting position and rigidly aligned. E) Compare the measured image quality with the target image quality of the optical imaging system to determine the image quality error; F) Determine the changes in the surface shape of the corrective mirror suitable for reducing the imaging quality error; G) The corrective mirror is processed to change its surface shape to a modified surface shape suitable for reducing the imaging error; H) Remove the tool module and install the optical module, which is designed as a replacement module, wherein the correction mirror has the modified surface shape; I) Perform system measurements to determine the imaging quality of the optical imaging system.

2. The method as described in claim 1, characterized in that, The results of the system measurement are evaluated, and if the system measurement indicates that the image quality is outside the tolerance range, an alignment operation is performed to align the mounted optical module in its rigid body degrees of freedom to improve the image quality, and further system measurements are performed, wherein the alignment operation and system measurements are repeated until the system measurement indicates that the image quality falls within the tolerance.

3. The method as described in claim 1 or 2, characterized in that, The shape measurement is performed at a first location, particularly at the location of the manufacturer of the imaging system, and at a second location, away from the first location, particularly at the location of the end user or system integrator.

4. The method as described in any one of the preceding claims, characterized in that, A wavefront measurement system is used to perform the system measurement, wherein spatially resolved wavefront measurements at multiple field points are preferably performed.

5. The method as described in any one of the preceding claims, characterized in that, The system measurements are performed in the EUV lithography apparatus, where the system measurement system is integrated into the EUV lithography apparatus.

6. The method as described in any one of the preceding claims, characterized in that, The shape measurement on the tool mirror is performed using the same component measurement system as the shape measurement on the mirror assigned to the selected correction mirror.

7. The method as described in any one of the preceding claims, characterized in that, A component measurement system performs shape measurements on each of the mirrors provided for constructing the imaging system to determine the surface shape.

8. The method as described in any one of the preceding claims, characterized in that, Operate the assisted imaging system with the installed tool module in assisted mode to perform tests and / or prepare for commissioning at the second position.

9. The method as described in claim 8, characterized in that, The auxiliary imaging system and the machining of the correction mirror for changing the surface shape are performed simultaneously in at least stages.

10. The method as described in any one of the preceding claims, characterized in that, To determine the drift effect at the component measurement system, the results of at least one other component measurement performed on the tool mirror using the same component measurement system, and the results of two or more shape measurements performed on the same tool mirror using the same component measurement system, are compared with the time distance.

11. The method as described in any one of the preceding claims, characterized in that, The same tool mirror was used multiple times in different imaging systems; When the tool's reflector is installed, the imaging system is subjected to system measurements to determine the imaging quality of the imaging system.

12. The method as described in any one of the preceding claims, characterized in that, This method is designed for manufacturing or repairing optical imaging systems, which are designed as projection lenses of EUV projection exposure devices or EUV mask inspection devices for inspecting EUV microlithography masks.

13. An optical imaging system for an EUV microlithography apparatus, comprising a plurality of optical modules, each optical module carrying a mirror, the plurality of optical modules being mounted at a distribution mounting position on a force frame along an imaging beam path guided from the object plane to the image plane of the imaging system, wherein at least one of the optical modules is designed as an interchangeable replacement module having a mirror selected as a correction mirror. Its features are, The optical imaging system is manufactured using the method described in any one of the preceding claims.

14. An EUV microlithography apparatus comprising the optical imaging system as described in claim 13.

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