Method for manufacturing the optical imaging system of a microlithography device

Abstract

In a method for manufacturing the optical imaging system of an EUV microlithography apparatus, a plurality of optical modules, each bearing a mirror, are installed at assigned installation positions on a force frame. At least one of the optical modules is designed as a replaceable replacement module that includes a mirror selected as a compensating mirror. The method comprises the steps of: A) performing shape measurements to determine the surface shape of the mirror selected as a compensating mirror using a component measurement system; B) preparing a tool module including a tool mirror, wherein i) the tool module has a mounting structure compatible with the installation position of the optical module designed as a replacement module, and ii) the tool mirror has the same or substantially the same surface shape as the compensating mirror according to the shape measurements in the component measurement system; and C) providing an auxiliary imaging system by installing the optical module including the mirror at the assigned installation position on the force frame, wherein the tool module is installed at the installation position of the optical module designed as a replacement module. The process includes the steps of: D) performing system measurements using a system measurement system to determine the image quality of the auxiliary imaging system after the optical module has been installed at the installation location and rigid alignment has been performed; E) comparing the measured image quality with the target image quality of the optical imaging system to determine the image quality error; F) determining the change in the surface shape of the corrective mirror suitable for reducing the image quality error; G) processing the corrective mirror to change its surface shape to a modified surface shape suitable for reducing the image error; H) removing the tool mirror and installing an optical module including a corrective mirror having a modified surface shape designed as a replacement module; and I) performing system measurements to determine the image quality of the optical imaging system.

Description

Technical Field

[0001] The following disclosure is based on German Patent Application No. 102023113819.7 filed on May 25, 2023, which is incorporated herein by reference in its entirety.

[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 of 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 an EUV mask inspection apparatus for inspecting a mask (reticle) for EUV microlithography.

Background Art

[0004] Today, microlithography projection exposure methods are mainly used in the manufacture of other microstructured components such as semiconductor devices and photolithography masks. In this case, a mask (reticle) or other pattern manufacturing device that holds or forms the pattern of the structure to be imaged, for example, the line pattern of a layer of a semiconductor device, is used. The pattern is positioned in the region of the object plane of the imaging system between the illumination system and an optical imaging system, usually 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 changed by the pattern travels along the imaging beam path through the imaging system, and the imaging system forms a reduced image on the substrate to be exposed. The surface of the substrate is arranged in the image plane of the imaging system that is optically conjugate to the object plane. The substrate is usually covered with a radiation-sensitive layer (resist, photoresist).

[0005] One of the goals in the development of projection lithography systems is to reduce the size of structures fabricated on substrates by lithography, for example, to achieve higher integration density in semiconductor devices. One approach is to use electromagnetic radiation with shorter wavelengths. For this purpose, optical systems using electromagnetic radiation in the range of 5 nanometers (nm) to 30 nm, particularly in the extreme ultraviolet (EUV) region with an operating wavelength of 13.5 nm, have been developed. Imaging systems for EUV microlithography systems use only mirrors to image structures from the object plane to the image plane, for example, from the reticle to the wafer.

[0006] Many modern EUV projection lenses are modular in design. The optical modules are installed in designated mounting positions within a common force frame, each equipped with a mirror, and are positioned in a completed mounting state at the respective mounting positions for the optical modules, with the optically effective surfaces of the mirrors forming the imaging beam path. To facilitate the assembly, maintenance, and, where appropriate, optimization of such imaging systems, (one or more) optical modules can be designed as interchangeable optical modules, i.e., replaceable modules (see, for example, Patent Document 1).

[0007] EUV microlithography equipment is a technically complex capital asset, and its procurement and operation incur significant costs for end-users. Such investments can only be justified if the microlithography equipment at the point of use reliably and permanently meets technical specifications and can operate productively without major interruptions. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] German Patent Application Publication No. 10 2021 201 162 [Overview of the project] [Problems that the invention aims to solve]

[0009] Against this backdrop, the present invention aims to provide a method for manufacturing an optical imaging system for an EUV microlithography apparatus, a corresponding optical imaging system, and an EUV lithography apparatus equipped therewith, so that the optical imaging system can reliably meet the manufacturer's expected specifications at its point of use, minimize downtime for maintenance purposes, and operate with high productivity. [Means for solving the problem]

[0010] To achieve these objectives, the present invention provides a method for manufacturing an optical imaging system having the features described in claim 1. Furthermore, an optical imaging system having the features described in claim 13 and an EUV microlithography apparatus having the features described in claim 14 are also provided.

[0011] This method is used to manufacture (or restore) the optical imaging system of an EUV microlithography apparatus. The optical imaging system comprises a plurality of optical modules, each carrying a mirror, along the imaging beam path from the object plane to the image plane of the imaging system. The optical modules are installed in assigned positions in a fixed spatial relationship with respect to the force frame. At least one of the optical modules is designed as a replaceable replacement module, including a mirror selected as a compensating mirror.

[0012] In this application, “optical module” includes a mirror and other components. The mirror includes a mirror substrate. A region of the substrate surface is optically precisely machined and thus substantially determines the surface shape of the mirror. In the case of an optical module completed for mass production operation of an EUV apparatus, this region of the substrate surface is covered with a reflective coating that has a reflective effect on EUV radiation to optimize reflectivity. Substrate fixing components, such as substrate-side components of a retaining frame, or carriers that support the mirror substrate on or within the retaining frame, and components of actuators and / or sensors, where appropriate, are also attached to the mirror substrate. Where appropriate, the position of the mirror substrate relative to the retaining frame can be set or changed by a suitable setting device, for example, for rigid alignment. Components of the setting device may be attached to the mirror substrate. The retaining frame may have mounting structures, such as flange-like mounting structures, for attaching the retaining frame to a force frame. Since the optical module may be designed to have interfaces between interchangeable optical modules and force frames, between retaining frames and force frames, or within the retaining frame, the optical module includes a mirror substrate and components of a retaining frame coupled thereto. The interface may be located within the area of ​​the retaining frame or the area of ​​the support device, in which case the optical module would consist only of the mirror substrate and the components to which the retaining frame or support frame is attached, and, where appropriate, the components of the sensor and / or actuator mounted and fixed to the substrate, and the components of the retaining frame or support device and / or the sensor and / or actuator components would not be replaced during replacement operations.

[0013] This method is based on the concept that at least one of the optical modules is designed as a replaceable module that includes a mirror selected as a corrective mirror. Such an optical module will be referred to here as the "optical module with corrective mirror" or the "replaceable module with corrective mirror." The underlying idea is that by changing the optically effective surface of the corrective mirror based on the results of system measurements, the level of residual aberration can be reduced to a degree that the imaging system meets its specifications, thereby allowing any residual aberration remaining after system alignment to be corrected using the corrective mirror. Therefore, the corrective mirror is a mirror prepared or selected as a corrective mirror, or a mirror whose surface shape needs to be modified to correct residual aberration remaining after rigid body alignment, if appropriate. This method involves multiple steps, and each step is also denoted by a capital letter here for simplification of reference.

[0014] According to step A, shape measurement is performed on the mirror selected as the corrective mirror to determine the surface shape of its optically usable area. Shape measurement is performed using a component measurement system, for example, by interference. For example, a Fizeau interferometer can be used as the component measurement system. An example of such an interferometer is disclosed in International Publication No. 2006 / 077145. From the shape measurement, the surface shape of the corrective mirror is determined within the measurement accuracy range of the component measurement system.

[0015] The substrate surface is processed to be extremely smooth in the optically used area, so it strongly reflects, for example, the visible light (VIS) of the spectrum. Therefore, especially for shape measurement using measurement light in the VIS region, it is not necessary for the substrate surface in the optically used area to already have a coating that reflects EUV radiation. Shape measurement can be performed on uncoated or coated substrates.

[0016] Furthermore, according to step B of this method, a tool module including a tool mirror is provided. Within the scope of the present invention, “tool module” is an optical module that is assigned to a specific optical module or a replacement module including a compensating mirror, has a mounting structure compatible with the installation location of the assigned optical module, and is equipped with a tool mirror that has the same or substantially the same surface shape as the compensating mirror of the assigned replacement module including the compensating mirror, according to shape measurements in the component measurement system.

[0017] Tool mirrors can have the same surface shape and optical effect as assigned compensating mirrors within manufacturing tolerances. However, this is not mandatory. Tool mirrors may have measurably different optical effects from the assigned optical module, including the compensating mirror. However, any differences in optical effects should be so small that they do not worsen the aberration level, and meaningful system measurements remain possible.

[0018] Each mirror for the EUV system comprises a mirror substrate, which may be made of, for example, glass or glass ceramic with a low coefficient of thermal expansion, and the mirror substrate has a substrate surface that is optically precisely machined and substantially determines the surface shape of the mirror. To optimize the reflectivity to incident EUV radiation, the substrate surface has a reflective coating that has a reflective effect on EUV radiation, such as a multilayer reflective layer composed of many individual layers. Tool mirrors are also configured in this way so that their reflective properties, particularly reflectivity and effect on the wavefront, are as similar as possible to those of assigned correction mirrors.

[0019] Tool mirrors can have relaxed specifications, for example, regarding mechanical handling, and can be designed to have minimal mechanical defects (e.g., scratches, protrusions, or cosmetic damage) or lifecycle impacts. In particular, it is important to have the surface geometry of the tool mirror as precisely as possible at the time of system measurement.

[0020] For example, an optical module that is nominally identical to the assigned optical module to be replaced, but has deteriorated to the point where it needs to be replaced after long-term use in a different location, can be used as a tool module. Therefore, even used optical modules can be used as a "means to achieve an objective," or more precisely, as an aid in system measurement. This saves resources and reduces costs.

[0021] To save time on manufacturing new tool mirrors, it is also possible to use mirrors with special tolerances as tool mirrors. In this sense, the tool mirror only serves as an auxiliary mirror for the optical imaging system, which needs to be realigned to ensure reliable system measurements.

[0022] According to step C, an auxiliary imaging system is provided by installing an optical module including a mirror at the relevant mounting position on the force frame, and the assigned tool module is installed at the mounting position of the optical module designed as a replacement module (a replacement module including an auxiliary mirror). The auxiliary imaging system is structurally different from the optical imaging system to be manufactured or restored, and is a temporary imaging system in particular, in that the assigned tool module is installed in place of the optical module including the auxiliary mirror.

[0023] According to Step D, a system measurement is then performed on the assembled auxiliary imaging system to determine the imaging quality of the auxiliary imaging system. This system measurement is performed after the optical module has been installed and the assembled module has been rigidly aligned at its installation location. The rigid alignment of the optical module is supported by the results of the system measurement, ensuring that the potential of the rigid alignment is maximized and that the imaging performance of the auxiliary imaging system is optimized toward the desired target imaging performance. The system measurement is performed using a system measurement system, such as a wavefront measurement system.

[0024] In step E, the imaging quality measured in step D is compared with, for example, the target imaging quality of the optical imaging system obtained from the specifications. The measured imaging quality is compared with the target imaging quality to obtain an imaging quality error. Based on the comparison, subsequently in step F, the way to change the surface shape of the (at least one) correction mirror is determined to reduce the imaging quality error and conform the auxiliary imaging system to the specifications.

[0025] The information obtained in step F is used in step G. In this method step G, the mirror selected as the correction mirror is processed such that its surface shape is changed towards a modified surface shape suitable for reducing the imaging quality error.

[0026] For this purpose, an uncoated mirror (a mirror substrate whose optical use area is polished to a high gloss or micro - machined to a high gloss by another method but has not yet been coated) can be used as the correction mirror, and the optical use area of its surface is changed by being processed by an appropriate shaping process, for example, by ion beam etching. A reflective coating designed for EUV radiation is subsequently 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 with the reflective coating already provided can be processed as described, for example, in U.S. Patent Application Publication No. 2012 / 212712, U.S. Patent Application Publication No. 2014 / 307308, U.S. Patent Application Publication No. 2012 / 300184, German Patent Application Publication No. 10 2014 225 197, U.S. Patent Application Publication No. 2019 / 018324, German Patent Application Publication No. 10 2021 213 148, German Patent Application Publication No. 10 2015 201 141, U.S. Patent Application Publication No. 2016 / 209750, or German Patent Application Publication No. 10 2011 076014.

[0027] After the machining to change the surface shape of the mirror selected as the correction mirror is completed, an exchange operation (swap operation) is performed according to step H. At that time, the installed tool module is removed from its installation position, and an assigned optical module including a correction mirror having a modified surface shape designed as a replacement module is installed at the installation position instead of the tool module. Therefore, the auxiliary imaging system becomes a desired imaging system with respect to the structural components, which is also referred to as the final imaging system.

[0028] The success of this measure is confirmed by performing a system measurement to determine the imaging quality of the optical imaging system for control purposes according to step I.

[0029] Generally, after the installation of the optical module provided with the correction mirror, in order to optimize the imaging quality of the imaging system, it is advantageous to perform a rigid body alignment of the optical module installed in the imaging system. Here, a plurality of further system measurements and alignment steps for optimizing the spatial position of the optical module at each installation position based on them may be useful in an iterative method. Therefore, this method preferably includes an evaluation of the results of the system measurement, an alignment operation for aligning the installed optical module in its rigid body degrees of freedom to improve the imaging quality when the results of the system measurement show an imaging quality outside the tolerance, and further system measurements, and the alignment operation and the system measurement are repeated until the system measurement shows an imaging quality within the tolerance.

[0030] Therefore, in this method, an assigned tool module, including a tool mirror, is used for at least one optical module designed as an interchangeable replacement module, including a compensating mirror. The tool module is temporarily mounted on the force frame of the optical imaging system. At the same time, further work can be performed on the resulting auxiliary imaging system in preparation for commissioning, and the mirror provided as the compensating mirror can be machined based on system measurements of the auxiliary imaging system with the tool mirror. This greatly improves time management, as many activities required in the manufacturing process can be performed simultaneously and, if appropriate, in different locations.

[0031] After system measurements of the auxiliary imaging system determine how the mirror selected as the corrector mirror must be processed to reduce the level of residual aberration, the processing of the mirror selected as the corrector mirror (step G) can already be started and executed while the auxiliary imaging system is still operational and can still be used for activities related to commissioning preparation, for example. This reduces the time required for commissioning the manufactured imaging system. This minimizes the downtime of the EUV microlithography equipment. Even if a processing recipe for changing the surface shape of the corrector mirror is already available, the tool mirror can remain in the auxiliary imaging system for a certain period until the optical module, which includes the processed corrector mirror designed as a replacement module, arrives and can be installed. This time can be used, for example, for testing or for necessary commissioning work that does not require perfect optical performance. Thus, the auxiliary imaging system, including the installed tool module, can be operated in auxiliary mode to prepare for testing and / or commissioning at a second location.

[0032] While there may be only one interchangeable optical module in the optical imaging system, including the mirror selected as the corrective mirror, it is preferable that multiple such optical modules are provided, for example, two, three, or four such interchangeable modules and corresponding assigned tool mirrors.

[0033] Shape measurements (one or more) and system measurements (one or more) can be performed in the same location, for example, at the manufacturer's premises of the mirror and / or imaging system, in the same measurement space or in different measurement spaces within the same manufacturing site.

[0034] However, according to one developmental form, shape measurement (singular or plural) is performed at a first location, and system measurement is performed at a second location located away from the first location. For example, the first and second locations may be located in different cities or regions of the same country, or in different countries and / or on different continents. For example, the distance between the first and second locations may be greater than 100 km and / or greater than 1,000 km and / or greater than 10,000 km.

[0035] This spatial separation between component measurements (measurements on individual components) and system measurements (measurements on assembled systems with multiple components) offers numerous technical and economic advantages, taking into account the many uncontrollable sources of error in the manufacturing of such highly complex optical imaging systems.

[0036] Shape measurement is preferably performed at the manufacturer's location (the first location), i.e., in spatial proximity to the mirror manufacturing site. This allows for efficient and low-error interaction between manufacturing and control using shape measurement. There is no need to maintain expensive system measurement technology at the first location.

[0037] The second location could be, for example, the location of a system integrator that constructs an EUV microlithography system consisting of many further components using an optical imaging system. The second location could also be the point of use, as system measurements can be performed at the end customer's point of use if appropriate. System measurements are used to determine the imaging quality of the constructed auxiliary imaging system. These measurements then record the effects of the operating environment and any effects that may occur during transportation between the mirror manufacturing location and the point of use of the entire system.

[0038] System measurements may be performed at the system integrator's site, while replacements (swap operations) may be carried out at the end customer's site. This allows for parallel manufacturing of the corrective mirrors and transportation of the entire system to the customer, which also saves time. Therefore, individual method steps can even be spread across more than two, especially three, different locations.

[0039] This method takes into account that both the individual components of the imaging system and the entire system, composed of many individual components, must meet their respective specifications. It also considers that environmental conditions at subsequent usage locations are likely to differ from those at the first location, and therefore, on-site system measurements by the component manufacturers may not always be sufficiently meaningful. Accordingly, this modification assumes a spatial separation between the first and second locations and optimizes the distribution of necessary measurement tasks.

[0040] It is preferable to perform system measurements using a wavefront measurement system. Preferably, spatially resolved wavefront measurements are performed for multiple field points. It is preferable that system measurements be performed directly on the EUV microlithography apparatus using integrated measurement techniques. This eliminates the need to construct a separate system measurement facility. Instead, integrated measurement techniques can be used, which can also be used in further operations of the EUV microlithography apparatus.

[0041] The optical effect of a mirror is determined, in particular, by its surface shape, which is also referred to here as the "surface shape." Therefore, deviations from the target surface shape specified according to the optical design can be called surface shape errors. Each mirror should have a precisely specified surface shape. The mirror surfaces of EUV systems are often designed as free-form surfaces, meaning they have surface shapes that are significantly different from spherical or rotationally symmetric aspherical shapes. Precise manufacturing is extremely complex. This makes accurate shape measurement using component measurement systems even more important.

[0042] Measurement errors can occur in these shape measurements. In addition, processing errors may occur during shaping. Furthermore, surface shape may change due to the introduction of deformation caused by measures taken during assembly, for example, due to adhesive effects or screw connection effects, which can also be called surface deformation error (SFD error). In addition, thermal effects that affect surface shape may occur in a fully assembled system. Positional errors may also occur, for example, if the installation position in the completed system differs from the installation position during component measurement.

[0043] This embodiment also takes into account that, generally, the type of environment in which such an EUV lithography system will operate at the end user's site is unknown before the completed optical imaging system is supplied to the system integrator, customer, or end user. For example, practical performance may be affected to such an extent that it cannot achieve the specifications required for mass production operation due to gravity conditions at the installation site, ground deformation caused by other surrounding machinery, etc. These conditions can be understood by measuring the system at the end user's site.

[0044] In the scenario outlined here, installing the tool mirror creates a basically functional imaging system, whose imaging performance may not meet the specifications for mass production operation, but is sufficient for, for example, performing further tests on the imaging system and / or preparing it for commissioning at a second location. By operating the imaging system with the installed tool module in auxiliary mode for testing and / or preparation for commissioning at a second location, downtime for the EUV microlithography equipment at the second location can be kept short, and useful measures can be taken during the time required to complete work on the selected mirror for surface shape modification or correction. In particular, the auxiliary mode of the auxiliary imaging system and the processing of the correction mirror for surface shape modification may be performed at least in stages, simultaneously, or in overlapping or parallel time.

[0045] Once this process is complete and the selected optical module with the corrected surface shape arrives at the second location, the tool module can be removed, and the optical module, including the selected mirror with the corrected surface shape, can be reinstalled in the same installation position.

[0046] The tool module can then be used further, for example, when constructing another nominally identical optical imaging system.

[0047] In another developmental form, a particular advantage is that the shape measurement to determine the surface shape of the tool mirror is performed using the same component measurement system that measures or is measuring the replacement mirror (corrective mirror) that is also being measured. This procedure takes into account that even in a component measurement system, measurements are not always error-free, and as a result, the measurement results may contain absolute errors that are undeterminable or require considerable effort to determine. However, when the corrective mirror and the assigned tool mirror are measured using the same component measurement system, the result of the shape measurements in both cases contains the same absolute error, resulting in the error disappearing or playing no role in the subsequent processes of the method. In other words, if the sole point of interest is the optical difference effect resulting from the change in surface shape, the absolute errors of the component measurements for both mirrors of the replacement pair cancel each other out in the formation of the optical difference.

[0048] Generally, these do not necessarily have to be the exact same measurement system; it may suffice if the measurement system is identical, i.e., one of two systems of the same design. It is important to ensure that the absolute errors between the two surface shape measurement systems are exactly the same. For this reason, using the exact same measurement system is advantageous.

[0049] In this method, it is not necessarily required to perform shape measurements to determine the surface shape of all mirrors used in constructing the optical imaging system. However, in one advanced form, shape measurements to determine the surface shape are performed on each mirror used in constructing the imaging system by a component measurement system. If the surface shape is determined within the range of measurement accuracy, more reliable conclusions and predictions can be derived from the system measurement results.

[0050] According to this concept, by using a tool mirror only temporarily, an auxiliary imaging system is temporarily provided, and this auxiliary imaging system can already be used for testing or other purposes while a replacement module containing a permanently installed auxiliary mirror is being manufactured and supplied to finalize its shape. After the replacement operation, the tool mirror can be used, if appropriate, as a temporarily used tool mirror in the construction of another optical imaging system that requires a mirror with an existing or modified surface shape, or as a "final-finished" mirror that will be finally placed in an imaging system for later use. Thus, according to one development, the same tool module can be used multiple times. Therefore, a tool mirror originally installed in an imaging system can be later reinstalled in another newly constructed imaging system, for example, as a tool mirror for initial wavefront measurement, or as a replacement mirror with a corrective surface shape for alignment. This concept is also referred to here as the "tool mirror cycle concept." Depending on the need, the components of the tool module, including the tool mirror, can be remanufactured, repaired, or swapped. The tool mirror cycle can be limited to, for example, two, three, four, five, or more uses.

[0051] This concept can also be advantageous in terms of knowledge regarding the measurement system. In one modified form, in addition to the initial component measurement, at least one further component measurement is performed on the tool mirror using the same component measurement system, and the results of two or more temporally separated shape measurements on the same tool mirror performed using the same component measurement system are compared, making it possible to identify the drift effects that may occur in the component measurement system. Thus, in this modification, a particular tool mirror acts as a reference element for the calibration of the component measurement system.

[0052] By utilizing the multiple uses of the same tool mirror in different imaging systems, system measurements can be performed for each imaging system to determine the imaging quality of the installed tool mirror(s). Simultaneously, reconstruction calculations can be performed based on the system measurement results to determine the tool mirror's shape error contribution. In other words, by mathematically combining the system measurement results for the imaging system, the surface shape error introduced by the tool mirror's surface shape can be reconstructed.

[0053] The use of such tool modules, including tool mirrors, offers further advantages to the manufacturing process of the optical imaging system of EUV microlithography equipment. For example, according to one development, at least one additional component measurement can be performed on the tool mirror using the component measurement system, and the drift effect in the component measurement system can be determined using the results of two or more time-separated shape measurements performed on the same tool mirror with the same component measurement system.

[0054] According to one developmental model, the same tool module is used multiple times in different imaging systems.

[0055] Furthermore, each time a tool module is installed, a system measurement is performed on the imaging system to determine its image quality.

[0056] The tool mirror originally installed can be reinstalled later in a newly supplied system for initial wavefront measurement (system measurement) and alignment. Depending on the need, tool module components can be remanufactured, repaired, or swapped. The multiple reuse of a tool module, including the tool mirror, is also referred to here as the "tool mirror recycling concept."

[0057] The present invention also relates to an optical imaging system for an EUV microlithography apparatus manufactured (initial manufacturing) or restored (after use) by this method, and to an EUV microlithography apparatus equipped therewith.

[0058] Further advantages and aspects of the present invention will become apparent from the claims and the description of exemplary embodiments of the invention, which will be described below with reference to the drawings. [Brief explanation of the drawing]

[0059] [Figure 1] The components of an EUV microlithography projection exposure apparatus equipped with a projection lens according to an exemplary embodiment are shown. [Figure 2A] This shows one step in a method for manufacturing an optical imaging system. [Figure 2B] This shows one step in a method for manufacturing an optical imaging system. [Figure 2C] This shows one step in a method for manufacturing an optical imaging system. [Figure 2D] This shows one step in a method for manufacturing an optical imaging system. [Modes for carrying out the invention]

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

[0061] A schematic Figure 1 shows the components of an EUV microlithography projection exposure system EXP, which exposes a radiosensitive substrate W, positioned in the image plane IS region of the projection lens PO, with at least one image of the pattern of a reflective mask M, positioned in the object plane OS region of the projection lens. The projection lens PO is an example of an optical imaging system for EUV lithography. The projection lens PO images the mask pattern to the image plane on which the substrate to be exposed, e.g., a semiconductor wafer, is positioned, by reducing its size.

[0062] The projection exposure system operates using radiation from a primary radiation source RS. The illumination system ILL receives the radiation from the primary radiation source and shapes the illumination radiation directed onto the pattern on the mask M. The projection lens PO images the pattern structure onto the radiation-sensitive substrate W.

[0063] The primary radiation source generates radiation in the extreme ultraviolet (EUV) region, particularly in the wavelength range of 5 nm to 15 nm. The illumination system and projection lenses consist of optical elements that reflect EUV radiation in order to operate in this wavelength range.

[0064] The illumination system shapes the radiation from the radiation source and uses it to illuminate the field of view positioned on or near the object surface OS of the projection lens PO. The shape and size of the field of view determine the shape and size of the effectively usable object field on the object surface OS. The field of view is generally slit-shaped with a large aspect ratio (width to height).

[0065] The device RST, which holds and manipulates the mask M (reticle), is positioned such that the pattern placed on the mask lies on the object plane OS of the projection lens PO, which is also referred to here as the reticle plane. On this plane, the mask can be moved in a scanning direction (y-direction) perpendicular to the reference axis (z-direction) of the projection lens using a scanning drive for scanner operation.

[0066] The substrate W to be exposed is held by a device WST that includes a scanner drive that moves the substrate in a scanning direction (y-direction) perpendicular to the reference axis in synchronization with the mask M. Depending on the design of the projection lens PO, these movements of the mask and substrate may be parallel or antiparallel to each other.

[0067] In this embodiment, the WST, also referred to as the "wafer stage," and the RST, also referred to as the "reticle stage," are part of a scanner device controlled by a scanning control device incorporated into the central control device CU of the projection exposure apparatus.

[0068] The projection lens PO in this example includes six mirrors M1 to M6, each having a convex or concave mirror surface. These can be free-form surfaces. An intermediate image is generated between the object field of view and the image field of view. Other configurations are possible, for example, with more or fewer mirrors, with or without the presence of an intermediate image.

[0069] All optical components of the projection exposure system EXP are housed in an evacuable housing H. The projection exposure apparatus operates under vacuum. EUV projection exposure apparatuses are known from, for example, Patent Document 1, which is a published patent application, and the disclosures of said document are incorporated herein by reference.

[0070] The projection exposure apparatus is equipped with a wavefront measurement system (WMS) which operates at the EUV operating wavelength and is designed to measure the wavefront of the projection radiation traveling from the mask to the substrate to be exposed within the projection lens. Preferably, spatially resolved measurements are performed for multiple field points. For example, a wavefront measurement system of the type described in U.S. Patent No. 7,333,216 or U.S. Patent No. 6,650,399 may be provided, and the disclosures of the above-mentioned documents are incorporated herein by reference.

[0071] Referring to schematic Figures 2A to 2D, the features of the method for manufacturing an optical imaging system in the form of an EUV projection lens presented herein are described. Figures 2A to 2D illustrate different method steps, some of which are performed in different locations. Figures 2A and 2C represent the process performed at the location of the projection lens manufacturer (first location LOC1). Figures 2B and 2D refer to the process and method steps performed at a remote second location LOC2 located on-site at the end-user's manufacturing facility for the projection lens.

[0072] In this schematic example, the projection lens has four mirrors (first mirror M1, second mirror M2, third mirror M3, and fourth mirror M4) mounted in appropriate positions on the force frame FF. In a ready-to-use assembled and aligned corrected state (Figure 2D), the projection lens PO meets its specifications for mass production operation and can be used for manufacturing microstructure components such as semiconductor chips. The projection radiation used for imaging travels along the projection beam path P schematicly shown in Figures 2B and 2D, from the pattern on the reticle M, through the reflective surfaces of the first mirror M1, second mirror M2, third mirror M3, and fourth mirror M4 to the surface of the structured semiconductor wafer W, where an image of the mask is generated.

[0073] At the final assembly and usage site (Second Location, LOC2), the system measurement system SMS is available. Using this system, the wavelength of projected radiation traveling from the object plane OS to the image plane IS can be measured by spatially resolved wavelength measurement and compared with the wavefront required according to the specifications. This allows the actual imaging quality to be compared with the target imaging quality according to the specifications.

[0074] The System Measurement System (SMS) is used as part of the assembly and alignment of projection lenses to prepare them for use, i.e., to meet specifications. In this example, the SMS is part of the projection exposure apparatus and is used for wavefront measurements, allowing the imaging characteristics of the projection lenses to be adapted to changed boundary conditions using a manipulator, if appropriate, even while the projection exposure apparatus is in operation.

[0075] The wavefront of the projected radiation P is particularly influenced by the surface shape of the optically used area of ​​the mirror surface. This surface shape is also referred to here as the "surface shape." Furthermore, the quality of the wavefront is greatly affected by the accuracy of the spatial position of the reflective surface within the projection lens. Deviations from the target position affect the wavefront propagation and the corresponding image quality error. This relationship is utilized in the alignment of rigid body degrees of freedom.

[0076] Each mirror has a target surface shape defined according to specifications, and this target surface shape should ideally exist to provide theoretically best imaging performance with perfect alignment. However, in reality, there are deviations from the target surface shape due to many sources of error, and these deviations are also called surface shape errors.

[0077] In the illustrated method, the surface shape of each mirror used in the manufacturing process is determined by the manufacturer at the first location LOC1 using a component measurement system (CMS) as part of the shape measurement. The illustrated component measurement system (CMS) is configured like a Fizeau interferometer. Since such measuring devices are publicly known, a detailed description is omitted here. An example can be found, for example, in International Publication No. 2006 / 077145.

[0078] In this example method, the surface shape of each mirror used during manufacturing is preferably determined by shape measurement using the same component measurement system (CMS). The advantages of this method are further explained below. In short, the negative impact of the unavoidable absolute error of the component measurement technique used can be eliminated if the only significant problem is the difference in surface shape, i.e., the difference in surface shape between mirrors. More details are explained below.

[0079] The projection lens PO is designed so that some or all of its mirrors can be replaced relatively easily, for example, for maintenance and repair purposes. For this purpose, each mirror is part of an optical element that can be installed in a fixed spatial relationship with the force frame and can be removed and replaced with minimal effort, as is the design. Therefore, in this example, all mirrors are incorporated into an optical module designed as a replaceable module. Other embodiments exist in which only some of the mirrors are so easily replaceable.

[0080] Regarding the method for manufacturing projection lenses, different types of optical modules are used, which are identified by different hatching in Figure 2.

[0081] In this example, the first mirror M1 and the third mirror M3 are mirrors that are installed once in their respective locations during the method and, if appropriate, their positions are changed during alignment within the area of ​​their installation locations, but they are not intended to be replaced again.

[0082] At least one of the optical modules is configured as a replaceable module and is designed to include a mirror selected as a corrective mirror. These optical modules or mirrors are denoted by the reference designation CM (corrective mirror). In this example, the projection lens includes two such corrective mirrors, namely a second mirror M2 and a fourth mirror M4. Their roles are described below.

[0083] Furthermore, two tool modules, each containing one tool mirror (reference designation TM), are also used in the manufacture of the projection lens PO. Each interchangeable optical module, including a corrective mirror CM, is assigned only one tool mirror TM.

[0084] The tool module, including the tool mirror, is designed as a replaceable module and has a mounting structure compatible with the installation location of the associated optical module equipped with a compensating mirror CM; therefore, in principle, it can be installed in the same installation location and in the same spatial position. Another essential criterion is that, according to the shape measurement in the component measurement system, the tool mirror should have the same or substantially the same surface shape as the assigned mirror CM selected as the compensating mirror.

[0085] Therefore, a tool module including a tool mirror TM can be a design twin of an allocation optical module including a compensating mirror CM. However, identity of surface shape and reflective coating is neither technically possible nor necessary as a whole. It is sufficient that the tool mirror has substantially the same optical effect as the allocation compensating mirror CM, so meaningful system measurements are possible even when an allocation tool module including an allocation tool mirror is installed in the compensating position instead of an optical module including a compensating mirror.

[0086] The projection lens, including the installed tool mirror, may possess optical performance within specifications, and as a result, in principle, the tool mirror can remain within the fully assembled projection lens.

[0087] However, such a correspondence between the tool mirror and the compensating mirror is unnecessary if the method is designed so that the tool mirror does not remain in the projection lens and is replaced with an assigned optical module including the compensating mirror before commissioning and the start of mass production.

[0088] In the illustrated example, Figure 2B shows an intermediate stage of the manufacturing process in which the tool mirror TM is installed relative to the second mirror M2 and the fourth mirror. In the completed system (Figure 2D), the corresponding optical module, including the compensating mirror CM, is subsequently installed in the same positions as the second and fourth mirrors.

[0089] For example, the following procedure can be used in manufacturing. First, all mirrors are measured using a component measurement system (CMS) to determine the surface shape (i.e., face shape) of the mirrors. It should be noted that the surface shape or face shape of the tool mirror TM and the assigned compensation mirror CM should be determined as precisely as possible. In this method, only the difference in face shape, i.e., the difference between the face shape of the installed tool mirror and the face shape of the replacement mirror to be installed, must be determined as precisely as possible. By measuring the compensation mirror and the tool mirror with the same component measurement system, potential problems caused by unavoidable absolute errors in face shape measurement technology or the component measurement system (CMS) are resolved. Typically, the surface shapes of all mirrors to be installed are preferably measured with the same component measurement system.

[0090] At a second location LOC2, for example at the end-user's location, the projection lens PO is subsequently assembled, which already partially corresponds to the projection lens to be manufactured, but retains the associated tool mirrors TM and their optical modules instead of the mirrors or their optical modules provided as corrective mirrors CM. The projection lens with the installed tool mirrors (see Figure 2B) can be considered an auxiliary imaging system, which may not yet have the imaging quality required for mass production operation within specifications, but should have sufficient imaging quality to begin operation in an auxiliary mode used, for example, for rigid body degree of freedom alignment.

[0091] Next, the auxiliary imaging system is mechanically aligned as much as possible under the control of the system measurement system (SMS) until the mirror achieves the best possible spatial position at its installation location. Subsequently, after the optical module is installed in its designated location and rigidly aligned, a final system measurement is performed to determine the image quality of the auxiliary imaging system with the best alignment.

[0092] The measured image quality is then compared to a target image quality desirable for mass production operation in order to determine the image quality error. These residual aberrations or remaining image quality errors usually cannot be significantly reduced further by rigid body degree of freedom alignment.

[0093] This is where the corrective mirror (CM) plays its role. Based on system measurements and comparison with the target imaging quality, the surface shape that the corrective mirror (CM) belonging to the tool mirror must have in order to reduce the measured wavefront error as much as possible is determined. In other words, for each installed tool mirror, the difference in surface shape or surface shape difference required to achieve imaging performance within specifications is calculated.

[0094] As soon as this information is obtained, the processing of the mirror selected as the corrective mirror (CM) can begin. This processing changes the surface shape to a modified surface shape, making it suitable for reducing imaging errors. During this normally very time-consuming process, the assembled auxiliary imaging system (Figure 2B) continues to operate, for example, to prepare for further commissioning steps or to test the projection lens against other specifications.

[0095] After the surface processing of the corrective mirror CM is completed, the corrective mirror CM is replaced with the corresponding tool mirror by a swap operation, which involves removing the optical module containing the tool mirror from the auxiliary imaging system and installing a replacement module containing the corresponding processed corrective mirror in its place.

[0096] Next, further system measurements are performed, and based on these, rigid body alignment is carried out to minimize the optical effect of mechanical installation tolerances. Generally, the image quality has already improved significantly. Experience shows that this can be optimized by further (small) alignment steps of the rigid body degrees of freedom, which are performed under the control of the system measurement system (SMS) until the maximum image quality is achieved.

[0097] As an example, the procedure presented here offers significant economic advantages over conventional concepts in terms of resource utilization and time consumption without compromising imaging performance. In particular, this concept has the advantage that the projection lens manufacturer (located in this case at the first location LOC1) does not need to provide expensive system measurement techniques, such as spatially resolved wavefront measurement systems, for these purposes. System measurements are performed by the end-user at the second location LOC2, where the system measurement techniques are already available in the projection exposure apparatus for subsequent mass production operation.

[0098] This method can also be described as initially supplying the end user with an imaging system that is largely unaligned, while the surface or plane geometry of the individual mirrors, including the tool mirror™, is well known through component measurements. The manufacturer does not require the user to utilize existing system measurement techniques, but this procedure allows for the precise fabrication of one or more corrective mirrors CM to reduce residual aberrations remaining after rigid body alignment. This system correction requires mirror swapping. However, this does not result in significant downtime for the user, as the projection lens can already be used as an auxiliary imaging system, including the tool mirror, which has been installed in many pre-commissioning tasks.

[0099] Tool mirrors, after their use in the manufacture of projection lenses, can be used for other purposes, for example, in connection with the manufacture of nominally structurally identical projection lenses. This is possible if the quality of the mirror surface, including the reflective coating, is sufficient; they can be used here as tool mirrors or as permanently installed corrective mirrors. Therefore, the originally installed tool mirrors™ can be reinstalled in newly supplied systems for initial wavefront measurement and alignment. Depending on the need, tool mirror components of optical modules can be remanufactured, repaired, or replaced with each other.

[0100] Tool mirrors can be used sequentially in multiple manufacturing processes, for example, two, three, four, five, six, or more manufacturing processes. If the quality is insufficient after use, the tool mirrors 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 comprises a plurality of optical modules, each carrying a mirror, along the imaging beam path from the object plane to the image plane of the imaging system, the optical modules being installed at assigned installation positions on a force frame, and at least one of the optical modules being designed as a replaceable replacement module including a mirror selected as a compensating mirror, A) A step of performing shape measurement using a component measurement system to determine the surface shape of the mirror selected as the correction mirror, B) This is the step of preparing a tool module that includes a tool mirror. i) The tool module has a mounting structure that is compatible with the installation location of the optical module, which is designed as a replacement module. ii) The tool mirror has the same or substantially the same surface shape as the compensating mirror, according to the shape measurement in the component measurement system, and has steps, C) A step of providing an auxiliary imaging system by installing an optical module including a mirror at the assigned installation position of the force frame, wherein the tool module is installed at the installation position of the optical module which is designed as a replaceable module. D) A step of performing system measurements using a system measurement system to determine the image quality of the auxiliary imaging system after the installation of the optical module at the installation position and rigid body alignment, E) A step of determining the image quality error by comparing the measured image quality with the target image quality of the optical imaging system, F) A step of determining the change in the surface shape of the correcting mirror that is suitable for reducing the image quality error, G) The step of processing the corrective mirror to change its surface shape to a modified surface shape suitable for reducing the imaging error, H) The steps of removing the tool mirror and installing the optical module, which includes the corrective mirror having the corrected surface shape designed as a replacement module, I) A step of performing a system measurement to determine the image quality of the optical imaging system. Methods that include...

2. The method according to claim 1, comprising: evaluating the results of the system measurement;, if the results of the system measurement indicate an out-of-tolerance image quality, an alignment operation to align the installed optical module with its rigid degrees of freedom to improve the image quality; and further system measurement, wherein the alignment operation and system measurement are repeated until the system measurement indicates an image quality within tolerance.

3. A method according to 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 the system measurement is performed at a second location located away from the first location, particularly at the location of the end user or system integrator.

4. A method according to any one of claims 1 to 3, characterized in that a wavefront measurement system is used to perform the system measurement, and preferably spatially resolved wavefront measurements are performed for a plurality of field points.

5. A method according to any one of claims 1 to 4, characterized in that the system measurement is performed in the EUV microlithography apparatus in which the system measurement system is incorporated.

6. A method according to any one of claims 1 to 5, characterized in that the shape measurement of the tool mirror is performed in the same component measurement system as the shape measurement of the assigned mirror selected as the correction mirror.

7. A method according to any one of claims 1 to 6, characterized in that shape measurement for measuring surface shape is performed on each of the mirrors for constructing the imaging system by a component measurement system.

8. A method according to any one of claims 1 to 7, characterized in that the auxiliary imaging system, including the installed tool module, is operated in auxiliary mode to perform the test and / or prepare for commissioning at the second location.

9. The method according to claim 8, characterized in that the auxiliary mode of the auxiliary imaging system and the processing of the corrective mirror for changing the surface shape are performed at least in steps and simultaneously.

10. In the method according to any one of claims 1 to 9, Perform at least one further component measurement on the tool mirror using the same component measurement system, and compare the results of two or more time-separated shape measurements on the same tool mirror performed using the same component measurement system to determine the drift effect in the component measurement system. A method characterized by the following.

11. In the method according to any one of claims 1 to 10, Multiple uses of the same tool module in different imaging systems, and When the tool mirror is installed, a system measurement is performed on the imaging system to determine the image quality of the imaging system. A method characterized by the following.

12. A method according to any one of claims 1 to 11, characterized in that it is designed for the manufacture or repair of an optical imaging system designed as a projection lens for an EUV projection exposure apparatus or an EUV mask inspection apparatus for inspecting masks for EUV microlithography.

13. An optical imaging system for an EUV microlithography apparatus, comprising a plurality of optical modules, each carrying a mirror, installed at assigned positions of the force frame along the imaging beam path from the object plane to the image plane of the imaging system, wherein at least one of the optical modules is designed as a replaceable module including a mirror selected as a correction mirror, An optical imaging system characterized by being manufactured using the method described in any one of claims 1 to 12.

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

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