EUV reflectometry measurement methods and EUV reflectometers

Abstract

Measurement method for measuring the reflectivity of a test object (PR) that reflects EUV radiation using an EUV reflectometer as a function of the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on a reflecting surface (OB) with the following steps: Generating a surface-directed measuring beam (STR) using EUV radiation, by imaging an EUV radiation-emitting source spot (QF) onto an exit slit (SP) of the monochromator by means of a first subsystem (TS1) of a beam shaping unit comprising a monochromator and by means of a second subsystem (TS2) of the beam shaping unit to image the exit slit onto the surface (OB) of the test specimen (PR) to generate the measurement spot (MFL), Holding the test specimen (PR) and positioning the test specimen in relation to the measuring beam (STR) in several degrees of freedom such that, during operation, the measuring beam (STR) hits the reflecting surface (OB) in the area of ​​a measuring spot (MFL) at a predefinable angle of incidence; Changing the position of the measurement spot on the surface of the test object by controlling the direction of the measurement beam (STR) in a beam direction control operation; Detecting a property of a beam reflected from the surface of the test object using a detector (DET) to generate detector signals representing the EUV radiation reflected from the test object; Evaluating the detector signals to determine reflectivity measurements, characterized by the fact that In the beam direction control operation, the position of the measurement spot on the surface of the test object is changed by a first mirror and at least one second mirror of the second subsystem connected downstream in the beam direction forming a reflective manipulator and being moved in a coordinated manner in at least one rigid body degree of freedom in response to control signals from the control unit.

Description

SCOPE OF APPLICATION AND STATE OF THE ART

[0001] The invention relates to a measuring method for measuring the reflectivity of a test object that reflects EUV radiation as a function of the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on a reflective surface of the test object, as well as an EUV reflectometer suitable for carrying out the method.

[0002] An "EUV reflectometer" is a measuring device for determining the reflection properties of a test object for electromagnetic radiation at wavelengths in the extreme ultraviolet (EUV) spectral range. The term EUV (Extreme Ultraviolet) refers to a wavelength range from approximately 6 nm to approximately 20 nm within the soft X-ray range, which is particularly important for optics in lithography systems.

[0003] An EUV reflectometer can be used to measure the reflectivity of a test object that reflects EUV radiation as a function of the wavelength of the EUV radiation (wavelength spectrum) and the angle of incidence of the EUV radiation (angular spectrum) on a reflective surface of the test object. Wavelength and angular spectra can be used, among other things, to characterize the materials involved in the reflection and their structure. EUV reflectometers are suitable, among other things, for investigating reflective test objects, such as mirrors or masks that have a multitude of material layers as a reflective coating (multilayer mirrors) or only one or a few layers, as is the case with mirrors designed for grazing incidence.

[0004] An EUV reflectometer should be able to determine the reflectance of a reflective surface with high accuracy in the EUV range.

[0005] In order to allow reliable statements about the spatial distribution of reflectivity in the entire usable area of ​​a reflective surface, measurements are usually taken at a large number of measuring points, the spatial position of which should be known and predefinable with high precision on the order of approximately 10 to 1000 µm.

[0006] German patent application DE 10 2020 216 337 A1 discloses an EUV reflectometer with a radiation source for EUV radiation, a monochromator for adjusting the wavelength of a measuring beam directed at the sample, wherein the monochromator comprises a first reflecting element arranged in the beam path of the measuring beam, a second reflecting element arranged in the beam path of the measuring beam, a first exit slit arranged in the beam path downstream of the second reflecting element, and a third reflecting element arranged in the beam path downstream of the first exit slit. The first reflecting element is configured to focus the measuring beam in a first direction in the region of the first exit slit or in the first exit slit, and the second reflecting element is configured to focus the measuring beam in a second direction perpendicular to the first direction in the region of the first exit slit or in the first exit slit.The second reflecting element is a concave grating. A detector for capturing radiation reflected from the sample is also provided. At least one of the reflecting elements is controllable. By controlling a steerable reflecting element, for example, the measuring beam can be adjusted with particular precision and ease. This allows the measuring beam to be aligned or adjusted with high accuracy to one of the elements in the beam path, such as one of the reflecting elements or the first exit slit and / or the sample. For example, the steerable third reflecting element, for instance through rotation and / or a change in position or translation, ensures particularly easy alignment of the measuring beam to the sample. The drive elements are controlled by a control unit.

[0007] The patent application US 2021 / 0389678A1 discloses a plasma position control system for EUV lithography light sources.

[0008] The precise measurement of the reflectivity of EUV optics is not merely an academic question, but also of economic interest, for example, because the reflectivity properties of EUV mirrors must be known with high accuracy if multi-component optical systems for EUV lithography are to be built according to their optimized optical design. Therefore, the measurements should be performed as quickly as possible without compromising measurement accuracy. TASK AND SOLUTION

[0009] Against this background, the invention aims to provide a measurement method of the type mentioned in the introduction and an EUV reflectometer configured for its implementation, which, compared to the prior art, offer the potential for high measurement accuracy with a relatively short overall time requirement for the work to be carried out in connection with a measurement. In particular, the EUV reflectometer should enable short measurement times even when the test specimens are large and heavy.

[0010] To solve this problem, the invention provides a measuring method with the features of claim 1 and an EUV reflectometer with the features of claim 6. Preferred embodiments are specified in the dependent claims. The wording of all claims is made clear by reference to the content of the description.

[0011] The measurement method according to the claimed invention is performed using an EUV reflectometer. The measurement method and the EUV reflectometer serve to measure the reflectivity of a test object that reflects EUV radiation as a function of the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on a reflective surface of the test object. For the measurement, a measuring beam containing EUV radiation is directed at the surface. The EUV reflectometer comprises an EUV radiation source with means for generating a source spot for emitting EUV radiation, as well as a beam shaping unit for receiving EUV radiation from the source spot and for generating the measuring beam. The beam shaping unit has a first subsystem and a downstream second subsystem. The first subsystem includes a monochromator for adjusting the wavelength of the measuring beam.The monochromator comprises a concavely curved reflection grating and an aperture assembly with an exit slit downstream of the reflection grating. The second subsystem is designed to generate an approximate image of the illuminated area of ​​the exit slit to form the measurement spot on the surface of the test specimen. The measurement spot on the surface of the test specimen should be homogeneously illuminated.

[0012] The test specimen is held and positioned by a positioning device so that the measuring beam can strike the reflecting surface at a predefined measuring point within the area of ​​a measurement spot at a predefined angle of incidence. Using a detector of the EUV reflectometer, (at least) one property of the beam reflected from the surface of the test specimen is detected. The detector generates detector signals that represent the EUV radiation reflected by the test specimen. An evaluation unit of the EUV reflectometer analyzes the detector signals and determines reflectivity measurements from them.

[0013] It is possible to change the position of the measurement spot on the surface of the test specimen by means of a beam direction control operation. This involves a controlled change in the direction of the measurement beam in response to control signals from a control unit.

[0014] A special feature is that, during the beam direction control operation, the position of the measurement spot on the surface of the test specimen is changed by coordinating the displacement of a first mirror and a second mirror downstream in the beam direction of a reflective manipulator of the second subsystem in response to control signals from the control unit, within at least one rigid-body degree of freedom. The first mirror and the (at least one) second mirror thus jointly form a reflective manipulator of the beam direction control system of the EUV reflectometer.

[0015] The invention also relates to an EUV reflectometer configured to perform the measurement method, in that the second subsystem comprises a reflective manipulator having a first mirror and at least one second mirror optically downstream of the first mirror, wherein the mirrors can be moved in a coordinated manner in at least one rigid-body degree of freedom. The reflective manipulator of the beam direction control system thus comprises at least two mirrors connected in series.

[0016] In other words, the concept can be realized in an EUV reflectometer by having a beam direction control system with a reflective manipulator arranged in the second subsystem, which has a first mirror and at least one second mirror downstream in the beam path, which, in response to control signals from a control unit, can be moved in at least one rigid body degree of freedom by means of an actuating device for the reversible change of the position of the mirrors with respect to a reference position.

[0017] The term "manipulator" here refers to an optomechanical device comprising at least one manipulable optical element and one or more actuators or control elements acting upon it. Based on corresponding control signals, the actuators or control elements can actively influence individual optical elements or groups of optical elements within a manipulator to alter the optical effect of the manipulable optical element in the beam path. In this case, the reflective manipulator changes the beam direction of the measuring beam without altering the shape of the reflective surfaces of the mirrors.

[0018] The measuring beam is thus reflected between the exit slit and the surface of the test specimen by at least two mirrors arranged in series in the beam path. By changing the reflection conditions at the first and second mirrors, the beam direction can be altered without significantly compromising the quality of the measurement spot. While manipulation is also possible by relocating only a single mirror in the second subsystem, this would result in considerably inferior measurement spot properties.

[0019] According to the inventors, several conditions must be met for precise and rapid measurement. Firstly, the desired measurement position on the test specimen should intersect as closely as possible with the axis of rotation of the positioning device. The surface normal of the test specimen at the location of the measurement spot should be as perpendicular as possible to this axis of rotation. These conditions can be met by adjusting the positioning device. Furthermore, the measurement spot should ideally intersect the axis of rotation of the positioning device. Positioning the measurement spot as close as possible to the axis of rotation of the positioning device can be achieved by changing the direction of the measurement beam. Finally, the measurement spot should have a defined, predefined size so that the measurement can take into account the area contributing to the intensity of the detector signal.A measurement spot can, for example, have a rectangular, or more specifically, a substantially square shape with side lengths on the order of a few hundred micrometers, such as 500 µm x 500 µm, 600 µm x 600 µm, or 700 µm x 700 µm, but also values ​​outside these ranges or intermediate values. The horizontal axis determines the monochromaticity, and the vertical axis influences the overall intensity or the location being measured. Finally, to achieve a good signal-to-noise ratio, as much power as possible should be concentrated in the measurement spot.

[0020] The requirements for a well-defined measurement spot and the highest possible power within the illuminated area can be met by optimizing the imaging properties of the second subsystem. According to the inventors, the claimed invention offers a good compromise between these differing requirements. For example, it would be conceivable to use only a single mirror as a reflective manipulator in the second subsystem, such as an ellipsoidal mirror. Compared to a reflective manipulator with at least two mirrors connected in series, this would theoretically have the advantage of higher transmission because transmission losses due to reflection would only occur once. However, according to the inventors' investigations, this would result in relatively unfavorable properties with respect to the measurement spot, potentially leading to light losses in the double-digit percentage range.

[0021] If, on the other hand, at least two mirrors connected in series are used in the second subsystem for beam direction control, the reflection losses are theoretically greater, but a significantly better spot quality can be achieved. This improved spot quality allows the light tube of the system to be enlarged, so that the overall performance can be considerably better than when using a single mirror as a reflective manipulator.

[0022] According to a further development, the reflective manipulator comprises a mirror arrangement of the type of a Wolter collector, i.e., a mirror arrangement with nested mirrors having rotationally symmetric surfaces that reflect EUV radiation, wherein preferably one of the mirrors, in particular the second mirror, is designed as a paraboloid of revolution or as an ellipsoid of revolution, and the other mirror, in particular the first mirror, is designed as a hyperboloid of revolution or as an ellipsoid of revolution. The Wolter collector can have a structure according to type I, type II, or type III of a Wolter collector.

[0023] Such a mirror arrangement makes it possible to create sharply defined measurement spots and at the same time to achieve the possibility of shifting the measurement spot by changing the position and / or orientation of the Wolter collector.

[0024] In some embodiments, the reflective manipulator includes, in addition to at least one other mirror, a plane mirror used in the grazing incidence direction. This plane mirror is positioned behind the other mirror in the beam path of the second subsystem and is pivotable about a rotational or tilting axis. The use of at least one pivotable plane mirror increases the degrees of freedom for spatial manipulation. Although the additional reflection introduces an unavoidable transmission loss, the plane mirror acts solely by convolution without altering the beam angle distribution and, consequently, without any substantial influence on the quality of the measurement spot.

[0025] Such a plane mirror can be provided in addition to a mirror arrangement similar to a Wolter collector, optically positioned between it and the positioning device, so that the measuring beam exiting the Wolter collector can be deflected. However, due to additional transmission losses, the additional plane mirror is usually omitted.

[0026] It is also possible that the second subsystem has a first mirror in the form of a revolution ellipsoid and the second mirror is formed by the plane mirror. In this arrangement, there are therefore only two reflections and correspondingly lower reflection losses, although possibly with compromises in the quality of the measurement spot. Such an embodiment can thus include a reflection element in the beam path of the second subsystem with a concavely curved reflective surface, which has a first curvature in a first direction and a second curvature in a second direction perpendicular to the first direction. This reflection element is designed as a component of the reflective manipulator and can be displaced in at least one rigid-body degree of freedom by means of at least one actuator of the reflective manipulator in response to control signals from the control unit.

[0027] Various manipulation options are provided, whereby the beam direction of the measuring beam can be changed by repositioning the mirrors without altering the shape of the reflective surface of the mirrors of the reflective manipulator. In one embodiment, the repositioning operation comprises a rotation of the complete second subsystem about a tilting axis located at the center of the exit slit. Accordingly, the reflective components of the second subsystem are preferably mounted with a fixed reference to a common reference system such that they can be tilted together about this tilting axis. This manipulation option has the advantage that the shape of the measuring spot remains essentially unchanged during tilting, at least for the required, relatively small tilting angles.

[0028] Another possible relocation operation involves rotating a plane mirror of the manipulator around a tilting axis running on or within the plane mirror. This also makes it possible to relocate the measurement spot on the test specimen surface without significantly altering its shape, size, or illumination.

[0029] It is also possible, if necessary, to perform the displacement operation in such a way that the mirrors of the reflective manipulator are translated in a direction perpendicular to the beam direction and / or perpendicular to an optical axis. However, this usually results in a deterioration of the measurement spot, since the optics are then operated off-axis. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Further advantages and aspects of the invention will become apparent from the claims and from the description of exemplary embodiments of the invention, which are explained below with reference to the figures. Fig. Figure 1 schematically shows components of an exemplary embodiment of an EUV reflectometer; Fig. Figure 2 shows an embodiment with components of a beam direction control system comprising a Wolter collector; Fig. Figure 3 shows an embodiment with components of a beam direction control system, which includes a tiltable plane mirror; Fig. Figure 4 shows an example of a Wolter collector used as part of a reflective manipulator; Fig. Figure 5 shows a beam direction control operation in which the entire second subsystem is tilted about a tilting axis located in the exit slit; Fig. 6A and Fig. Figure 6B shows, for lower and higher image quality respectively, a top view of a surface to be measured with a measurement spot formed there in the left subfigure and an intensity profile through the center of the measurement spot in the right subfigure; and Fig. Figure 7 shows a beam direction control operation in which a plane mirror is tilted. DETAILED DESCRIPTION OF THE EXECUTION EXAMPLES

[0031] The Fig. Figure 1 schematically shows components of an embodiment of an EUV reflectometer EUVR or a measuring device for measuring the reflectivity of a test object PR that reflects EUV radiation as a function of the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on a reflective surface OB of the test object. The test object can be, for example, a mirror for an EUV lithography lens, which has a flat or a generally concave or convex curved reflective surface. The positional relationships between the depicted components are derived from the right-handed Cartesian xyz coordinate system KS.

[0032] The EUV reflectometer allows, among other things, the measurement of the reflectivity of the test object at different wavelengths within a predefined wavelength range of extreme ultraviolet (EUV) radiation. This preferably refers to wavelengths in the range of 6 nm to 20 nm, and especially from 8 nm to 20 nm.

[0033] The ready-to-use EUV reflectometer includes an EUV radiation source SQ for emitting EUV radiation and a downstream beam shaping unit SFE, which is configured to receive EUV radiation from the EUV radiation source and generate a measurement beam STR from it, which, during operation of the measuring device, hits the reflecting surface OB of the test object PR at the test object end and forms a measurement spot MFL at a measurement point.

[0034] In this example, the EUV radiation source SQ comprises a pulsed laser whose laser beam LS is focused onto a gold target T or another suitable material using focusing optics (not shown). The laser beam generates a plasma PL at the surface of the target, which emits a quasi-continuous spectrum of electromagnetic radiation in the EUV range. The plasma forms a source spot QF, or emission spot, which emits the EUV radiation. This source spot QF serves as the effective radiation source. Alternatively, other EUV radiation sources can be used that emit a discrete or quasi-continuous spectrum of electromagnetic radiation in the EUV range, for example, a DPP source (DPP: "discharge-produced plasma"). Other EUV sources are also possible, such as HHG (high harmonic generation) sources. These are also based on lasers that fire at a target, but in this case, the target is gaseous.

[0035] The beam shaping unit SFE is in Fig. 1. Highly schematic representation. The Fig. 2 and Fig. Figure 3 shows exemplary embodiments with components of a beam direction control system SRSS.

[0036] The beam shaping unit SFE comprises a first subsystem TS1 and a downstream second subsystem TS2. The first subsystem TS1 includes a monochromator MC for adjusting the wavelength of the measuring beam. The monochromator has a concavely curved reflection grating RG and an aperture assembly BL with an exit slit SP downstream of the reflection grating. The aperture assembly can have a rectangular aperture whose width can be continuously adjusted in two mutually perpendicular directions. In the beam path upstream of the reflection grating RG, i.e., between the source spot QF or the radiation source SQ and the reflection grating, a front reflection element VRE is arranged. This element directly receives the divergent EUV radiation coming from the source spot QF and reflects it with a concavely curved reflective surface, focusing it in at least one plane towards the reflection grating RG.

[0037] Examples of the design of the first subsystem are described, for example, in DE 10 2018 205 163 A1 or WO 2021 / 156 411 A1. Their disclosure content is made by reference to the content of the description.

[0038] The second subsystem TS2 is designed to generate an approximate image of the illuminated area of ​​the exit slit SP on the surface OB of the test specimen PR, thereby forming the measurement spot MFL. In this embodiment, the extent of the area illuminated with EUV radiation within the measurement spot MFL can be sharply defined and continuously adjusted in two mutually perpendicular directions using the aperture arrangement BL.

[0039] The second subsystem TS2 includes a reflective manipulator MAN of a beam direction control system SRSS. This is in signal transmission communication with the control unit STE of the beam direction control system STSS and can be controlled by a control unit to change the beam direction of the measuring beam STR.

[0040] A positioning device of the EUV reflectometer is configured to hold the test specimen PR and to position it in several degrees of freedom with respect to the measuring beam STR such that, during operation of the EUV reflectometer, the measuring beam can strike the reflecting surface at a predefinable measuring point or location within the area of ​​a measuring spot MFL and a predefinable angle of incidence or angle of incidence range.

[0041] The EUV reflectometer also includes a detector DET sensitive to EUV radiation, configured to detect the EUV radiation reflected from the reflecting surface OB and to generate corresponding detector signals representing the EUV radiation reflected by the test object. In this example, the detector incorporates a measuring diode. An evaluation unit AW is connected to the detector DET via signal transmission and is configured to determine reflectivity measurements using the detector signals.

[0042] To account for unavoidable slight intensity fluctuations of the EUV radiation source during the acquisition and evaluation of measurement results, and to avoid resulting measurement errors, the EUV reflectometer EUVR includes a reference detector RDET located outside the measurement beam path and a beam splitter ST. The beam splitter ST serves to divert a portion of the incident EUV radiation from the measurement beam STR to the reference detector RDET and to allow another (larger) portion to pass through to the test object PR. In this example, the beam splitter ST is a geometric beam splitter in the form of a flat beam splitter comb; other configurations are possible.

[0043] The evaluation of the reference detector signals generated by the reference detector RDET and the detector signals generated by the detector DET takes place in the evaluation unit AW, which receives and processes these signals, in particular to obtain precise measurements of the reflectivity of the test specimen surface at the measurement spot. The reflectance (R) is calculated as the ratio between the intensity of the reflected radiation, measured by a detector DET, and the intensity of the incident radiation, the magnitude of which can be determined using signals from the reference detector RDET.

[0044] Measurements can be performed for angles of incidence in the range between 0° and 90° (excluding the limit values). The angle of incidence is defined here in relation to the surface normal at the point of impact. For example, there are mirrors designed for "normal incidence," i.e., for perpendicular or nearly perpendicular radiation incidence with correspondingly small angles of incidence (e.g., from 0° to approximately 20–35°). The angles of incidence can also be larger, as with mirrors for grazing incidence, where the angles of incidence can be greater than 60° and, in particular, can range from approximately 65° to approximately 89°.

[0045] According to the inventors, measuring the reflectivity of EUV optics presents, among other difficulties, the following challenges. When measuring EUV optics, the wavelengths should be determined to an accuracy of approximately 1–3 pm (picometers). Since the angle of incidence of the incident beam on the surface of the test object influences the wavelength position of the spectra, it follows that the angle of incidence should be set, or at least known, to an accuracy of approximately one hundredth of a degree. A further requirement concerns the measurement of the reflectivity, i.e., the intensity ratio of the reflected to the incident beam. This value, i.e., the reflectance, should be determined with an accuracy of fractions of a percent, if possible.

[0046] The reflectivity should ideally be measurable across the entire surface of the test specimen. Typically, the entire specimen is moved relative to the measuring beam STR using the positioning device until the measurement spot MFL is positioned at the intended measurement point. Particularly with relatively large and correspondingly heavy mirrors, it is challenging to position the mirror with high spatial accuracy in all required degrees of freedom using the positioning device.

[0047] The requirements for positioning accuracy on the part of the test specimen can be reduced if the optical system for generating the measuring beam STR is designed in such a way that the beam direction of the measuring beam can be controlled and changed within certain limits. This allows for a two-stage positioning operation to position the measuring spot at the designated measuring point. In the first stage, the test specimen is roughly positioned by moving it in at least one degree of freedom using the positioning device. In the second stage, a fine positioning of a measuring spot MFL on the now stationary test specimen is performed by controlling and changing the beam direction of the measuring beam STR while the specimen is stationary.

[0048] For this controlled change in the beam direction of the measuring beam STR, the beam shaping unit SFE includes a beam direction control system SRSS. This system comprises two mirrors arranged in the beam path of the beam shaping unit within the second subsystem TS2. These mirrors can be moved in a coordinated manner in response to control signals from the control unit, within one or more rigid body degrees of freedom. The at least two mirrors together form a reflective manipulator MAN of the beam direction control system SRSS.

[0049] The reflective manipulator is thus an opto-mechanical device comprising at least two manipulable optical elements in the form of mirrors, as well as one or more actuators or adjusting elements (not shown in detail) acting upon them. The reflective manipulator effects a change in the beam direction of the measuring beam without altering the shape of the reflective surfaces of the mirrors.

[0050] In the exemplary embodiment of the Fig. 2 Within the second subsystem TS2, a mirror arrangement WK of the Wolter type is arranged behind the exit slit SP of the monochromator. The mirror arrangement comprises nested mirrors with rotationally symmetric surfaces that reflect EUV radiation. An embodiment of a Wolter collector WK is shown in Fig. Figure 4 is shown schematically. In this example, the first mirror S1, which is struck first by the EUV radiation, is designed as a hyperboloid of revolution, in which the EUV radiation strikes a hyperboloid-shaped reflective surface on the outside of a correspondingly designed mirror substrate.

[0051] The second mirror S2, located downstream in the direction of the beam, is designed as a rotational ellipsoid or rotational paraboloid and has a rotationally ellipsoidal or rotationally paraboloidal mirror surface on the inner surface of a mirror substrate, which is coated with an EUV radiation reflecting coating.

[0052] The arrangement of the reflective surfaces is such that the two mirrors S1 and S2 of the Wolter collector WK form an imaging system that can project the illuminated area of ​​the exit slit SP onto the surface of the test specimen PR, thereby generating a relatively sharply defined measurement spot MFL. The two mirrors are mounted on a common support with a fixed relative spatial relationship to each other. The entire mirror assembly can be moved in various degrees of freedom using suitable actuators, for example, parallel to a reference axis of the beam shaping unit running in the x-direction between the light inlet and outlet, or perpendicular to this axis. Rotation of the entire mirror assembly about a tilting axis located outside the assembly is also possible.In particular, the actuators can be designed such that the Wolter collector WK as a whole can be tilted about a tilting axis perpendicular to the optical axis of the Wolter collector, which lies in the area of ​​the exit slit SP of the monochromator (cf. . Fig. 5).

[0053] In the exemplary embodiment of Fig. The second imaging subsystem TS2 also comprises two mirrors S1 and S2 of a reflective manipulator MAN. Here, the first mirror S1, immediately following the exit slit SP, is designed as a revolution ellipsoidal mirror with a first curvature in one direction and a second curvature in a perpendicular direction, differing from the first. This ellipsoidal mirror is the only reflective imaging element in the second subsystem. A second mirror S2 is arranged at a distance behind the first mirror S1. This second mirror is designed as a plane mirror with a flat reflective surface, which is used under grazing incidence (angle of incidence with respect to the surface normal, e.g., greater than 60°, in particular from approximately 65° to approximately 89°).

[0054] There are several manipulation possibilities. In one scenario, the first mirror S1 remains stationary, i.e., it is not manipulated with respect to the slit SP, while only the plane mirror S2 is tilted around a suitable tilting axis in order to move the measurement spot MFL to the desired location MFL'.

[0055] Another manipulation scenario is also possible, in which the first mirror S1 and the second mirror S2 have a fixed spatial relationship to each other, and the two mirrors as a whole are moved by a rotational movement whose axis of rotation is located near the slit SP. This also allows the measurement spot MFL to be moved on the surface of the test specimen.

[0056] The in Fig. Variant 5 shown, with tilting of a Wolter collector WK around a tilting axis lying within the exit slit SP, combines the advantage of relatively low transmission losses (only two reflections) with the advantage of a high spot quality of the measurement spot MFL, which makes precise measurements possible in the first place.

[0057] To better understand the meaning of the term "spot quality", it is important to refer to the Fig. 6A and Fig. 6B The following is noteworthy. The Fig. 6A and Fig. Figure 6B shows, in the left subfigure, a top view of a surface to be measured with the measurement spot formed there, which in this example is square. The right subfigure shows an intensity profile through the center of the measurement spot MFL in the x-direction.

[0058] Crucial for a precise measurement is, first and foremost, that the intensity of the EUV radiation strikes the surface exclusively within a measurement spot MFL of a defined size. The measurement spot size corresponds to the illuminated area, which in this example may have edge lengths of 600 µm. The measurement spot MFL forms an optical image of the illuminated exit slit SP of the monochromator onto the surface OB of the test specimen. The image quality of this image is of great importance for the quality of the measurement. For illustration, the following are shown in the Fig. 6A and Fig. Figure 6B shows theoretical image spots BI-1 and BI-2 within the measurement spot MFL, intended to illustrate the image quality. Consider an object point in the object plane of the imaging second subsystem, which corresponds to the plane of the illuminated slit of the monochromator. The sizes of the associated image spots illustrate how well the imaging system can generate an image point in the image plane (corresponding to the surface OB of the test specimen) from an object point.

[0059] With relatively good image quality (cf. Fig. 6B) the pixel is relatively small. However, if the image quality is worse ( Fig. 6A), resulting in a larger image spot BI-1. The image quality (represented by the sizes of the image spots) causes a more or less pronounced smearing of the images of the object's edges, i.e., the edges of the monochromator's exit slit image. The lateral extent of the smeared area is larger with poor image quality (6A) than with good image quality ( Fig. 6B).

[0060] Since the size of the measurement spot MFL is fixed for metrological reasons, poor imaging ( Fig. 6A) The size of the illuminated slit is reduced so that the (smeared at the edges) image of the exit slit remains within the permissible area of ​​the measurement spot MFL. This results in the Fig. 6A shows the intensity profile on the right with a drop in intensity over a wider area at the edge of the measurement spot. If, on the other hand, the image quality is good ( Fig. 6B), the smeared edge area becomes narrower. This can be used to work with a larger exit slit without intensity falling outside the desired measurement spot. This corresponds to better spot quality.

[0061] The improved image quality thus makes it possible to work with a larger exit slit SP, which allows more intensity to be accommodated within the specified limits of the measurement spot MFL. This is in Fig. 6B on the right can be seen from the steeper drop in intensity at the edges. With better image quality, the permissible illuminated area of ​​the exit slit is therefore larger. Since the exit slit is essentially homogeneously illuminated, a larger object field, proportional to its area, also delivers more power within the size-defined area of ​​the measurement spot (MFL). This increased power in the measurement spot area contributes to higher measurement accuracy.

[0062] In the exemplary embodiment of Fig. In section 7, the reflective manipulator MAN within the second subsystem TS2 comprises a mirror arrangement similar to a Wolter collector WK, followed by a tiltable plane mirror PL. This provides three reflections to achieve the relocation of the measurement spot MFL. Since the Wolter collector offers high image quality and the plane mirror merely folds the beam path without altering the imaging properties, a precisely defined measurement spot can be relocated to the desired measurement point without changing the spot quality. However, intensity losses are to be expected due to the additional reflection.

[0063] In addition to the manipulation options shown in the illustrations, it is also possible to shift the mirrors of the second subsystem TS2 as a group perpendicular to the optical axis or perpendicular to the beam direction of the measuring beam by translation. However, this results in the optics being operated off-axis, which impairs the spot quality.

Claims

[1] Measurement method for measuring the reflectivity of a test object (PR) that reflects EUV radiation using an EUV reflectometer as a function of the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on a reflecting surface (OB) with the following steps: Generating a surface-directed measuring beam (STR) using EUV radiation, by imaging an EUV radiation-emitting source spot (QF) onto an exit slit (SP) of the monochromator by means of a first subsystem (TS1) of a beam shaping unit comprising a monochromator and by means of a second subsystem (TS2) of the beam shaping unit to image the exit slit onto the surface (OB) of the test specimen (PR) to generate the measurement spot (MFL), Holding the test specimen (PR) and positioning the test specimen in relation to the measuring beam (STR) in several degrees of freedom such that, during operation, the measuring beam (STR) hits the reflecting surface (OB) in the area of ​​a measuring spot (MFL) at a predefinable angle of incidence; Changing the position of the measurement spot on the surface of the test object by controlling the direction of the measurement beam (STR) in a beam direction control operation; Detecting a property of a beam reflected from the surface of the test object using a detector (DET) to generate detector signals representing the EUV radiation reflected from the test object; Evaluating the detector signals to determine reflectivity measurements, characterized by , that In the beam direction control operation, the position of the measurement spot on the surface of the test object is changed by a first mirror and at least one second mirror of the second subsystem connected downstream in the beam direction forming a reflective manipulator and being moved in a coordinated manner in at least one rigid body degree of freedom in response to control signals from the control unit. [2] Measuring method according to claim 1, characterized bya two-stage positioning operation for positioning the measurement spot at a measuring point intended for measurement, wherein a first stage comprises a coarse positioning of the test object by moving the test object in at least one degree of freedom using a positioning device and a second stage comprises a fine positioning of the measurement spot by a controlled change of a beam direction of the measuring beam (STR) with the test object at rest, wherein preferably the fine positioning of the measurement spot comprises a positioning of the measurement spot on a rotational axis of the positioning device. [3] Measuring method according to claim 1 or 2, characterized bythat the reflective manipulator has a mirror arrangement of the type of a Wolter collector, wherein the mirror arrangement has nested mirrors with rotationally symmetric surfaces reflective for EUV radiation, and wherein the Wolter collector is displaced in at least one rigid body degree of freedom to change the beam direction of the measuring beam (STR), wherein preferably one of the mirrors, in particular the second mirror, is designed as a paraboloid of revolution or ellipsoid of revolution and the other mirror, in particular the first mirror, is designed as a hyperboloid of revolution or ellipsoid of revolution, and / or wherein the Wolter collector has a structure according to type I, type II or type III of a Wolter collector. [4] Measuring method according to claim 1, 2 or 3, characterized by , that the reflective manipulator has a plane mirror and that the beam direction control operation involves tilting the plane mirror. [5] Measuring method according to any one of the preceding claims, characterized by , that the mirrors of the reflective manipulator are moved in a coordinated manner in a displacement operation selected from the group: (a) a rotation of the entire second subsystem about a tilting axis located in the center of the exit slit; (b) a rotation of a plane mirror of the reflective manipulator about a tilting axis running on or in the plane mirror. (c) a translation of the mirrors of the reflective manipulator in a translation direction oriented perpendicular to the beam direction and / or perpendicular to an optical axis. [6] EUV reflectometer (EUVR) for measuring the reflectivity of a test object (PR) that reflects EUV radiation as a function of the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on a reflecting surface (PRO) of the test object comprising: an EUV radiation source with facilities for generating a source spot (QF) for emitting EUV radiation; a beamforming unit (SFE) for receiving EUV radiation from the source spot and for generating a measurement beam (STR), wherein the beamforming unit comprises a first subsystem and a downstream second subsystem, wherein the first subsystem has a monochromator with an exit slit and is configured to map the source spot onto the exit slit, and the second subsystem is configured to map the exit slit onto the surface of the test specimen to form the measurement spot; a positioning device for holding the test specimen (PR) and for positioning the test specimen in relation to the measuring beam (STR) in several degrees of freedom such that, during operation, the measuring beam (STR) hits the reflecting surface (PRO) at a predefinable measuring point in the area of ​​a measuring spot (MFL) at a predefinable angle of incidence, a beam direction control system configured to change the position of the measurement spot on the surface of the test object by a controlled change in the direction of the measurement beam; an EUV radiation sensitive detector (DET) for detecting the EUV radiation reflected from the reflecting surface (PRO) and for generating detector signals that represent the EUV radiation reflected from the test object; characterized by , that the beam direction control system (SRSS) has a reflective manipulator (MAN) arranged in the second subsystem (TS2), which includes a first mirror (S1) and has at least one second mirror (S2) downstream in the beam path, which can be moved in a coordinated manner in response to control signals from a control unit (STE) in at least one rigid body degree of freedom. [7] EUV reflectometer according to claim 6, characterized by , that the reflective manipulator (MAN) has a mirror arrangement of the type of a Wolter collector (WK) with nested mirrors with rotationally symmetric surfaces reflecting for EUV radiation, wherein preferably one of the mirrors, in particular the second mirror (S2), is designed as a paraboloid of revolution or ellipsoid of revolution and the other mirror, in particular the first mirror (S1), is designed as a hyperboloid of revolution or as an ellipsoid of revolution. [8] EUV reflectometer according to claim 6 or 7, characterized by , that the reflective manipulator (MAN) has, in addition to a first mirror (S1), a second mirror (S2) which is designed as a plane mirror used in grazing radiation incidence, is arranged in the beam path of the second subsystem (TS) behind the first mirror (S1) and is pivotable about a tilting axis. [9] EUV reflectometer according to claim 8, characterized by , that the plan mirror is provided in addition to a mirror arrangement of the type of a Wolter collector (WK) and is optically arranged between this and the positioning device. [10] EUV reflectometer according to claim 6, characterized by , that the reflective manipulator of the second subsystem (TS2) has a first mirror (S1) in the form of a revolution ellipsoid and the second mirror (S2) is formed by a plane mirror. [11] EUV reflectometer according to any one of claims 6 to 10, characterized by, that all components of the second subsystem (TS2) are mounted with fixed reference to a common reference system in such a way that they can be tilted together about a tilting axis which lies in the area of ​​the exit gap (SP).

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