Measurement method of EUV reflectometer and EUV reflectometer

Abstract

In a measurement method, the method uses an EUV reflectometer to measure the reflectance of a test object (PR) reflecting EUV radiation as a function of the wavelength of the EUV radiation and of the angle of incidence of the EUV radiation on the reflective surface (OB) of the test object, the reflectance is measured by means of a first partial system (TS1) and by means of a second partial system (TS2) of a beam shaping unit. The present invention relates to a method for measuring a surface (OB) of a test object (PR), comprising a first partial system (TS1) comprising a monochromator (MC) and being part of a beam shaping unit (SFE) for imaging an EUV radiation emission source spot (QF) on an exit gap (SP) of the monochromator, and a second partial system (TS2) for imaging the exit gap on the surface (OB) of the test object (PR), the first partial system (TS1) comprising a monochromator (MC) and being part of a beam shaping unit (SFE) for imaging the EUV radiation emission source spot (QF) on the exit gap (SP) of the monochromator. In this way, a measurement spot (MFL) is generated. According to the invention, the test object (PR) is positioned in a plurality of degrees of freedom with respect to the measurement beam (STR) such that during operation the measurement beam (STR) impinges on the reflective surface (OB) at a specifiable angle of incidence in the region of the measurement spot (MFL). A characteristic of the light beam reflected by the surface of the test object is detected by means of a detector (DET), wherein a detector signal representative of the EUV radiation reflected by the test object is generated. The detector signals are evaluated in order to determine reflectance measurements. In the beam direction control operation, by means of a reflection manipulator (MAN) formed by the first mirror (S1) and at least one second mirror (S2) of the second partial system located downstream thereof in the beam direction, and changing the position of the measurement spot on the surface of the test object by controlled modification of the beam direction of the measurement beam (STR) by means of these mirrors displaced in a mutually coordinated manner in at least one rigid degree of freedom in response to a control signal from the control unit.

Description

[0001] The following disclosure is based on the German patent application with the document reference number 10 2022 210354.8 filed on September 29, 2022. The disclosure content of this patent application is incorporated by reference into the content of the present application. Technical Field

[0002] The present invention relates to a measuring method for measuring the reflectivity of a test object having a reflective effect on EUV radiation based on the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on a reflective surface of the test object, and to an EUV reflectometer suitable for performing the method. Background Art

[0003] An "EUV reflectometer" is a measuring device for measuring the reflectance properties of a test object for electromagnetic radiation at wavelengths in the extreme ultraviolet (EUV) spectral range. Reference to EUV (extreme ultraviolet) refers to the wavelength range from about 6 nm to about 20 nm, in the range of soft x-ray radiation, which is particularly important for optical units of lithography systems.

[0004] EUV reflectometers can be used to measure the reflectivity of a test object, which has a reflective effect on the EUV radiation, based on the wavelength of the EUV radiation ("wavelength spectrum") and the angle of incidence of the EUV radiation on the reflective surface of the test object ("angular spectrum"). The wavelength spectrum and the angular spectrum can be used in particular to characterize the materials involved in the reflection and their structures. EUV reflectometers are particularly suitable for the inspection of reflective test objects, such as mirrors or masks, which include multiple material layers as reflective coatings (multilayer mirrors) or only one or a few layers, for example in the case of mirrors designed for grazing incidence.

[0005] An EUV reflectometer should be able to determine the reflectivity of a reflective surface or its reflectivity in the EUV range with high accuracy.

[0006] In order to allow reliable statements to be made about the spatial distribution of the reflectivity over the entire usable area of ​​the reflecting surface, measurements are usually performed at a plurality of measuring points, the spatial positions of which should be known and can be predetermined with a high accuracy of the order of magnitude of 10 to 1000 μm.

[0007] Document DE 10 2020 216 337 A1 discloses an EUV reflectometer having a beam source for EUV radiation, a monochromator for setting the wavelength of a measuring beam directed to a sample, the monochromator comprising a first reflective element arranged in the beam path of the measuring beam, a second reflective element arranged in the beam path of the measuring beam, a first exit gap arranged in the beam path downstream of the second reflective element, and a third reflective element arranged in the beam path downstream of the first exit gap. The first reflective element is designed to focus the measuring beam in a first direction in the region of the first exit gap or in the first exit gap, and the second reflective element is designed to focus the measuring beam in a second direction perpendicular to the first direction in the region of the first exit gap or in the first exit gap. The second reflective element is a concave grating. In addition, a detector for capturing radiation reflected from the sample is provided. At least one of the reflective elements is designed to be controllable. By controlling the controllable reflective element, for example, a particularly accurate and simple adjustment of the measuring beam can be performed. As a result, the measuring beam can be particularly precisely aligned or adjusted relative to one of the elements in the beam path, for example relative to one of the reflective elements and / or the first exit gap, and / or relative to the sample. For example, a particularly simple alignability of the measuring beam relative to the sample is ensured by the controllable third reflective element, for example by a rotation and / or a change in position or translation. The drive element can be controlled by a controller.

[0008] The precise measurement of the reflectivity of EUV optical units is not an academic question, but has economic significance, for example, because the reflectivity properties of EUV mirrors must be known with high accuracy if they should be used to construct a multi-component optical system for EUV lithography according to their optimized optical design. Therefore, the measurement should be able to be done as quickly as possible without compromising the measurement accuracy. Summary of the invention

[0009] Problems and Solutions

[0010] Against this background, the problem addressed by the present invention is to provide a measurement method of the type mentioned in the introduction and an EUV reflectometer configured to perform said measurement method, which, compared to the prior art, offer the potential for high measurement accuracy while requiring relatively little time overall for the work performed in conjunction with the measurement. In particular, the EUV reflectometer should be able to achieve short measurement times, even if the test object is large and heavy.

[0011] To solve this problem, the invention provides a measuring method having the features of claim 1 and an EUV reflectometer having the features of claim 6. Preferred developments are given in the dependent claims. The wording of all claims is incorporated into the content of the description by reference.

[0012] The measurement method according to the claimed invention is performed using an EUV reflectometer. In this case, the measurement method and the EUV reflectometer are used to measure the reflectivity of a test object (the test object has a reflective effect on the EUV radiation) based on the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on the reflective surface of the test object. For the measurement, the EUV radiation is used to generate a measurement beam that is directed to the surface. To this end, the EUV reflectometer comprises an EUV radiation source with a device for creating a source spot for emitting EUV radiation and a beam shaping unit for receiving EUV radiation from the source spot and for creating the measurement beam. The beam shaping unit comprises a first subsystem and a downstream second subsystem. The first subsystem comprises a monochromator for setting the wavelength of the measurement beam. The monochromator comprises a concave curved reflection grating and an aperture arrangement, the aperture arrangement having an exit gap arranged downstream of the reflection grating. The second subsystem is designed to create an approximate image of the illumination area of ​​the exit gap so as to form a measurement spot on the surface of the test object. The measurement spot on the surface of the test object should be uniformly illuminated.

[0013] The test object is held and positioned by a positioning device so that the measuring beam can be incident on a predeterminable measuring location on the reflecting surface in the area of ​​the measuring spot at a predeterminable angle of incidence. The detector of the EUV reflectometer is used to detect (at least) one property of the beam reflected off the surface of the test object. In the process, the detector creates a detector signal representing the EUV radiation reflected off the test object. The evaluation device of the EUV reflectometer evaluates the detector signal and determines a reflectivity measurement value therefrom.

[0014] There is an option to change the position of the measuring light spot on the surface of the test object by a beam direction control operation. The beam direction control operation comprises a controlled change of the beam direction of the measuring light beam in response to a control signal from a control unit.

[0015] The particularity lies in the fact that during the beam direction control operation, the position of the measurement spot on the surface of the test object is changed by means of a first mirror and a second mirror of the reflection manipulator of the second part system, which is located downstream in the beam direction, being displaced together in at least one rigid body degree of freedom in response to a control signal from the control unit. The first mirror and the (at least one) second mirror thus together form a reflection manipulator of the beam direction control system of the EUV reflectometer.

[0016] The invention also relates to an EUV reflectometer configured to perform a measurement method with the aid of a second partial system comprising a reflection manipulator, the reflection manipulator comprising a first mirror and at least one second mirror optically downstream of the first mirror, the mirrors being displaceable together in a coordinated manner in at least one rigid body degree of freedom. Thus, the reflection manipulator of the beam direction control system comprises at least two mirrors arranged in series.

[0017] In other words, the concept can be implemented in an EUV reflectometer with the aid of a beam direction control system comprising a reflector manipulator arranged in a second part system, the manipulator comprising a first mirror and at least one second mirror downstream in the beam path, the mirrors being displaceable together in a coordinated manner in at least one rigid body degree of freedom with the aid of an actuating device for reversibly changing the position of the mirror relative to a reference position in response to a control signal from a control unit.

[0018] In this context, the term "manipulator" denotes an optomechanical device comprising at least one steerable optical element and one or more actuators or actuating elements acting thereon. On the basis of appropriate control signals, the actuators or actuating elements can actively influence individual optical elements or groups of optical elements of the manipulator in order to change the optical effect of the steerable optical element in the beam path. In this case, the reflection manipulator causes a change in the beam direction of the measuring beam without changing the properties of the reflecting surface of the mirror.

[0019] The measuring beam is thus reflected by at least two mirrors arranged successively in the beam path between the exit gap and the surface of the test object. By changing the reflection conditions at the first and second mirrors, the beam direction can be changed without having to accept a substantial loss in quality with respect to the measuring spot. Although this is also possible by only displacing a single mirror in the second part of the system, this would be associated with significantly poorer properties of the measuring spot.

[0020] According to the inventors' insights, for accurate and fast measurements, several conditions should be met. First, the desired measurement position on the test object should intersect the rotation axis of the positioning device to the best possible extent. The surface normal of the test object at the position of the measurement spot should be as perpendicular to the rotation axis as possible. These conditions can be met by means of an actuation movement on the positioning device. In addition, the measurement spot should intersect the rotation axis of the positioning device, where possible. The best possible placement of the measurement spot on the rotation axis of the positioning device can be achieved by changing the beam direction of the measurement beam. In addition, the measurement spot should have a definable, predeterminable size so that the measurement can take into account the size of the surface that contributes to the intensity of the detector signal. For example, the measurement spot can have a rectangular, in particular substantially square, form with an edge length of approximately several hundred micrometers, such as 500μm×500μm or 600μm×600μm or 700μm×700μm, but can also be a value located outside these areas or an intermediate value within these areas. The horizontal line determines the monochromaticity, and the vertical line has an influence on the overall intensity or the measurement position. Finally, as much power as possible should be concentrated in the measurement spot to obtain a good signal-to-noise ratio.

[0021] The need for a measurement spot size that is as defined as possible and a power in the illuminated area that is as high as possible can be achieved by optimizing the imaging properties of the second part system. According to the inventors' insight, the claimed invention provides a good compromise between these different needs. For example, it is conceivable to provide only a single reflector (e.g. an ellipsoidal reflector) as a reflection manipulator in the second part system. In theory, this would have the advantage of greater transmission compared to a reflection manipulator with at least two reflectors arranged in series, since transmission losses due to reflection only occur once. However, studies by the inventors have shown that this would result in very unfavorable properties with respect to the measurement spot, which could result in light losses in the double-digit percentage range.

[0022] In contrast, if at least two mirrors arranged in series in the second part of the system are used for the purpose of beam direction control, the reflection losses are theoretically greater, but a substantially better spot quality can be achieved. The improved spot quality allows the light pipe of the system to be enlarged, and thus the total power can be significantly better than when using a single mirror as a reflective manipulator.

[0023] According to an improvement, the reflection manipulator comprises a reflector arrangement in the style of a Walter collector, i.e. a reflector arrangement with nested reflectors having a rotationally symmetric EUV radiation reflecting surface, wherein preferably one of the reflectors (in particular the second reflector) is designed as a parabola of revolution or a ellipsoid of revolution, and the other reflector (in particular the first reflector) is designed as a hyperboloid of revolution or a ellipsoid of revolution. The Walter collector can have a structure according to a type I, type II or type III Walter collector.

[0024] This mirror arrangement allows creating a measurement spot delimited by sharp edges and at the same time allows the option of shifting the measurement spot by changing the attitude and / or orientation of the Walter collector.

[0025] Some embodiments provide that the reflection manipulator includes, in addition to at least one other reflector, a plane mirror for use with grazing incidence of radiation, which is arranged downstream of the other reflector in the beam path of the second partial system and can be pivoted about a rotation axis or a tilt axis. The use of at least one pivotable plane mirror increases the degree of freedom of spatial manipulation. Although the additional reflection introduces unavoidable transmission losses, the plane mirror only acts in the sense of folding, without changing the beam angle distribution, and therefore correspondingly also does not have a substantial effect on the quality of the measurement spot.

[0026] In addition to a reflector arrangement of the Volta collector type, a plane mirror may be provided, optically precisely between the Volta collector and the positioning device, so that the measuring beam emerging from the Volta collector can be deflected. However, due to the additional transmission losses, the additional plane mirror will usually be omitted.

[0027] The second partial system may also include a first reflector in the form of a rotating ellipsoid, and the second reflector is formed by a plane mirror. Therefore, in this arrangement there are only two reflections and correspondingly lower reflection losses, although the quality of the measurement spot may be impaired. Therefore, such an exemplary embodiment may include a reflective element having a concavely curved reflective surface in the beam path of the second partial system, the concavely curved reflective surface having a first curvature in a first direction and a second curvature in a second direction perpendicular to the first direction, wherein the reflective element is designed as a component of a reflection manipulator and can be displaced in at least one rigid body degree of freedom by means of at least one actuator of the reflection manipulator in response to a control signal from a control unit.

[0028] Various steering options are provided, with which the beam direction of the measuring beam can be changed by displacing the mirror, without changing the properties of the reflecting surface of the mirror of the reflective manipulator. In an embodiment, the displacement operation comprises a rotation of the entire second part system about a tilting axis located in the center of the exit gap. Thus, the reflecting components of the second part system are mounted with a fixed reference to a common reference system, so that they can be tilted together about this tilting axis. This steering option has the advantage that in any case, for the relatively small tilt angles required, the shape of the measuring spot remains essentially unchanged during the tilting.

[0029] Another possible displacement operation includes a rotation of the plane mirror of the manipulator about a tilting axis extending on or in the plane mirror.A displacement of the measuring light spot on the test object surface can also be achieved thereby without significantly changing the shape and size of the measuring light spot and the illumination.

[0030] Alternatively, the displacement operation can also be performed in such a way that the translation of the mirror of the reflective manipulator is effected in a translation direction oriented transverse to the beam direction and / or transverse to the optical axis. However, this generally leads to a degradation of the measurement spot, since the optics unit is then operated off-axis. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Further advantages and aspects of the present invention are apparent from the claims and the description of exemplary embodiments of the invention, which will be explained below with reference to the drawings.

[0032] Figure 1 schematically illustrates components of an exemplary embodiment of an EUV reflectometer;

[0033] Figure 2 An exemplary embodiment having components of a beam direction control system including a Voltaic collector is shown;

[0034] Figure 3 An exemplary embodiment of a component having a beam direction control system including a tiltable plane mirror is shown;

[0035] Figure 4 An example of a Volta collector used as part of a reflection manipulator is shown;

[0036] Figure 5 showing a beam direction control operation in which the entire second part system is tilted about a tilt axis located in the exit gap;

[0037] Fig. 6A , 6B shows the beam direction control operation, where the plane mirror is tilted;

[0038] Figure 7A reflective manipulator is shown having a reflector arrangement in the style of a Walter collector with a tiltable flat mirror arranged downstream thereof. DETAILED DESCRIPTION

[0039] Figure 1 Components of an exemplary embodiment of an EUV reflectometer EUVR or measuring device for measuring the reflectivity of a test object PR (which has a reflective effect on EUV radiation) based on the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on the reflective surface OB of the test object are schematically shown. For example, the test object can be a mirror for an EUV lithography lens, which has a flat or substantially concave or convexly curved reflective surface. The pose relationships between the depicted components are from a right-handed Cartesian xyz coordinate system KS.

[0040] In particular, an EUV reflectometer allows measuring the reflectance or reflectivity of a test object at different wavelengths within a predetermined wavelength range of extreme ultraviolet (EUV) radiation. Preferably, this means a wavelength range of 6nm-20nm, in particular 8nm-20nm.

[0041] The EUV reflectometer in an operation-ready configuration comprises 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 to create a measurement beam STR from the EUV radiation, which during operation of the measuring device is incident on a reflecting surface OB of the test object PR at a side end of the test object and forms a measurement spot MFL at the measurement location.

[0042] In an exemplary case, the EUV radiation source SQ comprises a pulsed laser, whose laser beam LS is focused on a gold target T or any other suitable material using a focusing optical unit (not shown here). The laser beam generates a plasma PL near the surface of the target, which plasma emits a quasi-continuous spectrum of electromagnetic radiation in the EUV range. The plasma forms a source spot QF or an emission spot that transmits EUV radiation. This source spot QF serves as an effective radiation source. Alternatively, other EUV radiation sources that emit a discrete or quasi-continuous spectrum of electromagnetic radiation in the EUV range can also be used, such as DPP sources (DPP: "discharge produced plasma"). Other EUV sources are also possible, such as HHG (high harmonic generation) sources. These are also based on lasers emitted to a target, where the target is gaseous here.

[0043] exist Figure 1 The beam shaping unit SFE is depicted very schematically in FIG. Figure 2 and Figure 3 An exemplary embodiment of components with a beam direction control system SRSS is shown.

[0044] The beam shaping unit SFE comprises a first subsystem TS1 and a downstream second subsystem TS2. The first subsystem TS1 comprises a monochromator MC for setting the wavelength of the measuring beam, the monochromator comprising a concavely curved reflection grating RG and a stop device BL with an exit gap SP downstream of the reflection grating. The stop device can have a rectangular aperture, the width of which can be set continuously in two mutually perpendicular directions. The front reflective element VRE is arranged 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. It directly receives the divergent EUV radiation from the source spot QF and reflects the EUV radiation in the direction of the reflection grating RG and in such a way that it is focused in at least one plane using the concavely curved reflection surface.

[0045] Examples of configurations of the first partial system are described, for example, in DE 10 2018 205 163 A1 or WO 2021 / 156411 A1. The disclosure content of these documents is incorporated by reference into the content of the description.

[0046] The second partial system TS2 is designed to create an approximate image of the irradiated area of ​​the exit gap SP on the surface OB of the test object PR and to form the measurement spot MFL therefrom. In this exemplary embodiment, by means of the diaphragm device BL, the size of the area struck by EUV radiation in the region of the measurement spot MFL can be clearly defined and continuously set in two mutually perpendicular directions.

[0047] The second partial system TS2 comprises a reflection manipulator MAN of the beam direction control system SRSS. It has a signal transmission connection to a control unit STE of the beam direction control system SRSS and can be controlled to change the beam direction of the measuring beam STR via the control unit.

[0048] The positioning device POS of the EUV reflectometer is configured to hold the test object PR to be measured and to position the test object PR to be measured relative to the measuring beam STR with multiple degrees of freedom, so that during operation of the EUV reflectometer, the measuring beam can be incident on a predeterminable measurement part or a predeterminable measurement position on the reflecting surface in the area of ​​the measuring spot MFL at a predeterminable incident angle or a range of incident angles.

[0049] The EUV reflectometer further comprises a detector DET which is sensitive to EUV radiation and is configured to capture EUV radiation of a reflected EUV beam reflected off the reflective surface OB and to create a corresponding detector signal which is representative of the EUV radiation reflected off the test object. In the exemplary case, the detector comprises a measuring diode. The evaluation device AW is connected to the detector DET in a signal transmission manner and is configured to determine a reflectivity measurement value using the detector signal.

[0050] In order to take into account the unavoidable slight intensity fluctuations of the EUV radiation source during the capture and evaluation of the measurement results and to be able to avoid measurement errors caused thereby, the EUV reflectometer EUVR comprises a reference detector RDET arranged outside the measurement beam path and a beam splitter ST for output coupling a component of the incident EUV radiation of the measurement beam STR to the reference detector RDET and transmitting the other (larger) component to the test object PR. In the exemplary case, the beam splitter ST is a geometrical beam splitter in the form of a planar beam splitter comb; other configurations are also possible.

[0051] The evaluation of the reference detector signal generated by the reference detector RDET and the detector signal generated by the detector DET is carried out in an evaluation device AW which receives and processes these signals, in particular in order to obtain an accurate measurement of the reflectivity of the surface of the test object at the location of the measuring light spot. In this case, the reflectivity (R) results from the ratio between the intensity of the reflected radiation measured by means of the detector DET and the intensity of the incident radiation, the magnitude of which can be determined by means of the signal from the reference detector RDET.

[0052] The measurement can be performed for an angle of incidence in a range between 0° and 90° (without limiting values). Here, the angle of incidence is defined relative to the surface normal at the location of incidence. For example, there are mirrors designed for "normal incidence", i.e. for vertical or almost vertical incidence of radiation with correspondingly small angles of incidence (e.g. from 0° to about 20-35°). The angle of incidence can also be greater, for example in the case of a mirror for grazing incidence of radiation, where the angle of incidence is, for example, greater than 60° and can in particular be in the range from about 65° to about 89°.

[0053] According to the inventors' understanding, the difficulties explained below arise in particular when measuring the reflectivity of EUV optical device units. In the context of measuring EUV optical device units, the wavelength should be determined to an accuracy of about 1-3 pm (picometer). Since the angle of incidence of the incident light beam on the surface of the test object to be measured affects the wavelength position of the spectrum, the angle of incidence must then be set or at least should be known down to about one hundredth of a degree. Another requirement relates to the measurement of the reflectivity, i.e. the intensity ratio of the reflected measurement beam to the incident measurement beam. This value, i.e. the reflectivity, should be able to be determined with an accuracy down to a few percent.

[0054] As far as possible, it should be possible to measure the reflectivity over the entire surface of the test object. In the process, the test object as a whole is usually moved relative to the measuring beam STR by means of a positioning device POS until the measuring light spot MFL is at the envisaged measuring location. Particularly in the case of relatively large and correspondingly heavy reflectors, it is a challenge to position the reflectors with a high spatial accuracy in all required degrees of freedom using a positioning device.

[0055] The requirements relating to positioning accuracy on a part of the test object can be relaxed if the optical system for creating the measuring beam STR is constructed so that the beam direction of the measuring beam can be changed in a controlled manner within certain limits. In that case, a two-stage positioning operation can be performed in order to position the measuring light spot on the measuring part provided for measurement. In this case, a coarse positioning of the test object is performed in a first stage by displacing the test object in at least one degree of freedom by means of the positioning device POS. Then, in a second stage, a fine positioning of the measuring light spot MFL is performed on the stationary test object by means of changing the beam direction of the measuring beam STR in a controlled manner while the test object is stationary.

[0056] For such a controlled change of the beam direction of the measuring beam STR, the beam shaping unit SFE comprises a beam direction control system SRSS. The latter comprises two mirrors arranged in the beam path of the beam shaping unit within the second subsystem TS2, which mirrors can be displaced together in a coordinated manner in one or more rigid body degrees of freedom in response to control signals from the control unit. In this case, at least two mirrors together form a reflective manipulator MAN of the beam direction control system SRSS.

[0057] Thus, a reflection manipulator is an optomechanical device which comprises at least two steerable optical elements in the form of mirrors and one or more actuators or actuating elements (not depicted in detail) acting thereon. In this case, the reflection manipulator causes a change in the beam direction of the measuring beam without changing the properties of the reflecting surface of the mirrors.

[0058] exist Figure 2 In an exemplary embodiment of , a mirror arrangement WK in the style of a Walter collector is arranged in the second subsystem TS2 downstream of the exit gap SP of the monochromator. The mirror arrangement comprises nested mirrors with rotationally symmetric EUV radiation reflecting surfaces. Figure 4 An exemplary embodiment of a Walter collector WK is schematically depicted in . The mirror S1 , on which the EUV radiation is first incident, is designed in the exemplary case as a hyperboloid of revolution, on which the EUV radiation is incident on a hyperboloidal reflection surface on the outer side of a correspondingly designed mirror substrate.

[0059] The second reflector S2 downstream in the beam direction is designed as a rotation ellipsoid or a rotation parabola and has a reflector surface coated with an EUV radiation reflective coating and is in the shape of a rotation ellipsoid or a rotation parabola on the inner surface of the reflector substrate.

[0060] The arrangement of the reflecting surfaces has been chosen such that the two mirrors S1, S2 of the Walter collector WK form an imaging system which is able to image the illuminated area of ​​the exit gap SP onto the surface of the test object PR and to create there a measuring spot MFL delimited by relatively sharp edges. The two mirrors are assembled on a common support with a fixed relative spatial reference to each other. The mirror arrangement can be displaced as a whole with the aid of suitable actuators in various rigid body degrees of freedom, for example parallel to a reference axis of the beam shaping unit, which extends in the x-direction, between light incidence and light exit, or perpendicular to this axis. Rotation of the entire mirror arrangement about a tilt axis situated outside the mirror arrangement is also possible. In particular, the actuator can be designed such that the Walter collector WK can be tilted as a whole about a tilt axis extending perpendicularly to the optical axis of the Walter collector, said tilt axis being situated in the region of the exit gap SP of the monochromator (see Figure 5 ).

[0061] exist Figure 3 In an exemplary embodiment of the imaging second partial system TS2 further comprises two mirrors S1 and S2 of the reflective manipulator MAN. In this case, the first reflector S1 immediately after the exit gap SP is designed as a rotating ellipsoidal reflector having a first curvature in a first direction and a second curvature different from the first curvature in a direction perpendicular to the first direction. The ellipsoidal reflector is the only reflective imaging element in the second partial system. The second reflector S2 is arranged at a distance downstream of the first reflector S1, and the second reflector S2 is configured as a plane mirror with a plane reflective surface, which is used under grazing incidence of radiation (the angle of incidence relative to the surface normal is, for example, greater than 60°, in particular from about 65° to about 89°).

[0062] There are many steering options. In one scenario, the first mirror S1 remains stationary, ie is not steered relative to the gap SP, and only the plane mirror S2 is tilted about a suitable tilt axis in order to shift the measurement spot MFL to the desired location MFL'.

[0063] Another manipulation scenario is also possible, in which the first mirror S1 and the second mirror S2 have a fixed spatial relationship relative to each other and both mirrors are displaced as a whole using a rotational movement, the rotation axis of which is located near the gap SP. This also allows measuring the displacement of the light spot MFL on the surface of the test object.

[0064] Figure 5 The variant shown in which the Walter collector WK is tilted about a tilt axis located in the exit gap SP combines the advantages of relatively low transmission losses (only two reflections) with the advantages of a high spot quality of the measuring spot MFL, thereby making only precise measurements possible.

[0065] In order to better understand the meaning of the term "spot quality", reference should be made to Fig. 6A and Figure 6B Note the following. In the left part of the figure, Fig. 6A and 6B Each shows a plan view of the surface to be measured, where the measuring spot is formed there; the latter is square in the exemplary case. The right-hand partial figure in each case shows the intensity profile in the x-direction through the center of the measuring spot MFL.

[0066] It is primarily decisive for an accurate measurement that the intensity of the EUV radiation is incident only within a measurement spot MFL of defined size. The measurement spot size corresponds to the illuminated area, which in the exemplary case can have an edge length of 600 μm. The measurement spot MFL is formed as an optical image on the test object surface OB of the illuminated exit gap SP of the monochromator. The imaging quality of this image is very important for the quality of the measurement. For illustrative purposes, the measurement spot MFL is respectively Fig. 6A and 6B In the figure, theoretical image spots BI-1 and BI-2 are drawn within the measurement spot MFL, and these are intended to illustrate the imaging quality. In this case, object points in the object plane of the second part of the imaging system should be considered, which corresponds to the plane of the gap of illumination of the monochromator. The size of the associated image spot shows how well the imaging system is able to create image points in the image plane (corresponding to the surface OB of the test object) from the object points.

[0067] In the case of relatively good image quality, the image point is relatively small (see Figure 6B In contrast, in the case of worse imaging quality, a larger image spot BI-1 appears ( Fig. 6A ). The imaging quality (represented by the size of the image spot) results in a more or less blurred image of the edge of the object (i.e. the edge of the image of the exit gap of the monochromator). The lateral size of the blurred area is smaller in the case of poor imaging (6A) than in the case of good imaging ( Figure 6B ) is larger.

[0068] Since the size of the measuring light spot MFL is fixedly predetermined for measurement reasons, the size of the illuminated gap must be reduced in the case of poor imaging ( Fig. 6A ) so that the image of the exit gap (blurred at the edges) remains within the allowed area of ​​the measurement spot MFL. This produces an intensity distribution such as Fig. 6AAs shown on the right side of the image, there is a drop in intensity over a wider area at the edge of the measurement spot. On the contrary, if the imaging quality is good, the blurred edge area becomes narrower ( Figure 6B ). This can be used to work with a larger exit gap without the intensity being incident in areas outside the desired measurement spot. This corresponds to a better spot quality.

[0069] Therefore, better imaging quality allows working with larger exit gaps SP, whereby more intensity can be accommodated within the predetermined boundaries of the measurement spot MFL. Due to the steeper drop in intensity in the edge region, this can be achieved in Figure 6B . The permitted area of ​​the exit gap to be illuminated is therefore larger, with better imaging quality. Since the exit gap is illuminated essentially uniformly, a larger object field proportional to the area also supplies more power in the area of ​​the measurement spot MFL predetermined in terms of size. The increased power in the area of ​​the measurement spot contributes to a higher measurement accuracy.

[0070] exist Figure 7 In an exemplary embodiment of the invention, the reflection manipulator MAN in the second partial system TS2 comprises a reflector arrangement in the style of a Walter collector WK, downstream of which there is a tiltable plane mirror PL. Thus, three reflections are provided here in order to achieve a displacement of the measurement spot MFL. Since the Walter collector provides a high imaging quality and the plane mirror only folds the beam path without changing the imaging properties, this also allows a precisely defined displacement of the measurement spot to the desired measurement location without changing the spot quality. However, due to the additional reflections, intensity losses should be expected.

[0071] In addition to the illustrated steering options, there is also the option of displacing the mirrors of the second subsystem TS2 as a group by translation perpendicularly to the optical axis or perpendicularly to the beam direction of the measuring beam. In this case, however, the optical unit is operated off-axis, whereby the beam quality suffers.

Claims

1. A measuring method for measuring the reflectivity of a test object (PR) by means of an EUV reflectometer based on the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on a reflecting surface (OB), the test object having a reflective effect on the EUV radiation, the measuring method comprising the following steps: creating a measuring beam (STR) directed toward the surface (OB) using EUV radiation by means of an EUV radiation emitting source spot (QF), the EUV radiation emitting source spot (QF) being imaged by a first subsystem (TS1) of a beam shaping unit comprising a monochromator onto an exit gap (SP) of the monochromator, and the exit gap being imaged by a second subsystem (TS2) of the beam shaping unit onto the surface (OB) of the test object (PR) for creating a measuring spot (MFL), holding the test object (PR) and positioning it with a plurality of degrees of freedom relative to the measuring beam (STR) such that during operation the measuring beam (STR) is incident on the reflecting surface (OB) in the region of a measuring light spot (MFL) at a predeterminable angle of incidence; in a beam direction control operation, changing the position of the measurement light spot on the surface of the test object by a controlled change of the beam direction of the measurement light beam (STR); detecting a characteristic of the light beam reflected off the surface of the test object using a detector (DET) to create a detector signal representative of the EUV radiation reflected off the test object; evaluating the detector signal for the purpose of determining a reflectivity measurement value, It is characterized in that During the beam direction control operation, the position of the measuring light spot on the surface of the test object is changed by means of a first mirror and at least one second mirror which form the second partial system of a reflection manipulator and which are displaced together in at least one rigid body degree of freedom in response to control signals from the control unit, the at least one second mirror being located downstream in the beam direction.

2. The measuring method according to claim 1, characterized in that: A two-stage positioning operation for positioning the measurement spot at a measurement site provided for measurement, wherein a first stage comprises coarse positioning of the test object by displacing the test object in at least one degree of freedom by means of a positioning device, and a second stage comprises fine positioning of the measurement spot by controlled change of the beam direction of the measurement beam (STR) when the test object is stationary, the fine positioning of the measurement spot preferably comprising positioning the measurement spot on a rotation axis of the positioning device.

3. The measuring method according to claim 1 or 2, characterized in that: The reflection manipulator comprises a mirror arrangement in the form of a Walter collector, comprising nested mirrors having a rotationally symmetric EUV radiation reflecting surface, and the Walter collector is displaced in at least one rigid body degree of freedom for the purpose of changing the beam direction of the measurement beam (STR), wherein preferably one of the mirrors, preferably the second mirror, is designed as a rotating parabola or a rotating ellipsoid, and the other mirror, preferably the first mirror, is designed as a rotating hyperbola or a rotating ellipsoid, and / or the Walter collector has a structure according to a Type I, Type II or Type III Walter collector.

4. The measuring method according to claim 1, 2 or 3, characterized in that: The reflective manipulator comprises a plane mirror, and the beam direction control operation comprises tilting of the plane mirror.

5. The measuring method according to any one of the preceding claims, characterized in that The mirrors of the reflective manipulator are displaced in a coordinated manner in a displacement operation selected from the group consisting of: (a) rotation of the entire second part system about an inclined axis located at the center of the outlet gap; (b) rotation of a mirror of the reflective manipulator about a tilt axis extending on or in the mirror; (c) Translation of a mirror of the reflective manipulator in a translation direction transverse to the light beam direction and / or transverse to the optical axis orientation.

6. An EUV reflectometer (EUVR) for measuring the reflectivity of a test object (PR) based on the wavelength of the EUV radiation and the angle of incidence of the EUV radiation on a reflective surface (PRO) of the test object, the test object having a reflective effect on the EUV radiation, the EUV reflectometer comprising: an EUV radiation source having means for creating a source spot (QF) for emitting EUV radiation; a beam shaping unit (SFE) for receiving EUV radiation from the source spot and for creating a measurement beam (STR), the beam shaping unit comprising a first subsystem comprising a monochromator having an exit gap and configured to image the source spot onto the exit gap and a second subsystem downstream configured to image the exit gap onto a surface of the test object for the purpose of creating the measurement spot; a positioning device (POS) for holding the test object (PR) and for positioning the test object with a plurality of degrees of freedom relative to the measuring beam (MS) so that during operation the measuring beam (MS) impinges at a predeterminable angle of incidence on a predeterminable measuring location on the reflecting surface (PRO) in the region of a measuring light spot (MFL), a beam direction control system configured to change the position of the measurement light spot on the surface of the test object by controlled variation of the beam direction of the measurement light beam; a detector (DET) sensitive to EUV radiation and for capturing said EUV radiation reflected off said reflective surface (PRO) and creating a detector signal representative of said EUV radiation reflected off said test object; It is characterized in that The beam direction control system (SRSS) comprises a reflection manipulator (MAN), which is arranged in the second partial system (TS2) and comprises a first reflection mirror (S1) and at least one second reflection mirror (S2) arranged downstream in the beam path, and the reflection mirrors can be displaced together in a coordinated manner in at least one rigid body degree of freedom in response to a control signal from a control unit (STE).

7. The EUV reflectometer according to claim 6, characterized in that The reflection manipulator (MAN) comprises a mirror arrangement in the form of a Walter collector (WK), the mirror arrangement having nested mirrors having a rotationally symmetric EUV radiation reflecting surface, wherein preferably one of the mirrors, preferably the second mirror (S2), is designed as a rotating parabola or a rotating ellipsoid, and the other mirror, preferably the first mirror (S1), is designed as a rotating hyperbola or a rotating ellipsoid.

8. The EUV reflectometer according to claim 6 or 7, characterized in that: The reflection manipulator (MAN) comprises, in addition to a first reflection mirror (S1), a second reflection mirror (S2), which is designed as a plane mirror (PS) for use with grazing incidence of radiation, is arranged downstream of the first reflection mirror (S1) in the beam path of the second partial system (TS) and is capable of pivoting about a tilt axis.

9. The EUV reflectometer according to claim 8, characterized in that In addition to the mirror arrangement in the form of a Walter collector (WK), a plane mirror (PS) is provided and is arranged optically between the Walter collector (WK) and the positioning device (POS).

10. The EUV reflectometer according to claim 6, characterized in that: The reflection manipulator of the second partial system (TS2) comprises a first reflection mirror (S1) in the form of a rotational ellipsoid, and the second reflection mirror (S2) is formed by a plane mirror (PS).

11. The EUV reflectometer according to any one of claims 6 to 10, characterized in that All components of the second partial system (TS) are mounted with a fixed reference to a common reference system such that all components can be tilted together about a tilting axis located in the region of the exit gap (SP).

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