Method and apparatus for detecting optical reflectivity of a curved surface of an optical test piece

Abstract

The invention relates to a method for detecting the optical reflectivity of a curved surface (3) of an optical test piece (2), in particular a mirror or a lens, wherein the reflectivity (R) is detected by means of a reflectometer (4), in particular an extreme ultraviolet reflectometer, having a light source (5) and a light sensor (6), and wherein, for detecting the reflectivity by means of the light sensor (6), the light beam (7) reflected on the surface (3) by the light source (6) is measured at at least one predetermined distance relative to the test piece (2). It is provided that the light sensor (6) is moved at least once during a calibration process, such that the reflected light beam (7) is measured at different distances (x) relative to the surface (3) of the test piece (2), and the reflectivity (R) is detected for each distance (x), and the basic reflectivity of the surface (3) is determined from the measured reflectivities (R).

Description

[0001] The present invention relates to a method for detecting the optical reflectivity of a curved surface of an optical specimen, particularly a mirror or lens, wherein the reflectivity is detected by a reflectometer, particularly an extreme ultraviolet (EUV) reflectometer, the reflectometer having a light source and a light sensor, and wherein, in order to detect the reflectivity by the light sensor, a light beam reflected by the light source on the specimen or the surface of the specimen is measured relative to the specimen at at least a predetermined distance.

[0002] Furthermore, the present invention also relates to an apparatus for detecting the optical reflectivity of the curved surface of an optical specimen, particularly a mirror or lens, wherein the apparatus has a reflectometer, particularly an extreme ultraviolet reflectometer, the reflectometer having a light source and a light sensor, and wherein, in order to detect the reflectivity by means of the light sensor, the light sensor can be arranged at at least a predetermined distance relative to the specimen.

[0003] Methods and apparatuses of the above type are known in the prior art. For example, to measure the reflectivity of a curved mirror, it is known to aim a light beam at the mirror surface and detect the reflected beam using a photosensitive sensor. In this case, the light source and the photosensitive sensor are not on the same straight line, nor perpendicular to the sample surface; otherwise, the photosensitive sensor would block the light beam. Divergent scattered light from the reflectometer light source, such as light caused by impurities and / or roughness of the light source's optical elements, can cause light loss in the measurement beam, i.e., the beam reflected from the sample, and result in geometric overradiation of the photosensitive sensor. This excessive radiation can lead to errors or distortions in the reflectivity measurement. This can occur, for example, with convex mirrors. Previous corrections for this effect using calibrated samples have proven insufficiently accurate.

[0004] Therefore, the technical problem to be solved by the present invention is to provide an improved method for detecting the optical reflectivity of the curved surface of a test piece, the method being able to effectively compensate for the influence of the aforementioned scattered light.

[0005] The technical problem described herein is solved by a method having the features of claim 1. The method according to the invention has the advantage of being able to detect the influence of scattered light from the surface with particular precision and to perform corresponding precise compensation. To this end, according to the invention, the photosensor is moved at least once during calibration to measure the reflected beam at different distances relative to the specimen, and the reflectivity is measured for each distance, and the fundamental reflectivity of the surface is determined based on the measured reflectivity. Thus, the photosensor is moved during calibration to change its distance from the specimen. Since the photosensor still needs to detect the reflected beam at this time, this means that the photosensor needs to move along the beam direction, parallel to the beam, or along the beam. Since light loss increases with increasing distance between the photosensor and the specimen, and decreases with decreasing distance, the scattered light characteristics of the specimen can be advantageously detected by measuring multiple reflectivity values ​​of the specimen at different distances. During the measurement of the specimen's reflectivity values ​​after the calibration process, scattering loss is advantageously compensated for or eliminated in order to determine the actual reflectivity of the specimen. Here, the photosensor is preferably moved in steps to change the distance, or moved continuously. Optionally, the surface is an optical coating of the specimen.

[0006] Preferably, at least one reflectance value of the specimen measured during the test is corrected based on the determined basic reflectance. Preferably, correction is performed using a correction table with multiple control points, wherein interpolation may be performed between the control points.

[0007] To detect the influence of scattered light, it is particularly preferred that the reflectivity of the surface be measured relative to the specimen at at least two, preferably at least five, and especially more than ten different distances. The more reflectivity values ​​measured at different distances, the more accurate the determination of the scattered light characteristics of the specimen.

[0008] Particularly preferably, the fundamental reflectivity of the specimen is determined during calibration by extrapolation of reflectivity measured at different distances from the optical sensor relative to the specimen. Extrapolation allows for the direct calculation or determination of the effects of scattered light or optical loss on the reflective surface of the specimen. This fundamental reflectivity is then used to compensate for or correct reflectivity measurements taken in subsequent testing.

[0009] Particularly preferably, the sensor moves only along the direction of the light beam to change the distance between the sensor and the specimen. The orientation of the sensor relative to the light beam and / or the specimen remains unchanged. Therefore, it is particularly advantageous to extrapolate the effects of scattered light or optical loss.

[0010] According to a preferred improvement of the invention, before the calibration process begins, the light beam reflected by a reference planar element, the surface roughness of which is the same as or nearly the same as that of the test piece, is detected to compensate for the effect of roughness on scattered light during the calibration process. Since the surface roughness of the test piece itself also affects the scattered light effect, the degree of roughness's influence on scattered light can be determined by comparing and measuring the reference planar element—that is, a test piece with a surface that is flat / straight with the same or approximately the same roughness as the test piece. Therefore, by performing the above-described suggested calibration process on the reference planar element, this effect of scattered light can be detected and taken into account or compensated for during device calibration and measurement.

[0011] Preferably, the arrangement and / or orientation of the photosensor relative to the specimen is changed transversely to the direction of the reflected light beam. This allows for the detection and compensation of other characteristics of the scattered light. For this purpose, the photosensor may be displaced, oscillated, or rotated relative to the specimen, for example.

[0012] The device according to the invention, having the features of claim 8, is characterized by a controller specifically designed to execute the method according to the invention under normal use. This provides the advantages mentioned above.

[0013] Preferably, the light sensor is movably supported along a linear guide rail. Therefore, the light sensor is arranged on a guide rail extending in a straight line. Here, the guide rail can be aligned parallel to the light beam, and preferably oriented in such a way that the light sensor can move along or toward the light beam, especially without changing the orientation of the light sensor relative to the specimen or the light beam.

[0014] Preferably, the guide rail itself is movable in space to change the orientation of the photosensor or its position relative to the specimen and the light source, so that the light beam is reflected and detected by the photosensor, especially at different locations on the specimen.

[0015] The invention is described in detail below with reference to the accompanying drawings. In the drawings:

[0016] Figure 1 A simplified view of the preferred device is shown, and

[0017] Figure 2 A schematic diagram illustrating the method of operating the device is shown.

[0018] Figure 1 A simplified view of a preferred apparatus 1 for determining the reflectivity of a specimen 2 having a curved surface 3 whose reflectivity is to be measured is shown. Surface 3 may optionally be coated. The specimen 2 is, in particular, an optical element, such as a lens, or in this example, a mirror, especially an extreme ultraviolet mirror.

[0019] The device 1 also includes a reflector 4, which has a controllable light source 5, particularly a laser or plasma light source with a beam-shaping optical system, and a light sensor 6. The light source 5 is positioned such that the light beam 7 generated by the light source, particularly a laser beam, illuminates the surface 3 of the specimen 2 and is reflected from this surface to the light sensor 6. Due to the curved structure of the specimen 2, the light beam 7 is not incident perpendicularly on the surface 3, but at an angle to the vertical direction, causing the reflected beam to be in a different direction than its incident direction. Therefore, the light sensor 6 is also positioned next to the light source 5 to detect the reflected beam.

[0020] like Figure 1 As shown, beam 7 is typically not deflected without loss. Instead, reflection and the light source itself produce scattered light, in... Figure 1 The diagram shows a scattered light cone 8 emitted from the light source 5, a scattered light cone 9 emitted from the surface 3 of the specimen 2, and a scattered light cone 10 formed by the scattered light cone 8 reflected on the surface 3. Here, the scattered light cones 9 and 10 are particularly overlapping, such that the light sensor is at least partially over-illuminated on its sensor surface.

[0021] like Figure 1 As shown, the sensor surface of the light sensor 6 is smaller than at least Figure 1 The scattered light cone 10 is located at the position of the optical sensor 6, indicated by a solid line. This results in some scattered light bypassing the optical sensor 6, thus causing light loss. Figure 1 The image is shown in shaded area 11. Furthermore, as mentioned above, the overlapping scattering cones 9 and 10 also produce geometric overillumination of the light sensor 6, resulting in distortion of the reflectivity measurement. This is particularly noticeable on convex mirrors.

[0022] The controller 12 controls the light source 5 and analyzes the data collected by the light sensor 6 to determine the reflectivity value of the surface 3 of the specimen 2. Furthermore, the controller 12 is connected to an actuator 13, which is associated with the light sensor 6 and used to move the light sensor 6. Preferably, the light sensor 6 can slide on a guide rail 14 extending in a straight line. The sensor 6 can be moved to different positions along the guide rail 14 by the actuator 13. Here, the guide rail 14 is parallel to or oriented towards the reflected beam 7, so that when the light sensor 6 moves, the light sensor moves along the beam or towards the reflected beam 7, thereby maintaining different distances from the specimen 2 or its surface 3.

[0023] To calibrate the device 1, the controller 12 measures multiple reflectivities of the specimen 2, specifically detecting the reflectivity of the specimen 2 as the photosensor 6 moves along the guide rail 13 to different positions and maintains different distances from the surface 3. For this purpose, the photosensor 6 is moved stepwise to multiple positions via the actuator 13, whereby the reflectivity is detected at each position. This forms a set of analyzable reflectivity data.

[0024] Figure 2 A simplified view showing the relationship between the distance x between the light sensor 6 and the specimen 2 and the reflectivity R is presented. These measurements are distributed substantially along a linear path. The fundamental reflectivity value of the specimen 2 can be determined by extrapolating the measured values ​​or reflectivity.

[0025] By moving sensor 6, the excessive illumination on light sensor 6 is altered. Correspondingly, the reflectivity also changes, such as... Figure 2 As shown, by extrapolating the measured reflectance, when the sensor 6 moves to a position close to the specimen 2, the reflectance is directly measured on the surface 3. This reflectance value is referred to here as the basic reflectance value, which can be determined relatively accurately through extrapolation. However, the sensor 6 cannot get so close to the surface 3 because it would block the light beam 2 before it reaches the surface 3.

[0026] By determining the fundamental reflectivity of specimen 2 or its surface 3 in this way, the reflectivity values ​​measured during subsequent testing can be corrected, and the influence of scattered light can be compensated. Another advantage of the proposed method is that it eliminates the need to calibrate apparatus 1 using a calibration standard with a precisely defined curvature. Furthermore, the method allows for advantageous studies of the reflectivity of free surfaces with varying radii of curvature on surface 3. This method advantageously determines the fundamental reflectivity value of specimen 3 and the resulting correction value for compensating for reflectivity values ​​measured during testing under practical conditions, without requiring readjustment of the apparatus for calibration and testing. This simplifies the testing process and reduces testing time.

[0027] Optionally, the light sensor 6 moves continuously instead of in stages, and accordingly, the reflectivity of the specimen 3 is continuously detected and evaluated. Here, it is also preferable to determine the basic reflectivity value of the specimen 2 using an extrapolation method.

[0028] Furthermore, according to another embodiment, it is preferably specified that the effect of scattered light caused by the roughness of the surface 3 of the specimen 2 is compensated by measuring or detecting the reflectivity of a planar sample or reference planar element 15, either in place of or simultaneously with the specimen 2, before or during calibration. Successful compensation is contingent upon the roughness of the reference planar element 15 being the same as or substantially the same as that of the specimen. Since the reference element is a planar element, i.e., having a flat or non-curved surface, the effect of curvature on scattered light can be avoided when measuring the reflectivity of the reference planar element 15, thereby determining an optimized scattered light coefficient, which is considered for compensating or correcting the reflectivity value measured during the test.

[0029] Based on the causes and models of excessive exposure, Figure 2 The reflectance value varies along the curve, and the curve may also deviate from it. Figure 2 The straight line shown. In this case, the calculation or extrapolation of reflectance should preferably adapt to the actual curve of reflectance values.

[0030] Furthermore, the spatial angle of the light sensor 6 relative to the specimen 2 can be changed to generate more reflectivity measurements. Different light sensors 6 can be used here, or a large light sensor with a variable diameter aperture can be used. Device 1 can also be calibrated by combining light sensors 6 of different sizes with different distances x.

Claims

1. A method for detecting the optical reflectivity of an optical specimen (2), particularly a curved surface (3) of a mirror or lens, wherein, The reflectivity (R) is detected by a reflectometer (4), particularly an extreme ultraviolet reflectometer, which has a light source (5) and a light sensor (6), wherein, in order to detect the reflectivity by the light sensor (6), the light beam (7) reflected by the light source (6) on the surface (3) is measured relative to the specimen (2) at at least a predetermined distance, characterized in that the light sensor (6) is moved at least once during calibration so that the reflected light beam (7) is measured relative to the surface (3) of the specimen (2) at different distances (x), and the reflectivity (R) is detected for each distance (x), and the basic reflectivity of the surface (3) is determined based on the measured reflectivity (R).

2. The method according to claim 1, characterized in that, Based on the determined basic reflectance, at least one reflectance value of the specimen (2) measured during the test is corrected.

3. The method according to any one of the preceding claims, characterized in that, The reflectance (R) of the surface (3) was measured relative to the specimen (2) at at least two, preferably at least five, and especially more than ten different distances (x).

4. The method according to any one of the preceding claims, characterized in that, During the calibration process, the basic reflectance is determined by extrapolation of the measured reflectance (R).

5. The method according to any one of the preceding claims, characterized in that, The optical sensor (6) moves only along the direction of the light beam (7).

6. The method according to any one of the preceding claims, characterized in that, Before the calibration process begins, the beam (7) reflected by the reference plane element (15) is detected. The surface roughness of the reference plane element is the same as or almost the same as that of the specimen (2) in order to compensate for the effect of roughness on the scattered light during the calibration process.

7. The method according to any one of the preceding claims, characterized in that, The orientation and / or arrangement of the photosensor (6) relative to the specimen (2) is changed laterally to the direction of the reflected beam (7).

8. An apparatus (1) for detecting the optical reflectivity (R) of an optical specimen (2), particularly a curved surface (3) of a mirror or lens, wherein, The device (1) has a reflectometer (4), particularly an extreme ultraviolet reflectometer, the reflectometer having a light source (5) and a light sensor (6), and wherein, in order to detect reflectivity (R) by the light sensor (6), the light sensor (6) can be arranged at at least a predetermined distance relative to the specimen (2), characterized in that the device (1) has a controller (12) specifically designed to perform the method according to any one of claims 1 to 7 under normal use.

9. The apparatus according to claim 8, characterized in that, The optical sensor (6) is movably supported along the linear guide rail (14).

10. The apparatus according to claim 8 or 9, characterized in that, The guide rail (14) is capable of moving in space.

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