Method and apparatus for determining an optical reflectivity of a curved surface of an optical test object

Abstract

The invention relates to a method for determining an optical reflectivity of a curved surface (3) of an optical test object (2), in particular a mirror or lens, wherein the reflectivity (R) is determined using a reflectometer (4), in particular an EUV reflectometer, which comprises a light source (5) and a light sensor (6), and wherein, in order to determine the reflectivity, a light beam (7) from the light source (6) reflected on the surface (3) is captured by the light sensor (6) at least at a predetermined distance from the test object (2). According to the invention: the light sensor (6) is displaced at least once for a calibration process such that the reflected light beam (7) is captured at varying distances (x) from the surface (3) of the test object (2), and a reflectivity (R) is determined for each distance (x); and a baseline reflectivity of the surface (3) is determined depending on the captured reflectivities (R).

Description

[0001] DESCRIPTION

[0002] Method and device for determining an optical reflectivity of a curved surface of an optical test piece

[0003] The present invention relates to a method for determining an optical reflectivity of a curved surface of an optical test object, in particular a mirror or lens, wherein the reflectivity is determined using a reflectometer, in particular an EUV reflectometer, which has a light source and a light sensor, and wherein, in order to determine the reflectivity, a light beam of the light beam source reflected from the test object or its surface is detected by the light sensor at at least a predetermined distance from the test object.

[0004] Furthermore, the present invention relates to a device for determining an optical reflectivity of a curved surface of an optical test object, in particular a mirror or lens, wherein the device comprises a reflectometer, in particular an EUV reflectometer, which has a light source and a light sensor, and wherein the light sensor can be arranged at least a predetermined distance from the test object for determining the reflectivity by the light sensor.

[0005] Methods and devices of the type mentioned above are known from the prior art. To measure the reflectivity of a curved mirror, it is known, for example, to direct a light beam onto the mirror and detect the reflected light beam using a light sensor. The light source and light sensor are not located on the same line or perpendicular to the surface of the test object, as otherwise the light sensor would block the light beam from the light source. Divergent scattered light originating from the light beam source of the reflectometer, which arises, for example, due to impurity and / or roughness of the optical elements of the light beam source, can lead to light loss in the measuring beam, i.e., the light beam reflected from the test object, as well as geometric overexposure of the light sensor. Such overexposure then leads to false or falsified reflectivity measurements.This occurs, for example, with convexly curved mirrors. Correcting this effect using calibration samples has proven insufficiently precise in the past. The present invention is therefore based on the object of creating an improved method for determining the optical reflectivity of a curved surface of a test specimen, which allows for advantageous compensation of the above-described stray light influence.

[0006] The object underlying the invention is achieved by a method having the features of claim 1. The method according to the invention has the advantage that the influence of scattered light on the surface can be detected particularly precisely and thus compensated for with corresponding precision. For this purpose, the invention provides that the light sensor is moved at least once for a calibration process of the reflectometer, so that the reflected light beam is detected at different distances from the test object and a reflectivity is determined for each distance, and that a basic reflectivity of the surface is determined depending on the detected reflectivities. During the calibration process, the light sensor is therefore moved in such a way that its distance from the test object changes.Because it is still supposed to detect the reflected light beam, this means that the light sensor is moved in the direction of the light beam, parallel to the light beam, or along the light beam. Since the light loss increases with increasing distance between the light sensor and the test object and decreases with decreasing distance, the scattered light behavior of the test object is advantageously determined by detecting several reflectivities at different distances from the test object. When a reflectivity value of the test object is later recorded in a measurement process following the calibration process, the scattering loss is advantageously compensated for or subtracted in order to determine the actual reflectivity of the test object. The light sensor is preferably moved step by step to change the distance, or moved continuously.Optionally, the surface is an optical coating on the test specimen.

[0007] Preferably, at least one reflectivity value of the test object recorded during a test procedure is corrected depending on the determined base reflectivity. The correction is preferably carried out using a correction table with a plurality of support points, with optional interpolation for values ​​between the support points.

[0008] Particularly preferably, the reflectivity of the at least two, preferably at least five, in particular more than ten different distances between the light sensor and the test object is recorded in order to determine the influence of stray light. The higher the number of reflectivities measured at different distances from the test object, the more precise the determination of the stray light behavior of the test object. Particularly preferably, in the calibration process, the basic reflectivity of the test object is determined by extrapolating the reflectivities recorded at the different distances between the light sensor and the test object. By means of the extrapolation, in particular, the influence of stray light or light loss of the test object is calculated or determined directly on the reflective surface of the test object.This allows the basic reflectivity of the test specimen to be determined, which is then used in the subsequent testing process to compensate or correct a reflectivity measurement that has been carried out.

[0009] Particularly preferably, the sensor is moved only in the direction of the light beam to change the distance of the sensor from the test object. The orientation of the sensor relative to the light beam and / or the test object remains unchanged. This allows for particularly advantageous extrapolation of the influence of stray light or light loss.

[0010] According to a preferred development of the invention, before or during the calibration process, a reflected light beam is detected on a reference flat element whose surface roughness corresponds or almost corresponds to the roughness of the test piece in order to compensate for any scattered light influence of the roughness during the calibration process. Because the roughness of the surface of the test piece itself also influences the scattered light effect, a comparative measurement process on a reference flat element, i.e., on a test piece with a flat / straight surface that has the same or almost the same roughness as the test piece, can be used to determine the extent to which the roughness influences the scattered light. Thus, the proposed calibration process on a reference flat element can detect this scattered light influence and take it into account or compensate for it when calibrating the device and when performing the measurement.

[0011] Preferably, the arrangement and / or orientation of the light sensor relative to the test object is varied transversely to the direction of the reflected light beam. This allows further properties of the scattered light to be detected and compensated for. For this purpose, the light sensor is, for example, shifted, pivoted, or rotated relative to the test object.

[0012] The device according to the invention with the features of claim 8 is characterized by a control unit that is specifically designed to carry out the method according to the invention when used as intended. This results in the advantages already mentioned above. Preferably, the light sensor is mounted so as to be displaceable along a rectilinear rail. The light sensor is thus arranged on a rail that extends in a rectilinear manner. The rail can be aligned parallel to the light beam and is preferably aligned so that the light sensor can advantageously be displaced along the light beam or in the direction of the light beam, in particular without changing its alignment with the test object or the light beam.

[0013] Preferably, the rail itself is movable in space in order to change the orientation of the light sensor or its position relative to the test object and to the light source, so that in particular the light beam is reflected at different points on the test object and detected by the light sensor.

[0014] The invention will be explained in more detail below with reference to the drawings.

[0015] Figure 1 shows an advantageous device in a simplified representation and

[0016] Figure 2 is a diagram explaining a method for operating the

[0017] device

[0018] Figure 1 shows a simplified representation of an advantageous device 1 for determining the reflectivity of a test specimen 2 having a curved surface 3 whose reflectivity is to be measured. Surface 3 is optionally coated. The test specimen 2 is, in particular, an optical element such as a lens or, in the present case, a mirror, in particular an EUV mirror.

[0019] The device 1 further comprises a reflectometer 4, which has a controllable light source 5, in particular a laser beam source or plasma source with beam-shaping optics, and a light sensor 6. The light source 5 is arranged in a stationary manner, such that a light beam 7 generated by it, in particular a laser beam, strikes the surface 3 of the test piece 2 and is reflected by it in the direction of the light sensor 6. Due to the curved shape of the test piece 2, the light beam 7 strikes the surface 3, in particular not perpendicularly, but at an angle to a vertical, such that the light beam is reflected back in a different direction than the one from which it came. The light sensor 6 is thus also arranged next to the light source 5 in order to detect the reflected light beam.

[0020] As shown in Figure 1, the light beam 7 is not usually redirected without loss. Rather, scattered light is generated by reflection as well as by the light source itself, which in Figure 1 takes the form of a scattered light cone 8 emanating from the light source 5, a scattered light cone 9 emanating from the surface 3 of the test object 2, and a scattered light cone 10 resulting from the scattered light cone 8 reflected from the surface 3. In particular, the scattered light cones 9 and 10 overlap, so that the light sensor is out-illuminated, at least in some areas of its sensor surface.

[0021] As can be seen in Figure 1, the sensor area of ​​the light sensor 6 is smaller than at least the scattered light cone 10 in the position of the light sensor 6 shown by the solid line in Figure 1. This results in an area of ​​scattered light that bypasses the light sensor 6 and thus leads to a loss of light, which is shown in Figure 1 by a hatched area 11. The overlapping scattered light cones 9 and 10 also result in geometric overexposure of the light sensor 6, as already mentioned above, which leads to falsified reflectivity measurements. This occurs particularly with convexly curved mirrors.

[0022] A control unit 12 controls the light source 5 and evaluates the data acquired by the light sensor 6 to determine the reflectivity value of the surface 3 of the test object 2. In addition, the control unit 12 is coupled to an actuator 13, which is assigned to the light sensor 6 in order to displace the latter. Preferably, the light sensor 6 is displaceably mounted on a rectilinear rail 14. The actuator 13 enables the sensor 6 to be displaced along the rail 14 to different positions. The rail 14 is aligned parallel to the reflected light beam 7 or in the direction of the reflected light beam, so that when the light sensor 6 is displaced, it moves along the light beam or in the direction of the reflected light beam 7 and thereby assumes different distances from the test object 2 or its surface 3.

[0023] To calibrate the device 1, the control unit 12 detects multiple reflectivities of the test object 2 by determining the reflectivity of the test object 2 at different positions of the light sensor 6 along the rail 13 and thus at different distances from the surface 3. For this purpose, the light sensor 6 is moved step by step by the actuator 13 to multiple positions, with a reflectivity being determined at each position. This creates a set of reflectivities that can be evaluated.

[0024] Figure 2 shows a simplified representation of reflectivity R plotted against the distance x between the light sensor 6 and the test object 2. The numerous measured values ​​essentially extend along a straight line. By extrapolating the measured values ​​or reflectivities, a base reflectivity value of the test object 2 can be determined.

[0025] By moving the sensor 6, the glare at the light sensor 6 changes. The reflectivities change accordingly, as shown in Figure 2. It can be seen that by extrapolating the recorded reflectivities, the reflection value that would be recorded directly at the surface 3 if the sensor 6 could be moved sufficiently close to the test specimen 2—the value referred to here as the base reflectivity—can be determined relatively accurately by extrapolation. However, such a close approach to the surface 3 is not possible for the sensor 6 because it would then block the light beam 2 before it reaches the surface 3.

[0026] By determining the basic reflectivity of the test specimen 2 or its surface 3 in this way, the reflectivity measurements recorded in a subsequent test process are corrected and the influence of stray light is compensated. The proposed method also has the advantage that a calibration standard with a precisely defined curvature is not necessary for calibrating the device 1. Furthermore, the advantageous method can also be used to advantageously examine freeform surfaces with changing radii of curvature on the surface 3 with regard to their reflectivity. The determination of the basic reflectivity value and a resulting correction value for compensating the reflectivity values ​​recorded in the test process is advantageously carried out on the test specimen 3 under real conditions using the described method, so that no conversion of the device for calibration and testing is necessary.This simplifies the testing procedure and shortens the testing time.

[0027] Optionally, the light sensor 6 is not moved step by step, but continuously, whereby the reflectivity of the test object 3 is then continuously determined and evaluated accordingly. Here, too, the basic reflectivity value of the test object 2 is preferably determined by extrapolation.

[0028] Furthermore, it is preferably provided that, according to a further exemplary embodiment, the influence of scattered light caused by roughness on the surface 3 of the test piece 2 is compensated for by measuring a flat sample or a reference flat element 15 instead of the test piece 2 or together with the test piece 2 before or during the calibration or testing its reflectivity. A prerequisite for successful compensation is that the reference flat element 15 has a roughness that corresponds to or almost corresponds to the roughness of the test piece. Because the reference element is a flat element, i.e. has a straight or curvature-free surface, the influence of curvature on the scattered light is prevented when recording the reflectivity of the reference flat element 15, and thus an optimized scattered light factor is determined, which is taken into account to compensate for or correct a reflectivity value recorded during the testing process.

[0029] Depending on the cause and model of the overexposure, the curve along which the reflectivity values ​​move in Figure 2 may deviate from the straight line shown in Figure 2. In this case, the calculation or extrapolation of the reflectivity is preferably adjusted to the actual curve of the reflectivity values.

[0030] It is also possible to change the solid angle of the light sensor 6 relative to the test object 2 in order to generate additional reflectivity measurements. Different light sensors 6 can be used, or a large light sensor with a variable-diameter aperture. A combination of different sized light sensors 6 and different distances x can also be used to calibrate the device 1.

Claims

CLAIMS 1. A method for determining an optical reflectivity of a curved surface (3) of an optical test object (2), in particular a mirror or lens, wherein the reflectivity (R) is determined using a reflectometer (4), in particular an EUV reflectometer, which has a light source (5) and a light sensor (6), and wherein, in order to determine the reflectivity, a light beam (7) of the light source (6) reflected at the surface (3) is detected by the light sensor (6) at at least a predetermined distance from the test object (2), characterized in that the light sensor (6) is displaced at least once for a calibration process, so that the reflected light beam (7) is detected at different distances (x) from the surface (3) of the test object (2) and a reflectivity (R) is determined for each distance (x), and in that a basic reflectivity of the surface (3) is determined as a function of the detected reflectivities (R).

2. Method according to claim 1, characterized in that, depending on the determined basic reflectivity, at least one reflectivity value of the test object (2) recorded in a test process is corrected.

3. Method according to one of the preceding claims, characterized in that the reflectivity (R) of the surface (3) is recorded at at least two, preferably at least five, in particular more than ten different distances (x) from the test object (2).

4. Method according to one of the preceding claims, characterized in that in the calibration process the basic reflectivity is determined by an extrapolation of the detected reflectivities (R).

5. Method according to one of the preceding claims, characterized in that the light sensor (6) is displaced only in the direction of the light beam (7).

6. Method according to one of the preceding claims, characterized in that before the calibration process, the light beam (7) reflected on a reference flat element (15), the surface roughness of which corresponds or almost corresponds to the roughness of the test piece (2), is detected in order to compensate for a scattered light influence of the roughness in the calibration process.

7. Method according to one of the preceding claims, characterized in that an alignment and / or arrangement of the light sensor (6) to the test object (2) is varied transversely to the direction of the reflected light beam (7).

8. Device (1) for determining an optical reflectivity (R) of a curved surface (3) of an optical test piece (2), in particular a mirror or lens, wherein the Device (1) has a reflectometer (4), in particular an EUV reflectometer, which has a light source (5) and a light sensor (6), and wherein the light sensor (6) can be arranged at least a predetermined distance (x) from the test object (2) for determining the reflectivity (R) by the light sensor (6), characterized in that the device (1) has a control unit (12) which is specially designed to carry out a method according to one of claims 1 to 7 when used as intended.

9. Device according to claim 8, characterized in that the light sensor (6) is mounted displaceably along a rectilinear rail (14).

10. Device according to one of claims 8 or 9, characterized in that the Rail (14) can be moved in space.

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