Method and measuring system for determining the effective focal length of an optical system

Abstract

The invention relates to a method and to a measuring system for determining the effective focal length (EFL) of an optical system (10), in which an optical measuring device (11) is located in the image-side focal plane (BFP) of the optical system and an input beam (E1) is directed onto the optical system (10) at first and second angles (α1, α2) relative to the direction of the object-side optical axis (OA) and, in each case, the lateral position (x1, y1; x2, y2) of the points of incidence (AP1, AP2) of the associated output beams (A1, A2) on a measuring device (11) are ascertained and the effective focal length (EFL) of the optical system is calculated on the basis of the mutual spacing (formula (I)) of the points of incidence (AP1, AP2) of the first and the second output beams (A1, A2) and on the basis of the formula ΔL = EFL*tan(α2 - α1).

Description

[0001] Method and measuring system for determining the effective focal length of an optical system

[0002] Background of the invention

[0003] The invention relates to a method and a measuring system for determining the effective focal length of an optical system. The optical system can consist of one or more focusing, defocusing, or beam-deflecting optical elements such as lenses, mirrors, or the like. Such optical systems are particularly an essential component of laser processing machines that allow the precise focusing of a laser beam on a target object.

[0004] An optical system is characterized by several characteristic properties with which its input and output behavior can be described geometrically optically: the object-side focal plane FFP ("Front Focal Plane"), an image-side focal plane BFP ("Back focal plane"), the object and image-side principal planes and the object and image-side focal lengths FFL (Front focal length) and BFL (Back focal length), where the FFL indicates the distance of the object-side principal plane to the object-side focal plane FFP and the BFL indicates the distance of the image-side principal plane to the image-side focal plane BFP.

[0005] An optical system can also be summarized as a black box for calculating the input and output behavior by using a thin lens. The principal planes of a thin lens coincide and lie in the lens plane. Therefore, for a thin lens, the image-side focal length BFL and the object-side focal length FFL are identical and can be summarized as the effective focal length EFL. When considering an optical system as a black box, it is sufficient for most applications to describe the input and output behavior of the optical system using the effective focal length EFL and, if applicable, the position of the object-side focal plane FFP and the image-side focal plane BFP.

[0006] Various methods have been proposed for determining the effective focal length (EFL) of an optical system. However, these methods require the use of collimated input beams or input beams of known divergence to measure the system.

[0007] Object of the invention

[0008] The invention is based on the object of providing a method and a measuring system for the precise determination of the effective focal length of an optical system, which can be carried out with input beams whose divergence properties do not have to be known.

[0009] Description of the invention

[0010] This object is achieved according to the invention by a method for determining the effective focal length of an optical system, comprising the steps of: a) arranging an optical measuring device in the image-side focal plane of the optical system; b) directing an input beam from the object side of the optical system at a first angle (ai ) relative to the direction of the object-side optical axis onto the optical system and determining the lateral position (xi, yi) of the point of incidence of the associated output beam on the measuring device; c) directing the input beam at a second angle (oc2*ai) relative to the direction of the object-side optical axis from the object side onto the optical system and determining the lateral position (X2, y2) of the point of incidence of the associated output beam on the measuring device; d) calculating the effective focal length (EFL) of the optical system from the mutual distance AL = the

[0011] Impact points of the output rays and from the formula AL = EFL*tan(oc2 - ai).

[0012] The input beam is first directed at the optical system at a first angle ai and then, in a second step, deflected and directed at the optical system at a second angle a2. The corresponding points of incidence are measured by the measuring device arranged in the image-side focal plane, and their mutual distance AL is calculated. From this calculated distance AL and the known angles ai and a2, the effective focal length EFL can then be calculated. Since only one beam, which is deflected at an angle for the second measurement, is used instead of two input beams from different beam sources, the divergence properties of the input beam or the input beam bundle are irrelevant. Collimated, diverging, or converging beam bundles can therefore be used as input beams. The quality of the beam source therefore does not require high standards.The point of incidence of the corresponding output beams is determined in the same way in both measurement steps, so that, for example, any divergence in the input beam does not affect the measurement result of the effective focal length (EFL). For an input beam, the point of incidence of the principal ray is used for the measurement, with the principal ray being defined by the fact that it passes through the lens without geometrical influence.

[0013] Alternatively, the points of incidence of the output beams can be determined by determining the intensity center or the area center of the power density distribution of the output beam. Any diverging input beams do not affect the accuracy of the effective focal length EFL measurement result, since the optical measuring device is positioned in the image-side focal plane. However, the points of incidence must be determined in the same way for both measurements. Furthermore, it is useful to direct additional input beams at first and second angles onto the optical system and determine the lateral position of the points of incidence of the corresponding output beams on the measuring device in order to increase the accuracy of the effective focal length calculation by measuring the distances between the additional points of incidence.For example, the least squares method can be used when calculating the effective focal length to statistically compensate for any setting or measurement inaccuracies.

[0014] To perform the procedure, the optical measuring device must be positioned in the image-side focal plane. The position of the image-side focal plane of the optical system can be known in advance, for example, through reference marks in the optical system's installation space. However, if the position of the image-side focal plane is unknown, it can be determined using the following procedure:

[0015] - an input beam is directed from the object side onto the optical system and the lateral position of the intensity center of gravity or the area center of gravity of the corresponding output beam on the measuring device is determined for different positions of the measuring device in the direction of the image-side optical axis of the optical system (z-direction);

[0016] - the input beam is shifted in parallel and the lateral position of the intensity centers or area centers of gravity of the output beam of the shifted input beam on the optical measuring device are determined for different positions of the measuring device in the direction of the image-side optical axis of the optical system (z-direction);

[0017] - the lateral positions of the intensity centers of gravity or area centers of gravity of the output beam of the non-shifted input beam and the lateral positions of the intensity centers of gravity or area centers of gravity of the output beam of the shifted input beam for the different positions of the measuring device, plotted over the z-axis, are each connected by curves and the intersection point of the two curves is determined, the z-coordinate of which indicates the position of the image-side focal plane.

[0018] This procedure takes advantage of the fact that the output rays of exactly parallel principal rays (= central rays) of a beam incident on a focusing optical system intersect at the image-side focal plane BFP. By parallel shifting the input beam or the principal ray of an input beam and determining the lateral position of the points of incidence of the corresponding output rays on the measuring device for various positions of the measuring device in the z-direction, the intersection point of the output rays of the parallel input rays and thus the z-coordinate of the image-side focal plane can be calculated. The measuring device can then be placed in this z-position, and the procedure for determining the effective focal length can be carried out.To determine the image-side focal plane, several parallel input beam pairs can preferably be used and the image-side focal plane can be calculated with greater accuracy from the impact points of the corresponding output beams.

[0019] For the smooth functioning of the method according to the invention, the angles of the input beams on the object side of the optical system can be measured and corrected in case of deviations from a target value. This prevents erroneous results in both the calculation of the effective focal length and the determination of the image-side focal plane. Such a measurement can also be performed to check and, if necessary, correct the parallel shift of the input beams.

[0020] The measuring system according to the invention for carrying out the method according to the invention is characterized in that it has a beam source for generating input beams on the object side of the optical system, an optical measuring device on the image side of the optical system, which is arranged to be linearly adjustable in the direction of the image-side optical axis of the optical system, and at least one device for changing the direction of the input beams generated by the beam source.

[0021] The at least one device for changing the direction of the input beams preferably comprises a device for angularly deflecting an input beam, in particular a rotatable mirror. By slightly rotating the mirror, the two different angles of the input beam can be generated for calculating the effective focal length of the optical system.

[0022] If the direction-changing device is also arranged so that it can be adjusted transversely to the direction of the object-side optical axis of the optical system, the device can also perform a parallel shift of an input beam, which is necessary to determine the image-side focal plane of the optical system. Alternatively, two beam sources can be provided to generate the two parallel input beams.

[0023] Instead of a movable rotating mirror, the measuring system could also have two identical, separately rotatable wedge plates for angular deflection and / or parallel displacement of the input beams.

[0024] Further advantages arise if the measuring system includes an angle measuring device for the input beam(s). This ensures that the input beams have the desired angles for calculating the effective focal length and / or the image-side focal plane.

[0025] Preferably, the measuring system may comprise an autocollimator which serves as an angle measuring device and as a beam source.

[0026] The optical measuring device can advantageously be equipped with a camera chip connected to an image evaluation device. The image evaluation device records the lateral position of the points of incidence of the output beams and calculates the effective focal length and, optionally, the z-coordinate of the image-side focal plane. It is also possible to provide a beam splitter. This can be placed in front of the optical system and reflect back a portion—for example, 50%—of the input beam. From the reflected portion, the autocollimator can determine the angle or an angular change of the input beam. In the event of deviations from a target value for the angle, the device for changing the direction of the input beam can be corrected accordingly.

[0027] The optical system to be measured can be formed from one or more focusing, defocusing, or beam-deflecting optical elements such as mirrors, lenses, or apertures. The optical system can, for example, be part of a focusing unit, which is arranged in particular at the end of the beam path of a laser system for generating EUV (extreme ultraviolet) light. The focusing unit serves to focus the laser beam onto a target material, in particular a tin droplet. The desired extreme ultraviolet radiation is generated upon exposure of the target material. EUV systems are used, among other things, in the production of semiconductors. The invention therefore also relates to a focusing unit with an integrated measuring system according to the invention for monitoring and adjusting the focal point of a laser beam.The measurement of the focusing unit of, for example, an EUV system according to a method according to the invention with the aid of a measuring system according to the invention ensures smooth functioning of such EUV systems, since the target material can be reliably exposed.

[0028] Further features and advantages of the invention will become apparent from the drawings, the description of the drawings, and the claims. According to the invention, the above-mentioned and further-described features can be used individually or in combination in any convenient way. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention.

[0029] Detailed description of the invention and drawing

[0030] Fig. 1 shows a schematic representation of the principle of measuring the image-side effective focal length of an optical system;

[0031] Fig. 2 shows a schematic representation of a measuring system for determining the image-side effective focal length and the focal plane of an optical system;

[0032] Fig. 3 shows a schematic representation of the measurement of the image-side focal plane of an optical system;

[0033] Fig. 4 shows a schematic representation of the calculation of the z-coordinate of the focal plane of an optical system;

[0034] Fig. 5a, b show an alternative object-side design of the measuring system from Fig.2 in two different working positions.

[0035] Fig. 1 schematically illustrates the determination of the effective focal length EFL of an optical system 10, which has been replaced by a thin lens for black-box observation, but can actually consist of several optical elements such as lenses, mirrors, or apertures. For the measurement, an optical measuring device 11, which can in particular have a camera chip 12 (see Fig. 2), is arranged in the image-side focal plane BFP of the optical system 10. The position of the image-side focal plane BFP can be known in advance, for example by a marking on a housing of a focusing unit containing the optical system, or the position of the image-side focal plane BFP can be determined according to the method explained in Figs. 3 and 4 in order to be able to arrange the measuring device 11 in the image-side focal plane BFP for determining the effective focal length EFL.To measure the effective focal length EFL, an input beam El is sent through the optical system 10 at an angle oti relative to an object-side optical axis OA of the optical system 10. However, it is also possible to choose the angle ai = 0.

[0036] The optical axis OA defines the z-direction of the measuring arrangement. The output beam Al emerging from the optical system 10 impinges on the optical measuring device 11 at a point API. The distance Ai from the impingement point API to the point 0 where the optical axis OA passes through the image-side focal plane BFP is measured.

[0037] Subsequently, an input beam E2 is directed at the optical system 10 at a second angle β2. The beam E2 can originate from the same beam source and does not have to be parallel to the beam E1, as shown in Fig. 1, but merely inclined by the other angle β2 with respect to the optical axis OA. The displacement between the beams E1 and E2 shown in Fig. 1 is merely for the sake of clarity. The beam A2 emerging from the optical system strikes the measuring device 11 at a point AP2. The distance A2 from the point of incidence AP2 to the point 0 where the optical axis OA passes through the image-side focal plane BFP is measured. From the difference between the distances AL = A2 - Ai and the difference between the angles α2 - αi, the effective focal length EFL can be calculated using the following formula: AL = EFL * tan(α2 - ai).

[0038] Fig. 2 shows a basic measuring arrangement with which the effective focal length EFL of an optical system 10 according to Fig. 1 can be determined. On the image side of the optical system 10, an optical measuring device 11 with a camera chip 12 is provided. The measuring device 11 is arranged so as to be adjustable in the z-direction, as indicated by the double arrow 13. In this way, it is possible to arrange the measuring device 11 exactly in the image-side focal plane BFP of the optical system. On the object side of the optical system 10, an autocollimator AC is arranged, which has a beam source (not shown in detail here) and an angle measuring device. The input beams E1, E2 generated by the autocollimator AC are deflected by a device 14 for changing direction, here a rotating mirror - indicated by the double arrow 15 - and directed onto the optical system 10.The rotating mirror 14 allows the input beams E1 and E2 to be projected onto the optical system 10 at different angles ai, α2, thus allowing the effective focal length EFL to be measured as shown in Fig. 1. A beam splitter ST is also arranged in front of the optical system 10, which reflects back a portion of the input beams E1, E2. From this portion, the autocollimator AC can measure and, if necessary, correct the angle of the input beams E1, E2.

[0039] The rotating mirror 14 can also be moved in the x-direction—here to a position 14'—as indicated by the double arrow 16. This displacement is necessary for determining the image-side focal plane BFP according to the method explained in Figs. 3 and 4. However, the adjustment does not necessarily have to be in the x-direction. Any linear displacement of the direction-changing device 14 lateral to the z-direction allows the determination of the image-side focal plane BFP.

[0040] Fig. 3 shows how the image-side focal plane BFP of an optical system 10 can be determined. For this purpose, at least one pair of parallel input beams E1, E2; E3, E4; E5, E6 is sent through the optical system, and the points of incidence of the corresponding output beams A1, A2; A3, A4; A5, A6, or the intensity or area centers of gravity in the case of an output beam, are determined on the measuring device 11. This determination is performed for different positions of the measuring device in the z-direction (plane 1 to plane 3). In the image-side focal plane of a lens, parallel rays converge at a point at the input. This fact is used for the measurement according to Fig. 3. In the example shown, the position "plane 3" of the measuring device corresponds to the position of the image-side focal plane BFP. However, it is not necessary to position the measuring device 11 exactly in the focal plane to determine the BFP. As shown in Fig.4 shows, the measurement results from the positioning of the measuring device 11 in other positions in the z-direction can be used to determine the position of the image-side focal plane BFP in the z-direction. For this purpose, the lateral positions L of the impact points API, AP2 of the output beams A1, A2 of two parallel input beams E1, E2 are plotted over the z-direction axis, with these lateral positions L being determined for different positions of the measuring device in the z-direction. The impact points API and AP2 are each connected by a straight line, and the beam direction of the output beams A1 and A2 is thereby determined. The intersection point S of the beams A1 and A2 defines the position ZBFP of the image-side focal plane BFP in the z-direction. To increase the measurement accuracy, several parallel input beam bundles E1, E2; E3, E4; E5, E6 can also be used, as shown in Fig.3, and from the points of incidence of the corresponding output beams Al, A2; A3, A4; A5, A6 the course of the output beams Al, A2; A3, A4; A5, A6 and their mutual intersection points are determined.

[0041] This determination of the image-side focal plane (BFP) also works for non-collimated input beams. In this case, the focal point of the parallel-shifted beams lies behind or in front of the image-side focal plane, depending on whether the input beams are divergent or convergent. However, with a parallel shift of such input beams, the lateral position of the corresponding output beams on the measuring device does not change if the device is placed exactly in the image-side focal plane (BFP). This property can therefore be used to determine the image-side focal plane for all types of input beams.

[0042] Fig. 5 shows an alternative embodiment of the object side of the measuring system from Fig. 2, in which the device for changing direction 14 is not formed by a displaceable rotating mirror but by two identical parallel wedge plates 17, 18. The two wedge plates 17, 18 can be rotated independently of one another. Fig. 5a shows the two wedge plates 17, 18 in a position that allows a parallel displacement of an input beam E1. For this purpose, both wedges 17, 18 are rotated such that the input beam E1 remains parallel when entering the optical system 10. The parallelism can be controlled by the back reflection of the beam splitter ST and the autocollimator AC. The input beam E1 is shifted laterally into the positions 17', 18' by the rotation of the wedge plates 17, 18. The wedge plate pair 17, 18 acts as a "plane-parallel plate" which is rotated, ie the beam El is displaced parallel on a circular path.Preferably, the wedge plates 17, 18 are rotated together around a common axis of rotation with the same angle of rotation.

[0043] To set a defined angular offset of the input beam E1, both wedges 17, 18 are rotated relative to each other around their optical axes, as illustrated in Fig. 5b. The two wedge plates 17, 18 then act as a single wedge plate with an "adjustable" wedge angle. The angular offset of the input beam E1 can in turn be measured by the interaction of the beam splitter ST with the autocollimator AC.

[0044] However, the wedge plates 17, 18 can also be provided with a beam splitter coating on the inlet and outlet sides. This eliminates the need for a separate beam splitter (ST) in the measurement system.

[0045] Instead of providing a linearly adjustable rotating mirror 14 or wedge plates 17, 18, the beam source itself could of course also be moved and / or adjusted in angle to carry out the measurements.

Claims

Patent claims 1. A method for determining the effective focal length of an optical system, comprising the steps of: a) arranging an optical measuring device (11) in the image-side focal plane (BFP) of the optical system (10); b) directing an input beam (E1) from the object side of the optical system (10) at a first angle (ai) relative to the direction of the object-side optical axis (OA) onto the optical system (10) and determining the lateral position (xi, yi) of the point of incidence (API) of the associated output beam (A1) on the measuring device (11); c) directing the input beam (E2) at a second angle (α2*ai) relative to the direction of the object-side optical axis (OA) from the object side onto the optical system (10) and determining the lateral position (X2, y2) of the point of incidence (AP2) of the associated output beam (A2) on the measuring device (11); d) Calculate the effective focal length (EFL) of the optical system from the mutual distance AL = the Impact points of the first and second output beams and from the formula AL = EFL*tan(a2 - ai).

2. Method according to claim 1, characterized in that the points of incidence (Apl, Ap2) of the output beams (Al, A2) are each determined by determining the intensity center of gravity of the power density distribution of the output beams (Al, A2).

3. Method according to claim 1, characterized in that the points of incidence (API, AP2) of the output beams (Al, A2) are each determined by determining the center of gravity of the power density distribution of the output beams (Al, A2).

4. Method according to one of the preceding claims, characterized in that the points of incidence (API, AP2) of the output beams (AI, A2) are determined in the same way for all measurements.

5. Method according to one of the preceding claims, characterized in that further input beams are directed at first and second angles onto the optical system (10) and the lateral position of the points of incidence of the associated output beams on the measuring device (11) is determined in order to increase the accuracy of the calculation of the effective focal length (EFL) by measuring the distance of the further points of incidence.

6. Method according to one of the preceding claims, characterized in that the following steps are carried out to determine the image-side focal plane (BFP) of the optical system (10): - an input beam (El) is directed from the object side onto the optical system (10) and the lateral position (xi, yi) of the intensity center of gravity or area center of gravity (API) of the associated output beam (Al) on the measuring device (11) is determined for different positions (planes 1-3) of the measuring device (11) in the direction of the image-side optical axis (OA) of the optical system (z-direction); - the input beam (E2) is shifted in parallel and the lateral position (X2, yz) of the intensity center of gravity or area center of gravity (AP2) of the output beam (A2) of the shifted input beam (E2) on the optical measuring device (11) is determined for different positions (planes 1-3) of the measuring device (11) in the direction of the image-side optical axis (OA) of the optical system (z-direction); - the lateral positions (xi, yi) of the intensity centers or area centers of the output beam (Al) of the non-shifted input beam (El) and the lateral positions (x2, y2) the intensity centers of gravity or area centers of gravity of the output beam (A2) of the shifted input beam (E2) for the different positions (planes 1-3) of the measuring device (11), plotted over the z-direction axis, are each connected to one another by curves and the intersection point of the two curves is determined, the z-coordinate of which indicates the position of the image-side focal plane (BFP).

7. Method according to one of the preceding claims, characterized in that the angles (ai, az) of the input beams (El, E2) are measured on the object side of the optical system (10) and corrected in the event of deviations from a desired value.

8. Measuring system for carrying out a method according to one of claims 1 to 5, characterized in that it has at least one beam source for generating input beams (El - E6) on the object side of the optical system (10), an optical measuring device (11) on the image side of the optical system (10), which is arranged to be linearly adjustable in the direction (z direction) of the image-side optical axis (OA) of the optical system (10), and at least one device (14) for changing the direction of the input beams (El - E6) generated by the beam source.

9. Measuring system according to claim 6, characterized in that the at least one device for changing the direction (14) of the input beams comprises a device for angular deflection of an input beam (El, E2).

10. Measuring system according to claim 7, characterized in that the device for angular deflection is a rotatable mirror.

11. Measuring system according to one of claims 6 to 8, characterized in that the device (14) for changing the direction transversely to the direction of the object-side optical axis (OA) of the optical system (10) is arranged to be adjustable.

12. Measuring system according to claim 6, characterized in that it has two identical, separately rotatable wedge plates (17, 18) for angular deflection and / or parallel displacement of input beams (El, E2).

13. Measuring system according to one of claims 6 to 10, characterized in that it comprises an angle measuring device for the input beam(s) (E1 - E6).

14. Measuring system according to claim 6 and 11, characterized in that it comprises an autocollimator (AC) which serves as an angle measuring device and as a beam source.

15. Measuring system according to one of claims 6 to 12, characterized in that the measuring device (11) has a camera chip (12) which is connected to an image evaluation device.

16. Measuring system according to one of claims 6 to 13, characterized in that it comprises a beam splitter (ST).

17. Measuring system according to one of claims 6 to 14, characterized in that the optical system (10) is formed from one or more focusing, defocusing or beam-deflecting optical elements such as mirrors, lenses or diaphragms.

18. Focusing device for a laser processing system with a measuring system according to one of claims 6 to 15 for controlling and adjusting the focal point of a laser beam.

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