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

Abstract

The invention relates to a method and a measuring system for determining an effective focal length (EFL) of an optical system (10), an optical measuring device (11) being arranged in an image-side focal plane (BFP) of the optical system and an input beam (E1) being directed at the optical system (10) at a first and a second angle (alpha1, alpha2) with respect to the direction of an object-side optical axis (OA), x2, y2) of a point of incidence (AP1, Ap2) of the associated output beam (A1, A2) on the measuring device (11) is determined, and the effective focal length (EFL) of the optical system is calculated from the distance (formula (I)) of the points of incidence (AP1, AP2) of the first and second output beams (A1, A2) from one another and from the formula [delta] L = EFL * tan ([alpha] 2-[alpha] 1).

Description

Technical Field

[0001] This invention relates to a method and measurement system for determining the effective focal length of an optical system. The optical system can be formed by one or more optical elements that focus, defocus, or deflect a beam, such as lenses, mirrors, or the like. In particular, such an optical system is an important component of a laser processing machine, allowing a laser beam to be accurately focused onto a target object. Background Technology

[0002] The optical system is characterized by several distinctive properties that allow for the description of its input and output characteristics in terms of geometric optics: the object-side focal plane FFP ("Front Focal Plane"), the image-side focal plane BFP ("Back Focal Plane"), the object-side and image-side principal planes, and the object-side and image-side focal lengths FFL (Front Focal Length) and BFL (Back Focal Length), where FFL indicates the distance between the object-side principal plane and the object-side focal plane FFP, and BFL indicates the distance between the image-side principal plane and the image-side focal plane BFP.

[0003] To calculate input and output characteristics, the optical system can also be represented as a black box using a thin lens. The principal planes of the thin lens coincide and lie within the lens plane. Therefore, for a thin lens, the image-side focal length (BFL) and the object-side focal length (FFL) are the same and can be collectively described as the effective focal length (EFL). When observing the optical system as a black box, it is sufficient for most applications to describe the input and output characteristics of the optical system using the effective focal length (EFL) and, if necessary, the orientation of the object-side focal plane (FFP) and the image-side focal plane (BFP).

[0004] Different methods have been proposed for determining the effective focal length (EFL) of an optical system. However, known methods require the use of a collimated input beam or an input beam with known divergence to measure the system. Summary of the Invention

[0005] Invention Task The objective of this invention is to provide a method and measurement system for accurately determining the effective focal length of an optical system, which can be performed using an input beam whose divergence properties need not be known.

[0006] Aspect Description According to the present invention, this task is solved by a method for determining the effective focal length of an optical system, the method comprising the following steps: a) Arrange optical measuring instruments in the image-side focal plane of the optical system; b) Align the input beam with the optical system from the object side at a first angle (α1) relative to the direction of the optical axis on the object side, and determine the lateral position (x1, y1) of the incident point of the relevant output beam on the measuring instrument. c) Align the input beam with the optical system from the object side at a second angle (α2≠α1) relative to the direction of the optical axis on the object side, and determine the lateral position (x2, y2) of the incident point of the relevant output beam on the measuring instrument. d) The distance ΔL between the incident points of the output beam and each other = The effective focal length (EFL) of the optical system is calculated using the formula ΔL = EFL*tan(α2- α1).

[0007] First, the input beam is aligned with the optical system at a first angle α1. Then, in a second step, the input beam is deflected and aligned with the optical system at a second angle α2. The relevant incident points are measured by a measuring instrument arranged in the image side focal plane, and their relative distance ΔL is calculated. Then, the effective focal length (EFL) can be calculated from the calculated distance ΔL and the known angles α1 and α2. Since two input beams from different beam sources are not used, but only one beam deflected in angle for the second measurement, the divergence performance of the input beam or the input beam focusing is irrelevant. Therefore, a focusing beam that is used for focusing, defocusing, or deflecting can be used as the input beam. Therefore, there is no need to place high demands on the quality of the beam source. In both measurement steps, the incident points of the relevant output beams are determined in the same way, such that possible divergence in the input beam, for example, does not affect the measurement results of the effective focal length (EFL). In the case of input beam focusing, the incident point of the main beam is used for measurement, wherein the main beam is defined such that it passes through the lens without being geometrically affected.

[0008] Alternatively, the incident point of the output beam can be determined by finding either the intensity centroid or the area centroid of the power density distribution of the output beam convergence. A potentially divergent input beam does not affect the accuracy of the effective focal length (EFL) measurement because the optical measuring instrument is positioned on the image-side focal plane. However, the incident point must be determined in the same manner for both measurements.

[0009] Additionally, it is preferable to align the additional input beams with the optical system at first and second angles, and to determine the lateral position of the incident point of the relevant output beam on the measuring instrument, so as to improve the accuracy of the calculation of the effective focal length by measuring the distance between the additional incident points. For this purpose, for example, the least squares method can be applied when calculating the effective focal length to statistically compensate for possible setup or measurement inaccuracies.

[0010] To perform this method, the optical measuring instrument needs to be positioned in the image's side focal plane. The orientation of the image's side focal plane of the optical system can be known beforehand, for example, through reference marks in the mounting space of the optical system. However, if the orientation of the image's side focal plane is unknown, it can be determined using the following method: - Align the input beam with the optical system from the object side, and for different positions of the measuring instrument on the image side optical axis (z direction) of the optical system, determine the lateral position of the intensity centroid or area centroid of the output beam on the measuring instrument. - Move the input beam parallel to the optical axis and determine the lateral position of the intensity centroid or area centroid of the output beam of the moved input beam on the optical measuring instrument for different positions of the measuring instrument on the image side optical axis (z direction). - For different positions of the measuring instrument, the lateral positions of the intensity centroid or area centroid of the output beam of the unmoved input beam and the lateral positions of the intensity centroid or area centroid of the output beam of the moved input beam are plotted on the z-axis by connecting them with curves, and the intersection point of the two curves is obtained. The z-coordinate of the intersection point indicates the position of the image side focal plane.

[0011] This process utilizes the fact that the output beams of the precisely parallel main beams (=the intermediate beams) incident on the focusing optical system intersect in the image-side focal plane (BFP). By moving the input beams or the main beams of the input beam convergence parallel to each other, and by determining the lateral position of the incident point of the relevant output beams on the measuring instrument for different positions in the z-direction, the intersection of the parallel input beams' output beams can be calculated, and thus the z-coordinate of the image-side focal plane can be calculated. The measuring instrument can then be placed at this z-position, and a method for determining the effective focal length can be performed. Preferably, multiple parallel input beam pairs can also be used to determine the image-side focal plane, and the image-side focal plane can be calculated with high accuracy from the incident points of the relevant output beams.

[0012] To ensure the smooth operation of the method according to the invention, the angle of the input beam on the object side of the optical system can be measured, and said angle can be corrected if it deviates from the expected value, so as to avoid erroneous results not only when calculating the effective focal length, but also when determining the image-side focal plane. Such measurements can also be performed to check and, if necessary, correct for parallel movement of the input beam.

[0013] The measurement system according to the invention for performing the method according to the invention is characterized in that the measurement system has a beam source for generating an input beam on the object side of the optical system, an optical measuring instrument on the image side of the optical system, and at least one means for changing the direction of the input beam generated by the beam source, the optical measuring instrument being arranged in a manner that allows linear adjustment in the direction of the optical axis on the image side of the optical system.

[0014] Preferably, the at least one device for changing the direction of the input beam includes a device for deflecting the angle of the input beam, particularly a rotatable mirror. By slightly rotating the mirror, two different angles of the input beam can be generated for calculating the effective focal length of the optical system.

[0015] If the device for changing the direction is further arranged in a manner that allows for directional adjustment transverse to the object-side optical axis of the optical system, then parallel movement of the input beam, necessary for determining the image-side focal plane of the optical system, can also be achieved using this device. Alternatively, two beam sources can be provided, generating two parallel input beams.

[0016] Instead of a movable rotating reflector, the measurement system can also have two identical, separately rotating wedges for input beam angular deflection and / or parallel movement.

[0017] If the measurement system includes an angle measuring device for one or more input beams, additional advantages arise. This ensures that the input beams have the desired angles for calculating the effective focal length and / or the image-side focal plane.

[0018] Preferably, the measurement system may have an automatic collimator, which serves as an angle measuring device and as a beam source.

[0019] Advantageously, the optical measuring instrument may be equipped with a camera chip connected to an image analysis and processing device. The image analysis and processing device detects the lateral orientation of the incident point of the output beam, calculates the effective focal length, and optionally also calculates the z-coordinate of the image's side focal plane.

[0020] Alternatively, a beam splitter can be used. This beam splitter can be placed in front of the optical system and reflect a portion—for example, 50%—of the input beam back. An automatic collimator can determine the angle or angular variation of the input beam from the reflected portion. If there is a deviation from the desired value, the device used to change the direction of the input beam can be recalibrated accordingly.

[0021] The optical system to be measured can be formed by one or more optical elements that focus, defocus, or deflect the beam, such as mirrors, lenses, or light shields. For example, the optical system can be part of a focusing unit, particularly arranged at the end of the beam path of a laser system used for generating EUV (extrem ultraviolet) light. Here, the focusing unit is used to focus the laser beam onto a target material, especially a tin droplet. When the target material is exposed, the desired extreme ultraviolet radiation is generated. EUV systems are primarily used in semiconductor manufacturing. Therefore, the present invention also relates to a focusing unit having an integrated measurement system according to the invention for controlling and readjusting the focus of the laser beam. Measurement of the focusing unit of, for example, an EUV system using the measurement system according to the invention, according to the method of the invention, ensures the smooth functioning of such an EUV system because the target material can be reliably exposed.

[0022] Other features and advantages of the invention are derived from the dependent claims, the description of the drawings, and the claims themselves. According to the invention, the features mentioned above and further elaborated herein can be applied individually or in any suitable combination. The embodiments shown and described are not to be construed as an exhaustive enumeration, but rather as exemplary features used in the narrative of the invention. Attached Figure Description

[0023] Figure 1 A schematic diagram illustrating the principle of measuring the effective focal length on the image side of an optical system; Figure 2 A schematic diagram of a measurement system for determining the effective focal length and focal plane on the image side of an optical system is shown. Figure 3 A schematic diagram showing the measurement of the image-side focal plane of an optical system; Figure 4 The calculation of the z-coordinate of the focal plane of the optical system is shown in the schematic diagram; Figure 5 a and b are shown in two different working states. Figure 2 The alternative object-side configuration of the measurement system in the middle. Detailed Implementation

[0024] Figure 1 The determination of the effective focal length (EFL) of optical system 10 is illustrated schematically. For black-box observation, this optical system has been replaced with a thin lens, but in practice, it can consist of multiple optical elements, such as lenses, mirrors, or light-shielding plates. For measurement, an optical measuring instrument 11 is arranged in the image-side focal plane (BFP) of optical system 10. This optical measuring instrument may, in particular, include a camera chip 12 (see...). Figure 2 The orientation of the image-side focal plane (BFP) can be known in advance, for example, through markings on the housing of the focusing unit containing the optical system, or the orientation of the image-side focal plane (BFP) can be determined based on... Figure 3 and Figure 4 The method described herein is determined so that the measuring instrument 11 used to determine the effective focal length (EFL) can be arranged in the image side focal plane (BFP).

[0025] To measure the effective focal length EFL, the input beam E1 is transmitted through the optical system 10 at an angle α1 relative to the object-side optical axis OA. However, it is also possible to choose an angle α1 = 0.

[0026] The optical axis OA defines the z-direction of the measuring device. The output beam A1, emitted from behind the optical system 10, is incident on the optical measuring instrument 11 at point AP1. The distance Δ1 between the incident point AP1 and the penetration point 0 of the optical axis OA through the image-side focal plane BFP is measured.

[0027] Subsequently, the input beam E2 is aligned with the optical system 10 at a second angle α2. Here, the beam E2 can originate from the same beam source, and there is no need to... Figure 1 Instead of moving parallel to the beam E1, it only needs to be tilted at another angle α2 relative to the optical axis OA. The distance between beams E1 and E2... Figure 1 The movement shown is only for clarity in the schematic diagram. The beam A2 emanating from the optical system is incident on the measuring instrument 11 at point AP2. The distance Δ2 between the incident point AP2 and the penetration point 0 of the optical axis OA passing through the image-side focal plane BFP is measured. The effective focal length EFL can be calculated using the difference in distance ΔL = Δ2 - Δ1 and the difference in angle α2 - α1, according to the following formula: ΔL = EFL * tan(α2 - α1).

[0028] Figure 2 This illustrates the principle of the measurement arrangement, by which measurements can be performed according to... Figure 1The effective focal length (EFL) of the optical system 10 is determined. An optical measuring instrument 11 with a camera chip 12 is provided on the image side of the optical system 10. The measuring instrument 11 is arranged in a manner adjustable in the z-direction, as shown by double arrow 13. In this way, it is possible to precisely position the measuring instrument 11 in the image-side focal plane (BFP) of the optical system.

[0029] An autocollimator AC is arranged on the object side of the optical system 10. This autocollimator has a beam source (not shown in more detail here) and an angle measuring device. The input beams E1 and E2 generated by the autocollimator AC are deflected—as indicated by the direction-changing device 14 (here, a rotating mirror)—and aligned with the optical system 10. The rotating mirror 14 allows the input beams E1 and E2 to be directed onto the optical system 10 at different angles α1 and α2, thus performing alignment according to… Figure 1 The effective focal length (EFL) is measured. Furthermore, a beam splitter ST is arranged in front of the optical system 10, which reflects a portion of the input beams E1 and E2 back. An automatic collimator AC can be used by this component to measure and correct the angles of the input beams E1 and E2, if necessary.

[0030] Furthermore, the rotating reflector 14 can be moved in the x-direction—here, it moves to position 14', as indicated by the double arrow 16. This is in accordance with... Figure 3 and 4 The method described herein is necessary for determining the image side focal plane (BFP). However, the adjustment does not necessarily have to be performed in the x-direction. Any linear movement of the device 14 for orientation change in the lateral direction relative to the z-direction is sufficient to determine the image side focal plane (BFP).

[0031] exist Figure 3 The diagram illustrates how the image-side focal plane (BFP) of the optical system 10 can be determined. For this purpose, at least one pair of parallel input beams E1, E2; E3, E4; E5, E6 are sent through the optical system, and the incident points of the associated output beams A1, A2; A3, A4; A5, A6 are determined, or, in the case of beam convergence at the measuring instrument 11, the intensity centroid or area centroid is determined. This determination is performed for different positions of the measuring instrument in the z-direction (planes 1 to 3). In the image-side focal plane of the lens, the parallel beams at the input ends converge at a single point. This fact is used to determine the image-side focal plane of the lens. Figure 3 The measurement. In the example shown, the position of the measuring instrument, "plane 3," corresponds to the position of the image-side focal plane (BFP). However, to determine the BFP, it is not necessary to precisely position the measuring instrument 11 within the focal plane. Figure 4As shown, the measurement results from the positioning of the measuring instrument 11 at 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 incident points AP1 and AP2 of the output beams A1 and A2 of the two parallel input beams E1 and E2 are plotted on the z-axis, where these lateral positions L are obtained for different positions of the measuring instrument in the z-direction. The incident points AP1 and AP2 are connected to each other by straight lines, and the beam directions of the output beams A1 and A2 are thus determined. The intersection point S of beams A1 and A2 defines the position z of the image side focal plane BFP in the z-direction. BFP To improve measurement accuracy, multiple parallel input beams can also be used for focusing, such as E1, E2; E3, E4; E5, E6, etc. Figure 3 As shown, the directions and intersections of the output beams A1, A2, A3, A4, A5, A6 are obtained from the incident points of the relevant output beams A1, A2, A3, A4, A5, A6.

[0032] This determination of the image side focal plane (BFP) also applies to uncollimated input beams. In this case, depending on whether the input beam is divergent or convergent, the focal point of the parallel-moving beam is located behind or in front of the image side focal plane. However, when such an input beam moves parallel, if the measuring instrument is precisely placed in the image side focal plane (BFP), the lateral position of the associated output beam on the measuring instrument does not change. Therefore, this characteristic can be fully utilized to determine the image side focal plane in all types of input beams.

[0033] exist Figure 5 The middle shows Figure 2 An alternative configuration for the object side of the measurement system, wherein the device 14 for changing direction is not formed by a movable rotating mirror, but by two identical parallel wedge plates 17, 18. Here, the two wedge plates 17, 18 can rotate independently of each other.

[0034] Figure 5 Figure a shows two wedge plates 17, 18 in a position that allows for parallel movement of the input beam E1. For this purpose, the two wedge plates 17, 18 are rotated such that the input beam E1 remains parallel as it enters the optical system 10. The parallelism can be controlled by the back reflection of the beam splitter ST and the automatic collimator AC. The input beam E1 is moved laterally by rotating the wedge plates 17, 18 to positions 17', 18'. Here, the wedge plates 17, 18 act as “plane-parallel plates” that are rotated so that the beam E1 moves parallel along a circular track. Preferably, the wedge plates 17, 18 rotate together about a common axis of rotation at the same angle of rotation.

[0035] To adjust the defined angular offset of the input beam E1, the two wedges 17 and 18 are twisted relative to each other around their optical axes, as follows: Figure 5 As shown in b. Then, the two wedges 17 and 18 function like a single wedge with an "adjustable" wedge angle. The angular offset of the input beam E1 can be measured by the combined action of the beam splitter ST and the automatic collimator AC.

[0036] However, wedge plates 17 and 18 can also have beam splitter coatings on both the incident and exit sides. Then, a separate beam splitter ST can be omitted from the measurement system.

[0037] Instead of using a linearly adjustable rotating reflector 14 or wedge plates 17, 18, it is also possible to move the beam source itself and / or adjust it in terms of angle to perform measurements.

Claims

1. A method for determining the effective focal length of an optical system, the method comprising the following steps: a) An optical measuring instrument (11) is arranged in the image-side focal plane (BFP) of the optical system (10); b) Align the input beam (E1) with the optical system (10) from the object side of the optical system (10) at a first angle (α1) relative to the direction of the object-side optical axis (OA), and determine the lateral position (x1, y1) of the incident point (AP1) of the relevant output beam (A1) on the measuring instrument (11). c) Align the input beam (E2) with the optical system (10) from the object side at a second angle (α2≠α1) relative to the direction of the optical axis (OA) on the object side, and determine the lateral position (x2, y2) of the incident point (AP2) of the relevant output beam (A2) on the measuring instrument (11). d) The distance ΔL between the incident points of the first and second output beams relative to each other = The effective focal length (EFL) of the optical system is calculated using the formula ΔL = EFL*tan(α2- α1).

2. The method according to claim 1, characterized in that, The incident points (Ap1, Ap2) of the output beams (A1, A2) are determined by finding the intensity centroid of the power density distribution of the output beams (A1, A2).

3. The method according to claim 1, characterized in that, The incident points (AP1, AP2) of the output beams (A1, A2) are determined by finding the centroid of the area of ​​the power density distribution of the output beams (A1, A2).

4. The method according to any one of the preceding claims, characterized in that, The incident points (AP1, AP2) of the output beams (A1, A2) are determined in the same manner in all measurements.

5. The method according to any one of the preceding claims, characterized in that, The additional input beam is aligned with the optical system (10) at first and second angles, and the lateral position of the incident point of the relevant output beam on the measuring instrument (11) is determined in order to improve the accuracy of the calculation of the effective focal length (EFL) by measuring the distance between the additional incident points.

6. The method according to any one of the preceding claims, characterized in that, To determine the image-side focal plane (BFP) of the optical system (10), the following steps are performed: - Align the input beam (E1) with the optical system (10) from the object side, and for different positions of the measuring instrument (11) in the direction (z direction) of the optical axis (OA) on the image side of the optical system (plane 1-3), determine the lateral position (x1, y1) of the intensity centroid or area centroid (AP1) of the measuring instrument (11) on the measuring instrument (11). - Move the input beam (E2) in parallel, and for different positions of the measuring instrument (11) in the direction (z direction) of the image side optical axis (OA) of the optical system (plane 1-3), determine the lateral position (x2, y2) of the output beam convergence (A2) of the moved input beam (E2) on the optical measuring instrument (11) of the intensity centroid or area centroid (AP2). - For different positions (planes 1-3) of the measuring instrument (11), the transverse positions (x1, y1) of the intensity centroid or area centroid of the output beam convergence (A1) of the unmoved input beam (E1) and the transverse positions (x2, y2) of the intensity centroid or area centroid of the output beam convergence (A2) of the moved input beam (E2) are connected to each other by curves, and the intersection point of the two curves is obtained. The z coordinate of the intersection point indicates the position of the image side focal plane (BFP).

7. The method according to any one of the preceding claims, characterized in that, The angles (α1, α2) of the input beams (E1, E2) on the object side of the optical system (10) are measured, and the angles are corrected if they deviate from the expected values.

8. A measurement system for performing the method according to any one of claims 1 to 5, characterized in that, The measurement system has at least one beam source for generating an input beam (E1-E6) on the object side of the optical system (10), an optical measuring instrument (11) on the image side of the optical system (10), and at least one device (14) for changing the direction of the input beam (E1-E6) generated by the beam source, the optical measuring instrument being arranged in a manner that allows linear adjustment in the direction (z direction) of the optical axis (OA) on the image side of the optical system (10).

9. The measurement system according to claim 6, characterized in that, At least one device (14) for changing the direction of the input beam (E1-E6) has a device for deflecting the angle of the input beam (E1, E2).

10. The measurement system according to claim 7, characterized in that, The device used for angle deflection is a rotating reflector.

11. The measurement system according to any one of claims 6 to 8, characterized in that, The device (14) for changing direction is arranged in a manner that allows for directional adjustment laterally to the object-side optical axis (OA) of the optical system (10).

12. The measurement system according to claim 6, characterized in that, The measurement system has two identical, separately rotatable wedges (17, 18) for angular deflection and / or parallel movement of the input beam (E1, E2).

13. The measurement system according to any one of claims 6 to 10, characterized in that, The measurement system has an angle measuring device for one or more input beams (E1-E6).

14. The measurement system according to claims 6 and 11, characterized in that, The measurement system has an automatic collimator (AC), which serves as an angle measuring device and as a beam source.

15. The measurement system according to any one of claims 6 to 12, characterized in that, The measuring instrument (11) has a camera chip (12), which is connected to an image analysis and processing device.

16. The measurement system according to any one of claims 6 to 13, characterized in that, The measurement system has a beam splitter (ST).

17. The measurement system according to any one of claims 6 to 14, characterized in that, The optical system (10) is formed by one or more optical elements that focus, defocus, or deflect the beam, such as mirrors, lenses, or light shields.

18. A focusing device for a laser processing facility, the focusing device having a measuring system according to any one of claims 6 to 15 for controlling and readjusting the focus of a laser beam.

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