High-precision wavefront aberration detection method for a grating shearing interferometric projection objective

Abstract

Method for detecting wave aberration of high-precision projection objective by grating shearing interferometry. The grating shearing interferometer system adopted by this method includes: an optical and illumination system, an optical imaging system to be measured, a diffraction grating plate on the object plane, a diffraction grating plate on the image plane, a two-dimensional optoelectronic sensor, and a calculation and processing unit. The diffraction grating plate on the object plane and the diffraction grating plate on the image plane are respectively placed on the object plane and the image plane of the optical imaging system to be measured. During measurement, by means of axial scanning, the differential wavefronts at two or more positions in the axial direction are measured, and the Z2 coefficient of the differential wavefront in the x direction and the Z3 coefficient of the differential wavefront in the y direction at the axial position are obtained by fitting. The Z5 astigmatism error of the optical imaging system to be measured is directly obtained by linearly fitting the axial grating position using the Z2 and Z3 coefficients, eliminating the astigmatism error introduced by the grating defocus and position change in the shearing interferometer system. This method does not require the fabrication of additional gratings and does not require the additional addition of a turntable, which not only improves the wave aberration measurement accuracy of the optical imaging system to be measured but also does not increase the complexity of the measurement system structure.

Description

Technical Field

[0001] The present invention relates to the technical field of optical measurement, and specifically to a grating shearing interference projection objective wave aberration detection device and a detection method, which are applicable to a wave aberration detection device and a detection method for a projection objective of a lithography machine or other optical imaging systems based on a grating shearing interferometer. Background Art

[0002] Ronchi grating shearing interference is a shearing interferometer that uses an extended light source and modulates the coherence of the light source by a grating on the object plane. It has the advantages of common optical path, large dynamic range, no need for a separate ideal reference wavefront, high precision, and simple structure. By introducing phase-shifting interference technology, a series of interference patterns with different phase shifts are collected by laterally moving the grating, the differential wavefront is obtained and wavefront reconstruction is carried out to obtain the wave aberration of the system to be measured. When measuring the wave aberration by shearing interference, theoretically, it is required that the grating be located at the ideal focal plane position for measurement, such as in the prior art 1 (Yunjun Lu, Feng Tang, Xiangchao Wang, Wave aberration detection method for a grating shearing interference optical imaging system, Chinese invention patent, patent number: 109900201B). However, in the actual measurement process, due to the deviation of the grating from the focal plane position, and when measuring the differential wavefront in different directions, the position change caused by switching the grating will introduce additional systematic errors into the measurement results. The main component of this systematic error is astigmatism error (Z5).

[0003] The prior art 2 (Zhiqiang Liu et al, Astigmatism measurement by lateral shearing interferometer, JVST B - Microelectronics and Nanometer Structures, 2980 - 2983) proposed a wave aberration measurement method that adds a set of 45-degree gratings on the basis of 0-degree gratings, which solves the problem of astigmatism error introduced due to different grating positions during the measurement of differential wavefronts in different directions and can improve the wavefront reconstruction accuracy. This method can solve the systematic error (Z5) brought by the defocus of the grating position during the measurement, but it requires additional fabrication of gratings and an additional high-precision turntable, and switches between 0-degree gratings and 90-degree gratings during measurement, increasing the complexity of the measurement process.

[0004] Currently, there is no method for eliminating astigmatism error in a shearing interference wave aberration measurement system without increasing the complexity of the measurement process. Summary of the Invention

[0005] The object of the present invention is to overcome the deficiencies of the above-mentioned prior art, and a method for eliminating the astigmatism systematic error based on grating axial scanning is proposed. By using the Z-axis linear stage of the measuring device, a linear fitting is performed on the tilt term coefficient (Z2 / Z3) of the differential wavefront and the axial position of the grating, and the axial positions corresponding to the cases where the Z2 and Z3 coefficients are zero are obtained respectively. The astigmatism aberration (Z5) of the measured projection is directly calculated according to the distance between the two positions, effectively eliminating the astigmatism error of the shearing interferometer and improving the measurement accuracy of the astigmatism error of the projection object to be measured.

[0006] To achieve the above object, the technical solution of the present invention is as follows:

[0007] A method for detecting the wave aberration of a high-precision projection objective lens by grating shearing interference. The wave aberration measuring device used in this method includes: a light source and an illumination system, an object plane diffraction grating plate, a first three-dimensional displacement stage, an image plane diffraction grating plate, a second three-dimensional displacement stage, a two-dimensional photoelectric sensor, and a calculation and processing unit. The light source and the illumination system output spatially incoherent light. The object plane diffraction grating plate is fixed on the first three-dimensional displacement stage, and the image plane diffraction grating plate is fixed on the second three-dimensional displacement stage. The image plane diffraction grating plate is fixed on the second three-dimensional displacement stage. The object plane diffraction grating plate contains two sets of one-dimensional gratings with perpendicular grating directions. The image plane diffraction grating plate contains a set of checkerboard gratings or two sets of one-dimensional gratings with perpendicular grating line directions. The output end of the two-dimensional photoelectric sensor is connected to the calculation and processing unit. An xyz coordinate system is established, with the z-axis direction along the optical axis of the system, the x-axis along the grating line direction of the second grating on the object plane diffraction grating plate, and the y-axis along the grating line direction of the first grating on the object plane diffraction grating plate. Let the moving axes of the first three-dimensional displacement stage and the second three-dimensional displacement stage be the x-axis, y-axis, and z-axis respectively; the angle between the diagonal direction of the checkerboard grating and the x-axis (or y-axis) is 45 degrees. The steps of this method are as follows:

[0008] (1) Place the optical imaging system to be measured in this wave aberration measuring device, so that the light source and the illumination system are located on the object side of the optical imaging system to be measured, and the image plane diffraction grating plate is located on the image side of the optical imaging system to be measured. Adjust the first three-dimensional displacement stage to make the object plane diffraction grating plate located on the object plane of the optical imaging system to be measured, and adjust the second three-dimensional displacement stage to make the image plane diffraction grating plate located on the image plane of the optical imaging system to be measured. Using prior art 1, measure the wave aberration W of the optical imaging system to be measured, perform Zernike polynomial fitting on W to obtain the Z5-term wave aberration, and record the Z5-term error as W1;

[0009] (2) Divide the z-axis moving range of the second three-dimensional displacement stage into N equal parts (N >= 2) as the scanning positions, denoted as P i , where i = 1, 2, 3... N; P 1 is the starting height position;

[0010] (3) Move the first three-dimensional displacement stage to move the first grating of the grating lines on the object-plane diffraction grating plate along the y-axis direction into the position of the object-space field point of the optical imaging system to be measured. Move the second three-dimensional displacement stage to position the checkerboard grating or the one-dimensional grating in the corresponding direction on the image-plane diffraction grating plate in the z-direction to P 1 position;

[0011] (4) Through the phase shift of the object-plane or image-plane grating, obtain a series of shear interference patterns in the x-axis direction, and measure the differential wavefront in the x-axis direction For perform Zernike fitting, and extract the coefficient c of the Z2 term 2i , where i = 1, 2, 3... N;

[0012] (5) If the current position P i of the second three-dimensional displacement stage z is P N , then go to step 6), otherwise move the second three-dimensional displacement stage to the next position P i+1 , so that P i = P i+1 , and return to step 4);

[0013] (6) Move the first three-dimensional displacement stage to move the second grating of the grating lines on the object-plane diffraction grating plate along the x-axis direction into the position of the object-space field point of the optical imaging system to be measured; position the z-direction of the second three-dimensional displacement stage to P 1 position;

[0014] (7) Through the phase shift of the object-plane or image-plane grating, obtain a series of shear interference patterns in the y-axis direction, and solve for the differential wavefront in the x-axis direction For perform Zernike fitting, and extract the coefficient c of the Z3 term 3i where i = 1, 2, 3... N;

[0015] (8) If the current position P i of the second three-dimensional displacement stage in the z-direction is P N , then go to step 9), otherwise move the second three-dimensional displacement stage to the next position P i+1 , so that P i = P i+1 , and return to step 7);

[0016] (9) Perform linear fitting on the Z2 coefficient c 2i and the Z3 coefficient c 3i respectively with the axial position P i , and fit to obtain the axial position P 2 where c is zero and the axial position P S and c 3 where c is zeroT , from |P S -P T |, the distance ST between the two foci corresponding to astigmatism is obtained, the Z5 aberration coefficient of the optical imaging system to be measured is calculated according to formula (1), and the Z5 aberration W2 of the optical imaging system to be measured is obtained.

[0017]

[0018] (10) The wave aberration measurement result of the optical imaging system to be measured is corrected to W–W1+W2.

[0019] In the method for detecting the wave aberration of a high-precision projection objective lens by grating shearing interference, the ratio of the period of the one-dimensional grating on the object plane diffraction grating plate to the period of the checkerboard grating or one-dimensional grating on the image plane diffraction grating plate is equal to the magnification of the optical imaging system to be measured.

[0020] In the method for detecting the wave aberration of a high-precision projection objective lens by grating shearing interference, the duty cycle of the grating on both the object plane diffraction grating plate and the image plane diffraction grating plate is 50%.

[0021] The technical effect of the present invention is that by using the axial scanning method, the differential wavefronts at two or more axial positions are measured, and the Z2 coefficient of the differential wavefront in the x direction and the Z3 coefficient of the differential wavefront in the y direction at the axial position are obtained by fitting. The Z5 astigmatism error of the system to be measured is obtained by linearly fitting the Z2 and Z3 coefficients with respect to the z-direction position, eliminating the astigmatism error introduced by the grating defocus and position change in the shearing interferometer system. This method does not require the production of additional gratings, which not only improves the wave aberration measurement accuracy of the optical imaging system to be measured but also does not increase the complexity of the measurement system structure and measurement process. Description of the Drawings

[0022] Figure 1 It is a schematic diagram of a device for detecting the wave aberration of a high-precision projection objective lens by grating shearing interference;

[0023] Figure 2 It is a schematic diagram of the object plane diffraction grating plate;

[0024] Figure 3 It is a schematic diagram of the checkerboard grating of the image plane diffraction grating plate;

[0025] Figure 4 It is a schematic diagram of the optical path difference of grating defocus;

[0026] Figure 5 It is a schematic diagram of the astigmatic wave aberration imaging;

[0027] Figure 6 It is a schematic diagram of the Z2 coefficient of the differential wavefront in the x direction, the Z3 coefficient of the differential wavefront in the y direction, and the grating z-direction position;

[0028] Among them, 1. Object-plane diffraction grating plate; 2. First three-dimensional displacement stage; 3. Optical imaging system to be measured; 4. Image-plane diffraction grating plate; 5. Second three-dimensional displacement stage; 6. Two-dimensional photoelectric sensor; 7. Computing and processing unit; 8. Light source and illumination system. Specific implementation manner

[0029] To better understand the purpose, technical solution and advantages of the present invention, the present invention will be further described below in conjunction with the accompanying drawings and embodiments, but the protection scope of the present invention should not be limited thereby.

[0030] The high-precision wavefront aberration detection method based on the Ronchi shearing interferometer disclosed by the present invention, the Ronchi grating shearing interferometer adopted by this method is as Figure 1 shown. The system includes: light source and illumination system 8, object-plane diffraction grating plate 1, first three-dimensional displacement stage 2, image-plane diffraction grating plate 4, second three-dimensional displacement stage 5, two-dimensional photoelectric sensor 6 and computing and processing unit 7. The light source and illumination system 8 outputs spatially incoherent light. The object-plane diffraction grating plate 1 is fixed on the first three-dimensional displacement stage 2. The image-plane diffraction grating plate 4 is fixed on the second three-dimensional displacement stage 5. The output end of the two-dimensional photoelectric sensor 6 is connected to the computing and processing unit 7;

[0031] An xyz coordinate system is established. The z-axis direction is along the optical axis direction of the shearing interferometer. The x-axis is along the grating line direction of the second grating 102 on the object-plane diffraction grating plate 1. The y-axis is along the grating line direction of the first grating 101 on the object-plane diffraction grating plate 1. It is assumed that the moving axes of the first three-dimensional displacement stage 2 and the second three-dimensional displacement stage 5 are the x-axis, y-axis and z-axis respectively;

[0032] The first three-dimensional displacement stage 2 is used to move the first grating 101 and the second grating 102 in the object-plane diffraction grating plate 1 to the object-plane field center of the optical imaging system 3 to be measured;

[0033] The second three-dimensional displacement stage 5 is used to move the checkerboard grating in the image-plane diffraction grating plate 4 to the image-plane field center of the optical imaging system 3 to be measured, and perform specific periodic movements in the x-axis direction and y-axis direction on the image-plane diffraction grating plate 4;

[0034] The two-dimensional photoelectric sensor 6 can be a charge-coupled device CCD or a CMOS image sensor, and detects the shearing interference fringes generated by the diffraction of the checkerboard grating on the detection surface;

[0035] The computing and processing unit 7 is used to collect and store the interference pattern, and process and analyze the interference pattern;

[0036] Figure 2Schematic diagram of the object-plane diffraction grating plate 1, which includes two one-dimensional diffraction gratings, namely, the first grating 101 with grating lines along the y-axis direction and the second grating 102 with grating lines along the x-axis direction. The period of the one-dimensional diffraction grating is P1 and the duty cycle is 50%;

[0037] The first grating 101 and the second grating 102 are phase gratings or amplitude gratings;

[0038] Figure 3 Schematic diagram of the checkerboard grating of the image-plane diffraction grating plate 4, which is a checkerboard grating with a period of P2 and a duty cycle of 50%; the checkerboard grating is composed of square grids, and the diagonal direction of the square is along the x-axis direction or the y-axis direction;

[0039] The period P1 of the one-dimensional grating and the period P2 of the two-dimensional grating satisfy:

[0040] P1 = M·P2 (1)

[0041] where M is the magnification of the optical imaging system 3 to be measured.

[0042] Figure 4 The following shows the schematic diagram of the optical path difference at any point P on the CCD when the image-plane grating is defocused. The distance between the image-plane grating 4 and the focal plane is ΔZ. Taking the shear in the x direction as an example, the optical path difference introduced due to the grating defocus is:

[0043]

[0044] The main component of the optical path difference is the tilt term, which corresponds to the Z2 term of the Zernike polynomial. Similarly, a similar conclusion can be obtained. When shearing in the y direction, the optical path difference introduced by defocus is also mainly the tilt term, which corresponds to the Z2 term of the Zernike polynomial.

[0045] Figure 5 The following shows that when the optical imaging system 3 to be measured has astigmatism, when the ideal image point passes through the optical imaging system 3 to be measured, two focal points are formed on the image plane, corresponding to the S point of meridional imaging and the T point of sagittal imaging respectively.

[0046] Figure 6 The following shows the relationship curve between the Z2 term coefficient of the x-direction shear phase and the defocus size of the image-plane grating, and the relationship curve between the Z3 coefficient of the y-direction shear phase and the defocus size of the image-plane grating; among them, the S point when the Z2 coefficient is zero corresponds to the ideal image point of the meridional plane, and the ideal image point of the sagittal plane corresponds to when the Z3 coefficient is zero.

[0047] Using the above high-precision projection objective wave aberration detection method of the Ronchi grating shearing interferometer, this method includes the following steps:

[0048] (1) Place the optical imaging system 3 to be measured in this grating shearing interferometer, such that the light source and illumination system 8 are located on the object side of the optical imaging system 3 to be measured, and the image plane diffraction grating plate 4 is located on the image side of the optical imaging system (3) to be measured. Adjust the first three-dimensional displacement stage 2 to place the object plane diffraction grating plate 1 on the object plane of the optical imaging system 3 to be measured, and adjust the second three-dimensional displacement stage 5 to place the image plane diffraction grating plate 4 on the image plane of the optical imaging system 3 to be measured; using prior art 1, measure the wave aberration W of the optical imaging system 3 to be measured, perform Zernike polynomial fitting on W to obtain the Z5 term wave aberration, and denote the Z5 term error as W1;

[0049] (2) Divide the z-axis movement range of the second three-dimensional displacement stage 5 into N equal parts (N >= 2) as the scanning positions, denoted as Pi, where i = 1, 2, 3... N; P1 is the starting height position; i , where i = 1, 2, 3... N; P 1 is the starting height position;

[0050] (3) Move the first three-dimensional displacement stage 2 to move the first grating 101 along the y-axis direction on the object plane diffraction grating plate 1 into the object-side field point position of the optical imaging system 3 to be measured, and move the second three-dimensional displacement stage 5 to position the checkerboard grating or the one-dimensional grating in the corresponding direction on the image plane diffraction grating plate 4 in the z-axis direction to the Pi position; 1 position;

[0051] (4) Using prior art 1, through object plane or image plane grating phase shift, obtain a series of shearing interference patterns in the x-axis direction, measure the differential wavefront in the x-axis direction and extract the Z2 term coefficient ci, where i = 1, 2, 3... N; 2i , where i = 1, 2, 3... N;

[0052] (5) If the current z-axis position Pi of the second three-dimensional displacement stage 5 is PN, then proceed to step 6), otherwise the second three-dimensional displacement stage 5 moves to the next position Pi+1 such that Pi+1 = Pi + ΔP, and return to step 4); i is PN, N then proceed to step 6), otherwise the second three-dimensional displacement stage 5 moves to the next position Pi+1 i+1 , such that Pi+1 i = Pi i+1 + ΔP, and return to step 4);

[0053] (6) Move the first three-dimensional displacement stage 2 to move the second grating 102 along the x-axis direction on the object plane diffraction grating plate 1 into the object-side field point position of the optical imaging system 3 to be measured; re-position the z-axis of the second three-dimensional displacement stage 5 to the Pi position; 1 position;

[0054] (7) Using prior art 1, through object plane or image plane grating phase shift, obtain a series of shearing interference patterns in the y-axis direction, solve for the differential wavefront in the x-axis direction and extract the Z3 term coefficient ci 3iwhere i = 1, 2, 3... N;

[0055] (8) If the current z - direction position P of the second three - dimensional displacement stage 5 i is P N , then go to step 9), otherwise the second three - dimensional displacement stage moves to the next position P i+1 , make P i = P i+1 , and return to step 7);

[0056] (9) Linearly fit the Z2 coefficient c 2i and the Z3 coefficient c 3i with the axial position P i respectively, and the axial positions S where c 2 is zero and the axial position T where c 3 is zero are obtained by fitting. The distance ST between the two foci corresponding to astigmatism is obtained from |P S –P T |. Calculate the Z5 aberration coefficient of the optical imaging system to be measured according to formula (5), and obtain the Z5 aberration W2 of the optical imaging system to be measured.

[0057]

[0058] (10) Correct the wave aberration measurement result of the optical imaging system to be measured to W – W1+W2.

[0059] The method of axial scanning in the present invention measures the differential wavefronts at two or more axial positions, fits to obtain the Z2 coefficient of the x - direction differential wavefront and the Z3 coefficient of the y - direction differential wavefront at the axial position, and linearly fits the z - direction position using the Z2 and Z3 coefficients to obtain the Z5 astigmatism error of the system to be measured, eliminating the astigmatism error introduced by the grating defocus and position change in the shearing interferometer system. This method does not require making an additional grating, which not only improves the wave aberration measurement accuracy of the optical imaging system to be measured but also does not increase the complexity of the measurement system structure and measurement process.

Claims

1. A method for detecting the wave aberration of a high-precision projection objective by grating shearing interference. The wave aberration measuring device used in this method includes: a light source and illumination system (8), an object-plane diffraction grating plate (1), a first three-dimensional displacement stage (2), an image-plane diffraction grating plate (4), a second three-dimensional displacement stage (5), a two-dimensional photoelectric sensor (6), and a calculation and processing unit (7). The light source and illumination system (8) outputs spatially incoherent light. The object-plane diffraction grating plate (1) is fixed on the first three-dimensional displacement stage (2), and the image-plane diffraction grating plate (4) is fixed on the second three-dimensional displacement stage (5). The object-plane diffraction grating plate (1) contains two sets of one-dimensional gratings with perpendicular grating line directions. The image-plane diffraction grating plate (4) contains a set of checkerboard gratings or two sets of one-dimensional gratings with perpendicular grating line directions. The output end of the two-dimensional photoelectric sensor (6) is connected to the calculation and processing unit (7). An xyz coordinate system is established. The z-axis direction is along the optical axis direction of the shearing interferometer, the x-axis is along the grating line direction of the second grating (102) on the object-plane diffraction grating plate (1), and the y-axis is along the grating line direction of the first grating (101) on the object-plane diffraction grating plate (1). Let the moving axes of the first three-dimensional displacement stage (2) and the second three-dimensional displacement stage (5) be the x-axis, y-axis, and z-axis respectively. It is characterized in that The steps of this method are as follows: Step 1) Place the optical imaging system to be measured (3) in this wave aberration measuring device, make the light source and illumination system (8) located on the object side of the optical imaging system to be measured (3), and make the image-plane diffraction grating plate (4) located on the image side of the optical imaging system to be measured (3). Adjust the first three-dimensional displacement stage (2) to make the object-plane diffraction grating plate (1) located on the object plane of the optical imaging system to be measured (3), and adjust the second three-dimensional displacement stage (5) to make the image-plane diffraction grating plate (4) located on the image plane of the optical imaging system to be measured (3). Through the phase shift of the object-plane or image-plane grating, measure the wave aberration W of the optical imaging system to be measured (3), perform Zernike polynomial fitting on W to obtain the Z5-term wave aberration, and record the Z5-term error as W1. Step 2) Divide the z-direction movement range of the second three-dimensional displacement stage (5) into N equal parts, where N ≥ 2, as the scanning positions, denoted as Pi, where i = 1, 2, 3... N; P0 is the starting height position; i , where i = 1, 2, 3…N; P 1 is the starting height position; Step 3) Move the first three-dimensional displacement stage (2) to move the first grating (101) along the y-axis direction on the object surface diffraction grating plate (1) into the object field point position of the optical imaging system to be measured (3), and move the second three-dimensional displacement stage (5) to position the checkerboard grating or the one-dimensional grating in the corresponding direction on the image surface diffraction grating plate (4) in the z-direction to the P 1 position; Step 4) Obtain a series of shear interference patterns in the x-axis direction through object-plane or image-plane grating phase shift, and measure the differential wavefront in the x-axis direction For Perform Zernike fitting to extract the coefficient c of the Z2 term 2i , where i = 1, 2, 3…N; Step 5) If the current z-direction position P of the second three-dimensional displacement stage (5) i is P N , then proceed to Step 6); otherwise, the second three-dimensional displacement stage (5) moves to the next position P i+1 such that P i = P i+1 , and return to Step 4); Step 6) Move the first three-dimensional displacement stage (2) to move the second grating (102) along the x-axis direction on the object surface diffraction grating plate (1) into the position of the object field point of the optical imaging system (3) to be measured; position the z-direction of the second three-dimensional displacement stage (5) at P 1 Position; Step 7) Obtain a series of shear interferograms in the y-axis direction through object-plane or image-plane grating phase shift, and solve to obtain the differential wavefront in the y-axis direction For perform Zernike fitting and extract the coefficient c of the Z3 term 3i where i = 1, 2, 3…N; Step 8) If the current z-direction position P of the second three-dimensional displacement stage (5) i is P N , then proceed to Step 9); otherwise, the second three-dimensional displacement stage (5) moves to the next position P i+1 such that P i = P i+1 , and return to Step 7); Step 9) The Z2 coefficient c 2i and the Z3 coefficient c 3i are linearly fitted with the axial position P i respectively, and the axial positions P 2 at which c S is zero and the axial positions P 3 at which c T is zero are obtained. The distance ST between the two focal points corresponding to astigmatism is obtained from |P S -P T |. The Z5 aberration coefficient of the optical imaging system (3) to be measured is calculated according to formula (1), and the Z5 aberration W2 of the optical imaging system to be measured is obtained. Step 10) The wave aberration measurement result of the optical imaging system to be measured (3) is corrected to W–W1 + W2.

2. The method for detecting the wave aberration of a high-precision projection objective by grating shearing interference according to claim 1, It is characterized in that, The ratio of the period of the one-dimensional grating on the object-plane diffraction grating plate (1) to the period of the checkerboard grating or one-dimensional grating on the image-plane diffraction grating plate (4) is equal to the magnification of the optical imaging system to be measured (3).

3. The method for detecting the wave aberration of a high-precision projection objective by grating shearing interference according to claim 1, It is characterized in that, The duty cycle of the object-plane diffraction grating and the image-plane diffraction grating is 50%.

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