METHOD FOR COMPENSATION DURING THE PROCESS OF WAVEFRONT RECONSTRUCTION IN GRATING-BASED LATERAL SHEARING INTERFEROMETRY

Abstract

Method for simultaneously compensating pupil coordinate distortion and shear amount change in a process of wavefront reconstruction in grating transverse shear interference. Where a wavefront is diffracted by a grating, the shapes and light paths of the diffracted wavefronts of all the orders are different, so that on one hand, a coordinate system detected by a detector plane is distorted relative to a pupil coordinate system, and on the other hand, a shear amount changes along with a coordinate position.

Claims

1. A method for simultaneously compensating pupil coordinate distortion and shear amount change in a process of wavefront reconstruction in grating transverse shear interference, comprising (1) generating a differential wavefront interferogram of an optical system to be measured in X and Y-directions by using a grating transverse shear interferometer, and receiving differential wavefront interferograms I.sub.x(x.sub.d,y.sub.d) and I.sub.y(x.sub.d,y.sub.d) by using a two-dimensional photoelectric sensor, wherein (x.sub.d,y.sub.d) is a pixel position coordinate system on the two-dimensional photoelectric sensor; (2) extracting diffracted wavefronts of n.sub.1-order and n.sub.2-order, and differential phases S.sub.x(x.sub.d,y.sub.d) and S.sub.y(x.sub.d,y.sub.d) in X and Y-directions from the generated differential wavefront interferograms, wherein n.sub.1≠n.sub.2 and both are an integer; (3) using an inversing method to solve corresponding position coordinates of light rays received by different pixels on a two-dimensional photoelectric sensor plane in a pupil plane along diffraction light paths of n.sub.1 and n.sub.2 orders in the case of shearing in the X-direction and the Y-direction, respectively, wherein the position coordinates obtained in the case of shearing in the X-direction are (x.sub.n1x,y.sub.n1x) and (x.sub.n2x,y.sub.n2x), and the position coordinates obtained in the case of shearing in the Y-direction are (.sub.xn1y,yn1y) and (x.sub.n2y,y.sub.n2y); (4) selecting first m (m>1) polynomials to build a set of equations according to the obtained coordinate systems (x.sub.n1x,y.sub.n1x), (x.sub.n2x,y.sub.n2x), (x.sub.n1y,y.sub.n1y), (x.sub.n2y,y.sub.n2y), and solving to obtain a coefficient C.sub.i(i=1, 2, . . . , m) corresponding to each polynomial when the wavefront to be measured is expressed by the selected polynomials; and (5) reconstructing a wavefront W.sub.F according to the following Equation: $W_F=\underset{i=1}{\overset{m}{.Math.}}{C}_i{Z}_i,$ wherein Z, represents an i-th polynomial.

2. The method of claim 1, wherein the grating transverse shear interferometer is a four-wave grating shear interferometer, a Talbot grating shear interferometer, or a Ronchi grating shear interferometer.

3. The method of claim 1, wherein the differential phase of the diffracted wavefronts of the n.sub.1 and n.sub.2 orders is the differential phase between the diffracted wavefronts of 0 and ±1 orders, respectively, or the differential phase between the diffracted wavefronts of the −1 and +1 orders.

4. The method of claim 1, wherein the inversing method used is a light ray tracing method or an iterative method.

5. The method of claim 1, wherein the polynomial is a Zernike polynomial, a differential Taylor polynomial, or a Seidel aberration expression.

6. The method of claim 1, wherein the process of solving the coefficient C.sub.i in step (4) comprises the steps of building a set of equations for the differential phases S.sub.xk and S.sub.yk in the X-direction and Y-direction detected on a k-th pixel of the two-dimensional photoelectric sensor: $S_{\mathrm{xk}}=\underset{i=1}{\overset{m}{.Math.}}{C}_i.Math.\left(Z_{\mathrm{ik}}^{n1x}-Z_{\mathrm{ik}}^{n2x}\right)S_{\mathrm{yk}}=\underset{i=1}{\overset{m}{.Math.}}{C}_i.Math.\left(Z_{\mathrm{ik}}^{n1y}-Z_{\mathrm{ik}}^{n2y}\right),$ wherein upper right corner indices n1x, n2x, n1y, and n2y of Z respectively indicate that a value of a corresponding term is derived by substituting a k-th coordinate point in the coordinate systems (x.sub.n1x,y.sub.n1x), (x.sub.n2x,y.sub.n2x), (x.sub.n1y,y.sub.n1y), and (x.sub.n2y,y.sub.n2y) into a selected i-th polynomial expression; a lower right corner index i represents the i-th polynomial, and k represents that the value of the term is a value corresponding to the k-th coordinate in the selected coordinate system; sorting data detected by all pixels on the two-dimensional photoelectric sensor to obtain a set of equations as follows: $.Math.\begin{array}{cccc}{Z}_{11}^{n1x}-Z_{11}^{n2x}& Z_{21}^{n1x}-Z_{21}^{n2x}& .Math.& Z_{i1}^{n1x}-Z_{i1}^{n2x}\\ Z_{12}^{n1x}-Z_{12}^{n2x}& Z_{22}^{n1x}-Z_{22}^{n2x}& & Z_{i2}^{n1x}-Z_{i2}^{n2x}\\ .Math.& .Math.& & .Math.\\ Z_{1k}^{n1x}-Z_{1k}^{n2x}& Z_{2k}^{n1x}-Z_{2k}^{n2x}& & Z_{\mathrm{ik}}^{n1x}-Z_{\mathrm{ik}}^{n2x}\\ Z_{11}^{n1y}-Z_{11}^{n2y}& Z_{21}^{n1y}-Z_{21}^{n2y}& .Math.& Z_{i1}^{n1y}-Z_{i1}^{n2y}\\ Z_{12}^{n1y}-Z_{12}^{n2y}& Z_{22}^{n1y}-Z_{22}^{n2y}& & Z_{i2}^{n1y}-Z_{i2}^{n2y}\\ .Math.& .Math.& & .Math.\\ Z_{1k}^{n1y}-Z_{1k}^{n2y}& Z_{2k}^{n1y}-Z_{2k}^{n2y}& .Math.& Z_{\mathrm{ik}}^{n1y}-Z_{\mathrm{ik}}^{n2y}\end{array}.Math..Math..Math.\begin{array}{c}{C}_1\\ C_2\\ .Math.\\ C_i\end{array}.Math.=.Math.\begin{array}{c}{S}_{x1}\\ S_{x2}\\ .Math.\\ S_{\mathrm{xk}}\\ S_{y1}\\ S_{y2}\\ .Math.\\ S_{\mathrm{yk}}\end{array}.Math.,$ and solving a least-square solution of the above set of equations, i.e., the coefficients C.sub.1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 shows the experimental light path related to Example 1 of the present invention.

[0023] FIG. 2 shows the shear interferogram in an X-direction obtained in Example 1 of the present invention.

[0024] FIG. 3 shows the shear interferogram in a Y-direction obtained in Example 1 of the present invention.

[0025] FIG. 4 shows the shear interferogram in the X-direction in Example 1 of the present invention.

[0026] FIG. 5 shows the shear interferogram in the Y-direction in Example 1 of the present invention.

[0027] FIG. 6 shows the a fitting coefficient derived in Example 1 of the present invention.

[0028] FIG. 7 shows the phase to be measured reconstructed in Example 1 of the present invention.

[0029] FIG. 8 shows the experimental light path related to Example 1 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0030] The present invention is further described below with reference to examples and drawings, but the scope of the invention should not be limited thereto.

EXAMPLE 1

[0031] Wave aberration of a projection objective lens system is measured by using a Ronchi grating shear interferometer as shown in FIG. 1, where a focusing lens, an object plane grating (period T=20 μm), a projection objective lens system to be measured (NA=0.9), an image plane grating (period T=5 μm) and a two-dimensional photoelectric sensor are sequentially arranged along a light-ray-propagation direction of a light source (wavelength 80 =532 nm); the object plane grating is positioned in a rear focal plane of the focusing lens; a front focal plane of the projection objective lens system to be measured coincides with the rear focal plane of the focusing lens; the image plane grating is positioned on the rear focal plane of the projection objective lens system to be measured; the two-dimensional photoelectric sensor is parallel to the image plane grating, and a distance from the two-dimensional photoelectric sensor to the image plane grating meets the requirement that a sampling number of the diffraction light rays of the 0 order is 256×256 pixels. The measurement steps are as follows:

[0032] (1) a grating transverse shear interferometer is used for generating a differential wavefront interferogram of the wave aberration of the projection objective lens system to be measured in an X-direction and a Y-direction, and the two-dimensional photoelectric sensor is used for receiving interferograms I.sub.x(x.sub.d,y.sub.d) and I.sub.y(x.sub.d,y.sub.d) as shown in FIGS. 2 and 3;

[0033] (2) the differential phases S.sub.x(x.sub.d,y.sub.d) and S.sub.y(x.sub.d,y.sub.d) of the diffracted wavefronts of -1 and +1 orders in the X-direction and the Y-direction are extracted from the resulting interferogram by using the methods disclosed in Chinese Patent No. CN104111120B published on May 31, 2017, (inventors are Wu, Feibin et al.; applicant is Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences), which is incorporated by reference herewith, as shown in FIGS. 4 and 5;

[0034] (3) points where paths through which the light rays emitted from different positions on a pupil plane intersect a detector plane are tracked along paths of the diffracted light rays of the −1 and +1 orders in the case of shearing in the X-direction and the Y-direction, respectively, and the positions on the selected pupil plane are continuously adjusted to enable the points where the light rays intersect the detector plane to fall on designated pixels, so that the corresponding position coordinates of light rays received by different pixels on a two-dimensional photoelectric sensor plane in the pupil plane are obtained, wherein the position coordinates obtained in the case of shearing in the X-direction are (x.sub.−1x,y.sub.−1x) and (x.sub.+1x,y.sub.+1x), and the position coordinates obtained in the case of shearing in the Y-direction are (x.sub.—1y,y.sub.—1y) and (x.sub.+1y,y.sub.+1y);

[0035] first 100 Zernike polynomials are selected to build a set of equations according to the obtained coordinate systems (x.sub.−1x,y.sub.−1x), (x.sub.+1x,.sub.Y+1x), (x.sub.−1y,y−.sub.1y), and (x.sub.+1y,y.sub.+1y), and solving to obtain a Zernike coefficient C.sub.i(i=1, 2, . . . , 100) corresponding to each Zernike polynomial when the wavefront to be measured is expressed by the selected Zernike polynomials; specifically, a set of equations is built for the differential phases S.sub.xk and S.sub.yk in the X-direction and Y-direction detected on a k-th pixel of the two-dimensional photoelectric sensor:

[00004]$S_{\mathrm{xk}}=\underset{i=1}{\overset{100}{.Math.}}{C}_i.Math.\left(Z_{\mathrm{ik}}^{+1x}-Z_{\mathrm{ik}}^{-1x}\right)S_{\mathrm{yk}}=\underset{i=1}{\overset{100}{.Math.}}{C}_i.Math.\left(Z_{\mathrm{ik}}^{+1y}-Z_{\mathrm{ik}}^{-1y}\right)$

where upper right corner indices +1x, −1x, +1y, and −1y of Z respectively indicate that a value of a corresponding term is derived by substituting a k-th coordinate point in the coordinate systems (x.sub.−1x, y.sub.−1x), (x.sub.+1x,y.sub.+1x), (x.sub.−1y,y.sub.−1y), and (x.sub.+1y,y.sub.+1y) into a selected i-th Zernike polynomial expression; a lower right corner index i represents the i-th Zernike polynomial, and k represents that the value of the term is a value corresponding to the k-th coordinate in the selected coordinate system; data detected by all pixels on the two-dimensional photoelectric sensor are sorted to obtain a set of equations as follows:

[00005]$.Math.\begin{array}{cccc}{Z}_{11}^{+1x}-Z_{11}^{-1x}& Z_{21}^{+1x}-Z_{21}^{-1x}& .Math.& Z_{100,1}^{+1x}-Z_{100,1}^{-1x}\\ Z_{12}^{+1x}-Z_{12}^{-1x}& Z_{22}^{+1x}-Z_{22}^{-1x}& & Z_{100,2}^{+1x}-Z_{100,2}^{-1x}\\ .Math.& .Math.& & .Math.\\ Z_{1k}^{+1x}-Z_{1k}^{-1x}& Z_{2k}^{+1x}-Z_{2k}^{-1x}& & Z_{100,k}^{+1x}-Z_{100,k}^{-1x}\\ Z_{11}^{+1y}-Z_{11}^{-1y}& Z_{21}^{+1y}-Z_{21}^{-1y}& .Math.& Z_{100,1}^{+1y}-Z_{100,1}^{-1y}\\ Z_{12}^{+1y}-Z_{12}^{-1y}& Z_{22}^{+1y}-Z_{22}^{-1y}& & Z_{100,2}^{+1y}-Z_{100,2}^{-1y}\\ .Math.& .Math.& & .Math.\\ Z_{1,65536}^{+1y}-Z_{1,65536}^{-1y}& Z_{2,65536}^{+1y}-Z_{2,65536}^{-1y}& .Math.& Z_{100,65536}^{+1y}-Z_{100,65536}^{-1y}\end{array}.Math..Math..Math.\begin{array}{c}{C}_1\\ C_2\\ .Math.\\ C_{100}\end{array}.Math.=.Math.\begin{array}{c}{S}_{x1}\\ S_{x2}\\ .Math.\\ S_{x,65536}\\ S_{y1}\\ S_{y2}\\ .Math.\\ S_{y,65536}\end{array}.Math.,$

[0036] and a least-square solution of the above set of equations is solved to obtain the coefficient C.sub.i as shown in FIG. 6.

[0037] (4) A reconstructed wavefront W.sub.F obtained from the coefficient solution C.sub.i is:

[00006]$W_F=\underset{i=1}{\overset{100}{.Math.}}{C}_i{Z}_i,$

where Z.sub.i represents the i-th Zernike polynomial.

[0038] (5) The reconstruction result W.sub.F is output, as shown in FIG. 7.

EXAMPLE 2

[0039] Wave aberration of an optical system to be measured is measured by using a grating four-wave shear interferometer as shown in FIG. 8. A focusing lens, a filtering aperture, an optical system to be measured, a two-dimensional grating (with a period of 36 same in both the X and Y directions), and a two-dimensional photoelectric sensor are sequentially arranged along the light-ray-propagation direction of the light source (wavelength λ=532 nm); a center of the filtering aperture coincides with a rear focal point of the focusing lens and an object-side field-of-view point to be measured of the optical system to be measured; the two-dimensional grating is positioned near the rear focal plane of the optical system to be measured and is parallel to the rear focal plane; the two-dimensional photoelectric sensor is positioned behind the two-dimensional grating and is parallel to the two-dimensional grating, and a distance from the two-dimensional photoelectric sensor to the image plane grating meets the requirement that a sampling number of diffraction light rays of the 0 order is 256×256 pixels. The steps of the measurement are as follows:

[0040] (1) a grating four-wave shear interferometer is used for generating a differential wavefront interferogram of the wave aberration of the projection objective lens system to be measured in the X-direction and the Y-direction, and the two-dimensional photoelectric sensor is used for receiving the interferograms I.sub.x(x.sub.d,y.sub.d) and I.sub.y(x.sub.d,y.sub.d);

[0041] (2) the interferogram is subjected to FFT transformation to obtain a corresponding frequency spectrum, a first-order frequency spectrum in the X-direction and the Y-direction is filtered out of the frequency spectrum, the first-order frequency spectrum is translated to the center to carry out inverse FFT transformation and phase unwrapping to obtain differential phases S.sub.x(x.sub.d,y.sub.d) and S.sub.y(x.sub.d,y.sub.d) of the diffracted wavefronts of −1 and +1 orders in the X-direction and the Y-direction;

[0042] (3) the paths of the diffracted light rays of the −1 and +1 orders in the case of shearing in the X-direction and the Y-direction are deemed as coming from different point sources, and the corresponding pupil pane coordinates are directly derived through calculation according to the pixel coordinates of the detector, thereby obtaining the corresponding position coordinates of light rays received by different pixels on a two-dimensional photoelectric sensor plane in the pupil plane, wherein the position coordinates obtained in the case of shearing in the X-direction are (x.sub.−1x,y.sub.−1x) and (x.sub.+1x,y.sub.+1x) and the position coordinates obtained in the case of shearing in the Y-direction are (x.sub.−1y,y.sub.−1y) and (x.sub.+1y,y.sub.+1y);

[0043] (4) first 100 Taylor polynomials are selected to build a set of equations according to the obtained coordinate systems (x.sub.−1x,y.sub.−1x), (x.sub.+1x,y.sub.+1x), (x.sub.−1y,y.sub.−1y), and (x.sub.+1y,y.sub.+1y), and solving to obtain a Taylor coefficient C.sub.i(i=1, 2, . . . , 100) corresponding to each Taylor polynomial when the wavefront to be measured is expressed by the selected Taylor polynomials;

[0044] specifically, a set of equations is built for the differential phases S.sub.xk and S.sub.yk in the

[0045] X-direction and Y-direction detected on a k-th pixel of the two-dimensional photoelectric sensor:

[00007]$S_{\mathrm{xk}}=\underset{i=1}{\overset{100}{.Math.}}{C}_i.Math.\left(Z_{\mathrm{ik}}^{+1x}-Z_{\mathrm{ik}}^{-1x}\right)S_{\mathrm{yk}}=\underset{i=1}{\overset{100}{.Math.}}{C}_i.Math.\left(Z_{\mathrm{ik}}^{+1y}-Z_{\mathrm{ik}}^{-1y}\right),$

where upper right corner indices +1x, −1x, +1y, and −1y of Z respectively indicate that a value of a corresponding term is derived by substituting a k-th coordinate point in the coordinate systems (x.sub.−1x,y.sub.−1x), (x.sub.+1x,y.sub.+1x), (x.sub.−1y,y.sub.−1y), and (x.sub.+1y,y.sub.+1y) into a selected i-th Taylor polynomial expression; a lower right corner index i represents the i-th Taylor polynomial, and k represents that the value of the term is a value corresponding to the k-th coordinate in the selected coordinate system; data detected by all pixels on the two-dimensional photoelectric sensor are sorted to obtain a set of equations as follows:

[00008]$.Math.\begin{array}{cccc}{Z}_{11}^{+1x}-Z_{11}^{-1x}& Z_{21}^{+1x}-Z_{21}^{-1x}& .Math.& Z_{100,1}^{+1x}-Z_{100,1}^{-1x}\\ Z_{12}^{+1x}-Z_{12}^{-1x}& Z_{22}^{+1x}-Z_{22}^{-1x}& & Z_{100,2}^{+1x}-Z_{100,2}^{-1x}\\ .Math.& .Math.& & .Math.\\ Z_{1k}^{+1x}-Z_{1k}^{-1x}& Z_{2k}^{+1x}-Z_{2k}^{-1x}& & Z_{100,k}^{+1x}-Z_{100,k}^{-1x}\\ Z_{11}^{+1y}-Z_{11}^{-1y}& Z_{21}^{+1y}-Z_{21}^{-1y}& .Math.& Z_{100,1}^{+1y}-Z_{100,1}^{-1y}\\ Z_{12}^{+1y}-Z_{12}^{-1y}& Z_{22}^{+1y}-Z_{22}^{-1y}& & Z_{100,2}^{+1y}-Z_{100,2}^{-1y}\\ .Math.& .Math.& & .Math.\\ Z_{1,65536}^{+1y}-Z_{1,65536}^{-1y}& Z_{2,65536}^{+1y}-Z_{2,65536}^{-1y}& .Math.& Z_{100,65536}^{+1y}-Z_{100,65536}^{-1y}\end{array}.Math..Math..Math.\begin{array}{c}{C}_1\\ C_2\\ .Math.\\ C_{100}\end{array}.Math.=.Math.\begin{array}{c}{S}_{x1}\\ S_{x2}\\ .Math.\\ S_{x,65536}\\ S_{y1}\\ S_{y2}\\ .Math.\\ S_{y,65536}\end{array}.Math.,$

and

[0046] a least-square solution of the above set of equations is solved to obtain the coefficient C.sub.i;

[0047] (5) A reconstructed wavefront W.sub.F obtained from the coefficient solution C.sub.i is:

[00009]$W_F=\underset{i=1}{\overset{100}{.Math.}}{C}_i{Z}_i,$

where Z.sub.i represents the i-th Taylor polynomial; and

[0048] (6) The reconstruction result W.sub.F is output.

[0049] In the present invention, the method calculates and derives a transformation relation between a coordinate system of a wavefront to be measured in a pupil plane and a coordinate system of each diffracted wavefront in a detector plane by tracking an light path of each diffracted wavefront; a correspondence relation between a shear phase detected at each position on the detector plane and the phase in the wavefront to be measured can be obtained from the transformation relation between coordinate systems, a corresponding wavefront polynomial set can then be built, and a result of polynomial fitting for the wavefront to be measured can be obtained by fitting, whilst compensating the distortion of the pupil coordinate system and the shear amount change.