CN104807548A - Digital phase-shifting lateral shearing interferometer and optical system wave aberration measurement method - Google Patents
Digital phase-shifting lateral shearing interferometer and optical system wave aberration measurement method Download PDFInfo
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Abstract
Disclosed are a digital phase-shifting lateral shearing interferometer and an optical system wave aberration measurement method. The interferometer is composed of a light source, a small-hole mask, a first spatial light modulator, a second spatial light modulator, a two-dimensional photoelectric detector and a computer. The first spatial light modulator is arranged as a grating by computer programming to serve as a shearing light splitter, the second spatial light modulator is arranged as a double-window mask to serve as a filter for filtering high-order diffraction light at zero order and more than two orders, and only diffraction light at +1 order and -1 order takes part in interference. The problem that the high-order diffraction light takes part in interference is solved, the interferometer has the advantage that changeable shearing direction and adjustable shearing rate can be realized without replacing or rotating any device.
Description
Technical field
The invention belongs to field of optical detection, be specifically related to the measuring method of a kind of digital phase-shifting technique lateral shearing interferometer and optical system wavefront aberration.
Background technology
Lateral shearing interference tool has the following advantages:
1) wavefront to be measured and the transverse translation of himself produce and interfere, and eliminate the demand to high precision reference wavefront, therefore have very high measuring accuracy;
2) be a kind of concurrent path interference system, therefore insensitive to mechanical vibration, air turbulence, measuring system is stablized;
3) low to the spatial coherence requirement of light source, traditional two-beam interference based on reference wave requires wavefront to be measured all spatial coherence in whole pupil to be measured, and lateral shearing interference only requires that the Spatially coherent length of light source is not less than shear distance;
4) sensitivity of measuring system is adjustable, can change the sensitivity of measuring system by changing shearing displacement.Therefore, lateral shear is widely used in wavefront measurement field, as the surface topography measurement of optical element, the wave aberration measurement, X ray wavefront measurement etc. of optical system.
Particularly this technology has been applied in high-end litho machine, for the in site measurement of photoetching projection objective lens.Litho machine is the representative of global high-end equipment, it is the height fusion of ultraprecise, Iarge-scale system, the engineering limit, it is all very high to the index request of each subsystem, wherein the wave aberration of projection objective system has controlled at below 0.6nm, and this proposes high requirement to the precision of wave aberration detection technique.And the 193nm wave aberration of photoetching projection objective in situ detection technology that lateral shearing interference technology is commercial just, range of application contains the litho machine of 90nm node to 1x nm node.In addition, lateral shearing interference technology is also the mainstream technology that extreme ultraviolet lithography projection objective wave aberration detects.
The device producing wave front shearing has a lot of method, as flat plate shear, prism shearing, grating shearing, polarization shearing etc.For high-precision wavefront measurement, usually need to adopt phase shift interference technology, grating shearing is interfered owing to being convenient to introduce phase shift, become mainstream technology scheme in high precision wavefront measurement field, the projection objective wave aberration detection as 193nm photoetching and extreme ultraviolet photolithographic all have employed grating as shearing light-splitting device.Grating shearing is interfered by adopting PZT to drive spectro-grating at shear direction transverse shifting, phase differential between the secondary light of different diffraction level is changed, grating moves one-period, phase differential between 0 grade and ± 1 order diffraction light changes 2 π, is participating in introducing phase shift between the two-beam of interfering in this way.And use PZT generation phase shift usually to need to calibrate its voltage, and PZT phase shift is easily subject to the impact of nonlinear effect (as hesitation, thermal drift etc.).Document (S.Zhao andP.Chung, " Digital speckle shearing interferometer using a liquid-crystal spatial lightmodulator, " Opt.Eng.45, 105606 (2006)) a kind of digital speckle-shearing interferometry instrument is proposed, reflection type liquid crystal spatial light modulator (LC-SLM) is adopted to replace grating, and produce digital raster with calculating control LC-SLM, as shearing light-splitting device, can be moved by computer programming control LC-SLM grating lines, realize phase shift, avoid conventional P ZT phase shift Problems existing.But this theoretical method is two-beam interference, and the light participating in interfering is+1 and-1 order diffraction light of LC-SLM grating, but in fact there is Advanced Diffraction light, and the light participating in interfering not only has+1 grade and-1 light, also comprises the Advanced Diffraction light of more than 0 grade and 2 grades.
The method solving Advanced Diffraction optical issue has three kinds: (1) is by optimal design LC-SLM grating, suppress 0 grade and Advanced Diffraction light (in first technology [1]: A.Cornejo and D.Malacara, " Ronchi Test of AsphericalSurfaces; Analysis; and Accuracy; " Appl.Opt.9,1897-1901 (1970); (2) by optimal design LC-SLM grating, suppress the Advanced Diffraction light of more than 2 grades, retain 0 grade and+1 grade and-1 order diffraction beche-de-mer without spike and interference, i.e. Three-beam Interfere, phase place is extracted (at first technology [2] Si-Hong Zhai by special phase extraction method, Jianping Ding, Jing Chen, Ya-Xian Fan, and Hui-Tian Wang, " Three-wave shearinginterferometer based on spatial light modulator, " Opt.Express 17,970-977 (2009)); (3) by adopting Dual-window mask, filtering 0 grade and Advanced Diffraction light, ensure that two-beam interference is (at first technology [3]: YucongZhu, Katsumi Sugisaki, Katsuhiko Murakami, et al., Shearing Interferometry for atWavelength Wavefront Measurement of Extreme-Ultraviolet Lithography ProjectionOptics, Jpn.J.Appl.Phys 42,5844 – 5847 (2003)).In actual applications, for common LC-SLM, method (1) is difficult to the Advanced Diffraction light simultaneously eliminating more than 0 grade and 2 grades, method (2) needs the phase extraction method adopting particular design, range of application is restricted, and method (3) can be good at the Advanced Diffraction light of filtering more than 0 grade and 2 grades, makes+1 order diffraction beche-de-mer without spike and interference, also by changing Dual-window mask, 0 grade of light and+1 grade of (or-1 grade) beche-de-mer without spike and interference can be selected.But method (3) needs when changing shear direction or shearing rate to change (or rotation) wave filter, makes troubles, also can introduce extra measuring error to measurement simultaneously.
Summary of the invention
The object of the invention is to overcome the above-mentioned deficiency in first technology, a kind of digital phase-shifting technique lateral shearing interferometer and optical system wavefront aberration measuring method are provided, this interferometer structure adopts spatial light modulator as the digital space wave filter of image planes, can with Physical space digital filter with the use of, make the change of shear direction and shearing rate more convenient, eliminate because of the measuring error changed or rotary device brings.
Technical solution of the present invention is as follows:
A kind of digital phase-shifting technique lateral shearing interferometer, its feature is that this interferometer comprises light source, be little aperture mask, the first spatial light modulator, second space photomodulator and 2 D photoelectric detector successively along this light source output beam direction, the output terminal of this 2 D photoelectric detector is connected with the input end of computing machine; The first described spatial light modulator and second space photomodulator are placed on the first three-D displacement platform and the second three-D displacement platform respectively; The first described spatial light modulator is connected with the output terminal of computing machine respectively with the input end of second space photomodulator;
Described little aperture mask is one piece of square shadow shield, has the aperture of a printing opacity at the center of this shadow shield, and the diameter of this printing opacity aperture is less than the object space resolution of optical system to be measured;
The first described spatial light modulator is transmissive spatial photomodulator, control to produce digital raster by computing machine, as shearing light-splitting device, wavefront to be measured is diffracted to multi-level diffraction light, when carrying out the shearing measurement of x direction, the lines shearing grating distribute in the y-direction, comprise light transmission part and lightproof part; When carrying out the shearing measurement of y direction, the lines shearing grating in the x-direction, comprise light transmission part and lightproof part;
Described second space photomodulator is transmissive spatial photomodulator, control to produce Dual-window mask by computing machine, as spatial filter, filtering 0 grade and the Advanced Diffraction light of more than 2 grades, when carrying out the shearing measurement of x direction, in the x-direction, comprise two optical transmission windows, other parts are set to light tight the line of centres of Dual-window mask, when carrying out the shearing measurement of y direction, in the y-direction, comprise two optical transmission windows, other parts are set to light tight Dual-window mask center line;
Described two-dimension photoelectric sensor is CCD, CMOS or 2 D photoelectric detector array.
Digital phase-shifting technique lateral shearing interferometer described in utilization is to the measuring method of optical system wavefront aberration, and its feature is that the method includes the steps of:
1) interferometer system parameter is calculated:
The shearing rate s of setting interferometer, shearing rate s is value between 1% ~ 20% generally, measures for high-precision wave aberration, generally value between 1% to 5%, the interference fringe quantity N of setting interferometer, generally value between 20 ~ 100; According to the shearing rate s of setting, the image-side numerical aperture NA of optical system to be measured and the wavelength X of light source, calculate the screen periods p of the first spatial light modulator by formula p=λ/(s × NA); According to the number of interference fringes N of setting, the image-side numerical aperture NA of screen periods p and optical system to be measured, calculates the distance d between the first spatial light modulator and second space photomodulator by formula d=N × p/NA; By screen periods p, the wavelength X of light source, the distance d between the first spatial light modulator and second space photomodulator of the first spatial light modulator, calculate the distance, delta between second space photomodulator Dual-window mask two window center by formula Δ=2d λ/p; According to intending the phase shift algorithm adopted and the phase-shift phase ΔΦ often walked, calculate the grating lines amount of movement Δ p often walking phase shift by formula Δ p=p × ΔΦ/4 π;
2) interferometer system is adjusted:
Optical system to be measured is placed between little aperture mask and the first spatial light modulator, adjust optical system to be measured, little aperture mask is made to be in the object plane of optical system, optical axis is made to pass the center of the printing opacity aperture of little aperture mask, the displacement platform of adjustment second space photomodulator, make the surface of second space photomodulator perpendicular to optical axis, and be in the object plane of optical system to be measured, adjust the displacement platform of the first spatial light modulator, make the surface of the first spatial light modulator perpendicular to optical axis and and distance between second space photomodulator be step 1) the value d that calculates;
3) optical system wavefront aberration is measured:
1. x direction shear interference is measured:
Controlled by computing machine, the first spatial light modulator is set to y direction grating, namely grating lines in the y-direction, and screen periods is set to described p; Controlled by computing machine, second space photomodulator is set to Dual-window mask in the x-direction, namely the line of centres of two windows is parallel to x-axis, and the distance between two window center is set to described Δ, and makes optical axis be in the center of two window center lines; Open light source, detect interference light intensity signal I by photodetector
x1, and by I
x1import computing machine into preserve; Computing machine controls the first spatial light modulator grating, and make it along the translation of+x (or-x) direction, translational movement is described Δ p, detects interference light intensity signal I by photodetector
x2, and by I
x2import computing machine into preserve; The step of above-mentioned phase shift, interferogram detection, preservation is carried out n time, and n is the phase shift step number of phase shift algorithm, and n width phase shifting interference I preserved by final computing machine
x1, I
x2..., I
xn; By phase shift algorithm, by the interference signal I of computer calculate by preserving
x1, I
x2..., I
xncalculate the difference wavefront Δ W in x direction
x(see D.Malacara, Optical Shop Testing, 3rd ed, (CRC Press, Taylor & Francis, 2007));
2. y direction shear interference is measured:
Controlled by computing machine, the first spatial light modulator is set to x direction grating, namely grating lines in the x-direction, and screen periods is set to the value p that step 1 calculates; Controlled by computer programming, second space photomodulator is set to Dual-window mask in the y-direction, namely two window center lines are parallel to y-axis, and the distance between two window center is set to the value Δ that step 1 calculates, and make optical axis be in the center of two window lines; Interference light intensity signal I is detected by photodetector
y1, and by I
y1import computing machine into preserve; Computing machine controls the first spatial light modulator grating, and make it along the translation of+y (or-y) direction, translational movement is described Δ p, detects interference light intensity signal I by photodetector
y2, and by I
y2import computing machine into preserve; The step of above-mentioned phase shift, interferogram detection, preservation is carried out n time, and n is the phase shift step number of phase shift algorithm, and n width phase shifting interference I preserved by final computing machine
y1, I
y2..., I
yn; By phase shift algorithm, by the interference signal I of computer calculate by preserving
y1, I
y2..., I
yncalculate the difference wavefront Δ W in y direction
y;
3. wavefront reconstruction:
By described x direction difference wavefront Δ W
xwith the difference wavefront Δ W in y direction
y, wavefront W to be measured is reconstructed by wavefront reconstruction algorithm, described wavefront reconstruction algorithm can be that type method or field method are (see Fengzhao Dai, Feng Tang, Xiangzhao Wang, Osami Sasaki, and Peng Feng, " Modal wavefrontreconstruction based on Zernike polynomials for lateral shearing interferometry:comparisons of existing algorithms, " Appl.Opt.51, 5028-5037 (2012)), systematic error is eliminated (see Jie Li to wavefront W to be measured, Feng Tang, Xiangzhao Wang, Fengzhao Dai, Feibin Wu, " Calibration of system errors in lateral shearing interferometer for EUV-wavefrontmetrology, " Proc.SPIE 9422, 94222O (2015), obtain the wave aberration of optical system to be measured.
Described phase shift algorithm is from n width interferogram, extract the method for phase place, and n represents phase shift step number, and choosing of n is determined according to real needs.
Principle of the present invention is: by adopting spatial light modulator to produce digital Dual-window mask, the Advanced Diffraction light of filtering more than 0 grade and 2 grades, the light that participation is interfered is only+1 grade and-1 order diffraction light, avoids the impact of Advanced Diffraction light, improves measuring accuracy; Match with the first spatial light modulator, make measuring system more flexible, shearing rate is not realized and shear direction is adjustable under not needing to change the condition of any device, the equal half-twist of Dual-window mask making the grating of the first spatial light modulator and second space photomodulator is controlled by computing machine, realize the change of shear direction, by calculating the cycle of control break first spatial light modulator grating, and by the distance between computing machine control break second space optical modulation Dual-window mask two window, realize the adjustment of shearing rate.
The present invention, with compared with first technology, has the following advantages:
1, with compared with first technology [1], the present invention uses Dual-window mask filtering more than 0 grade and 2 grades Advanced Diffraction light, than the better effects if adopting the method for optimization system parameter designing to suppress more than 0 grade and 2 grades Advanced Diffraction light, improves measuring accuracy;
2, with compared with first technology [2], the present invention is two-beam interference, avoids the problem that Three-beam Interfere phase shift algorithm is limited, can adopt phase shift algorithm more flexibly for actual conditions;
3, with compared with first technology [3], the present invention controls spatial light modulator by computing machine and produces Dual-window mask, present system is more flexible, can under the condition do not changed or rotate any device, realizes that shearing rate is adjustable and shear direction is variable by means of only computer programming.
Accompanying drawing explanation
Fig. 1 is digital phase-shifting technique lateral shearing interferometer structural representation of the present invention;
Fig. 2 is the little aperture mask schematic diagram in the present invention's numeral lateral shearing interferometer;
Fig. 3 is the grating schematic diagram carrying out the y direction arranged in the first spatial light modulator when x direction is sheared and measured with the present invention's numeral lateral shearing interferometer;
Fig. 4 is the grating schematic diagram carrying out the x direction arranged in the first spatial light modulator when y direction is sheared and measured with the present invention's numeral lateral shearing interferometer;
Fig. 5 is the Dual-window mask schematic diagram carrying out the x direction arranged on second space photomodulator when x direction is sheared and measured with the present invention's numeral lateral shearing interferometer;
The y direction Dual-window mask schematic diagram that Fig. 6 is arranged when being and carrying out the shearing measurement of y direction with the present invention's numeral lateral shearing interferometer on second space photomodulator;
Embodiment
Below in conjunction with embodiment and accompanying drawing, the invention will be further described, but should not limit the scope of the invention with this.
First refer to Fig. 1, Fig. 1 is the light channel structure figure of the present invention's numeral lateral shearing interferometer, as seen from Figure 1, digital phase-shifting technique shearing interferometer of the present invention, comprise light source 1, be little aperture mask 2, first spatial light modulator 4, second space photomodulator 6 and 2 D photoelectric detector 8 successively along this light source 1 output beam direction, the output terminal of this 2 D photoelectric detector 8 is connected with the input end of computing machine 9; The first described spatial light modulator 4 and second space photomodulator 6 are placed on an XYZ three-D displacement platform 5 and the 2nd XYZ three-D displacement platform 7 respectively; The first described spatial light modulator 4 is connected with the output terminal of computing machine 9 respectively with the input end of second space photomodulator 6;
Described little aperture mask 2 is one piece of square shadow shield, has the aperture 21 of a printing opacity at the center of this shadow shield, and the diameter of this printing opacity aperture 21 is less than the object space resolution of optical system 3 to be measured;
The first described spatial light modulator 4 is transmissive spatial photomodulator, control to produce digital raster by computing machine 9, as shearing light-splitting device, wavefront to be measured is diffracted to multi-level diffraction light, when carrying out the shearing measurement of x direction, the lines shearing grating distribute in the y-direction, comprise light transmission part 401 and lightproof part 402; When carrying out the shearing measurement of y direction, the lines shearing grating in the x-direction, comprise light transmission part 411 and lightproof part 412;
Described second space photomodulator 6 is transmissive spatial photomodulator, control to produce Dual-window mask by computing machine 9, as spatial filter, filtering 0 grade and the Advanced Diffraction light of more than 2 grades, when carrying out the shearing measurement of x direction, the line of centres of Dual-window mask in the x-direction, comprise two optical transmission windows 501,502, other parts are set to light tight, when carrying out the shearing measurement of y direction, in the y-direction, comprise two optical transmission windows 511,512, other parts are set to light tight Dual-window mask center line;
Described two-dimension photoelectric sensor 6 is CCD, CMOS, or 2 D photoelectric detector array;
Use above-mentioned digital lateral shearing interferometer to carry out the measuring method of optical system wavefront aberration, comprise following steps:
1) interferometer system parameter is calculated:
The shearing rate s of setting interferometer, shearing rate s is value between 1% ~ 20% generally, measures for high-precision wave aberration, generally value between 1% to 5%, the interference fringe quantity N of setting interferometer, generally value between 20 ~ 100; According to the shearing rate s of setting, the image-side numerical aperture NA of optical system 3 to be measured and the wavelength X of light source 1, calculate the screen periods p of the first spatial light modulator 4 by formula p=λ/(s × NA); According to the number of interference fringes N of setting, the image-side numerical aperture NA of screen periods p and optical system to be measured 3, calculates the distance d between the first spatial light modulator 4 and second space photomodulator 6 by formula d=N × p/NA; By screen periods p, the wavelength X of light source 1, the distance d between the first spatial light modulator 4 and second space photomodulator 6 of the first spatial light modulator 4, calculate the distance, delta between second space photomodulator 6 Dual-window mask two window center by formula Δ=2d λ/p; According to intending the phase shift algorithm adopted and the phase-shift phase ΔΦ often walked, calculate the grating lines amount of movement Δ p often walking phase shift by formula Δ p=p × ΔΦ/4 π;
2) interferometer system is adjusted
Optical system 3 to be measured is placed between little aperture mask 2 and the first spatial light modulator 4, adjust optical system 3 to be measured, little aperture mask 2 is made to be in the object plane of optical system 3, optical axis is made to pass the center of the printing opacity aperture 21 of little aperture mask 2, the displacement platform 7 of adjustment second space photomodulator 6, make the surface of second space photomodulator 6 perpendicular to optical axis, and be in the object plane of optical system 3 to be measured, adjust the displacement platform 5 of the first spatial light modulator 4, make the surface of the first spatial light modulator 4 perpendicular to optical axis and and distance between second space photomodulator 6 be step 1) the value d that calculates,
3) optical system wavefront aberration is measured:
1. x direction shear interference is measured:
Controlled by computing machine 9, the first spatial light modulator 4 is set to y direction grating, namely in the y-direction, screen periods is set to described p to grating lines 401 and 402; Controlled by computing machine 9, second space photomodulator 6 is set to Dual-window mask in the x-direction, namely the line of centres of two windows 601,602 is parallel to x-axis, distance between two window 601,602 centers is set to described Δ, and makes optical axis be in the center of two window 601,602 lines of centres; Open light source 1, detect interference light intensity signal I by photodetector 8
x1, and by I
x1import computing machine 9 into preserve; Computing machine 9 controls the first spatial light modulator grating 4, and make it along the translation of+x (or-x) direction, translational movement is described Δ p, detects interference light intensity signal I by photodetector 8
x2, and by I
x2import computing machine 9 into preserve; The step of above-mentioned phase shift, interferogram detection, preservation is carried out n time, and n is the phase shift step number of phase shift algorithm, and n width phase shifting interference I preserved by final computing machine 9
x1, I
x2..., I
xn; By phase shift algorithm, calculate the interference signal I by preserving by computing machine 9
x1, I
x2..., I
xncalculate the difference wavefront Δ W in x direction
x;
2. y direction shear interference is measured:
Controlled by computing machine 9, the first spatial light modulator 4 is set to x direction grating, namely in the x-direction, screen periods is set to the value p that step 1 calculates to grating lines 411 and 412; By computing machine 9 programming Control, second space photomodulator 6 is set to Dual-window mask in the y-direction, namely two window 611,612 lines of centres are parallel to y-axis, distance between two window 611,612 centers is set to the value Δ that step 1 calculates, and makes optical axis be in the center of two window 611,612 lines; Interference light intensity signal I is detected by photodetector 8
y1, and by I
y1import computing machine 9 into preserve; Computing machine 9 controls the first spatial light modulator grating 4, and make it along the translation of+y (or-y) direction, translational movement is described Δ p, detects interference light intensity signal I by photodetector 8
y2, and by I
y2import computing machine 9 into preserve; The step of above-mentioned phase shift, interferogram detection, preservation is carried out n time, and n is the phase shift step number of phase shift algorithm, and n width phase shifting interference I preserved by final computing machine 9
y1, I
y2..., I
yn; By phase shift algorithm, calculate the interference signal I by preserving by computing machine 9
y1, I
y2..., I
yncalculate the difference wavefront Δ W in y direction
y;
3. wavefront reconstruction:
By described x direction difference wavefront Δ W
xwith the difference wavefront Δ W in y direction
yreconstruct wavefront W to be measured by wavefront reconstruction algorithm, described wavefront reconstruction algorithm can be type method or field method, after eliminating systematic error, can obtain the wave aberration of optical system to be measured to wavefront W to be measured.
Give the design parameter of an embodiment below:
Light source adopts He-Ne laser instrument, wavelength is 632.8nm, shearing rate is set to 1.5%, optical system image-side numerical aperture to be measured is 0.3, image space resolution is 1.05 μm, enlargement ratio is 1/4, object-side numerical aperture is 0.075, object space resolution is 4.2 μm, the diameter of the printing opacity aperture on the little aperture mask of object space is set to 1 μm, substrate thickness is 100nm, material is chromium, the amplitude type transmissive spatial photomodulator that first spatial light modulator and second space optical modulation all select Chinese Academy of Sciences's Xi'an ray machine to research and develop, model is TSLM029-A, Pixel Dimensions is 18 μm, pixel count is 1024 × 768, the screen periods of the first spatial light modulation is 144 μm, i.e. 8 pixels, distance between second space photomodulator two window center is 90 μm, i.e. 5 pixels, two windows are square window, the length of side is 18 μm, namely 1 pixel is 1 window, number of interference fringes is 20, distance between first spatial light modulator and second space photomodulator is 10mm, experiment employing three step phase shift algorithm, often walk phase-shift phase pi/2, computing formula is
for PHASE DISTRIBUTION to be measured, I
1, I
2, I
3be the interferogram intensity distributions that three phase shifts obtain, each moved further of grating distance p/8, namely 18 μm, 2 D photoelectric detector to be pixel count be 1024 × 768 CCD.
With compared with first technology, the present invention suppress 0 grade good with Advanced Diffraction light effect, measuring accuracy is higher, and system is flexible, does not need mobile or rotating optical element when changing shearing rate and shear direction.
Claims (4)
1. a digital phase-shifting technique lateral shearing interferometer, it is characterized in that this interferometer comprises light source (1), be little aperture mask (2), the first spatial light modulator (4), second space photomodulator (6) and 2 D photoelectric detector (8) successively along this light source (1) output beam direction, the output terminal of this 2 D photoelectric detector (8) is connected with the input end of computing machine (9); Described the first spatial light modulator (4) and second space photomodulator (6) are placed on the first three-D displacement platform (5) and the second three-D displacement platform (7) respectively; Described the first spatial light modulator (4) is connected with the output terminal of computing machine (9) respectively with the input end of second space photomodulator (6);
Described little aperture mask (2) is one piece of square shadow shield, have the aperture (21) of a printing opacity at the center of this shadow shield, the diameter of this printing opacity aperture (21) is less than the object space resolution of optical system to be measured (3);
Described the first spatial light modulator (4) is transmissive spatial photomodulator, control to produce digital raster by computing machine (9), as shearing light-splitting device, wavefront to be measured is diffracted to multi-level diffraction light, when carrying out the shearing measurement of x direction, the lines shearing grating distribute in the y-direction, comprise light transmission part (401) and lightproof part (402); When carrying out the shearing measurement of y direction, the lines shearing grating in the x-direction, comprise light transmission part (411) and lightproof part (412);
Described second space photomodulator (6) is transmissive spatial photomodulator, control to produce Dual-window mask by computing machine (9), as spatial filter, filtering 0 grade and the Advanced Diffraction light of more than 2 grades, when carrying out the shearing measurement of x direction, the line of centres of Dual-window mask in the x-direction, comprise two optical transmission windows (501,502), other parts are set to light tight, when carrying out the shearing measurement of y direction, in the y-direction, comprise two optical transmission windows (511,512), other parts are set to light tight Dual-window mask center line.
2. digital phase-shifting technique lateral shearing interferometer according to claim 1, is characterized in that described two-dimension photoelectric sensor (8) is CCD, CMOS or 2 D photoelectric detector array.
3. utilize digital phase-shifting technique lateral shearing interferometer described in claim 1 to the measuring method of optical system wavefront aberration, it is characterized in that the method includes the steps of:
1) interferometer system parameter is calculated:
The shearing rate s of setting interferometer, shearing rate s is value between 1% ~ 20% generally, measures for high-precision wave aberration, generally value between 1% to 5%, the interference fringe quantity N of setting interferometer, generally value between 20 ~ 100; According to the shearing rate s of setting, the image-side numerical aperture NA of optical system to be measured (3) and the wavelength X of light source (1), calculate the screen periods p of the first spatial light modulator (4) by formula p=λ/(s × NA); According to the number of interference fringes N of setting, the image-side numerical aperture NA of screen periods p and optical system to be measured (3), calculates the distance d between the first spatial light modulator (4) and second space photomodulator (6) by formula d=N × p/NA; By screen periods p, the wavelength X of light source (1), the distance d between the first spatial light modulator (4) and second space photomodulator (6) of the first spatial light modulator (4), calculate the distance, delta between second space photomodulator (6) Dual-window mask two window center by formula Δ=2d λ/p; According to intending the phase shift algorithm adopted and the phase-shift phase ΔΦ often walked, calculate the grating lines amount of movement Δ p often walking phase shift by formula Δ p=p × ΔΦ/4 π;
2) interferometer system is adjusted:
Optical system to be measured (3) is placed between little aperture mask (2) and the first spatial light modulator (4), adjust optical system to be measured (3), little aperture mask (2) is made to be in the object plane of optical system to be measured (3), the printing opacity aperture (21) of described little aperture mask (2) be centrally located on the optical axis of interferometer, the displacement platform (7) of adjustment second space photomodulator (6), make the surface of second space photomodulator (6) perpendicular to optical axis, and be in the image planes of optical system to be measured (3), adjust the displacement platform (5) of the first spatial light modulator (4), make the surface of the first spatial light modulator (4) perpendicular to optical axis and and distance between second space photomodulator (6) be d,
3) optical system wavefront aberration is measured:
1. x direction shear interference is measured:
Controlled by computing machine (9), make the first spatial light modulator (4) for y direction grating, namely in the y-direction, screen periods is set to described p to grating lines (401,402); Controlled by computing machine (9), second space photomodulator (6) is made to be Dual-window mask in the x-direction, namely the line of centres of two windows (601,602) is parallel to x-axis, distance between two windows (601,602) center is described Δ, and makes optical axis be in the center of two windows (601, the 602) line of centres; Open light source (1), detect interference light intensity signal I by photodetector (8)
x1, this I
x1import computing machine (9) into preserve; Computing machine (9) controls the first spatial light modulator grating (4), and make it along the translation of+x or-x direction, translational movement is described Δ p, detects interference light intensity signal I by photodetector (8)
x2, and by I
x2import computing machine (9) into preserve; Repeat n time altogether, n is the phase shift step number of phase shift algorithm, and n width interference light intensity signal I preserved by final computing machine (9)
x1, I
x2..., I
xn; By phase shift algorithm, by computing machine (9) by interference light intensity signal I
x1, I
x2..., I
xn,calculate, obtain the difference wavefront Δ W in x direction
x;
2. y direction shear interference is measured:
Controlled by computing machine (9), the first spatial light modulator (4) is set to x direction grating, and namely in the x-direction, screen periods is set to p to grating lines (411,412); Controlled by computing machine (9), second space photomodulator (6) is set to Dual-window mask in the y-direction, namely two windows (611, the 612) line of centres is parallel to y-axis, distance between two windows (611,612) center is set to Δ, and makes optical axis be in the center of two windows (611,612) line; Interference light intensity signal I is detected by photodetector (8)
y1, and by I
y1import computing machine (9) into preserve; Computing machine (9) controls the first spatial light modulator grating (4), and make it along the translation of+y or-y direction, translational movement is described Δ p, detects interference light intensity signal I by photodetector (8)
y2, and by I
y2import computing machine (9) into preserve; Repeat n time, n is the phase shift step number of phase shift algorithm, by computing machine (9) by phase shift algorithm, calculates interference signal I
y1, I
y2..., I
yn, obtain the difference wavefront Δ W in y direction
y;
3. wavefront reconstruction:
By described x direction difference wavefront Δ W
xwith the difference wavefront Δ W in y direction
y, reconstruct wavefront W to be measured by wavefront reconstruction algorithm, systematic error eliminated to wavefront W to be measured, obtains the wave aberration of optical system to be measured.
4. optical system wavefront aberration measuring method according to claim 3, is characterized in that described wavefront reconstruction algorithm can be type method or field method.
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