Non-linear error self-compensating laser interference nanometer displacement measuring device and method

Abstract

The invention discloses a non-linear error self-compensating laser interference nanometer displacement measuring device and method, and the device is characterized in that a light beam outputted by a single-frequency laser is converted into 45-degree linearly polarized light through a polarizing film, and the 45-degree linearly polarized light is emitted to a laser interference unit composed of a polarizing beam splitter, a quarter-wave plate, a fixed mirror and a movable mirror to form an interference signal. And then the light enters a nonlinear error self-compensating electro-optical control unit consisting of an electro-optical crystal modulator, a polarizing beam splitter, a photoelectric detector and a voltage controller. A difference value is obtained through light intensity signals of an orthogonal photoelectric detector, the sum of the difference value and working voltage is used as driving voltage of an electro-optical modulator, PID control of the light intensity difference value is adopted, automatic phase compensation is achieved, and therefore optical nonlinear errors mainly caused by polarization aliasing are compensated. According to the invention, a polarization modulation detection method of an electro-optical effect is combined with single-frequency polarization interference length measurement, and high-power subdivision of interference fringes is realized based on precise modulation and demodulation of polarized light by an electro-optical crystal.

Description

Technical Field

[0001] The present invention belongs to the field of precision measurement technology, and particularly relates to a laser interferometric nano-displacement measurement device and method with self-compensation for non-linear error. Background Art

[0002] As a core detection means in the field of ultra-precision manufacturing, laser interferometric nano-measurement technology plays an irreplaceable role in semiconductor lithography equipment, nanoimprint equipment, and precision optical element processing. The displacement resolution of a laser interferometer depends on the interference fringe subdivision method. Existing high-resolution interferometers often adopt an optical multiple-path subdivision method combined with electronic subdivision.

[0003] Traditional optical multiple-path methods have certain advantages, but such methods have many problems such as complex systems and sensitivity to collimation. Moreover, in optical multiple-path subdivision, due to the non-ideal optical elements, harmonic components are generated, and the influence of harmonic components limits the further improvement of the interferometer resolution. At the same time, optical non-linear error directly restricts the development of laser interferometers towards sub-nanometer accuracy. The measurement and compensation of non-linear error are research hotspots in interferometric measurement.

[0004] Currently, the main methods for compensating and correcting the non-linear error of a single-frequency laser interferometer are active compensation and passive compensation to suppress the three differential errors of unequal amplitude, non-orthogonal phase, and DC offset of the interference signal. The active compensation method adaptively corrects the non-linear error signal. The main problem is that the real-time performance is not strong, and it is difficult to achieve fast compensation and high-precision processing of orthogonal signals. The passive compensation method has strong real-time performance, but before measurement, the compensator needs to adjust factors such as the distance and surface reflectivity of the measured target, and is greatly affected by the measurement environment and requires regular calibration. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the present invention provides a laser interferometric nano-displacement measurement device and method with self-compensation for non-linear error.

[0006] In a first aspect, the present invention provides a laser interferometric nano-displacement measurement device with self-compensation for non-linear error, including:

[0007] A light source module for providing a single-frequency laser beam;

[0008] A polarized light generation module for converting the single-frequency laser beam into linearly polarized light;

[0009] A beam separation module for separating the linearly polarized light into two polarized lights and respectively guiding them to a fixed mirror and a moving mirror;

[0010] A polarization state conversion module for respectively converting the two polarized lights into circularly polarized lights and converting them back into linearly polarized lights after reflection;

[0011] An interference synthesis module for synthesizing the two linearly polarized light beams into one linearly polarized light beam;

[0012] A polarization state modulation module for modulating the polarization state of the synthesized linearly polarized light;

[0013] An optical intensity detection module for detecting the optical intensity in the X-axis direction and the optical intensity in the Y-axis direction of the modulated linearly polarized light;

[0014] A signal processing module for processing the detection signals of the optical intensity detection module to calculate the ellipticity angle and the orientation angle of the polarized light; the orientation angle is used to determine the displacement of the moving mirror to achieve a nanometer-level displacement measurement accuracy, and the ellipticity angle is used to monitor and compensate in real time for the optical nonlinear error caused by polarization aliasing;

[0015] An error compensation control module for dynamically adjusting the working state of the polarization state modulation module according to the ellipticity angle and the orientation angle information to achieve self-compensation of the optical nonlinear error.

[0016] In a second aspect, the present invention provides a laser interference nanometer displacement measurement method with self-compensation of nonlinear error, using the above measurement device, including the following steps:

[0017] Converting a single-frequency laser beam into linearly polarized light;

[0018] Separating the linearly polarized light into two polarized light beams and respectively guiding them to a fixed mirror and a moving mirror;

[0019] Converting the two polarized light beams into circularly polarized light respectively and converting them back into linearly polarized light after reflection;

[0020] Synthesizing the two linearly polarized light beams into one linearly polarized light beam;

[0021] Modulating the polarization state of the synthesized linearly polarized light;

[0022] Detecting the optical intensity in the X-axis direction and the optical intensity in the Y-axis direction of the modulated linearly polarized light;

[0023] Calculating the ellipticity angle and the orientation angle of the polarized light; the orientation angle is used to determine the displacement of the moving mirror to achieve a nanometer-level displacement measurement accuracy, and the ellipticity angle is used to monitor and compensate in real time for the optical nonlinear error caused by polarization aliasing;

[0024] Dynamically adjusting the parameters of the polarization state modulation according to the ellipticity angle and the orientation angle information to achieve self-compensation of the optical nonlinear error.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] 1. The present invention realizes a homodyne optical path polarization interferometer on a simple optical path by studying the conversion of the laser polarization state and the polarization state conversion by optical elements such as electro-optic crystal modulators, making the outgoing light of the interfering light be linearly polarized light. By measuring the polarization state of the linearly polarized light with an electro-optic crystal modulator, the interference fringe subdivision is realized. Taking the measurement resolution of the orientation angle of the linearly polarized light as 0.001° as an example, the interference fringe subdivision multiple can reach 360,000.

[0027] 2. The present invention realizes automatic phase compensation by double closed-loop controlling the driving voltage of the electro-optic crystal, realizes precise modulation of the polarization state of the synthesized interfering light, effectively controls the ellipticity angle to keep it at 0°, and compensates for the optical nonlinear error mainly caused by polarization aliasing.

[0028] 3. The present invention realizes real-time measurement of the ellipticity angle and orientation angle of linearly polarized light through precise modulation and demodulation of polarized light, and obtains a high-speed and high-multiple interference fringe subdivision method by means of the existing electro-optic crystal phase modulation technology. Description of the Drawings

[0029] Figure 1 It is the schematic diagram of the device in the embodiment of the present application.

[0030] Figure 2 It is the change amount of the ellipticity angle within the 1μm measurement range in the embodiment of the present application.

[0031] Figure 3 It is the displacement curve under different ellipticity angle numerical values in the embodiment of the present application. Detailed Embodiment

[0032] The following further describes the embodiments of the present application with reference to the drawings.

[0033] Based on the single-frequency polarization phase-shifting optical path, the present application completes the high-resolution and low-noise measurement of the orientation angle of the linearly polarized light emitted by the interferometer by means of the electro-optic crystal laser polarization parameter phase modulation and demodulation technology, thereby realizing high-multiple subdivision of the interference fringes. By controlling the light intensity difference between the circularly polarized lights of the measurement arm and the reference arm, the polarization state of the interferometer is controlled, and the optical nonlinear error mainly caused by polarization aliasing is compensated. While realizing high-multiple subdivision of the interference fringes, the self-compensation of the optical nonlinear error is completed, and the nano-measurement accuracy is realized.

[0034] As Figure 1 shown, the present application consists of a single-frequency laser 1, a polarizer 2, a first polarization beam splitter 3, a first quarter-wave plate 4, a fixed mirror 5, a second quarter-wave plate 6, a moving mirror 7, a third quarter-wave plate 8, a first electro-optic crystal modulator 9, a second electro-optic crystal modulator 10, a second polarization beam splitter 11, a voltage controller 12, a first photodetector 13 and a second photodetector 14.

[0035] The device of the present application can be divided into two parts: a laser interference unit and an electro-optical control unit for self-compensation of non-linear errors. The laser interference unit consists of a single-frequency laser 1, a polarizer 2, a first polarization beam splitter 3, a first quarter-wave plate 4, a fixed mirror 5, a second quarter-wave plate 6, a moving mirror 7, and a third quarter-wave plate 8. The beam output by the single-frequency laser 1 is converted into a linearly polarized light at 45° after passing through the polarizer 2 and then shoots at the first polarization beam splitter 3 to undergo reflection and transmission, forming an S-polarized light and a P-polarized light; the S-polarized light is reflected through the first quarter-wave plate 4 to become a circularly polarized light and then is sent to the fixed mirror 5, and the P-polarized light is transmitted through the second quarter-wave plate 6 to become a circularly polarized light and then is sent to the moving mirror 7; the two beams of light are respectively reflected by the fixed mirror 5 and the moving mirror 7 and then return to the first polarization beam splitter 3 after passing through the first quarter-wave plate 4 and the second quarter-wave plate 6 again to be combined into two linearly polarized lights, and then become a left-handed circularly polarized light and a right-handed circularly polarized light after passing through the third quarter-wave plate 8. The two circularly polarized lights are combined into a linearly polarized light, and this linearly polarized light is the outgoing light of the laser interference part.

[0036] Furthermore, the orientation angle of the linearly polarized light emitted by the laser interference unit will change by ±Φ / 2 with the displacement direction of the moving mirror 7 (Φ is the phase difference caused by the movement of the moving mirror), and the positive and negative signs are determined by the moving direction of the moving mirror. This phase change amount can be measured by detecting the orientation angle of the vibration plane of the polarized light so as to obtain the displacement amount of the moving mirror and the relationship expression with the orientation angle is:

[0037] (1)

[0038] In the formula is the vacuum laser wavelength, is the air refractive index.

[0039] The electro-optic control unit with self-compensation for non-linear error of the present application consists of a first electro-optic crystal modulator 9, a second electro-optic crystal modulator 10, a second polarization beam splitter 11, a voltage controller 12, a first photodetector 13, and a second photodetector 14. The outgoing light of the laser interference unit enters the electro-optic control unit with self-compensation for non-linear error. The linearly polarized light passes through the first electro-optic crystal modulator 9, the second electro-optic crystal modulator 10, and the second polarization beam splitter 11. The second polarization beam splitter 11 divides the light beam into two orthogonal P-polarized light and S-polarized light, and the X-axis direction light intensity and Y-axis direction light intensity are detected by the first photodetector 13 and the second photodetector 14. By taking the difference between the light intensity signals of the orthogonal photodetectors, if the difference is not zero, it means that the amplitudes of the outgoing light of the laser interference unit are not equal or the polarization state is non-circularly polarized light. The light intensity difference signal detected by the two photodetectors is fed back to the voltage controller 12, and the voltage controller dynamically adjusts the output voltage according to the feedback signal and acts on the two electro-optic crystal modulators to form a double closed-loop control system.

[0040] Furthermore, the electro-optic control unit with self-compensation for non-linear error realizes dynamic regulation of the polarization state through electro-optic crystal phase modulation. Based on the voltage-sensitive characteristic of the electro-optic crystal, by adjusting the applied voltage, the crystal birefringence coefficient is changed, thereby precisely controlling the state of the linearly polarized light output by the laser interference system. Among them, the phase delay amounts of the first electro-optic crystal modulator 9 and the second electro-optic crystal modulator 10 are controlled by the amplitude output by the EOM driver control circuit. The voltage controller 12 dynamically adjusts the output voltage according to the feedback signal and acts on the two electro-optic crystal modulators. Thereby, the driving voltages of the two electro-optic crystal modulators are changed, and further the phase delay amounts in the fast axis directions are changed. By changing the phase delay amounts of the EOM drivers of the two electro-optic crystal modulators, different phase delays are generated in the two electro-optic crystal modulators.

[0041] After the outgoing light of the laser interference unit is modulated by the first electro-optic crystal modulator 9 and the second electro-optic crystal modulator 10, it is divided into two orthogonal P-polarized light and S-polarized light by the polarization beam splitter 11, and the X-axis direction light intensity and Y-axis direction light intensity are respectively detected by the first photodetector 13 and the second photodetector 14. By changing the driving voltages of the first electro-optic crystal modulator 9 and the second electro-optic crystal modulator 10, the phase delay amounts in their fast axis directions can be changed. Then, the X-axis direction light intensity and Y-axis direction light intensity are detected by the first photodetector 13 and the second photodetector 14 again. The Stokes vector is calculated through the differences in the light intensities detected twice. Similarly, the Stokes vector can be calculated by changing the phase delay amounts of the other two groups. and . The ellipticity angle of the polarized light and the orientation angle of the polarized light can be obtained from , and Measured, ellipticity angle and orientation angle The relational expressions with the Stokes vector are as follows:

[0042] (2)

[0043] (3)

[0044] In the formula , the ellipticity angle of polarized light .

[0045] Furthermore, calibrate the relationship between the driving voltage of the electro-optic crystal and the phase delay amount. Change the voltage value multiple times. The optical intensity signal is received by the photodetector. The Stokes vector is obtained from the relational expression between the optical intensity signal and the Stokes parameters. Then, the ellipticity angle and orientation angle can be obtained from formulas (2) and (3), thereby realizing the measurement of polarization parameters.

[0046] Even further, according to the actual driving characteristics of the electro-optic modulator, a high-order nonlinear log function high-frequency signal is used to drive the electro-optic crystal modulator to make the phase response characteristic close to linear. While the interference signal is received by the photodetector and processed by the signal conditioning circuit hardware, the interference signal is collected for data to the computer. An important factor affecting the detection speed of polarization parameters is the intelligent algorithm of polarization parameters. According to formulas (2) and (3), the ellipticity angle and orientation angle are both obtained from the arctangent operation of the Stokes parameters. Considering the calculation speed, the CORDIC algorithm based on FPGA is adopted and the pipeline structure is used to realize the detection of polarization state parameters. Each iteration process of the CORDIC algorithm only has two algorithms: addition and shift. Therefore, a hardware circuit based on an iterative counter and a shift register is used to implement it.

[0047] In the electro-optic control unit with self-compensation for non-linear error, the intensity difference between the light intensity in the X-axis direction and the light intensity in the Y-axis direction detected by the orthogonal first photodetector 13 and second photodetector 14 is calculated. If the difference is not zero, the main reason is the optical non-linear error caused by polarization aliasing, resulting in the synthetic light being elliptically polarized light rather than linearly polarized light. When the difference is not zero, the light intensity difference signal detected by the two photodetectors is fed back to the voltage controller 12, and the voltage controller dynamically adjusts the output voltage according to the feedback signal and applies it to the two electro-optic crystal modulators. The sum of the difference signal and the working voltage serves as the driving voltage for the electro-optic crystal modulator. The PID control of the light intensity difference is adopted, and the phase is automatically compensated by double-loop controlling the driving voltage of the electro-optic crystal, realizing the precise modulation of the polarization state of the synthetic interference light. By precisely modulating the polarization state of the synthetic interference light, the ellipticity angle of the circularly polarized light can be effectively controlled to keep it at 0°, so that the displacement value is not affected by the ellipticity of the polarization synthetic light, thereby compensating for the optical non-linear error caused by polarization aliasing in the system.

[0048] Embodiment:

[0049] The laser source selected for the single-frequency laser 1 in the embodiment of the present application is a single-frequency He-Ne frequency-stabilized laser, and its wavelength calibration value is 632.991 nm. The interference signal is sampled with high precision by using a 16-bit ADC high-resolution analog-to-digital converter to digitize the analog signal, and then the accuracy is gradually improved through iterative CORDIC algorithm, and the subdivision effect is optimized each time. After 12 iterations of the CORDIC algorithm and 16-bit A / D subdivision, the orientation angle is measured to 0.001° step by step, so that each interference fringe period is divided into 360,000 parts. According to the theoretical relationship between the displacement and the orientation angle of the outgoing polarized light, substituting the numerical values can obtain the corresponding displacement measurement resolution of 17 pm. Due to the influence of the optical non-linear error on the polarization state of the synthetic interference light, it is reflected that the ellipticity angle is not always zero when the moving mirror moves. As Figure 2 shown, through multiple experiments, it is shown that the change range of the ellipticity angle on a 1-μm stroke is [-2.5°, 2.5°]. As Figure 3 shown, the displacement step measurement values are different under different ellipticity angles . Therefore, the phase is automatically compensated by double-loop controlling the driving voltage of the electro-optic crystal, realizing the precise modulation of the polarization state of the synthetic interference light, and effectively controlling the ellipticity angle to keep it at 0°, thereby compensating for the optical non-linear error mainly caused by polarization aliasing.

[0050] As can be seen from the embodiments above, the present application organically integrates the laser polarization interferometry and the electro-optic crystal phase modulation method, establishes a functional relationship between the displacement value of the moving mirror and the polarization parameters of the interference light, and through the precise modulation and demodulation of the polarized light, the ellipticity angle and the orientation angle value of the polarized light are obtained in real time. Combining with the electro-optic crystal phase modulation technology, a high-speed and high-magnification interference fringe subdivision method is obtained. At the same time, an electro-optic control system is constructed based on the electro-optic crystal phase modulation, and the polarization state of the interferometer is controlled by controlling the light intensity difference between the circularly polarized lights in the measurement arm and the reference arm, compensating for the optical nonlinear error mainly caused by optical polarization aliasing. Under strictly controlled experimental environmental conditions, the nano-level displacement measurement accuracy is achieved through the self-compensation of the optical nonlinear error.

[0051] The above specific embodiments are used to explain and illustrate the present invention, rather than limiting the present invention. Any modifications and changes made to the present invention within the spirit and scope of the claims of the present invention fall within the protection scope of the present invention.

Claims

1. A laser interference nano-displacement measurement device with self-compensation for non-linear error, characterized in that, Comprising: A light source module for providing a single-frequency laser beam; A polarized light generation module for converting the single-frequency laser beam into linearly polarized light; A beam separation module for separating the linearly polarized light into two polarized light beams and guiding them to a fixed mirror and a moving mirror respectively; A polarization state conversion module for converting the two polarized light beams into circularly polarized light respectively and converting them back into linearly polarized light after reflection; An interference synthesis module for synthesizing the two linearly polarized light beams into one linearly polarized light beam; A polarization state modulation module for modulating the polarization state of the synthesized linearly polarized light; A light intensity detection module for detecting the light intensity in the X-axis direction and the light intensity in the Y-axis direction of the modulated linearly polarized light; A signal processing module for processing the detection signals of the light intensity detection module to calculate the ellipticity angle and the orientation angle of the polarized light; the orientation angle is used to determine the displacement of the moving mirror to achieve a nanometer-level displacement measurement accuracy, and the ellipticity angle is used to monitor and compensate in real time the optical nonlinear error caused by polarization aliasing; An error compensation control module for dynamically adjusting the working state of the polarization state modulation module according to the ellipticity angle and orientation angle information to achieve self-compensation of the optical nonlinear error.

2. The laser interference nano-displacement measurement device with self-compensation for non-linear error according to claim 1, characterized in that, The beam separation module includes a polarization beam splitter for separating the linearly polarized light into S-polarized light and P-polarized light.

3. The laser interference nano-displacement measurement device with self-compensation of non-linear error according to claim 1 or 2, characterized in that, The polarization state conversion module includes a quarter-wave plate for converting the polarized light into circularly polarized light and converting it back into linearly polarized light after reflection.

4. The laser interference nano-displacement measuring device with self-compensation of non-linear error according to claim 3, characterized in that, The polarization state modulation module includes an electro-optic crystal modulator, and the phase delay amount of the electro-optic crystal modulator is dynamically adjusted by a voltage controller.

5. The laser interference nano-displacement measuring device with self-compensation of non-linear error according to claim 4, wherein The signal processing module uses the CORDIC algorithm based on FPGA to calculate the ellipticity angle and the orientation angle to improve the calculation speed and accuracy.

6. A laser interference nano-displacement measurement method with self-compensation for non-linear error, using the device described in any one of claims 1 to 5, characterized in that, Including the following steps: Converting the single-frequency laser beam into linearly polarized light; Separating the linearly polarized light into two polarized light beams and guiding them to a fixed mirror and a moving mirror respectively; Converting the two polarized light beams into circularly polarized light respectively and converting them back into linearly polarized light after reflection; Synthesizing the two linearly polarized light beams into one linearly polarized light beam; Modulating the polarization state of the synthesized linearly polarized light; Detecting the light intensity in the X-axis direction and the light intensity in the Y-axis direction of the modulated linearly polarized light; Calculating the ellipticity angle and the orientation angle of the polarized light; the orientation angle is used to determine the displacement of the moving mirror to achieve a nanometer-level displacement measurement accuracy, and the ellipticity angle is used to monitor and compensate in real time the optical nonlinear error caused by polarization aliasing; Dynamically adjusting the parameters of the polarization state modulation according to the ellipticity angle and orientation angle information to achieve self-compensation of the optical nonlinear error.

7. The laser interference nanometer displacement measurement method for self-compensation of nonlinear error according to claim 6, characterized in that: The step of modulating the polarization state of the synthesized linearly polarized light includes using an electro-optic crystal modulator, and the phase delay amount of the electro-optic crystal modulator is dynamically adjusted by a voltage controller.

8. The method for measuring nano-displacement by laser interference with self-compensation of non-linear error according to claim 6 or 7, characterized in that: The step of calculating the ellipticity angle and the orientation angle of the polarized light includes using the CORDIC algorithm based on FPGA for calculation to improve the calculation speed and accuracy.

9. The method for measuring nano-displacement by laser interference with self-compensation of non-linear error according to claim 7, characterized in that: The voltage controller includes a proportional-integral-derivative controller for dynamically adjusting the applied voltage according to the light intensity signal to achieve precise adjustment of the phase delay amount.

10. The method for measuring nano-displacement by laser interference with self-compensation of non-linear error according to claim 8, characterized in that: The method further includes calibrating the relationship between the drive voltage of the electro-optic crystal modulator and the phase delay amount, and establishing a mapping relationship between the voltage and the phase delay amount by changing the voltage value multiple times and measuring the corresponding light intensity signals.

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