Phase-shifted synchronous monitoring compensated interferometer and control method

Abstract

The application provides a phase-shifting synchronous monitoring compensation interferometer and a control method, which comprises a laser source, a first light splitting component, a main measurement light path, a CCD image acquisition module, a phase shifter, a phase-shifting driving module, a frequency monitoring module and a synchronous control module. The laser is split into two paths by the first light splitting component: one path enters the main measurement light path, and the other path enters the frequency monitoring module; the frequency monitoring module monitors the laser frequency drift and the phase change introduced by the phase shifter, and transmits the monitoring signal to the synchronous control module; the synchronous control module outputs the phase-shifting driving signal to the phase-shifting driving module according to the monitoring signal, and outputs the image acquisition trigger signal to the CCD image acquisition module when the phase reaches the preset step length; the phase shifter performs phase movement, and the CCD image acquisition module acquires the interference pattern. The application realizes the hard synchronization of phase shifting and image acquisition, reduces the timing jitter, and improves the measurement accuracy and stability.

Description

Technical Field

[0001] This application relates to the field of precision interferometry technology, and in particular to an interferometer and control method for phase-shift synchronous monitoring and compensation. Background Technology

[0002] The Fizeau interferometer, as a high-precision interferometric measurement device, is widely used in optical inspection, semiconductor manufacturing, and other fields due to its advantages of wide measurement range and high resolution. When using laser phase-shifting imaging technology for precision measurement, the consistency of the phase-shifting interval and the synchronization of the imaging timing are key factors determining the measurement accuracy.

[0003] Existing interferometer phase-shifting image acquisition control mostly adopts open-loop or semi-closed-loop control of the phase shifter, which has the following technical defects: First, the laser source has an inherent frequency drift, and environmental factors such as temperature and vibration can also cause laser frequency fluctuations. However, existing systems lack real-time monitoring methods for this frequency drift and cannot effectively perceive its impact during phase shifting. Second, the phase shifter itself has nonlinear and hysteresis characteristics, making the actual phase change nonlinear in relation to the driving signal. Existing systems cannot track the actual phase change introduced by the phase shifter in real time, resulting in uncontrollable phase shifting step size. Third, there is a lack of high-precision synchronization reference between phase shifting drive and CCD image acquisition. The phase shifting action and the image acquisition action are independent of each other, resulting in large timing jitter. The image acquisition time often cannot accurately correspond to the preset phase position, which easily causes the interferogram acquisition and imaging deviation. Summary of the Invention

[0004] In view of this, this application provides an interferometer and control method for phase-shift synchronization monitoring and compensation, in order to solve the problems of inconsistent phase-shift intervals, asynchronous acquisition timing, and laser frequency drift affecting measurement accuracy in the laser phase-shifting image acquisition process of the interferometer.

[0005] To solve the above-mentioned technical problems, one technical solution adopted in this application is: to provide an interferometer for phase-shifting synchronization monitoring and compensation, including a laser source, a first beam splitting component, a main measurement optical path, a CCD image acquisition module, a phase shifter, a phase-shifting drive module, a frequency monitoring module, and a synchronization control module; The laser output from the laser source is split into two paths by the first beam splitter. One path is input to the main measurement optical path, and the other path is input to the frequency monitoring module. The frequency monitoring module is electrically connected to the synchronization control module. The frequency monitoring module is used to monitor the laser frequency drift and the phase change introduced by the phase shifter in real time, and transmits the monitoring signal to the synchronization control module. The synchronization control module is electrically connected to the frequency monitoring module, the phase shift drive module, and the CCD image acquisition module, respectively. It is used to output a phase shift drive signal to the phase shift drive module according to the monitoring signal, and to output an image acquisition trigger signal to the CCD image acquisition module when the phase reaches the preset step size according to the monitoring signal. The phase shifter is electrically connected to the phase shift drive module, and the phase shifter performs phase shifting according to the phase shift drive signal; The CCD acquisition module acquires interferograms based on the acquisition trigger signal.

[0006] To solve the above-mentioned technical problems, another technical solution adopted in this application is: providing a control method applied to the interferometer for phase-shift synchronization monitoring and compensation in any of the above-mentioned methods, the method comprising: The first beam splitting component splits the laser output from the laser source into a main measurement beam and a monitoring beam, so that the main measurement beam is input into the main measurement optical path and the monitoring beam is input into the frequency monitoring module. The frequency monitoring module monitors laser frequency drift and phase changes introduced by the phase shifter in real time and generates monitoring signals; The synchronization control module outputs a phase-shifting drive signal to the phase-shifting drive module based on the monitoring signal, so as to drive the phase shifter to perform phase shifting; When the phase reaches the preset step size based on the monitoring signal, the synchronous control module outputs an image acquisition trigger signal to the CCD image acquisition module to acquire the interferogram.

[0007] The beneficial effects of this application are: This application's phase-shifting synchronous monitoring and compensation interferometer, by constructing a closed-loop monitoring and synchronization architecture consisting of a frequency monitoring module and a synchronization control module, is the first to fuse the interferometer's main measurement optical path with the real-time frequency / phase monitoring optical path, achieving end-to-end hard synchronization of phase-shifting drive and CCD image acquisition. Specifically, the frequency monitoring module captures the laser frequency drift and the actual phase changes introduced by the phase shifter in real time, generating a high-precision monitoring signal; the synchronization control module outputs a phase-shifting drive signal based on this monitoring signal, and immediately outputs an image acquisition trigger signal when the phase accurately reaches the preset step size, thereby ensuring that the acquisition time of each frame of interferogram strictly corresponds to the preset phase position, completely eliminating the timing asynchrony problem caused by open-loop or semi-closed-loop control in traditional interferometers. Since the image acquisition trigger depends entirely on the measured phase rather than the estimated time, timing jitter is significantly suppressed, providing a reliable image foundation for high-precision surface shape calculation. At the same time, real-time monitoring of laser frequency drift and phase shifter phase changes enables the system to perceive environmental disturbances and the nonlinear characteristics of the actuator, providing an accurate data foundation for subsequent closed-loop control. Attached Figure Description

[0008] Figure 1 This is a schematic diagram of the structure of an interferometer for phase-shifting synchronization monitoring and compensation according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the electrical connections of an interferometer for phase-shifting synchronization monitoring and compensation according to an embodiment of the present invention; Figure 3This is a schematic diagram of the frequency monitoring module of an embodiment of the interferometer for phase-shifting synchronization monitoring and compensation of the present invention; Figure 4 This is a flowchart of one embodiment of the control method of the present invention. Detailed Implementation

[0009] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0010] The terms "first," "second," and "third" in this application are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first," "second," or "third" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. All directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of this application are only used to explain the relative spatial positions and movements of components in a specific orientation (as shown in the figures). If the specific orientation changes, the directional indications also change accordingly. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or devices.

[0011] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0012] Figure 1 This is a schematic diagram of the interferometer for phase-shift synchronization monitoring and compensation according to an embodiment of the present invention. Figure 1 and Figure 2 As shown, the interferometer for phase-shifting synchronous monitoring and compensation includes: a laser source 1, a first beam splitting component 2, a main measurement optical path 3, a CCD image acquisition module 4, a phase shifter 8, a phase-shifting drive module 7, a frequency monitoring module 5, and a synchronization control module 6.

[0013] Laser source 1 is preferably a frequency-stabilized He-Ne laser with a center wavelength of 632.8 nm and a frequency stability better than 5 × 10⁻⁶. -9 The output power is 1mW. This laser source can output highly coherent and stable continuous laser light, providing an ideal light source for interferometric measurements.

[0014] The first beam splitter 2 uses a semi-transparent and semi-reflective beam splitter to split the laser output from the laser source 1 into two paths: one path is input to the main measurement optical path 3 to generate an interferogram for measuring the sample surface shape; the other path is input to the frequency monitoring module 5 to monitor the laser frequency and phase changes in real time.

[0015] The main measurement optical path 3 is a Fizeau interference structure, including a first reference arm 31, a first test arm 32 for placing the sample under test, and a dichroic mirror 34. After the main measurement light is incident on the main measurement optical path 3, part of the light is reflected by the first reference arm 31 to form reference light, and the other part of the light is transmitted to the surface of the sample under test (such as a semiconductor wafer) placed on the first test arm 32 and reflected to form test light. The reference light and the test light are superimposed in space to produce interference fringes. The interference light is then deflected by the dichroic mirror 34 and enters the CCD image acquisition module 4.

[0016] CCD image acquisition module 4 uses a global shutter type CCD camera, such as the JAI Spark SP-5000M-GE2 model, with a trigger response time of less than 5ns and a trigger accuracy of 5ns. It supports external trigger signal control. When an image acquisition trigger signal is received, CCD image acquisition module 4 immediately freezes the current interferogram and completes the acquisition, thereby obtaining an interferogram that precisely corresponds to the phase.

[0017] The phase-shifting drive module 7 is a high-voltage amplifier that amplifies the low-voltage phase-shifting drive signal (0~10V) output by the synchronous control module 6 to the drive voltage (0~150V) required by the PZT, and provides sufficient drive current to drive the phase shifter 8.

[0018] Phase shifter 8 employs a PZT piezoelectric ceramic phase shifter with a displacement resolution of 0.05 nm, a closed-loop control bandwidth of 2 kHz, and a maximum travel of 10 μm. Phase shifter 8 is electrically connected to phase shift drive module 7, generating nanometer-level displacement based on the phase shift drive signal, thereby changing the output wavelength of laser source 1 or directly pushing the reference arm to achieve phase shift.

[0019] The frequency monitoring module 5 is built based on the Michelson interferometry principle. Electrically connected to the synchronization control module 6, it monitors the laser frequency drift and the phase change introduced by the phase shifter 8 in real time, transmitting the monitoring signal to the synchronization control module 6. It should be noted that this monitoring signal, output by the frequency monitoring module 5, is an analog voltage signal containing laser frequency drift information and actual phase change information from the phase shifter 8. It is the result of converting the light intensity signal after interference between the reflected light from the reference arm and the test arm in the Michelson interferometer. This monitoring signal is a sinusoidal waveform that varies with time, with the period of amplitude change corresponding to a phase change of 2π. This monitoring signal serves as the primary basis for the synchronization control module to perform phase extraction and error judgment.

[0020] The synchronization control module 6 adopts an architecture combining an FPGA chip and a high-precision timing module. The FPGA chip receives the monitoring signal transmitted from the frequency monitoring module 5 and calculates the current phase value in real time using a built-in phase extraction algorithm. The synchronization control module 6 outputs a phase-shifting drive signal to the phase-shifting drive module 7 based on the monitoring signal. Simultaneously, it determines whether the phase has reached a preset step size (e.g., π / 2, π, 3π / 2, 2π) based on the monitoring signal. When the phase reaches the preset step size, it immediately outputs a map acquisition trigger signal to the CCD map acquisition module 4. All output signals are based on the same high-precision clock reference, ensuring strict synchronization between phase shifting and map acquisition. It should be noted that the phase-shifting drive signal is an electrical signal generated by the synchronization control module 6 and output to the phase-shifting drive module 7. It is typically an analog voltage (e.g., 0~10V), and its physical meaning represents the "expected displacement of the phase shifter." The synchronization control module 6 calculates the required displacement of the phase shifter 8 based on the preset step size (e.g., π / 2) and the current phase feedback, and converts this displacement into a corresponding voltage value as the phase-shifting drive signal. The phase-shifting drive module 7 amplifies this value and drives the phase shifter 8 to produce the corresponding displacement.

[0021] This embodiment constructs a closed-loop monitoring and synchronization architecture consisting of a frequency monitoring module 5 and a synchronization control module 6. For the first time, it integrates the main measurement optical path 3 of the interferometer with the real-time frequency / phase monitoring optical path, achieving end-to-end hard synchronization between phase-shifting drive and CCD image acquisition. Specifically, the frequency monitoring module 5 captures the laser frequency drift and the actual phase changes introduced by the phase shifter in real time, generating a high-precision monitoring signal. The synchronization control module 6 outputs a phase-shifting drive signal based on this monitoring signal, and immediately outputs an image acquisition trigger signal when the phase accurately reaches the preset step size. This ensures that the acquisition time of each interferogram frame strictly corresponds to the preset phase position, completely eliminating the timing asynchrony problem caused by open-loop or semi-closed-loop control in traditional interferometers. Since the image acquisition trigger depends entirely on the measured phase rather than the estimated time, timing jitter is significantly suppressed, providing a reliable image foundation for high-precision surface shape calculation. Simultaneously, real-time monitoring of laser frequency drift and phase changes of the phase shifter 8 enables the system to perceive environmental disturbances and the nonlinear characteristics of the actuator, providing an accurate data foundation for subsequent closed-loop control.

[0022] Furthermore, based on the above embodiments, in other embodiments, such as Figure 1 As shown, the phase shifter 8 is connected to the laser source 1 and is used to change the output wavelength of the laser source 1 to achieve phase shift.

[0023] Specifically, one end of the phase shifter 8 (PZT) is fixed to the base of the interferometer, and the other end is connected to the resonant cavity mirror or cavity length adjustment element of the laser source 1 via a precision mechanical coupling. This PZT piezoelectric ceramic phase shifter has a displacement resolution of 0.05 nm, a closed-loop control bandwidth of 2 kHz, and a maximum stroke of 10 μm, enabling sub-nanometer level precise displacement control. When the phase-shifting drive module 7 applies voltage, the PZT undergoes expansion and contraction, pushing the cavity mirror of the laser source 1 to move, thereby changing the optical length of the laser resonant cavity.

[0024] In this embodiment, the wavelength tuning range is approximately ±0.01 nm, which is sufficient to generate a phase change of more than 2π. Compared with the traditional method of driving the reference mirror to shift the phase, this embodiment avoids mechanical disturbance to the optical components in the main measurement optical path, reduces Abbe error and motion coupling, and is particularly suitable for measurement scenarios with high numerical aperture and long working distance. At the same time, since the phase shifter 8 is directly linked to the laser source 1, the second test arm in the frequency monitoring module 5 (see Embodiment 5) can indirectly monitor the phase change introduced by the phase shifter by sensing the wavelength change, realizing non-contact tracking and simplifying the mechanical structure.

[0025] Furthermore, based on the above embodiments, in other embodiments, such as Figure 1 As shown, the main measurement optical path 3 includes a first reference arm 31, a first test arm 32 for placing the sample to be tested, and a dichroic mirror 34 for transmitting interference light to the CCD image acquisition module 4.

[0026] The first reference arm 31 can be a first reference mirror, which is a silver-plated reflector with high flatness (λ / 20) and its reflecting surface is perpendicular to the optical axis. The first reference arm 31 is mounted on a finely adjustable frame, and its pitch and deflection angles can be changed through a precision adjustment mechanism to ensure the quality of the reference wavefront. The first reference arm 31 is used to generate a high-quality reference wavefront, and its surface accuracy directly affects the reference for interferometry.

[0027] The first test arm 32 is used to place the sample to be tested, such as a semiconductor wafer, an optical plane mirror, or a lens. The test arm has an adjustable sample stage that allows for translation and tilting of the sample in the X and Y directions to align the area under test with the measurement spot. The optical path of the test arm is coaxial with that of the reference arm. The test light is reflected from the sample surface and returns, interfering with the reference light.

[0028] The dichroic mirror 34 is located in the optical path after the reference light and the test light are combined. It can transmit the incident light, while simultaneously deflecting the interference light after the beam is combined by 90° before guiding it into the CCD image acquisition module 4. The incident angle of the dichroic mirror 34 is usually set to 45° to maximize the reflection efficiency.

[0029] In actual measurement, the sample to be tested is placed on the sample stage of the first test arm 32, and the sample position and tilt are adjusted to make the interference fringes clear. After the laser passes through the first beam splitter 2, the main measurement beam reaches the first reference arm 31 and the sample to be tested. The two reflected beams are superimposed in space to form an interference pattern, which is imaged onto the photosensitive surface of the CCD image acquisition module 4 by the dichroic mirror 34. The intensity distribution of this interference pattern reflects the surface shape information of the sample to be tested.

[0030] In other embodiments, the phase shifter 8 may also be directly mechanically connected to the first reference arm 31, and phase shifting is achieved by pushing the reference arm to change the optical path difference.

[0031] Furthermore, based on the above embodiments, in other embodiments, the synchronization control module 6 includes an FPGA chip and a high-precision timing module, the high-precision timing module being used to provide a clock reference for the FPGA chip.

[0032] Specifically, the core of the synchronization control module 6 consists of an FPGA chip and a high-precision timing module. The high-precision timing module uses a temperature-compensated crystal oscillator with a nominal frequency of 100MHz, frequency stability of ±2.5ppm, and phase jitter of less than 2ps. This timing module provides the master clock for the FPGA chip.

[0033] The FPGA chip incorporates multiple functional logic blocks, including phase extraction logic, phase-shift step counter, comparator, pulse generator, and compensation calculation unit. All logic is strictly synchronized with the clock provided by the high-precision timing module, ensuring that the timing jitter between the output signals (phase-shift drive signal and CCD trigger pulse) is less than 10ns and the signal delay is less than 1μs.

[0034] In practice, the FPGA chip receives the photodetector signal (converted to digital by an analog-to-digital converter) from the frequency monitoring module 5, and extracts the phase value in real time using a digital demodulation algorithm (quadrature demodulation). The quadrature demodulation is implemented as follows: the FPGA chip internally generates a pair of quadrature reference signals, which are multiplied by the input interference signal and then low-pass filtered to obtain the in-phase component I and the quadrature component Q. The phase... The phase value is updated once every clock cycle (10ns) to meet real-time requirements.

[0035] The FPGA chip compares the current phase with a preset step size (e.g., π / 2). When the phase error exceeds a threshold, a compensation voltage is calculated by the built-in PID controller. Simultaneously, when the phase is precisely equal to the preset step size (considering a certain quantization error), the FPGA immediately outputs a 1μs wide TTL trigger pulse to the CCD image acquisition module 4 in the next clock cycle. The high-precision timing module ensures the real-time performance and repeatability of these operations.

[0036] Furthermore, based on the above embodiments, in other embodiments, the second reference arm can be a second reference mirror, which is a highly flat mirror, such as... Figure 3 As shown, the frequency monitoring module 5 includes a second beam splitter 51, a second reference arm 52, a second test arm 53, and a photodetector 54. The second beam splitter 51 is used to split the input monitoring light into two beams, which are transmitted to the second reference arm 52 and the second test arm 53 respectively. The optical path of the second reference arm 52 is kept constant to provide a frequency reference. The second test arm 53 is linked with the phase shifter 8 driven by the phase shifting drive module 7 to track the phase change introduced by the phase shifter in real time. The photodetector 54 is electrically connected to the synchronization control module 6. The photodetector 54 is used to collect the interference signal after the reflected light from the second reference arm 52 and the second test arm 53 is combined, and convert it into a monitoring signal to be transmitted to the synchronization control module 6.

[0037] The second beam splitter 51 uses a non-polarizing beam splitter prism with a splitting ratio of 50:50. It splits the input monitoring light (from the first beam splitter 2) into two sub-beams of equal intensity, which are then guided to the second reference arm 52 and the second test arm 53, respectively.

[0038] The optical path of the second reference arm 52 remains constant, specifically implemented as a rigid optical path of fixed length (100 mm), with a highly stable reflector at its end. This reflector is fixed to a bracket made of Invar alloy, which has an extremely low coefficient of thermal expansion (approximately 1.2 × 10⁻⁶). -6 / ℃), thus ensuring the long-term stability of the optical path. Since the optical path of the second reference arm 52 remains constant, the interference signal it generates is only related to the laser frequency, and therefore can be used as a frequency reference. When the laser frequency drifts, the interference signal of the second reference arm 52 will change accordingly, and this change will be captured by the photodetector 54 and used for subsequent differential processing.

[0039] The second test arm 53 is linked to the phase shifter 8 driven by the phase shifting drive module 7. In this embodiment, the end mirror of the second test arm 53 is mechanically connected to the mover of the phase shifter 8 via a flexible hinge. When the phase shifter 8 pushes the laser source cavity mirror to change the wavelength, the mirror of the second test arm 53 remains stationary, but the wavelength change causes a change in the equivalent optical path; or, in an alternative, the mirror of the second test arm 53 moves directly with the displacement of the phase shifter 8. Regardless of the method used, the interference signal of the second test arm 53 can reflect the phase change introduced by the phase shifter 8 in real time. Specifically, when the phase shifter 8 causes a wavelength change Δλ or a mirror displacement Δl, the optical path difference change ΔOPD of the second test arm 53 will cause a phase change in the interference signal. This change was detected by photodetector 54.

[0040] The photodetector 54 employs a high-speed silicon photodiode with a response frequency of 20MHz (e.g., Thorlabs PDA10A2, response frequency 20MHz). Its photosensitive surface receives the interference light from the combined beams of light returned by the second reference arm 52 and the second test arm 53, converting the light intensity signal into an analog voltage signal (monitoring signal). The photodetector 54 is electrically connected to the synchronization control module 6, transmitting the monitoring signal to the FPGA in real time.

[0041] Furthermore, due to the nonlinearity, hysteresis, and drift caused by changes in ambient temperature, the actual phase shift step size of the phase shifter 8 often deviates from the theoretical value. To correct this deviation, in other embodiments, based on the above embodiments, the synchronization control module 6 is also used to calculate the phase deviation between the actual phase shift phase and the theoretical target phase based on the monitoring signal, generate a compensation signal using the phase deviation, and generate a new phase shift drive signal based on the compensation signal, which is then output to the phase shift drive module 7 to correct the phase shifting action of the phase shifter 8. It should be noted that the compensation signal is a compensation voltage generated by the synchronization control module during the closed-loop compensation process. The process is as follows: First, the synchronization control module calculates the phase deviation between the current actual phase shift phase value and the theoretical target phase value based on the phase signal output in real time by the Michelson frequency monitoring module. Then, the phase deviation is converted into a corresponding displacement compensation amount using a PID control algorithm or a proportional correction algorithm. Then, based on the voltage-displacement calibration coefficient of the phase shifter, the displacement compensation amount is converted into compensation voltage. The synchronization control module superimposes the compensation voltage onto the original phase-shifting drive voltage to form a corrected total drive voltage. This corrected total drive voltage generates a new phase-shifting drive signal and is output to the phase-shifting drive module in real time. The phase-shifting drive module controls the phase shifter's operation based on the corrected drive voltage. By accelerating or decelerating the phase shifting speed and increasing or decreasing the phase shift displacement, it compensates for phase deviations, ensuring that the actual phase-shifted phase quickly converges to the theoretical target phase, and guaranteeing that the phase shifting interval error between adjacent image acquisition moments does not exceed a preset threshold. Compensation voltage It can be zero (when there is no deviation) or positive / negative (when there is under-phase or over-phase shift).

[0042] Specifically, after each phase-shift drive signal is output, the synchronization control module 6 continuously monitors the monitoring signal returned by the frequency monitoring module 5. Once the phase change stabilizes (i.e., before or after the next image acquisition trigger signal is issued), the FPGA records the current actual phase value. and phase with the theoretical target Compare (e.g., π / 2, π, 3π / 2, 2π) and calculate the deviation. For example, assuming the theoretical target phase for a single phase shift is 90° (π / 2), the actual phase returned by the frequency monitoring module is 90.12°, a deviation of... It is -0.12°. When | When the preset threshold is exceeded, the synchronization control module 6 generates a corresponding compensation signal based on the phase deviation and generates a phase shift drive signal based on the compensation signal and outputs it to the phase shift drive module 7 to correct the phase shifting action of the phase shifter 8.

[0043] In an alternative approach, the compensation signal can be generated using voltage-displacement calibration coefficients. Specifically, based on the principle of interference, the phase change... With wavelength change The relationship is D is the optical path difference of the main measurement optical path. The wavelength change is related to the PZT displacement. The relationship is , This is the length of the laser cavity. Therefore, it can be derived that... ,in This is a proportionality coefficient, obtained through calibration experiments. In this embodiment, it is calibrated to obtain... PZT typically has an approximately linear piezoelectric coefficient, for example, 0.05 nm / V at 5 nm / 100 V. Assumption: , ,according to Solve And then according to The compensation voltage can then be calculated. The compensation voltage is superimposed on the driving voltage of the previous phase shift to form a corrected total driving voltage. A new motion driving signal is generated using the corrected total driving voltage and output to the phase shift driving module to correct the phase shifting action of the phase shifter.

[0044] In another alternative approach, the compensation signal can be generated using a built-in PID control algorithm. The discrete form of the PID controller is: + ; ; in, To compensate for voltage, The current output driving voltage (or the total driving voltage after compensation). This is the driving voltage output at the previous moment; This represents the phase deviation at the current moment (the difference between the actual phase shift and the theoretical target phase). This represents the phase deviation from the previous moment. This represents the phase deviation between the two previous moments; This is for the control period (i.e., the time interval between two phase shifts). This is the proportionality coefficient. The integral coefficient is... is the differential coefficient.

[0045] Based on the above embodiments, the preset threshold for phase shift interval error can be 0.1°, when the error|| When the value is 0.12, exceeding the preset threshold of 0.1, the PID control algorithm is triggered to calculate the compensation voltage. The PID parameters are tuned based on the system identification results as follows: =0.5V / °, =0.05V / (°·s), =0.01V·s / °. The compensation voltage calculated from this is... Compensation voltage This is superimposed on the driving voltage of the previous phase shift to form the corrected total driving voltage (i.e., the driving voltage output at the current moment). The corrected total drive voltage is used to generate a new motion drive signal and output to the phase shift drive module to correct the phase shifting action of the phase shifter. For example, a preset threshold of 0.1° is preferred and can be set in the configuration register of the synchronization control module 6; it can also be modified by the user via host computer software.

[0046] Furthermore, based on the above embodiments, in other embodiments, the synchronization control module 6 is configured with a phase extraction algorithm, a phase shift error compensation algorithm, and a resampling algorithm; the phase extraction algorithm is used to extract phase information from the monitoring signal of the frequency monitoring module in real time; the phase shift error compensation algorithm is used to compare the theoretical phase shift step size with the actual phase shift step size, and generate a compensation signal when the error exceeds a preset threshold; the resampling algorithm is used to record the precise phase value when each frame of the interferogram is acquired, and to resample and interpolate the interferogram with non-ideal phase shift to reconstruct the equal-step phase shift sequence.

[0047] Phase extraction algorithm: An orthogonal demodulation method is adopted. A pair of orthogonal reference signals (sin(ωt) and cos(ωt)) are generated internally by the FPGA, with frequencies matching the desired interference signal frequency. The interference signal I(t) output by the photodetector 54 is multiplied by sin(ωt) and cos(ωt) respectively, yielding: I·sin(ωt) = 0.5I0[sin(φ) + sin(2ωt+φ)], I·cos(ωt) = 0.5I0[cos(φ) + cos(2ωt+φ)]. The 2ωt component is filtered out by a low-pass filter, resulting in the in-phase component I_I = 0.5I0sin(φ) and the quadrature component I_Q = 0.5I0cos(φ). Then, the phase φ = atan2(I_Q,I_I) is calculated. This algorithm updates the phase value every 10ns, meeting real-time requirements.

[0048] Phase shift error compensation algorithm: A PID controller is used to calculate the compensation voltage in real time. Preferably, to improve the robustness of the algorithm, this embodiment also incorporates anti-integral saturation measures: when the integral term exceeds a preset limit value, the integration accumulation stops; and derivative-first: the target value is used as the input of the derivative term to avoid shocks caused by sudden changes in the given value. These enhancements ensure the smoothness and stability of the compensation process.

[0049] Resampling Algorithm: When the actual phase of an interferogram frame deviates from the theoretical phase by a preset threshold (e.g., 0.05°), the algorithm uses interferogram data from adjacent frames to perform cubic spline interpolation to reconstruct the interferogram corresponding to the theoretical phase. This resampling phase deviation threshold (0.05°) is a preferred parameter and can be set in the configuration register of the synchronization control module 6, or modified by the user via the host computer software. Let the acquired non-equiphase interval interferogram sequence be... , The actual phase is The target has a constant step size phase. (For example For each In the interval Internally constructed cubic spline function ,satisfy , And the first derivative and Continuous, second derivative and Continuous. Then calculate. The intensity of the interferogram is used after resampling. This algorithm can effectively correct errors introduced by non-ideal phase shift.

[0050] Furthermore, based on the above embodiments, in other embodiments, the synchronization control module 6 outputs a TTL trigger pulse to the CCD image acquisition module 4 according to the arrival status of the monitoring signal output by the frequency monitoring module 5.

[0051] In this embodiment, the arrival status of the monitoring signal output by the frequency monitoring module 5 is determined by the interference signal output by the photodetector 54. Specifically, the interference signal output by the photodetector 54 is an approximately sinusoidal waveform, and its zero-crossing point (i.e., the point where the signal intersects with the average value) corresponds to the phase φ = 0, π, 2π, ... . The signal slope is largest at these points, and it is most sensitive to phase changes, thus representing the optimal triggering timing. A high-speed voltage comparator (e.g., TI TLV3501, propagation delay 4.5ns) is used to compare the analog voltage signal output by the photodetector 54 with the reference voltage (average value). The reference voltage is extracted from the interference signal itself through a low-pass filter and voltage divider resistors to eliminate the influence of DC drift. When the interference signal crosses the reference voltage from negative to positive or from positive to negative, the comparator outputs an edge signal (rising edge or falling edge). The FPGA captures this edge signal and immediately outputs a 1μs wide TTL high-level pulse to the trigger input of the CCD image acquisition module 4 at the next clock rising edge (10ns period). Since the total delay of the comparator and FPGA is less than 0.5μs, and the triggering time strictly corresponds to the phase zero crossing point, the trigger jitter can be controlled within 5ns, ensuring that the image acquisition time is strictly synchronized with the phase reaching the preset step size.

[0052] Furthermore, to reduce the impact of environmental vibration and temperature changes on measurement accuracy, in other embodiments based on the above embodiments, the frequency monitoring module 5 and the main measurement optical path 3 adopt a common base and / or common temperature control design.

[0053] Specifically, the base is made of marble, measuring 600mm × 400mm × 100mm and weighing approximately 50kg. Marble is characterized by high rigidity, high damping, and a low coefficient of thermal expansion, effectively attenuating external vibrations. All optical components (including the laser source, beam splitter, mirrors, detectors, etc.) are rigidly fixed to the marble base with bolts, and locating pins are used to ensure repeatability. Rubber vibration damping pads are installed under the base to further isolate low-frequency vibrations.

[0054] Meanwhile, a common temperature control design is adopted: multiple PT100 temperature sensors and flexible heating films are embedded inside the marble base. The heating films are evenly distributed inside the base, and the base temperature is controlled at 23±0.1℃ by an external PID temperature controller. The controller adjusts the heating power based on sensor feedback, forming a closed-loop temperature control. In addition, the outer shell of all modules is wrapped with polyurethane foam insulation material to reduce the impact of external heat radiation and convection.

[0055] Furthermore, based on the above embodiments, in other embodiments, the first beam splitting component 2 splits the laser output from the laser source 1 to the frequency monitoring module 5 at a ratio of 5% to 10%.

[0056] Specifically, in this embodiment, a neutral density beam splitter or beam splitter prism is used to allocate 5% to 10% of the laser energy output from laser source 1 to frequency monitoring module 5, and the remaining 90% to 95% to main measurement optical path 3. Frequency monitoring module 5 only requires a weak light intensity to obtain a sufficient signal-to-noise ratio because the sensitivity of photodetector 54 can reach the nanowatt level; while main measurement optical path 3 requires sufficient light intensity to ensure that the interferogram has high contrast and a high signal-to-noise ratio. If the monitoring light ratio is too high (e.g., above 20%), the energy loss of the main measurement light will lead to a decrease in the grayscale value of the interferogram and a deterioration of the signal-to-noise ratio; if the monitoring light ratio is too low (e.g., below 5%), the monitoring signal amplitude will be too small and may be overwhelmed by noise.

[0057] Through experimental optimization, 5%–10% was found to be the optimal range. Actual measurement data shows that when the splitting ratio is 10%, the main measurement optical power is 0.9mW (initially 1mW), the maximum grayscale value of the interferogram reaches 80% of the CCD saturation value, and the signal-to-noise ratio (SNR) is greater than 40dB; the monitoring optical power is 0.1mW, the photodetector output signal amplitude is 200mV, and the SNR is 35dB. When the splitting ratio is 5%, the main measurement optical power is 0.95mW, the interferogram grayscale value reaches 85% of the saturation value, the monitoring optical power is 0.05mW, the detector output signal amplitude is 100mV, and the SNR is still greater than 30dB, meeting the monitoring requirements. Therefore, 5%–10% is the optimal range for balancing the main measurement SNR and the monitoring signal quality.

[0058] Figure 4 This is a flowchart illustrating the control method according to an embodiment of the present invention. Figure 4 As shown, this method is applied to the interferometer for phase-shift synchronization monitoring and compensation in any of the above embodiments. The control method includes: Step S1: The first beam splitting component splits the laser output from the laser source into a main measurement beam and a monitoring beam, so that the main measurement beam is input into the main measurement optical path and the monitoring beam is input into the frequency monitoring module.

[0059] Specifically, this step continues after system initialization to ensure optical path stability. For a detailed description of step S1, please refer to the above embodiments; it will not be repeated here.

[0060] Step S2: The frequency monitoring module monitors the laser frequency drift and the phase change introduced by the phase shifter in real time and generates a monitoring signal.

[0061] Specifically, the monitoring signal is an analog voltage, representing the intensity of the interference light, and its frequency is proportional to the rate of phase change. For details of step S2, please refer to the above embodiments; they will not be repeated here.

[0062] Step S3: The synchronization control module outputs a phase-shifting drive signal to the phase-shifting drive module according to the monitoring signal, so as to drive the phase shifter to perform phase shift.

[0063] Specifically, the FPGA obtains the phase value through quadrature demodulation, compares it with a preset step size, calculates the required digital value of the driving voltage using a PID controller, and then converts it into an analog voltage, i.e., a phase-shifting driving signal, via a DAC. The phase-shifting driving module amplifies this signal and applies it to the PZT, causing the PZT to shift, changing the laser wavelength, and thus altering the interference phase. For a detailed explanation of step S3, please refer to the above embodiment; it will not be repeated here.

[0064] Step S4: When the phase reaches the preset step size as determined by the monitoring signal, the synchronous control module outputs an image acquisition trigger signal to the CCD image acquisition module to acquire the interferogram.

[0065] Specifically, the trigger signal is a TTL pulse, and its rising edge corresponds to the instant the phase arrives. For details of step S4, please refer to the above embodiments; they will not be repeated here.

[0066] The above steps are executed cyclically, acquiring one interferogram frame after each step. In a typical four-step phase shift, four interferogram frames corresponding to phases π / 2, π, 3π / 2, and 2π are acquired sequentially for surface shape calculation.

[0067] Furthermore, based on the above embodiments, in other embodiments, step S5, when the phase reaches the preset step size according to the monitoring signal, the step of the synchronization control module outputting the image acquisition trigger signal to the CCD image acquisition module specifically includes: the synchronization control module outputting a TTL trigger pulse to the CCD image acquisition module according to the arrival status of the monitoring signal output by the frequency monitoring module.

[0068] In this embodiment, the arrival status of the monitoring signal output by the frequency monitoring module 5 is determined by the interference signal output by the photodetector 54. Specifically, the synchronization control module uses the interference signal output by the frequency monitoring module as the monitoring signal to monitor the timing of the phase crossing the zero-crossing point and outputs a TTL trigger pulse to the CCD image acquisition module. The phase zero-crossing point refers to the position where the interference signal crosses zero from negative to positive or from positive to negative, corresponding to phases φ=0, π, 2π, ... Since the phase shift step size is usually π / 2, the zero-crossing interval corresponds to two steps. In actual operation, the system can trigger once every two zero-crossing points, or trigger at each zero-crossing point but offset the triggering timing by π / 2.

[0069] Furthermore, based on the above embodiments, in other embodiments, the control method further includes a compensation step: using the synchronization control module to calculate the phase deviation between the actual phase shift phase and the theoretical target phase according to the monitoring signal, using the phase deviation to generate a compensation signal, and generating a new phase shift drive signal based on the compensation signal and outputting it to the phase shift drive module to correct the phase shifting action of the phase shifter.

[0070] Specifically, the compensation steps include: 1. The synchronous control module calculates the phase deviation between the current actual phase shift value and the theoretical target phase value based on the monitoring signal.

[0071] 2. The phase deviation is converted into the corresponding displacement compensation amount using the synchronous control module.

[0072] Specifically, according to the principle of interference, phase change With wavelength change The relationship is D is the optical path difference of the main measurement optical path. The wavelength change is related to the PZT displacement. The relationship is , This is the length of the laser cavity. Therefore, it can be derived that... ,in This is a proportionality coefficient, obtained through calibration experiments. In this embodiment, it is calibrated to obtain... .

[0073] 3. Using the synchronous control module, the displacement compensation amount is converted into compensation voltage based on the voltage-displacement calibration coefficient of the phase shifter.

[0074] Specifically, PZT typically has an approximately linear piezoelectric coefficient, for example, 0.05 nm / V at 5 nm / 100 V. .For example, , ,according to Solve And then according to Solving for the given information, we can obtain the following results: .

[0075] 4. The compensation voltage is superimposed on the original phase-shifting drive voltage using the synchronous control module to form the corrected total drive voltage. The corrected total drive voltage is then used to generate a new phase-shifting drive signal and output to the phase-shifting drive module.

[0076] Furthermore, based on the above embodiments, in other embodiments, the control method further includes a resampling correction step: using a synchronization control module to record the precise phase value corresponding to each frame of the interferogram, resampling and interpolating the non-ideal phase-shifted interferogram, and reconstructing the equal-step phase-shifting sequence for subsequent surface shape calculation.

[0077] Specifically, after acquiring all interferograms (e.g., four frames), the synchronization control module records the precise phase value corresponding to each frame of the interferogram (given by the phase extraction algorithm). If the actual phase of a certain frame of the interferogram deviates from the theoretical phase by more than a preset threshold (e.g., 0.05°), the resampling algorithm is used to interpolate and correct that frame of the interferogram. This resampling deviation threshold can be set in the configuration register of the synchronization control module, or it can be modified by the user through the host computer software.

[0078] Specifically, a cubic spline interpolation function is constructed using the measured phase of adjacent frames as the independent variable and the grayscale value of the interferogram as the dependent variable. Assume the actual phase of the current frame is... The grayscale value of the interferogram is The phase of the previous frame is grayscale value The phase of the next frame is grayscale value In the interval and Construct cubic spline functions to ensure continuity of function values, first derivatives, and second derivatives at the nodes. Then calculate the theoretical phase. Interpolation results at The grayscale values ​​of the interferogram are used as the resampled values. This process is performed independently for all pixels.

[0079] After resampling correction, a constant-step phase-shifting sequence is reconstructed and then transmitted to the data processing unit for surface shape calculation. The classic four-step phase-shifting algorithm is employed, substituting the corrected four-frame interferograms into the formula. This allows us to obtain the phase distribution of the sample under test, which can then be converted into a surface shape. I 1, I 2, I 3, I 4 refers to the brightness distribution of the interferograms of four different phase-shifting diagrams.

[0080] The above are merely embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. An interferometer for phase-shift synchronization monitoring and compensation, characterized in that, It includes a laser source, a first beam splitting component, a main measurement optical path, a CCD image acquisition module, a phase shifter, a phase shift driving module, a frequency monitoring module, and a synchronization control module; The laser output from the laser source is split into two paths by the first beam splitting component. One path is input to the main measurement optical path, and the other path is input to the frequency monitoring module. The frequency monitoring module is electrically connected to the synchronization control module. The frequency monitoring module is used to monitor the laser frequency drift and the phase change introduced by the phase shifter in real time, and transmit the monitoring signal to the synchronization control module. The synchronization control module is electrically connected to the frequency monitoring module, the phase-shifting drive module, and the CCD image acquisition module, respectively. It is used to output a phase-shifting drive signal to the phase-shifting drive module according to the monitoring signal, and to output an image acquisition trigger signal to the CCD image acquisition module when the phase reaches a preset step size according to the monitoring signal. The phase shifter is electrically connected to the phase shift driving module, and the phase shifter performs phase shifting according to the phase shift driving signal; The CCD image acquisition module acquires an interferogram based on the image acquisition trigger signal.

2. The interferometer for phase-shift synchronization monitoring and compensation according to claim 1, characterized in that, The phase shifter is connected to the laser source and is used to change the output wavelength of the laser source to achieve phase shift.

3. The interferometer for phase-shift synchronization monitoring and compensation according to claim 1, characterized in that, The main measurement optical path includes a first reference arm, a first test arm for placing the sample to be tested, and a dichroic mirror for transmitting interference light to the CCD image acquisition module.

4. The interferometer for phase-shift synchronization monitoring and compensation according to claim 1, characterized in that, The synchronization control module includes an FPGA chip and a high-precision timing module, wherein the high-precision timing module is used to provide a clock reference for the FPGA chip.

5. The interferometer for phase-shift synchronization monitoring and compensation according to claim 1, characterized in that, The frequency monitoring module includes a second beam splitter, a second reference arm, a second test arm, and a photodetector. The second beam splitter is used to split the input monitoring light into two beams, which are then transmitted to the second reference arm and the second test arm, respectively. The optical path length of the second reference arm remains constant, serving to provide a frequency reference; The second test arm is linked with the phase shifter driven by the phase shifting drive module to track the phase change introduced by the phase shifter in real time; The photodetector is electrically connected to the synchronization control module. The photodetector is used to collect the interference signal after the reflected light beams from the second reference arm and the second test arm are combined, and convert it into a monitoring signal and transmit it to the synchronization control module.

6. The interferometer for phase-shift synchronization monitoring and compensation according to claim 1, characterized in that, The synchronization control module is also used to calculate the phase deviation between the actual phase shift phase and the theoretical target phase based on the monitoring signal, generate a compensation signal using the phase deviation, generate a new phase shift drive signal based on the compensation signal, and output it to the phase shift drive module to correct the phase shifting action of the phase shifter.

7. The interferometer for phase-shift synchronization monitoring and compensation according to claim 6, characterized in that, The synchronization control module is equipped with a phase extraction algorithm, a phase shift error compensation algorithm, and a resampling algorithm; The phase extraction algorithm is used to extract phase information from the monitoring signal of the frequency monitoring module in real time; The phase shift error compensation algorithm is used to compare the error between the theoretical phase shift step size and the actual phase shift step size, and to generate a compensation signal when the error exceeds a preset threshold. The resampling algorithm is used to record the precise phase value when each frame of the interferogram is acquired, and to resample and interpolate the interferogram with non-ideal phase shift to reconstruct the equal-step phase shift sequence.

8. The interferometer for phase-shift synchronization monitoring and compensation according to claim 1, characterized in that, The synchronization control module outputs a TTL trigger pulse to the CCD image acquisition module based on the arrival status of the monitoring signal output by the frequency monitoring module.

9. The interferometer for phase-shift synchronization monitoring and compensation according to claim 1, characterized in that, The frequency monitoring module and the main measurement optical path adopt a common base and / or common temperature control design.

10. The interferometer for phase-shift synchronization monitoring and compensation according to claim 1, characterized in that, The first beam splitting component splits the laser output from the laser source into the frequency monitoring module at a ratio of 5% to 10%.

11. A control method, characterized in that, It is applied to the interferometer for phase-shift synchronization monitoring and compensation according to any one of claims 1-10, the method comprising: The first beam splitting component splits the laser output from the laser source into a main measurement beam and a monitoring beam, so that the main measurement beam is input into the main measurement optical path and the monitoring beam is input into the frequency monitoring module; The frequency monitoring module monitors the laser frequency drift and the phase change introduced by the phase shifter in real time, and generates a monitoring signal; The synchronization control module outputs a phase-shifting drive signal to the phase-shifting drive module based on the monitoring signal to drive the phase shifter to perform phase shifting; when the phase reaches a preset step size based on the monitoring signal, the synchronization control module outputs an image acquisition trigger signal to the CCD image acquisition module to acquire an interferogram.

12. The control method according to claim 11, characterized in that, When the phase reaches a preset step size based on the monitoring signal, the synchronization control module outputs an image acquisition trigger signal to the CCD image acquisition module, including: The synchronization control module outputs a TTL trigger pulse to the CCD image acquisition module based on the arrival status of the monitoring signal output by the frequency monitoring module.

13. The control method according to claim 11, characterized in that, The method further includes: The synchronization control module calculates the phase deviation between the actual phase shift phase and the theoretical target phase based on the monitoring signal, generates a compensation signal using the phase deviation, and generates a new phase shift drive signal based on the compensation signal and outputs it to the phase shift drive module to correct the phase shifting action of the phase shifter.

14. The control method according to claim 13, characterized in that, The monitoring signal calculates the phase deviation between the actual phase shift phase and the theoretical target phase, generates a compensation signal using the phase deviation, and generates a new phase shift drive signal based on the compensation signal, which is then output to the phase shift drive module, including: The synchronization control module calculates the phase deviation between the current actual phase shift value and the theoretical target phase value based on the monitoring signal. The synchronization control module is used to convert the phase deviation into a corresponding displacement compensation amount. The synchronous control module uses the voltage-displacement calibration coefficient of the phase shifter to convert the displacement compensation amount into a compensation voltage. The compensation voltage is superimposed on the original phase-shifting drive voltage using the synchronous control module to form a corrected total drive voltage. The corrected total drive voltage is then used to generate a new phase-shifting drive signal and output to the phase-shifting drive module.

15. The control method according to claim 11, characterized in that, The method further includes: The synchronous control module records the precise phase value corresponding to each frame of the interferogram, resamples and interpolates the non-ideal phase-shifted interferogram, and reconstructs the equal-step phase-shifting sequence for subsequent surface shape calculation.

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