A swept-frequency interferometric surface structure detection system and method

Abstract

The application discloses a kind of sweep frequency interference surface structure detection system and method, it is related to surface detection and sweep frequency laser field.The system mainly includes sweep frequency laser generation unit, interference unit, scanning detection unit and signal processing unit.Compared with prior art, in the present application, single-point photoelectric detector is used to collect interference light signal, phase solution is carried out by single-point calculation, distance information of each position of the surface to be measured is obtained by solving interference information obtained by each position of photoelectric detection system, high-precision surface structure detection is realized, and nanometer level can be achieved.Displacement table scanning mode is used to move detector to detect sweep frequency interference light, and large-scale, high-precision measurement can be realized.Because the receiving speed of single-point detector is much higher than that of area array detector, fast detection can be realized when the scanning range is small.

Description

Technical Field

[0001] This invention relates to the fields of surface shape detection and frequency sweeping laser, specifically to a frequency sweeping interference surface structure detection system and method. Background Technology

[0002] As modern equipment becomes increasingly complex, the number of structurally diverse components is also growing. Measuring parameters such as shape errors in the microstructure of product surfaces has become crucial for evaluating part quality and adjusting production processes. To achieve precise and rapid detection of product surface microstructures without damaging the product, high-precision, non-contact measurement methods have become a widespread market demand. Currently, axial measurement accuracy requirements have reached sub-micron or even nanometer levels; furthermore, due to the low optical reflectivity of many samples, the system's measurement sensitivity must be high. Traditionally, such minute features have been evaluated using scanning electron microscopy (SEM) or atomic force microscopy (AFM). However, the measurement speeds of AFM and SEM are too slow to meet the needs of large-area measurements. Therefore, in manufacturing, non-contact and high-speed optical measurement methods are emerging, continuously improving in accuracy and resolution, with interferometry, as a highly sensitive method, gaining widespread application.

[0003] For example, the invention with authorization number CN115327514B proposes a sweep frequency interferometric dynamic measurement system and method based on phase transfer. The optical path of the measuring interferometer is mainly used for calculating the absolute distance of the target and measuring the relative motion trajectory of the target. The optical path of the auxiliary interferometer 1 is used to provide the relative frequency coordinates for frequency modulation nonlinear correction. The optical path of the auxiliary interferometer 2 provides an acousto-optic frequency shift interference signal to perform phase demodulation of the signal of the auxiliary interferometer 1. The optical path of the gas cell is used for online calibration of the group delay of the auxiliary interferometer 1. This invention can realize real-time trajectory tracking of dynamic targets.

[0004] White light interferometry was the first proposed method based on wideband light source interference. The interference fringes can be considered as being generated by the superposition of interference patterns of different wavelengths. When the fringes are at their peak, all interference wavelengths have the same interference phase. That is, the fringes are at their peak when the optical path difference between the measurement light and the reference light is zero. Therefore, white light interferometry uses the position of maximum interference fringe contrast for surface reconstruction. To find this position, the surface to be measured needs to be moved longitudinally, resulting in a slow measurement speed and a small measurement range.

[0005] Optical coherence tomography (OCT) is an imaging technique proposed in the 1990s that can obtain three-dimensional information about the internal structure of a sample non-contactly and non-destructively. Sweep-frequency OCT (SS-OCT) uses a light source whose output wavelength changes over time, allowing spectral information to be obtained without axial scanning. Spectral interferometry is similar, and both methods can achieve sub-micron level precision.

[0006] Building upon the methods described above, it is of great significance to further improve the measurement accuracy and sensitivity of surface structures. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a swept-frequency interferometric surface structure detection system and method. Specifically, it provides an apparatus and method for high-precision surface structure detection based on swept-frequency interferometric signals received by scanning.

[0008] This invention is achieved through the following technical solution:

[0009] A swept-frequency interferometric surface structure detection system includes: a swept-frequency laser generation unit for generating a signal laser whose frequency varies with time; an interference unit including a reference surface, wherein the interference unit splits the signal laser beam and projects it onto the reference surface and the surface under test respectively, and then recombines the beams into swept-frequency interferometric light after reflection; a scanning detection unit for receiving the swept-frequency interferometric light and converting the optical signal into an electrical signal, wherein the scanning detection unit includes a single-point photodetector and a displacement stage for controlling the movement of the single-point photodetector, wherein the single-point photodetector receives a complete swept-frequency interferometric light from one position at a time, and then the displacement stage moves the single-point photodetector to receive a complete swept-frequency interferometric light from the next position, until the swept-frequency interferometric light from all positions on the entire surface under test is received by the single-point photodetector; and a signal processing unit for calculating the electrical signal converted from the swept-frequency interferometric light signal received by the scanning detection unit at each position, obtaining the phase difference between the surface under test and the reference surface, thereby obtaining the distance information of a single point on the surface under test, and recombining the obtained distance information at each position according to the scan number order to obtain the surface structure information of the surface under test.

[0010] The detection principle is as follows: the frequency of the interference light obtained by the sweep frequency signal illuminating the test surface and the reference surface changes continuously, containing more phases. After calculation, high-precision distance information can be obtained. Using a single-point photodetector to receive the sweep frequency interference signal can receive a set of sweep frequency light at a higher speed. A high-precision displacement stage is used to move the detector to scan and receive the sweep frequency interference signal light. After calculation, the surface structure information of the entire test surface is obtained. Its range and lateral accuracy are determined by the displacement stage.

[0011] Furthermore, the swept-frequency laser generation unit includes a swept-frequency light source for generating swept-frequency laser and a collimation and beam expansion device for collimating and expanding the swept-frequency laser to obtain spatial swept-frequency light that is easy to detect.

[0012] Furthermore, the frequency sweeping light source is a frequency sweeping light source based on a fiber optic Fabry-Perot tunable filter, with a sweeping range greater than 30 nanometers, a sweeping step size no greater than 10 pm, and a sweeping speed no less than 20 kHz. This ensures high interferometric resolution, high detection accuracy, and a short detection process, while minimizing the impact of environmental noise such as air and temperature changes.

[0013] Furthermore, the collimating and beam expanding device includes a collimator and a beam expander. The output swept laser propagates in the optical fiber and, after passing through the collimator and beam expander, obtains spatial swept light that is easy to detect.

[0014] Specifically, the interference unit includes a beam splitter. The reference surface and the surface to be tested are respectively located in the emission direction of the two laser beams split after the signal laser is irradiated by the beam splitter. After the two laser beams are irradiated by the reference surface and the surface to be tested, they are reflected and return to the beam splitter to be combined into a swept frequency interference beam. The combined swept frequency interference beam is emitted to the scanning detection unit.

[0015] Furthermore, the single-point photodetector includes a photodiode, an avalanche photodiode, or a photomultiplier tube.

[0016] Furthermore, the signal processing unit includes an analog-to-digital converter, a signal processor, and a display.

[0017] The analog-to-digital converter is used to convert electrical signals into digital signals and input them to the signal processor.

[0018] The signal processor is used to perform phase calculation on the digital signal to obtain the distance information of a single point, and restore the surface shape information of the entire surface to be measured, and then display it through the display.

[0019] Specifically, the method for solving the single-point phase is any one of the following: Fourier transform method, phase fitting method, Hilbert transform method, or envelope detection method.

[0020] Furthermore, the single-point phase is solved using the Fourier transform method. The intensity signal is Fourier transformed to the frequency domain, and the phase of the frequency components is extracted.

[0021] Specifically, the steps for solving a single-point phase using the Fourier transform method include:

[0022] The Fourier transform of signal x(t) is expressed by the following formula:

[0023]

[0024] X(f) is a complex function in the frequency domain, where f is the frequency and j is the imaginary unit;

[0025] The complex Fourier transform can be expressed in terms of amplitude and phase:

[0026] X(f)=|X(f)|e jφ(f)

[0027] Where |X(f)| is the amplitude and φ(f) is the phase.

[0028] The phase is calculated using the arctangent function, as follows:

[0029]

[0030] Where Im[X(f)] is the imaginary part of X(f), and Re[X(f)] is the real part of X(f);

[0031] Using the phase inverse calculation formula:

[0032]

[0033] By obtaining the distance difference information between the reference surface and the surface under test, and combining it with the known distance information of the reference surface, the distance information between the surface under test and the frequency sweep light source can be obtained.

[0034] The present invention also provides a method for detecting surface structures by sweep frequency interferometry, using the sweep frequency interferometry surface structure detection system, the method comprising the following steps:

[0035] (1) Place the surface to be measured;

[0036] (2) A signal laser with a frequency that varies with time, generated by the sweeping laser generation unit, enters the interference unit;

[0037] (3) The interference unit splits the signal laser beam and projects it onto the reference surface and the surface to be measured, respectively. After reflection, the beams are combined again to form a swept-frequency interference beam, which then illuminates the scanning detection unit.

[0038] (4) The single-point photodetector receives a complete sweep frequency interference light at one position each time, and then the displacement stage moves the single-point photodetector to receive the complete sweep frequency interference light at the next position until the sweep frequency interference light at each position of the entire test surface is received by the single-point photodetector.

[0039] (5) The signal processing unit calculates the frequency-scanning interference light signals received by the scanning detection unit at each position to obtain the phase difference between the surface to be tested and the reference surface, thereby obtaining the distance information of a single point on the surface to be tested. The distance information at each position is recombined according to the scan number order to obtain the surface structure information of the surface to be tested.

[0040] The beneficial effects of this invention are:

[0041] Compared with the prior art, the present invention uses a single-point photodetector to collect interference light signals, performs phase calculation through single-point calculation, and obtains the distance information of each position on the surface under test by calculating the interference information obtained at each position of the photodetector system, thereby achieving high-precision surface structure detection, which can reach the nanometer level.

[0042] By using a displacement stage to move the detector and detect swept-frequency interference light, large-scale, high-precision measurements can be achieved. Since the receiving speed of a single-point detector is much higher than that of an area array detector, this invention can achieve rapid detection when the scanning range is small. Attached Figure Description

[0043] Figure 1 This is a schematic flowchart of the sweep frequency interference surface structure detection system of the present invention.

[0044] Figure 2 This is a schematic diagram of the swept frequency interference surface structure detection system of the present invention.

[0045] Figure 3 This is a schematic diagram illustrating the principle of the single-point scanning reception of swept frequency interference signals in the swept frequency interference surface structure detection system of the present invention.

[0046] Figure 4 This is a schematic diagram illustrating the principle of a single-point photodetector receiving a swept frequency signal in the swept frequency interference surface structure detection system of this invention. Detailed Implementation

[0047] In this embodiment, as Figure 1 As shown, the swept frequency interferometric surface structure detection system includes a swept frequency laser generation unit 10, an interferometric unit 20, a scanning detection unit 30, and a signal processing unit 40.

[0048] The swept-frequency laser generation unit is mainly used to generate a signal laser whose frequency varies with time and to perform collimation and beam expansion operations. It includes a swept-frequency light source 11 and a collimation and beam expansion device 12. The swept-frequency light source 11 can be a swept-frequency light source based on a fiber Fabry-Perot tunable filter to obtain a swept-frequency laser with the required swept-frequency step size, swept-frequency speed, and swept-frequency light output energy. The center wavelength can be 532 nm, 780 nm, or near-infrared light. The choice of operating wavelength depends on the specific experimental requirements and application scenario. Short wavelengths can be used when a longer coherence length is required, while long wavelengths can be used when a wider interference fringe is required. The swept-frequency range of the swept-frequency light source 11 is greater than 30 nm to ensure high interference resolution; the swept-frequency step size is no greater than 10 pm to ensure high detection accuracy; and the swept-frequency speed is no less than 20 kHz to ensure a short detection process and minimal impact from environmental noise such as air and temperature changes. The output swept-frequency laser propagates in the optical fiber and passes through the collimator and beam expander in the collimation and beam expansion device 12 to obtain spatial swept-frequency light suitable for detection.

[0049] The interferometer unit 20 is used to obtain the interference signal between the reference surface and the surface under test, and includes an interferometer system 21 and a surface under test 22. In this embodiment, the interferometer system 21 can be a Michelson interferometer system, such as... Figure 2 As shown, the system includes a beam splitter 211 and a reference surface 212. The swept laser emitted from the swept light source 11 exits the collimating and expanding device 12 and enters the beam splitter 211, splitting into two beams that respectively irradiate the reference surface 212 and the surface under test 22. After reflection, the beams merge at the beam splitter 211, causing interference. This results in swept interference light containing distance information between the reference surface 212, the surface under test 22, and the swept light source, which then enters the scanning detection unit 30.

[0050] The scanning detection unit 30 is used to receive the spatial light of the swept frequency interference signal, and the specific process is as follows: Figure 3 As shown, the swept-frequency interference light at each location is received by a single-point photodetector 31, which can be a photodiode, avalanche photodiode, or photomultiplier tube. After a complete cycle of swept-frequency interference signal is received at a location, a high-precision displacement stage 32 is used to move the single-point photodetector 31 to achieve reception of the swept-frequency interference signal across the entire detection surface. The single-point photodetector 31 converts the received optical signal into an electrical signal, which then enters the signal processing unit 40.

[0051] The signal processing unit 40 is used to calculate the electrical signal converted from the swept-frequency interference signal received by the single-point photodetector 31. During data processing, it processes data separately for each location. Within a certain period, the signal received by a pixel is the swept-frequency interference signal illuminating a certain location on the object surface, such as... Figure 3 , Figure 4As shown. The signal processing unit 40 includes an analog-to-digital converter 41, a signal processor 42, and a display 43. The electrical signal is sampled and converted into a digital signal by the analog-to-digital converter 41, and then the converted digital signal is input into the signal processor 42. The defect information of the sample is obtained by restoring the scan signal and displayed on the display 43.

[0052] The signal processor 42 performs phase calculation on the swept-frequency interference signal to obtain the distance information of a single point, thereby reconstructing the surface shape information of the entire surface under test. In this embodiment, the Fourier transform method is used to calculate the phase of a single point. The intensity signal is Fourier transformed to the frequency domain, and the phase of the frequency components is extracted. The specific implementation method is as follows:

[0053] The Fourier transform of signal x(t) is expressed by the following formula:

[0054]

[0055] X(f) is a complex function in the frequency domain, where f is the frequency and j is the imaginary unit.

[0056] The complex Fourier transform can be expressed in terms of amplitude and phase:

[0057] X(f)=|X(f)|e jφ(f)

[0058] Where |X(f)| is the amplitude and φ(f) is the phase.

[0059] The phase is calculated using the arctangent function (arctan), as follows:

[0060]

[0061] Here, Im[X(f)] is the imaginary part of X(f), and Re[X(f)] is the real part of X(f).

[0062] Using the phase inverse calculation formula:

[0063]

[0064] The distance difference between the reference surface and the surface under test is obtained. Since the distance information of the reference surface is known, the distance information between the surface under test and the swept frequency light source can be obtained.

[0065] A method for detecting surface structures by sweeping frequency interferometry, using the sweeping frequency interferometry surface structure detection system of the present invention, includes the following steps:

[0066] (1) Place the surface to be tested in the designated position;

[0067] (2) The frequency of the signal laser generated by the sweeping laser generation unit 10 using the sweeping light source 11 based on the fiber Fabry-Perot tunable filter is changed with time. The signal laser is obtained by collimator and beam expander in the collimating and beam expanding device 12 to obtain spatial sweeping light that is easy to detect, and then enters the interference unit 20.

[0068] (3) In the interference unit 20, the signal laser enters the beam splitter 211 and is split into two beams, which are then irradiated onto the reference surface 212 and the surface to be measured 22 respectively. After reflection, they are combined again to form a swept interference light. The swept interference light contains the distance information between the reference surface 212 and the surface to be measured 22 and the swept light source. Then the swept interference light is irradiated into the scanning detection unit 30.

[0069] (4) In the scanning detection unit 30, the single-point photodetector 31 receives the complete sweep frequency interference light at one position each time. Then the displacement stage 32 moves the single-point photodetector 31 to receive the complete sweep frequency interference light at the next position until the sweep frequency interference light at each position of the entire surface under test is received by the single-point photodetector 31. The single-point photodetector 31 converts the received optical signal into an electrical signal and enters the signal processing unit 40.

[0070] (5) The signal processing unit 40 calculates the frequency sweep interference electrical signals received by the scanning detection unit 30 at each position. During data processing, each position is processed separately. Within a certain period of time, the signal received by a pixel is the frequency sweep interference signal that illuminates a certain position of the surface under test. The electrical signal is sampled and converted into a digital signal by the analog-to-digital converter 41. The converted digital signal is then input into the signal processor 42. The phase of the frequency sweep interference signal is calculated to obtain the distance information of a single point, thereby obtaining the distance information of a single point on the surface under test. The distance information of each position is recombined according to the scan number order to obtain the surface structure information of the surface under test, thereby restoring the surface shape information of the entire surface under test, and displaying it through the display 43.

[0071] When solving for single-point phase, Fourier transform, phase fitting, Hilbert transform, or envelope detection methods can be used.

[0072] In the method of the present invention, the Fourier transform method is used to solve the single-point phase, the intensity signal is Fourier transformed to the frequency domain, and the phase of the frequency component is extracted.

[0073] Specifically, the steps for solving a single-point phase using the Fourier transform method are as described above.

[0074] The distance difference between the reference surface and the surface under test can be obtained by solving the Fourier transform method. Combined with the known distance information of the reference surface, the distance information between the surface under test and the swept frequency light source can be obtained.

Claims

1. A swept-frequency interferometric surface structure detection system, characterized in that, include: A frequency-sweeping laser generation unit is used to generate a signal laser whose frequency varies with time, wherein the frequency sweeping speed of the frequency-sweeping laser generation unit is not less than 20kHz; An interference unit, including a reference surface, splits the signal laser beam and projects it onto the reference surface and the surface to be measured, respectively. After reflection, the beams are then combined again to form a swept interference beam. A scanning detection unit is used to receive the swept frequency interference light and convert the optical signal into an electrical signal. The scanning detection unit includes a single-point photodetector and a displacement stage for controlling the movement of the single-point photodetector. The single-point photodetector receives the complete swept frequency interference light at one position each time. Then the displacement stage moves the single-point photodetector to receive the complete swept frequency interference light at the next position until the swept frequency interference light at each position of the entire surface under test is received by the single-point photodetector. The signal processing unit is used to calculate the electrical signals converted from the frequency-scanning interference optical signals received by the scanning detection unit at each position, obtain the phase difference between the surface under test and the reference surface, and thus obtain the distance information of a single point on the surface under test. The obtained distance information at each position is recombined according to the scan number order to obtain the surface structure information of the surface under test.

2. The swept-frequency interferometric surface structure detection system according to claim 1, characterized in that, The swept-frequency laser generation unit includes a swept-frequency light source for generating swept-frequency laser and a collimation and beam-expanding device for collimating and expanding the swept-frequency laser.

3. The swept-frequency interferometric surface structure detection system according to claim 2, characterized in that, The frequency sweep light source is a frequency sweep light source based on a fiber Fabry-Perot tunable filter, with a frequency sweep range greater than 30 nanometers and a frequency sweep step size of no more than 10 pm.

4. The swept-frequency interferometric surface structure detection system according to claim 2, characterized in that, The collimation and beam expanding device includes a collimator and a beam expander.

5. The swept-frequency interferometric surface structure detection system according to claim 1, characterized in that, The interference unit includes a beam splitter. The reference surface and the surface to be tested are respectively located in the emission direction of the two laser beams split after the signal laser is irradiated by the beam splitter. After the two laser beams are irradiated by the reference surface and the surface to be tested, they are reflected and return to the beam splitter to be combined into a swept frequency interference beam. The combined swept frequency interference beam is emitted to the scanning detection unit.

6. The swept-frequency interferometric surface structure detection system according to claim 1, characterized in that, The single-point photodetector includes a photodiode, an avalanche photodiode, or a photomultiplier tube.

7. The swept-frequency interferometric surface structure detection system according to claim 1, characterized in that, The signal processing unit includes an analog-to-digital converter, a signal processor, and a display. The analog-to-digital converter is used to convert electrical signals into digital signals and input them to the signal processor. The signal processor is used to perform phase calculation on the digital signal to obtain the distance information of a single point, and restore the surface shape information of the entire surface to be measured, and then display it through the display.

8. The swept-frequency interferometric surface structure detection system according to claim 7, characterized in that, The method for solving the single-point phase is any one of the following: Fourier transform method, phase fitting method, Hilbert transform method, or envelope detection method.

9. The swept-frequency interferometric surface structure detection system according to claim 8, characterized in that, The Fourier transform method is used to solve for the single-point phase. This involves performing a Fourier transform on the intensity signal to the frequency domain and extracting the phase of the frequency components. The specific steps for solving for the single-point phase using the Fourier transform method include: The Fourier transform of signal x(t) is expressed by the following formula: ， X(f) is a complex function in the frequency domain, where f is the frequency and j is the imaginary unit; The complex Fourier transform can be expressed in terms of amplitude and phase: ， Where |X(f)| is the amplitude. It is phase. The phase is calculated using the arctangent function, as follows: ， Where Im[X(f)] is the imaginary part of X(f), and Re[X(f)] is the real part of X(f); Using the phase inverse calculation formula: ； By obtaining the distance difference information between the reference surface and the surface under test, and combining it with the known distance information of the reference surface, the distance information between the surface under test and the frequency sweep light source can be obtained.

10. A method for detecting surface structures using swept-frequency interferometry, characterized in that, Using the swept-frequency interferometric surface structure detection system according to any one of claims 1 to 9, the swept-frequency interferometric surface structure detection method includes the following steps: (1) Place the surface to be tested; (2) A signal laser with a frequency that varies with time, generated by the frequency-sweeping laser generation unit, enters the interference unit; (3) The interference unit splits the signal laser beam and illuminates the reference surface and the surface to be measured respectively, and after reflection, the beams are combined again to form a swept-frequency interference beam, which illuminates the scanning detection unit. (4) The single-point photodetector receives a complete sweep frequency interference light at one position each time, and then the displacement stage moves the single-point photodetector to receive the complete sweep frequency interference light at the next position until the sweep frequency interference light at each position of the entire test surface is received by the single-point photodetector. (5) The signal processing unit calculates the frequency-scanning interference light signals received by the scanning detection unit at each position to obtain the phase difference between the surface to be tested and the reference surface, thereby obtaining the distance information of a single point on the surface to be tested. The distance information at each position is recombined according to the scan number order to obtain the surface structure information of the surface to be tested.

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