A device and method for improving white light interferometry sampling rate based on automatic tracking

Abstract

The present invention discloses a device and method for improving the sampling rate of white light interference based on automatic tracking. The device is composed of a broadband light source, a reference arm, a measuring arm, a fiber coupler, a motion control unit, and a signal receiving and processing module. The method of the present invention first uses a linear displacement stage to find the equal optical path point and then stops the movement. Then the piezoelectric scanning stage starts working and continuously measures the surface of the sample to be measured. By setting an automatic tracking threshold, when the threshold is exceeded, the linear displacement stage moves to a new center stripe position, so that the center stripe position is again near the middle zero position of the piezoelectric scanning stage, and finally completes the automatic tracking. The method of the present invention will not cause the piezoelectric scanning stage to be unable to find the equal optical path point due to the fluctuation of the surface to be measured. The present invention can realize automatic tracking measurement of unknown surfaces without setting a measurement path and unknown measured surface information, and utilizes the high-frequency scanning characteristics of the piezoelectric scanning stage to improve the sampling rate and measurement speed.

Description

Technical Field

[0001] The present invention belongs to the field of interference measurement, and in particular relates to a device and method for improving white light interference sampling rate based on automatic tracking. Background Art

[0002] With the rapid development of ultra-precision machining, the precise measurement of precision parts and micro-nanostructures has become an important research direction. White light interferometry technology has the advantages of non-contact and high precision, making it a very suitable method for measuring the three-dimensional topography of micro-nanostructures. Summary of the Invention

[0003] The present invention aims to provide a method for improving the sampling rate of white light interference based on automatic tracking, so as to ensure that no light is lost during the measurement process and achieve high-speed measurement at the same time.

[0004] The objective of the present invention is achieved through the following technical solutions: A first aspect of an embodiment of the present invention provides a device for improving the white light interferometry sampling rate based on automatic tracking, comprising: a broadband light source, a reference arm, a measuring arm, a fiber optic coupler, a motion control unit and a signal receiving and processing module; one side of the fiber optic coupler is connected to the reference arm and the measuring arm respectively; the other side of the fiber optic coupler is connected to the broadband light source and the signal receiving and processing module respectively; the measuring arm and the reference arm output measuring light and reference light respectively; the signal receiving and processing module is used to receive analog signals of the measuring light and the reference light output by the measuring arm and the reference arm, and process and generate digital signals corresponding to the measuring arm and the reference arm; the motion control unit receives the digital signal and outputs an analog signal to control the measuring arm and the reference arm to measure the sample to be measured.

[0005] Furthermore, the broadband light source is one of an LED light source, an SLD light source, and an ASE light source.

[0006] Furthermore, the reference arm includes a linear translation stage, a piezoelectric scanning stage, a plane reflector and a collimating mirror; wherein the linear translation stage is connected to the piezoelectric scanning stage through an adapter plate, and the plane reflector is installed on the piezoelectric scanning stage.

[0007] Furthermore, the measuring arm includes an x-axis and z-axis translation stage, a transfer rod, a measuring probe, and a sample to be measured; a z-axis stage is provided on one side of the z-axis of the x-z two-axis translation stage, and is connected to the measuring probe through the transfer rod for vertical movement; an x-axis stage is provided on one side of the x-axis of the x-z two-axis translation stage, and the sample to be measured is placed on the x-axis stage for horizontal movement.

[0008] Furthermore, the reference light output by the fiber coupler is collimated by a collimator and then irradiated on a plane mirror. After being reflected by the plane mirror, it returns to the fiber coupler along the original optical path. The piezoelectric scanning stage drives the plane mirror to perform reciprocating motion to change the optical path of the reference arm. The fiber coupler is connected to the measuring probe through an optical fiber and outputs the measuring light to the sample to be measured, where it is reflected. The reflected light returns along the original path and interferes with the reflected light at the reference arm in the fiber coupler. Measurement is achieved by the equal optical path point corresponding to the central fringe of the interference pattern.

[0009] Furthermore, the signal receiving and processing module includes a host computer, a data acquisition and analysis instrument, and a photoelectric detector; the optical fiber coupler receives the reflected light of the reference arm and the measuring arm, and interference occurs in the optical fiber coupler, and the interference signal is transmitted to the photoelectric detector, which outputs an interference electrical signal. The data acquisition and analysis instrument converts the interference electrical signal into a digital signal and sends it to the host computer for processing. The host computer processes the digital signal to generate digital signals corresponding to the measuring arm and the reference arm; the motion control unit receives the digital signal and outputs an analog signal to control the stage in the measuring arm and the linear translation stage and piezoelectric scanning stage in the reference arm to measure the sample to be measured.

[0010] A second aspect of an embodiment of the present invention provides a method for improving the white light interferometry sampling rate based on automatic tracking, which is applied to the above-mentioned device for improving the white light interferometry sampling rate based on automatic tracking. The method comprises the following steps:

[0011] (1): Start the device, turn on the broadband light source, and preheat the light source to ensure stable light source output;

[0012] (2): Place the sample to be measured on the x-axis stage and adjust the pitch and yaw of the measuring probe so that the measuring light is incident vertically on the surface to be measured;

[0013] (3): Control the z-axis stage to drive the measuring probe to move in the vertical direction so that the surface to be measured is near the focus of the measuring probe;

[0014] (4): Set the power threshold a1 to control the linear stage at the reference arm to start horizontal reciprocating motion from the middle zero position; use the data acquisition analyzer to collect the light intensity signal received by the photodetector and the position of the linear stage in real time. When the power detected is greater than a1, it indicates that an interference pattern has appeared and the reflector has passed the equal optical path point. At this time, the linear stage stops moving, and the position d1 of the linear stage corresponding to the power peak is fed back to the motion controller, which controls the stage to move to d1 and then stop.

[0015] (5): Set the automatic tracking threshold a2 and start the piezoelectric scanning stage to scan. At this time, the position d2 of the piezoelectric scanning stage corresponding to the central fringe position d of the interference pattern is obtained by the envelope extraction algorithm;

[0016] (6): The host computer sends a command to control the movement of the x-axis translation stage to measure the entire surface of the sample to be measured. When the center stripe position d is less than d2-a2 or greater than d2+a2, it indicates that it exceeds the set threshold. The position d3 of the piezoelectric scanning stage at this time is fed back to the host computer and the linear translation stage is controlled to move to the d1+d3 or d1-d3 position and then stop, keeping the equal optical path point near the middle zero position of the piezoelectric scanning stage; completing automatic tracking.

[0017] Furthermore, in step (4), the data acquisition and analysis instrument further reduces noise and removes DC components by filtering the light intensity signal, and then estimates the power peak after Fourier transformation.

[0018] Furthermore, step (6) further includes: taking t as a period, detecting the central fringe position d of the last interference pattern among the multiple interference patterns collected in each period, and judging whether d exceeds a threshold value; if not, the linear translation stage remains stationary; if exceeded, the linear translation stage moves to a predetermined position in the next period.

[0019] The beneficial effects of the present invention are as follows: the method of the present invention combines the advantages of the wide range of motion of the linear translation stage with the high precision, fast response and high-frequency scanning of the piezoelectric scanning stage. First, the linear translation stage stops moving after finding the equal optical path point, and then the piezoelectric scanning stage starts working to continuously measure the surface of the sample to be measured. Since the undulation of the surface to be measured will cause the equal optical path point to change continuously, and the scanning range of the piezoelectric scanning stage is often less than 1 mm, when the undulation of the surface to be measured is large, the equal optical path point can easily exceed the scanning range. At this time, by setting an automatic tracking threshold, when the threshold is exceeded, the linear translation stage moves to a new center stripe position, so that the center stripe position is again near the middle zero position of the piezoelectric scanning stage, and finally completes the automatic tracking. The method of the present invention ensures that during the measurement process, the piezoelectric scanning stage will not be unable to find the equal optical path point due to the undulation of the surface to be measured; the present invention can realize automatic tracking measurement of unknown surfaces without setting a measurement path and unknown surface information through the combination of software and hardware, and utilizes the high-frequency scanning characteristics of the piezoelectric scanning stage to improve the sampling rate and speed up the measurement speed of the surface to be measured. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the detection device structure of the present invention

[0021] Figure 2 Schematic diagram of the measuring arm structure of the present invention

[0022] Figure 3 Schematic diagram of the reference arm structure of the present invention

[0023] Figure 4 Flowchart of the method for improving the white light interference sampling rate of the present invention DETAILED DESCRIPTION

[0024] To make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings. In the absence of conflict, the features of the following embodiments and implementations can be combined with each other.

[0025] like Figure 1 、 Figure 2 、 Figure 3 As shown, an embodiment of the present invention provides a device for improving the white light interferometry sampling rate based on automatic tracking, including a measuring arm 1, a reference arm 2, single-mode optical fibers 3 and 4, an optical fiber coupler 5, single-mode optical fibers 6 and 7, a photodetector 8, a motion controller 9, a broadband light source 10, a data acquisition and analysis instrument 11, and a host computer 12; the measuring arm 1 includes an xz two-axis translation stage 101, a z-axis stage 102, an adapter rod 103, a measuring probe 104, a sample to be measured 105, and an x-axis stage 106; the reference arm 2 includes a plane mirror 201, a piezoelectric scanning stage 202, an adapter plate 203, a linear translation stage 204, and a collimating mirror 205.

[0026] One side of the fiber coupler 5 is connected to the reference arm 2 and the measuring arm 1 respectively; the other side of the fiber coupler 5 is connected to the broadband light source 10 and the signal receiving and processing module through single-mode optical fibers 6 and 7 respectively; the measuring arm 1 and the reference arm 2 output the measuring light and the reference light respectively through the single-mode optical fibers 3 and 4; the signal receiving and processing module is used to receive the analog signals of the measuring light and the reference light output by the measuring arm 1 and the reference arm 2, and process them to generate digital signals corresponding to the measuring arm 1 and the reference arm 2; the motion control unit receives the digital signal and outputs an analog signal to control the measuring arm 1 and the reference arm 2 to measure the sample 105 to be measured.

[0027] Preferably, the broadband light source is one of an LED light source, an SLD light source, and an ASE light source.

[0028] like Figure 2 As shown, the measuring arm 1 includes an x-axis translation stage 101, a transfer rod 103, a measuring probe 104, and a sample to be measured 105; a z-axis stage 102 is provided on the z-axis side of the x-z translation stage 101, and is connected to the measuring probe 104 via the transfer rod 103, and the z-axis stage 102 moves in the vertical direction; an x-axis stage 106 is provided on the x-axis side of the x-z translation stage, and the sample to be measured 105 is placed on the x-axis stage 106, and the x-axis stage 106 moves in the horizontal direction.

[0029] like Figure 3As shown, the reference arm 2 includes a linear translation stage 204, a piezoelectric scanning stage 202, a plane mirror 201 and a collimating mirror 205; wherein the linear translation stage 204 is connected to the piezoelectric scanning stage 202 through an adapter plate 203, and the plane mirror 201 is installed on the piezoelectric scanning stage 202.

[0030] The signal receiving and processing module includes a host computer, a data acquisition and analysis instrument, and a photoelectric detector; the optical fiber coupler receives the reflected light of the reference arm and the measuring arm, and interference occurs in the optical fiber coupler, and the interference signal is transmitted to the photoelectric detector, which outputs an interference electrical signal. The data acquisition and analysis instrument converts the interference electrical signal into a digital signal and sends it to the host computer for processing. The host computer processes the digital signal to generate digital signals corresponding to the measuring arm and the reference arm; the motion control unit receives the digital signal and outputs an analog signal to control the stage in the measuring arm and the linear translation stage and piezoelectric scanning stage in the reference arm to measure the sample to be measured.

[0031] Furthermore, the reference light output by the fiber coupler is collimated by a collimator and then irradiated on a plane mirror. After being reflected by the plane mirror, it returns to the fiber coupler along the original optical path. The piezoelectric scanning stage drives the plane mirror to perform reciprocating motion to change the optical path of the reference arm. The fiber coupler is connected to the measuring probe through an optical fiber and outputs the measuring light to the sample to be measured, where it is reflected. The reflected light returns along the original path and interferes with the reflected light at the reference arm in the fiber coupler. Measurement is achieved by the equal optical path point corresponding to the central fringe of the interference pattern.

[0032] In an embodiment of the present invention, the light emitted by the broadband light source 10 passes through the single-mode optical fiber 6 and reaches the optical fiber coupler 5, where it is divided into two paths. One path is the reference light, which is emitted from the collimator 205 through the single-mode optical fiber 4 and hits the plane mirror 201. It is then reflected by the plane mirror and returns to the coupler 5 along the original path. The plane mirror 201 is mounted on a piezoelectric scanning stage 202, which is in turn mounted on a linear translation stage 204 through an adapter plate 203. The piezoelectric scanning stage 202 is used in conjunction with the linear translation stage 204 to drive the plane mirror 201 to reciprocate in the horizontal direction, thereby changing the optical path of the reference light. The other beam of light is the measurement light, which is vertically incident on the measured surface of the sample 105 through the single-mode optical fiber 3 by the measuring probe 104. In order to ensure that the reflected light from the measured surface is strong enough to cause interference, it is necessary to control the z-axis stage 102 and the x-axis stage 106 on the xz two-axis translation stage 101 through the motion controller 9 to drive The probe 104 and the sample to be measured 105 are in appropriate relative positions so that the surface to be measured is near the focus of the measuring probe 104; the reflected light from the reference arm and the reflected light from the measuring arm will meet and interfere with each other in the optical fiber coupler 5, and the generated interference signal is received by the photodetector 8 through the single-mode optical fiber 7, and is sent to the host computer 12 for processing and calculation through the data acquisition and analysis instrument 11, so as to obtain the position information of a certain point on the surface of the sample to be measured 105, and the measurement is achieved by corresponding the central fringe of the interference pattern to the equal optical path point; in order to obtain the complete morphology of the sample to be measured 105, it is necessary to control the x-axis stage 106 to drive the sample to be measured 105 to move in the horizontal direction. Since the surface of the sample to be measured 105 is not completely flat, the undulating surface may reach the equal optical path point and is not within the scanning range of the piezoelectric scanning stage 202, resulting in light loss. To avoid such a situation, the scanning position is continuously adjusted through automatic tracking to achieve no light loss and high-speed measurement during the measurement process.

[0033] Combine Figure 4 The embodiment of the present invention proposes a method for improving the white light interferometry sampling rate based on automatic tracking, which is applied to the above-mentioned device for improving the white light interferometry sampling rate based on automatic tracking. The method includes the following six steps:

[0034] (1): Start the device, turn on the broadband light source 10, and preheat the light source for about 30 minutes to ensure stable light source output.

[0035] (2): Place the sample 105 to be measured on the x-axis stage 106 of the xz two-axis translation stage 101, and adjust the pitch and yaw of the measuring probe 104 so that the measuring light is vertically incident on the surface of the sample 105 to be measured.

[0036] (3): Control the z-axis stage 102 on the xz two-axis translation stage 101 to drive the measuring probe 104 to move up and down, so that the surface of the sample to be measured 105 is near the focus of the measuring probe.

[0037] (4): Customize the power threshold a1 on the host computer 12, and control the linear translation stage 204 at the reference arm to move from the middle zero position to the negative direction to the end, and then move from the negative end to the positive end. During this process, the data acquisition analyzer 11 continuously and synchronously collects the light intensity signal received by the photodetector 8 and the position of the linear translation stage 204 in real time, and reduces noise and removes DC components by filtering the light intensity signal. After Fourier transformation, the power peak is estimated. When the power is detected to be greater than a1, it indicates that an interference pattern appears and the plane mirror 201 passes the equal optical path point. At this time, the linear translation stage 204 stops moving, and the position d1 of the linear translation stage 204 corresponding to the power peak is fed back to the motion controller 9, and the control translation stage stops after moving to d1, thereby ensuring that the plane mirror 201 is near the equal optical path point.

[0038] (5): Customize the automatic tracking threshold a2 and start the piezoelectric scanning stage 202. The piezoelectric scanning stage 202 quickly scans the full stroke from the -b position to the +b position. At this time, the position d2 of the piezoelectric scanning stage 202 corresponding to the center fringe position d of the interference pattern is obtained through the envelope extraction algorithm (such as Hilbert transform, wavelet transform, etc.); since the linear translation stage 204 stops at the equal optical path point position d1, d2 is close to d1 in size and is also close to the middle zero position of the piezoelectric scanning stage 202. In this way, the measurement range of the piezoelectric scanning stage 202 can be from the middle zero position to the negative end or the positive end, thereby improving the measurement capability.

[0039] (6): The host computer 12 sends a command to the motion controller 9 to control the x-axis stage 106 on which the sample to be tested 105 is placed to move horizontally to measure the entire surface of the sample to be tested 105. Since the surface to be tested is not absolutely flat, the undulating surface to be tested will cause the equal optical path point to change, so that the equal optical path point originally at the position d2 will shift to the positive or negative direction. When the center stripe position d is less than d2-a2 or greater than d2+a2, it indicates that the set threshold is exceeded, and the position d3 of the piezoelectric scanning stage 202 at this time is fed back to the host computer 12 and the linear translation stage 204 is controlled to move to the d1±d3 position and then stop. When the equal optical path point shifts toward the piezoelectric scanning stage 202 in the positive direction, the sign is positive, otherwise it is negative, so as to achieve the equal optical path point based on the piezoelectric scanning stage 202. The old one is near the middle zero position of the piezoelectric scanning stage 202; since the piezoelectric scanning stage 202 does not stop moving during the automatic tracking process, in order to prevent the linear translation stage 204 from being unable to reach the predetermined position normally due to the continuous change of the equal optical path point during the movement, the central fringe position d of the last interference pattern in the several interference patterns collected in each period is detected with t as a period, and it is judged whether d exceeds the threshold value a2. If not, the linear translation stage 204 is stationary. If it exceeds, the linear translation stage 1 is moved to the predetermined position in the next period, and the automatic tracking is finally completed. Through automatic tracking, the high-frequency scanning characteristics of the piezoelectric scanning stage are combined with the large motion range of the linear translation stage to achieve fast and accurate measurement of the sample to be measured.

[0040] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A device for improving white light interferometry sampling rate based on automatic tracking, characterized in that: include: Broadband light source, reference arm, measurement arm, fiber coupler, motion control unit and signal receiving and processing module; The reference arm includes a linear translation stage, a piezoelectric scanning stage, a plane reflector and a collimating mirror; wherein the linear translation stage is connected to the piezoelectric scanning stage through an adapter plate, and the plane reflector is installed on the piezoelectric scanning stage; The measuring arm includes an x-axis and z-axis translation stage, a transfer rod, a measuring probe, and a sample to be measured. A z-axis stage is provided on the z-axis side of the x-axis translation stage, and is connected to the measuring probe via the transfer rod for vertical movement. An x-axis stage is provided on the x-axis side of the x-axis translation stage, and the sample to be measured is placed on the x-axis stage for horizontal movement. The signal receiving and processing module includes a host computer, a data acquisition and analysis instrument, and a photoelectric detector; One side of the fiber coupler is connected to the reference arm and the measuring arm respectively; the other side of the fiber coupler is connected to the broadband light source and the signal receiving and processing module respectively; the measuring arm and the reference arm output measurement light and reference light respectively; the signal receiving and processing module is used to receive the analog signals of the measurement light and reference light output by the measuring arm and the reference arm, and process them to generate digital signals corresponding to the measuring arm and the reference arm; the motion control unit receives the digital signals and outputs analog signals to respectively control the x-axis stage and z-axis stage of the measuring arm, the linear translation stage of the reference arm, and the piezoelectric scanning stage to measure the sample to be measured; The reference light output by the fiber coupler is collimated by a collimator and then irradiated on a plane mirror. After being reflected by the plane mirror, it returns to the fiber coupler along the original optical path. The piezoelectric scanning stage drives the plane mirror to do reciprocating motion to change the optical path of the reference arm. The fiber coupler connects to the measuring probe via an optical fiber and outputs the measuring light to the sample to be measured, where it is reflected. The reflected light returns along the original path and interferes with the reflected light from the reference arm in the fiber coupler. The measurement is achieved by aligning the central fringe of the interference pattern with the equal optical path point. First, the linear translation stage is used to find the equal-path point and then stop moving. Then the piezoelectric scanning stage starts working and continuously measures the surface of the sample to be measured. Since the undulation of the surface to be measured will cause the equal-path point to change continuously, and the scanning range of the piezoelectric scanning stage is often less than 1mm, when the surface to be measured has large undulations, the equal-path point can easily exceed the scanning range. At this time, an automatic tracking threshold is set. When the threshold is exceeded, the linear translation stage moves to the new center stripe position, so that the center stripe position is again near the middle zero position of the piezoelectric scanning stage, and finally the automatic tracking is completed.

2. The device for improving white light interferometry sampling rate based on automatic tracking according to claim 1, characterized in that: The broadband light source is one of an LED light source, an SLD light source, and an ASE light source.

3. The device for improving white light interferometry sampling rate based on automatic tracking according to claim 1, characterized in that: The fiber optic coupler receives the reflected light from the reference arm and the measuring arm, and interference occurs in the fiber optic coupler, and the interference signal is transmitted to the photoelectric detector, which outputs an interference electrical signal. The data acquisition and analyzer converts the interference electrical signal into a digital signal and sends it to the host computer for processing. The host computer processes the digital signal to generate digital signals corresponding to the measuring arm and the reference arm; the motion control unit receives the digital signal and outputs an analog signal to control the x-axis stage and z-axis stage in the measuring arm and the linear translation stage and piezoelectric scanning stage in the reference arm to measure the sample to be measured.

4. A method for improving the sampling rate of white light interferometry based on automatic tracking, applied to the device for improving the sampling rate of white light interferometry based on automatic tracking according to any one of claims 1 to 3, characterized in that: The method comprises the following steps: (1): Start the device, turn on the broadband light source, and preheat the broadband light source to ensure stable output of the broadband light source; (2): Place the sample to be measured on the x-axis stage and adjust the pitch and yaw of the measuring probe so that the measuring light is incident vertically on the surface of the sample to be measured; (3): Control the z-axis stage to drive the measuring probe to move in the vertical direction so that the surface of the sample to be measured is near the focus of the measuring probe; (4): Set the power threshold a1 and control the linear translation stage of the reference arm to start horizontal reciprocating motion from the middle zero position; use the data acquisition analyzer to collect the light intensity signal received by the photoelectric detector and the position of the linear translation stage in real time. When the power detected is greater than a1, it indicates that an interference pattern appears and the reflector passes the equal optical path point. At this time, the linear translation stage stops moving, and the position d1 of the linear translation stage corresponding to the power peak is fed back to the motion controller, which controls the translation stage to move to d1 and then stop; (5): Set the automatic tracking threshold a2 and start the piezoelectric scanning stage to scan. At this time, the position d2 of the piezoelectric scanning stage corresponding to the central fringe position d of the interference pattern is obtained by the envelope extraction algorithm; (6): The host computer sends a command to control the movement of the x-axis translation stage to measure the entire surface of the sample to be measured. When the center stripe position d is less than d2-a2 or greater than d2+a2, it indicates that it exceeds the set threshold. The position d3 of the piezoelectric scanning stage at this time is fed back to the host computer and the linear translation stage is controlled to move to the d1+d3 or d1-d3 position and then stop, keeping the equal optical path point near the middle zero position of the piezoelectric scanning stage; completing automatic tracking.

5. The method for improving white light interferometry sampling rate based on automatic tracking according to claim 4, characterized in that: In step (4), the data acquisition analyzer further reduces noise and removes DC components by filtering the light intensity signal, and then estimates the power peak after Fourier transform.

6. The method for improving white light interferometry sampling rate based on automatic tracking according to claim 5, characterized in that: Step (6) further includes: taking t as a period, detecting the center fringe position d of the last interference pattern in the plurality of interference patterns collected in each period, and judging whether d exceeds a threshold value; if not, the linear translation stage remains stationary; if exceeded, the linear translation stage moves to a predetermined position in the next period.

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