Device for realizing large-stroke direct traceability type grating interferometer

Abstract

The invention discloses a device for realizing a large-stroke direct traceability type grating interferometer, which belongs to the technical field of precise displacement measurement, and comprises a grating bottom plate, a coherent light source and an optical fiber beam splitter, a plurality of gratings are arranged on the grating bottom plate, and the gratings comprise a first grating, a second grating, a third grating, a fourth grating and a fifth grating; the optical gratings are arranged on the optical grating bottom plate in a staggered manner, the optical gratings are all self-tracing optical gratings, the coherent light source is connected with the optical fiber beam splitter, the output end of the optical fiber beam splitter is connected with two reading heads, the two reading heads are a first reading head and a second reading head respectively, and the other ends of the first reading head and the second reading head are connected with a signal processing unit. The grating bottom plate is driven by the displacement table to perform one-dimensional translation, and the first reading head and the second reading head are fixed. By adopting the device, the problem that an existing large-stroke grating interferometer is difficult to directly trace is solved, and the device has the advantages of range self-calibration splicing, large expansion space and embedded use.

Description

Technical Field

[0001] This invention relates to the field of precision displacement measurement technology, and in particular to a device for realizing a large-stroke direct-trace grating interferometer. Background Technology

[0002] In integrated circuit manufacturing, the performance of the lithography machine directly determines the chip's fabrication process and integration density. As a core subsystem of the lithography machine, the stage bears the crucial task of precisely positioning the wafer; its motion and control accuracy are the fundamental guarantee for the chip's pattern overlay accuracy and linewidth uniformity. Currently, advanced lithography machines generally use 12-inch wafers, which requires the stage to have a large motion stroke of over 300mm. However, achieving and maintaining ultra-high positioning accuracy at the nanometer or even sub-nanometer level within such a large range is a formidable challenge.

[0003] Among numerous displacement measurement techniques, grating-based interferometry offers advantages over laser wavelength-based techniques, including higher precision, greater stability, and stronger anti-interference capabilities. Therefore, it is widely used in precision manufacturing and testing, as well as in the integrated circuit field. Grating-based displacement measurement relies on the grating pitch as its measurement reference; thus, the accuracy of the grating pitch is a crucial factor in displacement measurement. Ordinary gratings require metrological calibration to determine their pitch, and the uniformity of the grating pitch is difficult to guarantee.

[0004] A self-traceable grating is a high-precision grating manufactured using atomic lithography technology. Its grating pitch is directly traceable to the atomic transition frequency, thus offering advantages such as direct traceability and uniform grating pitch. Displacement measurement based on self-traceable gratings includes three methods: heterodyne measurement, zero-difference measurement, and self-mixing measurement. Grating interferometers using the grating pitch as the measurement reference offer advantages such as high stability and direct traceability, and existing self-traceable grating interferometers can serve as standards for calibration instruments. However, the limited size of a single self-traceable grating severely restricts the measurement range of the grating interferometer, making it difficult to meet the demands of large-stroke displacement measurement in the integrated circuit manufacturing field.

[0005] Based on this, the present invention proposes a device for realizing a large-stroke direct traceability grating interferometer. Summary of the Invention

[0006] The purpose of this invention is to provide a device for realizing a large-stroke direct traceability grating interferometer, which solves the problem that existing large-stroke grating interferometers are difficult to trace directly, and has the advantages of range self-calibration splicing, large expansion space, and embedded use.

[0007] To achieve the above objectives, the present invention provides an apparatus for realizing a large-stroke direct-traceability grating interferometer, comprising a grating base plate, a coherent light source, and an optical fiber beam splitter. The grating base plate is provided with multiple gratings, including a first grating, a second grating, a third grating, a fourth grating, and a fifth grating. The gratings are arranged in an alternating pattern on the grating base plate, with four overlapping regions (R1, R2, R3, and R4) between adjacent gratings in the grating period direction. All gratings are self-traceable gratings. The coherent light source and the optical fiber beam splitter are connected. The output end of the optical fiber beam splitter is connected to two reading heads, namely a first reading head and a second reading head. The other end of both the first and second reading heads is connected to a signal processing unit. The grating base plate is driven by a displacement stage to perform one-dimensional translation, while the first and second reading heads remain stationary.

[0008] Preferably, the coherent light source guides the laser into the first and second reading heads through an optical fiber beam splitter. The first and second reading heads incident the laser onto the grating surface, where it diffracts with the grating and returns to the reading heads to form an interference signal. The interference signal is transmitted to the signal processing unit, and the grating displacement and direction of motion are obtained through calculation.

[0009] Preferably, the periodic direction of the first grating is consistent with the motion axis of the displacement stage, and the reading head and the self-traceable grating are ideally installed. The spacing between adjacent self-traceable gratings is adjusted so that there is an overlapping area of ​​interference signals when the reading head passes through. The spatial attitude error of adjacent self-traceable gratings is calibrated by using the theoretical spacing of the self-traceable gratings.

[0010] Preferably, the process of calibrating the spatial attitude error of adjacent self-traceable gratings using the theoretical spacing of the self-traceable gratings is as follows: the cosine error is calculated by the ratio of the number of cycles of the original signals acquired by the two read heads in the overlapping area of ​​the interference signals, so that the length reference on all measurement strokes is the theoretical spacing of the self-traceable gratings. The ideal installation method for the self-traceable grating and the reading head is as follows: the laser is incident on the self-traceable grating at a Litterow angle and returns along the original path.

[0011] Preferably, the reading head adopts any displacement measurement method selected from zero-difference interferometry, heterodyne interferometry, and self-mixing interferometry, and the reading head and its corresponding self-traceable grating are set in the same motion plane.

[0012] Preferably, the coherent light source is a 405nm narrow linewidth laser; the fiber beam splitter is a single-mode polarization-maintaining fiber; the signal processing unit has four channels with a sampling rate of 100MHz and a sampling bit depth of 16 bits, used to convert the acquired voltage signals into digital signals, and to perform displacement calculation and error calibration on the four signals through an FPGA.

[0013] Preferably, the first reading head and the second reading head use the same optical path. When both the first and second reading heads use the zero-difference interference displacement measurement method, the optical path structure of the first and second reading heads includes an optical fiber collimator, a diffraction light generation unit, and a signal receiving unit. The diffraction light generation unit includes a fourth waveplate, a polarizing beam splitter, a first waveplate, a second waveplate, a first reflector, a second reflector, and a third reflector. The signal receiving unit includes a third waveplate, a non-polarizing beam splitter, a first polarizer, a second polarizer, a first detector, and a second detector. The optical fiber collimator is connected to the polarizing beam splitter through the first reflector and the fourth waveplate. The polarizing beam splitter is connected to the second waveplate, the first waveplate, and the third waveplate. The second waveplate is connected to the third reflector. The first waveplate is connected to the second reflector. The third waveplate is connected to the polarizing beam splitter. The polarizing beam splitter is connected to the first linear polarizer and the second linear polarizer. The first linear polarizer is connected to the first photodetector. The second linear polarizer is connected to the second photodetector. Both the first and second photodetectors are connected to the signal processing unit.

[0014] Preferably, the fourth waveplate is a half-waveplate, and the first, second, and third waveplates are quarter-waveplates. The fast axes of the first, second, and third waveplates are all at 45° to the plane of light incidence. The polarization direction of the first linear polarizer is at 45° to the plane of light incidence, and the polarization direction of the second linear polarizer is at 22.5° to the plane of light incidence.

[0015] Preferably, the laser beam enters from the fiber collimator, passes through the first reflecting mirror, and enters the diffraction light generating unit. The rotatable fourth waveplate is adjusted so that the polarization direction of the incident laser beam is at 45° with the incident plane. The laser beam is decomposed into two components, vertical polarization and horizontal polarization, by the polarization beam splitter. The vertically polarized light passes through the second waveplate and becomes right-hand circularly polarized light. The right-hand circularly polarized light passes through the third reflecting mirror and enters the self-tracing grating at the Littrow angle. The incident light diffracts at the self-tracing grating. The -1st order diffracted light returns along the original path, passes through the second waveplate again, and becomes horizontally polarized light. The horizontally polarized light is reflected after passing through the polarization beam splitter and enters the signal receiving unit. Another path of horizontally polarized light reflected by the polarizing beam splitter becomes left-handed circularly polarized light after passing through the first waveplate. The left-handed circularly polarized light is incident on the self-tracing grating at the Littrow angle after passing through the second mirror. The incident light is diffracted on the self-tracing grating, and the -1st order diffracted light returns along the original path, passes through the first waveplate again, and becomes vertically polarized light. The vertically polarized light is transmitted after passing through the polarizing beam splitter and merges with the previous diffracted light, and together they enter the signal receiving unit. Two perpendicularly polarized diffracted beams are converted into circularly polarized beams by a third waveplate. They are then split into two paths proportionally by an unpolarized beam splitter. One path passes through a first linear polarizer and enters the first photodetector, while the other path passes through a second linear polarizer and enters the second photodetector. The two orthogonal interference signals collected by the photodetectors are then fed into a signal processing unit. The arctangent of the interference signals is calculated and the phase is unwrapped to obtain the motion distance and motion reversal of the self-tracing grating.

[0016] Preferably, when the displacement stage moves at a uniform speed in one direction and the laser diffraction measurement area moves to any overlapping region, the first reading head and the second reading head simultaneously acquire the interference signals of the two gratings. Each reading head takes one signal, and the interference signal of the second reading head is calibrated using the interference signal period T1 of the first reading head. The calibration coefficients are as follows: ; in, This indicates the interference period of the first reading head. This indicates the interference period of the first reading head; The displacement data collected by the second reading head is multiplied by a coefficient. The measured displacement results are obtained.

[0017] Therefore, the present invention employs the above-described device for realizing a large-stroke direct-trace grating interferometer, which has the following beneficial effects: 1. Range extension of the self-traceable grating interferometer; 2. Enable direct traceability of large-stroke grating interferometers; 3. It has the advantages of self-calibrating splicing of measurement range, large expansion space, and embedded use.

[0018] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a device for implementing a large-stroke direct-trace grating interferometer according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the staggered arrangement of gratings according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the self-traceable grating deposition process according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the structure of the zero-difference self-traceable grating reading head according to an embodiment of the present invention; Figure 5 This is a graph illustrating the calibration of the overlapping region signal output signal in an embodiment of the present invention. Figure 6 This is a graph showing the output signal of the overlapping region before and after calibration, according to an embodiment of the present invention. Figure Labels 101. First grating; 102. Second grating; 103. Third grating; 104. Fourth grating; 105. Fifth grating; 106. First reading head; 107. Second reading head; 108. Grating base plate; 109. Coherent light source; 110. Fiber optic beam splitter; 111. Signal processing unit; 301. Chromium atomic beam; 302. Converging laser; 303. One-dimensional deposition grating; 401. First reflecting mirror; 402. Polarizing beam splitter prism; 403. First waveplate; 404. Second reflecting mirror; 405. Third reflecting mirror; 406. Second waveplate; 407. First photodetector; 408. First linear polarizer; 409. Second linear polarizer; 410. Second detector; 411. Unpolarizing beam splitter prism; 412. Third waveplate; 413. Fourth waveplate; 414. Fiber collimator. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0021] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0022] Example like Figure 1As shown, the present invention provides an apparatus for realizing a large-stroke direct traceability grating interferometer, including a grating base plate 108, a coherent light source 109, and an optical fiber beam splitter 110. Multiple gratings are disposed on the grating base plate 108, including a first grating 101, a second grating 102, a third grating 103, a fourth grating 104, and a fifth grating 105. The gratings are arranged in an alternating pattern on the grating base plate 108, and all gratings are self-traceable gratings. The coherent light source 109 and the optical fiber beam splitter 110 are connected. The output end of the optical fiber beam splitter 110 is connected to two reading heads, namely a first reading head 106 and a second reading head 107. The other ends of both the first reading head 106 and the second reading head 107 are connected to a signal processing unit 111.

[0023] Reference Figure 2 There are four overlapping regions, R1, R2, R3, and R4, between adjacent gratings along the grating period direction. The staggered self-traceable gratings are mechanically fixed to the grating base plate 108. Due to errors in the installation process, the grating period direction cannot be guaranteed to be perfectly parallel. The grating base plate 108 is made of zero-expansion glass material.

[0024] The grating base plate 108 is driven by a displacement stage to perform one-dimensional translation, while the first reading head 106 and the second reading head 107 remain stationary. The process of grating interference displacement measurement is as follows: the coherent light source 109 guides laser light into the first reading head 106 and the second reading head 107 through the fiber optic beam splitter 110. The first reading head 106 and the second reading head 107 incident the laser light onto the grating surface, where it diffracts with the grating and returns to the reading head to form an interference signal. The interference signal is transmitted to the signal processing unit 111, where the grating displacement and direction of motion can be obtained through calculation. When the measuring laser of the reading head moves outside the grating area, the interference signal is lost, making it impossible to measure the displacement. To achieve a larger measurement stroke, an interleaved arrangement of gratings is used to extend the stroke.

[0025] The process of measuring stroke extension is as follows: Taking the extension of the first grating 101 to the second grating 102 as an example, firstly, the installation angle of the grating base plate 108 on the displacement stage is adjusted so that the periodic direction of the first grating 101 is parallel to the movement direction of the displacement stage. Specifically, the displacement stage is made to reciprocate within a fixed stroke within the stroke of the first grating 101, and displacement data is collected using the first reading head 106 to maximize the displacement value. When the first reading head 106 moves to the overlap region R1, the second reading head 8 begins to generate an interference signal. However, due to the installation error between the second grating 102 and the first grating 101, under the same displacement conditions, The signal period T1 obtained by the first reading head 106 is not equal to the interference signal period T2 obtained by the second reading head 107. The interference signal period obtained by the second reading head 107 can be calibrated by the signal period of the first reading head 106. In this way, the measurement value of the second reading head 107 is eliminated from the installation error. The same calibration coefficient is also used at other positions where the second reading head 107 measures the second grating 102. Finally, the grating continues to move, the first reading head 106 loses the interference signal, and the range extension and installation error calibration are achieved entirely by measuring the second grating 102 with the second reading head 107.

[0026] In the overlapping region R2, the installation error of the third grating 103 is calibrated by the calibrated second reading head 107 signal. This process is repeated to achieve large-stroke measurement of the self-traceable grating and to calibrate the installation error, thus making the measured value traceable.

[0027] The periodic direction of the first grating is consistent with the motion axis of the displacement stage, and the reading head and the self-traceable grating are ideally installed. The spacing between adjacent self-traceable gratings is adjusted so that there is an overlapping area of ​​interference signals when the reading head passes through. The spatial attitude error of adjacent self-traceable gratings is calibrated using the theoretical spacing of the self-traceable gratings.

[0028] The process of calibrating the spatial attitude error of adjacent self-traceable gratings using the theoretical spacing of self-traceable gratings is as follows: the cosine error is calculated by the ratio of the number of original signal periods acquired by two read heads in the overlapping area of ​​the interference signal, and finally the length reference on all measurement strokes is the theoretical spacing of the self-traceable gratings.

[0029] The ideal method for installing the self-traceable grating and the read head is to have the laser incident at a Litterow angle onto the self-traceable grating and return along the same path.

[0030] The reading head employs any displacement measurement method selected from zero-difference interferometry, heterodyne interferometry, and self-mixing interferometry. The reading head and its corresponding self-traceable grating are positioned in the same motion plane.

[0031] The self-traceable grating is fabricated using atomic lithography, specifically including: heating and sublimating a metal material in a vacuum environment to form a gaseous metal; extracting a metal atomic beam through a cascading method; and depositing the metal atomic beam onto the surface of a zero-expansion glass substrate under the focusing effect of a laser standing wave field to form a grating structure.

[0032] All gratings used in this embodiment are chromium atom self-traceable gratings, referencing... Figure 3 The traceable grating deposition process is as follows: A crucible containing chromium powder is heated to between 1550℃ and 1650℃ in a vacuum environment until it reaches a sublimation state, and then drawn out through a 1mm effluent orifice to form a metal atom beam. Using a theodolite and altimeter, a one-dimensional cooling laser is adjusted to be orthogonal to the beam, and the divergence angle of the metal atom beam is reduced to within 1 mrad based on laser monitoring results. The collimated Cr atom beam 301 is then passed through a converging standing wave field of the orthogonal laser, and deposited onto the sample under the action of a dipole force to form a one-dimensional deposition grating 303. The wavelength of the converging laser 302 is 425.6 nm, corresponding to the resonant transition energy level of the Cr atom. This corresponds to a positive detuning (+20MHz) or negative detuning (-250MHz) position at the center frequency of the converging laser frequency adjusted to the resonant energy level. Therefore, the period of the formed one-dimensional chromium (Cr) atom photolithography grating structure is half the wavelength of the laser used, which is 212.8 nm. Because the one-dimensional chromium atom deposition grating prepared by laser focusing atom deposition technology requires a structure to be formed, the wavelength of the one-dimensional laser standing wave field must correspond to the wavelength of the atomic transition energy level. Furthermore, the wavelength of the formed one-dimensional chromium atom deposition grating is exactly half the wavelength of the one-dimensional laser standing wave field. Therefore, the one-dimensional chromium atom deposition grating is a self-traceable grating with metrological characteristics.4 In addition, to control the grating pitch stability at different temperatures, a zero-expansion glass material is chosen for the substrate.

[0033] The laser wavelength of the coherent light source 109 is less than twice the theoretical spacing of the self-traceable grating.

[0034] To meet the requirement of generating Littrow diffraction in the chromium atom self-tracing grating, a 405nm narrow linewidth laser was selected for the coherent light source 109.

[0035] The fiber optic beam splitter 110 uses single-mode polarization-maintaining fiber.

[0036] The signal processing unit 111 has four channels with a sampling rate of 100MHz and a sampling bit depth of 16 bits. It converts the acquired voltage signals into digital signals and then performs displacement calculation and error calibration on the four signals through the FPGA.

[0037] The first and second reading heads use the same optical path. Taking zero-difference interference as an example, their internal optical path is as follows: Figure 4As shown, the system includes an optical fiber collimator 414, a diffraction light generating unit, and a signal receiving unit. The diffraction light generating unit includes a fourth waveplate 413, a polarizing beam splitter 402, a first waveplate 403, a second waveplate 406, a first reflecting mirror 401, a second reflecting mirror 405, and a third reflecting mirror 404. The signal receiving unit includes a third waveplate 412, a non-polarizing beam splitter 411, a first polarizer 408, a second polarizer 409, a first detector 407, and a second detector 410. The optical fiber collimator 414 is connected to the polarizing beam splitter 402 via the first reflecting mirror 401 and the fourth waveplate 413. The polarizing beam splitter 402 is connected to the second waveplate 406, the first waveplate 403, and the third waveplate 412, respectively. The second waveplate 406 is connected to the third reflector 405, the first waveplate 403 is connected to the second reflector 404, the third waveplate 412 is connected to the polarizing beam splitter 411, the polarizing beam splitter 411 is connected to the first linear polarizer 408 and the second linear polarizer 409 respectively, the first linear polarizer 408 is connected to the first photodetector 407, the second linear polarizer 409 is connected to the second photodetector 410, and both the first photodetector 407 and the second photodetector 410 are connected to the signal processing unit 111.

[0038] Among them, the fourth waveplate 413 is a half-wave plate, and the first waveplate 403, the second waveplate 406, and the third waveplate 412 are quarter-wave plates. The fast axes of the first waveplate 403, the second waveplate 406, and the third waveplate 412 are all at 45° to the plane of light incidence. The polarization direction of the first linear polarizer 408 is at 45° to the plane of light incidence, and the polarization direction of the second linear polarizer 409 is at 22.5° to the plane of light incidence.

[0039] The process of acquiring the interference signal using the above-mentioned reading head in this embodiment is as follows: A 405nm laser beam enters from the fiber collimator 414, passes through the first reflecting mirror 401, and enters the diffraction light generating unit. The rotatable fourth waveplate 413 is adjusted so that the polarization direction of the incident laser beam forms a 45° angle with the incident plane. The laser beam is decomposed into two components, vertical polarization and horizontal polarization, by the polarizing beam splitter 402. The vertically polarized light is transmitted and then passes through the second waveplate 406, turning it into right-hand circularly polarized light. The right-hand circularly polarized light passes through the third reflecting mirror 405 and enters the self-tracing grating 415 at a Littrow angle. The incident light diffracts at the self-tracing grating 415, and the -1st order diffracted light returns along the same path, passes through the second waveplate 406 again, and becomes horizontally polarized light. The horizontally polarized light is reflected by the polarizing beam splitter 402 and enters the signal receiving unit. The other horizontally polarized light, reflected by the polarizing beam splitter 402, becomes left-hand circularly polarized light after passing through the first waveplate 403. Left-handed circularly polarized light is incident on the self-tracing grating 415 at a Littrow angle through the second reflecting mirror 404. The incident light is diffracted on the self-tracing grating 415. The -1st order diffracted light returns along the original path and passes through the first waveplate 403 again, becoming vertically polarized light. The vertically polarized light is transmitted through the polarizing beam splitter prism 402 and merges with the previous diffracted light, and together they enter the signal receiving unit.

[0040] Two perpendicularly polarized diffracted beams are converted into circularly polarized light by the third waveplate 412. They are then proportionally split into two paths by the unpolarized beam splitter 411. One path passes through the first linear polarizer 408 and enters the first photodetector 407, while the other path passes through the second linear polarizer 409 and enters the second photodetector 410. The two orthogonal interference signals collected by the photodetectors are input into the signal processing unit 111. By performing arctangent calculation and phase unwrapping on the interference signals, the motion distance and direction of motion of the self-tracing grating 415 can be obtained. The light intensity of a single interference signal... The following relationships exist: ; In the formula, The period of a chromium atom photolithography grating. This represents the distance the displacement stage travels.

[0041] Figure 5 As the displacement stage moves at a uniform speed in one direction, when the laser diffraction measurement area moves to any overlapping region (R1, R2, R3, or R4), the first reading head 106 and the second reading head 107 simultaneously acquire the interference signals of the two gratings. Each reading head takes one signal, and the interference signal of the second reading head 107 is calibrated using the interference signal period T1 of the first reading head 106. The calibration coefficients are as follows: ; The displacement data collected by the second reading head 107 is multiplied by a coefficient. The measured displacement results are obtained. (Refer to...) Figure 6 Without calibration, the velocity of the displacement curve changes abruptly after passing through the second reading head 107. After calibration, the displacement curve is consistent with the ideal uniform motion.

[0042] To extend the measurement range of the interferometer, this invention proposes a range splicing technique for adjacent self-traceable grating modules. This technique involves splicing multiple self-traceable gratings at intervals, adjusting the spacing between adjacent self-traceable grating modules to create an overlapping area of ​​interference signals when the reading head passes through. The spatial attitude error of adjacent self-traceable grating modules is calibrated using the theoretical spacing of the self-traceable gratings, thereby expanding the measurement range while ensuring displacement measurement accuracy.

[0043] Therefore, the present invention employs the above-mentioned device for realizing a large-stroke direct traceability grating interferometer, which achieves real-time error calibration by comparing the signal periods of two reading heads in the overlapping area, thereby effectively improving the measurement range of the grating interferometer.

[0044] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A device for realizing a large-stroke direct-trace grating interferometer, characterized in that: The system includes a grating substrate, a coherent light source, and an optical fiber beam splitter. The grating substrate has multiple gratings, including a first grating, a second grating, a third grating, a fourth grating, and a fifth grating. The gratings are arranged in a staggered pattern on the grating substrate, with four overlapping regions (R1, R2, R3, and R4) between adjacent gratings along the grating period direction. All gratings are self-traceable gratings. The coherent light source and the optical fiber beam splitter are connected. The output end of the optical fiber beam splitter is connected to two reading heads, designated as a first reading head and a second reading head. The other end of both the first and second reading heads is connected to a signal processing unit. The grating substrate is driven by a displacement stage to perform one-dimensional translation, while the first and second reading heads remain stationary.

2. The apparatus for realizing a large-stroke direct-trace grating interferometer according to claim 1, characterized in that, A coherent light source guides the laser into a first reading head and a second reading head through an optical fiber beam splitter. The first and second reading heads incident the laser onto the grating surface, where it diffracts with the grating and returns to the reading head to form an interference signal. The interference signal is transmitted to the signal processing unit, where the grating displacement and direction of motion are obtained through calculation.

3. The apparatus for realizing a large-stroke direct-trace grating interferometer according to claim 1, characterized in that, The periodic direction of the first grating is consistent with the motion axis of the displacement stage, and the reading head and the self-traceable grating are ideally installed. The spacing between adjacent self-traceable gratings is adjusted so that there is an overlapping area of ​​interference signals when the reading head passes through. The spatial attitude error of adjacent self-traceable gratings is calibrated using the theoretical spacing of the self-traceable gratings.

4. The apparatus for realizing a large-stroke direct-trace grating interferometer according to claim 3, characterized in that, The process of calibrating the spatial attitude error of adjacent self-traceable gratings using the theoretical spacing of self-traceable gratings is as follows: the cosine error is calculated by the ratio of the number of cycles of the original signals acquired by the two read heads in the overlapping area of ​​the interference signals, so that the length reference on all measurement strokes is the theoretical spacing of the self-traceable gratings. The ideal installation method for the self-traceable grating and the reading head is as follows: the laser is incident on the self-traceable grating at a Litterow angle and returns along the original path.

5. The apparatus for realizing a large-stroke direct-trace grating interferometer according to claim 1, characterized in that, The reading head adopts any displacement measurement method selected from zero-difference interferometry, heterodyne interferometry, and self-mixing interferometry. The reading head and its corresponding self-traceable grating are set in the same motion plane.

6. The apparatus for realizing a large-stroke direct-trace grating interferometer according to claim 1, characterized in that, The coherent light source uses a 405nm narrow linewidth laser; the fiber beam splitter uses single-mode polarization-maintaining fiber; the signal processing unit has four channels with a 100MHz sampling rate and 16-bit sampling bit depth, used to convert the acquired voltage signals into digital signals, and to perform displacement calculation and error calibration on the four signals through FPGA.

7. The apparatus for realizing a large-stroke direct-trace grating interferometer according to claim 1, characterized in that, The first and second reading heads use the same optical path. When both the first and second reading heads use zero-difference interferometric displacement measurement, the optical path structure of the first and second reading heads includes an optical fiber collimator, a diffraction light generation unit, and a signal receiving unit. The diffraction light generation unit includes a fourth waveplate, a polarizing beam splitter, a first waveplate, a second waveplate, a first reflector, a second reflector, and a third reflector. The signal receiving unit includes a third waveplate, a non-polarizing beam splitter, a first polarizer, a second polarizer, a first detector, and a second detector. The optical fiber collimator is connected to the polarizing beam splitter via the first reflector and the fourth waveplate. The polarizing beam splitter is connected to the second waveplate, the first waveplate, and the third waveplate. The second waveplate is connected to the third reflector. The first waveplate is connected to the second reflector. The third waveplate is connected to the polarizing beam splitter. The polarizing beam splitter is connected to the first linear polarizer and the second linear polarizer. The first linear polarizer is connected to the first photodetector. The second linear polarizer is connected to the second photodetector. Both the first and second photodetectors are connected to the signal processing unit.

8. The apparatus for realizing a large-stroke direct-trace grating interferometer according to claim 7, characterized in that, The fourth waveplate is a half-wave plate, while the first, second, and third waveplates are quarter-wave plates. The fast axes of the first, second, and third waveplates are all at 45° to the plane of light incidence. The polarization direction of the first linear polarizer is at 45° to the plane of light incidence, and the polarization direction of the second linear polarizer is at 22.5° to the plane of light incidence.

9. The apparatus for realizing a large-stroke direct-trace grating interferometer according to claim 7, characterized in that, The laser beam enters through the fiber collimator, passes through the first reflecting mirror, and enters the diffraction light generating unit. The rotatable fourth waveplate is adjusted so that the polarization direction of the incident laser beam is at 45° with the incident plane. The laser beam is decomposed into two components, vertical polarization and horizontal polarization, by the polarizing beam splitter. The vertically polarized light passes through the second waveplate and becomes right-hand circularly polarized light. The right-hand circularly polarized light passes through the third reflecting mirror and enters the self-tracing grating at the Littrow angle. The incident light diffracts at the self-tracing grating. The -1st order diffracted light returns along the same path, passes through the second waveplate again, and becomes horizontally polarized light. The horizontally polarized light is reflected after passing through the polarizing beam splitter and enters the signal receiving unit. Another path of horizontally polarized light reflected by the polarizing beam splitter becomes left-handed circularly polarized light after passing through the first waveplate. The left-handed circularly polarized light is incident on the self-tracing grating at the Littrow angle after passing through the second mirror. The incident light is diffracted on the self-tracing grating, and the -1st order diffracted light returns along the original path, passes through the first waveplate again, and becomes vertically polarized light. The vertically polarized light is transmitted after passing through the polarizing beam splitter and merges with the previous diffracted light, and together they enter the signal receiving unit. Two perpendicularly polarized diffracted beams are converted into circularly polarized beams by a third waveplate. They are then split into two paths proportionally by an unpolarized beam splitter. One path passes through a first linear polarizer and enters the first photodetector, while the other path passes through a second linear polarizer and enters the second photodetector. The two orthogonal interference signals collected by the photodetectors are then fed into a signal processing unit. The arctangent of the interference signals is calculated and the phase is unwrapped to obtain the motion distance and motion reversal of the self-tracing grating.

10. The apparatus for realizing a large-stroke direct-trace grating interferometer according to claim 1, characterized in that, When the displacement stage moves at a constant speed in one direction, and the laser diffraction measurement area moves to any overlapping region, the first and second reading heads simultaneously acquire the interference signals of the two gratings. Each reading head takes one signal, and the interference signal of the second reading head is calibrated using the interference signal period T1 of the first reading head. The calibration coefficients are as follows: ； in, This indicates the interference period of the first reading head. This indicates the interference period of the first reading head; The displacement data collected by the second reading head is multiplied by a coefficient. The measured displacement results are obtained.

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