Grating interferometer with angular error self-compensation function

Abstract

The application relates to the technical field of grating measurement, and discloses a grating interferometer with an angle error self-compensation function, which comprises a light source for emitting P-polarized light with a first frequency and S-polarized light with a second frequency; a first beam splitter for splitting the light into first P-polarized light and second P-polarized light, first S-polarized light and second S-polarized light; a reference light path, wherein the first P-polarized light and the first S-polarized light interfere to form a reference interference signal; a measurement light path, wherein +1-order diffracted light of the second P-polarized light and 0-order reflected light of the second S-polarized light interfere to form a measurement interference signal; a compensation light path, wherein 0-order reflected light of the second P-polarized light and +1-order diffracted light of the second S-polarized light interfere to form a compensation interference signal; and a computing device for calculating a target displacement with an angle error based on the reference interference signal, the measurement interference signal and the compensation interference signal. The application corrects the displacement value output by the interferometer in real time, so that the displacement deviation caused by movement deflection is eliminated.

Description

Technical Field

[0001] This application relates to the field of grating measurement technology, and more specifically to a grating interferometer with angular error self-compensation function. Background Technology

[0002] As a core device for precision displacement measurement, grating interferometers are widely used in high-end equipment fields such as lithography machines, precision machine tools, and micro / nano manufacturing due to their high resolution and high stability. They can achieve displacement measurements at the nanometer and even sub-nanometer levels. Building upon grating interferometry, heterodyne grating interferometry combines the advantages of gratings' resistance to environmental disturbances with the high signal-to-noise ratio of heterodyne interferometry, further reducing measurement noise. However, in actual measurement, the measured object will exhibit a small angular deflection along the measurement axis. If this is not corrected, it will severely affect the measurement accuracy. Furthermore, as the accuracy requirements of heterodyne interferometry measurements gradually transition from the nanometer scale to the sub-nanometer and even picometer scale, the problem of periodic nonlinear errors becomes increasingly prominent.

[0003] In practical measurement scenarios, the object being measured will experience a slight angular deflection as it moves along the measurement axis of the grating interferometer. This deflection causes a discrepancy between the displacement value directly read by the grating interferometer and the actual displacement of the object. Without compensation, this severely affects measurement accuracy and fails to meet the stringent displacement measurement requirements of high-end equipment. Integrating an independent angle measurement module into the interferometer system allows for the detection of angular deviations by the object using an angle sensor. By combining displacement measurement data and a compensation model, deviation correction can be achieved, resulting in a more accurate displacement value that closely approximates the true state of the object. However, this method requires an additional angle sensor, increasing system cost and integration complexity. The sensor must be precisely aligned with the rotation center of the object; otherwise, detection errors will be introduced. Existing spatially separated heterodyne grating interferometers modulate and generate two beams of different frequencies independently. The two beams need to be combined to produce an interference signal, which significantly increases the difficulty of optomechanical design and beam adjustment. In particular, the angular mismatch caused by beam combining greatly weakens the interference signal strength, reduces the signal-to-noise ratio, and further affects displacement measurement accuracy. Summary of the Invention

[0004] To address the above problems, this application proposes a grating interferometer with self-compensation for angular errors, thereby solving at least one of the aforementioned issues. The specific solution is as follows: According to a specific embodiment of this application, this application provides a grating interferometer with angular error self-compensation function, comprising: Light source, used to emit light at a first frequency f 1 P-polarized light and second frequency f 2 S-polarized light; The first beam splitter is used to separate the first frequency. f 1 The P-polarized light is split into first P-polarized light and second P-polarized light, and the second frequency is... f 2 S-polarized light is divided into first S-polarized light and second S-polarized light; In the reference optical path, the first P-polarized light and the first S-polarized light interfere to form a reference interference signal; In the measurement optical path, the +1st order diffracted light of the second P-polarized light and the 0th order reflected light of the second S-polarized light interfere to form a measurement interference signal; The compensation optical path is formed by the interference of the 0th order reflected light of the second P-polarized light and the +1st order diffracted light of the second S-polarized light to create a compensation interference signal. The computing device calculates the displacement of the target under test with angular error based on the reference interference signal, the measured interference signal and the compensated interference signal.

[0005] In some embodiments, the reference optical path includes a beam combiner prism and a first photodetector, wherein the first P-polarized light and the first S-polarized light pass through the beam combiner prism and enter the first photodetector to form a reference interference signal.

[0006] In some embodiments, the measurement optical path includes a second beam splitter, a first plane mirror, a second plane mirror, a one-dimensional reflection grating, and a second photodetector; In this process, the second P-polarized light passes through the transmission portion of the second beam splitter and is incident on the one-dimensional reflection grating via the first plane mirror. The +1st order diffracted light from the one-dimensional reflection grating returns to the second photodetector via the original path. The second S-polarized light passes through the reflection portion of the second beam splitter and is incident on the one-dimensional reflection grating via the second plane mirror. The 0th order reflected light from the one-dimensional reflection grating is incident on the second photodetector via the first plane mirror and the second beam splitter, and interferes with the +1st order diffracted light of the second P-polarized light to form a measurement interference signal.

[0007] In some embodiments, the compensation optical path includes a second beam splitter, a first plane mirror, a second plane mirror, a one-dimensional reflection grating, and an angle measurement component; The second P-polarized light, after passing through the transmission portion of the second beam splitter, is incident on the one-dimensional reflection grating via the first plane mirror. The 0th-order reflected light diffracted by the one-dimensional reflection grating is incident on the angle measuring component via the second plane mirror and the second beam splitter. The second S-polarized light, after passing through the reflection portion of the second beam splitter, is incident on the one-dimensional reflection grating via the second plane mirror. The +1st-order diffracted light diffracted by the one-dimensional reflection grating returns along the original path and is incident on the angle measuring component, interfering with the 0th-order reflected light of the second P-polarized light to form a compensated interference signal.

[0008] In some embodiments, the angle measurement assembly includes a lens and a four-quadrant photodetector.

[0009] In some embodiments, the angle measurement component includes a CCD image sensor.

[0010] In some embodiments, calculating the displacement of the target under test with angular error based on the reference interferometric signal, the measured interferometric signal, and the compensated interferometric signal includes: The measured displacement value is calculated based on the reference interference signal and the measured interference signal. S x ’ ; The compensation displacement deviation value is calculated based on the compensation interference signal. S(θ) ; The calculated compensated displacement of the target object S x for: .

[0011] In some embodiments, the second P-polarized light, after passing through the transmission portion of the second beam splitter, is incident on the one-dimensional reflective grating at a Littrow angle via the first planar reflector.

[0012] In some embodiments, the second S-polarized light, after being reflected by the second beam splitter, is incident on the one-dimensional reflective grating at a Littrow angle via the second plane mirror.

[0013] In some embodiments, the measured displacement value is calculated based on the reference interference signal and the measured interference signal. S x ’ The following relationship must be satisfied: ;

[0014] in, For the period of the grating, To measure the phase change in the interference signal caused by the motion of the grating.

[0015] Compared with the prior art, the above-described solutions of this application have at least the following beneficial effects: This application utilizes a beam incident on an angle measurement component in the optical path of a grating interferometer. The angle measurement component calculates the deflection angle of the object under test, and the measured angle is used to correct the displacement value output by the interferometer in real time, thereby eliminating displacement deviation caused by motion deflection. This method has a compact structure, high integration, and fast response speed, enabling dynamic real-time compensation. Attached Figure Description

[0016] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application. It is obvious that the drawings described below are merely some embodiments of this application, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. In the drawings: Figure 1 This is a schematic diagram of the structure of a grating interferometer with angular error self-compensation function provided in Embodiment 1 of this application.

[0017] Figure 2 This is a schematic diagram of the structure of a grating interferometer with self-compensation function for angular error provided in Embodiment 2 of this application.

[0018] Figure 3 This is a schematic diagram of the lens measurement principle structure provided in Embodiment 1 of this application.

[0019] Figure 4 This is a schematic diagram of the measurement principle structure of the four-quadrant photodetector provided in Embodiment 1 of this application.

[0020] Figure 5 This is a schematic diagram of the measurement principle structure of the CCD image sensor provided in Embodiment 2 of this application.

[0021] Explanation of reference numerals in the attached figures: Light source 1; first beam splitter 2; beam combiner prism 3; second beam splitter 4; first plane mirror 5; second plane mirror 6; one-dimensional reflective grating 7; first photodetector 8; second photodetector 9; lens 10; four-quadrant detector 11; CCD image sensor 12; angle measurement assembly 20. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a product or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a product or device. Without further limitation, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the product or device that includes that element.

[0024] The embodiments of this application are described in detail below with reference to the accompanying drawings.

[0025] According to the specific implementation of this application, such as Figure 1 , Figure 2 As shown, according to a specific embodiment of this application, this application provides a grating interferometer with angular error self-compensation function, including: a light source 1, used to emit light with a first frequency. f 1 P-polarized light and second frequency f 2 S-polarized light; first beam splitter 2, used to split the first frequency... f 1 The P-polarized light is split into first P-polarized light and second P-polarized light, and the second frequency is... f 2 The S-polarized light is divided into a first S-polarized light and a second S-polarized light; a reference optical path is formed by the interference of the first P-polarized light and the first S-polarized light to form a reference interference signal; a measurement optical path is formed by the interference of the +1st order diffracted light of the second P-polarized light and the 0th order reflected light of the second S-polarized light to form a measurement interference signal; a compensation optical path is formed by the interference of the 0th order reflected light of the second P-polarized light and the +1st order diffracted light of the second S-polarized light to form a compensation interference signal; and a computing device is used to calculate the displacement of the target under test with angular error based on the reference interference signal, the measurement interference signal, and the compensation interference signal.

[0026] This application provides a grating interferometer with self-compensation for angular errors. A light beam is incident on the grating at a Litrow angle. The diffracted light returns to the grating interferometer and interferes with a corresponding reference beam. The interference signal is received by a receiver, converted into an electrical signal by a photoelectric conversion module, and then processed by a signal processing system to obtain displacement data. Simultaneously, through a compensation optical path, the yaw angle information can be acquired in real time. The measured yaw angle is used to correct the displacement value output by the interferometer in real time, thereby eliminating displacement deviations caused by motion deflection. This method features a compact structure, high integration, and fast response speed. Accurate measurement can be completed with a single optical path system, and the displacement of the target object is obtained through dynamic real-time compensation.

[0027] Among them, light source 1 is a spatially separated heterodyne light source, which can generate a light source with a first frequency. f 1 P-polarized light and second frequency f 2 The light source 1 can be a dual-frequency laser, which outputs difference-frequency lasers with perpendicular orthogonal polarization. For example, a Gaussian light source with a wavelength of 628 nm is used, such as a heterodyne light source with wavelengths of 628.00682 nm and 628.00684 nm.

[0028] In some embodiments, the reference optical path includes a beam combiner prism 3 and a first photodetector 8, wherein the first P-polarized light and the first S-polarized light pass through the beam combiner prism 3 and enter the first photodetector 8 to form a reference interference signal.

[0029] In some embodiments, the measurement optical path includes a second beam splitter 4, a first plane mirror 5, a second plane mirror 6, a one-dimensional reflection grating 7, and a second photodetector 9; wherein, the second P-polarized light, after passing through the transmission portion of the second beam splitter 4, is incident on the one-dimensional reflection grating 7 via the first plane mirror 5, and the +1st order diffracted light diffracted by the one-dimensional reflection grating 7 returns to the second photodetector 9 along the original path; the second S-polarized light, after passing through the reflection portion of the second beam splitter 4, is incident on the one-dimensional reflection grating 7 via the second plane mirror 6, and the 0th order reflected light diffracted by the one-dimensional reflection grating 7 is incident on the second photodetector 9 via the first plane mirror 5 and the second beam splitter 4, interfering with the +1st order diffracted light of the second P-polarized light to form a measurement interference signal.

[0030] In some embodiments, the second P-polarized light passes through the transmission portion of the second beam splitter 4 and is incident on the one-dimensional reflective grating 7 at a Littrow angle via the first plane mirror 5; the second S-polarized light passes through the reflection portion of the second beam splitter 4 and is incident on the one-dimensional reflective grating 7 at a Littrow angle via the second plane mirror 6.

[0031] In some embodiments, such as Figure 1 As shown, the angle measurement component 20 includes a lens 10 and a four-quadrant photodetector 11.

[0032] When the grating has a yaw error, the light beam will move along the grating vector direction. The relationship between the height h of the light spot on the detector after passing through the lens and the incident light angle α is as follows: ,like Figure 3 As shown, the light spot on the photosensitive surface of the detector will move a distance x1 and x2 along the x-axis of the QPD detector, respectively, as shown. Figure 4As shown. The deflection angles of the 0th-order reflected light and the 1st-order diffracted light are measured using the position information output by the detector, which are α1 and α2 respectively. From this, the beam angle β = α1 + α2 is obtained.

[0033] Based on geometric relationships, the relationship between the included angle β of the measurement beam and the grating deflection angle θ is as follows:

[0034] θ L θ is the littrow angle, and θ is the grating tilt angle.

[0035] Therefore, the grating deflection angle θ can be calculated from the measured beam angle.

[0036] In some embodiments, such as Figure 2 As shown, the angle measurement component 20 includes a CCD image sensor 12. The CCD image sensor 12 can measure the grating deflection angle θ in real time.

[0037] The optical system collimates and focuses the incident laser light into a spot. The position of the spot on the photosensitive surface varies depending on the angle at which the laser strikes the photodetector. An algorithm can measure the spot position and obtain the laser angle information. The angle measurement system based on a photodetector array uses detection channels as basic units. Each channel is an independent laser receiver, typically consisting of a photodetector and a preamplifier circuit. The system determines the incident angle by receiving the laser signal. The relationship between the effective receiving area of ​​the detection channel and the laser incident angle is shown in Figure 5.

[0038] In the diagram, AB represents the laser receiving surface of the detector. When the laser is incident at an angle α, the effective laser receiving surface of the detector is AB'. Based on geometric relationships, we can obtain:

[0039] That is, the luminous flux received by the detector is sinusoidally related to the laser incident angle. The laser incident angle can be calculated from the output signal of the receiver.

[0040] In some embodiments, calculating the displacement of the target under test with angular error based on the reference interferometric signal, the measured interferometric signal, and the compensated interferometric signal includes: The measured displacement value is calculated based on the reference interference signal and the measured interference signal. S x ’ ; The compensation displacement deviation value is calculated based on the compensation interference signal. S(θ) ; The calculated compensated displacement of the target object S x for: .

[0041] The calculation process of the grating interferometer based on the angle error self-compensation function of this application is as follows.

[0042] The complex amplitudes of the two orthogonally linearly polarized beams with a certain frequency difference output from a dual-frequency laser can be expressed as:

[0043] In the formula, f 1 and f 2 These are the frequencies of the two orthogonally linearly polarized beams. and These represent the initial phases of the two beams of light. E 01 and E 02 The amplitudes of the two beams of light.

[0044] In the reference optical path, the complex amplitude of the laser entering the first photodetector 8 can be expressed as:

[0045] The intensity of the received interference signal is: (1) in, The frequency difference between the two lasers The initial phase difference is the difference between the two-frequency lasers.

[0046] In the measurement optical path, when there is an angular error in the grating, an additional optical path will be introduced into the optical path of the beam passing through the one-dimensional reflection grating 7, the first plane mirror 5, and the second photodetector 9. The complex amplitude actually received by the second photodetector 9 is expressed as:

[0047] In the formula, S x This represents the displacement value of the grating along the x-axis. These represent the additional optical path introduced by the grating angle error in the measurement optical path. The period of the grating.

[0048] The specific formula for calculating the additional optical path is as follows:

[0049] L is the propagation distance of the light beam from the one-dimensional reflection grating 7, the first plane mirror 5, and the second photodetector 9 when the grating is without deflection, and θ is the distance L travels without deflection. LLet θ be the littrow angle, and θ be the grating tilt angle.

[0050] In measuring the optical path interference signal, the interference signals received by the second photodetector 9 are as follows: (2) The measured phase is obtained by eliminating common terms through phase comparison using formulas (1) and (2):

[0051] in The systematic bias caused by the additional optical path introduced by the grating angle error. represents the phase of the diffracted light caused by the grating motion, and represents the effective measurement signal.

[0052] The measured displacement value is calculated based on the reference interference signal and the measured interference signal. S x ’ The following relationship must be satisfied:

[0053] in, For the period of the grating, To measure the phase change in the interference signal caused by the motion of the grating.

[0054] By measuring phase Calculate the displacement:

[0055] Among them, S x 'S' represents the uncompensated measurement value. x This represents the actual displacement.

[0056] The displacement deviation introduced by the grating yaw angle error is defined as:

[0057] The actual displacement after compensation is:

[0058] This application utilizes a beam incident on an angle measurement component in the optical path of a grating interferometer. The angle measurement component calculates the deflection angle of the object under test, and the measured angle is used to correct the displacement value output by the interferometer in real time, thereby eliminating displacement deviation caused by motion deflection. This method has a compact structure, high integration, and fast response speed, enabling dynamic real-time compensation.

[0059] Finally, it should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems or apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple, and relevant parts can be referred to the method section.

[0060] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A grating interferometer with angular error self-compensation function, characterized in that, include: Light source, used to emit light at a first frequency f 1 P-polarized light and second frequency f 2 S-polarized light; The first beam splitter is used to separate the first frequency. f 1 The P-polarized light is split into first P-polarized light and second P-polarized light, and the second frequency is... f 2 S-polarized light is divided into first S-polarized light and second S-polarized light; In the reference optical path, the first P-polarized light and the first S-polarized light interfere to form a reference interference signal; In the measurement optical path, the +1st order diffracted light of the second P-polarized light and the 0th order reflected light of the second S-polarized light interfere to form a measurement interference signal; The compensation optical path is formed by the interference of the 0th order reflected light of the second P-polarized light and the +1st order diffracted light of the second S-polarized light to create a compensation interference signal. The computing device calculates the displacement of the target under test with angular error based on the reference interference signal, the measured interference signal and the compensated interference signal.

2. A grating interferometer with angular error self-compensation function according to claim 1, characterized in that, The reference optical path includes a beam combiner prism and a first photodetector. The first P-polarized light and the first S-polarized light pass through the beam combiner prism and enter the first photodetector to form a reference interference signal.

3. A grating interferometer with angular error self-compensation function according to claim 1, characterized in that, The measurement optical path includes a second beam splitter, a first plane mirror, a second plane mirror, a one-dimensional reflection grating, and a second photodetector; In this process, the second P-polarized light passes through the transmission portion of the second beam splitter and is incident on the one-dimensional reflection grating via the first plane mirror. The +1st order diffracted light from the one-dimensional reflection grating returns to the second photodetector via the original path. The second S-polarized light passes through the reflection portion of the second beam splitter and is incident on the one-dimensional reflection grating via the second plane mirror. The 0th order reflected light from the one-dimensional reflection grating is incident on the second photodetector via the first plane mirror and the second beam splitter, and interferes with the +1st order diffracted light of the second P-polarized light to form a measurement interference signal.

4. A grating interferometer with angular error self-compensation function according to claim 1, characterized in that, The compensation optical path includes a second beam splitter, a first plane mirror, a second plane mirror, a one-dimensional reflection grating, and an angle measurement component; The second P-polarized light, after passing through the transmission portion of the second beam splitter, is incident on the one-dimensional reflection grating via the first plane mirror. The 0th-order reflected light diffracted by the one-dimensional reflection grating is incident on the angle measuring component via the second plane mirror and the second beam splitter. The second S-polarized light, after passing through the reflection portion of the second beam splitter, is incident on the one-dimensional reflection grating via the second plane mirror. The +1st-order diffracted light diffracted by the one-dimensional reflection grating returns along the original path and is incident on the angle measuring component, interfering with the 0th-order reflected light of the second P-polarized light to form a compensated interference signal.

5. A grating interferometer with angular error self-compensation function according to claim 4, characterized in that, The angle measurement assembly includes a lens and a four-quadrant photodetector.

6. A grating interferometer with angular error self-compensation function according to claim 4, characterized in that, The angle measurement component includes a CCD image sensor.

7. A grating interferometer with angular error self-compensation function according to claim 1, characterized in that, The calculation of the displacement of the target under test with angular error based on the reference interferometric signal, the measured interferometric signal, and the compensated interferometric signal includes: The measured displacement value is calculated based on the reference interference signal and the measured interference signal. S x ’ ; The compensation displacement deviation value is calculated based on the compensation interference signal. S(θ) ; The calculated compensated displacement of the target object S x for: 。 8. A grating interferometer with angular error self-compensation function according to claim 3 or 4, characterized in that, The second P-polarized light, after passing through the transmission portion of the second beam splitter, is incident on the one-dimensional reflective grating at a Littrow angle via the first plane mirror.

9. A grating interferometer with angular error self-compensation function according to claim 3 or 4, characterized in that, The second S-polarized light, after being reflected by the second beam splitter, is incident on the one-dimensional reflective grating at a Littrow angle via the second plane mirror.

10. A grating interferometer with angular error self-compensation function according to claim 7, characterized in that, The measured displacement value is calculated based on the reference interference signal and the measured interference signal. S x ’ The following relationship must be satisfied: ； in, The period of the grating, To measure the phase change in the interference signal caused by the motion of the grating.

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