CN107860318B - A planar grating interferometer displacement measurement system - Google Patents

Abstract

A planar grating interferometer displacement measurement system, including a single-frequency laser, a beam splitter, an acousto-optic modulator, a grating interferometer, a planar grating, a receiver, an electronic signal processing component, an optical fiber coupler and a frequency synthesizer; a grating interferometer Including polarizing beam splitter, refractive element, retroreflector and quarter-wave plate; the measurement system implements displacement measurement based on the principles of grating diffraction, optical Doppler effect and optical beat frequency. When the grating interferometer and the plane grating make linear relative motion with two degrees of freedom, the system can output two linear displacements. This measurement system can achieve sub-nanometer or even higher resolution and accuracy, and can measure two linear displacements at the same time. It has the advantages of high measurement accuracy and simple structure. As an ultra-precision workpiece stage position measurement system for lithography machines, it can improve the workpiece stage. Overall performance.

Description

A planar grating interferometer displacement measurement system

Technical field

The invention relates to a planar grating interferometer displacement measurement system, and in particular to a planar grating interferometer displacement measurement system for measuring the displacement of a photolithography machine workpiece stage.

Background technique

As a typical displacement sensor, grating measurement system is widely used in many electromechanical equipment. The measurement principle of the grating measurement system is mainly based on the moiré fringe principle and the diffraction interference principle. As a mature displacement sensor, the grating measurement system based on the moire principle has become the first choice for displacement measurement of many electromechanical equipment due to its long measuring range, low cost, easy installation and adjustment. However, the accuracy is usually in the micron level, which is common. For general industrial applications.

Photolithography machines in semiconductor manufacturing equipment are key equipment in semiconductor chip production. The ultra-precision workpiece stage is the core subsystem of the lithography machine, which is used to carry the mask plate and silicon wafer to complete high-speed and ultra-precision step scanning motion. The ultra-precision workpiece table has become the most representative type of ultra-precision motion system due to its motion characteristics such as high speed, high acceleration, large stroke, ultra-precision, and multiple degrees of freedom. In order to realize the above motion, the ultra-precision workpiece table usually uses a dual-frequency laser interferometer measurement system to measure the multi-degree-of-freedom displacement of the ultra-precision workpiece table. However, with the continuous improvement of motion indicators such as measurement accuracy, measurement distance, and measurement speed, dual-frequency laser interferometers suffer from environmental sensitivity, difficulty in improving measurement speed, large space requirements, high price, and poor dynamic characteristics of the measurement target workpiece table. A series of problems make it difficult to meet higher measurement requirements.

In response to the above problems, major companies and research institutions in the field of ultra-precision measurement in the world have launched a series of studies. The research mainly focuses on grating measurement systems based on the principle of diffraction interference. The research results have been disclosed in many patent papers.

U.S. Patent Document Publication No. US2011/0255096A1 (publication date October 20, 2011) discloses a grating measurement system applied to the ultra-precision workpiece stage of a photolithography machine. The measurement system uses one-dimensional or two-dimensional gratings to match specific readings. The head realizes displacement measurement and can measure horizontal and vertical displacement simultaneously, but the structure is complex; U.S. Patent Document Publication No. US2011/0096334A1 (publication date April 28, 2011) discloses a heterodyne interferometer. A grating is used as the target mirror, but this interferometer can only achieve one-dimensional measurement. U.S. Patent Document Publication No. US2013/0114087Al (published on May 9, 2013) discloses an interferometric measurement system applied to the ultra-precision workpiece stage of a photolithography machine. The measurement system uses a grating interferometer and a laser interferometer combined method, but the structure of this solution is too complex, the optical path is long, and it is difficult to integrate and miniaturize. U.S. Patent Document Publication No. US2016/0102999AL (publication date April 12, 2016) discloses a grating measurement system applied to ultra-precision workpiece stages of photolithography machines. The measurement system uses one-dimensional or two-dimensional gratings and a reading head. Displacement measurement, but the interferometer structure uses dual-frequency lasers and dual-frequency coaxial light transmission, which is prone to polarization aliasing and large measurement errors. Japanese scholar GAOWEI proposed a single encoder using the principle of diffraction interference in the research paper "Design and construction of a two-degree-of-freedom linear encoder for nanometric measurement of stage position and straightness. Precision Engineering 34(2010)145-155" Frequency two-dimensional grating measurement system, this grating measurement system can achieve horizontal and vertical displacement measurement at the same time. However, due to the use of a single-frequency laser, the measurement signal is susceptible to interference and the accuracy is difficult to guarantee. Chinese patent document application numbers 201210449244.9 (application date November 9, 2012) and 201210448734.7 (application date November 9, 2012) respectively disclose a heterodyne grating interferometer measurement system, and the readings in the two interferometer measurement systems A quarter-wave plate is used in the head structure to change the polarization state of the beam. The optical structure is complex, and the non-ideal nature of the optical elements will lead to measurement errors.

Contents of the invention

Taking into account the limitations of the above technical solutions, the purpose of the present invention is to provide a planar grating interferometer displacement measurement system, which not only has the advantages of high measurement accuracy, simple structure and easy miniaturization integration, but also can achieve sub-nanometer or even higher resolution and accuracy, and can measure two linear displacements at the same time, thereby improving the overall performance of the workpiece table.

The technical solution of the present invention is as follows:

A planar grating interferometer displacement measurement system, including a single-frequency laser, a beam splitter, a grating interferometer, a planar grating, an acousto-optic modulator, a receiver, an electronic signal processing component, an optical fiber coupler and a frequency synthesizer; characterized by: ; The grating interferometer includes a polarizing beam splitter, a refractive element, a first retroreflector, a second retroreflector and a third retroreflector, and a quarter wave plate; wherein the first retroreflector is located at the polarizing beam splitter At the top of the mirror, the second retroreflector and the third retroreflector are placed side by side at the bottom of the polarizing beam splitter; the single-frequency laser emitted from the single-frequency laser is split by the beam splitter and passes through the frequency synthesizer. The acousto-optic modulator modulates the light, and after splitting the light through two beam splitters, the two laser beams interfere with the fiber coupler and are input to the receiver as a compensation axis signal. After processing, an electrical signal is formed and input to the electronic signal processing component. ; The other two laser beams are incident on the polarizing beam splitter and then split. The two reflected lights are reference lights, and the two transmitted lights are measurement lights;

The two measurement beams pass through the quarter-wave plate and the refractive element for the first time and are incident on the plane grating at the Littrow angle. After reflection, they are incident on the polarizing beam splitter through the refractive element and the quarter-wave plate for the second time. , after reflection, the two measuring light beams pass through the second retroreflector and the third retroreflector respectively, and are reflected back to the polarizing beam splitter. After reflection again, they pass through the refractive element and the quarter-wave plate and then return to the Littrow angle. It is incident on the plane grating, and after reflection, it passes through the refractive element and the quarter-wave plate again, and then is incident on the polarizing beam splitter. After transmission, the two measurement beams emerge in parallel;

The two reference lights are reflected back to the polarizing beam splitter after passing through the first retroreflector, and after reflection, the two reference lights are emitted in parallel;

One beam of reference light and one beam of measurement light interfere to form an interference light signal, and the other beam of reference light and another beam of measurement light interfere to form another interference light signal. The two interference light signals are transmitted to the receiver through optical fibers for processing. Two measuring electrical signals are respectively formed, and the two measuring electrical signals are input to the electronic signal processing component for processing;

In the above technical solution, the plane grating adopts a two-dimensional reflective grating, the refractive element adopts a refractor with an isosceles trapezoidal cross-section, and the second retroreflector and the third retroreflector adopt parallel retroreflectors. Arranged side by side.

Another technical solution of the present invention is that the refractive element is composed of two reflecting mirrors.

Another technical solution of the present invention is that the refractive element adopts a lens.

The invention has the following advantages and outstanding technical effects: the measurement system uses a single-frequency laser and an optical fiber to separate and transmit light, and the interferometer adopts a special structure, thus suppressing polarization aliasing errors and improving measurement accuracy; after the interferometer is adopted, A wide range of angle measurements can be achieved behind the reflector, and the optical path is symmetrical; the planar grating adopts a two-dimensional reflective grating, and the Littrow structure is used to achieve the system's two-degree-of-freedom measurement and insensitivity to Z-direction motion; the interferometer structure uses It has fewer optical components and a simple structure, making it easy for miniaturization and integration.

Description of the drawings

Figure 1 is a schematic diagram of a planar grating interferometer displacement measurement system of the present invention.

Figure 2 is an optical path diagram of the grating interferometer of the present invention.

Figure 3 is an optical path diagram of two reference light paths of the grating interferometer of the present invention.

Figure 4 is an optical path diagram of the measurement light of the grating interferometer of the present invention.

Figure 5 is an optical path diagram of another measuring light of the grating interferometer of the present invention.

Figure 6 is a schematic diagram of the internal structure of the first grating interferometer of the present invention.

Figure 7 is a schematic diagram of the internal structure of the second grating interferometer of the present invention.

Figure 8 is a schematic diagram of the internal structure of the third grating interferometer of the present invention.

In the figure, 1—single frequency laser, 2a—first beam splitter, 2b—second beam splitter; 2c—third beam splitter; 3—grating interferometer, 4—plane grating, 5a—first acoustic light Modulator, 5b—second acousto-optic modulator; 6—receiver; 7—electronic signal processing component; 31—polarizing beam splitter, 32—refractive element; 32a—refractor; 32b—reflector; 32c—lens; 33 - first retroreflector; 34 - second retroreflector; 35 - third retroreflector; 36 - quarter wave plate.

Detailed ways

The structure, principle and specific implementation modes of the present invention will be described in further detail below with reference to the accompanying drawings.

Please refer to Figure 1. The planar grating interferometer displacement measurement system includes a single-frequency laser 1, a beam splitter, a grating interferometer 3, a planar grating 4, an acousto-optic modulator, a receiver 6 and an electronic signal processing component 7. The planar grating 4 It is a two-dimensional reflective grating.

Please refer to Figure 2. The grating interferometer 3 includes a polarizing beam splitter 31, a refractive element 32, a first retroreflector 33, a second retroreflector 34 and a third retroreflector 35. Wave plate 36, refractive element 32, in which the first retroreflector 33 is located at the top of the polarizing beam splitter, and the second retroreflector 34 and the third retroreflector 35 are placed side by side at the bottom of the polarizing beam splitter.

Please refer to Figure 1 and Figure 2. After the single-frequency laser emitted by the single-frequency laser 1 is split by the first beam splitter 2a, it passes through the first acousto-optic modulator 5a and the second acousto-optic modulator 5a supplied by the frequency synthesizer 9 respectively. After being modulated by the laser 5b, the light is split by the second beam splitter 2b and the third beam splitter 2c respectively, wherein a laser beam splitted by the second beam splitter 2b and a laser splitted by the third beam splitter 2c After the laser beam is interfered by the fiber coupler 8, it is input to the receiver 6 as a compensation axis signal. After processing, an electrical signal is formed and input to the electronic signal processing component 7; the other two laser beams are incident on the polarizing beam splitter 31 and then split. The reflected light beam is the reference light, and the two transmitted light beams are the measurement light;

Please refer to Figures 2 and 3. The two reference lights are reflected back to the polarizing beam splitter 31 after passing through the first retroreflector 33. After reflection, the two reference lights are emitted in parallel.

Please refer to Figures 2, 4, and 5. The two measurement beams pass through the quarter-wave plate 36 and the refractive element 32 for the first time and then are incident on the plane grating 4 at Littrow angle. After reflection, they pass through the quarter-wave plate 36 and the refractive element 32 for the second time. The refractive element 32 and the quarter-wave plate 36 are incident on the polarizing beam splitter 31. After reflection, the two measuring beams pass through the second retroreflector 34 and the third retroreflector 35 respectively, and are reflected back to the polarizing beam splitter 31. After reflection again, it passes through the refractive element 32 and the quarter-wave plate 36 and then is incident on the plane grating 4 at the Littrow angle. After reflection, it passes through the refractive element 32 and the quarter-wave plate 36 again and is incident on the polarizing beam splitter 31 , two beams of measuring light emerge in parallel after transmission.

One beam of reference light and one beam of measurement light interfere to form an interference light signal, and the other beam of reference light and another beam of measurement light interfere to form another interference light signal. The two interference light signals are transmitted to the receiver 6 via optical fibers respectively. The processing forms two measured electrical signals respectively, and the two measured electrical signals are input to the electronic signal processing component 7 for processing.

General dual-frequency laser interferometers usually have polarization mixing. The reasons are that the polarization mixing of the dual-frequency laser occurs at the light source due to the imperfection of the dual-frequency laser, and polarization mixing may also occur when using a coaxial optical path to transmit light. Stack. The planar grating interferometer measurement system uses a single-frequency laser 1 and uses an acousto-optic modulator 5 for frequency modulation, which avoids polarization mixing at the light source. Moreover, the measurement system uses two frequency lasers separated by optical fibers. Transmit light. Inside the grating interferometer, refer to Figure 3. The two reference beams are s-polarized light and emerge after the first retroreflector 33 and the polarizing beam splitter 31. There is no polarization leakage in the free space optical path; refer to Figures 4 and 5. , after the two measurement lights pass through the second retroreflector 34 and the third retroreflector 35 respectively, due to the depolarization effect of the retroreflector, partial polarization leakage will occur when passing through the polarizing beam splitter 3, but after passing through Analysis shows that the error caused by the polarization leakage in this part is on the order of picometers. It can be considered that this structure effectively suppresses the polarization mixing error. In summary, the planar grating measurement system suppresses polarization aliasing and effectively reduces measurement errors.

Three retroreflectors are used in the interferometer structure. Two reference lights pass through the first retroreflector 33, and two measurement lights pass through the second retroreflector 34 and the third retroreflector 35 respectively. Since Due to the retroreflective characteristics of the retroreflector, the angle between the optical paths caused by the grating angle deviation is converted into beam separation, thus increasing the angle measurement range and achieving symmetry of the optical path.

The planar grating 4 adopts a two-dimensional reflective grating. The Littrow structure is used to reflect the measurement light twice from the planar grating 4. Based on the grating Doppler effect, two-degree-of-freedom displacement measurement and Z-direction motion insensitivity are achieved.

When the plane grating 4 performs linear motion with two degrees of freedom in the horizontal direction and vertical direction (the vertical movement is a small movement, and the movement range is ±1mm) relative to the grating interferometer 3, the electronic signal processing component 5 will output the two degrees of freedom. linear displacement. The expression of the two-degree-of-freedom motion displacement is x=p*(α+β)/8π, z=(α-β)/16π*cosθ, where α and β are the reading values of the electronic signal processing card, and p is the grating The constant, θ, is the grating diffraction angle, taking p=0.833μm, and the x and z measurement resolutions of the grating interferometer are 0.415nm and 0.22nm respectively.

Please refer to Figure 6, which is a schematic diagram of the internal structure of the first grating interferometer of the present invention. As shown in Figure 6, the refractive element in the internal structure of the grating interferometer uses a refractive mirror 32a.

Please refer to Figure 7, which is a schematic diagram of the internal structure of the second grating interferometer of the present invention. As shown in Figure 7, the refractive element in the internal structure of the grating interferometer is composed of two reflecting mirrors 32b. Compared with the refractor 32a solution, this solution can eliminate the beam error caused by the uneven refractive index of the refractor, but the installation of the reflector takes up more space.

Please refer to Figure 8, which is a schematic diagram of the internal structure of the third grating interferometer of the present invention. As shown in Figure 8, the refractive element in the internal structure of the grating interferometer uses lens 32c to deflect the beam. Compared with the reflector 32b, the lens 32c takes up less space and can make the interferometer structure more compact, simple and easy to install.

The measurement system and structural solution given in the above embodiments can achieve simultaneous measurement of displacements of two linear degrees of freedom; simultaneously suppress polarization mixing errors; achieve wide-range angle measurement, and achieve optical path symmetry; and achieve two degrees of freedom. The measurement and Z-direction motion are not sensitive; and the interferometer has a simple structure and is easy to be miniaturized and integrated. It is applied to the displacement measurement of the ultra-precision workpiece table of the photolithography machine. Compared with the laser interferometer measurement system, on the basis of meeting the measurement needs, it can effectively reduce the volume and quality of the workpiece table, greatly improve the dynamic performance of the workpiece table, and make the workpiece table overall. Overall performance improvement. The planar grating interferometer displacement measurement system can also be used for precision measurement of multi-degree-of-freedom displacement of workpiece tables in precision machine tools, three-dimensional coordinate measuring machines, semiconductor testing equipment, etc.

Claims (6)

1. A planar grating interferometer displacement measurement system, including a single-frequency laser (1), a beam splitter (2a, 2b, 2c), a grating interferometer (3), a planar grating (4), and an acousto-optic modulator (5a , 5b), receiver (6), electronic signal processing component (7), optical fiber coupler (8) and frequency synthesizer (9); characterized in that: the grating interferometer (3) includes a polarizing beam splitter (31), Refractive element (32), first retroreflector (33), second retroreflector (34), third retroreflector (35) and quarter wave plate (36); wherein the first retroreflector (34) The retroreflector (33) is located at the top of the polarizing beam splitter (31), and the second retroreflector (34) and the third retroreflector (35) are placed side by side at the bottom of the polarizing beam splitter (31); single frequency laser (1) After the emitted single-frequency laser is split by the first beam splitter (2a), it passes through the first acousto-optic modulator (5a) and the second acousto-optic modulator (5b) supplied by the frequency synthesizer (9). ) is modulated, and then splitted by the second beam splitter (2b) and the third beam splitter (2c) respectively, wherein a laser beam split by the second beam splitter (2b) and the third beam splitter (2c) After interference by the optical fiber coupler (8), a beam of laser light is input to the receiver (6) as a compensation axis signal. After processing, an electrical signal is formed and input to the electronic signal processing component (7); the other two The laser beam is incident on the polarizing beam splitter (31) and then split. The two reflected light beams are the reference light, and the two transmitted light beams are the measurement light; The two measurement beams pass through the quarter-wave plate (36) and the refractive element (32) for the first time, and then are incident on the plane grating (4) at the Littrow angle. After reflection, they pass through the refractive element (32) for the second time. and the quarter-wave plate (36) are incident on the polarizing beam splitter (31). After reflection, the two measurement beams pass through the second retroreflector (34) and the third retroreflector (35) respectively, and are reflected back to The polarizing beam splitter (31) reflects again and passes through the refractive element (32) and the quarter-wave plate (36), and then is incident on the plane grating (4) at the Littrow angle. After reflection, it passes through the refractive element (32) again. and a quarter-wave plate (36) before being incident on the polarizing beam splitter (31). After transmission, the two measurement beams emerge in parallel; The two reference lights are reflected back to the polarizing beam splitter (31) after passing through the first retroreflector (33). After reflection, the two reference lights are emitted in parallel; One beam of reference light and one beam of measurement light interfere to form an interference light signal, and the other beam of reference light and another beam of measurement light interfere to form another interference light signal. The two interference light signals are transmitted to the receiver via optical fibers (6 ) are processed to form two measurement electrical signals respectively, and the two measurement electrical signals are input to the electronic signal processing component (7) for processing. 2. A planar grating interferometer displacement measurement system according to claim 1, characterized in that: the planar grating (4) adopts a two-dimensional reflective grating. 3. A planar grating interferometer displacement measurement system according to claim 1, characterized in that: the refractive element adopts a refractor (32a) with an isosceles trapezoidal cross-section. 4. A planar grating interferometer displacement measurement system according to claim 1, characterized in that: the refractive element adopts two reflectors (32b). 5. A planar grating interferometer displacement measurement system according to claim 1, characterized in that: the refractive element adopts a lens (32c). 6. A planar grating interferometer displacement measurement system according to any one of claims 1 to 5, characterized in that: the second retroreflector (34) and the third retroreflector (35) adopt parallel Arranged side by side.