Two-freedom homodyne grating interferometer displacement measuring system based on optical octave method

Abstract

A two-freedom homodyne grating interferometer displacement measuring system based on the optical octave method comprises a grating interferometer, a measuring grating, pre-processing unit and an electric signal processing unit. The grating interferometer comprises a laser tube, a polarizing film, a polarization splitting prism, a reference grating, a refraction element and a four-passage homodyne structure. On the basis of optical grating diffraction, optical Doppler and homodyne signal processing, the system realizes displacement measurement. The grating interferometer outputs optical signals to the pre-processing unit. The optical signals are converted into electric signals and are transmitted to the electric signal processing unit. When the interferometer and the measuring grating are in two-freedom linear relative motion, the system can output two linear displacements. The measuring system realizes optical octave through the second diffraction principle, can realize sub-nanometer and even higher resolution. Homodyne signal treatment is adopted, so that the influence brought by changes of direct components and amplitudes can be eliminated. The two-freedom homodyne grating interferometer displacement measuring system has the advantages of being insensitive to environments, high in measuring precision and the like, and can improve the performance of a workpiece table when used as a lithography machine ultra-precise workpiece table displacement measuring system.

Description

technical field [0001] The invention relates to a grating measurement system, in particular to a two-degree-of-freedom homodyne grating interferometer measurement system. Background technique [0002] In nanometer precision positioning and displacement monitoring, laser interferometry is the most widely used non-contact precision measurement technology. Heterodyne and homodyne laser interferometry are usually used. Heterodyne interferometry has the advantages of fast response, large measurement range, and strong anti-interference ability. However, due to the influence of factors such as light source, optical device adjustment error, processing error, and environment in the interferometer, the orthogonally polarized measurement light and reference light cannot be completely separated, resulting in the existence of nonlinear errors, even exceeding 10nm in large cases. And with the continuous improvement of motion indicators such as measurement accuracy, measurement distance, ...

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Netherlands ASML company US patent US7,102,729B2 (disclosure date August 4, 2005), US7,483,120B2 (disclosure date November 15, 2007), US7,940,392B2 (disclosure date December 24, 2009) , Publication No. US2010 / 0321665A1 (public date: December 23, 2010) discloses a planar grating measurement system and layout scheme applied to ultra-precision workpiece tables of lithography machines. The measurement system mainly uses a one-dimensional or two-dimensional plane The grating cooperates with the reading head to measure the horizontal large-stroke displacement of the workpiece table, and height sensors such as eddy currents or interferometers are used to measure the displacement in the height direction, but the application of various sensors limits the measurement accuracy of the workpiece table

US patent document US7,864,336B2 (published on January 4, 2011) discloses a grating interferometer measurement system applied to the ultra-precision workpiece stage of a lithography machine. The system uses the Littrow conditional grating reflection of two reading heads to realize Displacement measurement, but requires many components and large volume, and the measurement efficiency and accuracy are limited due to the calculation through the results of two reading heads

U.S. Patent Publication No. US2011 / 0255096A1 (published on October 20, 2011) of ZYGO Corporation of the United States discloses a grating measurement system applied to ultra-precision workpiece tables of lithography machines. The measurement system also uses one-dimensional or two-dimensional gratings to cooperate with A specific reading head realizes displacement measurement, which can simultaneously measure horizontal and vertical displacements, but the structure is complicated; Japanese CANON company US Patent Publication No. US2011 / 0096334A1 (publication date April 28, 2011) discloses a heterodyne interferometry In this interferometer, a grating is used as the objective mirror, but the interferometer can only realize one-dimensional measurement

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", Japanese scholar GAOWEI proposed a single encoder using the principle of diffraction interference. Frequency two-dimensional grating measurement system, which can realize horizontal and vertical displacement measurement at the same time, but due to the use of single-frequency laser, the measurement signal is susceptible to interference, and the accuracy is difficult to guarantee

Tsinghua University China Patent Application No. 201210449244.9 (application date November 9, 2012) and 201210448734.7 (application date November 09, 2012) respectively disclose a heterodyne grating interferometer measurement system, the two interferometer measurement systems The structure of the reading head uses a quarter-wave plate to change the polarization state of the beam, the optical structure is complex, and the non-ideality of the optical components will lead to measurement errors

In addition, Chinese Patent Literature Publication Nos. CN103307986A (disclosure date: September 18, 2013) and CN103322927A (disclosure date: September 18, 2013) respectively disclose a heterodyne grating interferometer measurement system. The structure of the reading head adopts the optical two subdivision optical path design, which leads to the problem of low resolution, and adopts the principle of heterodyne phase detection, which cannot eliminate the influence of DC component and amplitude change.

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