Volume 13 Issue 6
Dec.  2020
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Article Contents
YIN Yun-Fei, LIU Zhao-Wu, JIRIGALANTU, YU Hong-Zhu, WANG Wei, LI Xiao-Tian, BAO He, LI Wen-Hao, HAO Qun. Overview of 2D grating displacement measurement technology[J]. Chinese Optics, 2020, 13(6): 1224-1238. doi: 10.37188/CO.2020-0237
Citation: YIN Yun-Fei, LIU Zhao-Wu, JIRIGALANTU, YU Hong-Zhu, WANG Wei, LI Xiao-Tian, BAO He, LI Wen-Hao, HAO Qun. Overview of 2D grating displacement measurement technology[J]. Chinese Optics, 2020, 13(6): 1224-1238. doi: 10.37188/CO.2020-0237

Overview of 2D grating displacement measurement technology

Funds:  Supported by the National Key R & D Plan (No. 2016YFB0500100); Jinlin Province Science and Technology Development Plan (No. 20190201021JC, No. 20190103158JH, No. 20190103157JH, No. 20190302047GX), R & D Projects in Key Areas of Guangdong Province (No. 2019B010144001), National Natural Science Foundation of China (No. 61975255), Civil Aerospace Pre-research Project (No. D040101)
More Information
  • Corresponding author: leewenho@163.comqhao@bit.edu.cn
  • Received Date: 09 Dec 2019
  • Rev Recd Date: 21 Jan 2020
  • Available Online: 10 Nov 2020
  • Publish Date: 01 Dec 2020
  • Ultra-precision displacement measurement technology is not only the basis of precision machining, but also plays a decisive role in the chip manufacturing industry that is rapidly developing in Moore's Law. The grating displacement measurement system based on the grating pitch is widely used in multidimensional measurement system. Compared with the laser displacement measurement system, grating displacement measurement system greatly reduces the environmental requirements for humidity, temperature and pressure. In this paper, the development status of the optical structure of displacement sensing system based on two-dimensional grating in recent years is introduced. The principles of zero-difference and heterodyne grating interferometrys are introduced. The optical structure based on single-block two-dimensional grating is reviewed. The development history of the optical structure in single-block two-dimensional grating to coupling designs of multi-block two-dimensional gratings is summarized, the advantages and disadvantages of several two-dimensional grating displacement measurement systems are compared and analyzed, and the development trend of two-dimensional grating displacement measurement system is prospected. The engineering process of two-dimensional grating displacement measurement system is summarized.

     

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  • [1]
    BAI Y, HU P CH, LU Y F, et al. A six-axis heterodyne interferometer system for the Joule balance[J]. IEEE Transactions on Instrumentation and Measurement, 2017, 66(6): 1579-1585. doi: 10.1109/TIM.2016.2634758
    [2]
    HORI Y, GONDA S, BITOU Y, et al. Periodic error evaluation system for linear encoders using a homodyne laser interferometer with 10 picometer uncertainty[J]. Precision Engineering, 2018, 51: 388-392. doi: 10.1016/j.precisioneng.2017.09.009
    [3]
    DENG J L, YAN X N, WEI CH L, et al. Eightfold optical encoder with high-density grating[J]. Applied Optics, 2018, 57(10): 2366-2375. doi: 10.1364/AO.57.002366
    [4]
    卢兴吉, 曹振松, 黄印博, 等. 3.53 μm激光外差太阳光谱测量系统[J]. 光学 精密工程,2018,26(8):1846-1854. doi: 10.3788/OPE.20182608.1846

    LU X J, CAO ZH S, HUANG Y B, et al. Laser heterodyne spectrometer for solar spectrum measurement in the 3.53 μm region[J]. Optics and Precision Engineering, 2018, 26(8): 1846-1854. (in Chinese) doi: 10.3788/OPE.20182608.1846
    [5]
    羡一民. 激光干涉仪的应用——激光干涉仪技术综述之五[J]. 工具技术,2015,49(2):79-85. doi: 10.3969/j.issn.1000-7008.2015.02.023

    XIAN Y M. Applications of laser interferometer——Summary of laser interferometer technology[J]. Tool Engineering, 2015, 49(2): 79-85. (in Chinese) doi: 10.3969/j.issn.1000-7008.2015.02.023
    [6]
    VAN DER P E A F, LOOPSTRA E R. Position measurement unit, measurement system and lithographic apparatus comprising such position measurement unit: US, 7362446[P]. 2008-04-22.
    [7]
    FAN K CH, LIAO B H, CHUNG Y CH, et al.. Displacement measurement of planar stage by diffraction planar encoder in nanometer resolution[C]. Proceedings of 2012 IEEE International Instrumentation and Measurement Technology Conference, IEEE, 2012: 894-897.
    [8]
    王韵致, 谢芳, 陈龙辉, 等. 用于高精度测量位移等参量的光纤多波长激光器[J]. 光学 精密工程,2019,27(9):2036-2042. doi: 10.3788/OPE.20192709.2036

    WANG Y ZH, XIE F, CHEN L H, et al. Research on optical fiber multi-wavelength laser for measuring displacement precisely[J]. Optics and Precision Engineering, 2019, 27(9): 2036-2042. (in Chinese) doi: 10.3788/OPE.20192709.2036
    [9]
    高旭, 李舒航, 马庆林, 等. 光栅精密位移测量技术发展综述[J]. 中国光学,2019,12(4):741-752. doi: 10.3788/co.20191204.0741

    GAO X, LI SH H, MA Q L, et al. Development of grating-based precise displacement measurement technology[J]. Chinese Optics, 2019, 12(4): 741-752. (in Chinese) doi: 10.3788/co.20191204.0741
    [10]
    吕强, 李文昊, 巴音贺希格, 等. 基于衍射光栅的干涉式精密位移测量系统[J]. 中国光学,2017,10(1):39-50. doi: 10.3788/co.20171001.0039

    LV Q, LI W H, BAYANHESHIG, et al. Interferometric precision displacement measurement system based on diffraction grating[J]. Chinese Optics, 2017, 10(1): 39-50. (in Chinese) doi: 10.3788/co.20171001.0039
    [11]
    Optra Inc. The Nano grid principle of Measure[EB/OL]. [2019-04-12]. https://www.heidenhain.com.cn/zh_CN/company/.
    [12]
    GAO W, DEJIMA S, KIYONO S. A dual-mode surface encoder for position measurement[J]. Sensors and Actuators A:Physical, 2005, 117(1): 95-102. doi: 10.1016/j.sna.2004.06.004
    [13]
    KIMURA A, GAO W, ARAI Y, et al. Design and construction of a two-degree-of-freedom linear encoder for nanometric measurement of stage position and straightness[J]. Precision Engineering, 2010, 34(1): 145-155. doi: 10.1016/j.precisioneng.2009.05.008
    [14]
    KIMURA A, GAO W, ZENG L J. Position and out-of-straightness measurement of a precision linear air-bearing stage by using a two-degree-of-freedom linear encoder[J]. Measurement Science and Technology, 2010, 21(5): 054005. doi: 10.1088/0957-0233/21/5/054005
    [15]
    KIMURA A, GAO W, KIM W J, et al. A sub-nanometric three-axis surface encoder with short-period planar gratings for stage motion measurement[J]. Precision Engineering, 2012, 36(4): 576-585. doi: 10.1016/j.precisioneng.2012.04.005
    [16]
    GAO W, SAITO Y, MUTO H, et al. A three-axis autocollimator for detection of angular error motions of a precision stage[J]. CIRP Annals, 2011, 60(1): 515-518. doi: 10.1016/j.cirp.2011.03.052
    [17]
    LI X H, GAO W, MUTO H, et al. A six-degree-of-freedom surface encoder for precision positioning of a planar motion stage[J]. Precision Engineering, 2013, 37(3): 771-781. doi: 10.1016/j.precisioneng.2013.03.005
    [18]
    GAO W, KIM S W, BOSSE H, et al. Measurement technologies for precision positioning[J]. CIRP Annals, 2015, 64(2): 773-796. doi: 10.1016/j.cirp.2015.05.009
    [19]
    周维来. 光栅干涉仪在高精密测量中的技术和应用[J]. 工具技术,1994,28(1):37-42.

    ZHOU W L. Technology and application of grating interferometer in high precision measurement[J]. Tool Engineering, 1994, 28(1): 37-42. (in Chinese)
    [20]
    CASTENMILLER T, VAN DE MAST F, DE KORT T, et al. Towards ultimate optical lithography with NXT:1950i dual stage immersion platform[J]. Proceedings of SPIE, 2013, 7640: 76401N.
    [21]
    夏豪杰, 费业泰, 范光照, 等. 基于衍射光栅的二维纳米位移测量技术[J]. 纳米技术与精密工程,2007,5(4):311-314. doi: 10.3969/j.issn.1672-6030.2007.04.019

    XIA H J, FEI Y T, FAN G ZH, et al. 2D Nano-displacement measurement with diffraction grating[J]. Nanotechnology and Precision Engineering, 2007, 5(4): 311-314. (in Chinese) doi: 10.3969/j.issn.1672-6030.2007.04.019
    [22]
    夏豪杰. 高精度二维平面光栅测量系统及关键技术研究[D]. 合肥: 合肥工业大学, 2006.

    XIA H J. Research on precise 2-D plane grating measurement system and key technology[D]. Hefei: Hefei University of Technology, 2006. (in Chinese).
    [23]
    曹向群. 光栅计量技术[M]. 杭州: 浙江大学出版社, 1992.

    CAO X Q. Grating Measurement Technology[M]. Hangzhou: Zhejiang University Press, 1992. (in Chinese)
    [24]
    尚平, 夏豪杰, 费业泰. 衍射式光栅干涉测量系统发展现状及趋势[J]. 光学技术,2011,37(3):313-316.

    SHANG P, XIA H J, FEI Y T. Research status and developing trends of diffraction grating interferometer measurement system[J]. Optical Technique, 2011, 37(3): 313-316. (in Chinese)
    [25]
    张善钟. 计量光栅技术[M]. 北京: 机械工业出版社, 1985.

    ZHANG SH ZH. Metrology Grating Technology[M]. Beijing: Mechanical Industry Press, 1985. (in Chinese)
    [26]
    祝宏彬, 陈俊雹, 郭冬梅, 等. 零差激光干涉仪的大范围位移测量与精度分析[J]. 光电子技术,2016,36(1):5-11.

    ZHU H B, CHEN J B, GUO D M, et al. Large-range displacement measurement and accuracy analysis of homodyne laser interferometer[J]. Optoelectronic Technology, 2016, 36(1): 5-11. (in Chinese)
    [27]
    ELLIS J D. Field Guide to Displacement Measuring Interferometry[M]. Bellingham, UK: SPIE Press, 2014.
    [28]
    FREIDAH J T, CAHILL R F, JOSEPH A A, et al. Passive homodyne optical grating demodulator: principles and performance[J]. Proceedings of SPIE, 1986, 566: 114-121. doi: 10.1117/12.949774
    [29]
    WU C C, CHENG CH Y, YANG Z Y. Optical homodyne common-path grating interferometer with sub-nanometer displacement resolution[J]. Proceedings of SPIE, 2010, 7791: 779105. doi: 10.1117/12.860513
    [30]
    GUPTA P, SPEIRS R W, JONES K M, et al. Effect of imperfect homodyne visibility on multi-spatial-mode two-mode squeezing measurements[J]. Optics Express, 2020, 28(1): 652-664. doi: 10.1364/OE.379033
    [31]
    GAO W, KIMURA A. A three-axis displacement sensor with nanometric resolution[J]. CIRP Annals, 2007, 56(1): 529-532. doi: 10.1016/j.cirp.2007.05.126
    [32]
    WANG L J, ZHANG M, ZHU Y, et al.. A novel heterodyne grating interferometer system for in-plane and out-of-plane displacement measurement with nanometer resolution[C]. Proceedings of the 29th Annual Meeting of the American Society for Precision Engineering, ASPE, 2014: 173-177.
    [33]
    SHIMIZU Y, ITO T, LI X H, et al. Design and testing of a four-probe optical sensor head for three-axis surface encoder with a mosaic scale grating[J]. Measurement Science and Technology, 2014, 25(9): 094002. doi: 10.1088/0957-0233/25/9/094002
    [34]
    LIN J, GUAN J, WEN F, et al. High-resolution and wide range displacement measurement based on planar grating[J]. Optics Communications, 2017, 404: 132-138. doi: 10.1016/j.optcom.2017.03.012
    [35]
    金涛, 刘景林, 杨卫, 等. 线性位移台直线度高精密外差干涉测量装置[J]. 光学 精密工程,2018,26(7):1570-1577. doi: 10.3788/OPE.20182607.1570

    JIN T, LIU J L, YANG W, et al. High-precision straightness interferometer for linear moving stage[J]. Optics and Precision Engineering, 2018, 26(7): 1570-1577. (in Chinese) doi: 10.3788/OPE.20182607.1570
    [36]
    赵博, 晏磊, 郝德阜. 一种二维光栅干涉仪的研究[J]. 仪器仪表学报,2001,22(3):271-276. doi: 10.3321/j.issn:0254-3087.2001.03.013

    ZHAO B, YAN L, HAO D F. Experimental study about a type of gratings interferometer[J]. Chinese Journal of Scientific Instrument, 2001, 22(3): 271-276. (in Chinese) doi: 10.3321/j.issn:0254-3087.2001.03.013
    [37]
    朱煜, 张鸣, 王磊杰, 等. 一种双频光栅干涉仪位移测量系统: 中国, 102937411A[P]. 2013-02-20.

    ZHU Y, ZHANG M, WANG L J, et al.. Double-frequency grating interferometer displacement measurement system: CN, 102937411A[P]. 2013-02-20. (in Chinese).
    [38]
    朱煜, 王磊杰, 张鸣, 等. 一种二自由度外差光栅干涉仪位移测量系统: 中国, WO2014/201950A1[P]. 2013-03-20.

    ZHU Y, WANG L J, ZHANG M, et al.. A two-degree-of-freedom heterodyne grating interferometer displacement measurement system: CN, WO2014/201950A1[P]. 2013-03-20. (in Chinese).
    [39]
    王芳, 齐向东. 高精度控制光电光栅刻划机的光栅外差干涉仪[J]. 激光技术,2008,32(5):474-476, 526.

    WANG F, QI X D. Grating heterodyne interferometer of high accuracy controlling photoelectric grating ruling engine[J]. Laser Technology, 2008, 32(5): 474-476, 526. (in Chinese)
    [40]
    于梅, 刘爱东, 何闻, 等. 衍射光栅外差激光干涉法角振动校准技术研究[J]. 计量学报,2015,36(6):561-564. doi: 10.3969/j.issn.1000-1158.2015.06.01

    YU M, LIU A D, HE W, et al. Angle vibration calibration technology by diffraction grating heterodyne laser interferometry[J]. Acta Metrologica Sinica, 2015, 36(6): 561-564. (in Chinese) doi: 10.3969/j.issn.1000-1158.2015.06.01
    [41]
    王磊杰, 张鸣, 朱煜, 等. 超精密外差利特罗式光栅干涉仪位移测量系统[J]. 光学 精密工程,2017,25(12):2975-2985. doi: 10.3788/OPE.20172512.2975

    WANG L J, ZHANG M, ZHU Y, et al. A displacement measurement system for ultra-precision heterodyne Littrow grating interferometer[J]. Optics and Precision Engineering, 2017, 25(12): 2975-2985. (in Chinese) doi: 10.3788/OPE.20172512.2975
    [42]
    王磊杰, 张鸣, 朱煜, 等. 面向浸没式光刻机的超精密光学干涉式光栅编码器位移测量技术综述[J]. 光学 精密工程,2019,27(9):1909-1918. doi: 10.3788/OPE.20192709.1909

    WANG L J, ZHANG M, ZHU Y, et al. Review of ultra-precision optical interferential grating encoder displacement measurement technology for immersion lithography scanner[J]. Optics and Precision Engineering, 2019, 27(9): 1909-1918. (in Chinese) doi: 10.3788/OPE.20192709.1909
    [43]
    王磊杰, 张鸣, 朱煜, 等. 扫描干涉光刻机的超精密移相锁定系统[J]. 光学 精密工程,2019,27(8):1765-1773. doi: 10.3788/OPE.20192708.1765

    WANG L J, ZHANG M, ZHU Y, et al. Ultra-precision phase-shifting locking system of scanning beam interference lithography tool[J]. Optics and Precision Engineering, 2019, 27(8): 1765-1773. (in Chinese) doi: 10.3788/OPE.20192708.1765
    [44]
    LEE C B, LEE S K. Multi-degree-of-freedom motion error measurement in an ultraprecision machine using laser encoder-review[J]. Journal of Mechanical Science and Technology, 2013, 27(1): 141-152. doi: 10.1007/s12206-012-1217-6
    [45]
    HU P CH, CHANG D, TAN J B, et al. Displacement measuring grating interferometer: a review[J]. Frontiers of Information Technology &Electronic Engineering, 2019, 20(5): 631-654.
    [46]
    王选择, 郭军, 谢铁邦. 以正交衍射光栅为计量标准器的二维微位移工作台[J]. 光学 精密工程,2003,11(5):492-496.

    WANG X Z, GUO J, XIE T B. 2D-platform with cross diffraction grating as displacement measurement sensor[J]. Optics and Precision Engineering, 2003, 11(5): 492-496. (in Chinese)
    [47]
    WANG X Z, DONG X H, GUO J, et al. Two-dimensional displacement sensing using a cross diffraction grating scheme[J]. Journal of Optics A:Pure and Applied Optics, 2004, 6(1): 106-111. doi: 10.1088/1464-4258/6/1/019
    [48]
    LIN C B, YAN SH H, DING D, et al. Two-dimensional diagonal-based heterodyne grating interferometer with enhanced signal-to-noise ratio and optical subdivision[J]. Optical Engineering, 2018, 57(6): 064102.
    [49]
    邢旭, 常笛, 谭久彬, 等. 空间分离式外差二自由度平面光栅干涉仪[J]. 光学 精密工程,2019,27(8):1727-1736. doi: 10.3788/OPE.20192708.1727

    XING X, CHANG D, TAN J B, et al. Spatially separated heterodyne grating interferometer for in-plane displacement measurement[J]. Optics and Precision Engineering, 2019, 27(8): 1727-1736. (in Chinese) doi: 10.3788/OPE.20192708.1727
    [50]
    朱凡, 谭欣然, 谭久彬, 等. 高分辨力与高输出稳定性自准直系统设计[J]. 光学 精密工程,2016,24(10):109-116.

    ZHU F, TAN X R, TAN J B, et al. Design of high resolution and output stability autocollimation system[J]. Optics and Precision Engineering, 2016, 24(10): 109-116. (in Chinese)
    [51]
    崔继文, 刘雪明, 谭久彬. 超精密级二维工作台的自标定[J]. 光学 精密工程,2012,20(9):1960-1966. doi: 10.3788/OPE.20122009.1960

    CUI J W, LIU X M, TAN J B. Self-calibration for 2-D ultra-precision stage[J]. Optics and Precision Engineering, 2012, 20(9): 1960-1966. (in Chinese) doi: 10.3788/OPE.20122009.1960
    [52]
    HSIEH H L, PAN S W. Three-degree-of-freedom displacement measurement using grating-based heterodyne interferometry[J]. Applied Optics, 2013, 52(27): 6840-6848. doi: 10.1364/AO.52.006840
    [53]
    HSIEH H L, PAN S W. Development of a grating-based interferometer for six-degree-of-freedom displacement and angle measurements[J]. Optics Express, 2015, 23(3): 2451-2465. doi: 10.1364/OE.23.002451
    [54]
    HSIEH H L, CHEN W. Heterodyne Wollaston laser encoder for measurement of in-plane displacement[J]. Optics Express, 2016, 24(8): 8693-8707. doi: 10.1364/OE.24.008693
    [55]
    徐敏儿. 基于衍射光栅的高分辨力位移测量系统研究[D]. 哈尔滨: 哈尔滨工业大学, 2013.

    XU M E. Research on and high-resolution displacement measurement system based on diffractive grating[D]. Harbin: Harbin Institute of Technology, 2013. (in Chinese).
    [56]
    王超群. 具有绝对零位的三自由度光栅位移测量技术研究[D]. 哈尔滨: 哈尔滨工业大学, 2017.

    WANG C Q. Research on three-degree-of-freedom grating displacement measurement with absolute zero alignment[D]. Harbin: Harbin Institute of Technology, 2017. (in Chinese).
    [57]
    邸晶晶. 基于衍射光栅的高精度位移测量系统的设计[D]. 哈尔滨: 哈尔滨工业大学, 2012.

    DI J J. Design of high-precision displacement measurement system based on diffractive grating[D]. Harbin: Harbin Institute of Technology, 2012. (in Chinese).
    [58]
    关健. 基于二维光栅的高精度三维位移测量系统[D]. 哈尔滨: 哈尔滨工业大学, 2014.

    GUAN J. High-precision 3-dimensional displacement measurement system based on 2-dimensional grating[D]. Harbin: Harbin Institute of Technology, 2014. (in Chinese).
    [59]
    魏培培. 基于双光栅干涉的三维位移测量技术研究[D]. 哈尔滨: 哈尔滨工业大学, 2015.

    WEI P P. Research on 3-D displacement measurement technology based on double grating interference[D]. Harbin: Harbin Institute of Technology, 2015. (in Chinese).
    [60]
    温凤. 基于平面光栅的三维位移测头研究[D]. 哈尔滨: 哈尔滨工业大学, 2015.

    WEN F. The three-dimensional displacement scanner based on planar grating[D]. Harbin: Harbin Institute of Technology, 2015. (in Chinese).
    [61]
    陈航. 外差式光栅粗/细位移测量系统的研究[D]. 哈尔滨: 哈尔滨工业大学, 2017.

    CHEN H. Research on heterodyne grating displacement measurement system with high/low displacement resolution[D]. Harbin: Harbin Institute of Technology, 2017. (in Chinese).
    [62]
    WEI P P, LU X, QIAO D CH, et al. Two-dimensional displacement measurement based on two parallel gratings[J]. Review of Scientific Instruments, 2018, 89(6): 065105. doi: 10.1063/1.5024637
    [63]
    LIN D J, JIANG H, YIN CH Y. Analysis of nonlinearity in a high-resolution grating interferometer[J]. Optics &Laser Technology, 2000, 32(2): 95-99.
    [64]
    WANG L J, ZHANG M, ZHU Y, et al. Construction and accuracy test of a novel heterodyne grating interferomter system for two-dimensional displacement measurement[J]. Laser, 2013, 89(3): 69.
    [65]
    LV Q, LIU ZH W, WANG W, et al. Simple and compact grating-based heterodyne interferometer with the Littrow configuration for high-accuracy and long-range measurement of two-dimensional displacement[J]. Applied Optics, 2018, 57(31): 9455-9463. doi: 10.1364/AO.57.009455
    [66]
    吕强. 基于衍射光栅的外差Littrow式精密位移测量系统关键技术研究[D]. 长春: 中国科学院大学(中国科学院长春光学精密机械与物理研究所), 2019.

    LV Q. Study on key technology of heterodyne grating-based precision displacement measurement system with Littrow structure[D]. Changchun: University of Chinese Academy of Sciences (Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences), 2019. (in Chinese).
    [67]
    LU Y C, WEI CH L, JIA W, et al. Two-degree-freedom displacement measurement based on a short period grating in symmetric Littrow configuration[J]. Optics Communications, 2016, 380: 382-386. doi: 10.1016/j.optcom.2016.06.016
    [68]
    KURODA A. Optical displacement measurement system for detecting the relative movement of a machine part: US, 6166817[P]. 2000-12-26.
    [69]
    AKIHIRO K. Optical displacement measurement system: US, 6407815[P]. 2002-06-18.
    [70]
    HOLZAPFEL W. Advancements in displacement metrology based on encoder systems[C]. Proceedings of the 23rd Annual ASPE Meeting, 2008.
    [71]
    THIEL J, SPANNER E. Interferential linear encoder with 270 mm measurement length for nanometrology[C]. Proceedings of the 1st International Conference and general meeting of the European Society for Precision Engineering and Nanotechnology, 1999.
    [72]
    LOF J, DERKSEN A T A M, HOOGENDAM C A, et al.. Lithographic apparatus and device manufacturing method: US, 6819400B2[P]. 2005-10-21.
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