Research On Theory And Experiment System Of Two-dimensional Zero-reference Gratings

With the development of micro fabrication and nano technology, the demand for ultrahigh resolution positioning is becoming more and more important. It is also very important to provide a zero reference signal for absolute measurement in these areas. Supported by Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) of China (No. 20030358020), we studied a kind of specially coded two-dimensional zero-reference grating systematically.The zero reference signals are obtained from the non-periodic code in both x and y direction. When the two gratings are in the correct alignment position, the maximal output signal will be detected. The width of the peak is only one element width of the grating and each slope of this peak is of good linearity. If there is relatively small misalignment in either the x direction or the y direction, the output intensity will fall rapidly.The main research results are listed below:Firstly, the methods for the design of two-dimensional zero-reference codes based on autocorrelation theory are summarized and developed. A simple method for the design of two-dimensional zero-reference codes is presented: combined with a global method, one-dimensional zero-reference codes have been designed by means of a GA algorithm, then expand this optimum code to two-dimensional, the sensitivity keeps the same. This method proves to be a powerful tool, the laborious calculations are greatly reduced. According to different applications, several special codes are designed successfully use different methods.Secondly, due to the small dimensions of the grating, the diffraction effect is strong and it cannot be ignored. In this thesis, output characteristics based on the scalar diffraction theory are studied numerically, the transmission function obtained by diffraction analysis agrees with the result from the autocorrelation method by neglecting the diffraction effect under certain conditions, but the frequency of the laser, the gap between the two gratings and the dimension of the grating element affected the output intensity greatly. They will not only change the linearity and contrast of the output peak but also reverse it when the gap between the two gratings is large enough. Based on diffraction theory, the effects introduced by several kinds of fabricating and installation errors of the gratings are also analyzed in this paper, it is found that such grating alignment system has the function of reducing output error.Thirdly, the experiment setup of this kind of two-dimensional zero-reference grating is established. The arrangement of optical path, the design of circuit and the system software are presented in detail. The experiment results agree with the theoretic ones very well, and the repeated positioning accuracy is better than 20 nmwhen the width of the grating element is 5μm.Finally, some important applications of two-dimensional zero-reference grating are discussed, including the application in two-dimensional absolute displacement measurement, lithography alignment, time-stability measurement and compensation of a scanning probe microscope instrument, and a novel three-dimensional zero-reference system.