Research On Phase Subdivision For Interferometric Signal Measured By Motion Stage Of Lithography

With the continuous development of China's science and technology,the precision requirements of chip manufacturing equipment are constantly improving.The high-precision measurement of the displacement measurement system plays a key role in the accuracy of the equipment.In high-precision displacement measurement systems,optical interfero-meters have many unique advantages,but the displacement represented by the full-cycle signal is not sufficient for the current industry.In order to improve the accuracy of the displacement measurement,the measurement signal is subdivided,and the whole cycle signal is divided into a plurality of signals to achieve frequency multiplication of the signal.In this paper,the measurement signal of the grating interferometer is taken as the research object,and the method of extracting the instantaneous frequency by wavelet transform is used to study the scheme of signal phase subdivision in the time domain.The main research contents are as follows.First,the related theory of measuring the displacement of the motion table by the interferometer is introduced.The measurement signal of the interferometer is set according to the motion state of the motion stage for subsequent simulation processing.Through the comparison of several subdivision schemes,the phase segmentation scheme that is subdivided in the time domain is selected.By different methods for determining the frequency of subdivision points,two phase subdivision schemes with frequency of sampling point as frequency and uniform distribution between sampling points are designed.In the direct processing of the frequency variation function of the simulated signal,the average value of the phase difference of the single subdivision phase obtained by the subdivision method with the sampling point frequency as the frequency is 2.J92×10-6rad,and the subdivision frequency is distributed according to the uniform function.The average of the phase differences of the individual subdivision phases obtained by the subdivision method is 2.496×10-8rad.After comparing the two schemes,it can be found that the phase difference obtained by the subdivision point frequency according to the subdivision method of the uniform function distribution is smaller,and the subdivision method using the subdivision point frequency according to the uniform function distribution is selected.Second,the phase subdivision of the signal requires first determining the frequency of the signal.This paper chooses the frequency of the extracted signal by wavelet transform.After the wavelet transform processing is performed on the value of the simulated signal,the frequency of the signal is extracted,and then the phase is subdivided by the frequency of the obtained signal.The average value of the phase difference of the single subdivision phase obtained by the subdivision method according to the subdivision method of the uniform function distribution is 7.744×10-4rad,which is higher than the phase difference of the ideal signal frequency subdivision processing.It indicates that the error is brought into the process after the wavelet transform process,so it is necessary to analyze and optimize the process error.Thirdly,aiming at the error caused by the edge effect of the port and the discretization of the wavelet transform in the signal processing process,the five-point smoothing method and the extension method are respectively designed to deal with the edge effect of the port,and a multi-fitting function based on the fitting function is designed.After optimization,the phase error of the single subdivision phase is 2.831×10-5rad,which is nearly 10 times lower than the phase error of 1.911×10-4rad obtained without optimization.In the case where the displacement is measured by a grating interferometer composed of a grating having a selected pitch a = 833.3nm,and the subdivision factor is 1024 times.The single subdivision displacement is 0.8nm,and the displacement error is 1.870×10-3nm,which is the expected target of the study.It is also possible to transform the subdivision factor according to different needs and has good stability.