High precision double-interferometry displacement measurement by waveform transforming based on FFT

Abstract

The paper researches on a high precision displacement measurement system mixing double-wavelength interferometry and single-wavelength interferometry by waveform transforming based on Fast Fourier Transform (FFT) technology. The signal of double-wavelength interferometry is used for determining the amplitude of the measurand which makes the measurement range be as large as half a synthetic-wavelength, while that of single-wavelength interferometry is for measuring the value of the measurand precisely which endows the measurement resolution to be as high as less than 1nm, for the amount of the interference fringes of the signal of single-wavelength interferometry during the shifting range of the peak of the signal of double-wavelength interferometry demonstrates the value of the measurand. However, as the signal of double-wavelength interferometry is cosine amplitude modulated, the peak area of it is flatten and the peak position is difficult to be determined, which will influence the measurement precision directly. In order to address the peak position of the double-wavelength interferometric signal accurately, we transform using FFT technology the cosine amplitude modulated signal of double-wavelength interferometry into a triangle-wave amplitude modulated signal to make the peak position prominent. It is very easy to determine the peak position accurately and the amount of the interference fringes of the signal of single-wavelength interferometry during the shifting range of the peak will also be determined precisely. High precision displacement measurement with large range and high resolution could be realized. 1．INTRODUCTIONIn recent years, with the rapid development of semiconductor, advanced manufacturing and other scientific researches, there are great demands for displacement measurements applicable to millimeter range with sub-nanometer resolution. [1] Typically, conventional nanometrology can be divided into two kinds of approaches. One is the non-optical measurement techniques including atomic force microscope ,scanning tunneling microscope, and capactive sensors, etc, of which the measurement ranges are limited to the order of micrometer. Another is the optical measurement techniques including common-focus optical measurement technology and optical interferoemetry. Although the resolution of single-wavelength interferometry is as high as sub-nanometer resolution, the measurement range is only half a wavelength because of phase ambiguity.