Differential Motion Grating Displacement Measurement System Based on Moiré Fringe Projection Imaging
Objective With the continuous progress in semiconductor chip manufacturing processes,nanoscale-precision measurement technology has become an urgent scientific research problem to be solved.Grating displacement measurement technology has attracted the attention of domestic and foreign researchers owing to its high accuracy,robustness,and other advantages.In recent years,research on grating displacement measurement systems has continued,but some problems remain.Most systems use four split-phase sine-wave photoelectric signals for displacement measurement,which requires the introduction of a large number of optical components in the optical path,making it difficult to miniaturize the system.With this method,it is difficult to break the limitations of the grating processing level,and the demodulation error of phase edge information is also relatively large.In addition,the matching high frequency subdivision technology and subdivision error correction technology are relatively mature,making it difficult to continue improving the system resolution and accuracy.To further improve the resolution and accuracy of the grating displacement measurement system and achieve miniaturization of the system,this paper proposes a differential grating displacement measurement system based on Moire fringe projection imaging.Methods Image-based measurement systems based on the principle of grating polarized light interference have various advantages,and the traditional photoelectric conversion systems can be transformed into digital image processing systems.The use of image processing methods with the displacement amplification characteristics of Moiré fringes avoids the limitations of current grating processing systems for high-precision and high-resolution displacement measurements.Improving system measurement accuracy and resolution while reducing the use of multiple spectroscopic prisms makes miniaturization easier.Results and Discussions A high-resolution single-grating displacement optical sensing system was designed for the projection imaging of Moiré fringes(Fig.1).Compared with traditional systems,the special design of the reading head enables the system to achieve four optical subdivisions of the measured grating period with only one diffraction.The design improves the basic resolution of the system and replaces the traditional photoelectric conversion system with a digital image processing system.It avoids the problem of adding multiple light-splitting prisms and optical mirror groups in the optical path owing to the phase-shift measurement requirements of traditional photoelectric conversion systems,making it easier to achieve miniaturization.A mathematical model of the image-based displacement sensing signal was derived.Based on the optical mechanism of Moiré fringe imaging,specific calculations were carried out on the scribing,flatness,installation,and adjustment errors of the grating(Fig.4),which is the core component of the system.This further explained the relationship between the maximum allowable speed of the dynamic operation of the system and frame rate(Fig.5).Simulations and design performance analyses were conducted for the system(Figs.6-8).As shown in Fig.6,the spot energy generated by the system is concentrated at the center of the detector and has a good degree of overlap,providing high-quality image signals for the subsequent demodulation.As can be observed in Fig.7,the system ultimately presents a clear Moiré fringe pattern on the detector image plane.By using image processing methods to count and subdivide the stacked grating patterns,the displacement of the grating can be measured.As depicted in Fig.8,the distribution of the grayscale extreme value curve after interpolation and subdivision is consistent with that of the original image,and the stripe centerline is located at the brightest point of the stripe center.It can be observed from the demodulation results that when using a diffraction grating with a measurement grating of 1200 line/mm,the system has a resolution of 2.075 μm before subdivision.After subdivision,the resolution reaches the nanometer/sub-nanometer level.Conclusions A differential grating displacement measurement method based on Moiré fringe projection imaging is proposed to overcome the limitations of traditional grating displacement measurement systems that rely on four split-phase sine-wave photoelectric signal measurements.Based on the principle of grating polarized light interference,image processing methods,and displacement amplification characteristics of Moiré fringes,the limitation of the current grating processing level in high-precision displacement measurements is avoided.A high-resolution single-grating displacement optical sensing system for projection imaging of Moiré fringes is designed,which realizes the acquisition of four-fold optical subdivision displacement signals and completes the image conversion of displacement signals using a CMOS image detector.A mathematical model of the image-based displacement sensing signal is derived.Based on the optical mechanism of Moiré fringe imaging,specific calculations are performed on the scribing error,flatness error,installation,and adjustment error of the grating,which is the core component of the system.Simulation and design performance analyses are conducted on the system,which further explain the relationship between the maximum allowable speed of the dynamic operation of the system and the frame rate.At the maximum allowable speed of 0.048 mm/s,the basic resolution of the system can reach 2.075 μm;after subdivision,the resolution can reach the nanometer/sub nanometer level.
NETL
NSTL
万方数据
