大视场白光干涉测量系统及性能研究

Research on Large Field-of-view White Light Interferometry Measurement System and Performance

基于国产化的2倍迈克尔逊型干涉物镜组,优选配置0.5倍适配镜,对白光LED照明光源进行带通滤波参数的仿真估算和实验性能比较,构建了整套大视场白光干涉精密测量装置系统并进行了实验测试,通过白光干涉轴向响应实验曲线确定了中心波长。实验结果表明:通过光谱滤波获得了较为理想的白光干涉轴向响应曲线;系统的水平最大视场达到了14 mm;高度为2.04μm和20.43μm的标准台阶样品的测量结果分别为2.05μm和20.47μm,10次测量重复精度(标准差)分别为12 nm和16 nm。对粗糙度样板、微机电系统传感结构和半导体晶圆膜层进行了实测,表明所研制的系统装置在三维光学无损精密检测领域的应用具有可行性。

White Light Interferometry(WLI)is a classic low-coherence interferometry.The surface height information can be obtained with ultra-high precision through the positioning of zero optical path difference position of white light interference.At the same time,compared with point-sectioning laser confocal microscope,WLI is a surface-sectioning tomography technology.In particular,its vertical resolution is comparable to that of an atomic force microscope.White light interferometry measurement technology has been widely used in the inspection of semiconductor wafer defects,Micro-electromechanical System(MEMS)sensing structures,ultra-precision optical components,and film thicknesses.Generally,this technology originated in the late 1980 s in the United States,and domestic researches in this field started late,with the overall technology lagging behind.In terms of the instrumentation,although there have been breakthroughs,core components still rely on imports,such as interference objectives,nano-positioning scanners.A large fieldof-view white light interferometric measurement system was built and tested.In this system,a domestic white light interference objective with a lateral magnification of 2 was used and a 0.5×adapter lens was preferably configured in the imaging system in front of the image detector.For the white LED illumination source,a suitable bandpass filter was theoretically evaluated and experimentally confirmed.The center wavelength was determined through the experimental curve of white light interference along the axial direction.The actual magnification of WLI system and the distortion of the field of view were achieved through imaging of a twodimensional microscopic grid sample.The actual maximum field-of-view at the object side has reached 14 mm.By selectively filtering the spectrum of the white light source,the white light interference signal can be effectively modulated.According to this,the axial resolution and horizontal resolution can be changed within a certain range.For example,by changing the center wavelength of the incident light,the horizontal resolution can be changed according to the Rayleigh criterion.Experimental results show that:a more ideal white light interference axial response curve is obtained through suitable spectral filtering;the maximum field-of-view at the object side is as large as 14 mm;the measurement results of standard step samples with heights of 2.04μm and 20.43μm are 2.05μm and 20.47μm,and the repeatability(standard deviation)of 10 measurements is 12 nm and 16 nm,respectively.The measurement result of the 2.04μm height step was also compared with the result obtained by the atomic force microscope.Actual measurements were conducted on the roughness sample,MEMS sensing structure and semiconductor wafer film,demonstrating the feasibility of the developed system in the field of three-dimensional optical non-destructive precision inspection.For large field-of-view WLI systems,the horizontal or lateral resolution is on the order of a few micrometers,so it is difficult to apply WLI to the three-dimensional reconstruction of fine microstructures.This is an important shortcoming of large fieldof-view WLI systems.Besides,through research,it was found that for ultra-smooth surfaces,such as polished wafers,it is difficult to measure using traditional vertical scanning interferometry technology,and the phase shifting method should be used.Measurement of film thickness may be used to monitor morphological changes in biological transparent film layers.Further research can focus on high-performance white-light interference objectives,automation of the white-light interferometry measurement process,and implementation of large field-of-view high-resolution white-light interferometry methods.