Generation of a ruler marking every wavelength and its application to nano measurement by a wavelength-scanning interferometer with double feedback control

本文对采用正弦波长扫描干涉仪和双反馈控制系统实现波长以上距离的实时测量进行了分析和实验。除了常规的相位α外，慢波干涉仪的干涉信号还具有与波长扫描的光程差和幅度b成正比的相位调制幅度Z_b。施加到PZT上的电压使参考镜发生位移，并使相位α等于一个恒定值(2π或3π/2)，从而成为刻度小于波长的尺子。加在波长扫描装置上的电压，它决定了波长扫描的幅度b，并使幅度Z_b等于π或2π成为标记每一波长的尺子。通过双反馈控制，每隔一个波长改变OPD，即可实现每一波长刻度尺的自动标定。这两条规则…采用液晶法珀干涉仪(LC-FPI)作为双反馈控制的SWS干涉仪的波长扫描器件。通过引入双正弦调相技术，我们得到了较好的干扰信号，从而可以产生频带宽度为SWS频率两倍的反馈信号。由于该系统具有很高的分辨率，我们可以得到测量范围的上限为200μm，并且我们用Z_b=2π的锁相代替了Z_b=π，使得测量范围的上限达到了300μm，我们得到了100μm到300μm的所有稳定点，并且我们测量了270μm的相对距离，精度达到了纳米量级。此外，还采用了声光可调谐波长滤光器(AOTF)作为波长扫描装置。在这种情况下，稳定点的重复性比在LC-FP的情况下更高。使用线阵CCD图像传感器对测量点进行电扫描，并利用该干涉仪实时测量一维台阶轮廓。测量了台阶高度分别为1μm和20μm的两种不同的台阶轮廓，测量误差小于8 nm。一个测量点的测量时间是0.04 S。小于

This research deals analyses and experiments about real-time measurement of a distance longer than a wavelength using sinusoidal wavelength-scanning (SWS) interferometer and double feedback control system. In addition to a conventional phase α an interference signal of SWS interferometer has phase-modulation amplitude Z_b that is proportional to the optical path difference (OPD) and amplitude b of the wavelength-scanning. Voltage applied to the PZT which gives a displacement to a reference mirror and makes the phase α equal to a constant value (2π or 3π/2) becomes a ruler with scales smaller than a wavelength. Voltage applied to a wavelength-scanning device that determines the amplitude b of the wavelength-scanning and makes the amplitude Z_b equal to π or 2π becomes a ruler marking every a wavelength. The calibration of the ruler marking every a wavelength could be made automatically with the double feedback control by changing the OPD at intervals of about a wavelength. These two rul … More ers enabled us to measure an absolute distance longer than a wavelength in real-time.A liquid-crystal Fabry-Perot interferometer (LC-FPI) was adopted as a wavelength-scanning device in the SWS interferometer with double feedback control. By incorporating double sinusoidal phase-modulation we obtained a better interference signal from which we could generate the feedback signal whose frequency bandwidth was equal to twice the frequency of the SWS. Because of the high resolution of the LC-FPI in the SWS we could obtained the upper limit of the measurement range to 200 μm. Moreover we used the phase lock of Z_b=2π instead of Z_b=π, so that the upper limit of the measurement range became 300 μm. We obtained all of stable points from 100 μm to 300 μm, and we measured the relative distance around the OPD of 270 μm with a high accuracy of the order of nanometer. Moreover, an acousto-optical tunable wavelength filter (AOTF) was also adopted as a wavelength-scanning device. In this case the repeatability of the stable points was higher that in the case of the LC-FPI.A linear CCD image sensor was used to electrically scan a measuring point and measure one-dimensional step-profiles in real-time with this interferometer. Two different step profiles with a step height of 1 μm and 20 μm, respectively, were measured with the measurement error less than 8 nm. Measuring time for one measuring point was 0.04 s. Less