Three-dimensional optical profilometry with detection of weakly diffused light

为了实现准镜面或由不连续台阶或孤立部件组成的粗糙表面（如金属模具和微加工部件）的精确三维光学轮廓测量，我们开发了一种基于相位检测的新型波长扫描干涉测量方法，该方法在弱漫射光下成像具有宽动态范围。采用波长调谐范围为8nm的外腔激光二极管，以0.2 nm的步长进行波长调谐，利用迈克尔逊干涉仪中的PZT反射镜进行移相技术，通过检测每个像素点波长变化的相位斜率，实验获得了40 nm的高度分辨率。为了同时获得弱漫射光和镜面反射光的充分对比条件，我们采用了偏振选择技术。将偏振器的方位角设置为几乎垂直于线偏振方向，选择漫射光偏振状态的微小变化，以干扰一小部分来自镜面的线偏振分量。该技术实现了高精度的步进高度测量，并应用于测量孤立物体的高度分布，如放置在硅片表面的金属凹凸阵列。

To realize a accurate three-dimensional optical profilometry for quasi-specular or rough surfaces constituted of discontinuous steps or isolated parts, such as metallic molds and micro-machined parts, we developed a novel wavelength-scanning interferometry, which is based on a phase-detection and has wide dynamic-range in imaging against the weakly diffused light. By using a external-cavity laser diode with the wavelength tuning range of 8nm by 0.2 nm step, and phase-shifting technique using a PZT mirror in Michelson interferometer, a height resolution of 40 nm was experimentally attained by detecting a phase-slope against the wavelength change at each pixel. To obtain adequate contrast conditions between weakly diffused light and specularly reflected one simultaneously, we adopted a polarization selection technique. A small change in polarization state of the diffused light was selected with a polarizer whose azimuth was set to almost perpendicular to the linearly polarizing direction to be interfered with a small fraction of linearly polarizing component comes from the specular surface. This technique accomplished a precision step height measurement and was applied to a gauging of height distribution of isolated objects such as a metallic bump array placed on a silicon wafer surface.