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分组成的粗糙表面(如金属模具和微机械加工部分)的精确三维光学轮廓术，我们发展了一种新型的波长扫描干涉法，它基于相位检测，对弱漫射光具有较大的动态成像范围。采用波长调谐范围为8 nm×0.2 nm步长的外腔半导体激光器，在迈克尔逊干涉仪中采用PZT反射镜相移技术，通过检测每个像素处的相位斜率与波长变化的关系，获得了40 nm的高度分辨率。为了在弱漫射光和镜面反射光之间同时获得足够的对比度条件，我们采用了偏振选择技术。利用其方位被设置为几乎垂直于要干涉的线偏振方向的偏振器来选择漫射光的偏振态的微小变化，其中来自镜面的一小部分线偏振分量。该技术实现了精确的阶跃高度测量，并被应用于测量孤立物体的高度分布，例如放置在硅片表面上的金属凸起阵列。

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.