Reduction of motion axis by sensitivity calibration of sensor in shape measurement instrument using normal vector tracing

Abstract

High-precision free-form surface mirrors are required for synchrotron radiation facilities in the scientific field and semiconductor lithography systems in the industrial field. Previously, we developed a nano-profiler with the goal of achieving a measurement accuracy of 30 nm. The nano-profiler scanned and measured the slope angle of the surface to be measured with laser light and calculated the shape from the angle information. By driving the optical head and surface to be measured with four rotation axes and one translation axis, the surface could be scanned while keeping the optical path length constant. Although the rotation axis was controlled by a high-precision rotary encoder, pitching and yawing errors occurred in the translation axis. In this study, we attempt to eliminate the error of pitching and yawing from the conventional measurement operation of four axes of rotation and one axis of translation to the drive of only four axes of rotation. If the translation drive is eliminated, the optical path length will not be constant, and the sensitivity of the light-receiving element will change. Therefore, we propose a new method to calibrate the sensitivity of the receiving element and perform a comparative measurement with the conventional measurement method. Comparing the measured shapes obtained by both measurement results, it was found that the shapes had a maximum peak to valley difference of 6.2 nm. Thus, the proposed novel measurement method allows a significant reduction in pitching and yawing errors.