Sidewall Scallops Removal Method for End-Face Smoothing and its Application in the Fabrication of High-Temperature Optical Sensors

Optical pressure sensors are widely used in modern industries. In this paper, we present an optical pressure sensor with a deeply etched vertical Fabry-Perot (FP) cavity, which makes the sensor chip not only relatively small but also fully compatible with advanced packaging processes. In the fabrication of the pressure sensor, the smoothness of the sidewalls in the FP cavity significantly impacts the interference spectrum. The mechanism of cyclic alternations between isotropic etching steps and passivation steps in a deep reactive ion etching (DRIE) process can lead to the formation of sidewall scallops, which can cause high losses. The ranges of the sidewall roughness are usually from tens of nanometers to over a hundred nanometers. To reduce the sidewall roughness, we used a low-concentration potassium hydroxide (KOH)-based solution as a post-etching etchant. The experimental result showed that the sidewall roughness was reduced to below 5 nm after postetching in the KOH-based solution for 20 hours. A pressure sensor with a vertical FP cavity was then designed and fabricated. The FP interference spectrum was clearly observed under one atmosphere of pressure, which was attributed to the small losses of the smooth and vertical sidewalls. The measurement result indicates that various optical MEMS (Micro-Electromechanical Systems) sensors have the potential to be designed and fabricated based on the vertical FP cavity and the post-etching method.