Fast ultra-deep silicon cavities: Toward isotropically etched spherical silicon molds using an ICP-DRIE

Abstract

This paper investigates the parameter optimization of isotropic bulk silicon microscale etching using an inductively coupled plasma-deep reactive ion etching (ICP-DRIE) system. Etch profile characteristics, depending on the process and feature size, have been studied. The authors report detailed observations of the results for various etching parameters, including pressure from 30 to 70 mTorr, sulfur hexafluoride flow rate from 100 to 500 sccm, platen power from 20 to 150 W, and ICP power from 2000 to 2500 W. The goal here is to present anisotropic and isotropic etch processes for a wide range of applications in microfluidics, microelectromechanical-systems, and micro-optoelectronic mechanical system fabrication. A deep anisotropic etch through a 1.4 mm thick silicon wafer with smoothly etched surfaces has been achieved. Isotropic plasma etching is obtained, including a relation between the etching depth, the undercut, and the normalized radius of curvature of the profile. The authors have demonstrated an isotropic plasma potential that is much higher than those that can be produced by isotropic wet etching of silicon for the generation of more complex forms such as the manufacturing microlens molds. In particular, the authors have presented the possibility of creating aspheric shapes with a desirable negative conic constant (k<0) and a potential high numerical aperture. The choice of using photoresist masks provides better flexibility and economical processing. The presented results can be valuable for a wide range of applications, thus allowing a massive production using only a single commercial ICP-DRIE tool, which is of low cost and is compatible with an industrial perspective.