Variation of etch profile and surface properties during patterning of silicon substrates

Abstract

Patterned silicon substrates form the basis for various microfluidic and micromechanical devices. Recent developments in microfluidic and micromechanical systems for biomedical applications demand complex three-dimensional structure profiles and a well-defined surface quality of the etched patterns. The object of our investigations was to realize complex contours with different sidewall angles in silicon. The technology includes the combination of several etching steps of different character, possibly in combination with suitable coatings or oxidation. The investigations into plasma etching of silicon substrates were carried out using an Advanced Silicon Etch System from Surface Technology Systems Ltd. (STS), UK. Our experiments were based on three different silicon-etching processes: the Advanced Silicon Etch process (ASE™) introduced by STS; continuous anisotropic silicon etching; and silicon etching in pure SF 6. Variation in the ratio of etching to passivation causes drastic changes in the etch rate, tilt angle and quality of the etched silicon surfaces. Positive and negative edge profiles with tilt angles between 60 and 100° were realized by variation of the surface passivation and the intensity of etching. The roughness of the etched silicon surface can be varied by process modifications, within limits. A well-defined surface topology can be realized using different procedures and complex three-dimensional patterns can be created in silicon substrates. The main steps of the technology were the anisotropic patterning of silicon with the aid of sidewall passivation, deposition of an additional passivating layer, and subsequent isotropic silicon etching.