Cyclic deep reactive ion etching with mask replenishment

Abstract

A multi-step reactive ion etching (MS-RIE) process for silicon was developed for the fabrication of deep anisotropic, closely packed structures with vertical sidewalls. This process used repeated cycles of etching and the replenishment of masking layers, similar to the Bosch process (Laermer and Schilp 1996 US Patent 5,498,312) [1] that is employed in specialized etching tools. The process described here, however, can be used on conventional RIE tools, and is based on the isotropic deposition of an etch-inhibiting polymer to protect sidewalls, its anisotropic removal from the bottom etch front, and a subsequent isotropic etch into deeper layers. A conventional parallel plate etcher without fast gas management, cryogenic substrate cooling, or inductively coupled plasma density enhancement, produced these steps. Each process step was optimized for the maximal etch rate, minimal mask erosion, deposition of the thinnest polymer required to protect the sidewalls, and was tailored for use with 2 µm thick photoresist as the initial mask layer. This cyclic RIE process was used to fabricate photonic devices with high aspect ratios of etched depths over 100 µm and etch widths near 1 µm.