Fabrication of free-standing nanoscale SiNx membranes with enhanced burst pressure via improved etching process

Abstract

Fabrication of free-standing nanoscale SiNx membranes by the wet chemical etching of a thick Si substrate with KOH was investigated. During the long wet-etching process, localized etch rate differences within the substrate occur toward the end of the etching process, due to rapid stress increases at the edges, leaving a Si island at the center of the SiNx membrane. The thickness of this island, which is defined as the thickness of the central portion of the Si island in time at which the SiNx membrane begins to be exposed along the island boundaries, was monitored whilst varying the etching parameters. The parameters varied were the etching temperature, KOH concentration, and the concentration of an anionic surfactant. Bulge testing on SiNx membranes fabricated under various etching conditions indicated that the island thickness is a very useful figure of merit for monitoring etching uniformity and fracture strength of the fabricated membrane. The addition of a surfactant to the KOH solution at a concentration below its critical micelle concentration resulted in a decrease in the island thickness, thereby enhancing the fabrication yield for the etching process as well as the mechanical strength of the final membrane. The membrane fabricated under processing conditions including the use of a 35 wt% KOH etching solution, including 0.1 wt% Aerosol MA80, at 60 °C resulted in a yield of 100% and the average burst pressure of 1422 Pa, which was four times higher than that obtained for the membranes fabricated under one of the non-optimized conditions.