Deep reactive ion etching of fused silica using a single-coated soft mask layer for bio-analytical applications

In this note, we present our results from process development and characterization of reactive ion etching (RIE) of fused silica using a single-coated soft masking layer (KMPR® 1025, Microchem Corporation, Newton, MA). The effects of a number of fluorine-radical-based gaseous chemistries, the gas flow rate, RF power and chamber pressure on the etch rate and etching selectivity of fused silica were studied using factorial experimental designs. RF power and pressure were found to be the most important factors in determining the etch rate. The highest fused silica etch rate obtained was about 933 Å min^-1 by using SF₆-based gas chemistry, and the highest etching selectivity between the fused silica and KMPRR® 1025 was up to 1.2 using a combination of CF₄, CHF₃ and Ar. Up to 30 μm deep microstructures have been successfully fabricated using the developed processes. The average area roughness (R_a) of the etched surface was measured and results showed it is comparable to the roughness obtained using a wet etching technique. Additionally, near-vertical sidewalls (with a taper angle up to 85°) have been obtained for the etched microstructures. The processes developed here can be applied to any application requiring fabrication of deep microstructures in fused silica with near-vertical sidewalls. To our knowledge, this is the first note on deep RIE of fused silica using a single-coated KMPRR® 1025 masking layer and a non-ICP-based reactive ion etcher.