Fused silica microchannel fabrication with smooth surface and high etching selectivity

Channel fabrication technology has become increasingly important for microfluidic and nanofluidic devices. In particular, glass channels have high chemical and physical stability, high optical transparency, and ease of surface modification, so that there is increasing interest in glass microfluidic devices for chemical experiments in microfluidics and nanofluidics. For the fabrication of glass channels, especially those with a high aspect ratio (depth/width), lithography using a metal resist and dry etching have mainly been used. However, there are still issues involving the surface roughness of the etched channel and the low etching selectivity. In this study, a microchannel fabrication method with high etching selectivity that produces a smooth etched surface was developed. First, interference during dry etching by remaining Cr particles after the photolithography and Cr etching processes was assumed as the cause of the rough etched surface. Three different dry etching processes were introduced to verify this. In process 1 without removal of the Cr particles, the etched surface was not flat and had a 1 μm scale roughness. In process 2 where a cleaning process was included and high power etching was conducted, a smooth surface with a 1 nm scale roughness and a faster etching rate of 0.3 μm min⁻¹ were obtained. For this high-power etching condition, the etching selectivity (fused silica/Cr) was relatively low at approximately 39-43. In process 3 with a cleaning process and low-power etching, although the etching rate was relatively low at 0.1 μm min⁻¹, a smooth surface with 1 nm scale roughness (10 nm scale roughness deeper than 40 μm in the depth region) and a much higher etching selectivity of approximately 79-84 were obtained. The dry etching method presented in this study represents a significant contribution to microfluidics/nanofluidics for microchannel/nanochannel fabrication.