Reliable fabrication of buried microchannels via polymer trench passivation

The goal of lowering the material budget in microfluidics is often reached by fabrication of buried microchannels, typically achieved by deep reactive ion etching (DRIE) to create trenches, followed by sidewall passivation, and finally formation of hollow microchannels with isotropic etching. To avoid high temperatures, mask material or cleanroom restrictions, trench sidewalls can be passivated in the same DRIE tool with a fluorocarbon polymer layer using C4F8 as the source gas. Since the entire process is conducted within a DRIE tool, it significantly reduces fabrication time and complexity compared to conventional SiO2/Si3N4 passivation methods. Silicon microfabrication advancements often prioritize developing smaller or more precise features, while reliability and repeatability—crucial for large-scale production—are frequently overlooked. This study confirmed the development of a reliable, reproducible process for buried microchannel fabrication via polymer trench passivation, addressing key process instabilities and proposing functional solutions. Our optimized method employs a novel two-cycle approach of trench passivation by alternating the polymer deposition and anisotropic etching, enhancing uniformity and conformity of the passivation layer. This enables reliable fabrication of deep buried microchannels with a hydraulic diameter up to 40 μm at a depth of approximately 40 μm. In addition, the increased opening at the top of the trenches allows a significant reduction of time (above 20%) required for the subsequent isotropic etching which forms the final shape of the buried microchannels. The process demonstrates high reliability and uniformity on the entire wafer and can be adapted for different structures with minor parameter adjustments.