HF Etching of Sacrificial Spin-on Glass in Straight and Junctioned Microchannels for MEMS Microstructure Release

Abstract

Sacrificial spin-on glass (SOG) etching in straight and junctioned microchannels using hydrofluoric acid (HF) was investigated. SOG etch rates in both reaction-dominant and diffusion-dominant regimes for various HF concentrations were studied. An etching model based on a non-first-order chemical reaction/steady-state diffusion etching mechanism is presented to compensate for the etching effect at the channel junction. Straight microchannels in length and various widths were fabricated on silicon substrate by coating a hardened photoresist layer over rectangular-shaped SOG layers. Junctioned microchannels were fabricated on silicon by filling SOG into deep reactive ion etching (DRIE)-etched microchannels. The samples were time-etched in HF solution and etch-front propagation was observed under an optical microscope. It is observed that the SOG etch rate is linear in the reaction-limited region and drops approximately 70% in the diffusion-limited region. The SOG etch rate in microchannels is independent of channel width and depth. The SOG etch rate at the T-junction is 0.67 times lower than its etch rate in straight channels due to the instantaneous drop in HF concentration. This behavior is well embodied by the presented numerical model. Finally, 5% HF is suitable for release etch due to its acceptable etch rate while being less damaging to microelectromechanical system (MEMS) microstructures.