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Modelling of sacrificial spin-on glass (SOG) etching in non-straight microchannels using hydrofluoric acid

This paper studies spin-on glass (SOG) etching in T-shaped microchannels by hydrofluoric acid (HF). Since oxide etching by HF in microchannels is both reaction and diffusion limited, an etching model based on non-first order chemical reaction/steady-state diffusion sacrificial layer etching mechanism is presented to compensate for the etching effect at channel junction. Microchannels are formed on silicon substrate by deep reactive ion etching (DRIE). Samples with channel depth varying from 1μm to 6 μm are prepared by varying exposure time to reactant gas in DRIE chamber. Channel widths prior to the junction are varied from 2 μm to 10 μm while channel width beyond the junction is fixed at 5 μm. The channels are then filled with SOG by multiple spin, bake and cure processes. After etchback planarization using 5% HF solution, the samples are coated with 1.5 μm thick positive photoresist. An etch window is opened at channel fronts to expose underlying SOG. The samples are then time-etched in 5% HF solution and etch front propagation is observed under optical microscope through the transparent photoresist layer. It is observed that SOG etch rate in the microchannels is independent of channel width or channel depth. SOG etch rate at channel's T-junction is 0.67 times lower than etch rate in the straight channels preceding it due to HF concentration variation and etch product transfer rate variation effects. The proposed model fits experimental data well. Offset crosses vent pattern is determined as a good candidate for removing sacrificial oxide under an enclosed cap structure.