Metallization introduced corrosion and parylene protection of surface micromachined polysilicon film with submicron capacitive gap

Corrosion of polysilicon occurs in the hydrofluoric acid (HF), which may change the surface morphology, film thickness and even its mechanical strength; furthermore, long time exposure to HF and the presence of Au metallization appear to promote this corrosion. This unusual phenomenon was observed during the releasing of the clamped–clamped beams with a capacitive gap of 100 nm, where obvious color change of the surface polysilicon was revealed. Then the influence on electrical performance of the structure was evaluated as well as the relationship between the maximum releasing time and the concentration of HF acid. After that, a novel protective coating was developed to prevent the corrosion. Parylene was demonstrated to be an effective protection mask in HF wet-etching process, where thickness dominated the yield because parylene film swelled in HF etchant. With a parylene protection layer of 0.5–2 μm, Al pads can withstand 40 min exposure to 40% HF. Finally, a parylene layer of 2 μm was deposited and patterned on top of the metal electrode, which proved to be very effective for isolating the polysilicon from galvanic corrosion during releasing.

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