A High-Selectivity HNA Etching System for Bulk Micromachined Deep Holes with High Roundness

This paper reported a novel high-roundness deep-depth great-selectivity HNA, i.e. hydrofluoric acid + nitric acid + acetic acid, etching system for silicon deep-hole corrosion for the first time. A fluid model based on finite element method (FEM) is established to research HNA deep-hole corrosion mechanism based on the analysis of etchant flowing patterns and the transport mode of reactant and resultant. HNA deep smooth circular-hole corrosion is achieved, which can be widely used for pressure sensor, gyroscope and flowmeter, etc. The system can reinforce complex high-level microelectromechanical systems (MEMS) design with an effective fabrication technic. HNA ratio is designed as 2:7:1, the Si
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N
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mask selection is more than 2200, corrosion rate is close to 11 $\mu \mathbf{m}/\min$. Corrosion depth reaches $337.5\mu \text{m}$. The inhomogeneity of corrosion depth between different holes is less than 3.6%. The maximum height fluctuation in the hole is less than 0.88% of the corrosion depth. The roundness of the hole is less than 5%0.