The Role of BOE in MEMS Processes?

BOE (Buffered Oxide Etch) is a key wet etch chemistry in MEMS (microelectromechanical systems) processes, mainly used to selectively remove silica (SiO₂) sacrificial layers or dielectric layers, and can also be used for etching silicon nitride (Si₃N₄) under specific conditions.

Figure BOE bottled corrosion liquid

BOE is made up of an aqueous solution of hydrofluoric acid (HF) and ammonium fluoride (NH₄F) mixed in specific proportions. In typical formulations, HF concentrations range from 0.2%~20% and NH₄F concentrations range from 1.5%~40%, and some modified formulations also add surfactants (such as polyethylene glycol octyl phenyl ether) or amide additives (such as N-butylbutylamide) to adjust etch selectivity and uniformity. The addition of ammonium fluoride forms a buffer system (NH₄F-HF), which can stabilize the etch rate and inhibit the volatilization of HF, while increasing the etch selection ratio of SiO₂/Si (up to 100:1 or more) to reduce accidental corrosion on silicon substrates.

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Figure BOE corrosion tank

The concentration ratio of ammonium fluoride (NH₄F) solution commonly used by BOE in MEMS production lines is hydrofluoric acid (HF) solution≈ which is 7:1 (volume ratio), which is characterized by a fast etch rate (SiO₂ etch rate of about 10 nm/s), which is suitable for rapid removal of sacrificial layers, such as silicon oxide release under polysilicon structure. Another commonly used ratio is NH₄F : HF≈ 20:1 (volume ratio), which is characterized by a significantly lower etch rate (about 2-3 nm/s), but better uniformity, better sidewall protection, and a high SiO₂/Si selection ratio (>100:1) to reduce over-etching of silicon substrates, making it suitable for shallow oxide film removal or high-precision structures.

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Figure Schematic diagram of BOE corrosion process of SiO2

In the MEMS process, BOE can be used as a sacrificial layer to remove the chemical liquid formed with the cavity, such as accelerometers, bulk acoustic filters often need to form a cavity between the diaphragm and the backplate, and SiO₂ is deposited on the silicon substrate as a sacrificial layer during manufacturing, and after completing the structural layer processing, SiO₂ is selectively removed by BOE wet etching to release the movable structure. BOE can also be used to etch insulating layers (such as thermally oxidized SiO₂ or CVD deposited oxides) to create contact holes or isolation areas. In addition, BOE is used to remove the primary oxide layer on the surface and improve interfacial adhesion before wafer bonding or metal deposition. In the 6&8 inch MEMS production line, the BOE wet table is standard and is dedicated to the corrosion process of silica/silicon nitride.