Wet Etching of Silicon, Silicon Dioxide and Silicon Nitride

Silicon etch

• Single crystal silicon etch is used for forming the isolation blocks between neighboring transistors.

• Polysilicon etch forms the gates and local interconnections.

• Single-crystal and polycrystalline silicon can be etched isotropically with a mixture of nitric acid and hydrofluoric acid.

• First nitric acid oxidizes the silicon on the surface resulting in a thin layer of silicon dioxide, which can block the oxidation process.

• Hydrofluoric acid then reacts with silicon dioxide and dissolves it, exposing the silicon under it, which is oxidized again by nitric acid.

• The overall reaction can be written as:

Si + HNO₃ +6HF ® H₂SiF₆ + HNO₂ +2H₂O

Nitride Etch

• Nitride is widely used for isolation formation processes.

• Hot phosphoric acid is most commonly used to etch silicon nitride.

• This silicon nitride etch process has very good selectivity to thermally grown silicon dioxide and to silicon.

• The chemical reaction of the silicon nitride etch is:

Si₃N₄ + 4H₃PO₄ ® Si₃(PO₄)₄ +4NH₃

• Both of the byproducts are water-soluble.

Metal Etch

• A mixture of phosphoric acid, acetic acid, and nitric acid and water can etch aluminum.

• The etch mechanism of aluminum etch is very similar to silicon etch: Nitric acid oxidizes the aluminum to form aluminum oxide, and phosphoric acid dissolves Al₂O₃. Both oxidization and oxide dissolution in phosphoric acid proceed simultaneously.

• Another common metal wet etch process is titanium strip after the titanium silicide formation.

• The 1:1 mixture of hydrogen peroxide and sulfuric acid is commonly used to selectively etch away titanium while keeping silicon oxide and titanium silicide intact. [2]