The Chemistry of Wet Etching
Silicon-containing materials (Si, SiO2, Si3N4) are etched with fluoride-based aqueous etch solutions or by water. In the latter case, acids are required to catalyze the dissolution reaction, or a base is needed to keep the reaction product, silicic acid, in solution. For the etching of silicon, an oxidation agent accelerates the reaction rate. The dissolution reaction of all the materials is described by a repetitive two-step reaction cycle:
1. The activation of the surface monomolecular layer (Si-H to Si-OH or Si-F; Si-OH to Si-F; Si-NH2 to Si-OH or Si-F)
2. Dissolution of this activated layer that requires breaking Si-Si, Si-O, or Si-N bonds.
Selectivity is obtained by modifying the chemical solution and tuning the rate limiting step. Knowledge of these phenomena resulted in HF etching of Si3N4 being selective towards SiO2.
Keywords: Selective etching, reaction mechanism, oxidation potential, HF, water, phosphoric acid, buffered oxide etch, silicon, silicon dioxide, silicon nitride
3.1 Introduction and Overview
The origin of the word “etch” can be found in the Dutch word “etsen” and the German word “atzen”, both of which relate to the old high German word “azzen” that means to cause to eat. Etching is a corrosion process, in which material is slowly “eaten” away, usually in a chosen pattern. For semiconductor processing ...