Selective wet etching of Si3N4/SiO2 in phosphoric acid with the addition of fluoride and silicic compounds

Abstract

Various additives were added to H3PO4 in order to achieve a highly selective wet etching of Si3N4 to SiO2. Fluoride compounds such as HF, NH4F, and NH4HF2 were added to the H3PO4 in order to increase the etch rate of the Si3N4. In addition, silicic compounds, including H2SiF6, TEOS, and Si(OH)4, were added to decrease the etch rate of SiO2. The addition of the fluoride compounds into the H3PO4 increased the etch rate of the Si3N4, but the etch selectivity of the Si3N4 to SiO2 decreased due to the greater increase in the etch rate of the SiO2. Both the etch rate and the selectivity showed strong relationships with the amount of fluorine added in H3PO4. The addition of TEOS and Si(OH)4 increased the etch selectivity by reducing the etch rate of the SiO2. In particular, the addition of Si(OH)4 to H3PO4 in the presence of NH4F and NH4HF2 produced an etch selectivity greater than 104.