Various additives were added to H 3 PO 4 in order to achieve a highly selective wet etching of Si 3 N 4 to SiO 2 . Fluoride compounds such as HF, NH 4 F, and NH 4 HF 2 were added to the H 3 PO 4 in order to increase the etch rate of the Si 3 N 4 . In addition, silicic compounds, including H 2 SiF 6 , TEOS, and Si(OH) 4 , were added to decrease the etch rate of SiO 2 . The addition of the fluoride compounds into the H 3 PO 4 increased the etch rate of the Si 3 N 4 , but the etch selectivity of the Si 3 N 4 to SiO 2 decreased due to the greater increase in the etch rate of the SiO 2 . Both the etch rate and the selectivity showed strong relationships with the amount of fluorine added in H 3 PO 4 . The addition of TEOS and Si(OH) 4 increased the etch selectivity by reducing the etch rate of the SiO 2 . In particular, the addition of Si(OH) 4 to H 3 PO 4 in the presence of NH 4 F and NH 4 HF 2 produced an etch selectivity greater than 10 4 .