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

Various additives were added to H"3PO"4 in order to achieve a highly selective wet etching of Si"3N"4 to SiO"2. Fluoride compounds such as HF, NH"4F, and NH"4HF"2 were added to the H"3PO"4 in order to increase the etch rate of the Si"3N"4. In addition, silicic compounds, including H"2SiF"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"3PO"4 increased the etch rate of the Si"3N"4, but the etch selectivity of the Si"3N"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"3PO"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"3PO"4 in the presence of NH"4F and NH"4HF"2 produced an etch selectivity greater than 10^4.