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

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 NH ₄HF₂ 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 H ₃PO₄ in the presence of NH₄F and NH ₄HF₂ produced an etch selectivity greater than 10 ⁴.