Addition of Carboxylic Acids to Superheated Water for the Environmentally Benign Removal of Silicon Nitrides

The conventional Si3N4 etching process using H3PO4 or HF may cause environment, health, and safety issues. In this study, by adding ionic compounds and carboxylic acids to superheated water, the etching of Si3N4 was demonstrated without the use of H3PO4 and HF. In ionic-compound-containing superheated water, the etching rate of Si3N4 (RSi3N4) showed a strong dependence on the OH- concentration: RSi3N4 ≈ 170[OH-]0.12. However, the material loss of the Si substrate is inevitable because of the high OH- concentration in aqueous solution. The addition of carboxylic acid to superheated water also increased the etching rate of Si3N4. In carboxylic-acid-containing superheated water, the etching rate of Si3N4 was dependent on the concentration of the carboxylate ion, RCOO-, rather than the OH- concentration: RSi3N4 ≈ 351[RCOO-]0.49 = 7.6 × 10-4[OH-]-0.49. It is suggested that RCOO- breaks the Si-N bond of Si3N4 and forms Si-(OOCR)n in an SN2-like reaction. Finally, Si3N4 is etched in the form of Si(OH)4 in the superheated water containing carboxylic acids. In addition, this new aqueous process significantly reduced the material loss of the Si substrate that can occur during the etching of Si3N4. Since the use of H3PO4 or HF is avoided entirely, the eco-friendly Si3N4 etching process described in this study may reduce the potential environmental and safety problems associated with the conventional Si3N4 etching process.