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

Abstract

The conventional Si 3 N 4 etching process using H 3 PO 4 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 Si 3 N 4 was demonstrated without the use of H 3 PO 4 and HF. In ionic-compound-containing superheated water, the etching rate of Si 3 N 4 (R Si 3 N 4 ) showed a strong dependence on the OH − concentration: R Si 3 N 4 ≈ 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 Si 3 N 4 . In carboxylic-acid-containing superheated water, the etching rate of Si 3 N 4 was dependent on the concentration of the carboxylate ion, RCOO − , rather than the OH − concentration: R Si 3 N 4 ≈ 351[RCOO − ] 0.49 = 7.6 × 10 −4 [OH − ] −0.49 . It is suggested that RCOO − breaks the Si−N bond of Si 3 N 4 and forms Si−(OOCR) n in an S N 2-like reaction. Finally, Si 3 N 4 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 Si 3 N 4 . Since the use of H 3 PO 4 or HF is avoided entirely, the ecofriendly Si 3 N 4 etching process described in this study may reduce the potential environmental and safety problems associated with the conventional Si 3 N 4 etching process.