Wet etching mechanisms of ITO films in oxalic acid

This study examines how oxalic acid solutions affect indium tin oxide (ITO) etching. Experimental results show that the etching rate of ITO films increased linearly with the concentration of C2O42-. The open circuit potentials included in the potential-pH diagrams for indium and tin in aqueous oxalic acid systems helped determine that ITO films was dissolved by the formation of In(C2O4)(2)(-) in oxalic acid. However, the tin oxide in ITO films was difficult to dissolve, but could be removed by stripping. The EDS analysis and optical microscopic images indicate that the removal rate of SnO2 in oxalic acid etchants was slower than that of In2O3, and many residues were distributed around the ITO bars after the etching process. Moreover, kinetic experiments reveal the high activation energy of ITO etching, and it was found stirring or ultrasonic vibration of the etchants had no influence on the dissolution rate of ITO films at all. Therefore, the rate-determining step should be the chemical reaction on ITO surface. A mechanism including protonation and ligand adsorption was proposed to explain the observed results. (c) 2005 Elsevier B.V. All rights reserved.