Study of wet etching thin films of indium tin oxide in oxalic acid by monitoring the resistance

• Etching of indium tin oxide thin films by monitoring the resistance.
• Oxalic acid has 2–3 times lower etch rate than concentrated HCl.
• The etch rate in oxalic acid can be accelerated substantially by adding FeCl3.
• The proposed etching model for indium tin oxide was confirmed by X-ray analysis and scanning electron microscopy.
• Energy Dispersive X-ray Spectroscopy analyses showed preferential etching of In2O3, enriching the film with SnO2.

We describe a study on wet etching of thin films of indium tin oxide (ITO) using a simple method by monitoring the resistance of the thin film in aqueous solutions of oxalic acid and hydrochloric acid. Generally three different regimes can be distinguished during etching ITO in acids: (1) initial etching, which is slow, (2) a fast etching phase and (3) slow etching stage at the end. These regimes are explained in terms of a porosity–roughness model. This porosity model has been confirmed largely by X-ray reflection measurements at grazing incidence, roughness measurements and scanning electron microscopy (SEM).A reliable method for monitoring the resistance during etching has been developed. This method is based on a 2-strips measuring jig with a very low series contact resistance.The activation energy of the etch rate of ITO films was found to be 80 ± 5 kJ/mol for oxalic acid and 56 ± 5 kJ/mol for HCl. SEM analyses in the final stage of the etching process indicate an enrichment of Sn in the residual film material. These observations are explained in terms of preferential etching of In2O3. X-ray analyses showed that the density of the ITO film decreased by etching. By adding ferric chloride to the oxalic acid solution we could accelerate the etch rate substantially.