Wet etching of atomic-layer-deposited amorphous zinc–tin–oxide thin film using oxalic acid

Abstract

This study investigates the wet etching process of amorphous Zn–Sn–O (a-ZTO) thin films deposited by atomic layer deposition (ALD). It also examines the physical and electrical properties of corresponding thin-film transistors (TFTs). While most common wet etching acids, such as HF, HCl, HNO₃, and H₂SO₄, cannot etch the a-ZTO film, an oxalic acid (C₂H₂O₄) water solution can feasibly etch the film. Experimental results showed that a 0.5 M oxalic acid solution at 35 °C etched the a-ZTO film at a rate of 4.2 nm min⁻¹ while achieving a high etch selectivity of over 32 : 1 against various dielectric materials. Notably, the preferential dissolution of ZnO over SnO₂ by oxalic acid enabled the application of interface engineering at the source and drain electrode–channel junction. This process increased Sn concentration near the film's interface and improved electrode-channel contact characteristics by decreasing the electron injection barrier. Therefore, the contact resistance decreased from 4.9 kΩ to 1.5 kΩ, and the saturation mobility (μ_sat) increased from 15.0 cm² V⁻¹ s⁻¹ to 18.6 cm² V⁻¹ s⁻¹. These results suggest that oxalic acid not only enables effective wet etching of a-ZTO thin films but also enhances device performance by decreasing contact resistance.