Plasma-assisted deposition of indium tin oxide thin films by sublimation using an anodic vacuum arc discharge

Abstract

With the anodic vacuum arc discharge as a method of physical vapor deposition (PVD) excellent properties of thin transparent conductive oxide films can be achieved, but the relations between the process parameters and the coating properties have been insufficiently described in the literature. This paper is intended to narrow this gap by investigations on indium tin oxide (ITO) thin films deposited on borosilicate glass sheets by sublimation of indium tin oxide (90:10) using an anodic vacuum arc. The arc discharge is based on a hot hollow cathode electron source and a special evaporator connected as anode. The concentration of the discharge onto the sublimating material is achieved by combining the magnetic fields of an axially symmetrical permanent magnet system and a solenoid coil. Coating rates of about 15 nm/s were achieved with an arc current of 60 A. Measurements with a planar Langmuir probe have shown that the vapor is highly ionized (about 95%). The discharge voltages are relatively low (< 60 V), so it can be assumed that the particle energies are correspondingly low. Substrate temperature (30–300 °C), oxygen gas flow (0–100 sccm) and gas pressure (0.04–0.4 Pa) were varied to study their influence on the thin film properties. Specific electrical conductivity, density and mobility of charge carriers, as well as optical properties were determined and compared with those of other PVD processes. Low specific electrical resistance of 1.8–2.0 x 10–4 Ω cm was reached at a substrate temperature of 200–300 °C independently of the pressure. The mean optical transmission (in the spectral range between 400 nm and 1100 nm) of 100 nm thick ITO films deposited at a temperature of 200 °C and at an optimized oxygen flow was determined to 84%. The roughness of the layers determined by atomic force microscopy is significantly lower than typically for magnetron sputtered films. At a pressure of 0.04 Pa, the crystalline bixbyite phase can already be detected at a coating temperature of approx. 30 °C, whereby the layer growth begins with the formation of an amorphous structure and changes to the crystalline phase with increasing layer thickness. At a temperature above 100 °C and low coating pressure the layers are completely crystalline. At a higher pressure the transition from amorphous to crystalline phase is shifted towards higher temperature. For lower oxygen flows the crystallites exhibit mainly a preferred (211) orientation. For higher oxygen flows the texture of crystallites is changed and the intensity of (440) and (622) is increased. The formation of these special crystallographic textures can probably explain the very low roughness values of the ITO layers measured by atomic force microscopy. The investigations have shown how the film properties can be adjusted by the process parameters and that the ITO sublimation using an anodic vacuum arc is suited very well for plasma assisted ITO thin film deposition.