Sintering Behavior of Nano‐ and Micro‐Sized ZnO Powder Targets for rf Magnetron Sputtering Applications

Abstract

In this work, the nonisothermal sintering behavior of as‐received commercial high purity ZnO micrometric (m_ZnO), submicrometric (sm_ZnO) and nanometric (n_ZnO) powders was studied. The sintering behavior for sputtering target production was evaluated by changing the green density of samples from 62% of theoretical density (TD) to 35%. We observed that for n_ZnO powder, the maximum shrinkage rate (MSR) temperature (TMSR) was not affected by the green density, and that it was reached at lower temperatures (~710°C) compared with m_ZnO and sm_ZnO powders. For these powders, the temperature of MSR increased from 803°C to 934°C and from 719°C to 803°C as TD changed from 62% to 35% TD, respectively. Small grain size (~0.560 μm) and high density targets were obtained for n_ZnO when sintered at temperatures below the TMSR. Heating rate from 1°C to 15°C/min led to lower activation energy for n_ZnO (~201 ± 3 kJ/mol) than for the submicrometric (sm_ZnO) (~332 ± 20 kJ/mol) and micrometric (m_ZnO) (~273 ± 9 kJ/mol) powders. Using the model proposed by Bannister and Woolfrey, an n value of 0.75 was found, which was correlated with a combination of viscous flow and volume diffusion mechanisms that should control the initial stage of n_ZnO sintering. No significant differences were observed for n_ZnO powder in terms of density when the size of targets (scale‐up effect) was increased, while in the case of m_ZnO and sm_ZnO, a delay in the densification was observed, which was related to the higher sinterability of n_ZnO powder. Two inches ZnO ceramic targets with different particle sizes and final densities were used in an rf magnetron sputtering system to produce ZnO films under the same deposition conditions. Films with thickness around 100 nm and good uniformity were produced using those targets, and no variation was observed in the optical and morphological properties. However, low electrical resistivity (1.4 Ω·cm) films were obtained with n_ZnO targets, which could be explained in terms of a nonstoichiometric Zn:O composition of the started powders.