Populations of metastable and resonant argon atoms in radio frequency magnetron plasmas used for deposition of indium-zinc-oxide films

Abstract

This work reports optical absorption spectroscopy measurements of the number density of Ar atoms in resonant (3P1, 1P1) and metastable (3P2, 3P0) states in rf magnetron sputtering plasmas used for the deposition of ZnO-based thin films. While the density of Ar 3P2 and 3P0 was fairly independent of pressure in the range of experimental conditions investigated, the density of Ar 3P1 and 1P1 first sharply increased with pressure and then reached a plateau at values close to those of the 3P2 and 3P0 levels at pressures above about 50 mTorr. At such pressures, ultraviolet radiation from resonant states becomes trapped such that these levels behave as metastable states. For a self-bias voltage of −115 V and pressures in the 5–100 mTorr range, similar number densities of Ar resonant and metastable atoms were obtained for Zn, ZnO, and In2O3 targets, suggesting that, over the range of experimental conditions investigated, collisions between these excited species and sputtered Zn, In, and O atoms played only a minor role on the discharge kinetics. The metastable-to-ground state number density ratios were also fitted to the predictions of a global model using the average electron temperature, Te, as the only adjustable parameter. For all targets examined, the values of Te deduced from this method were in excellent agreement with those obtained from Langmuir probe measurements.