Measurements of sputtered neutrals and ions and investigation of their roles on the plasma properties during rf magnetron sputtering of Zn and ZnO targets

Abstract

Langmuir probe and optical absorption spectroscopy measurements were used to determine the line-integrated electron density, electron temperature, and number density of Ar atoms in metastable 3P2 and 3P0 levels in a 5 mTorr, rf magnetron sputtering plasmas used for the deposition of ZnO-based thin films. While the average electron energy and density of Ar atoms in 3P2 and 3P0 excited states were fairly independent of self-bias voltage, the Ar 3P2-to-electron number density ratio decreased by approximately a factor of 5 when going from −115 V to −300 V. This decrease was correlated to an increase by about one order of magnitude of the number density of sputtered Zn atoms determined by absolute actinometry measurements on Zn I using either Ar or Xe as the actinometer gas. These results were also found to be in excellent agreement with the predictions of a global model accounting for Penning ionization of sputtered Zn particles. The importance of the latter reactions was further confirmed by plasma sampling mass spectrometry showing a double peak structure for Zn ions: a low-energy component ascribed to thermalized ions created in the gas phase (by direct electron impact and by Penning ionization) and a high-energy tail due to ions ejected from the target and reaching quasi-collisionlessly the substrate surface.