In situ analyses on negative ions in the sputtering process to deposit Al-doped ZnO films

The origin of high energy negative ions during deposition of aluminum doped zinc oxide (AZO) films by dc magnetron sputtering of an AZO (Al2O3: 2.0 wt %) target was investigated by in situ analyses using the quadrupole mass spectrometer combined with the electrostatic energy analyzer. High energy negative oxygen (O−) ions which possessed the kinetic energy corresponding to the cathode sheath voltage were detected. The maximum flux of the O− ions was clearly observed at the location opposite to the erosion track area on the target. The flux of the O− ions changed hardly with increasing O2 flow ratio [O2/(Ar+O2)] from 0% to 5%. The kinetic energy of the O− ions decreased with decreasing cathode sheath voltage from 403 to 337 V due to the enhancement of the vertical maximum magnetic field strength at the cathode surface from 0.025 to 0.100 T. The AZO films deposited with the lower O− bombardment energy showed the higher crystallinity and improved the electrical conductivity.