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 O-2 flow ratio [O-2/(Ar+O-2)] 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. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3430556]