Negative Ion Density Measurements in Reactive Magnetron Sputtering

A combination of laser photo-detachment and conventional Langmuir probing has been used to obtain the bulk negative ion density in both a DC and radio frequency (RF) sputter magnetron. The argon and oxygen discharges were operated at low powers and over a range of pressures. The photo-detachment signal is expected to reach a limiting value; however, the signal continues to increase with laser energy density and this can be attributed to a laser ablation effect. In the RF magnetron the electron temperature (T-e) in oxygen decreased with increasing pressure, whereas the electron density (N-e) increased from 0.53 to 8.6 x 10(14)m (3). At around 12 mTorr, a sudden increase in Ne by 3.7 is accompanied by a small drop in T-e. The negative ion density (N_) also increases with pressure reaching a maximum of 1.7 x 10(14) m(3) between 5 and 10 mTorr. Under similar conditions, the DC magnetron negative ion fraction (N_/Ne) is estimated to be similar to 0.01, being significantly lower than in the RF magnetron where N_/Ne approximate to 1.