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 × 10 14  m −3 . At around 12 mTorr, a sudden increase in N e 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 × 10 14  m −3 between 5 and 10 mTorr. Under similar conditions, the DC magnetron negative ion fraction ( N − / N e ) is estimated to be ∼0.01, being significantly lower than in the RF magnetron where N − / N e  ≈ 1.