Negative ion density measurements in a reactive dc magnetron using the eclipse photodetachment method

The density of negative oxygen ions in the bulk plasma of a reactive dc magnetron has been determined for the first time using a combination of laser photodetachment and Langmuir probing. Experimental results are obtained for various O 2 /Ar gas mixtures (0–100%), applied powers (50–600 W) and total discharge pressures (2–25 mTorr). The measurements reveal that the O − ion dominates over with the latter less than 2% of the total observed. Variation of the operating parameters showed clear trends in the negative ion densities with maxima observed at particular powers (200 W) and oxygen partial pressures (10% O 2 ). The negative ion density was found to increase with the chamber pressure and the main loss reaction for O − was determined to be ion–ion recombination with O + , and Ar + . In this study, the maximum negative ion density obtained was found to be 7.7 × 10 15 m −3 at 200 W applied power, 25 mTorr total pressure and 50% oxygen partial pressure, giving the ratio of the negative ion to electron density, α = 1.4, indicating that the plasma is moderately electronegative. These new results show that significant concentrations of negative ions are present in the bulk magnetron plasma when operated in Ar/O 2 gas mixtures during dc sputtering. The influence of these ions on thin film growth is briefly discussed.