PLASMA DIAGNOSTICS BY ENERGY-RESOLVED QUADRUPOLE MASS-SPECTROMETRY OF A REACTIVE MAGNETRON SPUTTERING DISCHARGE FROM AN FE TARGET IN AR-H2S ATMOSPHERES

Reactive magnetron sputtering from an Fe target in Ar-H2S atmospheres was used to deposit thin pyrite (FeS2) films which are intended as absorber films in thin film solar cells. To measure the species arriving at the substrate and their respective energies, a quadrupole mass spectrometer in combination with an electrostatic energy analyser was used. Because of the sulphur chemistry, the plasma discharge in Ar-H2S produces a large variety of sulphur-hydrogen species SxHy with x up to 11 and y=0 to 5. This behaviour is completely different from reactive sputtering in oxygen or nitrogen and reflects the ability of sulphur to form S-x-rings or chains mainly with x=8 but also up to x=50. SxHy species with x=5 and x=8 have relative intensity maxima pointing to the higher structural stability of these molecules. From the energy distributions the plasma potential was determined for different discharge powers, total pressures and H2S to Ar ratios. The plasma potential shows only a weak variation with discharge power and total pressure, but a strong dependence on the depth of the erosion track. This influence is due to the increasing magnetic field in front of the target as the erosion proceeds. The addition of H2S to the discharge atmosphere reduces the effective ion energy by about a factor of 2. The metal ions sputtered from the Fe target display a high energy shoulder at pressures below 1 x 10(-2) mbar, at higher pressures the thermalization of the sputtered ions is visible from the strong decrease in this shoulder.