ENERGY-DISTRIBUTION OF IONS IN AN UNBALANCED MAGNETRON PLASMA MEASURED WITH ENERGY-RESOLVED MASS-SPECTROMETRY

The energy distribution of ions in the plasma during unbalanced magnet ron sputtering was measured with an energy-resolved mass spectrometer. Typical energy distributions of the main ion species were measured fo r sputtering of a Cu target in an Ar atmosphere in the pressure range from about 0.1 to 10 Pa. The effects of the discharge pressure and of the distance between target and mass spectrometer on the ion energy di stribution were studied. A low energy (thermalised) peak in the ion en ergy distribution function was observed at an energy corresponding to the plasma potential. In addition, a high energy tail was observed at energies of about 3-30 eV above the low energy peak, not only for sput tered ionised atoms but also for the Ar ions. The ion energy distribut ion of this high energy tail can be fitted with an exponential functio n; the mean energy was about 1-5 eV. The integrated intensity of the h igh energy tail was a function of the pressure-distance product p x d, where d is the distance of the mass spectrometer from the magnetron. The function is close to exponential decrease with increasing p x d pr oduct for the high energy tail of the Cu+ energy distribution. A simil ar exponential decrease was observed also for the high energy tail of the Ar+ energy distribution with the exception of lowest pressures (<0 .2 Pa) where the intensity of the Ar+ high energy tail increases with increasing pressure. An explanation of these observations is proposed based on energy transfer from sputtered atoms to Ar gas atoms during c ollisions.