CATHODIC BOMBARDMENT AND SPUTTERING IN THE 0.5-4.0 TORR PRESSURE REGION OF A GLOW-DISCHARGE ION-SOURCE

Cathodic ion sputtering energies have been studied in a glow-discharge ion source, operated in the ranges 0.5-2.0 Torr pressure and 100-1000 V discharge voltage using ion kinetic energy spectroscopy. Average energies in the range 10-20 eV were measured. These energies are very low compared with the theoretical prediction of the charge-transfer model. In view of this, the high sputtering yields measured are surprising. It is inferred that the sputtering is caused, not by the discharge gas ions as is still widely believed, but by fast discharge-gas atoms. A means of estimating the average energy of the neutral flux is found by comparing presently measured thresholds with Literature values directly obtained. The average energy seems to be about three times greater than that of the ions. The neutral flux is crudely estimated by comparison of the apparent nett sputter yields measured here with tabulated primary values and taking into account back-diffusion. It is found to be > 100 times larger than the ion flux. The flux created by charge transfer alone is therefore not enough to explain the high sputtering yields measured. Higher energy ions detected and composed mainly of ionized cathodic atoms appear to be formed in a region intermediate between the negative glow and the cathode, which again is different from the generally held view.