Cathodic bombardment and sputtering in the 0.5–4.0 torr pressure region of a glow‐discharge ion source

Abstract

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.Glow-discharge mass spectrometry (GDMS) has become an important technique €or the analysis of trace components in metals and semiconductor materials.',' Essentially, the GD consists of two electrodes in a gas with a sufficiently high voltage applied across them to maintain a continuous electrical discharge. In GDMS, the sample forms the cathode. The sputtered atoms of the sample are launched into the gas phase where they become ionized. These ions are sampled through an orifice in the anode and analysed by mass spectrometry.that the sputtering is caused by surface bombardment of the cathode, mainly involving cations of the discharge gas, accelerated from the negative glow (see Fig. 1) through the cathode dark space (the distance separating the negative glow from the surface of the cathode) onto the cathode. The degree of sputtering depends on the conditions prevailing in the discharge, viz: the nature of the cathode material, the type of discharge gas, the pressure and the discharge voltage. The pressure has two direct effects on the sputtering. First, the relatively high pressures of a glow discharge impede the complete escape of sputtered material away from the surface, causing backdiffusion onto the cathode, thereby decreasing the nett rate of erosion. Second, collisions and resonance charge-transfer reactions, such as (l), of the accelerating ions with the gas, considerably diminish their effective kinetic energy. It is generallyAr+ .+ +Are+ A P -+ + Ar'(1)