The charge state of the ions produced by a saddle field ion source

Abstract

The work described in this thesis is concerned with an analysis of the charge state and energy of the ions produced by a saddle field ion source and its application to the measurement of the sputtering yield. A review of the literature concerned with the production of ions is given together with a discussion of the fundamental processes and parameters involved. A magnetic analyser has been developed to measure the charge state of the ions produced by a water cooled source for hydrogen,helium, neon, nitrogen, oxygen and argon. The highest charge state observed in any significant proportion was the argon AR3+ ion and the ratio of Ar2+ to Ar+ was found to increase from 0.05 to 0.16 as the pressure increases from 4 to 14x10-5 mbar. The energy of the ions has been inferred from the charge spectra and directly measured using a retarding field analyser. It has been found that the energy of the ions is directly proportional to their charge state and thus all the ions must be formed in the same region in the source. This suggests that the higher charge states are produced by a single electron impact process, which is supported by the fact that the proportions of the observed quantities of Ar3+  ,Ar2+  and Ar+ the respective ionisation cross-sections for a single impact process are in reasonable agreement. The analyser has been modified to measure the sputtering yield of gold for the argon ions, Ar+ and Ar2+ and the yield was found to increase from 4 to 11 atoms per ion as the ion energy increases from 3 to 13.5 keV. However it has been shown that for ions of the same energy, the yield is independent of the charge state of the ions,which is in agreement with the momentum transfer theory for the sputtering process.