Studies of solid surfaces with 100 keV 4He + and H + ion beams

Abstract

Backscattering of 4He + and H + ions incident at 100 keV was used to detect foreign atoms on silicon and graphite surfaces, and also surface disorder on silicon. The ion beams were supplied by an ion-implantation accelerator. Energy spectra of the backscattered ions were measured by an electrostatic analyzer (ESA) which had 3 % energy resolution. Foreign atom sensitivity of this system, expressed as the atom density required to yield 10 counts, is 1.3 × 10 12/ cm 2 for gold, 9 × 10 12/ cm 2 for iron, and 3 × 10 14/ cm 2 for oxygen, at an ion dose of 0.8 microC per point. Use of an 8 microC dose lowers these limits an order of magnitude at the expense of greater surface damage and longer measurement time. The effective scattering yield as measured in the ESA departs somewhat from the Rutherford Z 2 dependence because of ion neutralization which depends on ion energy which in turn depends on the mass of the target atom. Ion channeling was used to supress scattering from silicon atoms beneath the surface which tends to obscure the peaks for light elements such as oxygen and carbon. The channeling phenomenon was also used to study surface disorder due to abrasion damage and damage done by the ion beam itself. Sputtering of metal films by the 100 keV 4He + beam was negligible. However, iodine was removed from silicon surfaces at a detectable rate. This technique at 100 keV, in common with backscattering at higher energies, does not require UHV conditions. It can therefore be used in a simple vacuum system to analyze surfaces of technological interest which might be altered by the heating or the heavy ion bombardment which are required to remove adsorbed background gas layers in several other surface analytical techniques.