Effect of electron-irradiation on void formation in helium-pre-injected aluminium films

Abstract

The effect of 200 kv electron-irradiation on evaporated aluminium films pre-injected with 35 kev helium ions has been studied. With a fluence of 1 × 10¹⁵ ions/cm² of helium, the damage produced consisted of voids 30–40 Å in diameter and dislocation loops 200 Å in diameter. Such damage was observed to be enhanced by in-situ electron-irradiation at room temperature (≃ 0.3 T_m ) with a flux of 1 × 10¹⁹ electrons/cm²-sec. The enhancement was in the form of increased void number density and void volume in addition ‘to increased interstitial dislocation loop density and loop area. This effect appeared to be temperature-dependent, since a decrease in void volume with a smaller increase in interstitial dislocation loop area was observed on electron-irradiation at 100°C (0°4 T_m ). The annealing of helium-pre-injected films, prior to electron-irradiation, at 100°C resulted in a large increase in void volume accompanied by a slight decrease in dislocation loop number density and loop area although loop size increased. Interstitial loops are found to play an important role in void formation. The preferential removal of interstitials at these loops tends to reduce recombination of interstitials and vacancies produced by electron-irradiation, thus creating a sufficient vacancy supersa turation to promote void growth.

Positive identification of voids 30-40 Å diameter in aluminium has been made using ‘through-the-focus’ sequences of micrographs to determine change in void contrast.