Mean free path of negative ions in diode sputtering

Abstract

Negative ions produced in the sputtering of certain compounds (e.g., SmAu, EuAu, CsAu, and LaF3) are capable of dominating the film-growth process in a thin-film depostion system. Under some conditions sputter etching of substrates is observed due to a high flux of negative ions accelerated from the target surface. The energy and collimation of these negative ions are examined by evaluating the cross sections for electron detachment and directed momentum loss. Under typical sputtering conditions the negative ions are found to retain their charge long enough to accelerate to the full dark space voltage. The mean free path at this energy is estimated from the Ar–Ar interaction potential and found to be many times longer than the thermal mean free path. These results explain the sharply outlined region of substrate etching by the negative ion flux and lead to a picture of a highly collimated, energetic particle beam directed away from the target surface. negative ion flux and lead to a picture of a highly collimated,