Mechanisms for highly ionized magnetron sputtering

Abstract

A simple model for ionization of sputtered metals by a high-density plasma is presented. Experimentally, ion flux fractions of greater than 80% can be obtained by sputtering aluminum into a region of dense plasma (ne∼1012 cm−3). Such a process has important applications in the filling of high-aspect-ratio features encountered in microelectronics fabrication. Both electron-impact and Penning ionization mechanisms are considered in this model. Under conditions of low electron density (ne≪1011 cm−3), Penning ionization is found to be the dominant ionization path. This is consistent with the accepted ionization mechanism for conventional diode sputtering. When high electron densities are generated, however, electron-impact ionization plays a significant ionization role. Langmuir probe measurements of the inductively coupled plasma indicate that the electron density lies between 2×1011 and 2×1012 cm−3. The model, in combination with measured plasma density, is used to calculate ion fractions. Modeled and experimentally measured aluminum ion fractions compare favorably. The effects of chamber dimensions, argon pressure, and sputtered metal density are investigated and shown to be important in optimizing the ionization fraction.