Comparative Study of an RF and a Microwave High-Density-Plasma Source for Plasma Immersion Ion Implantation

Abstract

A comparative experimental study is conducted of an RF and a microwave plasma source placed in the same reactor, the RF source using inductive coupling. The conditions correspond to actual fabrication processes: the plasmas are excited in BF3 at pressures between 0.5 and 20 mtorr and at applied powers ranging from 400 to 1500 W. Local values and radial profiles are measured near the wafer by Langmuir-probe method for electron density and temperature and for the densities of positive and negative ions. The electron energy distribution function is determined. It is shown that the density of charged species in an RF plasma is considerably larger than in a microwave one; in particular, the ion density is as high as about 1012 cm–3 at an applied power of about 1200 W. The RF source is found to ensure adequate radial uniformity for wider ranges of external parameters. It is established that the two sources differ significantly from each other in electron energy distribution function. The BF3 plasmas are found to be electronegative, the ratio of negative- to positive-ion density lying in the range 0.3–0.5 for both sources.