The Energies of Positive and Negative Ions in an RF Plasma in Nitrous Oxide

It is known that negative ions are important in the plasma oxidation of silicon and silicon nitride surfaces and there is interest in the use of nitrous oxide instead of oxygen as the plasma gas. The present paper describes an investigation into the distribution of energies with which the O - ions produced in a capacitive rf plasma impinge on the grounded discharge electrode and compares these energies with those of the positive N 2 O + ions produced under the same plasma conditions. It is shown that the maximum energy of the O - ions is largely determined by the d.c. bias of the driven electrode and is independent of changes in the plasma potential. It is, therefore, possible in such systems to select, independently, the energies of the positive and negative ions. The energy distribution of the O - ions shows interesting structure, which for a given d.c. bias, is a function of the input rf signal. The investigation confirms that, for strongly asymmetric geometries, negative ions produced in the sheath at the driven electrode of a capacitive rf plasma system travel through the plasma and impact on the counter electrode with a range of energies.