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 sil icon and silicon nitride surfaces and there is interest in the use of nitro us oxide instead of oxygen as the plasma gas. The present paper describes a n investigation into the distribution of energies with which the O- ions pr oduced in a capacitive rf plasma impinge on the grounded discharge electrod e and compares these energies with those of the positive N2O+ ions produced under the same plasma conditions. It is shown that the maximum energy of t he O- ions is largely determined by the d.c. bias of the driven electrode a nd is independent of changes in the plasma potential. It is, therefore, pos sible in such systems to select, independently: the energies of the positiv e and negative ions. The energy distribution of the O- ions shows interesti ng structure, which for a given d.c. bias, is a function of the input rf si gnal. The investigation confirms that, for strongly asymmetric geometries, negative ions produced in the sheath at the driven electrode of a capacitiv e rf plasma system travel through the plasma and impact on the counter elec trode with a range of energies.