MODEL FOR ION-INDUCED NUCLEATION BASED ON PROPERTIES OF SMALL IONIC CLUSTERS

A method is presented for calculating ion-induced nucleation rates. Wh ereas classical ion-induced nucleation theory makes the approximation that small ionic clusters are charged droplets whose properties equal their values for the bulk liquid, the method presented gives a nucleat ion rate in the form of a summation over discrete cluster properties. The summation converges rapidly around the critical cluster size, whic h is often as small as a few atoms. This approach allows the direct ut ilization of experimental and/or computational data for cluster proper ties. Sample calculations are presented for nucleation of silicon part icles via condensation of neutral silicon vapor onto silicon anions fo r conditions representative of microelectronics processing plasmas. As the temperature increases the predicted nucleation rates show a trans ition from the collision-limited regime to the condensation-evaporatio n regime, where nucleation rates drop sharply with temperature. The va lue of the temperature where this occurs depends on the condensible va por concentration. (C) 1996 American Vacuum Society.