Swarm data of O−, NO−, He+, N2O+, and ions in N2O, He and N2O–He mixtures for PECVD applications

Abstract

The aim is first to calculate and validate, from an ion swarm unfolding technique, the sets of collision cross sections of several ions (O−, NO−, He+, N2O+, , and ) colliding with N2O and He gases and their mixtures. The aim is then to calculate the corresponding ion swarm parameters in these pure gases and their mixtures in a large E/N range. These ion interactions and swarm data are very useful more particularly for the modelling of the electrical and energetic behaviour of RF discharge in N2O–He mixtures with a small admixture of SiH4 used in PECVD applications for thin film depositions. The sets of collision cross sections for these different ion/gas systems involve both elastic and inelastic processes in the case of an ion energy range varying from thermal energy up to about 100 eV. The elastic ion–gas interaction has been described by using a rigid core potential model which is well adapted for both polar and non-polar systems and also symmetric and asymmetric systems. Momentum transfer cross sections are then determined using a semi-classical JWKB approach while inelastic cross sections are taken from the literature and completed from empirical laws. Then by using these different cross section sets, it is possible to obtain the ion swarm data for different gas mixtures involving N2O and He whatever their relative proportions. These ion swarm data are obtained from an optimized Monte Carlo method well adapted for the ion transport in pure gas as well as gas mixtures. The specific behaviour of these swarm data (ion mobility or drift velocity, diffusion coefficient and reaction rates) is analysed in the case of each ion in different N2O–He mixtures over a quite large E/N range (1–1000 Td) showing more particularly the non-validity of the classical linear approximations such as Blanc law.