Computer simulation of a CF4 plasma etching silicon

A CF4 plasma etching silicon has been simulated to identify dominant chemical processes and to quantify the effects of various reaction and transport parameters. The model was a one-dimensional plug-flow reactor in which a packet of gas is followed through the plasma and into the afterglow region, allowing the simulation to be performed as an initial value problem in ordinary differential equations. Two temperature zones were used with all known significant reactions incorporated into the chemical mechanism with the best available rate constants. Adjustable parameters were included only for certain sticking coefficients, surface recombination rates, and surface polymerization rates. Appropriate adjustment of these parameters gives satisfactory agreement between the simulations and experimental measurements of downstream gas-phase composition. The model unambiguously shows that fluorine atoms are the main reactive species in the plasma, that gas phase chemistry is clearly dominated by neutral reactions, and that formation of surface polymer has a strong effect on the composition of the gas phase. A full sensitivity analysis of the mechanism reveals that transport processes, surface chemistry, and the formation of fluorocarbon polymer on the walls are among the dominant components of the mechanism, but adequate data for these are unavailable. It is concluded that improvements in the model will require the inclusion of three-dimensional spatial dependencies and better information on surface processes.