Plasma chemical behaviour of reactants and reaction products during inductively coupled CF4 plasma etching of SiO2

A two-dimensional fluid model has been developed to study plasma chemical behaviour of etch products as well as reactants during inductively coupled CF4 plasma etching of SiO2. The plasma fluid model consisted of Maxwell's equations, continuity equations for neutral and charged species including gas-phase and surface reactions and an energy balance equation for electrons. The surface reaction model assumed Langmuir adsorption kinetics with the coverage of fluorine atoms, fluorocarbon radicals and polymers on SiO2 surfaces. Numerical results indicated that etch product species occupy a significant fraction of reactive ions as well as neutrals in the reactor chamber during etching, which in turn leads to a change in plasma and surface chemistry underlying the processing. In practice, the density of SiF4 was typically about 10% of that of the feedstock CF4, being comparable to that of the most abundant fluorocarbon radical CF2; moreover, the density of SiF3+ was typically about 5% of that of the most abundant fluorocarbon ion CF3+. The density and the distribution of such product species in the reactor chamber were changed by varying the ion bombardment energy on the substrate surfaces, gas pressure, mass flow rate and coil configuration, which arises in part from gas-phase reactions depending on plasma electron density and temperature. Surface reactions on the chamber walls and on the substrate also affect the product density and distribution in the reactor; in particular, the surface reactions on the SiO2 dielectric window as well as substrate surfaces were found to largely affect the product density and distribution.