PLASMA PARAMETERS AND DENSITIES OF ACTIVE SPECIES IN MIXTURES OF FLUOROCARBON GASES WITH ARGON AND OXYGEN
Abstract
The comparative study of plasma electro-physical parameters and steady-state gas phase compositions in CF4 + O2 + Ar, CHF3 + O2 + Ar and C4F8 + O2 + Ar gas mixtures was carried out under the condition of 13.56 MHz inductive RF discharge. Constant processing parameters were fluorocarbon component fraction in a feed gas (50%), total gas pressure (6 mTorr), input power (700 W) and bias power (200 W). The investigation scheme included plasma diagnostics by Langmuir probes and 0-dimensional (global) modeling of plasma chemistry. It was found that polymerizing ability in both non-oxygenated (50% Ar) and oxygenated (50% O2) gas systems correlates with the x/z ratio in the original CxHyFz molecule. The substitution of Ar for O2 causes similar changes in electrons- and ions-related plasma parameters (electron temperature, plasma density, ion bombardment energy), always suppresses densities of polymerizing radicals and polymer film thickness, but has the different impact on the F atom kinetics. An increase in O2 fraction in 50% CF4 + O2 + Ar and 50% CHF3 + O2 + Ar gas mixtures results in monotonically increasing F atom densities. Mechanisms of these phenomena are increasing F atom formation rate and decreasing the F atom decay frequency, respectively. The addition of oxygen to 50% C4F8 + O2 + Ar gas mixture lowers the F atom formation rate, but does not result in sufficient changes in their decay frequency. This corresponds to monotonically decreasing F atom density toward O2-rich plasmas. As a result, the steady-state density of F atoms in gas systems with 50% O2 increases in the sequence of C4F8 - CHF3 - CF4.
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