On mechanisms influencing etching/polymerization balance in multi-component fluorocarbon gas mixtures

Abstract

In this work, we performed both experimental and model-based study of plasma parameters, steady-state gas phase compositions and heterogeneous process kinetics in CF4 + C4F8 + Ar and CF4 + CHF3 + Ar gas mixtures under the condition of 13.56 MHz inductively-coupled plasma. As constant input parameters we used gas pressure (6 mTorr) and input power (700 W) while variable ones were fluorocarbon component ratios and bias power. It was found that the substitution of CF4 for any second fluorocarbon gas a) results in rather weak effects on electrons- and ions-related plasma characteristics (especially in the case of C4F8); and b) causes sufficient changes in kinetics of neutral species that accelerate the formation of polymerizing radicals and suppress the F atoms density (especially in the case of CHF3). The latter is due to the effective decay of F atoms in the CHFx + F -> CFx + HF reaction family. On the contrary, the variation of bias power does not influence kinetics of plasma active species, but changes only the ion bombardment intensity (and thus, the ion-induced reaction kinetics) through the negative dc bias voltage. The phenomenological (based on the gas-phase-related tracing parameters) analysis of heterogeneous process kinetics indicated that the fluorocarbon gas ratio represents the more effective tool to adjust etching characteristics, polymerization rate and the fluorocarbon film thickness. The last two values were found to be always higher in C4F8 - containing plasmas.