Thermodynamic investigation of selective tungsten chemical vapour deposition: Influence of growth conditions and gas additives on the selectivity in the fluoride process

Abstract

Two steps in the fluoride-based selective tungsten deposition process were analysed thermodynamically: the initial step which is the silicon reduction of WF 6 and the continuation step which is H 2 reduction. The tendency of selective deposition was analysed from the calculated driving force values for the initial step, while chemical potentials were used for selectivity predictions for the continuation step. Three reaction gas mixtures were analysed: H 2-WF 6, H 2-WF 6-SiH 4 and H 2-WF 6-SiF 4. The results of the thermodynamic analysis were in good agreement with experiments. The predicted drastic increase in selectivity in addition of SiH 4 to the H 2-WF 6 gas mixture was attributed to compositional changes of the vapour. SiF 4 had only a minor influence on the selectivity but a reduced enroachment can be expected with the addition of SiF 4 to the H 2-WF 6 gas mixture. This investigation indicates that thermodynamics can be used to optimize selectivity as well as to select selectivity modifiers for chemical vapour deposition.