Deposition of NCD films using hot filament CVD and Ar/CH 4/H 2 gas mixtures

Abstract

Ar/CH 4/H 2 gas mixtures have been used in an attempt to deposit nanocrystalline (NCD) diamond and ultra-nanocrystalline (UNCD) diamond films using hot filament (HF) chemical vapour deposition (CVD). A detailed composition map has been developed for the type of films deposited in the Ar/CH 4/H 2 system. It was found that the standard gas mixtures of 1%CH 4/Ar (+ 1–2%H 2) that are used successfully to grow UNCD films in microwave plasmas produce only graphitic film growth in a HF system. A 2-dimensional computer model was used to calculate the gas phase composition for these conditions. The non-uniform temperature distribution arising from the hot filament produces a substantial decrease in gas phase H atoms near to the substrate surface, whilst [CH 3] remains almost constant. We find that the [H] : [CH 3] ratio near the surface decreases from ∼5 : 1 for 1%CH 4/H 2 gas mixtures to 1 : 36 for 1%CH 4/Ar mixtures, and that this can explain the decrease in growth rate and the reduction in film quality toward nanocrystalline or graphitic films. Increasing the H 2 content in the gas mixture improves the situation, but NCD growth was confined to a limited composition window at the boundary of the microcrystalline diamond growth region and ‘no growth’ region. A 2D model of a microwave CVD system has also been developed which gives the gas phase composition for the various Ar-rich gas mixtures. We find that due to the higher temperatures within the plasma ball, plus the fact that the gas temperature close to the substrate surface is in excess of 2000 K ensures that the [H] : [CH 3] ratio remains ≫ 1, and thus permits growth of diamond, NCD or UNCD. Furthermore, since the model shows that [CH 3] and [C 2H] are always much greater than [C 2], this suggests that CH 3 and C 2H species may be more important growth precursors than C 2 under typical UNCD deposition conditions.