Hysteresis behaviour of filament temperature versus methane concentration and its influence on diamond film morphology

This study is being conducted to determine the influence of deposition reaction conditions on diamond purity, morphology and degree of crystallinity. The HF-CVD process has been chosen because it allows the uniform coating of large areas. All deposition parameters being equal, an irregular tantalum filament temperature behaviour is observed when the methane concentration changes at constant power. By enhancing [CH4], the temperature of the filament suddenly increases by approximately 100 degrees C. The methane concentration at which this jump occurs depends on the power consumption of the filament. By subsequently lowering [CH4], a sudden decrease in the temperature is observed although at a lower [CH4]. A hysteresis curve can therefore be drawn. This means that for a given [CH4], two filament operating regimes are possible that influence strongly the crystallinity of the obtained diamond films. The position of the hysteresis loop strongly depends on the total gas flow. SEM observations of as-deposited diamond surfaces show that the diamond morphology depends not only on the methane concentration but also on the filament operating regime. An explanation is given that relates the filament temperature variation to the chemical composition of the filament surface. (C) 1998 Elsevier Science S.A.