Revisiting the gas flow rate effect on diamond films deposition with a new dome-shaped cavity type microwave plasma CVD reactor

With the newly built dome-shaped cavity type microwave plasma CVD reactor, the gas flow rate effect observed in diamond films deposition was reinvestigated. Experimentally, a maximum in diamond films growth rate was observed when the gas flow rate was set at an intermediate value. To explain the experimental results, a two-folded mechanism was taken into consideration, i.e., the catalytic effect of nitrogen impurity in enhancing diamond films deposition, and depletion of carbon containing species in diamond depositing environment. Experimental evidence has been collected by analyzing the diamond films and measuring optical emission spectra of the microwave plasma. It is shown that with the proposed mechanism, the experimental results could be explained, and at the same time, changes in morphology and quality of diamond films with the gas flow rate are discussed.

In depositing diamond films with the microwave plasma chemical vapor deposition (MPCVD) method, a dependence of growth rate of diamond films on gas flow rates may be observed. The phenomenon has been reported by Ralchenko et al. (Quality of diamond wafers grown by microwave plasma CVD: effects of gas flow rate, Diamond Relat. Mater. 8 (1999) 189-193), and no detailed explanation has been given.In this investigation, the gas flow rate effect is revisited, with our newly built MPCVD reactor. Then based on experimental results, a two-folded mechanism is used to explain the effect. It is shown that nitrogen impurity enrichment is responsible for growth rate enhancement as the gas flow rate at first decreases, and then depletion in carbon concentrations is responsible for decrease in growth rate of diamond films at lower gas flow rates. With such a two-folded mechanism, experimental results could be well described.62