Quality of diamond wafers grown by microwave plasma CVD: effects of gas flow rate

Abstract We found a strong impact of gas flow rate on diamond growth process in a 5 kW microwave plasma chemical vapour deposition reactor operated on CH 4 -H 2 gas mixtures. Diamond films of 0.1–1.2 mm thickness and 2.25 in. in diameter were produced at H 2 flow rates varied systematically from 60 sccm to 1000 sccm at 2.5% CH 4 . The highest growth rate, 5 μm h −1 , was observed at intermediate F values (≈300 sccm). Carbon conversion coefficient (the number of C atoms going from gas to diamond) increases monotonically up to 57% with flow rate decrease, however, this is accompanied with a degradation of diamond quality revealed from Raman spectra, thermal properties and surface morphology. High flow rates were necessary to produce uniform films with thermal conductivity >18 W cm −1  K −1 . Diamond disks with very low optical absorption (loss tangent tg δ −5 ) in millimetre wave range (170 GHz) have been grown at optimized deposition conditions for use as windows for high-power gyrotrons.