Experimental methods enabling the efficient control of high pressure microwave discharges

Summary form only given. High pressure microwave discharges (150 Torr to one atmosphere) have been the focus of numerous recent experimental investigations such as microwave plasmoid/microwave fireball investigations
¹
, microwave plasma assisted combustion
²
, and microwave plasma assisted gas reforming, and also recent commercial applications such as CVD diamond synthesis
³
. In each of these experiments discharge control and the coupling efficiency become very important as the pressure is increased. This is especially true as applications move toward commercial realization. Microwave plasma assisted chemical vapor deposition of diamond at high pressures (150-400 Torr) is one microwave discharge application that is now being commercialized throughout the world
⁴
. Thus, in this presentation the diamond synthesis application will be used as an example of how high pressure microwave discharges can be efficiently utilized in a commercial material synthesis setting. Experimental methods and coupling technologies that have been developed to enable CVD diamond synthesis in the high pressure regime will be described. A versatile, mechanically internally tuned reactor technology has been developed
⁵
and is employed to synthesize single crystal diamond (SCD). High power density (~200-500W/cm
³
), high pressure (150-400 Torr), CH
₃
/H
₂
, stable discharges are produced via efficient coupling. These discharges have gas temperatures of 2500-4000K and electron densities of 5x10
¹¹
to 3x10
¹²
/cm
³
. SCD is synthesized without the formation of detrimental, dusty plasmoids
^1,6
. The discharge is located in good contact with the substrate and the plasma substrate boundary layer is controlled and varied as the input power, pressure, gas flow rate, gas mixture, substrate temperature etc. are all independently varied. Microwave coupling efficiencies of greater that 90% are easily achieved over the entire experimental input variable range, and once the optimum synthesis process conditions are identified, coupling efficiencies greater that 98% are achieved. Using the same reactor technology, diamond synthesis process control strategies will also be presented. Discussion of how these techniques can be applied to other potential high pressure microwave discharge applications will also be presented.