Yajun Gu

Determining the microwave coupling and operational efficiencies of a microwave plasma assisted chemical vapor deposition reactor under high pressure diamond synthesis operating conditions

The microwave coupling efficiency of the 2.45 GHz, microwave plasma assisted diamond synthesis process is investigated by experimentally measuring the performance of a specific single mode excited, internally tuned microwave plasma reactor. Plasma reactor coupling efficiencies (η) > 90% are achieved over the entire 100-260 Torr pressure range and 1.5-2.4 kW input power diamond synthesis regime. When operating at a specific experimental operating condition, small additional internal tuning adjustments can be made to achieve η > 98%. When the plasma reactor has low empty cavity losses, i.e., the empty cavity quality factor is >1500, then overall microwave discharge coupling efficiencies (η(coup)) of >94% can be achieved. A large, safe, and efficient experimental operating regime is identified. Both substrate hot spots and the formation of microwave plasmoids are eliminated when operating within this regime. This investigation suggests that both the reactor design and the reactor process operation must be considered when attempting to lower diamond synthesis electrical energy costs while still enabling a very versatile and flexible operation performance.

Microwave plasma reactor design for high quality and high rate diamond synthesis

Since the first demonstration of microwave plasma assisted chemical vapor deposition (MPACVD) of diamond by Matsumoto [1] microwave plasma reactor technologies have evolved into a variety of commercially available technologies which have been applied to numerous applications. Some of the most notable applications are the synthesis: (1) of optical quality large area (~10cm diameter), thick, free standing polycrystalline diamond (PCD) plates and (2) of multi-carat, single crystal diamond (SCD) plates with properties that suggest potential and significant, gem, optical, and electronic applications. Recently the evolution of MPACVD diamond reactor technologies has resulted in diamond synthesis occurring at pressures of 150-300 torr and associated high discharge power density ranges of 150-1000 W/cm3 [2,3]. These new reactor designs have resulted in the production of high quality diamond at rates of 10-100 microns/hour and have involved the development of new and potentially important commercial MPACVD reactor configurations that operate over the 50-400 torr regime.

A description of the experimental microwave discharge behavior versus pressure, power and reactor geometry for MPACVD diamond synthesis reactors

Summary form only given. Recently two new microwave plasma-assisted chemical vapor deposition (MPACVD) diamond synthesis reactors [1,2] were designed, built and experimentally evaluated and their performance was compared to earlier MPACVD reactor designs [3]. In order to take advantage of the improved CVD diamond synthesis conditions that occur within the high pressure regime (160–300 Torr) the two reactors were designed to operate with high power densities and at high pressures. When these reactors operate at high pressures single crystal MPACVD diamond synthesis occurs over a large range of reactor conditions [1,2] such as: (1) pressure, p, 100->300 Torr. (2) input power 1–6 kW, (3) flow rates, (4) methane to hydrogen concentrations of < 1% to greater than 9%, (5) discharge power densities of 200–1000 W/cm3, (6) substrate holder design/geometry, (7) reactor design/geometry and (8) plasma/substrate position, ZS.

Microwave plasma assisted synthesis of single crystal diamond at high pressures and high power densities

Summary form only given. It is now widely understood that CVD synthesized diamond quality and growth rates can be improved by using high power density microwave discharges operating at pressures above 160 Torr. Thus we have developed microwave plasma reactor designs and associated process methods that are both robust and are optimized for high pressure and high power density operation and thereby take advantage of the improved deposition chemistry and physics that exist in the high pressure (180-320 Torr) regime. Here we will present the single crystal diamond (SCD) synthesis performance of two 2-5 kW, 2.45 GHz microwave plasma assisted CVD reactor designs, Reactors B and C, that have been specifically designed and optimized for operation in the high pressure regime. The reactors have incorporated design features that are adaptable such as reactor tuning that allows discharge matching and also the control of the position, size and shape of the discharge. Reactor B has been described earlier and Reactor C further improves performance by increasing applicator size and by positioning the discharge away from the reactor walls. The experimental performance of the new reactor designs will be presented and will be compared with each other and with a related reactor, Reactor A, that operates at lower pressures of 10-160Torr.

Microwave plasma assisted reactor design for high deposition rate diamond synthesis

In view of the important, recent, opportunity to commercially synthesize high quality single crystal diamond (SCD) there is a need to continue to improve existing microwave plasma assisted reactor designs that enable high quality and high deposition rate SCD synthesis. It is now widely recognized that both the quality and growth rates of microwave plasma assisted CVD (MPACVD) synthesized diamond are improved by using high power density microwave discharges operating at pressures above 160 Torr [1]. Thus we are developing microwave plasma reactor designs and associated process methods that are both robust and are optimized for high pressure and high power density operation [2], and thereby take advantage of the improved deposition chemistry and physics that exist at high pressures. These reactors operate in the 160–320 Torr pressure regime. Here we describe our the design methodologies, and then we present the specific design details of a 2.45 GHz MPACVD reactor design that enables optimized reactor performance and high growth rate diamond synthesis in the high pressure regime.

Microwave plasma-assisted diamond synthesis reactor design for large deposition areas at high rates

The Microwave plasma assisted CVD synthesis of diamond was first demonstrated during the early 1980s by Kamo et al [1]. Diamond synthesis was achieved in a small ( 2–4 cm ), tubular reactor where microwave energy was coupled into a quartz tube that was inserted through a waveguide. This reactor produced high radical densities and high quality diamond films and was inexpensive, simple to design, construct and operate. Hence it was utilized by many early diamond researchers to experimentally investigate and understand CVD diamond synthesis. However this reactor type had a number of inherent limitations such as small deposition area and low operating pressure regime. Since these early investigations the significant potential of industrial applications of CVD diamond synthesis has spurred numerous, innovative, microwave plasma reactor designs.