Microwave engineering of plasma-assisted CVD reactors for diamond deposition

Abstract

The unique properties of CVD diamond make it a compelling choice for high powerelectronics. In order to achieve industrial use of CVD diamond, one must simultaneouslyobtain an excellent control of the film purity, very low defect content and a sufficientlyrapid growth rate. Currently, only microwave plasma-assisted chemical vapour deposition(MPACVD) processes making use of resonant cavity systems provide enough atomichydrogen to satisfy these requirements.We show in this paper that the use of high microwave power density (MWPD)plasmas is necessary to promote atomic hydrogen concentrations that are highenough to ensure the deposition of high purity diamond films at large growth rates.Moreover, the deposition of homogeneous films on large surfaces calls for theproduction of plasma with appropriate shapes and large volumes. The production ofsuch plasmas needs generating a fairly high electric field over extended regionsand requires a careful design of the MW coupling system, especially the cavity.As far as MW coupling efficiency is concerned, the presence of a plasma load represents amismatching perturbation to the cavity. This perturbation is especially important at highMWPD where the reflected fraction of the input power may be quite high. This mismatchcan lead to a pronounced heating of the reactor walls. It must therefore be taken intoaccount from the very beginning of the reactor design. This requires the implementation ofplasma modelling tools coupled to detailed electromagnetic simulations. This is discussed insection 3.We also briefly discuss the operating principles of the main commercial plasma reactorsbefore introducing the reactor design methodology we have developed. Modelling results fora new generation of reactors developed at LIMHP, working at very high powerdensity, will be presented. Lastly, we show that scaling up this type of reactorto lower frequencies (915 MHz) can result in high density plasmas allowing forfast and homogeneous diamond deposition on up to 160 mm diameter surfaces.