Interfacial properties in the deposition of diamond films by combustion assisted CVD

Abstract

Combustion flame assisted CVD of diamond has been carried out on variety of substrates to examine the mechanism of nucleation and growth and the role of the interface in this process. Initial studies were carried out on native oxide covered Si (100), and Si (100) with thin thermal oxide films (100 nm, 200 nm). After pretreatment under reducing flame conditions, a SiC layer, identified by X-ray Photoelectron Spectroscopy, was produced on all three substrates. The SiC/SiO2 ratio varied systematically with deposition time. Atomic Force Microscopy (AFM) suggests SiC particles were randomly distributed over the substrates early in the nucleation phase. Pretreatments in the fuel rich flame induce SiC formation after an induction time dictated by the thickness of the oxide film. Following SiC formation, diamond crystals, detected by photoelectron spectroscopy and Micro-Raman spectroscopy, can be grown on all three substrates with similar growth rates. In a laser pyrolysis study, with the subsequent deposition of diamond films in mind, the laser powered decompositions of silacyclobutane (SCB). 1,3-disilacyclobutane (DSCB) have been investigated. By changing the deposition conditions and the mixing ratio of the precursors, amorphous silicon carbide CVD films (Si1-xCx) have been deposited as buffer layers for subsequent diamond film deposition. Atomic Force Microscopy (AFM) shows that the deposits consist of very uniform films. XPS shows the stoichiometry of the deposits obtained from SCB and DSCB are SiC2.4O0.3 and SiC1.1O0.2 respectively. The laser pyrolysis technique has the advantages of producing desirable and controllable Si/C stoichiometry at lower deposition temperatures than conventional CVD methods. However, the instability of the films on exposure to air makes the method unsuitable as a precursor to CA-CVD diamond deposition. In the last part of thesis, the role of carbide and oxy-carbide films in the adhesion of CA-CVD deposited diamond films on commercial pure Ti metal, and a Ti, 6%Al,4%V substrates was examined. It was found that the carbon to oxygen ratio in an oxy-carbide structure formed at the interface broadly correlates with the adhesive strength of the diamond/substrate bond. Deposition experiments suggest the nature of the substrate is important in aiding carbide and subsequently diamond formation. The formation of a stable carbide clearly assists in the nucleation phase and in enhancing bonding at the interface, but it is far from clear that it is an essential precursor to the formation of diamond. Rather it appears to act to stabilize the substrate surface while diamond growth is established. SIMS depth profiles also show that the interfacial surface on Ti-6Al-4V contains several percent of aluminium and nitrogen, both at the outermost surface and in the interior of the interfacial layer. The aluminum is enriched at the surface as compared with the bulk concentration, and diamond nucleation is accelerated on this surface relative to pure Ti- These experiments with CA-CVD diamond deposition confirm that despite the very different gas composition used in diamond deposition, the initial mechanisms of nucleation and growth are very similar to those observed in other CVD diamond growth technologies. The major difference is the much higher rate at which these steps occur. This has made it a significant challenge to examine the nature and role of the interface in these experiments. It is clear that this zone is critical in controlling adhesion and growth of diamond on the various substrates used.