Low-Temperature Plasma Synthesis of Cubic Boron Nitride

Low-temperature plasma synthesis of cubic boron nitride (cBN) coatings on diamond seeded silicon substrates was carried out in a microwave plasma chemical vapor deposition (MPCVD). Low-temperature plasmas (LTP) are severely non-equilibrium systems that use free electrons to create a novel physical and chemical environment at low gas temperatures. In MPCVD, hydrogen (H₂), argon (Ar), a mixture of diborane in H₂ (95% H₂, 5% B₂H₆), and nitrogen (N₂) were used as the feed gas. A direct current (DC) bias system was used to externally bias the sample negatively where the MPCVD chamber wall was grounded. The presence of sp³ bonded BN in the synthesized coatings was demonstrated by X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR). The cBN content in the coating increases with increasing Ar flow, reaching a maximum at the maximum Ar flow of 400 sccm used in this study. High-resolution XPS scans for B1s and N1s indicate that the deposited coating contains more than 70% cBN. The surface roughness of the BN coatings was also measured using Atomic Force Microscopy (AFM). This study found that energetic argon ions generated in microwave-induced plasma are beneficial for cBN formation in MPCVD.