Synthesis of cubic boron nitride thin films by microwave PECVD

Abstract

Boron nitride can be synthesised in two major crystalline polytypes, the hexagonal (h-BN) and the cubic one (c-BN), related respectively to the sp2 and the sp3 hybridation of the chemical bondings of both atomic species. The cubic form is very attractive, due to its extreme properties, similar to those of diamond in term of hardness, thermal conductivity, chemical inertness and optical transparency, It is even more stable against oxidation up to higher temperature, and can be doped whether P or N type, making it a candidate for applications in power electronics. Classically, c-BN can be synthesised under high pressure and high temperature, but more recently PVD and CVD have been successfully employed to obtain thin films. In these later cases, the growth process requires bombardment with energetic particles, and general ways the structure is left in compressive stress after deposition and delamination or cracking of the film can occur. We have deposited boron nitride thin films on silicon substrates at low temperature (below 300/spl deg/C) in a microwave plasma enhanced chemical vapor deposition (PECVD) apparatus, An organometallic compound, borane dimethyl amine, was used as boron precursor. To promote the growth of the cubic phase a negative self bias was applied to the sample holder by mean of a 13.56 MHz RF signal. Characterisation of the samples involve infra red and Raman spectrometries, and atomic force microscopy. Films containing a fraction of c-BN, as high as 98%, have been obtained, as can be seen on the infra red spectra. The ratio has been determined according to the relative intensity of the active IR mode of the cubic and the hexagonal phases. At normal incidence only the transversal optical (TO) modes are observed, whereas at oblique incidence the longitudinal optical (LO) modes are also evidenced, according to the Berreman effect. The c-BN TO absorption band is observed at 1071 cm/sup -1/ and its associated LO mode is located at 1267 cm/sup -1/. The films exhibit a very low roughness, as observe by atomic force microscopy, and contain nanocrystals of about 10 nm averaged size, as deduced from the Raman measurements. These deposits are very adherent and do not delaminate even after more than one year. Also, PECVD seems to be a promising method for the synthesis of c-BN layers.