Boron nitride has become the focus of a considerable amount of interest because of its properties which relate closely to those of carbon. In particular, the cubic boron nitride phase has extreme hardness, chemical inertness, high resistivity, thermal conductivity and transparency and resistance to oxidation up to 1600°C. The conventional methods of synthesis use highly toxic and inflammable source materials at high deposition temperature. A hot filament activated PACVD technique was developed to deposit BN films under a wide range of conditions. The source material was borane-ammonia (BH3-NH3) which is a non-toxic crystalline solid, free of carbon and oxygen. Using this technique, mixed-phase boron nitride (BN) films, containing crystallites of the cubic phase embedded in a hexagonal matrix, were deposited at a substrate temperature of 350°C. These films showed good thermal and chemical stability with smooth surface topography. A good correlation was obtained between the properties of the films and plasma diagnostics. With increasing rf power, the ratio of cubic to hexagonal phase, determined by infra-red spectra, was increased with a shift towards lower wave number. In the growing and phase stabilization of the cubic phase, ion bombardment plays an important role in forming the sp3-microstructure of BN films. The ion-density and thus ion flux to the more negatively biased substrate was increased with rf power corresponding to an increase o f N2+ species in the plasma, determined by OES. This increased ionbombardment contributed in increasing the volume fraction of cubic phase in the film. For a specific substrate and filament temperature, if the deposition conditions are measured in terms of ion-bombardment, a sharp threshold value exists where the phase of the films changes from being hexagonal to being cubic. At a substrate temperature o f 350°C, this threshold value was found to be 160 W of rf power. A substrate temperature o f 300°C was necessary for enhancing the growth of cubic phase. The shift towards lower wave number at higher power was considered to be a transition to the cubic phase from the wurtzite phase of BN which grows under less highly activated conditions. Hydrogen free film was deposited at a temperature greater than 300°C and a rf power greater than 200 W.