Thin films of boron nitride (BN) have been deposited at low temperature by microwave plasma enhanced chemical vapour deposition (PECVD), using borane dimethylamine as boron precursor. Chemically stable and smooth films of hexagonal boron nitride (h-BN) have been synthesised. Infrared transmittance spectroscopy is used for the phase identification, and also for the determination of the orientation of the c-axis in the samples. The influence of the deposition conditions on the film's morphology have been studied, by varying independently the plasma power, the pressure and the precursor flux.