Boron nitride thin films have been deposited in a magnetic field r.f glow discharge by the decomposition of diborane and nitrogen. The influence of ion current density, bias voltage, deposition time and gas composition are measured and can be explained in terms of the subplantation model developed by Robertson for the formation of highly tetrahedral bonded amorphous carbon. The ion bombardment of the growing film creates defects (interstitials) leading to an increase in the compressive stress which is consistent with the compressive stress model suggested by McKenzie et al. The crystallization process can be explained by rearrangement processes within the high temperature region of the ion impact induced thermal spike. The validity of the subplantation model is supported by computer simulations of the ion bombardment of a hexagonal boron nitride layer. For a description of the densification process a term n(max) is defined which was tested for carbon and boron nitride layers.