An electron cyclotron resonance (ECR) microwave plasma source has been used to deposit boron nitride thin films onto silicon substrates. Borane-ammonia (BH3 . NH3) vapor was introduced in a controlled fashion by congruent thermal sublimation of the non-toxic white crystalline solid and resulted in the deposition of stoichiometric films without the need for additional nitrogen-bearing precursors. Deposition parameters involved a constant microwave power, substrate bias (effective ion energy) ranging from -10 to -100 V de, and substrate temperatures ranging from 350 degrees C to 575 degrees C. The films were characterized with respect to their crystal structure, composition and nature of bonding using a variety of techniques. The crystalline structure and bonding type were weak functions of substrate bias and temperature. At modest bias (-100 V) there was evidence for the existence of the wurtzitic and/or cubic phase, but in general the films were predominantly sp(2) bonded and amorphous.