Boron nitride, in the form of a sub-micrometre thick film, has become the main interphase material used in SiC/SiC ceramic matrix composites (CMCs) developed for aeronautical applications. These low-density thermostructural materials are paving the way for new aircraft engines that are lighter and allow for more complete combustion due to their high operating temperatures. The BN interphase controls the mechanical interfacial bond between the SiC fibrous reinforcement and the SiC matrix. It is thanks to the BN interphase that the CMC exhibits non-brittle behaviour while its constituents are individually brittle. For the synthesis of the BN interphase, an alternative to the traditional CVI (chemical vapour infiltration) batch process is to continuously coat the fibres with the interphase before weaving the fibrous preform by feeding a multifilament tow into a CVD (chemical vapour deposition) reactor. For this purpose, a cold-wall reactor, in which the SiC fibre tow is heated by microwave radiation, has been developed. This process allows the exploration of a wide range of deposition temperatures while avoiding the difficulties of infiltrating a whole fibrous preform. Thanks to the volumetric heating of the substrate by microwave radiation, it is possible to control the direction of the infiltration gradient of the BN interphase within the tow by modifying the process parameters and thus the CVD deposition regimes. Thus, while the mass transfer-limited regime gives the infiltration gradient usually encountered in CVI in porous substrates, the chemical reaction-limited regime combined with the thermal gradient specific to microwave heating gives an inverted infiltration gradient in the fibre tow. The use of triethylamine borane complex with ammonia as a precursor instead of the usual boron trichloride avoids the formation of HCl or undesirable solid by-products such as NH₄Cl corroding or clogging gas lines and pumping/exhaust pipes. However, this molecular precursor can lead under certain conditions to a particular microstructure of the BN coating. While it allows a dense and anisotropic deposit to be obtained at very low pressure as with the usual precursors, it can also lead to the formation of a porous and isotropic BN layer if the pressure in the CVD reactor is increased.