Implementations of the present disclosure generally relate to methods for forming thin films in high aspect ratio feature definitions. In one implementation, a method of processing a substrate in a process chamber is provided. The method comprises flowing a boron-containing precursor comprising a ligand into an interior processing volume of a process chamber, flowing a nitrogen-containing precursor comprising the ligand into the interior processing volume and thermally decomposing the boron-containing precursor and the nitrogen-containing precursor in the interior processing volume to deposit a boron nitride layer over at least one or more sidewalls and a bottom surface of a high aspect ratio feature definition formed in and below a surface of a dielectric layer on the substrate.