Oxygen might not be aggressive enough. The beauty of carbon is oxygen turns carbon into CO2 which is easily pumped from the chamber. Boron forms a stable solid oxide. Fluorine can form a boron gas but it is highly toxic and fluorine attacks almost everything. The key question is the etch rate vs the deposition rate. Fluorine is commonly used in semiconductor chamber cleaning so the recipe exists. An question to pose would be using a mixture of BF3 (toxic boron gas) and hydrogen as fuel. The fluorine would attack the electrodes, but it would also keep the boron in gas form. I guess it would be a trade off in which gets you first, boron build up or electrode/chamber erosion.
