As far as I know, there is a process used in high voltage diode, which drives
the boron through the wafer from one side to the other side about 250um. It
might need one week in a diffusion furnace.

Roger Brennan  wrote:  I believe most (perhaps all)
people driving boron to 25 um do use a furnace
"pre-deposition". Boron disks (usually boron-nitride) might be less
expensive to set up than say, BBr3 or B2O3. You probably need a dose in the
range of 5E15 to 1E16 atoms/ cm2. Watch out for "boron skin" during the
deposition. You may need to employ a low temperature "wet" oxidation to
remove it.