SYNTHESIS OF CUBIC BN FROM THE GAS-PHASE BY A NEW PLASMA CHEMICAL VAPOR-DEPOSITION METHOD USING RF WAVES AND A TUNGSTEN FILAMENT

Several types of plasma chemical vapour deposition (CVD) apparatus have been developed which enable cubic BN (c-BN) films to be formed. In the development, it was essential to introduce a high energy into the reaction space electromagnetically and thermally for plasma excitation. A recently developed plasma CVD apparatus is composed of a Knudsen-cell-type crucible in which a tungsten filament is mounted. The crucible is covered by a cap with a small pinhole for the high energy gas plasma to jet out. An r.f. coil is placed around the crucible. With this apparatus the yield of c-BN formation has been increased remarkably and a BN film of about 100% c-BN (volume fraction) was formed in the optimum range of conditions. It was found that tungsten filament heating contributes to c-BN formation by excitation of the plasma gas to create high energy ions, radicals and atoms. The atomic hydrogen created was found to etch the deposited turbostratic BN structure considerably to give a high volume fraction of c-BN in the BN films. A mechanism for c-BN formation is discussed qualitatively in relation to the atomic potentials with sp2 and sp3 electron orbitals.