Selective etching of boron nitride phases

Cubic boron nitride (c-BN) films can be used as hard coatings and for elect ronic devices due to their outstanding material properties, but the gas pha se deposition of c-BN is still a challenging task. Until now it has only be en possible to achieve nanocrystalline c-BN layers via physical vapor depos ition (PVD) methods with rather weak film qualities. Only a chemical vapor deposition (CVD) process for c-BN can produce high quality films with mater ial properties similar to those of the product achieved by high pressure. h igh temperature professes (HPHT) conventional routes. Therefore it is essen tial to tune the individual steps in the CVD process (nucleation, growth an d selective etching) in a similar manner to that for diamond CVD to enable continuous growth of c-BN. Since selective etching of hexagonal boron nitride (h-BN) and sp(2) phases is still a major problem, we investigated the interaction of h-BN and c-BN with different reactive gases - ammonia (NH3), chlorine (Cl-2), hydrogen ch loride (HCl) and boron trifluoride (BF3) - regarding their etching behaviou r and surface stabilisation properties. Etching ratios from approximate to 10:1 up to 450:1 were found in the temperature range 600-1300 degrees C for the h-BN/c-BN system, clearly indicating a high selectivity due to kinetic effects. The reaction mechanisms will be discussed with respect to the kinetic diffe rentiation of the degradation of c-BN and h-BN (selective etching). The mor phological changes and the quality of the remaining BN phases was studied b y scanning electron microscopy (SEM), X-ray diffraction (XRD), and infrared and Raman spectroscopy and these indicated a homogeneous decay of the indi vidual phases. Since a homogeneous decay of c-BN resembles the reversed gro wth, the study of the interaction of both BN phases with reactive gases all owed us to collect more detailed information of the molecular mechanisms in volved in the formation of the individual phases. These results will provid e new routes for growing c-BN in a CVD process. (C) 2000 Elsevier Science S .A. All rights reserved.