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MATERIALS RESEARCH

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Selective chemical etching of hexagonal boron nitride compared to cubic boron nitride Stephen J. Harris, Anita M. Weiner, Gary L. Doll, and Wen-Jin Meng Physics and Physical Chemistry Department, General Motors R&D Center, Warren, Michigan 48090-9055 (Received 20 May 1996; accepted 13 September 1996)

A BN film containing comparable amounts of sp2 and sp3 phases was subjected to a gas-phase chemical etch in a hot-filament environment containing 1% CH4 in H2 . After a partial etch, examination by FTIR shows that the sp2 was preferentially etched, leaving a larger sp3 fraction than in the unetched film. The possibility that preferential etching could be used to increase the purity of cBN films is discussed.

I. INTRODUCTION

Because of its superlative properties, there have been many efforts to grow films and coatings of sp3 bonded cubic boron nitride (cBN), most successfully with PVD (physical vapor deposition) processes.1,2 In spite of some success, a significant fraction of sp2 bonded hexagonal BN (hBN) and amorphous BN (aBN) is often found in these films. In contrast, CVD (chemical vapor deposition) techniques, which are unsuccessful at growing cBN, are generally very successful in depositing sp3 -bonded diamond with little or no detectable sp2 bonded carbon, even though graphite is more stable than diamond. It is not obvious why diamond and cBN are so different in this respect. However, diamond is nearly always grown with high concentrations of atomic hydrogen,3 and the purest diamond is often grown with traces of oxygen.4 These species etch nondiamond (sp2 ) carbon more readily than diamond, so that even if both diamond and non-diamond carbon are formed, only the diamond survives. Etching strategies for growing cBN have been considered. For example, Kiel et al.5 discussed the possibility that aBN may be etched more readily than cBN during PVD deposition, while Yarbrough6 has speculated that one reason for the low concentrations of cBN in films deposited by CVD is that no effective sp2 etchant is present during its growth. We recently showed7 that aBN films are chemically etched in a hot filament environment in which the input gas contains a small amount of methane in hydrogen. The most abundant active species in such an environment, where ions and electrons are absent and where species have thermal energies ,0.1 eV, are H atoms and CH3 radicals.8 One or both of these species is likely to be the principal etchant.7 Although the estimated etching efficiencies are very low,7 the ability to etch BN films may nevertheless find practical use if sp2 -bonded BN is etched more efficiently than sp3 -bonded BN. In that case, BN films grown in or exposed to an etching environment may have higher sp3 contents than otherwise. In this 412

J. Mater. Res., Vol. 12, No. 2, Feb 1997

work we examine the etching of a film containing both sp2 - and sp3 -bonded BN. II. EXPERIMENTAL

Etching took place in a chamber made from a stainless steel cross which could be evacuated with a mechanic