ANALYSIS OF RESIDUAL-STRESS IN CUBIC BORON-NITRIDE THIN-FILMS USING MICROMACHINED CANTILEVER BEAMS

We have investigated the residual stress of cubic boron nitride (cBN) thin films using a cantilever-beam deflection method. The residual str esses were determined by measuring the deflection of cantilever beams micromachined from (100)-oriented silicon, and the hardness and elasti c modulus of sp(2)-bonded BN (sp(2)-BN) were determined from nanoinden tation experiments. Residual stresses were compressive, and varied fro m approximately 0.15 GPa to 1.2 GPa for sp(2)-BN films and from 0.5 to 5.5 GPa for cBN-containing films; the lower stresses are below a thre shold stress level for cBN formation previously suggested by McKenzie et al. A correlation between the relative cBN content and the compress ive film stress was also established: larger residual stresses occurre d in the films containing a greater percentage of the cubic phase. We find that both sp(2)-BN and predominantly cBN films have approximately the same elastic strain. Thus, assuming that the strain is independen t of composition, we develop a quantitative model that well describes the observed correlation of compressive firm stress with cBN content. Within the model, the film stress increases with increasing cBN conten t simply because the elastic modulus of cBN is substantially larger th an that of sp(2)-BN. The relevance of this analysis to the mechanism o f cBN formation is discussed.