Analysis of residual stress in cubic boron nitride thin films using micromachined cantilever beams

Abstract

We have investigated the residual stress of cubic boron nitride (cBN) thin films using a cantilever-beam deflection method. The residual stresses were determined by measuring the deflection of cantilever beams micromachined from (100)-oriented silicon, and the hardness and elastic modulus of sp 2-bonded BN (sp 2-BN) were determined from nanoindentation experiments. Residual stresses were compressive, and varied from 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 threshold stress level for cBN formation previously suggested by McKenzie et al. A correlation between the relative cBN content and the compressive film stress was also established: larger residual stresses occurred 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 independent of composition, we develop a quantitative model that well describes the observed correlation of compressive film stress with cBN content. Within the model, the film stress increases with increasing cBN content simply because the elastic modulus of cBN is substantially larger than that of sp 2-BN. The relevance of this analysis to the mechanism of cBN formation is discussed.