Substrate influence in Young's modulus determination of thin films by indentation methods

This paper deals with the determination of Young's modulus of thin films by indentation methods using cubic boron nitride (c-BN) as an example. This is a non-trivial task because the Young's modulus of this material comes close to that of the diamond indenter. An analysis of results from literature revealed that Young's moduli of c-BN thin films are generally distinctly smaller than bulk material values. We give reasons that this is at least to a large extent due to measuring problems, in particular the absence of a proper correction of substrate influence. In the experimental part we have used nanoindentation with spherical and three-sided pyramidal (Berkovich) indenters to obtain the data, and employed both standard (Oliver and Pharr) and own theoretical methods (Image Load Method, ILM) for data evaluation. Two groups of samples were investigated: i) a 1.8-µm thick polycrystalline c-BN film on silicon which was well-adherent thanks to a graded (B,C,N) interlayer, and ii) heteroepitaxial c-BN thin films on single-crystal diamond. The polycrystalline c-BN film had a Young's modulus of 800 ± 30 GPa as determined using the ILM. In contrast the standard method mentioned above delivered a depth-dependent modulus reaching 775 GPa for a indentation depth to film thickness ratio of 1.3%. For the 456 nm heteroepitaxial film on diamond a modulus of 900 ± 100 GPa was measured which is very close to c-BN bulk material values. It was demonstrated that the ILM delivers trustworthy results without noticeable substrate influence also in the complicated cases of a superhard film on a much more compliant substrate (silicon) as well as on the even stiffer diamond substrate. Concluding the article we discuss limits of the method and possibilities to overcome them.