Elastic modulus-density relationship for amorphous boron suboxide thin films

Boron suboxide thin films nave been deposited on Si(100) substrates by reactive RF magnetron sputtering of a sintered B target in an Ar/O2 atmosphere. Elastic recoil detection analysis was applied to determine the film composition and density. Film structure was studied by X-ray diffraction and transmission electron microscopy. The elastic modulus, measured by nanoindentation, was found to decrease as the film density decreased. The relationship was affected by tuning the negative substrate bias potential and the substrate temperature during film growth. A decrease in film density, by a factor of 1.55, caused an elastic modulus reduction by a factor of 4.5, most likely due to formation of nano-pores containing Ar. It appears evident that the large scattering in the published data on elastic properties of films with identical chemical composition can readily be understood by density variations. These results are important for understanding the elastic properties of boron suboxide, but may also be qualitatively relevant for other B-based material systems.