Stress relaxation by ion bombardment in cubic boron nitride thin films

Ion bombardment plays a special role in the deposition of cubic boron nitride (cBN) for hard coating applications. On one hand, low energy ion bombardment (100-500 eV) of the growing film surface is required to produce the hard cubic phase, on the other hand this implements extremely high compressive stresses (~10 Gpa) limiting film thickness and adhesion. Recently, high energy ion implantation (from keV to MeV range) has been successfully used to release the compressive stress in the films.
In this work the stress relaxation mechanism was investigated using the cantilever bending principle for in situ real time stress measurement during ion beam assisted deposition and Magnetron sputter deposition with simultaneous high energy ion bombardment. The amount of the relaxation is dependent on the product of the energy and the flux of the high energy ions. Using a variety of structural investigations, it can be shown that the relaxation takes place within the cBN grains, likely via the relaxation of interstitials. A relaxation due to the transformation of cBN to the more ductile hexagonal BN modification is less likely. This suggests a stress relaxation mechanism that is driven by a collisional relocation of strained film atoms due to high energy ion impact. A theoretical description of the stress relaxation based on TRIM simulation is presented and discussed. Assuming the stress in the layer to be proportional to the density of unrelaxed atoms, the calculation of the collisional relocation yield enables the calculation of a relative stress relaxation rate. The relocation yield is dependent on the incident ion energy and on the chosen relocation threshold energy. The experimental data are consistent with the model for different deposition parameters and processes for a relocation threshold energy of 5 eV.