Effect of Al additions and AlN interlayers on the stabilization of cBN sputtered thin films

Cubic boron nitride thin films were prepared by sputtering a hexagonal BN target with an unbalanced magnetron using pulsed d.c. biasing of the crystalline silicon substrate. An additional balanced magnetron was used to co-sputter aluminum in order to test the effect of small Al additions on the stabilization and adhesion of the cubic boron nitride thin films. Rutherford backscattering spectroscopy and neutron depth profiling were performed on the samples and used to determine the amounts of boron, nitrogen and aluminum in the films. The results showed that the addition of an atom fraction of approximately 2% aluminum produces no significant de-stabilization of the BN cubic phase, and that a significant reduction in the amount of cBN occurs when the atomic concentration of aluminum is a fraction of approximately 4%. Several of these BN:Al films were prepared using AlN interlayers. These interlayers did not influence the stabilization window for deposition of the BN cubic phase but did allow for growth of thicker cBN/AlN bilayers that did not delaminate from the substrate. Neutron depth profiling showed that boron atoms diffused into the aluminum nitride/boron nitride interlayer, suggesting the formation of a diffusion interface between the cubic boron nitride and the aluminum nitride. A mechanism of incorporation of small amounts of additives at grain boundaries and boron sites during critical nuclei formation is suggested in order to explain the observed de-stabilization of the cBN phase. (C) 2004 Elsevier B.V All rights reserved.