Stress reduction in boron carbonitride films by ion energy-modulated multilayers

A multilayer concept for boron carbonitride films has been developed with optimization by stepwise graduation of the substrate bias. For this purpose, multilayers with a varying number of layers and with different single layer thicknesses were deposited by reactive magnetron sputtering in combination with ion bombardment. The energy of the ions was varied systematically. Film deposition was carried out in an argon/nitrogen gas mixture at a gas pressure of 0.35 Pa and a constant r.f. power of 400 W. During deposition, the substrates were cooled to a temperature below 100 deg C. The films produced were investigated by means of AES, XRD and TEM. The film properties and the film/substrate composite were characterized using different mechanical test methods. In particular, the film stress was determined by the bending of thin silicon substrates (0.18 mm). According to the AES measurements, the carbon concentration of the films amounted to about 22.5 at.%, with lower values being measured at the higher ion energies. The boron/nitrogen ratio was constant at 1.4 in all films. Graded multilayers were found to have stresses that were far below the theoretical values obtained by mere superpositioning of the stresses of deposited monolayers. This is attributed to the influence of the interface in the layer composite. A 2 mu m thick multilayer consisting of seven single layers has a hardness of 25 GPa, a residual stress of 1.67 GPa (instead of 5.3 GPa) and a good adhesion with critical loads of failure of 44 N, as determined in a scratch test. Substrates included WC+Co hard metal.