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Growth of multilayer or superlattice thin films has shown various degrees of hardness enhancements often exceeding the individual hardness of the materials involved. Typically the hardness increases with decreasing wavelength until a maximum value is reached in the nm range, after which the hardness decreases with further decrease in wavelength 1,2 . Different theories have been developed in order to explain the observed increase in hardness. Koehler 3 showed theoretically a hardness increase for materials with a lamellar structure. This increase arises from image forces on dislocations due to a shear modulus difference between the layers. Other theories that tries to explain the observed increase in hardness are, coherency stress hardening 4-6 , where dislocation movement is restricted by the stress fields present at coherent interfaces within the multilayer. Also, the epitaxial stabilization effect has been demonstrated 7 , where a metastable structure for one of the layer materials is formed by pseudomorphic forces to the surface of the other layer during nucleation and growth thus creating a coherent interface, e.g., a normally amorphous material assuming crystalline structure for small layer thicknesses. For Orowan-like strengthening 8-11 , plastic deformation occurs by dislocation movement and bowing inside layers. Finally, in the case of Hall-Petch strengthening 12-13 hardness increases due to a reduction in grain size and thereby an increase in grain boundary density, grain boundaries which acts as dislocation obstacles.Multilayer thin films consisting of titanium nitride (TiN) and silicon nitride (Si 3 N 4 ) layers with compositional modulation periodicities between 3.7 and 101.7 nm have been grown on silicon wafers using reactive magnetron sputtering. Electron microscopy and X-ray diffraction studies showed that the layering is flat with distinct interfaces. According to the XRD studies (figure 1), the deposited TiN layers were crystalline and exhibited a preferred 002 orientation for layer thicknesses of 4.5 nm and below. For larger TiN layer thicknesses, a mixed 111/002 preferred orientation was present as the competitive growth favored 111 texture in monolithic TiN films. The TEM studies (figure 2) revealed that the Si 3 N 4 layers exhibited amorphous structure for layer thicknesses ≥ 0.8 nm, however, for the first time cubic crystalline silicon nitride phase was observed for layer thicknesses ≤ 0.3 nm. Formation of this metastable SiN x phase is explained by epitaxial stabilization to TiN. The microstructure of the multilayers displayed columnar growth within the TiN layers with intermittent TiN renucleation after each Si 3 N 4 layer. A nano-brick-wall structure was thus demonstrated over a range of periodicities. As-deposited films exhibited relatively constant residual stress levels of 1.3±0.7 GPa (compressive) independent of the layering. Nanoindentation was used to determine the hardness of the films, and the measurements showed an increase in hardness for the multilayered films compared to the ...

The formation of cubic-phase SiNx is demonstrated in TiN∕SiNx multilayers deposited by reactive dual magnetron sputtering. Transmission electron microscopy examination shows a transition from epitaxially stabilized growth of crystalline SiNx to amorphous growth as the layer thickness increases from 0.3to0.8nm. The observations are supported by ab initio calculations on different polytypes, which show that the NaCl structure has the best lattice match to TiN. Calculations also reveal a large difference in elastic shear modulus between NaCl–SiNx and TiN. The results for phase structure and shear modulus offer an explanation for the superhardening effect determined by nanoindentation experiments.

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