Multilayer Stress Research Articles

Understanding of stress and its correlation with microstructure near the layer continuous limit in nano-scaled multilayers

The evolution of residual stress and its correlation with microstructure are investigated systematically in nano-scaled periodic W/B4C multilayers (MLs) as a function of individual layer thicknesses at the ultra-thin limit (∼0.4–3 nm). Details of the microstructure are accessed through hard X-ray reflectivity and X-ray diffuse scattering (rocking scan) measurements. To understand the contributions of stresses in the layers of each type of material to the total stress in ML films, both the total stress in MLs and the stress in nanocrystalline W layers are analyzed and correlated. It is observed that the physical properties of the materials as well as their interfacial morphology undergo significant modification as the layer thickness varies from the continuous to the quasi-discontinuous regime. A non-monotonic variation of compressive total residual stress in the MLs is observed as a function of thicknesses of W and B4C and explained using a model of the mechanism of film growth. The observed value of in-plane total compressive residual stress of W/B4C MLs is less than the residual stress in W layers in the MLs, which indicates that the net combined stress from B4C layers and interfaces is tensile in nature. The observed compressive stress and the increase of lattice spacing with respect to the stress-free structure in W layers provide evidence of a peening effect. The observed higher surface density of grains with smaller average size and phase formation also provide high compressive stress in W layers.

Evolution of Residual Stress and Structure in YSZ/SiO2 Multilayers with Different Modulation Ratios

The multilayer (ML) YSZ/SiO2 films with different modulation ratios ranging from 1:3, 4:9, 1:1 to 3:1 (the thickness ratio of the YSZ to SiO2) are deposited on BK7 glass substrates by electron beam evaporation under the same processing conditions. The effect of modulation ratio on the residual stresses and structure are investigated by an optical interferometer and Grazing incidence x-ray diffraction. The results show that the total residual stress in MLs is compressive and decreases to tensile when the modulation ratio is changed from 1:3 to 3:1. The YSZ films are of cubic phase structure and the SiO2 films are amorphous in all the MLs. The change of residual stress in these MLs can be attributed to the variation of an individual layer?s stress with thickness, which indicates that adjusting the thickness ratio of two materials is an effective measure for depositing near-zero stress MLs.