Structure zone model for extreme shadowing conditions
Previously reported data on the microstructure of glancing angle deposited (GLAD) metal layers is used to extend the qualitative arguments of the structure zone model for physical vapor deposition to growth conditions with exacerbated atomic shadowing. At low growth temperatures Ts relative to the melting point Tm, the microstructural development is governed by atomic shadowing for both normal deposition and GLAD, resulting in fibrous grains with voided boundaries (Zone I). As the homologous growth temperature θ = Ts / Tm is raised above approximately 0.3, GLAD layers continue to exhibit well separated columns while conventional thin films show dense columnar microstructures (Zone II). θ > 0.5 leads to equiaxed grains independent of deposition angle (Zone III). Therefore, strong shadowing during GLAD suppresses Zone II microstructures, causing a direct transition from Zone I to Zone III. GLAD microstructures can be divided into four distinct zones: rods, columns, protrusions, and equiaxed grains: separated self-affine rods form for θ < θc = 0.24 ± 0.2, while considerably broader columns develop at θ > θc, due to exacerbated self-shadowing associated with an increased growth front roughness, causing larger growth exponents. Above θ ≈ 0.35, protrusions develop on top of some columns as they capture an overproportionate amount of deposition flux and grow much higher than the surrounding layer. At θ > 0.5, diffusion processes dominate over atomic shadowing, leading to faceted rough layers with equiaxed grains. In addition, the large mass transport facilitates the formation of whiskers that form for many metal GLAD layers at θ > 0.4. © 2012 Elsevier B.V.
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