The Influence of Limitation of Sputtered Tin Area Through Mesh Grids on Sn Whiskering

An important question in Sn whisker research concerns the role of lateral Sn diffusion in the film to the whisker root, providing the feedstock for whisker growth. In an elegant tracer experiment on diffusion in Sn films, Woodrow (2006) observed lateral atomic Sn diffusion over distances of hundreds and thousands of microns through Sn grain boundaries during whisker growth. In this work, we examine the effect of limiting the lateral extent of the Sn feedstock available during whiskering by employing extremely thin, micron-sized patterned Sn deposits, and compare to whisker growth from an comparatively large, essentially infinite (1cm x 1cm) thin Sn film. A series of metal, mesh grids of varying sizes were used as masks to define the area of sputtered tin deposited on Si substrates. The defined areas were primarily in the form of squares, with sizes ranging from7.5µm x 7.5µm to 153µm x 153µm. Sample areas also included 2 circles, with radii of 1µm and 4µm, and a rectangular strip with dimensions 46µm x 3000µm. A thickness of ~500Å of sputtered Sn was deposited on each specimen. The samples were subsequently incubated for 34 days under three incubation conditions: 23oC (isothermal); 100oC (isothermal); and thermal cycling over -40oCoC. Characteristically, higher whisker densities were observed as the patterned Sn area size increased, e.g., the 153µm patterns had ~ 3X more whiskers compared to the 23µm patterns under the same incubation conditions. There was a discontinuous transition in whisker density at the smallest patterned area sizes, with few whiskers found on the 7.5µm-23µm dimensioned square patterns and no whiskers found on the 1µm and 4µm patterns. Further, the whiskers obtained from micron-size areas of sputtered Sn are predominantly hillock in nature, in contrast to the high-aspect ratio whiskers normally grown from sputtered Sn on Si using standard sized films (1cm2) under identical growth conditions.