MSE Seminar: Dr. Carol Handwerker, Purdue University
Wednesday, November 16, 2022
3117 Computer Science Instructional Center (CSI) Bldg #406
301 405 5240
Title: Heterogeneous Stress Relaxation in Tin Thin Films: Whiskers, Hillocks, and Beyond
Abstract: When tin polycrystalline thin films are stressed, the film microstructures can become unstable, leading to the formation of long, single crystal whiskers that can create short circuits in electronic circuits and destroy MEMS devices in electronic assemblies. The conditions for whisker formation are inherently local as indicated by their low frequency: there is typically 1 whisker for every 103 -106 film grains. Relaxation of compressive stresses occurs by diffusion to the base of specific grain boundaries in the plane of the film and leads to growth of that individual grain out of the plane of the film. Hillocks form when grain boundary migration accompanies growth out of the plane of the film and whiskers in the absence of grain boundary migration. Experiments in tin whisker formation at low strains and strain rates allowed us to develop a simple model of whisker growth based on stress gradients between the base of shallow grains and the compressively stress columnar grains and on grain boundary sliding limited diffusional creep. However, many features of whisker growth, even under these conditions, cannot be explained by this simple model or with these film geometries. These include grain rotation and tilting of pre-existing grains, nucleation of new grains, incubation times before whiskers start growing suggesting changes in sub-surface features, such as grain boundary structures and geometries, and whiskers that stop growing. The mechanisms underlying some of these phenomena have been isolated by changing the film microstructure to pseudo-bicrystal films with millimeter-scale in-plane grain sizes and using other stressing conditions, such as cyclic bending, rapid thermal cycling, and thermal cycling using in-situ SEM. Finite element models of elastic and thermoelastic anisotropy, finite deformations and elasto-plastic anisotropy were used to simulate the mechanical response of polycrystalline thin films, including the formation of sub-grains due to lattice rotation. Finally, our research on stress relaxation in β-Sn can be put into the broader context of stress relaxation processes in Ag, Au, and Al thin films as a function of microstructure, film geometry, and stress conditions, and can start to be used as the basis for constructing microstructure-dependent deformation maps.