Templated fingering during solid state dewetting

Abstract

Solid thin films formed on substrates are normally metastable in the as-deposited state and will dewet via capillarity-driven surface self-diffusion when heated, while remaining in the solid state. Solid state dewetting proceeds through retraction of film edges that develop rims that either pinch-off to form wires that run parallel to the retracting edges or breakdown to develop fingers that propagate into the films. We have investigated the mechanisms that lead to fingering instead of pinch-off using lithographically patterned single-crystal Ni(110) films formed on MgO. We find that fingering tends to occur when edges are initially rough and demonstrate that fingering can be induced by lithographically pre-patterning film edges with periodic roughness. We also find that periodic roughness provides a template that controls the steady state finger spacing and leads to the formation of periodic arrays of parallel wires. Templated fingering was investigated in films patterned into rectangular patches with a wide range of experimental variables; roughness period (16 different values, 2.7 μm - 12.5 μm); edge orientation (20o, 33o and 45o with respect to [11¯0]); and film thickness (90, 120 and 150 nm). For each of these experimental variables, the finger spacing, propagation direction, and propagation rate was determined. A model based on mechanisms that control the rate of finger propagation is developed and shown to be consistent with the measured propagation rates over the wide range of experimental conditions that were investigated.