Quantitative analysis for hillocks grown from electroplated Sn film

Tin (Sn) films of 1 μm thick and 2–3 μm grain size were electroplated over a silicon (Si) wafer. The wafer was first deposited with an adhesion layer of chromium (Cr) followed by a nickel (Ni) using the evaporation technique. A special fixture was used to apply a biaxial compressive stress to the Sn film. After exposing the sample to 160 °C under vacuum for seven days, large hillocks were observed having dimensions of 10–30 μm in diameter and 30–150 μm in length. The hillock density was approximately seven per square mm. A quantitative method was developed using surface mapping and MATLAB™ to estimate the volume of hillocks per unit area of the Sn film and thus, to relate that volume to the local compressive stress. The rate of growth was modeled as a flow of Sn atoms along the Sn/Ni interface under the applied pressure gradient. Reasonable viscosity and self-diffusivity were calculated, which suggested that Sn/Ni interface diffusion was potentially the mechanism for hillock growth.