Realization of high A/R and fine pitch Cu pillars incorporating high speed electroplating with novel strip process

Abstract

Fan-out wafer level packaging (FOWLP) enables high density heterogeneous integration of distinguished functions into single chip by 3D stacking logic and memory chips. High density 3D FOWLP requires Cu pillars to deliver power and signal between stacked chips. However, realization of reliable Cu pillars is a challenge due to its distinguished features including extreme height with high aspect ratio (A/R) and its exceptionally long process time for electroplating. Herein, this study reports realization of high A/R and fine pitch Cu pillars incorporating high speed electroplating with novel strip process. Process conditions including electroplating current, ion concentration, process temperature, and mechanical agitation were studied and experimentally evaluated to accelerate electroplating rate. Gradual modulation of applied current in electroplating process helps to resolve non-uniform ion distribution. Elevation of process temperature enhances diffusion and flow of Cu ions. Surface modification for photoresist leads the superior tolerance in high temperature electroplating bath through preventing leaching and deformation of the photoresist. High A/R structure of Cu pillar requires novel strip process, and identification and modeling of the process relating nozzle and spray position leads drastic improvement of its performance than conventional process. The derived knobs demonstrates mass-productive and reliable Cu pillar for 3D heterogeneous packaging.