Maskless shaping of gold stud bumps as high aspect ratio microstructures

Micro/nanoimprinting is a simple and economical way of patterning polymeric structures over large areas. This paper seeks to extend this technique to fabricate three dimensional (3D) metallic microstructures, even in trenches and constrained areas using a flip chip bonder in conjunction with a wire bonder. In this two step process, gold stud bumps were placed first on sputtered metal at appropriate locations using a wire bonder capillary tool. The second step involved flattening of the said gold bump followed by in situ restructuring into high aspect ratio microstructures using deep reactive ion etched (DRIE) silicon molds coated with an anti-stiction agent. This process produced microstructures of differing geometries and sizes ranging in height from 1–50 μm and aspect ratios from a low 0.3:1 to as much as 4:1 (with uniformity). The data obtained for 26 different templates, with varying imprint areas, were analyzed and a strong correlation of 1.62X (SD = 0.3) was observed between the force applied and the heights of the resulting microstructures. Final microstructure yield enhancement was around 70% with this technique in comparison to the traditional electroplating through mask approach.