Thiol-Based Self-Assembled Monolayers (SAMs) as an Alternative Surface Finish for 3D Cu Microbumps

With scaling beyond 40um pitch 3D interconnects, cost, performance and reliability become ever more critical. Thiol-based self-assembled monolayers (SAM) were applied before to enable Cu-Cu connection in dual damascene vias [1]. In this study, we are researched a parallel application, for an alternative, low-cost organic surface finish for electroplated Cu pads/pillar/bumps to enable 3D interconnects [2]. The effects of pre-cleaning, deposition times and self-assembled monolayer (SAM) type (C3, C10, C18) on oxidation resistance and electrical continuity were studied with Voltammetry and X-ray Photoelectron Spectroscopy (XPS). Experiments were performed on electroplated Cu flat samples and process conditions were selected for further processing of 3D patterned dies and subsequent stacking and thermo-compression bonding in a face-to-face configuration. Overall, C18 SAM showed better electrical continuity and lower electrical resistance than C3 and C10, a result which is consistent with the longer C chain and higher thermal stability of C18. A second result of this study — consistent in both flat and patterned samples — was that microwave plasma cleaning prior to SAM deposition was more effective than wet cleaning, indicating either better oxide cleanability or better affinity of SAM’s with more pristine Cu.