Thin-Film Signal and Power Redistribution Layers Based on AL-X and Cu

Abstract

Use of unpackaged die in advanced integrated systems (i.e., 3-D integrated systems) calls for dense interconnection schemes with controlled impedance for high-speed signal routing and minimal impedance for efficient power distribution. We have evaluated a new material set for use in a thin-film-based redistribution layer (RDL) that consists of Asahi Glass AL-X spin-on low-k dielectric polymer and electroplated copper metallization. This technology allows fan-out and interconnection of high-speed signals and power to/from die pads on pitches sufficiently less than 100 μm directly to companion die over short distances or for transition to underlying board metallization for longer transmission distances that may require lower signal loss. This technology is demonstrated using Si wafers onto which the thin-film RDL is fabricated. We have developed and described the fabrication procedures used to construct multiple interconnected layers of AL-X / Cu, which are compatible with standard wafer level packaging (WLP) processes. We have also evaluated the performance of this technology for high-speed digital signal transmission by characterizing frequency parameters (i.e., S parameters) of single-ended and differential strip-line transmission line structures. We have optimized transmission line geometries for transmission of signals at rates greater than 25 Gbps. In addition to high-speed signal redistribution capabilities, we have characterized power redistribution capabilities of this technology. Results of the signal and power integrity measurements and simulations performed in this work are presented.