Barrier/seed system for electroless metallization on complex surfaces using (aminomethylaminoethyl)phenethyltrimethoxysilane self-assembled films

Abstract

High frequency signals propagate along the edges of conductors. If the conductors are electroplated, then a conducting seed layer is needed at least on one edge, so care must be taken to ensure the electrical quality of these layers. A poor, high resistance seed layer may carry all the current at 10 GHz due to reduced skin depth. In this work, we study the initial quality of self-assembled monolayer (SAM)-based seed layers that are compatible with complex surfaces including through-silicon vias (TSVs), as are used in via-last three-dimensional semiconductor device packaging. In particular, morphology, adhesion, and resistivity are found to vary with the electroless catalyst and electroless metal deposition parameters; inductance-induced losses are also influenced by edge resistivity and metal choice. The seed layer must be fabricated on a barrier that will withstand diffusion, yet be thin enough to provide a conformal surface that allows for continuous seed layer deposition. Standard barrier and seed layer deposition methods such as evaporation or sputtering require either a line of sight from the source or aspect ratios large enough to provide scattering from the background gas within the structure to coat all surfaces. Such via holes are difficult to reliably fabricate and rely on tight parameter control. We propose a barrier layer based on an aromatic self-assembled monolayer (SAM) that also aids catalyst and high-quality electroless copper seed-layer attachment. The viability of the SAM barrier layer is determined by the quality of the deposited copper seed film, judged quantitatively by thin film resistivity and qualitatively by surface adhesion and morphological properties such as cracks and bubbles. Insights to the origins of problems are described and an optimal scheme identified. Atomic force microscopy (AFM) is used to verify results at each fabrication step. Extensions for use as a photolithographic resist layer are suggested. Our SAM approach for TSV applications yields a “smart” seed layer that can be used with a “simple,” scalloped, easy to fabricate, via hole.