A barrier/seed system for electroless metallization on complex surfaces using (aminomethylaminoethyl)phenethyltrimethoxysilane self-assembled films
High frequency signals propagate along the edges of conductors. If the conductors are electroplated, then the seed layer forms at least one edge, so care must be taken to insure the electrical quality of these layers. In this work, we study the initial quality of 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. The conformal and electrical quality of the seed metal is very important. Also important for a multifunction seed layer is its ability as a barrier layer, which protects the substrate from high temperature diffusion of the deposited metal. Thus, the barrier layer must be robust enough to withstand diffusion, yet thin enough to provide a conformal surface that allows metal seed layer deposition. Standard barrier 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. Electrochemical and chemical vapor deposition provide alternatives, but concerns arise about contamination and compatibility with radio frequency or high-speed digital signals. We propose a barrier layer based on an aromatic self-assembled monolayer (SAM) for use in electroless copper seed layer deposition. 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 and an optimal scheme are described. 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.