High Adhesion Glass Seed Process for Glass Core Packages Using Metal Oxide as Binder Layer

The core package substrates for establishing chiplet technology is expanding. When using conventional organic materials, suppressing warpage associated with increased mounting area has reached its limits. As a solution, the use of glass substrates with low CTE as the core layer is gaining momentum. Conductivity on glass substrates is primarily achieved through seed layer formation using sputtering. Sputtering is widely used as a technology for forming uniform, high quality metal thin films and holds an indispensable position in semiconductor manufacturing. However, since core substrates for advanced packaging require the formation of high aspect ratio through-holes (aspect ratio ≥ 10), sputtering makes it difficult to form seed layers within these through-holes. While various seed layer formation methods on glass have been proposed, attention is focused on achieving wet-process treatment due to considerations of mass producibility and cost when considering panel sizes. This report introduces a new high-adhesion glass seed process. It encompasses a film deposition technology using a wet process with a metal oxide binder layer on high aspect ratio through-hole substrates, and a future plating process compatible with narrow-pitch through-holes.

Zinc oxide-based binder layer with high-adhesion to glass was employed. The existing zinc oxide film deposition technique [1] applied an aqueous solution electroless plating method using Pourbaix diagram. Zinc oxide can be electrochemically deposited from a simple zinc nitrate (Zn(NO₃)₂) aqueous solution [2], [3], [4]. Zinc oxide is formed when OH⁻ ions generated by the reduction of nitrate ions using dimethylamineborane (DMAB) as a reducing agent raise the pH near the substrate surface. The zinc oxide produced by this process is highly dense and precipitates in a pillar-like structure. Such dense precipitation cannot be achieved using the conventional sensitization method [1]. The film thickness was approximately 200 nm. Generally, zinc oxide, being a semiconductor, exhibits electrical conductivity, and its resistivity varies significantly depending on the excess zinc content in the lattice [2], [3], [4]. Therefore, by controlling the zinc content within the zinc oxide, it became possible to form a binder layer with high surface resistivity. As the zinc oxide film thickness increases, the surface resistivity decreases. Especially, it decreases extremely sharply around the 200 nm threshold. Considering ionic migration during wiring formation, a binder layer thickness of 200 nm or less is desirable.

EDS analysis results revealed copper within the zinc oxide layer when observing the coating after electroless copper plating. Zinc oxide possesses a pillar-like structure with gaps between particles, allowing the electroless copper plating solution to penetrate and deposit inside, forming a nanoscale anchor structure. Furthermore, in addition to this nano anchor structure, heat treatment causes copper to diffuse into the zinc oxide, generating Zn-O-Cu bonds, suggesting improved adhesion. It also exhibits good coverage even on narrow-pitch, high aspect ratio through-hole, demonstrating a throwing power of 90%. This was achieved by enhancing the permeability of the reduction process and the electroless copper plating bath composition.

An average strength exceeding 5 N/cm was obtained on all substrates, achieving adhesion strength surpassing that of Ti/Cu sputtered films on silicon wafers (3.2 N/cm). The improvement in adhesion strength is attributed to the pillar-like zinc oxide nano anchor structure, combined with metal diffusion between the glass/seed layer/conductive layer during high temperature heat treatment in a nitrogen atmosphere after electroplating. Furthermore, this process enables line/space = 10/10 μm wiring formation. No undercut was observed in the wiring after seed etching, demonstrating that a layer with excellent corrosion resistance is formed due to metal diffusion during thermal processing. 

We developed a high-adhesion seed formation process suitable for narrow-pitch, high aspect ratio through-hole substrates by forming a dense zinc oxide binder layer, and improving the permeability of electroless copper plating solutions. This process enables the formation of fine wiring without ionic migration, making it a promising new process for future AI-HPC packaging.