About this document
This Technical Note describes how to remove the adhesive-opening step from bottom-up plating of through-glass vias that use a copper foil as the via-bottom electrode. It covers PDA bonding, a pressed electrode, a sealing jig and the plating bath layout.
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Summary
In foil-based bottom-up plating, the adhesive that holds the copper foil covers the via bottoms, so a separate plasma step is needed to open them before plating. Coating the via-drilled glass with PDA first and then bonding the foil leaves copper exposed directly at each via bottom. The note covers a process in which a pressed electrode and a sealing jig keep the foil in contact during plating.
Conclusion
Bottom-up plating, which fills through-glass vias from the bottom, needs a continuous electrode across the via bottoms. The published patent used as the reference (Company E) bonds a copper foil instead of sputtering a metal seed; because the adhesive holding the foil covers the via bottoms, the adhesive is removed by plasma before plating. The patent example uses a 2 µm adhesive layer, 30 µm copper foil, 130 µm glass and 80 µm vias. This adds a dry plasma step and substrate transfer ahead of plating, leftover adhesive can block local plating initiation, and the glass–copper interface has to be designed separately from foil fixing. Cools reverses the order. The via-drilled glass is first wet-coated with PDA (about 50 nm, design example) and then bonded to an uncoated copper foil. Because the bond line breaks at each via, the copper foil is exposed directly at the via bottoms and the plasma opening step drops out. PDA provides initial fixing and interfacial bonding between glass and foil and also strengthens the glass–copper interface on the via walls. A pressing jig keeps the foil in contact during plating. A metal electrode plate feeds current directly to the back of the foil (current path: electrode plate, foil, growing copper), while an elastic support layer and pressure plate spread the load. An upper frame leaves the plating face open to the bath and constrains the edges, and a continuous gasket outside the active area keeps the back-side electrode contact dry. Two bath layouts are described: a horizontal upward-facing type with a dry electrode below and the plating space above, and a vertical cassette type that hangs sealed cassettes in parallel on a common rack. Both share a rectifier, copper anode, circulation pump and filter. The process runs in six steps: PDA pre-coating, foil bonding, cassette loading, wetting and plating, rinsing and separation, and surface finishing (foil etching and CMP). Whether the foil is removed or kept depends on the downstream circuit design. The document presents the difference from the patent embodiment as a change in process configuration, namely the removal of the plasma step for opening the via-bottom adhesive, and states that it is not a measured cost reduction, yield or throughput figure. Interchangeable electrodes and frames, plus additional standard cassettes, extend the setup from small coupons to large panels.
Topics
- Plating
- Glass substrates
