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This Technical Note reviews research on when sputtered metal becomes electrically connected on a polymer surface and applies it to sputtered seed layers in PDA-coated glass vias. It covers a connection-depth calculation and an integrated process design.
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Summary
Sputtered metal grows as islands and becomes electrically connected the moment they touch, and the critical thickness for connection falls as reactivity with the interface rises. On the same polymer interface, Cu connected at 2.5 nm and Au at 6.2 nm. The note covers the principle and process by which a PDA coating over the whole via wall lowers that threshold so the seed stays connected even on deep walls where line-of-sight sputtering deposits little metal.
Conclusion
The note starts from work by Schuster et al. (Nanomaterials 2021, 11, 589). The researchers formed a cross-linked PCGF–PEI polymer film on glass and DC-magnetron-sputtered Au, Ag, Cu and Ni onto it while measuring nominal thickness by QCM and sheet resistance between electrodes at the same time. Metal grows through nucleation, island growth, electrical connection and a continuous film, and the point where resistance drops sharply marks the critical connection thickness dc at which the first conductive path forms between islands. Resistance follows R ∝ (d − dc)^−t with a critical exponent of about 1.3, supporting the island-growth and network-connection picture. On the same polymer surface, dc was 2.5 ± 0.2 nm for Cu, 2.9 ± 0.3 nm for Ni, 4.8 ± 0.1 nm for Ag and 6.2 ± 0.1 nm for Au (mean of two runs each, QCM nominal thickness). The researchers found dc correlated more clearly with metal reactivity (oxide formation enthalpy) than with surface energy, interpreting that interfacial bonding lowers atomic mobility so the metal spreads as small, branched clusters that connect at a lower thickness. GISAXS-based shape estimates likewise showed Au gathering into large clusters and Cu forming small, branched shapes. Cools carries this result over to glass vias. PDA, a solution process, coats the via conformally to its full depth and provides catechol and amine metal-binding sites, compensating for the thin deposition of line-of-sight sputtering on deep walls by lowering the threshold on the interface side. The seed connects where tf·F(z/D) ≥ dc: at fixed depth the required deposition scales with dc, and at fixed deposition the connection depth scales with dc to the power −1/3. For a 50 μm diameter, 350 μm deep (AR 7) glass via sputtered from both sides with 1.0 μm of field Cu per side, an analytical model (cosine incidence, sticking coefficient 1, line-of-sight flight) gives the following when dc drops from 6.2 nm to 2.5 nm: the single-side connection depth grows from 130 μm to 180 μm (2.6D to 3.6D), the combined two-side connection length from 260 μm to 360 μm, and the 90 μm unconnected center disappears. The field Cu needed for full-depth connection falls from 2.30 μm to 0.93 μm, a 60% cut per side that also reduces overhang at the opening. These are model calculations. The integrated process activates the glass surface with O₂/Ar plasma (water contact angle 10° or less), coats 10–20 nm of PDA from 2 mg/mL dopamine at pH 8.5, sputters a Ti 50 nm / Cu 1.0 μm seed sequentially from both sides, and plates at 2 A/dm² (0.44 μm/min growth by Faraday's law) to bridge a 10 μm gap in about 11 minutes.
References
- 1. Schuster, P. A. et al. Nanomaterials 2021, 11, 589. doi:10.3390/nano11030589
Topics
- Glass substrates
- Plating
