About this document
This Technical Note examines why copper plating slows down in deep through-glass vias from the standpoint of ion supply, and sets out a development concept that uses nitrogen nanobubbles to replace the liquid inside the via, together with a staged verification plan. The full report is available in Korean. An English summary is provided on this page.
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Summary
In deep vias, diffusion alone cannot replace the copper ions consumed at the growth surface, so plating slows down. Filling the via with a nitrogen-nanobubble solution and then applying a local stimulus repeatedly replaces the liquid inside, shortening the supply distance and thinning the boundary layer at the growth surface. The note covers design calculations based on the Péclet number, development targets, a liquid-exchange pretest and the plating verification plan.
Conclusion
As a via gets deeper, the characteristic diffusion time grows with the square of its depth. Assuming a diffusion coefficient of 5×10⁻¹⁰ m²/s, it is 20 seconds over a 0.1 mm transport distance and about 2,000 seconds over 1 mm. These figures are time scales for ion transport, not the plating time needed to fill the via with copper. Agitating the plating bath does not help if the liquid at the growth surface deep in the via is not exchanged. The document starts from a comparison case in which filling 1 mm took about 20 hours. Nanoflushing is a concept in which a nitrogen-nanobubble solution is first supplied to the via and a local stimulus then drives repeated cycles of bubble expansion, liquid discharge and inflow of fresh plating solution. The aim is to carry solution deep into the via by convection so that only the thin boundary layer in front of the growth surface has to be crossed by diffusion. Whether a stimulus at one point triggers bubble responses across many vias, and how far and how repeatably it does so, remains to be confirmed by experiment. The design criterion is the Péclet number (Pe = UL/D), the ratio of convective to diffusive transport. For a 1 mm via with an effective supply velocity of about 0.5 mm/s, Pe is about 1,000, setting a 2-second convection time against a 2,000-second diffusion time. Under mass-transfer-limited conditions, reducing the boundary layer from 100 μm to 1 μm raises the limiting current density from about 3.9 to 386 A/dm² in a film-model calculation (assuming a copper ion concentration of 0.40 mol/L). These are order-of-magnitude design calculations, not measurements. A larger Pe does not raise the plating rate by the same factor or guarantee uniform filling, so flow and stimulus distribution are verified together with fill quality. Cutting the 1 mm fill time from 20 hours to 12 minutes, up to 100 times faster, is a follow-on development target, with an intermediate target of 10 times. The current density needed for a 12-minute fill is about 378 A/dm², calculated at 100% current efficiency. Verification begins with a pretest in which open vias filled with plain water and with nitrogen-nanobubble water receive the same stimulus. Exchange speed is quantified by the time for the dye signal to fall by 90%, spatial reach by distance from the stimulus point and position at the via bottom, and repeatability by at least three independent comparisons. Testing then moves to bottom-fill conditions with one end closed and, depending on results, to center fill. Pre-verification from October to December 2026 is intended to establish the effect and repeatability of liquid exchange inside the via, with actual plating left to the next stage. In the plating stage, current is raised step by step after the exchange effect is confirmed, and growth, fill time and cross-section quality are measured. The pass criterion is a shorter plating time for the same geometry and fill target while meeting predefined cross-section, void and resistance criteria. The document also presents the effect of shorter plating time on equipment throughput as a calculated scenario that includes auxiliary process time.
Topics
- Plating
- Glass substrates
