Through-Glass Via (TGV) is a core technology for three-dimensional interconnection in advanced glass-based packaging. Leveraging its superior high-frequency and low-loss characteristics, it has become a key process for high-end chip packaging. The electroplated metallization filling of TGV is a critical step for achieving vertical electrical interconnects, presenting dual process challenges of high-aspect-ratio filling and uniform global filling consistency, which directly determine the conductivity and reliability of the packaged device. Currently, the mainstream TGV electroplated filling is divided into two major process systems: conformal filling and solid filling. Meanwhile, differences in the etched via morphology also significantly affect the electroplating filling performance.
Conformal filling essentially preserves the original shape characteristics of the TGV via. Its advantages are short plating time and relatively easy control of process uniformity. However, its disadvantage is that the allowable current through the via is limited, leading to a narrow range of product applications. Moreover, after conformal filling, the via center needs to be filled with a polymer material before subsequent redistribution layer (RDL) processes. Apart from uniform sidewall growth, conformal filling can also be achieved using a unilateral sealing technique, resulting in an arch-like conformal structure.
It should be noted that although TGV typically features a through-hole structure, the actual via shape after processing is not perfectly uniform. In addition to relatively regular straight-through holes, double-bottleneck features with enlarged openings and tapered sidewalls can also occur. The key control points during electroplating differ for various via shapes. For vias with regular cross-sections, focus is placed on wetting, filling uniformity, and residual stress control. For double-bottleneck structures, localized current concentration at the via entrance is more likely, affecting ion replenishment and deposition uniformity inside the via. Therefore, discussions on TGV electroplating should not only distinguish between conformal and solid filling, but also incorporate specific via shape characteristics to understand process differences.
The solid electroplating filling of TGV vias is more complex. The mainstream approach first forms a bridge at the via center, followed by bottom-up filling based on two blind vias. To achieve bridging at the center first, two methods can be used. One involves additives to enhance sensitivity to convection, creating a non-uniform concentration distribution inside the TGV via, resulting in different polarization levels. Consequently, the deposition rate is fastest at the via center, completing the bridge first. The other method uses pulsed current to control the difference in dissolution rate inside the TGV via. The electric field is stronger near the via openings, so the dissolution rate is faster there, while it is slower inside the via due to a weaker field. After a number of pulse cycles, bridging is achieved at the via center.
Currently, three waveform control strategies for TGV bridging exist. The first is synchronous pulse control, where identical forward and reverse current pulses are applied to both sides of the TGV. The second is asynchronous pulse control, where forward and reverse current pulses of different amplitudes are applied to the two sides. The third is intermittent current control, which adds a current pause step to the forward and reverse current pulses.
In addition to the bridge-first-then-bottom-up filling method, some research institutions directly employ a bottom-up filling approach. For example, EXTOL (Korea) first temporarily bonds a conductive layer to one side of the glass substrate. The original TGV vias then resemble bump-like features, and the vias grow bottom-up from the conductive layer. After via filling is complete, the conductive layer is separated. National Chiao Tung University (Taiwan) adopts a different bottom-up TGV growth method: first, a metal layer is sputtered on one side of the TGV, followed by photolithography to expose both RDL and TGV patterns. Electroplating simultaneously completes the RDL on one side and partial TGV filling, after which bottom-up electroplated metallization filling of the TGV is performed.
In summary, the selection and optimization of TGV electroplated metallization filling processes should balance process efficiency, filling quality, and device application requirements. Conformal filling is simple and controllable, suitable for low-frequency, lightweight packaging scenarios. Solid filling, with its excellent current-carrying capacity and structural stability, meets the stringent performance requirements of high-end chips and high-frequency, high-speed packaging.