Double-Sided Copper Filling of Small Diameter, High-Aspect Ratio Through-Glass Vias in High-Density Glass Interposers

Glass substrates offer significant advantages over current organic substrate, particularly in high-density, high-performance chip packaging for data-intensive applications such as artificial intelligence (AI). Glass with ultralow flatness enhances the depth of focus in lithography, which helps pattern precisely at advanced metal interconnects. In addition, their superior thermal stability minimizes pattern distortion, and their outstanding mechanical stability supports ultralarge package sizes. These exceptional dimensional stability properties facilitate precise layer-to-layer interconnect alignment, ultimately enabling glass substrates to achieve ten times higher interconnect density compared to organic substrates. However, fabricating high-density, small-diameter, high-aspect ratio (AR) through-glass vias (TGVs) remains a significant challenge. The current state-of-the-art technology for vertical TGVs achieves an AR of 12, with a via diameter of $30~\mu $ m. In this work, we present the first demonstration of straight TGVs with 20- $\mu $ m diameters on 300- $\mu $ m thick borosilicate glass, achieving a record-high AR of 15. Thanks to the low large-area packaging cost, low thermal expansion coefficient, excellent thermal stability, and low electrical dissipation in high-frequency operation, borosilicate is chosen as the glass substrate in our research. For straight, high AR TGVs, this study explores a double-sided seed layer enhancement (SLE) approach using electroless deposition to reinforce the seed layer, combined with an electroplating strategy to produce void-free, fully filled straight TGVs metal interconnects. The parameter study of the SLE process provides valuable insights and guidelines for fabricating high AR TGVs for future high interconnect density 3-D integration systems.