Fabrication and characterization of advanced through glass via interconnects
Citation
Authors: Timothy Clingenpeel, Arian Rahimi, Seahee Hwangbo, Yong-Kyu Yoon, and Aric Shorey
Title: Fabrication and characterization of advanced through glass via interconnects
Conference: Proceedings of the International Symposium on Microelectronics (ISOM)
Year: 2016
Page numbers: 288-295
Publisher: International Microelectronics Assembly and Packaging Society (IMAPS)
Keywords
- Through Glass Vias (TGVs)
- Radio-frequency (RF) applications
- Copper Conductors
- Superlattice metaconductors/Cu/NiFe Superlattice
- Corning SGW3
- 3D-Interposer/3D Interconnects
- Circuit Model
Brief
This article presents the fabrication process and electrical characteristics of Through Glass Via (TGV) structures, including a novel design using Cu/NiFe superlattice metaconductors to reduce radio-frequency losses for applications like 5G communications.
Summary
This article presents the fabrication and characterization of Through Glass Vias (TGVs) with copper and composite conductors for use in next-generation communication applications such as 5G.
TGVs are of particular interest for radio frequency (RF) applications due to their lower dielectric loss compared to Through Silicon Vias (TSVs). Pure copper is the predominant conductor used in both TGV and TSV technologies; however, at high frequencies, copper's low electrical resistance is outweighed by the skin effect, where alternating current tends to flow on the surface of the conductor. This effect increases resistance, and in turn, power consumption in RF systems.
The article focuses on research using metaconductors, specifically superlattice structures, which are formed by stacking alternating layers of non-ferromagnetic and ferromagnetic metals, to address high frequency power loss in RF applications. The authors developed a modified TGV using a Cu/NiFe superlattice metaconductor and low loss glass (Corning SGW3). The goal is to combine the low loss properties of the glass substrate with the low loss properties of the metaconductor to create a highly power efficient RF/microwave system.
One of the primary benefits of this superlattice structure is a reduction in resistance over certain frequencies due to the canceling out of eddy currents. This effect is achieved when the permeability of the non-ferromagnetic and ferromagnetic layers average to zero, which sets the effective skin depth to infinity. This relationship was illustrated by calculating the resistance of a 150 μm-wide stack of 10 paired Cu/NiFe layers (150 nm/25 nm-thick Cu/NiFe), with the peak resistance located at the ferromagnetic resonance frequency.
The authors detail the fabrication process, highlighting the importance of minimizing damage and stress on the 115 μm-thick glass substrate (Corning SGW3).6 The process involves several steps such as double-sided photolithography, sputter deposition of the conductor layers (copper-only and superlattice structures), and a lift-off process.
The electrical characterization of the fabricated structures was conducted using a two-port vector network analyzer with a frequency range of 300 kHz to 20 GHz. The results were analyzed using a circuit model to understand the behavior of the TGV. The developed model demonstrated a good representation of the measurement results; however, some discrepancies in resistance were noted. The authors suggest that these discrepancies might be attributed to factors like TGV-TGV coupling and variations in conductor thickness inside the TGV.
Overall, the article presents a promising approach to improving the performance of TGVs for high-frequency applications. The authors highlight that future work will focus on alternative deposition methods to improve the conformity of the conductor coating on the TGV sidewalls.