Design and Demonstration of Fine-Pitch and High-Speed Redistribution Layers for Panel-Based Glass Interposers at 40-μm Bump Pitch

摘要

This article analyzes redistribution layer (RDL) technologiesneeded for 2.5-dimensional (2.5-D) die integration on thinglass interposers and developed using low-cost processes. Thedesign, fabrication, and characterization of a four-metal layer RDLbuildup required for wide input/output (I/O) routing at 40-μmbump pitch and a two-metal layer RDL buildup fabricated directlyon glass for high-speed, off-package signaling are described. SuchRDL technologies are targeted at 2.5-D glass interposer packagesto achieve up to 1 Tb/s die-to-die bandwidth and off-interposerdata rates > 400 Gb/s, driven by consumer demand of online servicesfor mobile devices. Advanced packaging architectures including2.5-D and 3-D interposers require fine-line lithography beyondthe capabilities of current organic package substrates. High electricalloss and high cost are characteristic of silicon interposers fabricatedusing back-end-of-line (BEOL) processes that can achieveRDL wiring densities required for 2.5-D die integration. Organicinterposers with high wiring densities have also been demonstratedusing a single-sided, thin-film process. This article goes beyond siliconand organic interposers in demonstrating fine-pitch RDL onglass interposers fabricated by low-cost, double-side, and panelscalableprocesses. The high modulus and smooth surface of glasshelp to achieve lithographic pitch close to that of silicon. Furthermore,the low permittivity and low loss tangent of glass reducedielectric losses, thus improving high-speed signal propagation. Asemiadditive process flow and projection excimer laser ablationwere used to fabricate four-metal layer (2 + 0 + 2) fine-pitch RDLand two-metal layer RDL directly on glass. A minimum of 3 μmlithography and 20 μm microvia pitch was achieved. Highfrequencycharacterization of these RDL structures demonstratedsingle-ended insertion losses of −0.097 dB/mm at f = 1 GHz anddifferential insertion losses of −0.05 dB/mm at f = 14 GHz.