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

Abstract

This article analyzes redistribution layer (RDL) technologies needed for 2.5-dimensional (2.5-D) die integration on thin glass interposers and developed using low-cost processes. The design, fabrication, and characterization of a four-metal layer RDL buildup required for wide input/output (I/O) routing at 40-μm bump pitch and a two-metal layer RDL buildup fabricated directly on glass for high-speed, off-package signaling are described. Such RDL technologies are targeted at 2.5-D glass interposer packages to achieve up to 1 Tb/s die-to-die bandwidth and off-interposer data rates > 400 Gb/s, driven by consumer demand of online services for mobile devices. Advanced packaging architectures including 2.5-D and 3-D interposers require fine-line lithography beyond the capabilities of current organic package substrates. High electrical loss and high cost are characteristic of silicon interposers fabricated using back-end-of-line (BEOL) processes that can achieve RDL wiring densities required for 2.5-D die integration. Organic interposers with high wiring densities have also been demonstrated using a single-sided, thin-film process. This article goes beyond silicon and organic interposers in demonstrating fine-pitch RDL on glass interposers fabricated by low-cost, double-side, and panel-scalable processes. The high modulus and smooth surface of glass help to achieve lithographic pitch close to that of silicon. Furthermore, the low permittivity and low loss tangent of glass reduce dielectric losses, thus improving high-speed signal propagation. A semiadditive process flow and projection excimer laser ablation were used to fabricate four-metal layer (2 + 0 + 2) fine-pitch RDL and two-metal layer RDL directly on glass. A minimum of 3 μm lithography and 20 μm microvia pitch was achieved. High-frequency characterization of these RDL structures demonstrated single-ended insertion losses of −0.097 dB/mm at f = 1 GHz and differential insertion losses of −0.05 dB/mm at f = 14 GHz.