High-frequency characterization of through package vias formed by focused electrical-discharge in thin glass interposers

This paper presents the modeling, design, fabrication and characterization, up to 30 GHz, of low loss and high aspect-ratio 55 μm diameter through package vias (TPVs) in 300 μm thick glass interposers. These TPVs were fabricated using a novel, high-throughput, focused electrical discharge method and low cost panel-based double-side metallization processes. Such a glass interposer is targeted at two emerging applications, (a) large 30 mm to 60 mm body size 2.5D interposers to achieve 28.8 Gbps logic-memory bandwidth and (b) 3D interposers for mm wave applications at 28 GHz local multipoint distribution service (LMDS) for future 5G networks. Accurate measurement of the electrical performance of fine pitch metallized through vias in glass up to 30 GHz and beyond is critical for both these high performance interposer applications. In this paper, two novel characterization methods are applied: 1) the short-circuit-and-open-circuit method and 2) the dual-via-chain method. The resistance and the inductance of a single via are extracted by using a short-circuit structure along with an open-circuit structure. At 10 GHz, the values for the series resistance and inductance have average values of 0.1 Ω and 160 pH respectively. Long dual-via chains were designed to evaluate their performance in insertion loss, delay and eye diagram. The insertion loss achieved with the longest dual-via chain was found to be less than 1 dB/cm up to 30 GHz with only a 6.2 ps delay in the TPVs, and the simulations indicate a wide open eye.