Co-Fabrication of Microcoaxial Interconnects and Substrate Junctions for Multichip Microelectronic Systems

Abstract

RF and millimeter-wave designers continue to use wire bonds and flip chip in their designs when bonding chips or multichip systems. Due to electrical and mechanical constraints of wire bonding and flip chip, designers are forced to compensate for mismatched impedances, high loss, or mechanically unreliable interconnects. In this article, we propose microcoaxial cables (MCCs) as an alternative interconnect, specifically, 30–75- $\Omega $ MCCs for signals, and low-inductance MCCs for bias and supply. In addition, each shielded interconnect provides sufficient isolation to prevent electromagnetic interference (EMI) and crosstalk. The in situ fabrication method presented here utilizes only conventional wire bonding and microfabrication techniques, providing a high-feasibility path toward a new interconnect paradigm based on MCCs. Each cable measured consists of a 25.4- $\mu \text{m}$ gold bond wire coated first with a dielectric and then with a 5.0- $\mu \text{m}$ -thick gold shield. MCCs appropriate for signals have a 38- $\mu \text{m}$ -thick Parylene C dielectric and a characteristic impedance of 39– $68~\Omega $ . For the first time, to the best of our knowledge, we introduce low-inductance MCCs, where the dielectrics evaluated are 1.0- $\mu \text{m}$ -thick Parylene C dielectric and 100-nm-thick HfO2. Their inductances are measured to be no more than 140 pH/mm. For 3.5-mm-long wire with a 0.51-mm pitch, the crosstalk is −62 dB at 1 GHz for signal MCCs. Crosstalk increases to −30 dB at 26.5 GHz.