Air cavity low-loss signal lines on bt substrates for high frequency chip-to-chip communication

Chip-to-chip communication across conventional printed circuit boards is limited to low frequencies (a few GHz), thus limiting the aggregate bandwidth without the use of optics. Low-loss, air insulated transmission lines can extend the frequency range of electrical circuitry on fiberglass-epoxy substrates (e.g. FR4 and BT). Described in this paper are electrical signal lines separated only by an airgap fabricated on BT substrates. The structures demonstrated greater than 46 percent reduction in capacitance and greater than 90 percent in loss tangent, which translates to substantially lower losses at high frequencies (10 GHz or greater). Attenuation in these structures have been simulated to be eight times lower than conventional microstrip lines. Air cavity signal lines are currently being integrated on multilayer BT substrates to demonstrate chip-to-chip communication using chips fabricated at the 65-nm node employing cross-talk cancellation techniques.