Chip-To-Chip Electrical Interconnect Transmits 1.8-ps Pulses From 50 To 300 GHz

In the search for a high-bandwidth, electrical chip-to-chip interconnect, many optically interconnected electronic chips and devices have been created. Yet the optical link needs non-silicon devices to generate and modulate optical signals and then couple to optical fibers and waveguides to reduce complexity and cost. Recently, chip-to-chip electrical interconnects were demonstrated and characterized in an experimental fashion by Korea Maritime University’s Jeong Sang Jo and Tae-In Jeon together with Daniel R. Grischkowsky from Oklahoma State University.

The team connected two optoelectronic chips with air-spaced two-wire transmission lines. In doing so, it showed essentially undistorted, transform-limited propagation of a 1.8-ps (FWHM) pulse to a distance of 2000 mm. The electrical pulses were generated by photo-conductively switching a charged coplanar transmission line on the optoelectronic transmitter chip. They were then coupled to the two-wire transmission line by two matched probes. After propagation over that transmission line, the pulses were transferred to the receiver’s coplanar transmission line via near-contact coupling with two probes (one for each wire). Finally, the coupled pulses were optoelectronically measured on the receive chip.

The demonstration featured the team’s latest Type-II, two-wire transmission line with the relatively low power loss of 5.8 dB/m from 50 to 300 GHz. The transmission line comprises two copper wires with diameters of either 0.4 or 0.5 mm. Their centers are separated by 1.0 mm. For the coupling, two tungsten probes with 1-μm diameter tips were used in near contact(with a gap of 5 μm).

Impressively, the air-spaced, two-wire line’s measured pulse-amplitude attenuation—albeit relatively small—was 1.31X larger than the theoretical prediction for transverse-electromagnetic (TEM) mode of the air-spaced, two-wire copper line. This difference is a result of the reduced conductivity in the skin-depth layer and some scattering by the variable wire separation. See “Prototype 250 GHz Bandwidth Chip to Chip Electrical Interconnect, Characterized with Ultrafast Optoelectronics,” IEEE Transactions On Terahertz Science And Technology, July 2013, p. 453.