Selete Achieves 5 GHz Pulses on Silicon Photonics IC
Kenji Tsuda, Asia Contributing Editor -- Semiconductor International, 6/5/2008 9:07:00 AM
Optical interconnects using an optical fiber and on-chip light guide are being developed as interconnects in servers, circuit boards, and on silicon chips, leveraging advantages such as higher speed transmission and lower power consumption. To that end, the Selete (Tsukuba, Japan) consortium said it has succeeded in transmitting a 5 GHz pulse waveform through a 4 mm light guide produced on a silicon IC. The consortium made the announcement at the recent Selete Symposium 2008.
The light guide serves as an optical interconnect using silicon as the base material, SiON as the core light guide, and SiO2 for the cladding layer. It connects to a light-emitting diode (LED) or laser diode made of a compound semiconductor. A Schottky barrier diode with a silver electrode is used as the photodetector. The light guide layer is produced on a silicon on insulator (SOI) wafer that is attached to the wafer holding the logic circuits, both with face-to-face electrodes.
Connections between the light-guide chip and logic IC are created with a gold-tin (Au-Sn) alloy, which allows for alignment of both electrodes. A previous proposal included optical connections between an optical interconnect wafer and circuit wafer, but that approach was deemed unrealistic because of the difficulty of aligning both light guides.
 |
| A comb-like silver pattern is used to create a waveguide-coupled plasmon photodiode with a silicon photodetector (PD). The detailed image at right shows the surface-plasmon antenna with a silver electrode array. |
In the technology developed by Selete, bump electrodes on the silicon wafer are connected to bumps on a light guide wafer, then the silicon substrate of the wafer with the light guides is etched off. The light guide consists of a SiON layer with a refractive index that is slightly higher than the surrounding SiO2 cladding layer. The refractive index of the silicon layer is ~40% higher than that of SiO2. The silicon core has a higher transmission loss of 1-3 dB/cm, compared with 0.2-0.3 dB/cm for a SiON core.
 |
| The Selete solution achieves 5 GHz transmission speeds using an optical clock. Circuits in the logic LSI are triggered by periodic optical pulses. |
The electromagnetic wave of the light within the silver pattern resonates with a vibration that is almost the same as the wavelength that produces the plasmon. When the light permeates to a depletion region between the silver electrode and silicon substrate, electron-hole pairs are separated. The two detector electrodes separate the electrons and holes, with the plus electrode attracting the electrons.
In the experiment, the engineers confirmed a 20 psec single pulse response and generated 5 GHz clock pulses to activate the ICs. In the future, Selete plans to improve the transmission’s eye pattern and move to a higher data rate.
