On-Chip Optical Interconnects: The Next Step
Peter Singer, Editor-in-Chief -- Semiconductor International, 2/1/2001
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According to Professor Lionel Kimerling and his research team at the Massachusetts Institute of Technology's (MIT) Microphotonics Center, optical interconnects - or what they call silicon microphotonic circuits - are an excellent alternative to electrical interconnects. They offer high bandwidth, low power consumption, low noise and reliable data transfer with minimal crosstalk, thereby allowing higher interconnection densities. Additionally, they introduce the signal transmission advantages of optics while maintaining the computational capabilities of electronics.
"Microphotonics is the next revolutionary technology," said Kimerling. "The field of photonic research has emerged from unprecedented demands for more bandwidth created by communications applications such as the Internet, and for faster speeds for silicon chips. Light-based technologies are the logical, cost-effective way to meet these demands."
Kimerling and his team have, among other things, been working on polysilicon/SiO2 waveguides for optical interconnects (see Semiconductor International, January 2001). This material combination is effective because of the large refractive index contrast (3.5/1.5) between the core (the polysilicon) and the cladding (the oxide), enabling excellent optical confinement. This allows submicron interconnect dimensions (0.5 µm or less).
However, many challenges need to be addressed, said Kimerling. Limitations include photon scattering and photon absorption within the core and at the core/cladding interface, both of which lead to transmission losses.
To address these limitations, the MIT research team evaluated the effects of waveguide thickness, thermal treatment and hydrogen passivation on relevant material and optical properties of polysilicon, in an effort to lower the transmission losses. Two wavelengths of operation were investigated because they are the communication wavelengths of choice in optical fibers: 1.55 µm is the absorption minimum and 1.32 µm is the dispersion minimum in optical fibers.
The results of this study, reported in the Journal of Electronic Materials (December 2000), show that thinner waveguides (0.2 µm) show lower losses than thicker ones (1.0 µm). This is attributed to lower bulk losses. A high-temperature (1100°C) thermal treatment yielded lower losses than a lower-temperature (600°C) anneal because of an improved degree of crystallinity, larger grain size, fewer grain boundaries and fewer light-absorbing dangling bonds. The oxide has a lower index of refraction compared with polysilicon; therefore, when it replaces polysilicon within the waveguide it leads to poorer confinement of light. Several kinds of hydrogenation techniques were used to passivate the dangling bonds at the grain boundaries.
The best the researchers were able to achieve was a transmission loss of 9 dB/cm, and experiments are currently underway to isolate and explain this loss.
The MIT Microphotonics Center, announced in the fall of 1998, was made possible by a joint agreement with Nanovation Technologies Inc. (Miami, Fla.) With funding of $90M over six years, Nanovation is sponsoring research on photonic, microphotonic and nanophotonic devices, circuits and systems, and other photonics-related technologies for telecommunications, data communications and computing applications.
Examples of research areas to be undertaken at the MIT Microphotonics Center include optical networking devices, systems and methods, such as optical amplifiers, array transmitters and receivers and photonic bandgap materials. In the Input/Output coupling project, for example, monolithic devices are used as optical junctions for highly confined optical systems. The goal of the team is the creation of specific coupling methodologies for in-line transmission in monolithic circuits and efficient hybrid waveguide-to-filter connections. Future developments in photonics are expected to expand bandwidth and carry hundreds of times more information.
For additional information on wafer processing, go to www.semiconductor.net/wafer
