A group of researchers from MIT, the University of California at Berkeley and Boston University have developed a technique for assembling on-chip optics and electronic separately, which enables the use of more modern transistor technologies.
The technique can be used by existing manufacturing processes and according to Amir Atabaki, one of the research scientists involved in the project, “The most promising thing about this work is that you can optimise your photonics independently from your electronics.”
Moving from electrical communication to optical communication is an attractive proposition for chip manufacturers because it could significantly increase chips’ speed and reduce power consumption, an increasingly important advantage as chips’ transistor count continues to rise:
The integration of optical – or “photonic” – and electronic components on the same chip reduces power consumption. The optical communications devices currently on the market tend to consume too much power and generate too much heat to be integrated into an electronic chip such as a microprocessor.
Atabaki and his colleagues have used a more space-efficient modulator design, based on a photonic device called a ring resonator.
“We have access to photonic architectures that you can’t normally use without integrated electronics,” Atabaki explains. “For example, today there is no commercial optical transceiver that uses optical resonators, because you need considerable electronics capability to control and stabilize that resonator.”
In addition to millions of transistors for executing computations, the researchers’ new chip includes all the components necessary for optical communication: modulators; waveguides, which steer light across the chip; resonators, which separate out different wavelengths of light, each of which can carry different data; and photodetectors, which translate incoming light signals back into electrical signals.
Silicon must be fabricated on top of a layer of glass to yield useful optical components. The difference between the refractive indices of the silicon and the glass is what confines light to the silicon optical components.
Earlier work on integrated photonics involved a process called wafer bonding, in which a single, large crystal of silicon is fused to a layer of glass deposited atop a separate chip. The new work, in enabling the direct deposition of silicon on top of glass, uses polysilicon, which consists of many small crystals of silicon.
Single-crystal silicon is useful for both optics and electronics, but in polysilicon, there’s a trade-off between optical and electrical efficiency. Large-crystal polysilicon is efficient at conducting electricity, but the large crystals tend to scatter light, lowering the optical efficiency. Small-crystal polysilicon scatters light less, but it’s not as good a conductor.
Using the manufacturing facilities at SUNY-Albany’s Colleges for Nanoscale Sciences and Engineering, the researchers tried out a series of recipes for polysilicon deposition, varying the type of raw silicon used, processing temperatures and times, until they found one that offered a good trade-off between electronic and optical properties.
“I think we must have gone through more than 50 silicon wafers before finding a material that was just right,” Atabaki said.