IBM’s technology can be more compact than either of these, says Assefa. “We’re integrating on the same chip as the electronics, using the same piece of silicon, for both transistors and photonics,” he says. “That means we’re able to make much finer features and build the much denser and power-efficient structures needed to target future high-end systems.” IBM’s technology can fit a photonic transceiver—able to send and receive optical signals—into a space 10 times smaller than has been demonstrated before.
That’s possible using new designs of photonic components that can be made at the same stage in the CMOS process in which transistors are etched, when lithography techniques precise to just tens of nanometers can be used. But it required some creative thinking to allow optical and electronic components to be built side by side.
For example, to create the last component, IBM researchers had to reinvent their photodetector, which receives incoming optical signals. “We wanted to use a layer of germanium, which is already used in CMOS processing, but had to find a way not to use too thick a layer, which would inhibit the transistors,” says Assefa. The team figured out that carefully spaced tungsten “plugs” in contact with a germanium layer thin enough not to harm nearby transistors gave it the desired electronic properties.
Finding ways to design very small photonic components is impressive, says Jalali, because they have typically been orders of magnitude larger than electrical ones, such as transistors. “They have done well to lower, if not remove, that particular barrier,” he says. “IBM has emerged as the industry leader at this stage.” However, he points out that further big leaps in miniaturization are unlikely. The light-carrying portions of IBM’s components have been scaled down to near the diffraction limit, the fundamental limit physics places on the size of optical components for any given wavelength of light. “That is a more difficult barrier to get around,” says Jalali.