This packaged electronic-photonic processor microchip under illumination reveals the chip's primary features. The light rays emanating from the chip are drawn to show that the processor talks to the outside world using light. Images by Glenn J. Asakawa, University of Colorado
Engineers at Berkeley, MIT, and Boulder have successfully brought together electrons and photons within a single-chip microprocessor, a landmark development that opens the door to ultrafast, low-power computer processing
The researchers packed two processor cores with more than seventy million transistors and eight hundred fifty photonic components onto a three-by-six-millimeter chip. They created the microprocessor in a foundry that mass-produces high-performance computer chips, showing that their design can be easily and quickly scaled up for commercial production.
The new chip, described in a paper to be published December 24, 2015 in the print issue of the journal Nature, marks the next step in the evolution of fiber optic communication technology by integrating into a microprocessor the photonic interconnects, or inputs and outputs (I/O), needed to talk to other chips.
Compared with electrical wires, fiber optics support greater bandwidth, carrying more data at higher speeds over greater distances with less energy. While advances in optical communication technology have dramatically improved data transfers between computers, bringing photonics into the computer chips themselves had been difficult.
That's because no one until now had figured out how to integrate photonic devices into the same complex and expensive fabrication processes used to produce computer chips without changing the process itself. Doing so is key since it does not further increase the cost of the manufacturing or risk failure of the fabricated transistors.
The researchers verified the functionality of the chip with the photonic interconnects by using it to run various computer programs, requiring it to send and receive instructions and data to and from memory. They showed that the chip had a bandwidth density of 300 gigabits per second per square millimeter, about 10 to 50 times greater than packaged electrical-only microprocessors currently on the market.
The photonic I/O on the chip is also energy-efficient, using only 1.3 picojoules per bit, equivalent to consuming 1.3 watts of power to transmit a terabit of data per second. In the experiments, the data was sent to a receiver 10 meters away and back.
The researchers came up with a number of key innovations to harness the power of light within the chip.
Each of the key photonic I/O components - such as a ring modulator, photodetector and a vertical grating coupler - serves to control and guide the light waves on the chip, but the design had to conform to the constraints of a process originally thought to be hostile to photonic components. To enable light to move through the chip with minimal loss, for instance, the researchers used the silicon body of the transistor as a waveguide for the light. They did this by using available masks in the fabrication process to manipulate doping, the process used to form different parts of transistors.
After getting the light onto the chip, the researchers needed to find a way to control it so that it can carry bits of data. They designed a silicon ring with p-n doped junction spokes next to the silicon waveguide to enable fast and low-energy modulation of light.
Using the silicon-germanium parts of a modern transistor - an existing part of the semiconductor manufacturing process - to build a photodetector took advantage of germanium's ability to absorb light and convert it into electricity.
A vertical grating coupler that leverages existing poly-silicon and silicon layers in innovative ways was used to connect the chip to the external world, directing the light in the waveguide up and off the chip. The researchers integrated electronic components tightly with these photonic devices to enable stable operation in a hostile chip environment.
