All the magic of modern computing technology, from the smartphone in your pocket to the laptop you may be reading this on, exists thanks to the semiconductor chip. These tiny pieces of silicon contain a collection of electronic circuits, which manipulate electric current in order to store and transfer data. However, the electrons that generate electric current can only move so fast across a chip. Even though electrons are miniscule subatomic particles that weigh almost nothing and can move very quickly in a vacuum, the speed of a transmitted electric signal tends to be only a few hundredths of an inch per second. This relatively slow movement limits the performance of today’s computer processors—but if we could find a way to transport data faster, we could break the barrier of electron movement and make computers way more powerful.
What’s faster than an electron, you may ask? Well, the fastest thing we know of is the speed of light, which travels more than a hundred thousand MILES per second. What if we could harness the speed of light in our computer technology? That’s what a team of researchers at Berkeley, MIT, and the University of Colorado Boulder are trying to achieve, and their new work published in Nature Letters has introduced a silicon chip system that utilizes both electronics and laser light to execute complex computer programs. The result is a major step toward the large-scale manufacturing of light-driven microprocessors, which may be the next computing revolution.
Efforts to create a chip like this in the past have faced a number of challenges, the main one being that it’s extremely hard to combine components for manipulating both electrons and light beams on the same chip. Scientists and engineers have worked to develop special fabrication processes to achieve this purpose, but those processes aren’t very compatible with the standard processes used at a microelectronics foundry where semiconductor chips are made.
This latest study adopts a different approach: the researchers attempted to devise new optical components that could be fabricated using the current foundry processes. Their design consists of a layer of crystalline silicon, into which positively charged ions have been implanted to form a thin layer of metal oxide which separates it from the silicon wafer support material. The top crystalline layer contains an intricate pattern of transistors and structures for guiding light waves, while the bottom layer is etched away underneath each optical structure so that none of the light is lost by leaking out into the silicon.
In order to transmit laser light and electric current between different chips, a special ring-shaped system takes in an electronic voltage signal from the first chip and converts it to a light signal that can be directed to the second chip some distance away. This works because the electrons entering the system as a result of the voltage change cause refraction in the ring, which changes its wavelength—light can get through only when the wavelength matches that of the laser. With this setup, the researchers can move data along a series of chips using only light, and without requiring any electronic connection between them.
The control and manipulation of lasers is exciting, but does it really work? The scientists utilized the laser-connection capability of their chips to hook one chip up to a second chip serving as a 1 MB memory system. Then they ran computer programs on it—both the programmer’s standard “Hello World” script, which simply prints those two words as output, and a more complicated three-dimensional rendering program that returns a picture of a teapot. Both programs were executed flawlessly with a maximum processor speed of 1.6 Gigahertz, comparable to many modern desktop computers. When the system was operated in optical mode, the laser light could transfer data at a dizzying speed of 2.5 gigabytes per second, twice the speed of the fastest wireless connection around today!
Although this exciting breakthrough is still only demonstrated with a tiny device, enabling the integration of optics into electronics on a larger scale would be a huge advance for modern computing technology. For one thing, the addition of light to the mix opens up infinite possibilities for new integrated circuit designs, perhaps more complex and efficient than any we have seen before. The supercomputer Deep Thought in Douglas Adams’s The Hitchhiker’s Guide to the Galaxy took 7.5 million years to calculate the meaning of life…equipped with the speed of light, maybe we can shave that processing time down a bit.
Top image: Eight Major Steps to Semiconductor Fabrication, Part 6: The Addition of Electrical Properties (CC BY-NC-SA 2.0)
Chen Sun, Mark T. Wade, Yunsup Lee, Jason S. Orcutt, Luca Alloatti et al. Single-chip microprocessor that communicates directly using light. Nature 528 (2015), 534-538.
Jing Chen, Xi Wang, Yemin Dong, Xiang Wang, Wanbing Yi, Zhihong Zheng, Zixin Lin. The formation of thick buried oxide layers using ion implantation from water plasma. Thin Solid Films 472 (2005), 309-316.
“Circuits and the Speed of Light: Chapter 14 – Transmission Lines.” All About Circuits. http://www.allaboutcircuits.com/textbook/alternating-current/chpt-14/circuits-and-the-speed-of-light/ (accessed 7 Dec 2016).
Yang, Sarah. Engineers demo first processor that uses light for ultrafast communications. Berkeley News. http://news.berkeley.edu/2015/12/23/electronic-photonic-microprocessor-chip/ (accessed 7 Dec 2016).