Photonics for microelectronics

Researchers at the University of Colorado Boulder, the Massachusetts Institute of Technology and Micron Technology Inc have developed a new technique that allows microprocessors to use light, instead of electrical wires, to communicate with transistors on a single chip. The system could lead to extremely energy-efficient computing, allowing for increasingly faster electronics, from supercomputers to laptops to smartphones.

The research team, led by CU-Boulder researcher Milos Popovic, an assistant professor of electrical, computer and energy engineering, created two different optical modulators - structures that detect electrical signals and translate them into optical waves - that can be fabricated within the same processes already used in industry to create today’s state-of-the-art electronic microprocessors.

While transistors continue to get smaller, halving their size today no longer leads to a doubling of computing speed. That’s because the limiting factor in microelectronics is now the power that’s needed to keep the microprocessors running. The vast amount of electricity required to flip on and off tiny, densely packed transistors causes excessive heat build-up.

Microelectronics also are limited by the fact that placing electrical wires that carry data too closely together can result in ‘crosstalk’ between the wires.

In the last half-dozen years, microprocessor manufacturers, such as Intel, have been able to continue increasing computing speed by packing more than one microprocessor into a single chip to create multiple ‘cores’. But that technique is limited by the amount of communication that then becomes necessary between the microprocessors, which also requires hefty electricity consumption.

Optical communication circuits, known as photonics, have two main advantages over communication that relies on conventional wires: using light has the potential to be brutally energy efficient and a single fibre-optic strand can carry a thousand different wavelengths of light at the same time, allowing for multiple communications to be carried simultaneously in a small space and eliminating crosstalk. Optical communication is already the foundation of the internet and the majority of phone lines. But to make optical communication an economically viable option for microprocessors, the photonics technology has to be fabricated in the same foundries that are being used to create the microprocessors. Photonics have to be integrated side by side with the electronics in order to get buy-in from the microprocessor industry, Popovic said.

“In order to convince the semiconductor industry to incorporate photonics into microelectronics you need to make it so that the billions of dollars of existing infrastructure does not need to be wiped out and redone,” Popovic said.

Last year, Popovic collaborated with scientists at MIT to show, for the first time, that such integration is possible. “We are building photonics inside the exact same process that they build microelectronics in,” Popovic said.

“We use this fabrication process and instead of making just electrical circuits, we make photonics next to the electrical circuits so they can talk to each other.”

In two papers published in Optics Letters with CU-Boulder postdoctoral researcher Jeffrey Shainline as lead author, the research team refined their original photonic-electronic chip further, detailing how the crucial optical modulator, which encodes data on streams of light, could be improved to become more energy efficient. That optical modulator is compatible with a manufacturing process - known as silicon-on-insulator complementary metal-oxide-semiconductor, or SOI CMOS - used to create state-of-the-art multicore microprocessors such as the IBM Power7 and Cell, which is used in the Sony PlayStation 3.

The researchers also detailed a second type of optical modulator that could be used in a different chip-manufacturing process, called bulk CMOS, which is used to make memory chips and the majority of the world’s high-end microprocessors.