Stretching the limits of flexible electronics

Posted on 01. May, 2013 by in Academic Departments, Electrical and Computer Engineering, Issues, Magazine, Research

A sample of stretched silicon

A sample of stretched silicon. Photo: Jung-Hun Seo.

 

Stretched-out clothing might not be a great result of laundry day, but in the case of microprocessor manufacturing, stretching out the atomic structure of the silicon in the critical components of a device can be a good way to increase a circuit’s performance.

Creating “stretched” semiconductors with larger spaces between silicon atoms, commonly referred to as “strained silicon,” allows electrons to move more easily through the material.

Historically, the semiconductor industry has used strained silicon to squeeze a bit more efficiency and performance out of the conventional microprocessors that power the desktop and laptop computers we use each day. However, manufacturers’ inability to introduce strained silicon into flexible electronics has limited their theoretical speed and power to, at most, approximately 15 gigahertz.

Thanks to a new production process being pioneered by UW-Madison engineers, that speed could double. “This new design is still pretty conservative,” says Zhenqiang (Jack) Ma, a professor of electrical and computer engineering. “If we were more aggressive, it could get up to 30 or 40 gigahertz, easily.”

Ma and his collaborators reported their new process in Nature Scientific Reports on Feb. 18, 2013.

The straining process is similar to stretching out a t-shirt: The researchers pull a layer of silicon over a layer of atomically larger silicon germanium alloy, which stretches out the silicon and forces spaces between atoms to widen. This allows electrons to flow between atoms more freely, moving through the material with ease.

In semiconductor manufacturing processes, doping introduces impurities that provide electrons that ultimately flow through the circuit. However, doping also distorts the flexible, free-standing silicon sheet, limiting its stability and usefulness as a material for integrated circuits.

Ma and his collaborators—Max Lagally, the Erwin W. Mueller Professor and Bascom Professor of Surface Science and Materials Science and Engineering; and Paul Voyles, an associate professor of materials science and engineering—developed a process through which they dope a layer of silicon, then grow a layer of silicon germanium on top of the silicon, then grow a final layer of silicon over that. Now, the doping pattern stretches along with the silicon.

The researchers call the new structure a “constrained sharing structure.” Ma believes that using the material to design next-generation flexible circuits will yield flexible electronics that offer much higher clock speeds at a fraction of the energy cost.
The next step will be to realize processors, radio frequency amplifiers, and other components that would benefit from being built on flexible materials, but required more advanced processors to be feasible.

“We can continue to increase the speed and refine the use of the chips in a wide array of components,” says Ma. “At this point, the only limit is the lithography equipment used to make the high-speed devices.”

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