New process give flexibility to strained silicon

Stretching the atomic structure of silicon in the critical components of a device can increase a processor’s performance.

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

The semiconductor industry has used strained silicon to gain better efficiency and performance from the conventional microprocessors that power everyday desktop and laptop computers.

So far, however, manufacturers’ inability to introduce strained silicon into flexible electronics has limited their theoretical speed and power to approximately 15GHz.

Thanks to a new production process being pioneered by University of Wisconsin-Madison engineers, that cap could be lifted.

‘This new design is still pretty conservative,’ said Zhenqiang Ma, a professor of electrical and computer engineering. ‘If we were more aggressive, it could get up to 30 or 40GHz, easily.’

Ma and his collaborators reported their new process today in Nature Scientific Reports.

To create stretched silicon a layer of the material is pulled over a layer of atomically larger silicon germanium alloy, forcing the spaces between atoms to widen.

This allows electrons to flow between atoms more freely but problems can occur during the doping process, a step in semiconductor manufacturing that introduces impurities to improve electrical performance.

Doping a sheet of strained silicon can cause distortions, limiting its stability and usefulness as a material for integrated circuits.

‘We needed to dope this material in a way that the lattice structure within would not be distorted, allowing for silicon that is both strained and doped,’ Ma said in a statement.

To avoid distortion, Ma and his UW-Madison collaborators - Max Lagally and Paul Voyles - 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 so the doping pattern stretches along with the silicon.

‘The structure is maintained, and the doping is still there,’ said Ma.

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 realise processors, radio frequency amplifiers, and other components that would benefit from being built on flexible materials, but previously have 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,’ said Ma. ‘At this point, the only limit is the lithography equipment used to make the high-speed devices.’