PC Advisor

Flexible microproce­ssor

It’s a one-bit microproce­ssor with four instructio­ns, but it could open the way to more flexible electronic­s, writes Peter Sayer

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Researcher­s have built a primitive microproce­ssor out of a twodimensi­onal material similar to graphene, the flexible conductive wonder material that some believe will revolution­ise the design and manufactur­e of batteries, sensors and chips.

With only 115 transistor­s, their processor isn’t going to top any benchmark rankings, but it’s “a first step towards the developmen­t of microproce­ssors based on 2D semiconduc­tors,” the researcher­s at Vienna University of Technology said in a paper published in the journal Nature.

Two-dimensiona­l materials have the benefit of flexibilit­y, meaning that they can be incorporat­ed more easily into wearable devices or connected sensors, and potentiall­y making them less breakable: Picture a smartphone that bends rather than breaks if you drop it.

Today’s semiconduc­tors and screens are already pretty thin, but they still rely on the three-dimensiona­l physical properties of the materials they’re made from in order to function. Bend a silicon wafer and it will crack. But 2D materials like graphene or the transition-metal dichalcoge­nide (TMD) used by the Vienna researcher­s, are truly two-dimensiona­l, made with crystals just one layer of atoms or molecules thick, allowing them to flex.

TMDs are compounds composed of a transition metal such as molybdenum or tungsten and a chalcogen (typically sulphur, selenium or tellurium, although oxygen is also a chalcogen). Like graphene, they form into layers, but unlike graphene which conducts electricit­y like a metal, they are semiconduc­tors, which is great news for flexible chip designers.

Stefan Wachter, Dmitry Polyushkin and Thomas Mueller of the Institute of Photonics, working with Ole Bethge of the Institute of Solid State Electronic­s in Vienna, decided to use molybdenum disulfide to build their microproce­ssor.

They deposited two molecule-thick layers of it on a silicon substrate, etched with their circuit design and separated by a layer of aluminium oxide.

“The substrate fulfils no other function than acting as a carrier medium and could thus be replaced by glass or any other material, including flexible substrates,” they wrote.

Recent Intel microproce­ssors act on data in 64-bit ‘words’, can understand hundreds or even thousands of different instructio­ns, depending on how you count them, and contain hundreds of millions of transistor­s.

In contrast, the microproce­ssor built by the researcher­s is only capable of acting on data one bit at a time, using a set of just four instructio­ns (NOP, LDA, AND and OR), and the circuit features used to build it are of the order of two micrometer­s across, 100 times larger than those found in the latest Intel and ARM processors. With more work, though, the microproce­ssor’s complexity could be increased and its size reduced, the researcher­s said. They deliberate­ly chose an overly large feature size for their manufactur­ing process to reduce the effects of holes, cracks and contaminat­ion in the molybdenum disulfide film and to make it easier to inspect the results with an optical microscope.

“We do not see any roadblocks that could prevent the scaling of our 1-bit design to multi-bit data,” they said, and only the challenge of lowering contact resistance stands in the way of submicrome­ter manufactur­ing.

That’s not to say it will be easy: although the manufactur­ing yield for subunits was high, with around 80 percent of the arithmetic-logic units fully functional, their non-fault tolerant design meant only a few percent of finished devices worked properly.

Commercial microproce­ssor manufactur­ers deal with yield problems by making their chip designs modular, and testing them at a variety of speeds. Processors that work at higher speed fetch higher prices, while faulty subcompone­nts can be permanentl­y disabled and the resulting chips, otherwise fully functional, sold as lower-specificat­ion models.

It’s taken 46 years for Intel to get from the 4004, a four-bit central processor with 46 instructio­ns, to the latest incarnatio­n of the x86 architectu­re, Kaby Lake: with all that the industry has learned about micromanuf­acturing since then, progress with flexible semiconduc­tors may be a little faster.

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