Beyond silicon: the future of the CPU

29th Oct 2007 | 00:00

Beyond silicon: the future of the CPU

The silicon processor needs a successor, says PC Plus

Today's chips are manufactured from silicon, sourced from sand, one of the most abundant materials on Earth. But with miniaturisation techniques reaching their limit, and physics standing in the way of endless speed enhancements, an alternative is needed.

The processor in your computer needs to evolve. New and exotic alloys such as indium gallium arsenide are being considered and researchers are looking at other materials and even biological components to squeeze more longevity out of good old silicon.

A little history lesson

Intel's first microprocessor, the 4004, contained just 2,300 transistors mounted on a 12mm-square silicon wafer. Compare that to the latest Dual-Core Xeon 7100 series of processors has a mind-bending 1.3 billion transistors on the same sized wafer.

The practical upshot of this is not only that the Complementary Metal Oxide Semiconductor (CMOS) processors of today's CPUs cost less to manufacture, but that processor performance has increased exponentially (Moore's Law) as more transistors are packed together.

The nemesis of this drive to even greater transistor densities is heat. Just take a look at the heat sink that dwarfs your computer's CPU next time you install some more RAM.

Eventually, though, the exponential growth of microprocessor complexity will lead to the chip's integrity failing. Estimates vary, but silicon has perhaps another ten years before a descendant must be found that can continue to provide the exponential growth in processing power we're all used to.

If not silicon, then what?

Back in 2005, Intel announced that along with researchers at QinetiQ they had successfully demonstrated a new transistor based on indium antimonide (InSb). You can read QinteiQ's case study here.

InSb is in a class of materials called III-V compound semiconductors which are in use today for a variety of discrete and small scale integrated devices such as radio-frequency amplifiers, microwave devices and semiconductor lasers.

The prototype transistor is much faster and consumes less power than previously announced transistors. Intel anticipates using this new material to complement silicon, further extending Moore's Law.

PC Plus (issue 256) has previously reported on MIT's attempts to use indium gallium and other compounds to create more efficient microprocessors. The race is now on to discover a material that can push Moore's Law to the middle of the century.

The diamond age?

Challenges could come from the most unexpected of directions. They say that diamonds are a girl's best friend, but if some researchers are able to perfect their techniques, your next PC could come with a carat rating!

The industrial production of artificial diamonds has now been perfected by two companies that can take ordinary carbon and subject it to 2,200°F under 50,000 atmospheres to produce a diamond that's indistinguishable from those produced naturally.

Diamond happens to be a great semiconductor. It has high thermal conductivity, which means - in theory, at least - you could build a microprocessor out of diamond that wouldn't fail at around 200°F as today's CPUs do.

With the arrival of Gemesis and Apollo Diamond CPU design could move in a new direction once the political wrangling about the devaluation of natural diamond is resolved. Silicon is already running at the temperatures we demand simply to play hi-def games. Diamond could eventually prove to be the computer chip industry's saviour.

Plastic fantastic?

The much-heralded plastic CPU has been conspicuous by its absence on the materials lists of AMD and Intel. But with recent advances in Thin Film Transistor Circuits (TFTCs) the plastic CPU suddenly doesn't seem that far fetched at all.

If you could print a circuit onto a flexible plastic substrate that replaces the silicon dielectric of today's microprocessors, you could push Moore's Law several decades into the future. Many computer components that are currently built using different processes could all be completed using TFTC techniques, helping to reduce costs.

Printed electronics, as this science has come to be known, uses techniques that are more familiar to inkjet or laser printers than hi-tech chip manufacturers, as all the process really consists of is printing your circuits onto a flexible material. What is now familiar as a screen technology could soon become the basis for a future generation of microprocessors.

Next-generation processors

The commercial application of plastic printed circuits is already being realised. Plastic Logic is gearing up to produce sheets of semiconductor that can be used in a wide variety of applications.

First off, the production line in Dresden, Germany will produce A4-sized plastic sheets of polyethylene terephthalate substrate, the same material used to make drinks bottles. Manufacturing these new circuits also doesn't need complex plant and machinery, toxic chemicals or the high temperatures and pressures that traditional silicon-based chips require.

It's early days: the Plastic Logic plant will be fabricating to five to 10 micrometres scale, whereas today's silicon processor plants use nanometre scales.

Over time though, fabrication will improve as techniques are refined: Plastic Logic's R&D is already prototyping 60nm circuits. The technology could reduce the cost of devices by up to 90 per cent and herald a new age of consumer electronics. The first use of this new technology is likely to be in hands-on devices such as PDAs, mobile phones, eBook readers and ultra-portable PCs.

Chasing Moore's Law

How we build the next generation of microprocessors is evolving rapidly. There's still plenty of mileage in good old silicon. New fabrication techniques such as immersion lithography are coming online. However, eventually, silicon will have to be abandoned, and there are plenty of pretenders to its throne.

It's likely we'll see new compounds and alloys developed that can be a direct replacement for the silicon dielectric as we understand it today. But with nano science also developing rapidly, integrated circuit manufacture towards the middle of this century will have changed out of all recognition and consigned silicon to the world's science and technology museums.

This article has been adapted from an article published in PC Plus 262 (Dec 2007) and written by David Howell.

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