Bernie's Basics

The end of the line for silicon?

Silicon chips can't keep getting smaller forever, but will we be crunching data and defriending people on quantum/graphene thingies any time soon?

Thanks to some clever physics, engineering, and the odd multi-billion dollar research budget, transistors — the workhorse of all computers — have consistently gotten smaller, so more of them can be squeezed onto a silicon chip.

More transistors mean more processing power in smaller packages, which means our mobile phones can run rings around the supercomputers of a few decades ago. (Read about how transistors work).

^ to top

Less is Moore

Gordon Moore is probably even more famous for his 'law' of doubling than for co-founding Intel. In 1965, seven years after the integrated circuit (silicon chip) was invented, Moore noticed that the number of transistors that could fit on a chip doubled roughly every two years.

A master of extrapolation, Moore predicted that miniaturisation would allow the doubling trend continue into the 70s. Almost fifty years later his 'law' doesn't just still hold, it's become the unofficial motto for the International Technology Roadmap for Semiconductors — the layers down of the law in terms of development in the silicon chip industry.

The shrinking can't go on forever — it's not like we can miniaturise atoms. But with so much money and manufacturing infrastructure invested in it, every stop will be pulled out to make sure we milk the silicon pony for all it's worth.

For most of the last 50 years, miniaturisation has meant improving manufacturing processes, silicon crystal purity and dealing with heat. But the gate of a standard transistor right now is a ridiculously small 32 nanometres — that's a quarter of the size of a flu virus, and about 150 times the size of a silicon atom. And when you're working at the nano scale, tiny interactions start messing with the system and, not surprisingly, the word 'quantum' rears its head.

^ to top

Leaky electrons and quantum tunnels

Transistors are all about controlling current flow. But as the different components get smaller and thinner, electrons start appearing in places they're not wanted — they do a quantum version of walking through walls.

Electrons aren't the solid little lumps we were brought up to think they were — they get around as disembodied probability distributions. That just means that instead of occupying a specific bit of space at every moment, they occupy a cloudy sort of region. The electron can materialise anywhere in that region — so if the insulating barrier is thinner than the probability distribution, there's a good chance the electron will suddenly pop up on the other side of the barrier. Which is exactly what goes on when the insulating oxide layer beneath the gate gets too thin — electrons start 'quantum tunnelling' from the channel to the gate.

The quantum electron leak doesn't produce a huge amount of current, but it does suck a bit of power and generate a bit of heat. That bit of heat times a billion transistors puts a real spanner in the shrinkage works. And as the oxide gets thinner, the problem gets worse. But as with every miniaturisation or scaling challenge to date, researchers have come up with a solution. Instead of using a skinny layer of silicon dioxide (SiO 2 ) they hunted around for a different material that could be made thick enough to block paranormal quantum electron manoeuvres without blocking the gate's small electric field. (Are you sure you don't want to read more about how transistors work?). Enter hafnium dioxide (HfO2).

Hafnium dioxide has a nice high dielectric constant (K) — which means it ramps up the effectiveness of the gate's electric field. But while it plugs the electron leaks, it makes for some sloppy joinery when it meets silicon surfaces. And at this scale, tidy interfaces are all important.

Thanks to some loose electrons, HfO 2 doesn't make as neat an edge as SiO 2 , so it can't line up squarely with the silicon gate. After 50 years in the shrinking game, the researchers aren't fazed by a few dangling bonds - they've changed the gate material to a nice hafnium-friendly metal, so things stay neat.

Which is lucky because it means they can focus their attention on one of the other key problems in the nano-scopic realm — getting the dose of phosphorus atoms exactly right when mixing up a batch of N-type silicon.

^ to top

The dopant head-count

At the 32 nanometre transistor scale, you're only talking a handful of dopant phosphorus atoms in the source and the drain. If the ratio gets even slightly out of whack, the current flow in the channel from source to drain is affected. Getting it right is hellishly difficult, and one idea that's sucking up a fair swag of those R&D billions is building chips that do without dopant altogether. No doping means no counting problem — it's nothing if not lateral thinking.

These new dopeless chips could be skinny layers of silicon with a second gate or vertical layers poking up of out the chip — two of the 'renovations' being hammered out in the big players' labs. But whatever these new chip architectures look like, it brings us one step closer to the end of the line for Moore's law in silicon transistors.

^ to top

Life after silicon?

With the engineers destined to run out of workarounds for silicon once they hit the atomic scale, there is no shortage of contenders for the Next Big Thing in computing. There are two big fields of research — those looking at new materials, and those looking at quantum computing.

Materials like graphene (literally a single layer of the graphite in pencils) have phenomenal conducting properties that could make them handy for speeding up data transmission between chips.

And there will always be a cluster of old-school semi-conductor fans who want to see germanium or gallium arsenide take their rightful place. (They'll be the ones counting down the End of Silicon days with their slide rules).

The only trouble is, silicon has got a hell of a head start. No matter how promising a new material is in the lab, ramping it up to be anywhere near competitive with today's billion transistor silicon chips is a massive call. The decades of work and dollars that have gone into nurturing, cajoling and even hafnium-ing silicon make the challenge for other materials that much greater.

Quantum computers are an entirely different kettle of fish from our transistor-based models. Much has been written about their mind-blowing potential for tackling massive data crunching tasks and encryption — if someone ever makes one they'll have massive markets in climate modelling, finance and defence for starters.

Australian scientists have already ticked off three of the five major requirements for making a scalable quantum computer. And their choice of material might give them an edge when it comes time to scale up production — they've been working with phosphorous atoms trapped in a silicon crystal: the same set-up that silicon transistors are based on.

Instead of using the loose electron of phosphorous as a charge carrier like a transistor does, the spin of the electron (which can be up, down or both at once) becomes the quantum 'bit' of information, or qubit. There's still a way to go to get their qubits to do a calculation, and further yet to rolling out the first quantum computer. But the combination of industry-standard materials and some pretty spectacular success to date might give them the edge when it comes time to make the quantum version of ENIAC.

Whatever the future of computing holds, it's still a way off. We won't be strapping graphene-based devices on our arms or downloading tv episodes with them anytime soon. And considering today's quantum bits are only stable at an extremely chilly fraction above absolute zero (-273°C), quantum laptops and phones will be about as practical as hand-held black holes.

Thanks to Professor Michelle Simmons, director of the, Atomic Fabrication Facility, Centre for Quantum Computer Technology at UNSW and Dr Patrick Kluth from the Department of Electronic Materials Engineering at Australian National University.

^ to top