“WE BELIEVE that what we’ve got – and I say this after a lot of thought – is something that will change the world,” says Tracey Stephens of Plastic Logic. What the company has is a way to make plastic electronics. Rather than spend billions on an ultrahigh-tech chip fabrication plant, it reckons it can make electronic gadgets with a technology dating back to 1st-century China – printing.
Plastic electronics have been under development for some time, so somebody must want them. But why? Silicon chips are already cheap and ubiquitous, with wearable computers and smart fridges only the latest marvels. So why all the excitement? The aim, say advocates, is not to replace silicon electronics but to complement them. Polymers can be colourful or transparent, they can be made in huge sheets, not just in small, discrete chips. They are light and flexible so they’re a “go anywhere” technology. And above all else, they promise to be so cheap as to be disposable. Anywhere you find print today, you could see tiny circuits tomorrow. Soup cans, newspapers and advertising hoardings could all carry designs and messages that shimmer and change, glowing with polymer LEDs. At the end of their life, they would simply be thrown away.
But before any of this can happen, the price of making plastic electronics needs to tumble. That’s where printing comes in. Yet what form of printing is by no means a done deal. Major players in the electronics and print industry have waded in on the side of ink-jet printing. They’ve set up plants to produce high-tech machines to help turn printed devices in the lab into commercial reality. Japan’s printer giant Seiko Epson has teamed up with Plastic Logic, which is based in England, on the outskirts of Cambridge. Meanwhile in Pleasanton, California, Litrex is investing more than $5 million in machines to print polymer components. But some scientists fear the huge gamble these firms are taking could go badly wrong.
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The idea of plastic electronics was born in the late 1970s, when Alan Heeger, Alan MacDiarmid and Hideki Shirakawa, then all at the University of Pennsylvania, found that adding tiny amounts of iodine to a simple polymer called polyacetylene gradually forced it to change from an insulator to a conductor. Since then, a host of other polymers and organic molecules have been discovered that can serve as conductors, and even replace silicon and other semiconductors.
The leading contenders for making plastic circuits are conjugated polymers. In simple polymers, a carbon atom shares each of its four outer electrons with different neighbours, but in conjugated polymers every carbon shares two of these electrons with just one of its neighbours, creating a backbone of alternating single and double bonds (see Diagram). Add more electrons to the chain or steal them to leave positively charged “holes” and charge races along the chain, flipping the double bonds as it goes.
They may be slow, but…
Just how good a conductor the polymer is depends not only on how easily electrons flow along these chains, but also on how easily they can hop between adjacent molecules. To ease their passage, molecules need to be close enough for weak electrostatic forces to act as bridges across which electrons can move.
But despite decades of development, semiconducting polymers are still not a patch on silicon. The all-important property is speed: how fast electrons can zip through the material. Right now, electrons travel through silicon a thousand times as fast as through the best semiconducting polymers. And this is unlikely to change in the foreseeable future, says Richard Friend, Cavendish Professor of Physics at Cambridge University and chief scientist at Plastic Logic. “You’re never going to make a Pentium 4 from polymers,” he says.
Silicon circuits are made using a technique called photolithography, that involves a lot of high-precision steps. First, the silicon wafer is coated with a layer of polymer called photoresist. Next, a mask that acts as a template for the first layer of circuitry is positioned above the wafer. Shining ultraviolet light through the mask projects the pattern onto the photoresist and changes its chemical structure – either making it more soluble than the material left in the shade, or less soluble, depending on the photoresist used.
The more soluble regions of the polymer coating are etched away using an organic solvent. The wafer is then washed in water and baked, before a layer of conductor, say, is deposited.
For every layer, more photoresist is added and another mask used. Because each component is so small – wires between them are now just one-tenth of a micrometre across – the precision with which the masks are lined up is paramount. The machinery that works to such tolerances is becoming ever more expensive as chip makers move to larger wafers and pack the transistors more densely. This year, when IBM set up a new chip plant in East Fishkill, New York, it parted with $3 billion.
Don’t etch, print
Photolithography can be used to make polymer circuits, too, but the process has to be completely revamped because conventional photoresists and organic solvents dissolve polymers. So with these practical problems and the gargantuan cost, many scientists are leaving standard chip-making methods behind. “If you’re going to have some sort of electronic circuit on a yogurt pot, how are you going to do it?” asks Friend. “Printing is the only process which in the end is cheap enough.”
Printing has some big advantages over etching. While silicon circuits can only be spread over square centimetres, polymer circuits can cover square metres. Instead of having to line up masks for every step, you can just spray or stamp your material with the electronics as it rolls through the presses. And because the electronics are plastic, there’s no reason why you can’t print them onto flexible sheets or strangely shaped surfaces.
At Plastic Logic, Friend’s money is on ink-jet printing. But it’s not simply a case of loading up your office printer with semiconductor inks and hitting the print button. The coloured blobs spat out by conventional ink jets splatter when they hit paper, making the droplets spread out. And every droplet follows a different trajectory. As a result, it’s impossible to print with sufficient precision to make intricate circuits.
The bullseye doesn’t matter
But two years ago, Friend’s team worked out how to improve the precision of ink-jet printing roughly tenfold and place a dot to within 5 micrometres of a target – closer to the dimensions of today’s silicon circuits. The trick is to fire the ink droplets onto a surface precisely patterned with regions that attract or repel ink (Science, vol 290, p 2123). “You don’t have to hit a bullseye,” says Friend. “Droplets are steered into position because the ink likes one part of the surface and hates the other.”
To demonstrate the technique, Friend first made a patterned surface on which to build an array of polymer transistors – the digital switches of microchips. He coated a sheet of glass with water-repellent polyimide and then used a customised photolithographic process to etch a series of tiny, glass-bottomed pits bordered by polyimide walls. Then he began to print the transistors (See Diagram).
Transistors have three electrodes, called the source, drain and gate. (Varying the voltage to the gate switches the current between source and drain on or off.) First down were the source and drain, made of a water-soluble, conductive polymer ink. Droplets of the ink migrated to the glass-bottomed pits, repelled by the polyimide walls. Next, a few drops of semiconducting polymer were squeezed on and the sheet was spun at 5000 revolutions a minute to form an even layer no more than 30 nanometres thick. Then, an insulating layer of another polymer was laid down, also by “spin coating”. Finally, a droplet of conductive polymer was ink-jet printed on top to make the gate electrode.
Circuits can also be built on top of one another. Transistors that overlay one another are joined by making vertical shafts with a suitable solvent and then filling them with droplets of conductive polymer.
Many of the companies joining the race to make plastic circuits see lightweight, flexible computer and TV screens as their first major application. If you think it would be great to have a lighter laptop, how much better would it be to be able to roll up your screen or even your TV and put it in your pocket? To make such a screen, plastic transistors would be overlaid by, say, an array of polymer LEDs. Already Friend has set up another company to create these arrays, again by ink-jet printing (żěè¶ĚĘÓƵ, 10 July 1999, p 38)
The first job is to refine the procedure for making the transistors. Spin-coating can be done by ink-jet printers, but to be efficient they would need an ink-jet head with several nozzles that could hold the different polymer solutions and be steered with great accuracy. Printing some features, such as long, fine lines could also be a problem – get it wrong and you could end up with a string of dots that need joining up to complete the circuit. Ways to print onto flexible materials instead of glass and avoid costly photolithography to do the initial patterning have been found, says Stephens, but she won’t give details.
While the electronics industry has faith that the inadequacies of ink-jet printing can be sorted out, some academics are not convinced. “There are companies that have set about making printers that can cover square metres,” says James Sturm, who works on polymer electronics at Princeton University in New Jersey.
These printers have to print droplets of around 5 micrometres, a fraction of the 30 micrometres that office printers spit out. Their jets must also be controllable to a high accuracy, and be reliable enough to churn out circuits day in, day out. And they must withstand the mostly organic solvents needed to dissolve the polymers. “There are still fundamental problems,” says Sturm. “This is a huge bet industry is taking right now.”
Another potential fly in the ointment has persuaded Sturm to look for a different way to print circuits. Check out your desk or kitchen table and you might get an idea of what’s bothering him. “If you look at a coffee stain, you’ll find that as it dries, you get material building up at the edge,” he says. When other liquids dry, material gathers in the centre. You can get both with ink-jet printed polymers. This is bad news, because for efficient circuits you want smooth, flat surfaces every time. Fans of ink-jet printing are trying to iron out these wrinkles by tweaking solvents, print temperatures and drying methods.
Sturm believes the future lies in dry printing. His idea is simple. Just spin-coat a thin layer of “host polymer”, then use a soft stamp to print inks that diffuse into the polymer, modifying tiny regions of it to give the properties needed.
Earlier this year, Sturm showed how the technique could be used to print full-colour polymer displays. After laying down a coating of a host polymer called poly (9-vinylcarbazole) (PVK) containing a blue fluorescent dye, he used a rubber stamp patterned with tiny dots to push first red, then green fluorescent dyes into the coating. The result was a sheet of polymer patterned with regularly spaced pixels of red, green and blue dots. (Proceedings of the Materials Research Society, vol 708, p 210). By laying his sheet over an array of fine electrodes, Sturm created a working display. He says that with the right kind of inks, his technique could be used to make transistors.
A world away from silicon
So far, neither ink-jet printing nor dry printing alone has produced the best polymer electronics. To make useful polymer circuits, most scientists use a combination of different techniques. Last year, Dago de Leeuw at Philips Research Laboratories in Eindhoven in the Netherlands used photolithography and spin coating to produce a circuit with more than 300 transistors. The circuit could be used to generate 15-bit codes – not fantastically useful on its own, but it proved that complete circuits could be made. The team used the same techniques to fit 4096 transistors into a grid measuring 5 centimetres square. They used this matrix to drive a liquid crystal display capable of showing 256 shades of grey.
Another team, Walter Fix and Wolfgang Clemens of Siemens in Erlangen, Germany, also used photolithography and spin coating to make the fastest polymer circuits yet. At 100 kilohertz, they still lag well behind gigahertz silicon, but they are fast enough for simple applications, including cheap electronic security tags (see “Chips with everything”).
These tags, together with flexible polymer screens, are where polymer electronics are expected to make their first inroads into our lives. But those in the business say this is just the start. Because they can be printed onto almost any surface, plastic electronics could be embedded inconspicuously in anything from work surfaces to wallpaper. And because the circuits can be made so thin as to be virtually transparent, they could even sprawl across windows.
But just because plastic electronics can be put almost anywhere doesn’t mean we know what to do with it yet. “It’s so fundamentally different from silicon, we think people are going to come up with applications no one’s even considered yet,” says Stephens. “That’s how it’s going to change the world.”
From memory to medicine
Making polymer circuits takes more than just transistors. Ways of making resistors and capacitors are being developed, and now even plastic memory is on the cards. Earlier this year, Howard Katz at Bell Labs in Murray Hill, New Jersey, discovered that under normal conditions a small amount of charge sticks to the insulating layer of organic transistors.
He further found that this charge could be added or removed by changing the currents between the electrodes, creating an opportunity to make a simple memory. The charge doesn’t disappear when the power is switched off, so in effect he had a permanent memory.
Katz is already using this phenomenon to make sensors. Because they are all organic, each transistor can be tuned by altering its chemical make-up so the current passing through it changes when a particular molecule sticks to its surface. “You can make them so they pick up medically relevant compounds in your breath,” he says.
A host of conditions, from diabetes to liver and kidney disease, can be detected by analysing breath. Earlier this year, Katz showed his transistors could even work underwater, detecting molecules at concentrations as low as 1 part per million.