Inexpensive, Unbreakable Displays
Researchers at HP are scaling up a process for making silicon electronics on rolls of plastic.
Carl Taussig unfurls a roll of silvery plastic patterned with arrays of small iridescent squares, each a few centimeters across. The plastic in his hands, along with the scraps and scrolls of the material scattered on benchtops and desks in the rooms of Hewlett-Packard Labs in Palo Alto, CA, may look like silver wrapping paper, but each square contains thousands of silicon transistors. The transistors can switch pixels in displays on and off as fast as those in conventional flat-screen monitors and televisions, but they’re far cheaper to fabricate and more resilient.
In today’s displays, whether they’re flat-panel TVs or iPads, the electronics that control the pixels are made of amorphous silicon on glass. Taussig’s goal is to replace these heavy, fragile, expensive displays with lightweight, rugged, inexpensive ones made on plastic–without compromising performance. He is using high-volume roll-to-roll mechanics, the type of high-speed manufacturing process used in newspaper production, to make high-performance transistor arrays on the 33-centimeter-wide plastic rolls. HP researchers are now engineering a process for a planned pilot plant, where the company will produce the arrays at volumes of about 46,500 square meters a year through a partnership with Phicot, a manufacturer of thin-film electronics based in Ames, IA.
The idea is to combine these transistor arrays with flexible “frontplanes”–the part of a display that creates the images and that the transistors control. “Our goal is to make displays at a cost of $10 per square foot,” Taussig says. That’s about a 10th the price of today’s displays. Silicon-on-plastic displays might be used in laptops, or a few thin sheets might be stuffed into briefcases, replacing printouts and pads of paper. Taussig also imagines “ginormous displays” pasted to walls to show videos and ads.
HP may not be the first company to market with a plastic display–Plastic Logic, which plans to release an e-reader soon, is likely to earn that distinction. But Plastic Logic’s display isn’t flexible–the plastic frontplanes and backplanes are protected by a rigid case. It also doesn’t use silicon; it uses lower-performance organic transistors that aren’t fast enough for video. HP hopes to gain an edge, Taussig says, by lowering the cost of the display while producing speedy, video-capable transistors.
Part of the reason silicon-on-plastic displays haven’t been produced before is that it’s difficult to deposit high-quality silicon at temperatures low enough to avoid melting the plastic. Hewlett-Packard ‘s partner, Phicot, has managed to solve that problem. HP has addressed another challenge: unlike glass, which provides a mechanically stable surface, plastic tends to distort. By finding a way to make nanoscale features on plastic, HP is opening the way to the large volumes and low costs that Taussig has in mind.
On a Roll
The key to forming nanoscale electronics on distortion-prone plastic is a process called self-aligned imprint lithography, which Taussig’s team invented in 2001, and which his group first applied to displays in 2006. Thin-film transistors have several layers, and in conventional manufacturing, the materials in each layer are deposited separately. After each layer is deposited, it’s carved into precise patterns before the next layer is added. This process requires careful alignment of the photolithographic masks used to outline each pattern. Taussig’s process, on the other hand, uses a single, three-dimensional template to pattern all the layers, eliminating the need to align different masks. “It’s immune to distortion, which is the biggest challenge when making electronics roll-to-roll,” Taussig says.
Researcher Albert Jeans shows off the starting material: a roll of plastic film coated with several thin layers of metal, amorphous silicon, and other materials needed to make the electronic circuits. To create the three-dimensional template, he loads the roll onto a spindle and threads it into a machine. The film moves through the machine, and a stationary blade spreads a uniform coating of liquid polymer over it. A stamp creates intricate impressions in the polymer coating, and these are instantly frozen in place by an ultraviolet light, which solidifies the polymer.
From the edge, the three-dimensional template that’s been created resembles a microscopic city skyline. What follows is a series of etching steps, with the template controlling how far the underlying layers of metal and silicon are carved into at each one. Each step erodes the template uniformly, gradually eating into the thin films until the three-dimensional pattern of the polymer template is transferred to the layers below.
To start this carving process, the patterned plastic film is fed into a wet-etching machine. Once inside, the template is thoroughly coated with chrome etchant, which eats away roughly one micrometer from the entire surface. The thinnest parts of the template disappear, exposing parts of a thin film of metal below. Then the etchant carves into that exposed metal. The film is transferred to a plasma etching machine, which bombards the template with fluorine ions. They carve deeper into the film, this time cutting away parts of a silicon layer. Next it’s back to the wet etcher to carve more metal layers. Each step carves still further into the thin films of metal, insulating material, and silicon until parts of the bottommost layer of metal are exposed and etched, and the circuits are complete.
At each stage of the etching, Taussig uses a digital microscope to scan the surface for defects. He stores the images for examination during what he calls a “display autopsy.” In that process, a computer screen shows rows and rows of transistors and capacitors interconnected with perpendicular conductive lines that will convey image data. These images also reveal subtle defects. The roll-to-roll process tolerates more variation than the processes for manufacturing traditional silicon electronics, but things can still go wrong.
If a transistor in the display doesn’t work, the researchers can review the images and related data about the equipment settings (such as the tension on the roll, or the temperature) to identify the source of the problems: a faulty etching bath or a maladjusted spindle, for example.
To finish the display, researcher Hao Luo cuts a sheet of flexible “electronic paper,” which contains microscopic black and white capsules; HP’s transistors will control which capsules move to the surface of a pixel, making it appear black or white. He peels off the back of the e-paper to expose its adhesive and lays the material on top of the transistor array–a process about as simple as putting Scotch tape on a piece of paper, he says. This prototype is a step toward wristband displays that could show maps and other information to soldiers. The picture changes at a rate dictated by the e-paper, which is slow. But the electronics perform well enough to be combined with other, faster flexible pixel technologies. One leading candidate is a reflective, full-color, video-capable display that HP is developing.
As Taussig carries the finished rolls of plastic circuitry past rows of his colleagues’ offices, he sees paper everywhere–internal motivational posters, comics, the usual stuff of cubicle walls. He envisions a day when the plastic he’s carrying could replace them all.