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.