Any fan of the Harry Potter books knows that the walls of Hogwarts are lined with portraits whose subjects move around and talk inside their picture frames. And one of Harry’s most prized possessions is the Marauder’s Map, which shows the location of anyone on the school grounds at a given moment.
Wafers, such as the one EECS Professor Vivek Subramanian is holding, are used to fabricate printed circuits. (Photo by Bart Nagel) Moving portraits and maps with moving icons may sound like fantasy, but they could soon be reality. Vivek Subramanian is working on printable electronics—that is, real circuits that can be printed onto plastic by a standard inkjet or gravure printer. One application he has in mind is a flexible display that could be rolled up like a poster. Other applications include cheap, flexible radio-frequency ID (RFID) tags or printable biosensors that could detect milk that has soured.
Subramanian's interest in printable electronics was motivated by the inherent wastefulness of conventional integrated circuit fabrication, in which layers of material are deposited on top of a silicon wafer and then most of each layer is etched away again. A printer, by contrast, deposits ink only where it is needed. “Printed electronics cost more per transistor than silicon electronics, but in terms of cost per square centimeter, they are cheaper,” Subramanian says.
For printable electronics, engineers need inks that become functional electric materials after they dry—conductors, semiconductors, and dielectrics. Subramanian’s close ties with Berkeley's chemistry department help him to find candidate materials, and his research group does their own synthesis and manufacturing. “We synthesize our own chemicals, make our own printers, build our own transistors, diodes, and capacitors, and design our own circuits,” says Subramanian. “We’re near the leading edge, not because we’re doing things in a dramatically different way, but because we do all of our own engineering.”
Subramanian has developed a promising new material called “liquid gold”—a solution of extremely small gold crystals, each only 20 atoms across and encapsulated in an organic shell. “Normally gold doesn’t dissolve, but when you make it into really small particles it does,” he says. “The melting point also goes down from about 1000 degrees Celsius to 100 degrees. That means we can print it out on plastic and then heat it up. The gold melts and forms a continuous film.” Presto—a printed wire!
At present, the finest details that Subramanian can print are more than 100 times larger than the components in a Pentium chip. For that reason, he is working on applications that do not require nanometer-scale electronics. Displays, for example, are one of the few areas of electronics where bigger is better, and they also enable him to exploit the economics of printed electronics.
Subramanian’s group split the challenge of making a display into two smaller problems: First, to print liquid crystals—the kinds used in laptop displays—onto a flexible substrate such as plastic or rubber. Second, to print out transistors that can tell the liquid crystals what to do. He has achieved both of these goals and he is now in the process of integrating them into a working prototype.
Another promising application for printable electronics is RFID tags—identifiers containing tiny chips and antennas that enable them to listen for radio queries and respond with identification data. RFID tags have become increasingly popular with businesses, which use them for tracking inventory in large warehouses, but the tags are relatively costly. For the industry to really take off, Subramanian believes, the price must come down to under a half a cent per tag, which printable electronics could achieve. At that point, he says, RFID tags could be attached to every item in a supermarket: tracking inventory would be effortless and checkout lines would become obsolete.