An Industry First: Jet-Printed Plastic Transistor Arrays

The Palo Alto Research Center (PARC), a subsidiary of Xerox Corp., has developed the first plastic semiconductor transistor array entirely patterned using jet printing. This approach, similar to that found in the familiar office inkjet printer, is inexpensive and could eliminate photolithography and expensive deposition processes now used to fabricated active-matrix display backplanes. Ultimately, it could also open up new markets for wall-sized televisions, unbreakable cell phone displays, rollable displays, and electronic paper. "PARC contributed greatly to the amorphous silicon transistor that is at the heart of all active-matrix liquid crystal displays," said Mark Bernstein, president and center director of PARC. "With this breakthrough, PARC is well positioned to revolutionize display technology yet again."

The demonstration arrays of 128 × 128 pixels were printed using two techniques — additive and subtractive methods — on flexible and rigid substrates. In the additive process, polymer inks are jetted directly onto substrates just where they are needed. In the subtractive process, other materials are blanket-deposited and a mask is jet-printed on top. The material is then dissolved away except where protected by the printed mask. Figure 1 shows the printed array, Figure 2 the printing system.

Both methods require precise layer-to-layer registration. The PARC printer is controlled by a patent-pending computer vision system to ensure proper alignment of the layers, even if the substrate warps or deforms during processing — a well-known complication with flexible substrates. "The process is analogous to color registration," explained Raj Apte, research scientist at PARC. "The printer correctly positions each layer of color with respect to the other layers, even if the paper or substrate has shrunk or warped. In this case, the layers to align are metal, dielectric and semiconductor, which create the transistor arrays." The linewidths on the printed array measure about 40 µm, with 10 µm spacing. "That's not a 'best'... that's a design rule," Apte said.

1. PARC scientists have successfully integrated the jet-printed polymer into a prototype display circuit, in which printing techniques define all the patterns.

Apte said there are also some possibilities with organic light emitting diodes (OLEDs), which many see as the successor to LCDs. "What is exciting is some of the OLED materials have improved, particularly the high-efficiency materials, such that they are now drivable by amorphous silicon at a mobility of 1 (cm²/V-sec). That's a controversial point, but if that is true, we would obviously like to make up that last factor of 10. That would be our drive to get to the next higher performance."

2. A researcher works on the jet-printer system used to fabricate the transistor arrays.

The scientists at PARC perfected the technique of jet-printing polymer semiconductors to make high-performance transistor arrays. This advancement builds on the invention of polythiophene-based semiconducting polymer ink developed at the Xerox Research Centre of Canada (XRCC) by Beng Ong. It is one of several joint research projects conducted within the Xerox Innovation Group. As Bob Street, research fellow at PARC, described, "These printed transistors have exceptional performance for polymers and meet all the requirements for addressing displays: high mobility, low leakage and good stability."

For additional information on emerging technologies, go to www.semiconductor.net/emerging