Technology News
04/01/2002
Organic FETs emerging out of OLED technology
While just a niche in overall microelectronics, emerging flexible manufacturing is growing significantly in importance, says Robert Pinnel, CTO for the US Display Consortium (USDC).
"The Department of Defense, for example, is increasingly inserting flat panel displays into platforms because over 80% of information from these systems is obtained visually. The growing interest in 'flexible' displays comes from the fact
that high resolution active-matrix displays fabricated on flexible substrates improve durability, lessen packaging weight, and improve form factor of portable devices," he says. Pinnel spoke recently at the USDC Flexible Manufacturing Workshop held in Phoenix, AZ.
The workshop provided an interesting cross section of fabrication technologies being developed for flexible manufacturing. While early concentration on organic materials has been for organic LEDs (OLEDs), clearly the technology is now evolving to include organic FETs (OFETs). For example, Universal Display Corp. (UDC), Ewing, NJ, is developing high efficiency small-molecule OLEDs (also flexible OLEDs or FOLEDs) based on electro-phosphorescence rather than more conventional fluorescence. The small molecule OLEDs provide up to four times higher power efficiencies: brighter (e.g., red at 6-25 cd/A) longer-life LEDs with lower power consumption (e.g., deep red OLEDs have a 100,000 hr life at 70 cd/m2).
FOLEDs provide unique and exciting opportunities for applications such as UDC's pen-like Inernet communication device (Fig. 1). UDC's Mike Hack also showed recent results with a video-rate plastic phosphorescent OLED display — a 240 ¥ 64 passive matrix display with 80 dpi resolution, 120 Hz refresh rate, 32 gray levels, fabricated on 0.175mm low cost polyethylene terephthalate (PET).
E Ink Corp., Cambridge, MA, uses materials found in inks and paper to create contrast by printing fluid-filled microcapsules with positively charged white pigment chips and negatively charged black pigment chips. This technology, developed in partnership with Toppan Printing, achieves paper-like Lambertian reflective optics that are insensitive to the position of the illumination source (a common problem with reflective LCD based displays).
Peter Kazlas of E Ink says, "Electronic ink combined with an amorphous-silicon TFT backplane on foil provides a viable pathway to high-resolution, low cost flexible paper-like displays — technology for smartcards, cell phones, PDAs, peripheral displays, electronic books and newspapers, wearable displays, large area displays." Late in 2001, E Ink opened a fab facility in Woburn, MA, for commercializing this technology.
According to Rag Apte from Xerox PARC, Palo Alto, CA, "Present manufacturing is too expensive for ubiquitous displays, medical imaging, smart cards, ID tags, etc." Using ink jet printing techniques, PARC engineers have demonstrated ~20μm minimum features (determined by drop size) and layer registration on discrete and self-aligned TFTs in a matrix addressing structure.
Combined with these basic technologies, some companies are pursuing roll-to-roll fabrication methods. Robert Priano from Vitex Systems, San Jose, CA, noted that for the needed growth in the flexible OLED display market, cost advantages are as important as technical innovations.
"We need to look closely at the cost advantages available with roll-to-roll manufacturing." Along with the continued evolution required in materials, Priano sees the need for other processes such as extreme cleanliness. Some issues need to be solved, including vacuum-to-atmosphere integration and process speed matching.
Participants in one of the workshop's breakout sessions tagged "dynamic advertising" as an ideal applications opportunity to develop roll-to-roll processing for large area, low power displays. Participants concluded that weight advantage gained through the use of OLED or electrophoretic technologies on plastic substrates should be compelling. The serious conflict between good color gamut and wide angle viewing that is faced by manufacturers of reflective LCDs provides a clear opening for alternative technologies.
Another intriguing side of the application of organic materials in microelectronics fabrication is the debate over OFETs. Some think that the technology is not good enough yet. Others, like Tom McLean, director of business development at Aveica Electronic Materials, Manchester, UK, says "The attitude is changing to 'it is good enough.' While OFETs still need to be delivered in practice, today the view is that the relatively low carrier mobility of OFETS [i.e., ~10-1 cm2/Vs] compared to silicon-based technology is potentially good for non-high performance applications where OFETs are seen as 'solutions' for larger area, low cost/unit area, flexible substrate, and short runs [up to 2000] applications."
McLean noted that Aveica has made progress with ambient stable processing from solution coating processes without precursors to achieve p-type FET materials where the target in 2002 is z>10-2 cm2/V mobility. Aveica engineers are also working on n-type, dielectric, and conductor materials. Perhaps the most difficult issue with dielectrics is preventing the solvent base, potential cross linking, and kinetics from damaging the layer on which it is deposited.
 Figure 2. An OFET — polymer thin film transistor — from Plastics Logic. |
But clearly, transistors are emerging out of OLED technology. At the conference, Stuart Evans of Plastic Logic, Cambridge, UK, showed his company's work in demonstrating PLL IJ-TFTs on flexible substrates that achieved an ~10-2cm2/Vs mobility. The Plastic Logic fabrication process is all additive to a modified flexible substrate using ink jet printing for the source, drain and gate, and spin coating for the semiconductor and dielectric layers (Fig. 2). Company engineers have developed a process for channel lengths of 5-10μm and have shown experimental lengths of 1μm. The Plastic Logic printing process for active polymer electronics is inherently scaleable in substrate size and economic batch size.
MEMS chain smaller than a hair
Researchers at the Department of Energy's Sandia National Laboratories, Albuquerque, NM, have developed a MEMS surface micromachined chain link system. Initial designs (see illustration) used a 50μm pitch (the distance between link centers) and 50 links with the longest free span of 0.5mm.
Because each link has a 1μm clearance for the hubs, each span can sag by 20μm. Each link is capable of ±52° of rotation with respect to the preceding link, enabling a great deal of freedom in locating devices driven by the MEMS chain.
Aside from its intriguing appearance — resembling a tiny bicycle chain — this accomplishment has significant implications for MEMS applications. Because a single microchain can be fabricated to rotate many drive shafts, it eliminates the need to place multiple tiny MEMS motors close to one another.
 Optical image of the same device below. |
Sandia technician Ed Vernon says, "The primary problem with multiple motors is that they are so large compared to the devices they drive. On a typical 3 x 8mm module, one drive can take up to one-third the available real estate." Vernon has received a patent for the silicon microchain. Sandia Labs will be licensing this technology soon.
Beyond the initial work, Vernon believes it is possible to fabricate links with a 35μm or as large as 200μm pitch. "Larger links would require many design considerations, such as release holes. In addition, a chain tensioner would be required to accommodate longer spans," he says.
The microchain also makes it possible to drive a MEMS device from a motor placed some distance away, again saving considerable space on a MEMS-bearing chip. "This chain could allow drive access to an entire 3 x 8mm module," says Vernon. "Currently used rack-and-pinion systems allow only for back and forth motion; the chain allows rotary and continuous motion to any location on the die. Sliding translation devices tend to wedge and bind in operation; therefore the ability to translate motion long distances without slides could be advantageous. Chain systems do not have stroke limitations and allow for both continuous and intermittent translation."
Envisioning potential applications, Vernon believes his microchain could be used to power microcamera shutters or for mechanical timing and decoding, but currently no applications are being developed. "But, I have had many contacts from the medical and engineering communities that lead me to think someone will soon have an application for it." This device could also be used to control concurrent (parallel) events or consecutive (serial) events.
Vernon fabricated a microchain rather than a microbelt because although polysilicon belts are tough and flexible, they are spring-like and produce too much torque on gears not aligned in a straight line. The chain's degrees-of-rotation tolerance prevents pressure on the support structure. The wide angle of rotation means MEMS designers can be relatively unconstrained in positioning multiple devices.
The multilevel surface-micromachined silicon device was constructed with the aid of Sandia's patented Summit IV and Summit V technologies. This five mechanical layer process begins with a 150mm silicon wafer to which alternating layers of doped polysilicon and sacrificial oxide are applied. Photolithography and etching defines each structural layer of the desired MEMS.