Chris Chinnock
New fabrication techniques for creating screens based on the polysilicon technology that's used in high-resolution, diminutive camcorder viewing screens could result in higher-resolution LCDs than those now found on laptop computers. But researchers must overcome several obstacles before this technology is suitable for notebook displays.
Most LCD transistors are made with amorphous silicon. Although suitable for the colorful screens found in high-end portable computers, amorphous silicon can't deliver the high-pixel densities that polysilicon fabrication can achieve. Companies like Hitachi and Seiko-Epson have introduced new polysilicon products for the projection and head-mounted display markets. NEC, Sharp, Toshiba, and others have demons
trated prototype polysilicon displays for the HDTV market. But until manufacturers perfect new processing techniques for large format polysilicon, the technology will be relegated to virtual reality and projection applications.
Although polysilicon technology has many benefits, it also has its challenges. The high-temperature fabrication process that is the most mature for creating polysilicon transistors requires the use of quartz substrates, because glass substrates would melt. In addition to being more expensive to produce than glass substrates, quartz substrates are limited to 6 to 8 inches in diameter, which also limits the size of the resulting display. Companies in Japan and the U.S. are investigating low-temperature processes, but researchers don't expect larger-format polysilicon displays until the next century.
Another obstacle to polysilicon is the demand for active-matrix displays, which is driving manufacturers to expand amorphous capacity, overpowering consideration of polysilicon.
``Virtual reality hasn't taken off yet, so there isn't a market for these [polysilicon] displays,'' says Joel Pollack, display product-marketing manager at Sharp Microelectronics (Camas, WA). ``The projector market is experiencing steady growth, but it is still minuscule compared to the notebook or small TV market.''
Tim Patton, business planning manager at Hitachi America's Electron Tube and Devices Division (Norcross, GA), acknowledges the risk in developing polysilicon technology: ``We know there is a need in the projection market, and there is an unknown, but potentially large, market in virtual reality and head-mounted displays. We want to learn how to make polysilicon displays and position ourselves for the development of a large-format polysilicon process.'' Until that happens, high-resolution polysilicon displays will be relegated to the small screen.
Illustration: Amorphous vs. Polysilicon Displays
Polysilicon displays offer advantages in performance and assembly costs compared to
amorphous silicon-based displays. Amorphous silicon quality is good enough to fabricate screen transistors that control the light, but it is not good enough to fabricate the drive circuits that regulate the amount of light that passes through each display pixel--the driver circuits must be fabricated separately and connected to the LCD glass using standard interconnect methods. Polysilicon quality is good enough to support the simultaneous fabrication of LCD transistors and drive circuits on the same substrate. Because driver ICs are now integral to the design, pixel density is not limited by interconnect technology, as it is with amorphous silicon. Thus, polysilicon delivers screens with much higher resolution than amorphous technology.
Polysilicon: The Path to Better Displays?
