FROM digital watches to portable computers, flat-panel displays form an integral part of a myriad of consumer and military products. The $8-billion annual worldwide market for flat-panel display technology, now overwhelmingly dominated by active matrix liquid crystal displays (AMLCDs), is projected to grow to more than $20 billion by the end of the decade.|
However, as anyone using a portable computer can attest, liquid crystal displays have significant limitations in brightness, angle of viewability, and power consumption. For U.S. security and economic experts, an even more significant factor is the fact that liquid crystal display technology is dominated by Japanese companies; American firms control less than 3% of the market. A breakthrough for display manufacturing by Lawrence Livermore researchers, however, may well put U.S. flat-panel producers in a position to lead the market with a simple, cost-effective way to produce field-emission displays (FEDs).
Consuming less power than AMLCDs, FEDs are a new kind of flat-panel display technology that can be thinner, brighter, larger, and lighter. They have numerous potential applications in portable and large area displays and can, in principle, cost much less to manufacture.
Moving to FEDs
Active matrix technology uses liquid crystals sealed between two thin plates of glass, with the display divided into thousands of individual pixels that can be charged to transmit or block light from an external source to form characters or images on a screen. In contrast, each pixel in an FED acts as a microscopic cathode ray tube (CRT) and produces its own light. Instead of a single electron beam sweeping across an array of phosphor pixels as in a conventional CRT, the FED has millions of individual CRT-accelerated electrons crossing a vacuum gap and impinging upon a phosphor-coated screen to emit light.
Switching on blocks of emitters that comprise a pixel in a given sequence achieves the same effect as changing a selected pixel in a liquid crystal display. What's more, FEDs produce high brightness over the full range of color, but could require only one-tenth to one-half the power of a conventional liquid crystal display.
The main problem with FEDs has been that their fabrication requires a micromachining technology with the ability to pattern very small structures over large areas. The display's electron-generating field emitter tips are less than 100 atoms wide and must be made precisely and uniformly over the entire screen area. Now, only small-scale (1 sq. in.) FEDs can be produced by the extremely slow and expensive process of electron-beam lithography. Conventional photo-lithographic techniques, while capable of producing larger arrays (approximately 10 sq. in.) cannot produce sufficiently small emitters.
Citing U.S. firms' mediocre penetration into the critical flat-panel display market, the federal government formed the U.S. Display Consortium and assembled a White House Flat Panel Display Task Force. Both the consortium and the task force concluded that to develop a viable domestic flat-panel display industry, U.S. firms could either partner with an established Japanese manufacturer or "leapfrog" the technology with a new approach.
Leapfrogging the Competition
A leapfrog approach was demonstrated by a Lawrence Livermore team headed by Laser Programs physicist Michael Perry. The team perfected the process, called laser interference lithography, and they demonstrated its applicability to large (>2500 cm2) patterning. The process is expected to aid substantially in the successful commercialization of high-performance FEDs and enable the technology to capture a significant share of the flat-panel market.
Interference lithography has been used in a variety of other applications for more than 15 years, especially for fabricating diffraction gratings.* The technology offers the promise of low-cost, high-resolution, bright, and energy-efficient displays that are ideal for applications ranging from portable computers and instruments to virtual-reality headsets and large workstations. What's more, the technology may have direct applications to lower manufacturing costs of other products, such as computer memory chips.
The LLNL process can easily produce a high-density array of posts or holes 0.2 to 0.5 micrometers wide in a photosensitive material, perfect for creating densely packed and precisely arrayed patterns required for FED production. The technology allows the use of inexpensive substrates such as silicon and glass and works with proven photoresist materials and processes that are used in traditional lithography techniques.
Using Lasers to Produce Precise Patterns
The laser interference technique is based on the pattern produced by two interfering laser beams of a given wavelength. The standing wave interference pattern produces alternating light and dark fringes with a spacing determined by the angle at which the beams intersect. For a typical near-ultraviolet or violet laser operating in the range 0.3 to 0.4 micrometers, lines down to 0.2 micrometers can be fabricated, a resolution easily exceeding that required for FED manufacturing. With multiple exposures, essentially any pattern that can be formed by intersecting lines can be fabricated.
In order to apply interference lithography to array areas larger than 6,000-cm2 (1,000-in.2), the LLNL researchers further developed specialized techniques. For example, meniscus coating allows the substrate (such as silicon or glass) to be coated with the liquid photoresist solution to exactly the desired thickness.
Another technique that the team developed indicates when the pattern geometry is optimized. The growth of the features is monitored in real time during the development step. This process is critical because although the Livermore technology is relatively straightforward, many variables such as laser intensity, coating thickness, and temperature come into play simultaneously.
With the integration of these new fabrication procedures, the team has succeeded in fabricating 2,500-cm2 (400-in.2) arrays of submicrometer photoresist material suitable for the production of field emitters. The large arrays contain about 1 trillion submicrometer structures, with better than ±5% spacing uniformity.
Impressive Results Attracting Industry
The technical results have been so impressive that several major U.S. display producers and lithography vendors are collaborating with the LLNL development team. Some of these firms have successfully converted the pattern left by the photoresist material into functioning emitters by a series of etching and evaporation steps.
Team members say the new technique will find direct use in other applications requiring deep submicrometer patterns. The most significant may be a new method for the critical lithography steps in DRAM (dynamic-random-access-memory) chip manufacture, a $150-billion-per-year market. LLNL researchers are currently discussing the approach with major U.S. manufacturers to evaluate the DRAM application.
The first commercial products with FEDs manufactured with the Lawrence Livermore process may be high-resolution units for military needs such as in aircraft and ground vehicles. Somewhat later, FEDs should start appearing in such consumer products as portable and desktop computers and even flat-screen televisions with picture quality comparable to that from the best conventional cathode ray tube TV displays.
The laser interference lithography process is part of a much larger effort involving a dozen industrial collaborations working to advance flat-panel display technology, with funding provided by the Department of Energy, Department of Defense, and industry. All of the flat-panel efforts take advantage of LLNL expertise in lasers, optics, and materials science and state-of-the-art facilities.