Researchers at Lawrence Livermore National Laboratory (LLNL, Livermore, CA) are producing arrays of tiny holes or dots without masks with optical inteference lithography. Such holes and dots, which are tiny built-up areas, could be used to make field-emission displays (FEDs). Now that they have demonstrated the engineering feasibility of the process, LLNL researchers are evaluating how the technique can be commercialized and integrated with other FED manufacturing methods.
In interference lithography a laser beam is split into two beams and recombined at an angle onto a photosensitive material, exposing an interference pattern. Constructive and destructive interference produces a pattern of very fine lines. This technique, originally developed in the 1970s, is still used to make gratings today.
Recently, scientists in the inertial confinement fusion program at LLNL scaled up the technique to permit exposure of large-area gratings for pulse compression of high-power lasers. There are now three operational exposure facilities at LLNL. One uses an argon-ion laser at 351 nm for process development. A krypton-ion laser at 413 nm in another facility can expose plates up to about 10 in. in diameter. The largest facility also uses a krypton-ion laser but can work with 1-m plates.
Field-emission displays work similarly to cathode ray tubes (CRTs), except they are flat. Instead of having a large electron gun that is swept across a phosphor-coated screen, FEDs have an array of millions of tiny electron emitters. In one type of FED architecture, a thousand to perhaps ten thousand emitters make up a single pixel. Each emitter is on the order of 1 µm in diameter. Pixels in the display are then addressable with row and column voltages. An array of colored phosphors is mounted in close proximity to the emitters to form a display.
FED Corp. (East Fishkill, NY) worked with LLNL researchers to develop the process for making FED holes and dots (see photo). First, the plate is exposed in the interference lithographic system to form a series of lines, then rotated 90° and exposed again. The intersection of these two sets of lines forms a series of tiny holes. By adjusting the laser wavelength and angle of intersection, the periodicity of the lines can be varied.
Andrew Hawryluk, deputy program manager of LLNL`s advanced microtechnology program, says, "We can expose features with a periodicity from a few tenths of a micron to several microns." Although interference lithography has the advantage of not needing a mask to define the features, it is limited to the type of structures it can produce. "The technique is good for gratings, zone plates, and arrays of holes, dots, or lines," says Hawryluk.
Commercializing the technique for FED production still faces many challenges. Hawryluk points out that at this stage they have shown that the process can be scaled u¥to make exposures over large areas, which is a necessity for displays. Also, the exposure can be done in about a minute, which is compatible with commercial throughput requirements. However, questions remain as to the optimization, cost-performance characteristics, and market potential for a commercial instrument.
For example, manufacturers want to be sure they can sell more than a couple of machines before they spend a lot of money on development. But to answer this, questions on the configuration of the lithographic system must be answered. The choice of lasers is one. Most photoresist work is centered on g (436 nm) and i (365 nm) lines of mercury arc lamps, so Hawryluk thinks it makes sense to pursue those wavelengths. Higher-power lasers would mean faster throughput. Also, the size of the equipment must be be reduced. Today, the large exposure facility is 12 × 18 ft, but Hawryluk hopes to get that down to the footprint of a standard lithographic system. Estimates of the cost of an optical interference lithographic tool range from $500,000 to $2 million.
The technique may also be appropriate for surface preparation in LCD manufacturing. "Rubbing of the surface to orient the liquid-crystal molecules is a rather uncontrolled brushing operation today. Interference lithography could produce much more uniform lines for this application," says Jones.