TECHNOLOGY ON DISPLAY

As with any industry in which the profit lies in volume, the focus in the display industry has shifted to the development of production-line technology that will bring better, more competitive displays to market.

Today, liquid crystal displays (LCDs) are assembled by treating the glass face sheet with a polyimide thin-polymer film to anchor the liquid crystal (LC), then brushing the resultant coated sheet with a special cloth to align the LC molecules. Physically brushing the film, however, leaves defects on the glass surface, which reduce screen quality. Such defects can prove especially grievous to high-pixel-count displays while slowing down production lines and increasing unit product cost. A noncontact solution would be far more acceptable, lending itself to speedy processes and fewer defects. Researchers have investigated ultraviolet (UV) curing and ion-beam bombardment methods to fix the polymer and align the LC material.

At several IBM locations, including California, New York, and Yasu and Yamato, Japan, researchers have taken the first commercial step toward automating the LC anchoring step using a linear ion-beam gun and scanning stage. First, using chemical vapor deposition (CVD), researchers apply a structurally ordered, 3- to 15-µm thick diamond-like carbon (DLC) thin film to the glass cover. Next, in a separate vacuum chamber, they bombard the film with a directed ion beam. Researchers expect that for commercial systems, both processes could be completed in the same chamber.

As an alternative to polyimide films and mechanical alignments (right), ion-beam bombardment (left) minimizes defects.

In comparison with the polyimide anchoring method, prototype displays fabricated using ion bombardment show a significant decrease in yield problems related to the uniformity of the carbon film or brush tracks. The ion/DLC procedure also aligns the LC material to ±0.5° necessary for the advanced in-plane switching (IPS) LC displays, as well as an adjustable (0° to 10°) pre-tilt angle for the LC material in twisted nematic LC displays.

Organic-light-emitting-diode (OLED) technology offers the prospect of flexible displays on plastic substrates and roll-to-roll manufacturing processes. One of the biggest challenges to the OLED display industry is not competition from the incumbent LCD industry but from simple water and oxygen.

The materials involved in small molecule and polymer OLEDs are vulnerable to contamination by oxygen and water vapor, which can trigger early failure. The obvious solution, plastic encapsulation, is not as easy as it seems. Plastics are permeable materials, often with holes many microns in diameter that allow water and oxygen to reach the OLED material. Experimental results predict that to achieve a 10-year device lifetime, encapsulating material for OLED displays cannot allow more water vapor than 10^-6 g/m²/day (at 38°C and 90% RH).

The problem is bigger than just developing new materials. As the saying goes, you can't make it if you can't test it, but current instrumentation can only detect the presence of water and oxygen at 10^-3 g/m²/day. Now Battelle National Labs (Battelle, WA) spinout Vitex Systems (Sunnyvale, CA) has teamed with Philips Research (Eindhöven, The Netherlands) and others to develop new test methods using a thin layer of water-reactive calcium metal to verify that Vitex's flexible multilayer plastic material passes water vapor and oxygen between 10^-6 g/m²/day and 10^-7 g/m²/day.

To fabricate the barrier material, Barix 200, engineers deposit a monomer vapor on a plastic polymer substrate. The liquid monomer fills in the peaks and valleys, and a UV light polymerizes the layer. Next, they deposit a layer of aluminum oxide (AlO₂) a few hundred angstroms thick on the polymer surface. The group repeats the process five times until the entire substrate measures approximately 2-µm thick and has permeability characteristics as good as those of glass, according to the recent Philips/Vitex findings.

"It's also much cheaper and faster than encapsulating the OLED in a glass lid or can," explains John McMahan, vice president of sales and marketing. "It's as fast as the fastest OLED production tool, [which produces] about a panel a minute. Our process will keep up with that. Metal cans take about four or five minutes per can; with our process, the throughput of the production line goes way up."

One unknown remains, however. In the study, both Barix-encapsulated and glass-encapsulated control samples used adhesive to sandwich the LC material between two layers. In production, Vitex says it would deposit the AlO₂ layer in the Barix beyond the monomer layer, creating a seal. What sort of permeability this process would permit as smooth monomer layers fall back to pitted plastic is unknown. Compared to the difficulty of developing Barix, though, it seems a surmountable challenge. oe