by Debra Vogler, Senior Technical Editor, Solid State Technology
Replacing traditional TFT-LCD manufacturing with a lower-cost alternative that is also a more efficient technology is (forgive the pun) a bright idea that should be attractive to manufacturers. Uni-Pixel is aiming to fulfill this wish with its time-multiplexed optical shutter (TMOS), a thin-film membrane (Fig. 1) that oscillates in and out when in contact with a lightguide. Leveraging support from Philips’ open innovation center in The Netherlands, Uni-Pixel recently announced the successful assembly of six display prototype devices.
Unlike TFT-LCD technology in which the panel is back-lit, TMOS panels are edge-lit (so the light source is on the edge), Tassone told WaferNEWS. In a TMOS system, light is injected into the lightguide (or a sheet of TFT mother glass) and maintained in the lightguide by way of total internal reflection (TIR) (Fig. 2). “By injecting light into the lightguide, our objective is to create a uniform dispersion within it, and we ensure that the coupling at any given pixel is detuned so that individual pixels do not extract too much light too early,” said Tassone. “Each individual pixel’s coupling efficiency is tuned down to a level where it allows light to travel throughout the lightguide without being lost too early, or too much, at any given point.”
Fig. 1.Micrograph of TFT backplane with Opcuity film bonded on top. Source: Uni-Pixel [Opcuity is a trademark of Uni-Pixel]
Control over the maintenance of the TIR angles over time is achieved by precision injection in the z-dimension (the lightguide/glass plane forms the x/y dimensions), Tassone explained. Some light is lost due to scattering, but at insertion the light is controlled at specific angles and maintains them as it travels through, until it finds an open pixel, couples out, and is extracted to the viewer.
While there is some degradation (loss of light over time and distance traveled within the system), the individual light bursts are in such a short amount of time and are exhausted very quickly, that the amount of degradation is not significant enough to affect the overall performance of the system.
Fig. 2.Conceptual cross-section of a single pixel actuated during a red sub-frame. Source: Uni-Pixel.
According to Tassone, a TMOS system is ~61% efficient, meaning that ~61% of the light that is injected into the system is ejected. “Because of the detuning of the individual pixels, any given light photon ensemble will only lose a portion of its energy in the first bounce,” he explained. “That ensemble, or portion of that ensemble, is available to pixels downstream, so it’s a multi-pass system where light is recycled until it is used or exhausted.” In contrast, today’s LCDs are about 5%-8% efficient, meaning that ~5%-8% of the backlight makes its way through the system to the viewer in a single pass, he noted (Fig. 3). The company’s models show that a TMOS system can be tuned to get an efficiency >61%, but uniformity is adversely affected as a result, Tassone noted.
Fig. 3.Comparison of TMOS and LCD output based on light losses per layer. Source: Uni-Pixel.
A major advantage of a TMOS-based panel is that polarizers, color filters, and liquid crystals, which make up a significant portion of the bill-of-material (BOM) costs, are not needed. Tassone said the company’s model shows that, depending on formfactor, its technology can achieve 40%-60% lower BOM costs than an LCD panel.
The thin film itself is made up of a carrier substrate, similar to a mylar PET film, although the company is also examining other materials. Micro-optic structures are added to the thin film followed by a conductor layer. “Each individual pixel is a “drumhead,” and mating the thin film to the TFT glass creates the “drum” — so the MEMS structure is the drum created by adding the thin film to the mother glass, explained Tassone. “That thin film is now complete in the sense of what a manufacturer needs to integrate it into a panel,” he said.
Because the TMOS-based panel uses a limited number of LEDs for light insertion at the edges, the panel is power efficient — models show on average, ~90% less power consumption than in an LCD system, said Tassone. “So you have a much higher light efficiency in transmission at a much lower power.” An example of how this combination would be useful is in a handheld device, where a daylight-readable cell phone could still consume very little power, thereby extending battery life.
Two key challenges were overcome in the TMOS prototypes and will be further optimized as the company develops a volume manufacturing process: ensuring a sufficiently low pixel-drive voltage, and stiction mitigation. The capacitor structure (the conductor on the thin film and the conductor on the TFT glass) is driven by the voltage differential, which creates the Coulomb attraction that actuates the pulling action on the thin film. “At the individual pixel level within this capacitor structure, the distance between the conductive plates is directly tied to the voltage required to drive the pixel,” noted Tassone. “The challenge is making sure that these two conductors are close enough so that the voltage is low enough to be compatible with current TFT manufacturing.” The company says it has created a system in which the two conductors are close enough so that the voltage required to actuate is <20V. "If you look at LCD manufacturing, there are falloffs in yield if there is an actuation >20V,” noted Tassone.
With respect to overcoming stiction, Tassone told WaferNEWS that a “little bit of stiction is good” as it provides for good light coupling (when the thin film is in contact with the lightguide). However, stiction has to be overcome when the membrane needs to be released so the device can go into the off-state. He said the company’s prototypes show that its methods overcome stiction, and the challenge now is to migrate them into high-volume manufacturing, “and we believe we have a path to do that.”
The business model
The company will license the overall architecture and various aspects of the subsystems, and will also supply the single thin film. The films are delivered to the company’s licensees in either sheets or rolls; alternatively, fabs/panel manufacturers can license the recipe. TMOS-based panels can be built in existing LCD fabs using existing TFT-LCD equipment.
The company is currently negotiating with licensees and also considering JDPs with various LCD manufacturers. “These agreements will allow us to place projects into R&D development fabs that will allow us to transition to production fabs,” said Tassone. “The timeline for roll-out will be tied to these agreements.” The company is targeting having systems enter the market in late 2009, and “we see no significant hurdles toward compatibility with production,” Tassone said. — D.V.