Digital Paper display technology holds promise for portables

Digital Paper display technology holds promise for portables
SAN FRANCISCO — A new technology called Digital Paper may give liquid-crystal displays some competition. Engineers are using micromechanics to create the alternative technology at Iridigm Display Corp. Digital Paper technology has already demonstrated reflectivities of more than 80 percent, and experimental systems have achieved contrast ratios of 12:1, viewing angles of ±60° and drive voltages less than 5 V.

In addition, Iridigm says new displays can be made using fabrication processes already developed for the LCD industry, which means displays should be cheap. Major OEMs have already expressed interest in using the new technology, including Palm Inc. (Santa Clara, Calif.), Motorola Inc. (Phoenix) and Handspring Inc. (Mountain View, Calif.).

"Displays based on this technology are capable of both color and black-and-white performance with the potential to achieve 1,000-dot-per-inch resolutions," said Mark Miles, chief technology officer at Iridigm, based here. He said resolution is "limited only by driver chip count."

Another advantage, Miles said, is that the basic component of Digital Paper — the Interferometric Modulator, or IMod — "requires only the simplest of driver and controller electronics due to the binary nature of the IMod itself. This, in conjunction with the single-substrate fabrication, makes Digital Paper particularly attractive to hardware makers trying to reduce system costs and size." Miles said that "the high speeds of the basic IMod device provide a path to bring video-rate imagery to handheld and other portable electronics devices."

The main applications under consideration are reflective displays for portable electronic devices, he said, a product area that includes advanced cell phones, Palm-like organizers, Internet appliances and electronic books. Miles called this new class of consumer products — which require low-power, high-performance displays — "the least well-served" by today's LCD industry.

Superior image

Dave Blakely, smart-products studio leader at Ideo Inc. (Palo Alto, Calif.), which is responsible for the external design of the Palm V, is considering the Iridigm scheme for use in personal digital assistants. "Noticeably higher resolution, higher contrast and lower power are the main advantages of Iridigm's display technology," said Blakely. "The better resolution and contrast combine to produce a more pleasing and superior-looking image than any LCD."

The IMod component that drives Digital Paper differs from devices such as Silicon Light Machines' Grating Light Valve, which recently caught Sony's interest as licensee. In the GLV, the diffraction is caused by surface relief structures, working in much the same way as embossed holograms like those on credit cards.

To combine the desired diffractive orders and eliminate unwanted colors, these products require a relatively sophisticated optical system. Plus, the angle of view is narrow. For these reasons, development has been geared around the projection display market, where the cost of the optical system and the geometrical restriction can be accommodated.

The IMod, on the other hand, uses interference in a way that has much more in common with reflection holography. The device consists of a conducting metallic membrane, a thin-film stack protected by an insulating layer and a transparent substrate.

The stack and metal membrane act as two mirrors in an optical resonator. The optical distance between them determines which wavelengths constructively interfere with each other, and consequently are reflected by the device, and which destructively interfere and are absorbed. Where no current is flowing, the optical distance between the two mirrors is determined by the thickness and refractive index of the insulator and the height of the membrane.

When the device is switched, however, the current causes the membrane to be electrostatically attracted to the insulator, stack and substrate. When it makes contact, the optical distance drops to that of the insulator, and the color changes. Using this method instantly solves some of the problems inherent to diffractive systems. In particular, there is almost no "rainbow effect," where the color changes with viewing angle.

The IMod device can operate at low power thanks to its inherent hysteresis, caused by the fact that restoration force of the membrane is linear, whereas the electrostatic force opposing it is nonlinear. As a result, if the voltage is kept at the correct level — somewhere between 3 and 5 V — the device only needs to be switched when its state is actually required to change. It will maintain that state until it is switched again. According to Rickson Sun, Ideo's director of research and development, this is another factor that makes the technology very attractive for PDAs.

"Extremely low power consumption in standby modes is especially interesting if we consider possibilities like personalized standby screens," said Sun. "How many times have you put your PDA on a table next to several identical PDAs? Iridigm's displays could be left in standby mode with a user-customized screen that help PDAs and their owners find each other."

Credibility gap

Consultant Charles McLauglin agreed that the Iridigm technology looks good on paper, but he is not convinced that manufacturing it will be easy. "The question is can they deliver on the potential," he said. "There are two gaps of credibility for me. First, I need to see and measure a prototype to gauge the challenge of delivering hardware that comes close to the theoretical maximum. Second, can the displays be manufactured?"

Achieving high yields on any display technology is far from easy, McLaughlin said, "and MEMS-based approaches have been extremely difficult. Clearly Iridigm is years and hundreds of millions of dollars away from a high-volume, low-cost product. Who knows if they can overcome the manufacturability hurdles."

Iridigm's Miles agreed there are problems to be overcome but does not accept the proposition that new technology is necessary to do so. The IMod concept "exploits the phenomenon of interference by the use of thin-film stacks incorporated into the device," he said. "Consequently, manufacturers must monitor film-thickness uniformity to a greater degree than usual. Film stress is also an important parameter to be monitored: The residual stress of the mechanical films impacts the electromechanical behavior of the fundamental IMod device and thus must be tracked."

Manufacturing, said Miles, "is expected to occur using existing LCD lines around the world. But while such manufacturers may not be used to tracking and controlling these parameters, the techniques exist, are mature and are well used in the IC and thin-film optics industry. The challenge will be in educating manufacturers about these techniques."

Brightness advantage

The potential gains are large. "The exceptional efficiency of interference makes possible theoretical IMod reflectivities approaching 100 percent," said Miles, compared with "the immediate 50 percent loss experienced by LCDs due to the inherent polarizer." This provides the technology with "a potential two to three times brightness advantage over LCDs. In general, LCDs remain hampered by their polarizers, color filters and other compensation films required to achieve modulation and color, significantly reducing their light-handling efficiency."

Another boon to the user will be the high resolution, made possible because the micromechanical devices are so small: just 40 to 30 microns currently. According to Ideo's Blakely, however, this may be a double-edged sword. "Another potential problem is the catch-up needed in computing power of PDAs to take full advantage of the increased resolution," he said. "With megapixels on your PDA you will need much more than today's Dragonball processors to fully exploit the display."

Iridigm has produced a number of test displays using the first-generation design, including a Palm Pilot-size screen. In these systems, tens or hundreds of IMods were used within each pixel. Though most of the demonstrations so far have been monochromatic, the company says that, using conventional RGB additive color mixing, full-color displays are a straightforward extension of the existing technology.

Iridigm has already fabricated and tested a second-generation design. Here, a large membrane is raised up using a number of support posts, rather than having lots of small separate devices. The new design not only supports a fill factor of up to 95 percent, but it has also has good long-term behavior, according to the company. In tests, it has been shown to be reliable for a number of switch cycles equivalent to using a Palm Pilot four hours a day over 18 years.