Never Too Thin
Thanks to advances in flat panel display technologies, prices are falling and performance is rising. As a result, flat displays are in the thick of things.
To paraphrase the Duchess of Windsor, displays can never be too thin or too rich. Flat
panel displays take up less room and consume less power than their cathode-ray-tube
kin. However, in the past, flat panel displays were too expensive and their image
quality too poor for many applications. Advances in technology have changed all
“In 2002, we had flat panel displays at
$29 billion revenue worldwide, and that’s growing to $62 billion by 2006,”
said Barry Young, vice president of the Austin, Texas-based flat panel display market
research and consulting firm DisplaySearch.
Most flat panel displays today employ
liquid crystal display (LCD) technology but other methods — notably organic
LEDs (OLEDs), micromirrors and liquid crystal on silicon — offer some advantages
and potential. Organic LEDs, for example, may allow displays that can be rolled
up like a piece of paper. Liquid crystal on silicon could power headsets that give
doctors a three-dimensional close-up during endoscopic surgery. Micromirrors can
not only provide large-screen, high-definition digital color, but also might enable
high-speed optical switching.
But before such possibilities can become
commercial reality, a few challenges must be met. Organic LEDs need to grow in size
and solve some material lifetime and performance issues. Micromirrors need to cut
costs, and liquid-crystal-on-silicon displays must take advantage of their relatively
high pixel density. A look reveals each technology’s promises and pitfalls
as well as how LCDs are fighting back.
Improving the standard
At present, there’s little for LCDs to battle.
Analysts report that LCDs of various types capture more than $4 out of every $5
spent on flat panel displays worldwide. One implementation, the active-matrix LCD,
accounts for 75 percent of the total market. In an active-matrix configuration,
thin-film transistors, capacitors, resistors and other semiconductor elements sit
beneath each and every pixel and actively switch it on and off.
Stanford Resources is a San Jose, Calif.-based
research firm specializing in the electronic-display industry. Kimberly Allen, director
of technology and strategic research, noted that any flat panel display that measures
more than 7 or 8 inches across is likely in the active-matrix LCD category. “LCDs
are essentially unchallenged in the areas of notebooks and monitors and small flat
panel TVs, those less than 30 inches in size,” she said.
Many factors lie behind this LCD dominance,
and these have an impact on the prospects for other flat panel display technologies.
The first, and probably most important, element is ongoing cost reduction, which
is driven by manufacturing innovations and economies of scale. Intertwined with
this, but also distinct, are such secondary aspects as improvements in the thin-film
transistors and the liquid crystal itself.
The steady decline in the cost of a
liquid crystal display has been due to a steady gain in manufacturing know-how.
Display manufacturers speak of generations and mother glass. In fabricating an active-matrix
display, manufacturers deposit transistors on a slab of mother glass, cut it up
into pieces, and attach connections to the outside world. This is then capped with
another piece of glass held off by spacers. The edges are sealed and the gap filled
with liquid crystal material. A backlight is often added as a light source.
Most of the initial fabrication is
done in cleanrooms comparable in size, cost and capability to leading-edge semiconductor
facilities. In the case of flat panel display manufacturing, however, the state-of-the-art
substrate size is quite a bit bigger than the 300-mm-diameter silicon wafers found
in the most advanced semiconductor factories. Japan’s Sharp Corp., for example,
has announced a seventh-generation fab that uses an 1800 x 1500-mm piece of glass.
“Put Shaquille O’Neal on
the diagonal, and there’s three inches of headroom. That’s how big it
is,” said Joel Pollack, vice president of the display business unit for Camas
Sharp Microelectronics of the Americas in Camas, Wash.
Besides better manufacturing know-how,
Sharp and other vendors, most of which are located in Japan or Taiwan, are working
on thin-film transistor innovations. Because these transistors are deposited on
glass, they must be fabricated at a lower temperature and using a different base
material than that employed to make standard semiconductors. Traditionally, this
has meant a performance hit and the necessity of using off-glass electronics.
To combat this, Sharp has developed
what it calls continuous-grain silicon, which reportedly boosts the switching speed
of the transistors on the glass. Because it allows more of the display electronics
to be built on the glass itself, this could lead to higher-performance, less expensive
and more reliable displays. Putting more electronics on the glass also reduces the
number of connections that need to be made to the outside world. This eliminates
a bottleneck, allowing greater pixel density and higher information content on smaller
Sharp is also pursuing transflective
LCDs. Besides a backlight, such displays use shiny thin-film-transistor regions
to create a reflective display. Careful engineering of the LCD cell gap allows the
total amount of liquid crystal material traversed to be the same in either transmissive
or reflective modes. In the former, light transits the cell once, while in the latter,
it crosses it twice. According to Pollack, this optical engineering allows fabrication
without compromises to contrast or transmissivity. He added that the display looks
the same despite moving from bright sunshine to full darkness.
Future so bright?
While LCDs dominate the middle of the display
spectrum, new contenders are nibbling at both edges. Organic LEDs are targeting
small screens, such as those found in cameras, cell phones and cell phone subdisplays.
The latter are used to present caller ID and other information.
Organic LEDs break down into several
competing variants. Eastman Kodak Co. of Rochester, N.Y., has a small-molecule version,
Cambridge Display Technology Ltd. of Cambridge, UK, a polymeric version, and Universal
Display Corp. of Ewing, N.J., a phosphorescence-based one.
What an organic LED offers over an
LCD is the ability to directly generate light. This capability eliminates the need
for a backlight and the associated extra thickness, weight and power drain, which
can be particularly important in cell phone and other mobile applications. Organic
LEDs also have wider viewing angles, lower overall power consumption, faster image
response, higher contrast, greater immunity to temperature extremes and, according
to adherents, eventually lower manufacturing costs than LCDs. Despite these advantages,
proponents of this technology are not complacent.
“We have a very active chemistry
group that is developing next-generation enhancements,” said Dan d’Almeida,
vice president of sales and marketing of the Display Group. “We envision a
100-times improvement in a combination of power, lifetime and brightness over the
next two to three years.”
Kodak has the largest commercial organic
LED display presence. Full-color active-matrix displays, which are based on the
company’s technology, have just appeared in one of its digital cameras (Figure
1). According to d’Almeida, this type of display enhances the consumer experience
by providing a large viewing angle and good contrast. That, in turn, enables a more
faithful reproduction of the image actually captured by the camera.
Figure 1. Eastman Kodak recently employed its active-matrix organic LED technology in one
of its digital cameras.
All organic LEDs consist of a stack
of thin organic layers sandwiched between electrodes (Figure 2). When an appropriate
voltage — typically a few volts — is applied, both positive and negative
charges are injected into the light-emitting layer within the organic LED cell.
These charges recombine in the emissive layer to produce light. That process can
be improved by tailoring the materials used and by doping the emissive layer with
a small amount of highly fluorescent or phosphorescent molecules.
Figure 2. Organic LEDs are
built using a stack of thin organic layers sandwiched between electrodes. Courtesy
of Eastman Kodak.
On the other hand, the polymeric method
can literally coat a surface employing low-cost, solution-based processes such as
spin coating and ink-jet printing, taking advantage of these other methods to manufacture
displays (Figures 3 and 4).
“That’s one of the very
strong attractions of this technology, the idea of actually printing displays or
lighting devices,” commented Stewart Hough, vice president of business development
at Cambridge Display Technology.
Figure 3. Displays, such as this
one from Cambridge Display Technology, are made using light-emitting
Today’s organic LEDs are, of
course, less than perfect. Manufacturers of these devices have started small because
of the production challenges associated with scaling up the displays. It’s
easier to build a cell phone screen than one that goes in a laptop or television.
Vendors are investing heavily in the hope that their products will follow the same
upward trend in size that LCDs took.
Other problems with these devices include
material lifetime and finding the right emitters, particularly in the blue. A few
years ago, there was some skepticism that any organic LED could last long enough
to be useful in even a short-lived consumer product. That has been answered by demonstrations
of tens of thousands of hours of life by several materials. A more subtle form of
the same problem is the different aging of red, green and blue emitters. Today,
vendors compensate by carefully balancing the size of the different subpixel components.
Figure 4. The structure of a polymeric organic LED includes a
light-emitting polymer layer. Courtesy of Cambridge
These problems are being attacked through
ongoing research and development at all of the companies involved. Investigators
at Princeton University in New Jersey and the University of Southern California
are associated with Universal Display. A few years ago they developed phosphorescent
organic LEDs, which are said to be more efficient than their fluorescent counterparts.
The technique is being developed and has already borne some fruit.
“We’ve been able to demonstrate
that 100 percent of the electrical input into a device can be converted into light
output,” explained Janice Mahon, vice president of technology commercialization
at Universal. “We’ve demonstrated some world-record power efficiencies
through phosphorescent OLED technology.”
Mahon added that her company has been
able to create well-saturated reds and greens that are highly efficient and have
greater than 10,000 hours of operational life. For now, this has been done only
with the small molecule approach, but she said that the company hopes eventually
to do the same in a solution-processible, and thus printable, formulation. Another
challenge lies in creating a long-life blue with good saturation and efficiency
characteristics. It’s a problem that faces the entire organic LED industry
to some degree, no matter the technology, Mahon said.
Despite these issues, the market for
these displays is expected to expand substantially. Analysts peg the annual growth
rate at 65 percent per year over the next five years or so.
Upon further reflection
As organic LEDs strive to penetrate the small
end of the LCD domain, the other extreme is facing its own contenders. Standard
direct-view LCDs stop at sizes of 40 inches or so. Large-screen LCD televisions
of 50-plus inches have been demonstrated, but poor mother-glass yields make such
devices more proof of principle than anything else. So for very large screens, there
are a few other flat — or, perhaps more accurately, thin — display technologies
currently being employed.
Plasma displays are one such. These
are truly flat, emissive, bright and colorful. They’ve also been around for
years. However, the consensus is that plasma displays are too expensive for widespread
consumer acceptance and possibly of too low a quality compared with other approaches.
An alternative to plasma displays is
a whole class of what might be called thin displays. These use optics to magnify
a small microdisplay into one that fills an entire screen and so are not flat. Optical
constraints force them to have a ratio of screen size-to-cabinet depth.
“About a quarter to a fifth of
the screen size is about how deep the cabinet will initially be,” said Sandeep
Gupta, CEO of MicroDisplay Corp. of San Pablo, Calif. Future improvements and optical
engineering innovations, he said, should lead to even smaller ratios of cabinet
MicroDisplay, along with such companies
as Three-Five Systems Inc. of Tempe, Ariz., is pushing liquid crystal on silicon
(Figure 5). In this approach, silicon is used for the back plane, so all imaging
is dependent on reflected light.
Figure 5. This image chip was built using liquid-crystal-on-silicon
technology. Using projection optics, it will be enlarged to fit a 50-in. TV screen.
Courtesy of MicroDisplay Corp.
Another consequence is that the pixel
density can be quite high. Pixel sizes of 12 μm are routine, and it’s
possible to get devices with pixels as small as 8 μm. What’s more, the
gap between pixels is measured in fractions of a micron. Three-Five Systems’
vice president and chief technology officer Bob Melcher noted that, as a result,
the image is smooth. There is no screen-door effect that is found in some transmissive
display technologies where dark spaces between the pixels are visible, much like
the wire in a screen. On the other hand, the tight packing and tiny pixel size can
lead to imaging chips that are almost too small for the other system components.
“As you make the device smaller,
they can be made at a lower cost without sacrificing performance. However, the smaller
they are, the more difficult they become to illuminate efficiently,” said
Today, liquid-crystal-on-silicon microdisplays
from all vendors are a small fraction of the total sold. That may change in the
future, but for now, a different microdisplay technology, called digital light processing,
or DSP, from Texas Instruments Inc. of Dallas is a significant and rapidly growing
part of the overall market. According to analysts, this company has captured 25
percent of the large-screen-projection market, a category that includes televisions
and business gear. That figure will grow to 33 percent in the next three years.
Digital light processing devices use reflection off miniature aluminized mirrors.
On command, the mirrors tilt and send a spot of reflected light into and out of
the optical path. This effectively creates a digital image.
The latest generation of digital micromirrors
measure 13.8 μm center to center. Ian McMurray, European marketing manager
for Texas Instruments’ digital light processing products, said that this
represented a nearly 20 percent reduction in mirror spacing from the devices introduced
a few years ago.
The company also has enhanced the contrast
by coating the substructure of the mirrors with a light-absorbing material. Other
improvements have been made to supporting circuitry to allow faster and more complex
signal processing. Finally, the capabilities of the mirrors have undergone changes.
“They feature a 12°, rather than a 10°, mirror-tilt angle. The effect
of this is to increase brightness by, typically, up to 20 percent,” McMurray
He added that the manufacturing process
has recently transitioned to a larger wafer size and that the company has entered
into agreements with another firm for manufacturing assistance. All of these moves
are designed to help meet what it sees as rapidly rising demand.
However, for all the flat panel display
advances and the resulting rapid growth, it’s important to keep in mind that
the cathode-ray tube is not dead. IDC, a market research firm based in Framingham,
Mass., has predicted that in 2003 flat panel displays will, for the first time,
pass the venerable tube in terms of computer monitor revenue. Next year, IDC predicts,
the displays will pass tubes in terms of units sold.
The same can’t be said for televisions.
Plain vanilla direct-view cathode-ray tubes make up about 95 percent of the units
shipped, according to Stanford Resources’ Allen. The reasons are the same
as those behind the acceptance of any other display technology — and a reminder
of what all flat panel displays have to strive for.
As Allen observed, “In the TV
world, what matters are what it looks like in the store and how much it costs. That’s
how consumers buy TVs. The CRT is perfect — it looks great and it’s
cheap. What’s not to like?
Flexibility Is a Must
Organic LEDs promise flexibility, but that potential is hard to realize.
The problem lies in the materials that make up these devices. They’re flexible
in that they can bend and move, but they’re inflexible in their intolerance
of water and oxygen, which can damage them, leading to black or inactive spots on
the screen. As a result, today’s organic LED display is sealed by two pieces
of glass sandwiched with a desiccant interlayer.
It’s this glass that makes the display as
unyielding as the one found on every laptop. For true flexibility, the glass must
be eliminated, which is what Vitex Systems Inc. of San Jose, Calif., would like
The company has a vacuum polymer deposition
process that puts down a 3-μm coating of alternating polymers and ceramics.
The result, according to company officials, is a thin, flexible film that offers
the moisture protection of a piece of glass and that can be put down right on top
of an organic LED.
“When you’re finished,
you can come right out into atmosphere, and it’s protected,” said John
McMahon, Vitex’s vice president of sales and marketing.
For small displays, this means that
a single piece of glass is sufficient. That effectively cuts the thickness of the
display in half and trims the weight by a like amount. There are also cost savings,
according to McMahon. All of these are important considerations in cell phones and
in other mobile applications.
But the same technique, he noted, can
be extended even further. The coating can be done on a piece of plastic, leading
to what Vitex calls flexible glass. Depositing an organic LED and the associated
electronics on such plastic and then encapsulating it would lead to a truly flexible
Near — Not in — the Eye of the Beholder
Microdisplays can do more — or actually
less — than make a wall-size image. They also can make one considerably smaller.
But, thanks to the magic of optics, such an image can appear to be a full-size one
floating in space. These are so-called near-to-eye applications because they require
the user to peer through some sort of goggles or glasses.
This approach has a number of possible applications.
One is as a mini-beamer, a small device that would project a cell phone or personal
digital assistant display onto a piece of paper. In this way, people could wirelessly
surf the Web without having to work through a tiny screen.
Another possible near-to-eye application
is in ultraportable DVD players. Other uses of the technology involve the military
or biomedical fields.
A near-to-eye liquid-crystal-on-silicon microdisplay, such as the one in this conceptual
drawing, allows the viewer to see a full-size image in a portable product. Courtesy
of Three-Five Systems.
“You can build a 3-D stereo binocular
headset so that a surgeon actually gets a feel for depth during endoscopic surgery.
That’s not possible with a CRT or a standard direct view display,” said
Three-Five Systems’ vice president and chief technology officer Bob Melcher.
Such applications, he readily admitted,
have not existed before. So the technology is developing along with the markets.
He also acknowledged that many of the initial markets will be small but could provide
high value to, and command a high premium from, potential customers.
While any microdisplay manufacturer
could pursue such markets, many have chosen not to. This is true even for those
with liquid-crystal-on-silicon-based products. MicroDisplay, for example, specifically
does not target near-to-eye use, instead choosing to concentrate on big-screen televisions
and home entertainment systems — applications with existing and large markets.
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