Although some new display technologies for mobile computers are on the horizon, LCDs are likely to dominate for years to come
Chris Chinnock
Mobile-computer screens are roughly midway through a transition from monochrome to color. Five years ago, color screens on laptops were quite rare; five years from now, they will be not only commonplace, but taken for granted. Monochrome displays might still be offered on the lowest-end laptops in the future, and palmtop devices will cling to monochrome displays for a while longer. But for the most part, color screens will soon be the accepted standard, just as they are on desktop systems.
User demand for larger and brighter color displays seems relentless, and it poses great challenges for the designers who struggle to cram these screens into smaller and lighter mobile systems (which users
also expect to run longer on a battery charge). Newer display technologies could prove to be their salvation, but LCDs still have plenty of room for improvement and will continue to dominate the market for the immediate future. As new display and materials technologies evolve, prices will drop, and innovative combinations could lead to new categories of mobile-computing products.
LCD Is King
Today's LCDs are essentially valves that regulate the amount of light emitted from an internal backlight. This light passes through several layers of polarizers, liquid crystal materials, and color filters. A screen image is controlled by a grid of electrodes that determines how much light passes through each grid point, or pixel.
AMLCDs (active-matrix LCDs) have a transistor at each pixel site, while PMLCDs (passive-matrix LCDs) don't. These transistors provide better control over the light that passes through the grid, enabling more vivid colors and faster updates without ghosting
. This is particularly important for multimedia applications that use motion video. (See the table
"Comparing Today's LCD Technologies"
.)
Notebook computers represent a significant portion of the mobile-computing market, and both types of LCDs are almost universal in these systems. Passive displays cost less, of course, so they're generally found in lower-end notebooks. Active displays are likely to cost about twice as much as their passive counterparts for several years to come (see the figure
"LCD Price Trends"
).
Up to now, the vast majority of AMLCDs have been manufactured in Japan. High-volume manufacturing capacity is rapidly expanding--not only in Japan, but also in Korea and Europe. (The U.S. has only a few low-volume factories.) This expansion in Asia and Europe is being encouraged by the maturity and stable demand of LCD technology. "Customers have finally realized that active-matrix displays can do the job," says Dave Kuty, display project ma
nager at Apple. "They have moved past the `needing improvement' phase to the `acceptable' stage."
Steve Depp, manager of home electronics and the subsystem technical and applications lab at IBM Research, agrees. "The development of AMLCDs is beginning to look like silicon's. There's a technology road map showing what levers you pull at what time. That's a sign of a reasonably robust technology."
With so much money and brainpower being invested in AMLCDs, suppliers expect to make progress in all aspects of production: screen quality and size, power consumption, weight, and cost.
"The primary emphasis this year will be to shift the 10.4-inch VGA-resolution [640- by 480-pixel] screens to SVGA [800 by 600 pixels] and to add more bits of color," says Joel Pollack, senior product marketing manager at Sharp Electronics. "We are also talking about offering an 11.3-inch-diagonal display toward the end of the year."
There are many technical challenges involved in improving AMLCDs. For exampl
e, to pack more pixels into the same area, manufacturers are reducing the size of the transistors and line widths and are moving toward self-aligning photolithographic techniques. They are also developing improved fabrication processes and better test and repair procedures.
Many manufacturers are now on their third generation of fabrication equipment. The latest equipment reduces the number of steps required in the manufacturing process, resulting in faster, cleaner, and more uniform processing. The latest equipment can also process larger glass substrates. This permits more (or larger) displays to be processed on a single substrate. All these factors help to improve yields and reduce costs.
At the same time, both weight and power consumption are decreasing. "Today's 10.4-inch AMLCD is 11.5 mm thick, weighs 500 grams, and consumes 2.9 watts of power," explains Pollack. "By next year, we intend to get that down to 8 mm thick, weighing about 400 grams, and consuming less than 2 watts of power."
Many technical improvements are driving this progress, including more efficient backlights, better optical light guides, and more transmissive color filters. But the best gains appear to come from shrinking the transistors. This allows more light to pass through the LCD, so the screens can be brighter and also use less power. "A few years ago, I would have thought we were getting to the point of diminishing returns for AMLCD, but I now see steady progress being made throughout this year into next [year]," says Pollack.
The Future of Passive-Matrix
As mentioned earlier, passive displays still cost less and consume less power than active displays, so they're expected to command a considerable market share at the lower end of the notebook market. They will also dominate the market for low-cost hand-held devices. But as AMLCDs advance rapidly along the price/performance curve, manufacturers of PMLCDs will have to match those gains to remain competitive. Although they can take
advantage of many of the same advances in manufacturing and materials as AMLCDs, the manufacturing techniques for PMLCDs are quite different.
For example, PMLCD manufacturers must pay special attention to the thickness of the cell gap through which liquid crystal material flows, which is located between the two sheets of glass that form the display. Preparing the glass surfaces for this material is a particularly delicate process because any small variations are clearly visible on the screen.
It has taken some time for the Japanese manufacturers to solve these difficult problems. But as they gain better control of their processes, new options become available. For instance, a technique known as
electrically controlled birefringence
, or ECB, can produce a color PMLCD without filters. If the cell gap is precisely controlled, it's possible to obtain various colors by applying different voltages. This wasn't feasible before, due to variations in the thickness of the cell gap.
"Now Ja
panese manufacturers are routinely holding this thickness to 0.1 micron, which is actually flatter than the glass itself," says Apple's Kuty. "While ECB is still in the research phase, if we can get adequate speed and some color capability, this technology might make an excellent ultra-low-power reflective display in a few years."
Another challenge faced by makers of passive displays is the limited number of addressable lines in the LCD grid. To overcome this, most manufacturers are adopting a dual-scan approach that adds a second set of drive electronics to the screen.
Low contrast is yet another problem. Some liquid crystal materials yield high contrast, but at the expense of slower response. For example, rapid mouse movements cause the cursor to "submarine" (i.e., temporarily disappear), and ghost images ruin the playback of motion video.
New addressing schemes may be able to solve these problems. For example, Motif, a joint venture of In Focus Systems and Motorola, will soon introduce
passive displays that use a new technique called active addressing. This scheme updates the LCD rows semirandomly under the control of a special ASIC.
"Our first generation of products will carry a 30 percent to 35 percent price premium over traditional passive-matrix displays, but this buys you display performance that is near active-matrix levels," says Motif vice president Kevin Cornelius.
Motif has lined up an impressive array of strategic partners, including Asahi Glass/Optrex, Kyocera, Standish Industries, and Tottori Sanyo. Together, they'll promote active addressing in the marketplace. Motif will supply ASICs to its partners and receive display panels in return. All the involved parties can then combine the ASICs and panels to assemble their final products. (In late March, In Focus announced that it would buy back almost half of Motorola's shares in Motif and that Motorola expected to sell its remaining Motif shares. At press time, it was unclear what effect this would have on the rollo
ut of Motif's active-addressing technology.)
Other novel addressing schemes are likely to appear. Arithmos, a West Coast semiconductor start-up, is reportedly working on a passive-matrix technique called "transform addressing." In Japan, Asahi Glass is developing a method known as MLS (multiline selection). Motif's Cornelius says that MLS and
active addressing
are complementary technologies. "Both schemes increase response time and contrast, but MLS does not have a discrete gray-scale capability, which is necessary for displaying video images," he notes.
Another interesting addressing scheme, which is currently under development at Positive Technologies, has been dubbed
adaptive scanning
by company president Robert Hotto. "In standard passive-matrix addressing, each pixel is updated approximately 30 times per second, row by row," he explains. "But adaptive scanning modifies this update rate, customizing it for each row or group of rows. Higher-intensity pixels, or pixels that a
re changing rapidly--due to fast motion, for example--are updated more frequently than slow-moving or static areas. This smart-addressing method essentially optimizes the pixel-update rate to conform to the type of imagery being displayed."
Adaptive scanning works for both passive- and active-matrix LCDs. Positive Technologies is working on ways to use the technique in automotive displays as well as in hand-held computing devices for package-delivery services.
Additional advancements may come from experiments with new kinds of materials, such as
guest-host
liquid crystals. With these, dye is added to the liquid crystal to increase contrast. Some of these displays dramatically boost the brightness of the screen without the use of polarizers. When used in a passive-reflective mode, they consume little power, sometimes in the range of 50 milliwatts. Guest-host liquid crystals work with both active- and passive-matrix displays.
Ferroelectric LCDs have been under development for a lon
g time, especially at Canon. However, product introductions have been delayed for more than 10 years because of persistent problems with manufacturing and the mechanical instability of the liquid crystal material. Although prototypes are once again expected to appear this year, they face a skeptical audience.
"Ferroelectric LCDs require different driving electronics," says Apple's Kuty. "You just can't replace an active-matrix [display] with them. Unless they have some real advantage over active-matrix, people will be reluctant to invest the engineering time in them."
Emerging Markets
Today, the vast majority of mobile-computing devices are notebook computers, along with personal organizers and a few thousand PDAs (personal digital assistants). The market is likely to expand in coming years as new technologies enable the production of high-resolution displays that measure 3 inches (diagonal) or less. These displays will be the visual interfaces for communications devices
designed to be carried around or even worn on the body. This market is just now emerging and could become quite significant in five years.
For example, HMDs (head-mounted displays) are being touted as a new kind of interface for some types of portable computers. These aren't the full-immersion goggles associated with virtual-reality systems, but rather a single eyepiece that obscures only part of the user's field of vision. While these systems can be used for traditional applications, such as spreadsheets and word processors, new uses include the display of such things as technical manuals, blueprints, exploded diagrams, and even video clips that show how to, say, repair complex machinery (
see the photo
).
When HMDs are linked to voice-activated computers worn on the body, the result is a system that gives workers hands-free access to volumes of information from almost anywhere. For instance, a helicopter-repair technician equipped with an HMD wouldn't have to lug a heav
y technical manual to the top of an aircraft to work on the rotor blade. Technicians working in cramped quarters could view schematics right on the repair scene, possibly even getting advice from a supervisor monitoring the situation through a small video camera and a wireless communications link. Factory workers could view a virtual template that helps them assemble complex components. CPSI and InterVision are among the companies developing systems for these types of applications.
New kinds of projection devices are coming, too. There's already a clear shift under way from CRTs to LCDs for projection panels. Most of today's projection devices are merely peripherals for laptop or desktop computers, but a few are starting to acquire some computing capabilities of their own. For example, In Focus Systems already offers a projector with a built-in floppy drive that doesn't have to be tethered to a computer to show a presentation.
Communications devices will benefit from improved display technologie
s as well. Portable phones will probably integrate small displays and enough computing power to enable the user to view E-mail messages, news bulletins, and other information.
The first device to explore this territory is the Simon, which was introduced last year by BellSouth Cellular and designed by IBM. The Simon is a combination cellular phone/PDA that includes an x86-compatible CPU, a fax modem, a PCMCIA Type II PC Card slot, 11 built-in programs, and a 4 1/2- by 1 1/2-inch touch-sensitive LCD screen. Better displays will almost certainly lead to more hybrid devices like the Simon.
New Types of LCDs
By experimenting with different types of silicon, manufacturers are now producing new LCDs that have greater resolution than before, as well as improved electrical characteristics. Some of these LCDs are better suited for the tiny displays in miniaturized devices, while others show promise for the full-size, direct-view screens on laptop systems.
The AMLCDs used in
today's laptop computers are fabricated with
amorphous silicon
. This type of silicon requires discrete ICs for the off-screen electronics, which increases the display's weight, bulk, cost, and power consumption. These ICs are made in traditional silicon foundries from
single-crystal silicon
, which has excellent electrical characteristics. Another type of silicon is
poly-silicon
, which isn't as efficient as single-crystal silicon but is better than amorphous silicon.
All the on-screen transistors and off-screen electronics that are made for displays out of single-crystal silicon and poly-silicon can be fabricated simultaneously on a single substrate. This not only improves reliability and cuts costs but also offers some performance advantages. The result is a display with lots of pixels in a small area.
One company that's trying to commercialize single-crystal LCD technology is Kopin, which recently established a plant for mass production. "Right now we are shipping 1
1/2-inch VGA monochrome display toolkits that allow systems designers to evaluate the displays, but 1280- by 1024-pixel displays will follow soon," says Jeff Jacobsen, Kopin's vice president for business development. "Last May, we received an ARPA contract to demonstrate an AMLCD with 2560 by 2048 resolution. No one has done anything like that before," he adds.
LCDs based on poly-silicon have many of the advantages of single-crystal-silicon LCDs, but their poorer electrical characteristics may prevent them from achieving the highest density displays. The viewfinders in camcorders often use this type of LCD.
Manufacturers such as Hitachi and Seiko-Epson are large-volume suppliers of poly-silicon LCDs, but that situation is changing. Camcorder manufacturers recently established their own poly-silicon lines, so outside suppliers are looking for new market opportunities, such as HMDs and projection devices. Hitachi and Seiko-Epson have increased the resolutions of their viewfinder displays and, alon
g with Sony, are now sampling VGA-resolution products. A U.S.-based company named Sarif (a joint venture of In Focus and the David Sarnoff Research Center) was recently formed to commercialize poly-silicon displays.
Other companies are developing new technology for larger, direct-view displays based on poly-silicon. Current fabrication techniques use a high-temperature process that requires expensive quartz substrates, because the heat would warp a flat glass panel. Consequently, the goal is to find a low-temperature process. ARPA is currently funding this development in the U.S. "It will be two to three years before low-temperature processes are commercialized, but pilot line production might begin within 18 months," explains ARPA program manager Dave Slobodin.
Display technology is evolving rapidly on many fronts. And each advancement potentially enables the development of new kinds of mobile-computing devices. Although it's difficult to predict what these devices will look like, or even what
they will do, today's mobile computers stand to benefit as well. As Sharp's Pollack puts it, "In five years, I think we'll be looking at a whole new paradigm in display technology. Displays may look the same, but it won't be today's technology."
WHERE TO FIND
Apple Computer, Inc.
Cupertino, CA
(800) 776-2333
(408) 996-1010
fax: (408) 996-0275
Arithmos
Santa Clara, CA
(408) 982-4480
fax: (408) 982-4481
Asahi Glass Co.
Tokyo, Japan
+81 3 3218 5250
fax: +81 3 3287 1047
BellSouth Cellular
Atlanta, GA
(800) 746-6672
(404) 705-1880
fax: (404) 717-2111
CPSI
Fairfax, VA
(703) 631-6925
fax: (703) 631-6734
IBM Research
Yorktown Heights, NY
(914) 945-3000
fax: (914) 945-2141
In Focus Systems, Inc.
Wilsonville, OR
(800) 294-6400
(503) 685-8888
fax: (503) 685-8887
InterVis
ion
Raleigh, NC
(800) 850-2556
(919) 850-2511
fax: (919) 850-0562
Kopin Corp.
Taunton, MA
(508) 824-6696
fax: (508) 822-1381
Kyocera
Kyoto, Japan
+81 75 592 3851
Motif
Wilsonville, OR
(503) 682-7700
fax: (503) 682-7036
Motorola, Inc.
Northbrook, IL
(708) 480-6842
fax: (708) 205-3872
E-mail:
g10839@email.mot.com
Optrex
Tokyo, Japan
+81 3 5688 8265
fax: +81 3 5688 8270
Optrex America
Plymouth, MI
(313) 416-8500
fax: (313) 416-8520
Planar America
Beaverton, OR
(503) 690-1100
fax: (503) 690-1244
Positive Technologies
San Diego, CA
(619) 457-5151
fax: (619) 457-4646
E-mail:
postech@powergrid.electriciti.com
Sarif
Vancouver, WA
(206) 750-0242
fax: (206) 750-0244
David Sarnoff Research Center
Princeton, NJ
(609) 734-2000
fax: (609) 734-2221
Sharp Electronics
Mahwah, NJ
(800) 237-4277
(201) 529-8200
fax: (201) 529-8413
Standish Industries
Lake Mills, WI
(414) 648-1000
fax: (414) 648-1001
Tottori Sanyo Electric Co.
Tottori, Japan
+81 857 21 2001
fax: +81 857 21 2034
DISPLAY COLOR VIDEO
TYPE QUALITY RESPONSE CONTRAST PRICE POWER
Active-matrix **** **** **** High **/****
Passive-matrix ** * ** Low **/****
Active-addressing *** *** *** Moderate **
Key:
Excellent ****
Very Good ***
Good **
Poor *
illustration_link (10 Kby
tes)
photo_link (23 Kbytes)
Small, high-resolution LCDs will enable a new class of head-mounted displays that give hands-free access to volumes of technical data.
Chris Chinnock is a technology writer based in Norwalk, Connecticut, who specializes in emerging and cutting-edge technologies. He can be contacted on the Internet at
76061.3671@compuserve.com
or on CompuServe at 76061,3671.
Color to Go
