Information Display (Dec 2002)

A New Angle on ReflectiveDisplays
lridigm has demonstrated something truly different: a direct-view MEMS
device that creates a reflective color display by optical interference.
by M. Miles, E. Larson, C. Chui, M. Kothari, B. GaIly, and J. Batey
Ww.
T THE WORLD NEEDS NOW
is a betterdisplay technologyfor portable
applicationsthat is lessexpensive,consumes
lesspower,andprovidesfull color. As digital
informationbecomesmore portable,thereare
moreopportunitiesto incorporatedigital displaysinto our daily lives. In addition to
imagequality, two main obstaclesstandingin
the way of broaderacceptanceare size and
cost. The sizeand weight of a portabledevice
arelargelydeterminedby the amountof
powerthat it consumes,and for most devices
the displayconsumesthe lion's shareof the
power. If the display is mademore efficient,
then lessbatterypower is requiredto provide
the sameoperatingtime, so smallerand
lighter batteriescan be used. But even if an
innovativedisplay technologycould deliver
powersavings,it would haveto be inexpensive to produceandintegrateinto products.
LCD-basedsolutionsfor reflective displays
haveimproveddramaticallyin recentyears,
gainingin both brightnessand contrast. The
rateof progressappearsto haveslowed,
however,anddevelopersare now forced to
achievegainsby improving the characteristics
Marlc W. Miks is the Founderand Chief
TechnologyOfficer of lridigm Display Corp..
2415 Third Street,Suite235. SanFrancisco.
CA 94107:telephone415/626-8800xJO2.fax
415/626-9775,e-mail: [email protected].
URL www.iridigm.com.Theother authors
havevariouspositionsat lridigm. This article
is basedon a paper presentedat the 7th Asian
Symposiumon Information Display (Society
for InformationDisplay, 2002: ISBN 981-046983-7).
of ancillary componentssuchas anti-reflective coatingsor by addingbacklightsto
achievetransflectivemodesof operation.
Bistabledisplaysbasedon liquid-crystal and
other materialsshow somepromise.but have
beenslow to reachfull-scale productionand
commercialavailability.
Iridigm Display Corp. is currently developing a microelectromechanica1
systems
(MEMS) approachto the problemwith a technology they call interferometricmodulation.
Their new devicesare basedon a family of
structuresreferredto as interferometricmodulators,or iMoDsTM.Thesemodulatorsoffer
high brightnessandcontrastand. unlike other
MEMS, are suitablefor large-areamanufacture and application. The simple structures
provide the functionsof modulation.colorselection,and memory.while eliminating
components such as active matrices, color
filters, and polarizers. The result is a highperformance display that provides activematrix functionality at the cost of supertwisted-nematic liquid-crystal displays
(STN-LCDs).
iMoOTMTheory and Structure
lridigm's iMoDsTM are viewed through the
substrate upon which they are fabricated.
Individual structures are binary and, for color
applications, switch between black and reflective red, green, or blue states. The concept
exploits the combined attributes of MEMS
and thin-film optics. MEMS provide the ability to modulate a microscopic structure at high
speeds, while consuming very little power and
demonstrating inherent hysteresis. Thin films
modulate incident light with high efficiency.
Fig. 1: An attractive characteristicoftM iMoDTMis its very simplearchitecture.
26 InformationDisplay 12AJ2
0362-09721O211812.026S1.00 + .00 C sm 2002
Color-Display Prototype
Iridigm Display Corp. hascreatedthe fmt
color direct-view reflectivedisplayto use
MEMS technology(Fig. 3). This displayconsistsof 240 x 160color pixels at 100dpi; the
resulting arraycontainsnearly2 million
iMoDsTM. Color-pixel dimensionsare257 x
257 J.IDl.The pixels aredivided into red,
green,and blue color stripes. Eachsubpixel
hasa different air-gapspacing,which producesthe desiredcolors(Fig. 4). The sizeof
the gap is controlledby the useof sacrificial
spacersof different thicknessesthat are
removedduring the fabricationprocess.
Entirely driven by commerciallyavailable
STN driver componentsin a passivematrix,
the display is addressedby'scanninga line at
a time using drive voltagesof approximately
II V. It could be driven with muchlower
voltagesif suitabledriver componentswere
available. The lower iMoDTMactuation
voltagesprovidethe potentialfor lower costs
and power consumptionin future models.
Optical Performance
IrkIgm Dt8pIay Corp.
Fig. 2: Thismonochromedisplay maintainedits imagefor more than 10 hours with thepower
disconnected.
demonstratingits bistableoperation.
The resulting family of devices are planar
microscopic structures that can achieve black.
white, and colored reflective states based on
the size of the air gap within this optically resonant cavity (Fig. I). Because of their small
size, response times of several microseconds
and sub-5-V drive waveforms have been
demonstrated. Early designs have been
improved by reducing the size of the support
structure - which increases the active area and the development of a color process.
Current fill factors are approximately 70%.
Based on the dimensions of the structure,
resolutions of 300+ dpi (mono) or 200+ dpi
(color) can be fabricated with the existing
lithographic tools used in LCD toolsets.
The performance of the iMoDTM is such that
it lends itself quite readily to display applications, particularly those in which incident
light and power are scarce. Power consump-
I tion in these devices has been measured to
be hundredsof microwattsper squareinch,
evenat video rates. Becausethe device
exhibits memoryand high resolution,gray
scalecan be achievedvia frame-widthmodulation and/orspatialdithering. This allows
for a greatdeal of flexibility in the kinds of
applicationsthat may be addressedand
options in the tradeoffsa display designer
can make.
iMoOsTMare also capableof true bistable
operation. The electronicchargecan be readily manipulatedand maintainedin the array to
the point at which written imagerycanpersist
indefmitely. Imageshavebeendemonstrated
to persistfor tensof hours(Fig. 2). Although
completeremovalof power is unlikely in
mostpracticalapplications,the iMoO's
bistablenaturecanplaya role in reducing
powerconsumption.
The optical performance of the lridigm panel
is superior to commercially available state-ofthe-art polysilicon reflective LCDs, such as
those found in "pocket PC" products. Comparing the white-state performance of one
such LCD with the Iridigm panel shows that
the iMoDTM display is approximately twice as
reflective (Fig. 5). This is due to its efficiency
in modulating light by means of optical interference and the elimination of polarizers. In
these tests, both displays were measuredwithout intervening components, such as a touch
screen or front light. Optical models indicate
that this performance difference may be
increased as the iMoDTM design is refined.
The iMoDTM panel also works well at different v:ewing angles. Unlike the image on
most LCDs, the Iridigm panel's image does
change in contrast or brightness as the
viewer's angle changes. Viewability is limited only by the off-axis reflectivity of the
intervening components; the viewing angle
of a bare iMoD TMdisplay easily exceeds
60°.
The analysis of the optical performance of
several different reflective media provides
additional perspective on the role of brightness and contrast in reflective displays. Traditionally, display measurement - both emissive
and reflective - has emphasized the value of
contrast ratio as an indicator of absolute perInfomUJlionDisplay 12102 27
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MEMS displays
.1
l'
IrtdIgm DI8pI8y Corp.
Fig. 3: This iMoD Matri?" color display has a resolution of240 x 160pixels.
formance. For reflective displays,brightness
may be a bettermetric.
A comparisonof the brightnessand contrast
of severalstaticandactivereflective display
mediarevealsan interestingpoint National
Geographicmagazinearguablyrepresentsone
28 InformationDisplay I W2
of the higheststandardsin reflective media
and providesa userexperiencethat easily
transcendsthat of commerciallyavailable
reflective thin-film-transistor (TFI') displays.
Contrastmeasurements
were 16:1 for the
magazineand 15:1for the TFr display.
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Fig. 4: Thepixels are divided into red, green,and blue color stripes. Each subpixelhas a dilferem air-gap spacing.which producesthe desiredcolors.
an active matrix, polarizers,or color filters, so
the materialscostsshouldbe lower andthe
processlesscomplexthanthosefor LCD production.
Work is under way to rapidly advance the
state of the art of this new display, and
improvements in fill factor and contrast ratio
are already being incorporated. Continued
reductions in drive voltages will allow the use
of CMOS-based drive circuits, providing further decreases in cost, system complexity, and
power consumption. Further improvements
are also in the works: gains in brightness, contrast, gray scale, and refresh rates are possible.
The result should be color displays with
paper-like whites and little or no color shift
that are suitable for full-color video cellular
phones, photo-quality PDAs, and ultra-Iowpower e-books. Large formats such as
HDTVs may be possible in the future.
Advanced prototypes showing many of these
improvements are to be made during 2003.
It is not easy for a new technology to
unseat an established one, especially one that
is as deeply entrenched as are LCDs in the
portable-device market. Because of the
potential for lighter, thinner, brighter reflective displays with lower power consumption
and a lower production cost, we believe
iMoDTM displays show promise of being a
viable competitor in a wide variety of highresolution full-color display applications. .
Fig. 5: Whenwhite-stateperforma1l(;eis measured.the iMoD Matrix"" color display outperforms a polysilicon reflectiveLCD.
InformationDisploy IW2
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