Solid State Devices 4B6
Solid State Devices
4B6
Lecture 13 – Projection and 3D displays:
LCD, DLP and LCOS
Daping Chu
Lent 2015
Liquid crystal display
Liquid crystal display (LCD)
A 105 inch TFT-LCD 4k2k curved panel by Samsung
January 2014
A 105 inch OLED 3k2k curved panel by LG
January 2014
A 98 inch TFT-LCD QUHD panel by BOE
March 2014
A 152 inch plasma 4k2k 3D panel by Panasonic
October 2010
$600,000
Sky is the limit?
Liquid crystal display
Typical liquid crystal molecules
A Chemist’s view:
Methoxybenzilidene Butylanaline (“MBBA”)
p-decyloxybenzylidene p'-amino 2-methylbutylcinnamate ("DOBAMBC")
A Physicist’s view:
Liquid crystal display
Some liquid crystal phases
Isotropic
Nematic
Smectic A
External electrical field can induce dipoles in liquid crystal molecules, resulting their
alignment. Some liquid crystal molecules may even have a ferroelectric phase. For
example, the chiral nature of DOBAMBC permits the molecule to exhibit a “ferroelectric
phase”. The size of the polarization and the response of the molecule to an applied
voltage depends on the chemical structure of the molecule, largely the carbonyl [C=O]
group near the chiral carbon.
Liquid crystal display
Polarising filters
Liquid crystal display
Effect of twisted nematic liquid crystal
After 1st polarizer
Through liquid crystal
The two arrows represent the directions of the
polarisations of the components of left and
right circularly polarized light at a particular
point in time. After passing through the liquid
crystal, the two components sum to create
linear polarization in a different direction,
resulting in a non-zero emission from the 2nd
polarizer.
Liquid crystal display
Colour and intensity adjustment
Average voltage
for off-pixel
Average voltage
for on-pixel
Response curve of a twisted nematic LCD cell.
The range of voltage (and therefore brightness)
felt by on and off pixels is much less than the full
range possible.
Liquid crystal display
Fabrication of amorphous Si TFT pixel transistor
Liquid crystal display
Cross section of a TFT-LCD cell
Liquid crystal is filled into the gap between bottom TFT substrate and top colour filter
substrate using fast vacuum filling process.
Liquid crystal display
Energy efficiency
Input energy
0.5%
Liquid crystal display
Different types of LCDs
Reflective TFT-LCD with internal Diffuser
Trans-Reflective TFT-LCD
Reflective TFT-LCD with Front Light
Transmissive TFT-LCD
Liquid crystal display
Active matrix TFT-LCD
As liquid crystal material is dielectric in nature, each LCD
cell has the property of a capacitor under external bias.
Liquid crystal display
Driving AM-LCD
Liquid crystal display
Effect of storage capacitor
Actually liquid crystal should be driven by AC voltage. For active LCD, the voltage is only
supplied in the state when switch turning on, then the switch turns off immediately. In some
cases, the liquid crystal voltage will fall due to voltage leakage, stage by stage. To avoid
this situation, we can use one parallel storage capacitor to make up for the leakage voltage.
The larger the capacity of Cst is, the closer the voltage waveform will be to the square wave.
Liquid crystal display
HTPS LCD panels
HTPS is an abbreviation of High Temperature Poly-Silicon, an active matrix transmissive LCD.
It's superior in that it is smaller in size, has higher resolution and higher contrast, and can embed
drivers on panel. The main function of HTPS is as a light valve for projectors.
HTPS has a thin-film transistor (TFT) generated by poly-silicon in each pixel. These pixel transistors
act as a conduction switch by changing the scan line's voltage. They are produced in the same way
as semiconductors. They are small and highly reliable because they can easily be miniaturized
(pixelation or high open area ratio) and drivers can be generated on substrates by processing at a
high temperature.
Liquid crystal display
High brightness – Aperture ratio
The aperture ratio is between the transparent area and the whole pixel area, excluding the
pixel's wiring area and the transistor area (they are usually hidden by a black matrix). The
higher the aperture ratio is, the more efficient the light penetrates the panel.
The use of micro fabrication technology in HTPS and optimisation of the wiring and element
design can improve the open area ratio by reducing the black matrix area, increasing the
projector's luminosity. For example, the aperture ratio is improved by more than 10% with the
0.7 inch XGA (1024 x 768 pixels).
Liquid crystal display
High brightness – Micro lens array (MLA)
LCD (HTPS) panels have transparent areas and lightproof areas. This technology has
improved the brightness of panels between 1.5 and 1.6 times.
Liquid crystal display
High brightness – Integrated lens
This lens technology projects the whole screen brightly by transmitting light evenly from the
first lens array to the second lens array.
Liquid crystal display
High brightness – Polarisation changer unit
Lamp emits light waves with variety of orientations, but HTPS transmits only longitudinal waves.
Polarizer technology converts shear waves to longitudinal waves so that the light can penetrate
through HTPS as much as possible. This technology improves brightness by about 1.5 times.
Liquid crystal display
Dichroic mirrors
Two dichroic mirrors are used to divide the light from the lamp into red, green, and blue. The base
glass of these two mirrors is coated with a thin film that reflects only light of a specific wavelength.
Liquid crystal display
Dichroic prism
A dichroic prism is used to recompose a colour image from red, green and blue images. The prism
is formed by combining four triangular poles to create one rectangular solid. High precision is
required in the processing and adhesion of poles to avoid dark lines and double images.
Liquid crystal display
High brightness – 3LCD projector system
– using dichroic mirrors and dichroic prism
3LCD is the most widely used projection technology system in the world. 3LCD technology uses
three high-temperature polysilicon liquid crystal displays (HTPS LCDs), hence the name 3LCD.
Liquid crystal display
Inside a 3LCD projector unit
Liquid crystal display
LCD panel classification
Using HTPS TFTs instead of a-Si TFTs in LCD panel for projectors is mainly because (a) n-/p-types
are possible using HTPS and only n-type is possible using a-Si and the mobility of HTPS TFTs is
much higher than that of a-Si TFTs (>100:1), hence, integration of drivers on panel is possible; (b)
HTPS are much more stable than a-Si, particularly when operating for long time at high temperature.
Digital light processing
Digital mirror device (DMD)
At the heart of every DLP® projection system
is an optical semiconductor known as the
Digital Micromirror Device, or DLP® chip,
which was invented by Dr. Larry Hornbeck of
Texas Instruments in 1987.
The DLP® chip is probably the world's most
sophisticated light switch. It contains a
rectangular array of up to 2 million hingemounted microscopic mirrors; each of these
micro-mirrors measures less than one-fifth the
width of a human hair.
When a DLP® chip is coordinated with a digital
video or graphic signal, a light source, and a
projection lens, its mirrors can reflect an alldigital image onto a screen or other surface.
The DLP® chip and the sophisticated
electronics that surround it are what is called
the Digital Light Processing™ (DLP)
technology.
Digital light processing
Digital light processing – Timer dithering for grey scales
A DLP® chip's micro-mirrors are mounted on
tiny hinges that enable them to tilt either
toward the light source in a DLP® projection
system (ON) or away from it (OFF)-creating a
light or dark pixel on the projection surface.
Electrostatic
attractor electrode
Tilting mirror
Si substrate with conventional circuits
The bit-streamed image code entering the
semiconductor directs each mirror to switch on
and off up to several thousand times per
second. When a mirror is switched on more
frequently than off, it reflects a light grey pixel;
a mirror that's switched off more frequently
reflects a darker grey pixel.
In this way, the mirrors in a DLP® projection
system can reflect pixels in up to 1,024
shades of grey to convert the video or graphic
signal entering the DLP® chip into a highly
detailed greyscale image.
Digital light processing
Digital light processing – Interlaced colours
The white light generated by the lamp in a
DLP® projection system passes through a
colour wheel as it travels to the surface of the
DLP® chip. The colour wheel filters the light
into red, green, and blue, from which a singlechip DLP® projection system can create at
least 16.7 million colours. And the 3-chip
system found in DLP Cinema® projection
systems is capable of producing no fewer than
35 trillion colours.
Time dithering for both colour selection (RGB)
and intensity (grey scales).
The on and off states of each micromirror are
coordinated with these three basic building
blocks of colour. For example, a mirror
responsible for projecting a purple pixel will
only reflect red and blue light to the projection
surface; our eyes then blend these rapidly
alternating flashes to see the intended hue in
a projected image.
Digital light processing
1 Chip DLP projection
Digital light processing
3 Chip DLP projection
Digital light processing
Bio-application of DLP – Advantage of UV capability
A Digital Optical Chemistry (DOC) System is a unique UV light projector that can be used to
manufacture biological/chemical arrays using UV photochemistry or semiconductors using
standard photoresist chemistry.
Digital light processing
Bio-application of DLP – Some examples
Generated digital mask
High resolution image
A chip that was custom
made to re-sequence the
cancer gene
The DOC system can manufacture high density oligo arrays for DNA re-sequencing or expression studies. These
arrays can be readout using a fluorescent scanner, including the UTSW MAGNA scanner or commercial units. The
advantages are: fast to make a unique mask using “digital masking”, high density array (> 2million pixels), small
size machine and optical readout can be done using standard commercial equipments.
Simple comparison of LCD and DLP
AM-TFT-LCD:
Majority transmissive type – considerable light loss.
Driver integration on panel.
Direct image display possible – flat panel displays.
Response speed limited but multi-levels.
Back light energy efficiency problem.
Mature technology – low cost, but low yield for large size.
DLP:
Reflective type – less light loss.
Separated driving circuitry.
Image only formed through projection.
Fast response possible but binary.
Complicated fabrication, packaging and yield problems.
Relatively new process – higher cost.
Liquid crystal on silicon
Liquid crystal on silicon (LCOS)
Aluminium
Edge
Glass Cover
Contact
ITO
Pattern
Glue
Seal
Silicon Chip
Mirrors
Liquid crystal on silicon
Merits of LCOS
Planarised (flat) surface
(CMP process, filled vias)
Highly Reflective Pixels, > 91%
(Thin top metal layer, low temperature
process for small grain size and good
reflectivity)
Small inter-pixel gap
(Good Fill-factor, low light-loss)
Anti-reflective coating on buried
metal layer
(To give good light shielding for
underlying circuits)
Small silicon feature size
(For high-speed loading and small
inter-pixel gap)
Additional High Voltage Process
(To drive liquid crystal )
Liquid crystal on silicon
LCOS – Amplitude vs Phase-only
Gen 1: Amplitude
HDTV projection
Gen 2: Phase-only
Holography
Phase-only
– highest possible efficiency
+
Real-time
– ultimate image manipulation
holography
An ideal phase
hologram can
manipulate light beams
without
loss of a single photon.
Displaying images using phase is the future.
Liquid crystal on silicon
Holographic projection – 2D
Phase-only
hologram
LCOS SLM
screen
Diffraction
diffraction
Replay
field
Input form laser
CUED Photonics & Sensors Group
Liquid crystal on silicon
Holographic projection – 3D
“An updatable holographic three-dimensional display”, Nature, 8 Feb 2008
CUED
Photonics & Sensors Group
and
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