January, 2009
CLEANROOM NEWS
Over the university intersession, we were busy making improvements to the cleanroom. In particular,
all of the stainless steel tables in the cleanroom have been resurfaced, and new material handling
policies implemented to prevent future staining and corrosion. Some equipment reorganization has
taken place, with more to follow in the coming month.
The Class 10,000 South Area has been reorganized and now includes the TNP Instruments Laser
Repair Station, JA Woollam Spectroscopic Ellipsometer, Tencor Profilometer, LC Technologies
liquid crystal filling station, VPI scriber, Olympus polarized transmission microscope, and light
box/driver circuits. We hope to install an Ocean Optics Spectrometer in the next few weeks for
quick cell gap measurements.
The Brewer GX100 Large Area Spincoater has been returned to Brewer for repair/overhaul.
Electrical issues have been the primary problem. In the mean time, large plates can still be processed
using the Headway IT22 Large Area Spincoater.
New tooling is being designed for use with the Clean Air Systems Wet Bench. The new tooling will
accommodate 6” and 7” glass sizes, for photolithographic processing using the in-bench tanks. Training
sessions for usage of the new setup will begin in February.
The GenVac Aerospace SC1 ion-assisted e-beam coater will be addressed in the coming weeks. At
this time it is not functional. Ion gauges and several cooling fans will be replaced, and staff will
troubleshoot the ion gun control system and repair safety interlocks.
If there are particular pieces of equipment that are of interest to you, or if you would like to see
particular capabilities added to the room, please contact Doug Bryant.
PROCESS SPOTLIGHT: Glass Substrates for LCDs
Nearly all liquid crystal devices made today are made on glass substrates. Glass has a variety of
properties that make it a good substrate choice, including chemical resistance, ease of handling, barrier
to moisture and gas permeation, reasonably high processing temperatures, mechanical stability,
reasonable cost, good transmission, no birefringence, etc. Despite years of hype for plastic substrates as
a glass replacement, materials issues have prevented direct substitution in all but a few niche markets,
where flexibility or curved surfaces are required, and mostly in polarizer free displays where
birefringence is acceptable. This may change in coming years, but for now glass is the substrate of
choice.
There are a wide variety of glass types available today with properties optimized for just about any
application. Soda lime glass accounts for the largest volume of glass, due largely to its use in
architectural and automotive windows. It also sees application in passive matrix LCDs. Active matrix
LCDs are typically fabricated on borosilicate or aluminosilicate (alkali free) glass, which has suitable
thermal properties. In the following sections, we will describe the different types of glass substrates
used for liquid crystal devices.
Soda lime glass
Soda lime glass is made via a float process, developed by Pilkington in the 1950s. Raw materials (silica
sand, calcium oxide, soda ash, and magnesium) are mixed in a furnace at 1500°C, then continuously
poured onto a 1000°C bath of molten tin. The viscous glass does not mix with the tin, but rests on top,
where surface tension forces cause the glass to settle into a uniform thickness—sheet thickness is
controlled by the rate of travel. As it cools to 600°C, the ribbon becomes rigid enough to pass onto a set
of rollers, where it passes into an annealing oven, or lehr. The annealing process carefully controls
temperature in order to relieve stresses in the glass. After annealing, the substrates can be cut to sheet
form. The surfaces of soda lime glass are called “fire finished”, and do not typically require grinding or
polishing.
Table 1, Figure 1. Composition of soda lime
glass, from Pilkington website.
Glass
Material Composition Reason for Adding
(Percent)
Sand
72.6
-
Soda Ash
13.0
Easier melting
Limestone
8.4
Durability
Dolomite
4.0
Working &
weathering
properties
Alumina
1.0
-
Others
1.0
-
Figure 2.
Schematic of the float glass process (from http://www.tangram.co.uk/TI-GlazingFloat%20Glass.html )
Despite the surface quality, there is typically some detectable corrugation to the glass, which may be
visible upon inspection with a monochromatic light source. This is due to the formation process, and
typically does not pose a problem for the low end passive LCDs that are usually fabricated from soda lime
glass. High end passive displays may require polished soda lime glass.
For display applications, soda lime glass is a very affordable substrate, with good flatness and optical
properties. A number of manufacturers produce soda lime glass for display applications, with available
thicknesses of 1.1mm, 0.9mm, 0.7mm, 0.55mm being most common.
Issues with soda lime glass
There are several major problems with using soda lime glass for LCDs:
The first issue is sodium ion contamination. Over time, sodium ions in the glass itself can diffuse into the
liquid crystal cell. Ionic contamination of liquid crystals can lead to ghost images (image sticking, due to
trapping of free ions at one surface) and even galvanic corrosion, and this effect can worsen with time.
To prevent this, a barrier layer (usually SiO2) can be vacuum deposited on the glass before ITO
deposition, effectively blocking the ions from reaching the liquid crystal. This adds a small cost to the
substrates, but since the transparent electrode ITO is also vacuum deposited, both coatings can be done in
a single in line process. For most passive matrix LCD applications, this barrier coating is sufficient to
overcome the ion problem.
The second major issue is temperature, specifically thermal expansion. During the manufacturing
process, glass substrates are heated to temperatures of as much as 500°C (for polysilicon TFTs). Relative
to the newer glass types below, soda lime glass has a much higher coefficient of thermal expansion.
Processes carried out at different temperatures can result in misalignment of patterns, and high
temperature processing can cause shrinkage of the glass substrate. More on temperature effects later.
Borosilicate glass
Borosilicate glasses (such as Corning’s 7059 glass) eliminate some of the problems inherent in soda lime
glass. These types of glass are sometimes referred to as being low alkali, as it is alkali earth ions that are
the biggest problem for AMLCD manufacturers. A common way to make glass with low alkali content is
to use boric oxide to replace the calcium and magnesium oxides in the glass. Glass with a high
percentage of boric oxide typically has better thermal and chemical resistance for LCD processing.
Corning pioneered the fusion process for formation of low alkali content display glass (NEG uses a nearly
identical process they refer to as overflow). Molten glass flows into a trough made of a refractory
material, then flows over two edges of the trough, and down the sides. The refractory trough tapers to a
ridge on the bottom, at which point the two glass flows recombine or fuse to form a single sheet of glass.
This sheet is then drawn downward. Because the two faces of the glass never touch the trough, the
surface quality is very high, and polishing is not required.
Figure 3. Schematic of fusion or overflow process; both glass surfaces are untouched, eliminating the
need for polishing (from NEG website)
Boro-aluminosilicate and other alkali-free glass
Most of the display glass used today has a higher alumina content, which again gives better high
temperature performance. These materials can be made by a float process and polished (manufacturers
such as Asahi Glass or NH Techno) or by the fusion process (Corning, NEG). These glass types are
much harder, and require much more attention during scribe and break than soda lime glass. Much of
today’s glass is supplied in thicknesses considerably less than one millimeter, typically 0.7mm or 0.5mm.
By going to thinner glass substrates, not only is the weight of the end device reduced, but sag during
substrate handling is also minimized. Since today’s large generation size glass can be up to 2 meters
square, handling becomes a huge consideration. Minimizing glass density and thickness can reduce
problems here.
More on temperature effects
When assembling large glass panels (today’s LCDs are primarily manufactured on glass panels 1 to 2
meters on a side), even small differences in processing temperatures can leave small features displaced
from one another on top and bottom glass. This restricts the accuracy with which a black matrix can be
aligned to a TFT array—by making the matrix larger to accommodate misalignment, the clear aperture
(and thus brightness) of the display is reduced. Table 2 is based on data from Corning (reference 7); the
offset indicated is for a 1°C difference in processing temperature between the TFT and color filter plate:
Table 2. Effect of thermal expansion for various display glasses (data from ref. 7)
Glass Type
CTE (x10-7/°C)
Offset between TFT
and CF plates for
680x880mm glass
(microns)
Soda-lime glass (typ.)
90
10
Corning 7059
46
5.1
Corning 1737F
37.6
4.2
Corning Eagle2000
31.8
3.5
Density (g/cc)
2.53
2.76
2.54
2.37
In October 2007 at the FPD International show, Samsung’s exhibit included six 19” SXGA panels
fabricated on soda-lime glass (see ref. 8). Samsung has not indicated how the TFT LCDs were fabricated,
but low temperature TFT processing was certainly necessary. Industry experts have previously indicated
that a return to the much cheaper soda-lime glass might be possible in the 2011 or 2012 timeframe, so this
could be a huge breakthrough. Charles Annis of DisplaySearch has estimated that switching to soda-lime
glass could save 6% on the total cost of a 32” VA mode display.
Table 3. Liquid Crystal Display glass substrate market share
LCD Market Share, 2003
Area Capacity
Corning
Samsung Corning Precision
Subtotal Corning Family
Sq. Meters
13,710
13,710
27,420
Asahi Glass
13,392
Nippon Electric Glass
Sq. Feet (Mil)
148
148
295
Dollars (Mil)
595
580
1,175
Yen (Mil)
68,770
67,036
135,807
Share
26%
26%
52%
144
581
67,175
26%
5,124
55
217
25,054
10%
NHT
3,636
39
156
18,008
7%
Others
Total
2,820
52,392
30
564
122
2,251
14,145
260,190
5%
100%
Source: DisplaySearch, Banc of America Securities estimates
Environmental Concerns
Although LCD glass does not pose the lead hazard that CRT glass does, it still has historically contained
heavy metals such as arsenic and antimony. In the past 5 years, there has been increasing pressure on
manufacturers to make their materials more environmentally friendly, both in the manufacturing process
and in waste disposal. Corning has taken a leading position in this regard, having eliminated several
hazardous ingredients (arsenic, antimony, and barium) from their Eagle XG glass. Some of these
materials were historically used to eliminate bubble formation during glass processing.
At the LCDRF
The LCI Cleanroom uses ITO coated soda-lime glass standard, with Corning 1737F and Eagle2000 glass
available as necessary. All are available in 14”x14” sheets. If you would like more information on the
materials available at the LCI, please contact Doug Bryant.
We welcome your suggestions: any industrial partners who would like to see a particular topic covered in
a future newsletter can forward ideas to Doug Bryant or Phil Bos.
A Sampling of Resources:
1. http://www.pilkington.com/pilkington/corporate/english/education/float+process/default1.htm
with links to raw materials and step by step process
2. http://www.glassresource.com/sneakpeek/sample15.htm Information on soda lime float glass
fabrication process
3. Corning website, http://www.corning.com/ , has an extensive library of technical articles that
require registration on the website
4. http://www.borax.com/pioneer17.html Boric oxide usage in low alkali glass
5. “Alkali extraction as a determinant in the selection of a glass for displays”, Faulkner, E.K.;
Whitney, R.K.; Zeman, J.E., IEEE Transactions on Electron Devices, Volume 30, Issue 5, May
1983 Page(s): 545 – 548
6. “Flat Panel Display Materials 2007, Trends and Forecasts,” Fuji Chimera Research Report,
available at http://www.fpdonline.com/
7. “AMLCD Substrates Trends and Technology”, Corning Technical Information Paper, J.C. Lapp,
available on Corning website
8. http://www.displaysearchblog.com/2007/10/tft-lcds-produced-on-soda-lime-glass-substrates/
Glass Manufacturer Websites
1. Corning: http://www.corning.com/
2. Asahi Glass: http://www.agc.co.jp/english/products/products_05.html
3.
4.
5.
6.
7.
Nippon Electric Glass (NEG): http://www.negamerica.com/
Pilkington: http://www.pilkington.com/
Glaverbel: http://www.glaverbel.com/
Schott: http://www.schott.com/
Nippon Sheet Glass: http://www.nsg.co.jp/en/it/index.html
LCI NEWS
New Faces at LCI
Lu Zou is a new postdoctoral fellow for Professor Chanjoong Kim
Antal Jakli’s recent publication:
J. Harden, N. Diorio, A. G. Petrov, A. Jakli, ” Chirality Of Lipids Makes Fluid Lamellar Phases
Piezoelectric”, Phys. Rev. E, 79, 011701 (2009)
Upcoming LCI Seminars
January 28: Prof. Monica Olvera de la Cruz, Northwestern University, IL, "Nano-Patterns in Gels of
Charged Chains"
February 2: Dr. Cynthia Reichhardt, Los Alamos National Laboratory, NM, "Local Probes of Melting,
Decoupling, and Jamming Transitions"
February 4: Prof. Judit E. Puskas, University of Akron, "Novel Carbon Nanocomposite Biomaterials"
February 11: Prof. Matthew Glaser, Department of Physics, University of Colorado, Title: T.B.A
February 18: Prof. Sergij Shiyanovskii, Liquid Crystal Institute, Kent State University, Title: T.B.A.
February 23: Prof. Victor Reshetnyak, National Taras Shevchenko University of Kyiv, Title: T.B.A.
February 25: Dr. Matt Lynch, Proctor & Gamble, Cincinnati, Ohio, Title: T.B.A.
Seminar videos
In case you could not attend some of the Fall semester LCI seminars, you can still watch them on your
computer by going to this IPP web site link: http://www.lci.kent.edu/ipp/07/lciconnection.htm and typing
in the username and password that was provided to you recently.
Recently recorded presentations include:
January 26: Dr. Charles Reichhardt, Los Alamos National Laboratory, NM, "Using Colloids to Explore
Complex Matter with Competing Interactions"
James Fergason passes away at 74 (1934-2008)
Dr. James L. Fergason died recently at age 74. Dr. Fergason was one of the first employees at the Liquid
Crystal Institute, joining it in 1966.
Dr. Fergason's work associated his name with a number of seminal achievements in the field of liquid
crystal displays, most notably with the first commercially successful low-power, field-operated LC
display, known as the "twisted nematic cell." Fergason discovered the effect in 1969 at the LCI; his
technology was patented in the United States in 1971; Martin Schadt and Wolfgang Helfrich patented
similar technology in Europe. Dr. Fergason received the 2006 Lemelson-MIT Prize, the largest single
award for invention in the United States. He donated a portion of the award to the LCI to establish the
Saupe Scholarship Fund for graduate students.
Fergason saw the importance of transforming scientific knowledge into business opportunities, creating in
Kent the International Liquid Xtal Company (ILIXCO), as early as 1968. ILIXCO produced the first field
operating LCD, originally used in wrist watches. The invention impact was enormous, as "LCD
technology, starting with quartz watches and calculators, has completely redefined many industries, such
as computer displays, medical devices, and the vast array of consumer electronics", as stated at the
website of the National Inventors Hall of Fame, into which Fergason was inducted in 1998.
National Instruments LabVIEW hands-on workshop held at Liquid Crystal Institute
On January 21, the Liquid Crystal Institute hosted a free National Instruments LabVIEW hands-on
workshop. Approximately 20 students and faculty members received three hours of hands-on technical
training on real hardware from Hillary Emer, a field sales engineer from National Instruments.
Participants explored the latest features and capabilities of NI LabVIEW and learned how to create
complete NI LabVIEW applications from scratch with interactive Express VIs and I/O.
Be sure to let us know if you would like more detailed information about any activities at the LCI.
Phil Bos
pbos@lci.kent.edu
330-672-2511