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Description  |
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This invention relates to a method of making a color filter array element
which is temperature-resistant and fade-resistant.
In recent years, thermal transfer systems have been developed to obtain
prints from pictures which have been generated electronically from a color
video camera. According to one way of obtaining such prints, an electronic
picture is first subjected to color separation by color filters. The
respective color-separated images are then converted into electrical
signals. These signals are then operated on to produce cyan, magenta and
yellow electrical signals, which are then transmitted to a thermal
printer. To obtain the print, a cyan, magenta or yellow dye-donor element
is placed face-to-face with a dye-receiving element. The two are then
inserted between a thermal printing head and a platen roller. A line-type
thermal printing head is used to apply heat from the back of the dye-donor
sheet. The thermal printing head has many heating elements and is heated
up sequentially in response to the cyan, magenta and yellow signals. The
process is then repeated for the other two colors. A color hard copy is
thus obtained which corresponds to the original picture viewed on a
screen. Further details of this process and an apparatus for carrying it
out are contained in U.S. Pat. No. 4,621,271, the disclosure of which is
hereby incorporated by reference.
Liquid crystal display devices are known for digital display in electronic
calculators, clocks, household appliances, audio equipment, etc. Liquid
crystal displays are being developed to replace cathode ray tube
technology for display terminals. Liquid crystal displays occupy a smaller
volume than cathode ray tube devices with the same screen area. In
addition, liquid crystal display devices usually have lower power
requirements than corresponding cathode ray tube devices.
There has been a need to incorporate a color display capability into such
monochrome display devices, particularly in such applications as
peripheral terminals using various kinds of equipment involving phototube
display, mounted electronic display, or TV-image display. Various attempts
have been made to incorporate a color display using a color filter array
element into these devices. However, none of the color array elements for
liquid crystal display devices so far proposed have been successful in
meeting all the users' needs.
One commercially available type of color filter array element that has been
used in liquid crystal display devices for color display capability is a
transparent support having a gelatin layer thereon which contains dyes
having the additive primary colors red, green and blue in a mosaic pattern
obtained by using a photolithographic technique. To prepare such a color
filter array element, a gelatin layer is sensitized, exposed to a mask for
one of the colors of the mosaic pattern, developed to harden the gelatin
in the exposed areas, and washed to remove the unexposed (uncrosslinked)
gelatin, thus producing a pattern of gelatin which is then dyed with dye
of the desired color. The element is then recoated and the above steps are
repeated to obtain the other two colors. Further details of this process
are disclosed in U.S. Pat. No. 4,081,277.
Color liquid crystal display devices generally include two spaced glass
panels which define a sealed cavity that is filled with a liquid crystal
material. For actively-driven devices, a transparent electrode is formed
on one of the glass panels, which electrode may be patterned or not, while
individually addressable electrodes are formed on the other of the glass
panels. Each of the individual electrodes has a surface area corresponding
to the area of one picture element or pixel. If the device is to have
color capability, each pixel must be aligned with a color area, e.g., red,
green or blue, of a color filter array. Depending upon the image to be
displayed, one or more of the pixel electrodes is energized during display
operation to allow full light, no light or partial light to be transmitted
through the color filter area associated with that pixel. The image
perceived by a user is a blending of colors formed by the transmission of
light through adjacent color filter areas.
In forming such a liquid crystal display device, the color filter array
element to be used therein may have to undergo rather severe heating and
treatment steps during manufacture. For example, a transparent conducting
layer, such as indium tin oxide (ITO), is usually vacuum sputtered onto
the color filter array element which is then cured and patterned by
etching. The curing may take place at temperatures as high as 200.degree.
C. for times which may be as long as one hour or more. This is followed by
coating with a thin polymeric alignment layer for the liquid crystals,
such as a polyimide, followed by another curing step for up to several
hours at an elevated temperature. These treatment steps can be very
harmful to many color filter array elements, especially those with a
gelatin matrix using the prior art technique described above.
Polycarbonate dye image-receiving layer materials for color filter array
elements are described in U.S. Pat. No. 4,962,081. In using these
materials to form a color filter array element, the polymeric material is
typically coated on a glass support, using spin coating in order to obtain
a smooth coating. Alternatively, the dye image-receiving layer can be
applied to the support in a pattern, just slightly larger than the viewing
area of the liquid crystal display device, by means of screen printing, as
disclosed in U.S. Pat. No. 5,079,214.
U.S. Pat. No. 4,923,860 discloses that the dyes which color each pixel of a
color filter array can be thermally transferred by means of a patterned
flash of light onto a dye-donor sheet held in close contact with a
polycarbonate receiving layer coated on a glass support. However, there is
a problem with this manufacturing method because the finite thickness of
the dye-donor sheet support used causes some blurring and rounding of the
edges of the transferred dye spots. If the blurring is too extensive, it
can lead to mixing of some of the color from one pixel to the next, with
concomitant loss of color purity. Because of this, the support for the
dye-donor sheet must be as thin as possible, which in turn makes it
fragile and difficult to handle without creasing.
As described above, a useful color filter array should have good thermal
resistance so that subsequent high-temperature processing steps, such as
vacuum sputtering of conductive layers and curing of polymeric alignment
layers will not degrade the color quality of the pixels making up the
array. The dyes which color the pixels of the array should also be chosen
to have good fade resistance to the viewing light which illuminates them.
These dyes must have good color purity, and the overall transmissivity of
the color filter array should be as high as possible, consistent with good
color quality and saturation, so that the power of the illuminating source
need not be excessively high. Additional requirements for a high-quality
color filter array are that resolution be high so that images appear sharp
and detailed to the eye and that overall image uniformity be good.
It is an object of this invention to provide a method for making color
filter arrays which would not involve the need for using fragile dye-donor
sheets from which dyes are transferred into a polymeric receiving layer,
and which would still provide formation of temperature- and
fade-resistant, sharp pixels in such an array.
The present invention provides a method of preparing a color filter array
element comprising the following steps:
a) coating a glass support with a polymeric dye image-receiving layer;
b) coating the polymeric dye image-receiving layer with at least one
additive primary color dye from a solvent that does not swell or penetrate
the polymeric dye image-receiving layer;
c) placing a stencil mask of a desired pixel shape in intimate contact with
the surface of the polymeric dye image-receiving layer;
d) fusing the dye into the polymeric dye image-receiving layer by heating
or by using a solvent vapor treatment using a solvent which will swell or
penetrate the dye image-receiving layer;
e) removing the stencil mask;
f) removing all unfused dye with a solvent wash; and
g) repeating steps b) to f) twice using different additive primary color
dyes.
When the above steps b) to f) are repeated using other additive primary
color dyes, the patterned stencil mask is obviously offset in order to
produce the colors in a different image pattern. The resulting pixels in
the color filter array prepared by the process of the invention excel in
sharpness, well-defined edges, high color purity, and saturation. The
resulting color filter array is also heat- and fade-resistant.
Another embodiment of the invention relates to a method of preparing a
color filter array element comprising the following steps:
I) coating a glass support with a polymeric dye image-receiving layer;
II) coating said polymeric dye image-receiving layer with at least one
additive primary color dye from a solvent that does not swell or penetrate
said polymeric dye image-receiving layer;
III) fusing said dye into said polymeric dye image-receiving layer by
irridiating with a focused, modulated laser beam to cause dye to penetrate
into said dye image-receiving layer in accordance with the modulation
pattern;
IV) removing all unfused dye with a solvent wash; and
V) repeating steps II to IV twice using different additive primary color
dyes.
A prefatory step to the pattern-wise fusing of the dye into the polymeric
dye image-receiving layer is to coat or otherwise distribute the unfused
dye onto the surface of the dye image-receiving layer. Various methods may
be employed to place the dye onto the surface of the dye image-receiving
layer. A powder of the solid dye may be sprinkled or dusted onto the
surface or the dye may be evaporated onto the surface in a vacuum
deposition apparatus.
A preferred method to lay down the dye is by spin coating the dye from a
solvent such as water that does not swell or dissolve the polymeric dye
image-receiving layer. For example, a very fine dispersion or emulsion of
hydrophobic dye particles in water can be used to spin coat the dye onto a
hydrophobic polymeric dye image-receiving layer.
The process of the invention provides a dye image-receiving layer which
contains a thermally transferred image comprising a repeating pattern of
colorants, preferably in a mosaic pattern.
In a preferred embodiment of the invention, the mosaic pattern consists of
a set of red, green and blue additive primaries.
In another preferred embodiment of the invention, each area of primary
color and each set of primary colors are separated from each other by an
opaque area, e.g., black grid lines. This has been found to give improved
color reproduction and reduce flare in the displayed image.
The size of the mosaic set is not critical since it depends on the viewing
distance. In general, the individual pixels of the set are from about 50
to about 600 mm and do not have to be of the same size.
In a preferred embodiment of the invention, the repeating mosaic pattern of
dye to form the color filter array element consists of uniform, square,
linear repeating areas, with one color diagonal displacement as follows:
RGBRG
BRGBR
GBRGB
In another preferred embodiment, the above squares are approximately 100
mm.
The color filter array elements prepared according to the invention can be
used in image sensors or in various electro-optical devices such as
electroscopic light valves or liquid crystal display devices. Such liquid
crystal display devices are described, for example, in UK Patents
2,154,355; 2,130,781; 2,162,674 and 2,161,971.
Liquid crystal display devices are commonly made by placing a material,
which is liquid crystalline at the operating temperature of the device,
between two transparent electrodes, usually indium tin oxide coated on a
substrate such as glass, and exciting the device by applying a voltage
across the electrodes. Alignment layers are provided over the transparent
electrode layers on both substrates and are treated to orient the liquid
crystal molecules in order to introduce a twist of, e.g., 90.degree.,
between the substrates. Thus, the plane of polarization of plane polarized
light will be rotated in a 90.degree. angle as it passes through the
twisted liquid crystal composition from one surface of the cell to the
other surface. Application of an electric field between the selected
electrodes of the cell causes the twist of the liquid crystal composition
to be temporarily removed in the portion of the cell between the selected
electrodes. By use of optical polarizers on each side of the cell,
polarized light can be passed through the cell or extinguished, depending
on whether or not an electric field is applied.
The polymeric alignment layer described above can be any of the materials
commonly used in the liquid crystal art. Examples of such materials
include polyimides, polyvinyl alcohol and methyl cellulose.
The transparent conducting layer described above is also conventional in
the liquid crystal art. Examples of such materials include indium tin
oxide, indium oxide, tin oxide and cadmium stannate.
The dye image-receiving layer used in forming the color filter array
element of the invention may comprise, for example, those polymers
described in U.S. Pat. Nos. 4,695,286, 4,740,797 and 4,775,657, and
4,962,081, the disclosures of which are hereby incorporated by reference.
In a preferred embodiment, polycarbonates having a glass transition
temperature greater than about 200.degree. C. are employed. In another
preferred embodiment, polycarbonates derived from a methylene-substituted
bisphenol A such as 4,4'-(hexahydro-4,7-methanoindan-5-ylidene)-bisphenol
are employed. In general, good results have been obtained at a coverage of
from about 0.25 to about 5 mg/m.sup.2.
The support used in the invention is glass such as borax glass,
borosilicate glass, chromium glass, crown glass, flint glass, lime glass,
potash glass, silica-flint glass, soda glass, and zinc-crown glass. In a
preferred embodiment, borosilicate glass is employed.
Various dyes or mixture of dyes can be used in the process of the
invention. Especially good results have been obtained with the following
dyes:
##STR1##
or any of the dyes disclosed in U.S. Pat. No. 4,541,830, the disclosure of
which is hereby incorporated by reference. The above dyes may be employed
singly or in combination to obtain a monochrome.
The above subtractive dyes can be employed in various combinations to
obtain the desired red, blue and green additive primary colors, as
disclosed in U.S. Pat. Nos. 4,957,898, 4,975,410, and 4,988,665, the
disclosures of which are hereby incorporated by reference. The dyes can be
mixed within the dye layer or transferred sequentially if coated in
separate dye layers and can be used at a coverage of from about 0.05 to
about 1 g/m.sup.2.
Various methods can be used to fuse dye into the polymeric dye
image-receiving layer on the support to form the color filter array
element. For example, a stencil mask, with fine holes corresponding to the
desired color array pixels, may be placed over the polymeric dye
image-receiving layer. If the stencil is made of a metal such as iron or
nickel, it can be held in good contact with the dye image-receiving layer
by means of a magnet placed behind the support. Such an assembly may then
be exposed to a solvent vapor which will swell and soften the polymeric
dye image-receiving layer through the holes in the stencil, thereby
effecting a pattern of the stencil.
Instead of using an external metal stencil, an in situ stencil may be
prepared by means of coating the dye image-receiving layer with a
water-based photoresist, such as dichromated gelatin. The pattern of holes
in the resist may then be prepared by exposure to a pattern of
ultra-violet radiation that will crosslink the resist. The unexposed
resist can then be washed off, leaving areas of the dye image-receiving
layer polymer into which the dye can be fused by means of the solvent
vapor described above.
Another way to effect imagewise fusing of the dye into the dye
image-receiving layer is by using heating such as a thermal resistive head
as described in U.S. Pat. No. 4,621,271, the disclosure of which is hereby
incorporated by reference.
Another kind of heating is the use of an array of metal prongs, or needles,
having the dimensions and spacing of the desired pixel elements of the
colorfilter array. Fusing the dye into the dye image-receiving layer may
be accomplished by placing a heated array of needles into contact with the
dye layer coated on the dye image-receiving layer for sufficient time to
allow the heat to fuse the dye into the dye image-receiving layer.
Yet another way to effect the fusing of the dye into the dye
image-receiving layer is by heating through irradiation of the pixel
element with an intense beam of radiation absorbed by either or both the
dye and the polymeric dye image-receiving layer. A convenient way of
supplying the radiation is by flash discharge of an electrically charged
capacitor through an argon-filled quartz tube. Such flash lamp tubes are
widely employed as electronic photographic flash bulbs and, in larger
sizes, as illuminating flash lamps for airport runways.
The following examples are provided to illustrate the invention:
Example 1
A small square of glass (approximately 5 cm on a side) was spin coated at
200 rev/min with a 10% solution of
4,4'-(hexahydro-4,7-methanoindan-5-ylidene)bisphenol polycarbonate in
toluene and allowed to dry while spinning. This was overcoated by spinning
at 1000 rev/min with a saturated solution of the above cyan dye in
ethanol. When dry, two pennies were placed on the surface of the dye, and
the assembly was placed in a fusing chamber saturated with toluene vapor.
After 5 minutes, the assembly was removed from the chamber and the unfused
dye was removed by washing with ethanol, leaving two clear areas where the
pennies were in contact with the dye layer surrounded by fused cyan dye.
This shows that the technique of the basic steps of the invention is
feasible.
Example 2
Another square of glass was prepared in the same way as in Example 1, but
the fusing was effected by touching the surface of the dye layer with a
soldering iron tip heated to 450.degree. F. for 2 seconds. The layer was
then washed with ethanol, leaving a small, sharply defined spot of cyan
dye where the soldering iron tip had contacted the dye image-receiving
layer. This again shows that the technique of the basic steps of the
invention is feasible.
Example 3--In situ method
Another square of glass was prepared in the same way as in Example 1, and
was then overcoated with a solution of 5% poly(vinyl alcohol) and 1%
ammonium dichromate in water by spinning at 100 rev/min. When dry, the
plate was covered with a photographic mask pattern of small holes, and the
assembly was exposed to a high intensity UV lamp for 60 seconds. Then the
resist image was developed by spinning with water to remove the unexposed
resist. The developed image was dried and then overcoated by spinning with
a solution of the above cyan dye in ethanol.
When dry, the image was placed in a chamber filled with anisole vapor for
10 minutes and then baked in an oven at 133.degree. F. for 10 minutes. The
plate was then washed with ethanol on the spinner and the resist removed
by rubbing under hot water. The result was a clear image of blue dots
corresponding to the photographic mask pattern.
This shows the feasibility of the invention using the in-situ preparation
of the mask.
Example 4--Patterned Fusing by Laser Heat
A 5 cm square of soda lime glass was overcoated by spinning (2000 rev/min)
with a 5% solution of Butvar B76.RTM. (Monsanto Co.) polymer in methyl
isobutyl ketone as a dye image-receiving layer. When dry, the dye
image-receiving layer was overcoated with a dispersion of the first cyan
dye illustrated above in water by spinning at 2000 rev/min for 2 minutes.
The coated glass was placed on the exposure platen of a diode laser
scanning devise described in U.S. Pat. No. 5,017,547, hereby incorporated
by reference.
The 50 mW (37 mW on the exposure plane) beam (7.times.9 micron oval at half
power) was swept across the exposure plane at 165 mm per second, and the
power of laser was modulated between full power and threshold power in
accordance with a pattern stored on a computer magnetic disk. After
exposure, the sample was washed with water and lightly scrubbed with a
cotton swab. A photomicrograph of the resulting image showed sharp, well
defined, high contrast pixel elements 200.times.300 microns in size, with
50 micron clear gaps between the pixels.
Example 5--Patterned Fusing by Masked Radiant Heat
The coated glass of Example 4 was covered by a metal mask with 5 mm holes
in it and the assembly was placed beneath an infrared source (Model 5610
from Research Inc., Minneapolis, Minn.) at a distance of 15 cm for 4
minutes. Then the unfused cyan dye was removed by washing with water
leaving a high contrast image of fused cyan dye corresponding to the holes
in the mask.
Example 6--Patterned Fusing by a Heated Metal--Three Colors
The coated glass of Example 4 was placed on a 0.95 cm thick aluminum block
that had 0.32 cm channels cut in it to a depth of 0.32 cm in orthogonal
directions, thus generating an array of 0.16 cm square posts on 0.32 cm
centers. The aluminum block was placed on a hot plate held at 220.degree.
C. and allowed to equilibrate before the coated glass was placed on the
aluminum. The coated side of the glass was in contact with the aluminum
posts.
The glass was allowed to remain in contact with the hot aluminum posts for
about 1 second, then removed and washed with water to reveal a high
contrast image of the aluminum post pattern.
The glass was then overcoated with a dispersion of the first magenta dye
illustrated above (12%) in water by spinning at 5000 rev/min for 1 minute.
The coated glass was placed on the heated aluminum post block for about 1
second, removed and washed with water to reveal the second color pattern.
The glass was then coated with the third yellow illustrated above (12% in
water), and the heating was repeated to give the third color pattern.
Example 7--Patterned Fusing by Masked Solvent Vapor
The coated glass of Example 4 was placed on an aluminum plate (0.64 cm
thick) which had 4 holes drilled in it (0.64 cm diameter). The aluminum
plate was placed over a well of methylene chloride for 10 seconds.
Then the unfused cyan dye was washed off the plate with water, leaving a
high contrast image of cyan dye where the holes in the aluminum plate
allowed the methylene chloride vapor to fuse the dye into the dye
image-receiving layer.
The invention has been described in detail with particular reference to
preferred embodiments thereof, but it will be understood that variations
and modifications can be effected within the spirit and scope of the
invention.
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