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FIELD OF THE INVENTION
The invention relates to a cell, divided from front to back into two or
more separate compartments with minimum separation between the
compartments, suitable for use in flat panel displays.
BACKGROUND
The performance of a flat panel liquid crystal display may be inferior to
that required if a single display cell is employed. This can be the case
for both reflective and transmissive, or backlit, displays. In the case of
reflective displays, either the contrast ratio (CR) or the reflectivity
(R), or both may be lower than required, or some other property, such as
the display operating voltage, may be outside the required range, as the
following examples of existing art will show.
EXAMPLE 1
Nematic Guest-Host Display
A nematic Guest-Host display (Heilmeier GH and Zanoni LA, Appl. Phys.
Lett., Vol. 13, p 91 (1968)) uses a solution of a dichroic dye (guest) in
a nematic liquid crystal (host). The liquid crystal is aligned
substantially parallel to the display substrates. The properties of the
guest dye molecules are chosen so that they align parallel to the liquid
crystal molecules and so that the transition dipole for the absorption of
visible light is substantially parallel to the geometric axis (i.e., the
alignment direction) of the molecule.
The alignment direction on the upper and lower substrates may be either
parallel or, for optimum performance, orthogonal (Lowe AC, Mol. Cryst.
Liq. Cryst., Vol. 66, p 295, 1981). The display is viewed through a
polarizing film oriented with its transmitting axis parallel to the
alignment direction on the upper substrate. Thus the guest-host layer is
illuminated only with light of the polarization state which can be
absorbed by the guest dye molecules; absorption of light is efficient and
the display appears dark. When an electric field is applied across the G-H
layer by means of electrodes on the upper and lower substrates, the liquid
crystal and therefore also the dye molecules are rotated perpendicular to
the plane of the display, in which orientation little light is absorbed by
the dye and the display appears light. Contrast ratios in excess of 10:1
can be obtained in such displays. However, the light state reflectivity is
greatly reduced by the presence of the polarizer which reduces
reflectivity to a practical maximum of about 30%. The operating voltage of
such a display is low at about 4 volts for full turn-on.
EXAMPLE 2
Cholesteric Guest-Host Display
This is a development of the nematic guest-host display described in
Example 1. By the addition of a chiral material to the guest-host mixture
to produce a helical pitch length a few times greater than the wavelength
of visible light (White DL and Taylor GN, J. Appl. Phys., Vol. 45, p 4718,
(1974)) both polarizations of incident light can be absorbed and a front
polarizer is no longer required. This increases the light-state
reflectivity. However, the threshold voltage of a chiral system is
inversely proportional to the chiral pitch. This has the effect of
increasing the threshold by a factor of 4-5 over the nematic G-H described
in Example 1. Moreover, the voltage required for complete turn-on of the
device is also increased and the absorption of unpolarized light in the
field-off state is less efficient than the absorption of polarized light
in Example 1, so either contrast ratio or reflectivity must be
compromised.
EXAMPLE 3
Cholestenc Texture Effects
These effects make use of the fact that chiral nematic systems reflect
light of wavelength, .lambda., where .lambda. is related to the pitch
length, P, and the average refractive index, n.sub.a, of the liquid
crystal by .lambda.=n.sub.a P/2. (Crooker et al, U.S. Pat. No. 5,200,845,
Doane et al, U.S. Pat. No. 5,453,863, Doane et al, U.S. Pat. No.
5,437,811, West et al, U.S. Pat. No. 5,453,863). However, the liquid
crystal layer reflects circularly polarized light such that the electric
vector of the reflected light is a mirror image of the chiral helix; i.e.,
a right-handed helix will reflect only right-handed circular polarized
light. Unpolarized light can be considered to be composed of two
oppositely-handed circular polarized components. Thus the theoretical
limit of the reflectivity of a chiral layer is 50%. In practice, it is
less than this--about 40%.
These examples serve to show that an adequate combination of high contrast,
high reflectivity and minimum operating voltage cannot be obtained by the
known art.
The contrast and/or the reflection performance of displays can be improved
by using a stacked structure of two or more cells. Other examples of known
art relate to the use of stacked cells (Haim et al U.S. Pat. No.
4,637,687; Crooker et al U.S. Pat. No. 5,200,845). These are formed by
stacking separate cells, or by making more complex cells in which an
intermediate substrate is shared by both cells, forming the rear surface
of the front cell and the front surface of the rear cell. Because such
substrates have significant thickness (approx 1 mm), substantial parallax
results between the images formed in the two cells when the display is
viewed at off-normal angles. This limits the use of such displays to
applications where the pixel size is substantially greater than the
separation distance between the two cells. Alternatively (Yoshimisu U.S.
Pat. No. 5,329,388), a light-guiding film can be used to constrain the
angle at which light propagates from one cell to the other to an angle
close to the normal to the display surface. The disadvantage of this
approach is that the optical efficiency of such films is not high, and
reflectivity or transmittance are degraded.
It is an object of the present invention to provide an improved liquid
crystal display cell.
It is another object of the present invention to provide an improved liquid
crystal display cell which avoids the adverse effects of parallax.
Thus, the known art is unsuitable for applications where high pixel density
and high contrast and/or reflectivity performance are simultaneously
required, such as in reflective or highly transmissive backlit computer
displays.
SUMMARY
This invention relates to a novel cell construction which enables stacked
cells to be formed in which the intermediate substrate is so thin that
little or no parallax occurs between the images formed in the two or more
cells. In addition, the thickness of the intermediate substrate is such
that the field drop across it is low (about 10%) compared to that across
the liquid crystal layers, and the stacked cell can be addressed as a
single cell. This greatly simplifies and reduces the cost of the
interconnections and drive electronics. The avoidance of the need for
transparent electrodes on both surfaces of the intermediate substrate also
improves transmittance (and reflectivity) of the device. Of course, should
it be required, the intermediate substrate can be patterned with
transparent electrodes in the conventional way. The advantage of
substantially zero parallax would remain.
Further objects, features, and advantages of the present invention will
become apparent from a consideration of the following detailed description
of the invention when read in conjunction with the drawing FIGS., in which
:
DESCRIPTION OF DRAWINGS
FIG. 1A and 1B show a cross-section of a cell 10 comprising a transparent
front substrate 11 and a transparent or opaque rear substrate 12. A very
thin intermediate substrate 13 is supported between substrates 11 and 12,
forming two compartments 14 and 15, which are filled with liquid crystal
material. Substrate 11 is coated with a transparent electrically
conducting material 16 on its inner surface. Substrate 12 is coated with a
plurality of electrodes 17 made from a transparent or an opaque
electrically conducting material on its inner surface. The opaque
conducting material can be reflective or light absorbing. The surfaces of
16, 17 and 13 which are exposed to the liquid crystal may be coated with
an aligning material and rubbed or treated in some other way known in the
art to promote alignment of the liquid crystal. The alignment layers 18 in
the upper portion of the cell may or may not be different from the
alignment layers 19 in the lower portion. An electrical field may be
applied between the layers 16 and 17 by means of an external source 20 and
a switch 21. FIG. 1A shows the cell in the field-off state. The shading in
the upper and lower portions of the cell is intended to show the liquid
crystal aligned perpendicular to the plane of the page in the upper
portion and parallel to the plane of the substrates in the lower portion.
FIG. 1B shows the cell in the field-on state, in which all of the liquid
crystal material is aligned substantially perpendicular to the plane of
the substrates.
FIG. 2 shows the arrangement of spacers 30, 31 on the substrates 11 and 12
respectively. The size and number of the spacers shown in this and
subsequent figures is for illustrative purposes only and does not
represent the actual size and number of spacers. FIG. 2 also shows a
possible method whereby adhesive 32, 33 is pre-screened around the
periphery of 11 and 12 to form an edge seal. This avoids any need to
process the thin intermediate substrate 13 other than for liquid crystal
alignment.
FIG. 3A shows how the spacers 30, 31 overlap when the cell is assembled.
FIG. 3B shows in detail a section through the cell at a point where the
intermediate substrate 13 effectively clamped in position at the
intersection point of the spacers 30 and 31.
FIG. 4 shows the intermediate substrate 13 mounted on a supporting frame 35
which enables the film to be handled and kept flat, with its surfaces out
of contact with solid objects 13 may be fixed to 35 with adhesive or
mechanically clamped or held by any other suitable means under suitable,
uniform tension. During cell assembly, the upper and lower substrates
11,12 are aligned with respect to each other with 13 in place between
them. By suitable mechanical means they are then moved into contact as
shown by the arrows and the cell is sealed by curing the peripheral
adhesive.
FIG. 5A shows a detailed cross-sectional view of the spacer overlap region
of a three compartment cell comprising a front substrate 11, a rear
substrate 12 and two intermediate substrates 13 and 40. Spacers 30, 31 and
45 are deposited on to substrates 11, 12 and 40 respectively. FIG. 5B
shows a detailed cross-sectional view of the spacer overlap region of a
four compartment cell comprising a front substrate 11, a rear substrate 12
and three intermediate substrates 13, 40 and 41. Spacers 30, 31 are
deposited on to substrates 11 and 12 respectively. Also shown are two
alternative configurations for the deposition of spacers 45, 46. In the
left hand illustration of FIG. 5B, spacers 45 and 46 are deposited on to
substrates 40 and 41 respectively. In the right hand illustration, spacers
45, 46 are deposited on to opposite sides of substrate 40.
DETAILED DESCRIPTION
FIG. 1 shows a cross section view of a duplex liquid crystal cell 10 in
which a transparent front substrate 11 and a transparent or opaque rear
substrate 12 are separated by and spaced apart from a transparent
intermediate substrate or membrane 13. The inner surface of substrate 11
is coated with a transparent electrode material 16 e.g., indium-tin oxide
(ITO). The inner surface of the rear substrate is coated with an electrode
material 17 which can be transparent or opaque, reflective or light
absorbing, depending on the particular liquid crystal effect employed.
This invention relates to a novel cell structure and the means for making
it. It can be used with several different liquid crystal effects and
therefore with several different addressing schemes. For example, when
using a monostable liquid crystal effect in a high resolution display, the
pixels will be addressed by an active matrix in which each pixel is driven
via an integrated circuit switch, which can be a transistor, a diode or a
metal/insulator/metal (MIM) device. In all such cases, the conductor 17
will be etched into a multiplicity of electrodes, one for each pixel. In
the case of TFT-driven display, the electrode 16 will be a continuous
sheet and in the case of diodes and MIMs it will be etched into strips,
one for each column of the display.
In the case of bistable liquid crystal effects, such as PSCT, an active
matrix may not be required. Consequently, 16 will be etched into column
strips and 17 will be etched into row strips, in a direction orthogonal to
16. The nematic guest-host effect will be used as an example in the
remainder of this description. This effect is suitable for being addressed
with an active matrix and the electrodes 17 will be formed from a highly
reflective metal such as, for example, Aluminum. Alternatively, they could
be made from a dielectric mirror stack coated with a transparent
conducting material. Referring now to FIG. 2, spacers 30, 31 are deposited
on to substrates 11 and 12. The spacers are precisely located so that,
when substrates 11 and 12 are brought together, pairs of spacers on
opposite substrates overlap as shown in FIG. 3. There are several methods
for depositing such spacers know in the art. A preferred method is to coat
the substrate with a polymer such as polyimide, with a thickness
substantially equal to the required spacer height. The film is then cured,
patterned by photolithographic means and etched by a reactive ion etching
process to leave the required spacers. The dimensions of the spacers will
be determined by the liquid crystal effect used, the pixel pitch and the
precision of the tooling used to align the substrates during cell
assembly. In this example, their shape and size has been chosen so that,
simultaneously, the alignment accuracy of 11 to 12 is minimized and the
spacers remain sufficiently narrow in width to be hidden in the interpixel
gaps. However, this invention is not restricted to the particular size and
shape shown, and any suitable alternative size and shape may be used.
Typical values in the present case will be height between 3 and 10 .mu.m,
length between 20 and 250 .mu.m and width less than the interpixel gap
(5-20 .mu.m). There is an advantage in positioning the spacers in the
interpixel gap since they do not then interfere with or degrade the visual
performance of the display. The substrates are then spin-coated with a
thin layer (a few tens of nanometers thick) of an aligning polymer such as
polyimide, cured and rubbed, preferably in the direction of the long axis
of the spacers.
The intermediate substrate is a polymer film which must be as thin as
possible, consistent with handling and processing requirements. A
preferred example is Mylar (.TM.), a polyester film manufactured by
DuPont, which is available in thickness down to 0.9 .mu.m, but any film
with the required optical and mechanical properties will be suitable.
Other polymer films such as, but not limited to, polyimide, polyamide and
polyacrylates, and ultra thin glass, oxide or other transparent dielectric
films would also be suitable within the handling and processing
constraints already mentioned. Mylar will be used as the preferred
material in this description. Referring now to FIG. 4, a sheet of Mylar 13
is attached to a rigid, flat frame 35 either by adhesive or mechanical
means such that it is held under light, uniform tension. The film so
mounted is then subjected to a heating cycle up to about 200 degrees
Centigrade. This causes a permanent shrinkage of the film to occur which
tensions and flattens it. Mylar film is optically anisotropic, and for the
present example, it is important that the liquid crystal is aligned either
parallel to or perpendicular to the optic axis. The axial direction is
determined by some suitable method such as examining the film between
crossed polarizing prisms although in practice, in manufacture when the
film is cut from a bulk roll, the direction of the optical axis will be
substantially constant and will not need to be determined for every
sample. Then the film is subjected to an alignment process. Any process
consistent with the handling properties of the film can be used.
Non-contact methods are to be preferred but contact methods are not
excluded. An example of a non-contact method is to spin-coat both sides of
the film with a thin polyimide aligning layer and then expose each surface
to collimated UV radiation at the appropriate direction to produce
orthogonal alignment directions on the two surfaces of 13.
However, a preferred method, which avoids both the need to pre-coat the
film 13 with an alignment layer and the need for rubbing contact, is to
expose each side of the film to a beam of low energy ions, such as Argon
ions, at an incident angle between 45.degree. and 85.degree. with respect
to the substrate normal, in such directions as to provide the required
orthogonal alignment directions, parallel and perpendicular to the optic
axis, on the two surfaces of 13. In order to carry this out a thin mylar
sheet is stretched across a frame. The film is free-standing. The mylar is
irradiated directly. Note no polyimide is spun on the mylar. Thus, we
protect the use of mylar, a free-standing film, a frame, and its use in a
display. Such methods are described in U.S. patent application Ser. No.
08/664,884 filed on the same day herewith entitled, "Atomic Beam Alignment
of Liquid Crystals" to Chaudhari et al., which is assigned to the assignee
of the present invention and is incorporated herein by reference.
Referring now to FIG. 2, adhesive 32,33 is deposited around the periphery
of substrates 11 and 12. In practice a gap is left in the adhesive to form
an opening through which the cell can subsequently be filled. Substrate 12
is placed in an alignment tool. The film 13, still attached to the frame
35 is lowered over 12. Substrate 11 is then positioned so that the spacers
30, 31 overlap as shown in FIG. 3. It is then lowered and forced into
contact with 13 and 12 and the adhesive is cured. The Mylar film 13 is now
held in tension by the peripheral adhesive seal, and the cell spacing is
maintained to the required degree of precision because the film is clamped
between spacers at each spacer intersection. The density of spacers can be
adjusted to ensure that this requirement is met. Excess Mylar is trimmed
from around the periphery of the cell and the cell is now filled with a
nematic guest-host mixture. Because the same mixture is used in both
compartments 14,15 of the cell, the fill holes in the periphery can be
adjacent. In other examples, when a different liquid crystal mixture is
used in each compartment, the fill holes must be well separated,
preferably on opposite sides of the display. The cell is held under excess
external pressure to ensure that positive contact occurs at spacer
intersection at all points on the display, and the fill holes are sealed
with adhesive or some other suitable material.
The configuration of the cell is such that, in the field-off state,
compartment 14 contains an untwisted homogeneously aligned nematic
guest-host layer which will efficiently absorb light polarized in the
direction of the arrow H--H in FIG. 3. Compartment 15 contains a similar
nematic guest-host layer, but with the alignment direction orthogonal to
that in compartment 14. This layer efficiently absorbs light polarized in
the direction of the arrow V--V. Thus the cell effectively absorbs all the
light incident on it. When a field is applied across the cell between
substrates 11 and 12, the liquid crystal molecules, which have positive
dielectric anisotropy, rotate substantially perpendicular to the plane of
the display. In this orientation absorption of the incident light by the
dye molecules is minimized and the display becomes highly reflective. This
display demonstrates the very low field-off state reflectivity and low
drive voltage of the known art of Example 1 and the high field-on state
reflectivity of the know art in Example 2. Because of the very thin
intermediate substrate 13 used, there is effectively no parallax between
the images formed in the upper and lower compartments of the cell. Better
that 12:1 contrast is obtainable with an on-state reflectivity in excess
of 60%.
This embodiment relates to the use of thematic materials with positive
dielectric anisotropy, which align with the molecules parallel to the
direction of an applied electric field.
Another embodiment of the stacked guest-host effect relates to the use of
nematic materials with negative dielectric anisotropy, which align with
the molecules perpendicular to the direction of an applied electric field.
The general cell construction and properties for this embodiment are
substantially the same as for the first described embodiment. What is
different is that the alignment layers 18 in FIG. 1A are such as to cause
substantially homeotropic (perpendicular) alignment of the liquid crystal
molecules in compartment 14, with a small tilt bias in the direction of
the arrow H--H in FIG. 3A. In similar fashion, the alignment layers 19 in
FIG. 1A are such as to cause substantially homeotropic alignment of the
liquid crystal molecules in compartment 15 but with a small tilt bias in
the direction of the arrow V--V in FIG. 3A. Thus the field-off state of
the display is light. When an electric field is applied between substrates
11 and 12, the liquid crystal molecules align substantially parallel to
the substrates and parallel to the direction H--H in compartment 14 and
parallel to V--V in compartment 15. Thus the field-on state of this
embodiment is dark. The methods of producing homeotropic alignment with a
tilt bias are known in the art and are not claimed here. The reflectivity
and contrast ratio achievable by this embodiment are substantially the
same as that achievable in the first embodiment, but this embodiment is
light in the field-off state as opposed to dark in the case of the first
preferred embodiment. FIG. 3B also illustrates an embodiment wherein
electrodes 38 are placed on one or both surfaces of the thin transparent
membrane.
Another embodiment of this invention relates to effects in which the liquid
crystal layer itself reflects light as discussed in example 3 of the prior
art. Here the cell structure is substantially the same as that in the
first preferred embodiment. However, the properties required of the
intermediate substrate 13 are different in this case. Compartments 14 and
15 are filled with chiral nematic materials with opposite twist senses,
e.g., a right-handed twist layer in 14 and a left-handed twist layer in
15. Thus the light which is transmitted by 14 will be left circularly
polarized, and this polarization state must be preserved as it is the
state which is reflected by the material in 15. Thus 13 must be
substantially optically isotropic. In other words, it must have
substantially zero birefringence. In the case of Mylar films, this will be
difficult to achieve with a single film. However, two substantially
identical birefringent films oriented orthogonally to each other will
achieve the required optical properties since the birefringence in the one
film will be cancelled by the other. A suitable composite film can be made
by simply bringing together two such films by evacuating the space between
them. They will then remain in intimate contact by the action of surface
forces. Alternatively, a film which is substantially isotropic in the
plane of the film can be made by casting or spin coating a polymer
material and subsequently curing it and removing it from the substrate,
which could be solid or liquid. The cell is processed in substantially the
same manner as in the first preferred embodiment, with the fill holes for
the two compartments separated and the compartments filled separately with
the left-handed and right-handed materials. It should be noted that the
alignment requirements may differ, according to the nature of the effect
used, but these are already described in the prior art. A cell such as the
one described here will exhibit approximately double the reflectivity of
the cells described in the prior art.
Another embodiment of the invention relates also to the use of effects in
which the liquid crystal layer reflects light. Here, the birefringence of
the intermediate substrate 13 is exploited by adjusting its optical
anisotropy such that the optical path lengths parallel to and
perpendicular to its optic axis differ by one half wavelength of light. In
this case, compartments 14 and 15 are filled with the same chiral nematic
material. We will use right-handed material as the example, but the
argument applies equally to left-handed material. The left circular
polarized light transmitted by 14 now becomes converted to right circular
polarized light on transmission through 13 and is now reflected by 15.
Because it will be difficult to make a film which is exactly a half-wave
retarder throughout the visible region of the spectrum, the performance of
this example may be inferior to that in the preceding example.
Yet another embodiment of this invention involves displays which require
more than two compartments. For example, application of this invention to
a stacked color cell similar to that described by Crooker et al in U.S.
Pat. No. 5,200,845 would require a total of six compartments. However, the
principle involved is similar for any number of compartments greater than
two. In such cases, spacers must be deposited on to the appropriate number
of surfaces of the intermediate substrates. Referring now to FIG. 5A, a
three compartment cell is made by using an additional intermediate
substrate 40 on to which must be processed spacers 45. Note that in this
configuration, spacers 31 deposited on to substrate 12 are now rotated to
a direction perpendicular to the case for a two compartment cell.
FIG. 5B shows a four compartment cell. Here there are two options for the
deposition of spacers on to the intermediate substrates 40, 41. Either one
set of spacers can be deposited on each (spacers 45, 46 on to substrates
40, 41 respectively) (left hand illustration) or both sets of spacers 45,
46 can be processed on to substrate 40 (right hand illustration). The
former procedure will be simpler from the processing standpoint, but the
latter will allow for simpler cell assembly as the number of substrates
which must be precisely aligned is reduced from four to three.
While the present invention has been described with respect to preferred
embodiments, numerous modifications, changes, and improvements will occur
to those skilled in the art without departing from the spirit and scope of
the invention.
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