CA1056936A

CA1056936A - Long life high contrast liquid crystal display

Info

Publication number: CA1056936A
Application number: CA241,003A
Authority: CA
Inventors: Akira Yasutake; Richard K. Oshiro
Original assignee: Omron Tateisi Electronics Co
Current assignee: Omron Corp
Priority date: 1974-12-04
Filing date: 1975-12-03
Publication date: 1979-06-19
Also published as: CH609467A5; FR2293761A1; DE2554417A1; FR2293761B1; JPS5182648A; GB1540838A; NL7514148A

Abstract

ABSTRACT OF THE DISCLOSURE
A field effect or twist nematic liquid crystal display is assembled having two transparent substrates with light polarizers affixed to the outwardly facing sur-faces of the substrates. The inwardly facing surfaces of -the substrate each have deposited thereon predetermined arrays of transparent electrodes. A transparent coating is applied overlying the transparent electrodes on each in-wardly facing surface of the two substrates by sputtering or vacuum deposition processes. The inwardly facing sur-faces of the coatings are formed to carry uniform unidirec-tional score marks throughout which are substantially parallel to marks on the same surface. The score marks are placed on the coating surfaces by buffing therefore requiring a moderately soft coating surface. A field effect liquid crystal is placed between the coatings in contact with the surfaces having the score marks thereon.
A fluid inpervious seal is placed surrounding the field effect liquid crystal and fused in place for containing the liquid crystal in the assembly and for isolating it from the surrounding environment.

Description

iOS~936 This invention relates to a high contrast liquid crystal display assembly and more particularly to such an assembly exhibiting non-degradable display characteristics over a long period of use.
Liquid crystal display cells in the past using field effect or twist nematic liquid crystals have carried a coating over electrodes on a glass substrate on either side of the liquid crystal. ~he coating has been an or-ganic film having typically a 100 to 400 Angstrom thick-ness, or a silicon dioxide film which may be placed on the glass substrates by vacuum deposition. When an organ-ic material is used as a coating, phenoxy or polyvinyl chloride are typical choices. An adhesion problem exists . between organic films and underlying electrodes and glass substrates. This organic coating or "surface agent" is used oftentimes in spite of its disadvantages, because the coating material contains long molecular chains with which the adjacant liquid crystal molecules tend to align in the absence of an electric field. When the liquid crystal display cell must be used in a high temperature environ-ment, or must be sealed in the assembly process at temper-atures from 300C to 400C, silicon dioxide is used for the coating overlying the electrodes and the surface on the glass substrates. Silicon dioxide is a very hard material, and it is diffucult to score the surface thereof in a uniform and unidirectional fashion so that any tend-ency for liquid crystal molecular alignment is absent.
Resulting performance provides lower display contrast levels and slightly slower visual response due to the lack ; 30 of preferred liquid crystal molecular orientation when the

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electric field is absent.
There is therefore a need for a liquid crystal display cell assembly which may be subjected to high temperatures during assembly for providing a liquid crystal seal which will assure long cell life. Moreover, a liquid crystal display assembly is needed which will provide high visual contrast and somewhat faster visual response to the application and removal of an electric field to the crystal. The need also exists for a readi~y applied strong coating having high adherent qualities so - that manufacturing yield of display assem~lies is improved.
A liquid crystal display has first and second transparent substrates and light polarizers attached to the outwardly facing surfaces of the substrate. The .
inwardly facing surfaces of the transparent substrates have predetermined patterns of electrodes attached there-- to with externally accessible electrical contact means. -A liquid crystal is positioned between the inwardly facing surfaces of the su~strates which each carry a transparent coatlng overlying the arrays of electrodes. The coating ` readily adheres to the electrodes and the inwardly facing ; -, surfaces of the substrates and carries uniform unidirec-tional score marks on the inwardly facing coating surface which is coterminous with the liquid crystal on each side . .~ . . .
thereof. A fluid impervious seal surrounds the arrays of electrodes and the liquid crystal at the edges thereof, for containing the liquid crystal and providing isolation from the surrounding environment. The fluid impervious seal is formed at a relatively high temperature readily adhering to the coatings.

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It is an object of the present invention to provide a liquid crystal display assembly which allows a high manufacturing yield.
It is another object of the present invention to provide a field effect liquid crystal display assembly which provides high contrast displays due to preferred molecular alignment within the liquid crystal.
It is another object of the present invention to provide a liquid crystal display assembly which may be subjected to relatively high tempera-tures during assembly or operation without reduced performance character-istics.
It is another object of the present invention to provide a liquid crystal display assembly which is isolated from the surrounding environment for providing a long period of nondegradable operation.
Thus, in accordance with one broad aspect of the invention there is provided, in a liquid crystal display having a liquid crystal element with opposing broad surfaces disposed between predetermined patterns of electrodes for application of electrical potentials thereto for creating electric fields therebetween to alter light transmission therethrough, the improvement comprising a transparent coating overlying the predetermined patterns of electrodes, said transparent coating being selected from the group consisting of magnesium fluoride, barium titanate, and titanium dioxide and being mechanically stable at temperatures up to 1000C, a uniform unidirectional pattern of score marks on said transparent coatings, said patterns of score marks being coterminous with the broad surfaces of the liquid crystal element, said liquid crystal molecules assuming a preferred orientation relative to said score marks in the absence of an electric field, ` whereby light transmission contrast is increased between liquid crystal `~ portions excited by an e~ectric field and adjacent quiescent liquid crystal portions.
In accordance with another broad aspect of the invention there is ~I_ provided a method of forming a liquid crystal display of the type using two opposing predetermined patterns of transparent electrodes arrayed in facing position on two substrates, comprising the steps of depositing a trans-parent coating selected from the group consisting of magnesium fluoride, barium titanate and titanium dioxide, having a substantially uniform thick-ness and a mechanicalb stable surface over the two patterns of electrodes, buffing the mechanically stable surfaces of the transparent coatings to obtain a uniform pattern of unidirectional score marks, disposing a liquid crystal between the buffed surfaces coterminous therewith, and sealing the periphery of the liquid crystal between the buffed surfaces, whereby the liquid crystal molecules at the boundaries coterminous with the buffed surfaces align with the score marks as a preferred orientation in the quiescent state and are reoriented by an electric field caused by electrical potential applied between electrodes on said two opposing patterns of electrodesO
Additional objects and features of the invention will appear from the following description in which the preferred embodiment has been set forth in detail in conjunction with the accompanying drawings.
ERIEF DESCRIRTION OF THE DRAWINGS
Figure 1 is a sectional view of one type of liquid crystal display assembly using the disclosed invention.
Figure 2 is an isometric view of one substrate in a liquid crystal assembly having a fluid impervious seal line applied thereto.
` Figure 3 is a sectional view along the line 3-3 of Figure 2.
Figure 4 is a sectional view of an additional liquid crystal display assembly.
Figure 5 is a sectional view of yet another liquid crystal display assembly.
A liquid crystal display assembly is shown in Figure 1. The liquid 1l)5~936 crystal display assembly of Figure 1 uses a field effect or twist nematic liquid crystal 11. The assembly of Figure 1 is of the type using reflected light for providing a visual display. One display assembly embodiment dis-closed herein includes a top polarizer 12 attached to the outwardly facing side of a transparent top substrate 13. Attached to the inwardly facing side of transparent substrate 13 is a transparent electrode 14 representing a predetermined pattern of such electrodes 14. Overlying electrode 14 is a - transparent coating 16 which adheres to the inwardly facing surfaces of top substrate 13 and transparent electrodes 14. Coating 16 has an inwardly facing surface in contact with liquid crystal 11. A lower substrate 17 has an outwardly facing surface with a bottom polarizer 18 attached thereto.
The inwardly facing surface of bottom substrate 17 also has formed thereon a predetermined pattern of transparent electrodes 19. Overlying transparent electrodes 19 and the exposed inwardly facing surface of bottom substrate 17 is another transparent coating 21 which adheres to the underlying surfaces.
Coating 21 has an inwardly facing surface contacting one side of liquid crystal 11. Liquid crystal 11 is confined between coatings 16 and 21 coterminous with -5a-,:~

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the surfaces thereof and confined by a seal 22 at the edges thereof. In this embodiment light is transmitte~ -through the assembly thus far described, and is reflected from the surface of a reflector 23 to traverse a path back through the liquid crystal display assembly for providing an image to an observer in accordance with electric fields within the liquid crystal 11 caused by electrical potentials applied across predetermined ones in the patterns of electrodes 14 and 19.
Figure 2 shows bottom substrate 17 with-trans-parent electrodes 19 deposited thereon and coating 21 overlying electrodes 19 and the inwardly facing surface of bottom substrate 17. Externall~ accessible conducting paths 20 are shown leading to the outside of the assembly for allowing electrical contact to be made with separate ones of the electrodes 14 and 19. Coating 21 is of a material which is mechanically stable, but is soft enough so that a uniform unidirectional pattern of score marks may be placed on the inwardly facing or exposed surface thereof by means of buffing, for example. It has been found that coating 21 should have a hardness less than 7 and preferably from 5 to 7 on the Mohs scale. Hardness levels less than this range appear too soft and smearing rather than scoring takes place. Hardness levels above this range present problems in obtainina uniform patterns of score marks due to the resistance of the surface to scoring.
Coating 21 may have the score marks aligned in a direction indicated by arrow 24 in Figure 2. A preferred material for coating 21 as well as coating 16 is magnesium .
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fluoride which may be placed overlyinq transparent eiec-trodes 19 and bottom substrate 17 by vacuum deposition or by the process of sputtering. The vacuum deposition process may typically take place in the range of 150C to 400C depending upon desired characteristics of the coating. The manner in which these characteristics are controlled is well-known in the art and is not a part of this disclosure. Vacuum deposition may take place at any .
temperature below the melting point of substrate 17. In general, strong adhesion to the underlying surface is exhibited by coatings 16 and 21, which adhesion is improved at higher temperatures. Magnesium fluoride coatings have been found to be mechanically stable at temperatures up to 1000C. A lower heat process is provided by sputtering which provides excellent adhesion to substrate 17 and transparent electrodes 19 as well as - allowing a wider range of substrate and electrode materials due to the low heat requirement.
Seal 22 in Figure 2 is shown surrounding the area on bottom substrate 17 which contains the predeter- -mined pattern of transparent electrodes l9. Seal 22 is placed on the assembly at this stage by means such as silk screening an emulsion of silicon dioxide in place, for example. Figure 3 shows bottom substrate 17 ! trans-parent coating 21 and the surrounding seal gasket 22, which is yet uncured.
Turning to Figure 4 top substrate 13 having the predetermined pattern of transparent electrodes 14 deposited thereon and the coating 16 overlying the pattern of electrodes 14 is shown. Liquid crystal 11 is shown :

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lOS~;936 disposed between the inwardly facing surfaces on coatings 16 and 21, each of which have been buffed to provide the uniform unidirectional pattern of score marks thereon as described above. Coating 16 may have the direc.tion of score marks as indicated by arrow 26 in Figure 4 for a purpose to be hereinafter descrihed. At this point in the assembly of the liquid crystal display the a.ssembly is subjected to a temperature in the range of 30~C to 500C for curing or fusing the silicon dioxide emulsion . 10 seal 22. Seal 22, through the process of fusion at high heat, becomes impervious to fluids and also permanently .
adheres to the inwardly facing surfaces of coatings 16 and 21, electrodes 14 and 19 or substrates 13 and 17 depending on which surface i.s adjacent seal 22. Fusion `~ temperature for seal 22 is limited by the softening '. temperature of substrates 13 and 17. Typical fusion .. steps for seal 22 include subjecting the assembly to

3~0C for 45 minutes or 400C for 10 minutes.
Seal 22 prolongs the life of the liquid crystal ` 20 display by barring passage of water molecules or gases which might migrate through the lattice provided by the large molecules of a typical low temperature plastic seal for a liquid crystal display. When water molecules migrate across the boundary of a seal in a liquid crystal display they are absorbed by the liquid crystal ' which lowers the resistivity of the liquid crystal causing a rise in current therethrough when an electrical poten-.
tial is applied between electrodes 14 and 19 for creating . an electric field. Levels in the range of 12 or 14 nanoamperes cause overheating in the thin transparent :

1~5~;936 electrodes 14 and 19 causing them to become opaque thus destroying the visual utility of the display. Gas migration across a seal such as seal 22 provides discontinuities in the display as gas pockets are formed therein.
Figure 5 shows top polarizer 12 and bottom polarizer 1~ assembled to the display. The assembly of Figure S may be used in the light trans- ~ -missive mode. When used in the light transmissive mode with top polarizer 12 having its axis of polarization aligned with arrow 26 and with bottom polarizer 18 having its polarization axis aligned orthogonal to that of polarizer 12 and parallel to arrow 24 of Figure 2, operation is as follows.
A light source located above top polarizer 12 will be observed by an observer positioned on the side of the assembly below bottom polarizer 18 since the twist nematic liquid crystal 11 twists the polarized light passing through top polarizer 12 through 90. When an electric field exists due to an electrical potential applied between electrodes 14 and 19, light polarized by the top polarizer 12 will proceed in the same polarized orientation due to the fact that the liquid crystal molecules become aligned with the electric field. The light polarized by top polarizer 12 then impinges orthogonal to the axis of bottom polarizer 18 and will be blocked by the . .
display assembly. Score marks in the directions of arrows 26 and 24 on coatings 16 and 21 respectively, provide preferred alignment direction~s .
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lOS~936 for the elongate molecules of the field effect li~uid - crystal in the absence of an electric field. The liquid crystal molecules also realign to the quiescent state somewhat more quickly with the score marks when they are present upon the removal of an electrical potential from transparent electrodes 14 and 19 thereby reducing display response time.
The embodiment of Figure 1 may have the polar-izing axes of top polarizer 12 and bottom polarizer 18 oriented 90 relative to one another. For that reason ; the angle between the directions of the score marks represented by arrows 24 and 26 on the surfaces of coat-ings 21 and 16 respectively has been represented as 90.
Other angles between the score marks on the surfaces of the two coatings may be utilized depending upon the pre-ferred direction of molecular orientation in the liquid crystal 11 in the quiescent state upon the removal of an electric field.
The embodiments of Figures 1 or 5 may contain a dynamic scattering type of liquid crystal 11. With this type of assembly the molecules in the liquid crystal 11 in the quiescent state assume an internal orientation which is approximately perpendicular to the broad surfaces of the crystal element 11. The surface molecules tend to assume a preferred orientation relative to the score marks 24 and 26. Upon application of control signals to ones of the externally accessible conducting paths 20 on sub-strates 13 and 17 the molecules assume a random orienta-tion between the associated electrodes 14 and 19 to scat-ter light, causing contrast differences which provide a visual display.
Magnesium fluoride is a preferred material for coatings 16 and 21 which must present a relatively soft mechanically stable surface. Barium titanate presents a harder surface, but such a material may be used for transparent coatings 16 and 21 and still be buffed to provide a uniform unidirectional pattern of score marks on the surface. Titanium dioxide has also been found to be useful as a material for coatings 16 and 21. Ti-tanium dioxide being a relatively hard material is also more difficult to buffuniformly than magnesium fluoride but has been found to produce satisfactory results. ~he titanium aloxide and barium titanate coatings lend them- -selves to the sputtering process for overlying and adher-ing to the transparent electrodes 14 and 19 and the sub-strates 13 and 17. Barium titanate and titanium dioxide coatings have also been found tD be mechanically stable at temperatures up to approxima~ely 1000C.
Referring to Table I ~elow, a comparison is made between the old art organi~ ~surface agents" and the disclosed coatings. As may be seen, thick organic transparent coats adjacent to the liquid crystal element may produce high contrast displays. Thickness from 5000 Angstroms to 1 micron are generally considered in the "thick" range. Such organic coatings are capable of exposure to relatively low temperatures only however, which limits the seal which may ~e utilized in the display assembly to more porous materials than those which may be used in seal 22. A thin coating of organic material is undesirable since manufacturing yield drops. Contrast '~' .: .

lV56936 suffers also since uniformity of the coating in such a display is difficult to obtain, thereby causing variation in contrast in the display.
The disclosed coatings 16 and 21 may be used in a variety of display assembly configurations with a var-iety of liquid crystal element types. Table I shows that thick coatings 16 and 21 of magnesium fluoride, barium titanate or titanium dioxide provides high contrast, high manufacturing yield and high temperature capability.
The manufacturing yield advantages accrue from the high degree of adhesion between the coatings 16 and 21 and the underlying surfaces as well as from the internal struc-tural strength of the coatings.
Thin coatings 16 and 21 also provide the thick coat advantages plus desira~le ~ow display operating voltages. The thin coatings may be in the range of 5 to 50 Angstroms. Coatings have been produced ranging from ... . . . .
several angstroms to one micron in thickness. The coatings 16 and 21 disclosed herein are adjacent to the liquid crystal element 11 in the display and have a uni-form density of unidirectional score marks on the coating surfaces which are coterminous with the broad surfaces of the liquid crystal element 11. ~he elongate molecules of the unexcited liquid crystal element, whether it be a twist nematic or dynamic scattering type, generally align with the score marks. Thus, when a portion of the liquid crystal element between two electrodes 14 and 19 is excited by an electric field, the aligned molecules at the surfaces of the quiescent portions of the liquid crystal provide high light transmission contract between 11~5~936 the excited and quiescent portions.

TABLE I
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COATING REL. COAT RELATIVE REL. MFR TYP. SEAL OPER
THICKNESS CONTRAST YIELD CURE TEMP VOLT

ORGANIC THICK HIGH INCREASED 150C HIGH , DISCLOSED ~HIN HIGH INCREASED 400C LGW

A liquid crystal display assembly has been disclosed which provides a long usable life due to the fluid inpervious seal provided about the periphery of the liquid crystal, which provides a high contrast between light and dark areas due to the preferred alignment of the liquid crystal molecules with the direction of the score marks on the internal coatings in the quiescent state. Reduced display response time is also provided due to the aforementioned preferred alignment tendency of the elongate liquid crystal molecules.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A field effect liquid crystal display com-prising first and second substrates substantially impervious to fluids and having respective first and second opposing surfaces thereon, a pretetermined pattern of the transparent electrodes attached to each of said first and second opposing surfaces, contact means for providing external electrical access to ones of said predetermined patterns of electrodes, a coating selected from the group consisting of magnesium fluoride, barium titanate and titanium dioxide overlying each of said predetermined patterns of electrodes and said first and second opposing surfaces, a field effect liquid crystal disposed between said coatings, a seal gasket surround-ing said patterns of electrodes extending between said coatings and adhering thereto for containing said liquid crystal in isolation from ambient environments, each of said coatings having a uniform pattern of substantially parallel score marks in contact with said liquid crystal for providing a preferred liquid crystal molecular align-ment direction in the absence of an electric field, and top and bottom polarizers disposed on opposite sides of said field effect liquid crystal, whereby visual contrast between areas passing light through the layers of the display and areas blocking light from passing there-through may be observed throughout long usage in accordance with electric fields resulting from appli-cation of electrical potentials between ones of said predetermined patterns of transparent electrodes through said contact means, and fast visual response is provided due to the preferred molecular alignment direction induced by said score marks.

2. A field effect liquid crystal display as in Claim 1 wherein said seal gasket comprises a particu-late glass deposition which is fused in place at a high temperature during assembly of the display to provide a substantially fluid impervious seal, said coating being free of adverse effect due to said high temperature.

3. A field effect liquid crystal display as in Claim 1 wherein said score marks on the coatings on said first and second surfaces are aligned with the polarizing axes of said top and bottom polarizers respectively, whereby liquid crystal molecules are urged away from alignment with the polarizing axes by an electric field.

4. In a liquid crystal display of the type having first and second transparent substrates, light polarizers attached to the outwardly facing surfaces of the substrates, a predetermined array of externally accessible transparent electrodes attached to the inwardly facing surface of the substrates, and a liquid crystal element positioned between the first and second substrates, the improvement comprising a transparent coating overlying each array of transparent electrodes adhering to the electrodes and the substrates and having a surface in contact with the liquid crystal, said transparent coating being selected from the group consisting of magnesium fluoride, barium titanate, and titanium dioxide, a seal surrounding the arrays of electrodes and adhering to said transparent coatings for protecting the liquid crystal from the surrounding environment, said surface in contact with the liquid crystal having uniform unidirectional score marks thereon in contact with the liquid crystal for causing preferred alignment directions for molecules in the liquid crystal element, whereby application of electrical signals causing an electric field between electrodes on opposing sides of the liquid crystal causes the liquid crystal molecules to tend to depart from the preferred alignment with the direction of said score marks.

5. A liquid crystal display as in Claim 4 wherein said seal is substantially fluid impervious glass material having a fusion temperature above 300°C
for curing the seal, thereby providing prolonged isolation from surrounding environments.

6. In a liquid crystal display having a liquid crystal element with opposing broad surfaces disposed between predetermined patterns of electrodes for applica-tion of electrical potentials thereto for creating elec-tric fields therebetween to alter light transmission therethrough, the improvement comprising a transparent coating overlying the predetermined patterns of electrodes, said transparent coating being selected from the group consisting of magnesium fluoride, barium titanate, and titanium dioxide and being mechanically stable at temper-atures up to 1000°C, a uniform unidirectional pattern of score marks on said transparent coatings, said patterns of score marks being coterminous with the broad surfaces of the liquid crystal element, said liquid crystal mole-cules assuming a preferred orientation relative to said score marks in the absence of an electric field, whereby light transmission contrast is increased between liquid crystal portions excited by an electric field and adjacent quiescent liquid crystal portions.

7. A liquid crystal display as in Claim 6 wherein said transparent coating has a thickness in the range of several Angstroms to one micron in thickness.

8. A method of forming a liquid crystal display of the type using two opposing predetermined patterns of transparent electrodes arrayed in facing position on two substrates, comprising the steps of depositing a trans-parent coating selected from the group consisting of magnesium fluoride, barium titanate and titanium dioxide, having a substantially uniform thickness and a mechani-cally stable surface over the two patterns of electrodes, buffing the mechanically stable surfaces of the trans-parent coatings to obtain a uniform pattern of unidirec-tional score marks, disposing a liquid crystal between the buffed surfaces coterminous therewith, and sealing the periphery of the liquid crystal between the buffed surfaces, whereby the liquid crystal molecules at the boundaries coterminous with the buffed surfaces align with the score marks as a preferred orientation in the quiescent state and are reoriented by an electric field caused by electrical potential applied between electrodes on said two opposing patterns of electrodes.

9. A method as in Claim 8 together with the step of fusing the seal at the periphery of the liquid crystal at a temperature above 300°C to form a fluid im-pervious barrier for isolating the liquid crystal from ambient environments.

10. A liquid crystal electro-optical element which comprises a pair of electrode plates confronting each other and a thin layer of nematic liquid crystal material having a positive dielectric anisotropy positioned between the two plates, each of the said electrode plates being coated on its interior face with a thin layer of magnesium fluoride.

11. In a liquid crystal display of the type having first and second transparent substrates, light polarizers attached to the outwardly facing surfaces of the substrates, externally accessible transparent electrodes attached to the inwardly facing surface of the substrates, and a liquid crystal element positioned between the first and second substrates, the improvement comprising a transparent coating overlying and adhering to each electrode, and having a surface in contact with the liquid crystal, said transparent coating consisting of magnesium fluoride, a seal at the edges of the electrodes and adhering to said transparent coatings for protecting the liquid crystal from the surrounding environment, said surface in contact with the liquid crystal having uniform unidirectional score marks, thereon in contact with the liquid crystal for causing preferred alignment directions for molecules in the liquid crystal element, whereby application of electrical signals causing an electric field between the electrodes on opposing sides of the liquid crystal causes the liquid crystal molecules to tend to depart from the preferred alignment with the direction of said score marks.

12. A method of forming a liquid crystal display of the type using two opposing transparent electrodes arranged in facing position on two substrates, comprising the steps of depositing a transparent coating consisting of magnesium fluoride having a substantially uniform thickness and a mechanically stable surface over the two electrodes, buffing the mechanically stable surfaces of the transparent coatings to obtain a uniform pattern of unidirectional score marks, disposing a liquid crystal between the buffed surfaces coterminous therewith, and sealing the periphery of the liquid crystal between the buffed surfaces, whereby the liquid crystal molecules at the boundaries coterminous with the buffed surfaces align with the score marks as a preferred orientation in the quiescent state and are reoriented by an electric field caused by electrical potential applied between said two opposing electrodes.