GB2366874A

GB2366874A - Liquid Crystal Display

Info

Publication number: GB2366874A
Application number: GB0110320A
Authority: GB
Inventors: Stephen Buchwalter; Christos Dimit Dimitrakopoulos; Kang-Wook Lee; Shui-Chih Alan Lien; Sampath Purushothaman
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 2000-06-26
Filing date: 2001-04-27
Publication date: 2002-03-20
Anticipated expiration: 2021-04-27
Also published as: CN1330282A; GB2366874B; GB0110320D0; JP2002023169A; KR20020014996A; TW548490B

Abstract

In the invention, the technology of the formation of self assembled monolayers is employed in imparting vertical alignment and domain favoring features in homeotropic pixel elements in the technology of liquid crystal type displays. Substituted silane compounds, such as octadecylsilane when applied on a transparent conductive layer surface such as that of indium tin oxide in a hydrolyzable alcohol solution, and then followed by a baking temperature cycle, form essentially single thickness molecule layers (monolayers) of molecules that are bonded at one end to the indium tin oxide surface and have the other end of the molecule operable to influence molecular orientation and domain preference of a liquid crystal.

Description

2366874 LIQUID CRYSTAL DISPLAY The technical field of the invention is in

liquid crystal displays and in particular to the fabrication of liquid crystal displays having a 5 wider viewing angle through the use of homeotropic pixel cells with self assembled monolayer alignment.

Liquid crystal displays, which among their attributes provide the ability to be configured as a flat panel, have become increasingly 10 important in industries including the computer industry where the display of information is involved. These types of displays provide unique opportunities for lowering the weight, size and eventually the cost of displaying information.

15 One of the problems in the art, however, involves the range of movement that a viewer of the display can have. In the display, light at each pixel, is permitted to pass or is inhibited from passing through the liquid crystal by the orientation of the molecules in the liquid crystal in response to a signal at the pixel location. The range of movement of the 20 viewer, while still having display quality, is related to how well the assembly, at each liquid crystal pixel, is able to shut off light passing through the liquid crystal when there is no signal, and how well it permits light to pass when there is a signal.

25 At the present state of the art attention is being given to the use in display assemblies of vertically aligned, known as homeotropic in the art, liquid crystal polymer materials in pixel increments between essentially parallel transparent conductor bearing surfaces wherein the molecules of the polymer are aligned perpendicular to the surface plane 30 when there is no applied signal, and particular domain favoring features are imparted to the liquid crystal.

In one reference in the art, US patent 5,309,264, of A. Lien and R.

John, there is described how one can use a fringe field produced by

35 electrode cutouts to impart multi-domains in a liquid crystal display (LCD) to improve the viewing angle range of conventional LCDs. In another reference in the art, US patent 5,907,380, A. Lien describes the use of a fringe field and polymer wall (ridge) to produce a multi-domain liquid crystal display (LCD) for the purpose of improving the viewing angle of 40 conventional LCDs. In each of the above described multi-domain homeotropic liquid crystal displays, a homeotropic (vertically aligned) polymer was used as the liquid crystal layer, which layer aligns the liquid crystal molecules perpendicular to the surface plane, when there was no voltage applied.

There is a problem in the art however in that deposition of such a polymeric alignment layer and the imparting of particular domain favoring features involves multi-step and somewhat complex processes such as casting, curing and rubbing so that simplifcation of the alignment layer 5 deposition process and the lowering of its cost is of substantial importance.

In totally different arts, involving interfaces used in such examples as lubrication and biological membranes, work has been reported in forming 10 supported monolayers of organic molecules with ordered structures in which the order is a self assembled result of the fabrication process.

In one example of such work, reported by R.G. Nuzzo and D.L. Allara in the Journal of American Chemical Society vol. 105, pp. 4481-3, 1983, 15 self assembled monolayers, which are essentially single layers of organic molecules, are described as having end groups selectively adsorbed to the surface of a solid. The solid is usually an inorganic material or a metal.

In other examples of such work,self assembled monolayers of thiols, 20 adsorbed on gold, have been found to have ordered structures, with methylene (CH2) groups that have all trans configurations. In work reported by J. Sagiv, in the Journal of the American Chemical Society, vol. 102, pp.92-98, 1980) self assembled monolayers of n-octadecylsilane were prepared on glass surfaces using hydrolyzed n-octadecyltrichlorosilane. in 25 work reported by C.R. Kessel and S. Granick, Langmuir, vol 7, pp. 532- 538, 1991 well ordered monolayers are formed on cleaved mica by hydrolyzing n-octadecyltrimethoxysilane.

In the invention, the technology of the formation of self assembled 30 monolayers is employed in imparting vertical alignment and domain favoring features in homeotropic pixel elements in the technology of liquid crystal type displays. Silane compounds, such as octadecylsilane when applied appropriately on a transparent conductive layer surface such as that of indium tin oxide, and followed by a baking temperature cycle, form 35 essentially monolayers of molecules that are bonded at one end to the indium tin oxide surface and have the other end of the bonded molecule operable to influence molecular orientation of a liquid crystal.

The drawings illustrate an embodiment of the invention.

FIG. 1 is a top view of the substrate in the formation of a conventional crossover type active-matrix liquid crystal display arrangement.

FIG. 2 illustrates at a larger scale a partial cross section in the direction of the line (A-A) in Fig.1 of the liquid crystal display arrangement with the electrodes and the liquid crystal material in place.

S FIGS. 3 (A) and 3 (B) illustrate the orientation of the liquid crystal molecules in a homectropic liquid crystal cell.

Referring to Figure 1 there is shown a top view of the substrate in the formation of a conventional crossover type active-matrix liquid crystal 10 display arrangement in a conventional liquid crystal display device employing the invention, wherein an exemplary two row, three column array of six pixel electrodes 26 is initially formed that will be below the pixels of the display. The pixels are formed in the conventional arrangement between gate lines 32 and data lines 31. The active matrix is elements such as thin film transistors (TFT) are shown in Fig. 2.

Referring to both Fig. 1 and Fig 2; in Fig. 2 there is illustrated at a larger scale a partial cross section in the direction of the line (A-A) in Fig.1 of the liquid crystal display arrangement with the electrodes and 20 the liquid crystal material in place and illustrating the application of self aligned monolayer technology. The array of Fig. 2 shows a substrate member 22 and a support member 24 of a transparent material such as glass.

The two members 22 and 24 are arranged so as to be parallel to one another with a high degree of precision. Typically, the members 22 and 24 are 25 separated from one another by a distance of approximately one to twenty micrometers, and are sealed at their edges (not shown) so as to provide a closed interior space there between. The substrate member 22 has deposited thereon an array of pixel electrode films 26 which define pixels of the liquid crystal display. Also formed on substrate member 22 in selected 30 areas not having electrode films 26 deposited thereon are semiconductor devices 30 such as diodes or thin film transistors (TFTs). In standard practice in the art, there will be one or more TFTs 30 for each pixel. The TFTs 30 will each be controlled by a conductive gate line 32, as shown in Fig. I and a conductive data line 31 as shown in both Figs 1 and 2, which 35 are typically deposited on substrate member 22 in a manner so as not to be electrically connected to electrodes 26 except that the source electrode of each TFT 30 is electrically connected to one respective electrode film 26.

Gate lines 32 (not shown in Fig. 2) and data lines 31 are also electrically insulated from one another at crossover regions. The support member 24 40 typically has deposited thereon a color matrix layer 23. The color matrix layer 23 typically has a black matrix material 23-1 interleaved with R, G, or B color matrix material 23-2. The black matrix material 23-1 is disposed opposite the TFTs 30, data line 31 and gate line 32 (visible only in Fig.1) in order to block the devices from ambient incident light and prevent light leakage through outside the pixel area. The color matrix material 23-2 is disposed opposite the pixel electrode 26. In addition, a continuous electrode 28 is typically formed on the color matrix layer 23.

The continuous electrode 28 is preferably formed of a thin transparent 5 layer of a conductive material, such as indium tin oxide (ITO) or other suitable material The liquid crystal material 36 fills the space between members 22 and 24. In some applications the nature of the liquid crystal material will 10 vary with the mode of operation of the liquid crystal display 20.

In accordance with the invention the interior surfaces in contact with the liquid crystal material 36 are coated with at least one of alignment layers 38 and 40 that are layers of ordered molecular structure 15 that operate to vertically align the liquid crystal molecules of the material 36 in the cell gap between the electrodes 26 and 28 and to impart at least one domain preference.

The molecular orientation for a homeotropic liquid crystal is 20 depicted in Figures 3A and 3B.

Referring to Fig. 3A, the orientation of the liquid crystal molecules 50 at least in the vicinity of the members 26 and 28 are aligned so that the long axes of the molecules 50 are almost perpendicular to the 25 respective member 26 and 28 surfaces when no electric field ' V = 0) is applied between the pixel electrode 26 and the electrode 28. The molecules have a small pretilt angle, typically one to fifteen degrees of tilt, away from the substrate normal.

30 Referring to Fig. 3B, when an electric field V 0 is applied between the respective member 26 and 28 surfaces, at least a layer portion 51 of the liquid crystal molecules are caused to be oriented in a direction that is substantially perpendicular.

35 In accordance with the invention, in the application of the self assembled monolayer technology to the liquid crystal display technology, a monolayer forming self assembly material is selected for the transparent layer material to be used in the liquid crystal display technology that meets the interdependent criteria that it can be suspended in a vehicle, 40 with the suspension being susceptible to a deposition operation through dipping or spinning wherein surface tension establishes a thickness and in turn the vehicle can then be driven off within a normal time and temperature excursion process window.

The material indium tin oxide is the most frequently used at this stage of the art for transparent conductive members.

A satisfactory vehicle is an alcohol.

There is a family of silane compounds that when hydrolyzed and in an alcohol vehicle will dissociate into two compounds one of which will react more favorably with the transparent conductor material and satisfactorily provide a thin uniform layer on dipping or spinning and will react in 10 temperature cycle of about 100 degrees C for a fairly wide time range in forming self assembled monolayers on an indium tin oxide surface.

The family of silane compounds include substitutions of R- Si X where R is an akyl group and X is a member of the group consisting of Cl, 15 OCH and OC H. The akyl groups typically contain twelve to twenty two carbon atoms.

In the practice of the invention the following example process may be employed. Referring to Figures 1 -3, in the fabrication of alignment layers 20 38 and 40 on the transparent layer 28 using the indium tin oxide; the material n-octadecyltrimethoxysilane, known in the art as (OTMS), is hydrolyzed to n-octadecyltrihydroxysilane in a mixture of isopropanol and water or in ethanol and coated by dipping onto a substrate with a layer of indium tin oxide on it. The coated substrates are then baked at 85 degrees 25 C for 10 to 30 minutes. In the baking, the silane end molecular groups of n-octadecylsilane become bound to the surface of the indium tin oxide layer whereas the methyl end molecular groups of n-octadecylsilane are oriented away from the surface and the n- octadecyl molecules are oriented almost perpendicular to the surface. The configuration of the methylene groups in 30 the n-octadecylsilane is all trans.

The n-octadecyl chains can be either densely packed or loosely packed. It has been observed that loosely packed self assembled n-octadecylsilane monolayers tend to produce better homeotropic alignment 35 of liquid crystal materials, particularly nematic type.

Returning to Figures 1,2 and 3, the exterior surfaces of members 22 and 24 may have respective optical compensating films 42 and 44 disposed thereon. Finally, respective polarizing films 46 and 48 may be applied 40 over compensation films 42 and 44 respectively, where compensating films are used; or applied over members 22 and 24 respectively, where compensating films are not used.

Conventional liquid crystal displays of the type illustrated in Figures 1 and. 2 are illuminated by a light source (not shown) that is located below the array on the substrate 22 side and viewed from above on the support member 24 side. The invention is applicable to transmissive 5 light and reflective light liquid crystal display technologies.

Liquid crystal cells typically are characterized by a pixel area and cell gap. The pixel area of a given cell is defined by the width W and the length L of the pixel electrode film 26 pattern of the cell as illustrated 10 in FIG.S I and 2. In addition, the cell gap is defined by the distance between the layers 38 and 40 as shown in FIG. 2.

Homeotropic liquid crystal cells require a liquid crystal material that exhibits negative dielectric anisotropy. Examples of such materials 15 are ZLI-4788 or ZLI-2857 manufactured by the E. Merck company of Darmstadt, Germany and available in the United States through EM Industries.

Heretofore in the art the alignment of the liquid crystal molecules of homeotropic cells has typically been provided by placing alignment 20 layers of an abrasion responsive material such as polyimide in the cell gap adjacent the electrodes and then having a rubbing step that is usually manual among the fabrication operations. An example of such rubbing is described in K-W. Lee et al-, "Microscopic Molecular Reorientation of Alignment Layer Polymer Surfaces Induced by Rubbing and its Effects on LC 25 Pretilt Angles", Macromolecules, Vol. 29, Number 27, Pages 8894-8899.

In accordance with the invention a valuable and important benefit is that the alignment layers 38 and 40 permit self assembly of the molecular arrangement and thereby the removal of any need for the rubbing type 30 operation that has been used heretofore in the art.

It will further be apparent that the application of the self assembling monclayer technology of the invention can be practiced with and to improve the fringe techniques of the referenced prior art as described

35 in U.S. Patents 5,309,264 and 5, 907,380.

In any manufacturing process employing the self assembled monolayer technology will be almost the same as the current technology with the exception of the structural variation and process of applying alignment 40 layers.

As is well known in the art, homeotropic liquid crystal cells typically use a compensating film to reduce dark state light leakage for light that travels through the liquid crystal display panel in a direction other than perpendicular to the substrates. For best results, the product of the thickness of the liquid crystal material layer in the liquid crystal display cell and the difference between the extraordinary and ordinary indexes of refraction for the liquid crystal display material is equal to 5 or close to the product of the total thickness of the compensating films and the difference between extraordinary and ordinary and extraordinary indexes of refraction of the compensating film.

It will be apparent that there will be many variations in the 10 application of the principles of the invention as may be seen from the following examples.

As one example substrates bearing a transparent conductive layer such as indium tin oxide are dipped into an alcoholic solution of self 15 assembling monolayer precursor materials The materials n-octadecyltrimethoxysilane, known in the art as (OTMS), and nhexadecyltriethoxysi lane known in the art as (HTMS), are hydrolyzeable into n-octadecyltrihydroxysilane and n-hexadecyltrihydroxysilane, respectively, by mixing in a solvent of isopropanol and water or in 20 ethanol. The concentration of OTMS or HTMS will be in the range of 0.11.0 volume %. Either partially or fully hydrolyzed solutions may be applied onto the indium tin oxide layer on a substrate by the operations of dipping or spinning, and then the indium tin oxide bearing substrates are baked at 85 IC for 10-30 min. Nematic liquid crystal material such as 25 would meet the 95-463 type specifications, with negative dielectric anisotropy is then placed between two substrates in a sandwich arrangement, each substrate having coated indium tin oxide layers over which self assembling alignment layers have been placed. The quality of the liquid crystal vertical alignment is then verifiable by observing light 30 transmission through two polarizers positioned, one at the bottom and the other at the top, where, if a sandwich assembly has homeotropic alignment, light would be blocked, As another example, indium tin oxide coated glass is cleaned in 35 several steps including initial degreasing in an organic solvent such as chloroform. The cleaning is then followed by a 10 minute UV-ozone treatment of each side of the glass substrate to eliminate any existing organic contamination. The substrate is then rinsed thoroughly with deionized water and it should be wetted completely by it, an indication 40 that there is no presence of organic contaminants on the surface. The glass substrate is dried with clean nitrogen and subsequently placed in an oven at 100 degrees C for an hour after which treatment there will be on the indium tin oxide surface, a bound monolayer of water. An OTMS solution in ethanol is then prepared.

A concentration of precursor in alcoholic solution in the range of 0.01 to 5.0 vol % provides satisfactory results. A concentration of 3.59 v/v is preferred.

Satisfactory results are achieved using spin coating and dipping deposition techniques for different amounts of time.

Following deposition of OTMS on the indium tin oxide surface, an 10 optional cleaning step can be employed involving immersion in chloroform and/or rinsing with deionized water. The substrates are then dried with clean nitrogen.

Where spinning is employed, an about 2 ml of solution is placed on is the indium tin oxide coated glass substrate as it rests on the spinner head, and left there for about 3.0 minutes before being spun.

The substrate with the indium tin oxide coating and with the deposited solution is then dried with clean nitrogen and baked at 100 20 degrees C for an hour. This time is sufficient to create a self assembled monolayer on the indium tin oxide that operates to induce satisfactory vertical alignment of the liquid crystal molecules.

In the fabrication process there is some flexibility with higher 25 quality being achieved with solution concentrations in the vicinity of 3.59 -.v/v with a tradeoff being observed with dipping times and concentrations.

The extent and quality of the vertical alignment is conveniently evaluated by observing the light transmission through two crossed 30 polarizers, placed above and below the cell. In case of perfect homeotropic alignment light would be blocked. If there is disorder or misorientation of the molecules, light will pass through the crossed polarizers.

35 While the invention has been described in connection with specific embodiments, it will he understood that those with skill in the art may be able to develop variations of the disclosed embodiments without departing from the spirit of the invention or the scope of the following claims.

Claims

1. Liquid crystal display technology wherein the transmission of light, passing through a pixel area quantity of liquid crystal material having 5 signal responsive alignable molecules positioned between and forming an interface with each of first and second essentially parallel transparent conductive member surfaces, is caused to change in response to signals applied between said transparent conductive members, characterised by:

at least one alignment layer positioned in at least one said 10 interface between at least one of said first and second transparent conductive members and said liquid crystal material, each said alignment layer having at least one layer of elongated molecules having first and second end groups of atoms, said first end group of atoms of each molecule of said layer of 15 elongated molecules having a chemical bond at a surface of an adjacent said at least one of said first and second transparent conductive members, and, each said elongated molecule with second end group of atoms, extending essentially perpendicular to said surface and into said liquid 20 crystal material.

2. The liquid crystal display technology of claim I wherein said transparent conductive members are of indium tin oxide and said elongated molecules are substituted silane compounds.

3. The liquid crystal display technology of claim 2 wherein said substituted silane compounds include R- Si X where R is an akyl group and X is a member of the group consisting of Cl, OCH 3 and OC H.

30 4. Liquid crystal display technology wherein the transmission of light, passing through a pixel area quantity of liquid crystal material having signal responsive alignable molecules positioned between and forming an interface with each of first and second essentially parallel transparent conductive member surfaces, is caused to change in response to signals 35 applied between said transparent conductive members, characterised by:

a monolayer alignment member positioned in at least one said interface between a transparent conductive member and said liquid crystal material, each said alignment member being operable, to orient said alignable molecules of said liquid crystal perpendicular to said 40 essentially parallel transparent conductive member surfaces, and, to impart a preferential domain to said alignable molecules.

S. A liquid crystal display pixel area cell wherein the transmission of light, passing through a quantity of liquid crystal material having signal responsive alignable molecules positioned between and forming an interface with each of first and second essentially parallel transparent conductive member surfaces of said cell, is caused to change in response to signals applied between said transparent conductive members, characterised by:

5 a monomolecular thickness layer of substituted silane molecules positioned in each interface between a transparent conductive member surface and said liquid crystal material with one end of each of said substituted silane molecules bonded to a surface of said transparent conductive member and the remaining end of each said substituted silane 10 molecule extending into said liquid crystal.

6. A method of fabricating a homeotropic liquid crystal pixel including a layer of liquid crystal material with a molecular orientation essentially perpendicular to an alignment layer and with at least one preferential 15 domain, for use in a display, comprising in combination the steps of:

providing, a first transparent conductive material member having an exposed surface, on a supporting substrate, applying, to said exposed surface of said first transparent conductive material member, a thin coating from a solution of a compound, that has an elongated molecule that 20 forms a monolayer of said elongated molecule to said first transparent conductive member, baking the combination, of said substrate, said first transparent conductive member and said coating of said compound in said solution for a time and at a temperature operable to drive off solvent from said solution 25 and bond said compound in said coating to said first transparent conductive material member, placing a first surface of an approximately 20 micrometer thick layer of liquid crystal material on the exposed surface of said baked combination, and, 30 positioning a second transparent conductive material member on the second and remaining surface of said layer of said liquid crystal material 7. The method of claim 6 wherein said providing, applying and baking steps are applied to said second transparent conductive member and said 35 layer of liquid crystal material is placed between them.

B. The method of claim 6 wherein said first transparent conductive material is indium tin oxide.

40 9. The method of claim 8 wherein said applying step is taken from the group of processes consisting of immersing and spin coating.

10. The method of claim 9 wherein said solvent is an alcohol.

11. The method of claim 10 wherein said supporting substrate is glass.

12. The method of claim 6 wherein in said applying step said compound is taken from the group of R-Si X silane compounds where R is an akyl group 5 and X is a member of the group consisting of Cl, OCH and OC H.

13, A method of fabricating a homeotropic liquid crystal pixel including a layer of liquid crystal material with a molecular orientation essentially perpendicular to the alignment layer and with at least one preferential 10 domain, for use in a display, comprising in combination the steps of:

providing, a layer of indium tin oxide having an exposed surface, on a supporting substrate, applying, to said exposed surface a thin coating of a substituted silane compound that has been hydrolyzed to have a molecule with a favored reactivity with indium tin oxide for one end group, said 15 compound being in a mixture of isopropol alcohol solvent and water, baking the combination, of said substrate, said first transparent conductive member and said hydrolyzed coating of said substituted silane compound for a time and at a temperature operable to drive off said solvent and whereby end molecular groups of n-octadecylsilane become bound at said exposed 20 surface to said indium tin oxide forming an approximately monomolecular layer, placing a first surface of an approximately 20 micrometer thick layer of liquid crystal material on the exposed surface of said baked combination, and, positioning a second transparent conductive material member on the 25 second and remaining surface of said layer of said liquid crystal material.

14, The method of claim 13 wherein said silane compounds are of the R-Si X where R 2 is an akyl group and X is a member of the group consisting of Cl, OCH and OC H.

15. The method of claim 14 wherein said substrate is of glass.

16. The method of claim 1S wherein said solvent is isopropanol.

35 17. The method of claim 16 wherein the concentration of said substituted silane compound is 0.01-5 vol' 18. The method of claim 17 wherein said liquid crystal is the nematic type.