WO1998052096A1

WO1998052096A1 - Lcd device

Info

Publication number: WO1998052096A1
Application number: PCT/GB1997/002696
Authority: WO
Inventors: Andrew Carrington
Original assignee: Central Research Laboratories Limited
Priority date: 1997-05-09
Filing date: 1997-09-30
Publication date: 1998-11-19
Also published as: GB9709457D0

Abstract

An LCD device comprises two spaced transparent plate assemblies, each comprising a plate (1, 2), barrier layer (6), alignment layer (7), array of electrodes (4, 5), and spacers (10, 12) for maintaining a space between the plate assemblies for a liquid crystal material (3). The spacers (10) comprises spaced elongate strips extending across the plate assemblies; and elongate adhesive strips (12) for bonding the assemblies together. The adhesive strips extend across the plate assemblies parallel to the spacers and defining together with the spacers parallel channels for the liquid crystal material (3). One of the plates is provided with a colour filter (2a) comprising elongate regions of coloured pixels (2b) and elongate non-pixel regions (2c), and the strips of said spacer means and said adhesive are located at the non-pixel regions. The regions of coloured pixels are arranged in a repeating pattern of parallel lines of red, green and blue pixels, and the strips of said spacers and said adhesive are located between each line of coloured pixels.

Description

LCD DEVICE

This invention relates to Liquid Crystal Display devices and in particular to a method of manufacturing the same. The invention is particularly concerned with Digital Ferroelectric Liquid Crystal Displays (DFLCD), although it is not limited to DFLCD devices.

A LCD device of the general type to which this invention relates is described in a paper entitled "An 8.5"Digital Ferroelectric Video Rate Colour Display" by PWH Surguy et al presented to the 1990 Eurodisplay conference and published in "Eurodisplay '90" at page 146 et seq.

A further prior known LCD device is known from GB-B 2157451. In such devices the facing surfaces of the two optically transparent plates are normally provided with respective arrays of optically transmissive electrodes, for example, mutually orthogonal grids of indium tin oxide electrodes, by means of which portions of the liquid crystal material can be selectively addressed. Such addressing enables optical properties of selected portions of the material to be changed. If the assembly of the two plates is positioned between crossed polarisers, the said addressing can result in modification of the optical transmissivity of selected areas of the resulting device, thereby enabling it to be used, for example, as a television display device.

In the device known from GB-G-2157451 the spacer members take the form of parallel solid walls all of which are secured to one plate and extend from one side of the plate to the other. According to the disclosure of GB-B -2157451 the walls of the device displaced therein serve the additional purpose of improving monodomain formability in the liquid crystal material, and to this end they are oriented parallel with or perpendicular to a monaxial orientation treatment (conventionally a rubbing treatment) which has been applied to the face of at least one of the two plates contacting the liquid crystal material.

Prior known LCD devices can be subjected to substantial temperature differences and differences in thermal expansion co-efficients of the materials used in the construction of LCD devices can give rise to structural movements and variation of the spacing between the plates. With cell spacings of the order of 1.3 to 1.5μ, which is common with such devices, only minor variations in the spacings can disrupt the correct working of the LCD device. US Patents 5,285,304 and 5,581,384 acknowledged that cell thickness variations caused a yellowing effect and created voids in the liquid crystal material.

Hence the design of cell spacers, and the bonding of the plates together to form accurate spacing is very critical and important. LCD devices can, moreover, be subjected to mechanical deformation forces which cause parts of the liquid crystal material to move relative to other parts thereof. If the spacing between the plates is small and the liquid crystal material has a high viscosity this can result in changes in physical properties of parts of the material which have a deleterious effect on operation of the device and which can only be reversed with difficulty.

In the main, the most common methods of maintaining accurate spacing of the plates and bonding of the plates, is to use a random mixture of precisely dimensioned glass sphere spacers and thermoplastic spheres (of slightly larger diameter than the glass spheres) as the adhesive. A mixture of these spheres are deposited by blowing a cloud of mixed glass spheres and thermoplastic spheres onto one of the plates, and laying the other plate on to the deposited spheres. Bonding is achieved by heating the assembly to cause melting of the thermoplastic beads. Examples of this type of construction are described in US Patents 4,732,961, 5,581,384 and 5,285,304. In our US Patent 5,459,598 we describe an alternative method of manufacturing a ferroelectric LCD device which comprises forming the spacers as spaced blocks alternately on each sheet or substrate, so that when assembled, the blocks form elongate parallel walls and define between them a channel in which the liquid crystal materials is contained. Located in each of the channels where the liquid crystal material is contained are thermoplastic sticky adhesive beads and glass spacer beads which are randomly scattered along the channels.

One of the problems with all of the prior known devices mentioned above, is that at least the sticky adhesive beads, and sometimes the glass spacer spheres, are scattered randomly throughout the regions where the liquid crystal material is contained and this causes local variation of the contrast or brightness levels of the finished LCD Device, and can also give rise to localised crystal alignment defects.

An object of the present invention is to provide an LCD device with spacer means and adhesive means that are located in the non-pixel regions of the display.

A further object is to provide a LCD device in which the adhesive means and the spacer means are precisely located relative to each other and not randomly located.

A further object is to provide an economic method of manufacture which is robust and resistant to mechanical shocks which might otherwise affect the operation of the LCD device.

According to one aspect of the invention there is provided a LCD device comprising two spaced optically transparent plate assemblies each of which comprises a plate, barrier layer alignment layer and array of electrodes; a spacer means for maintaining a space between the plate assemblies for receiving a liquid crystal material, said spacer means comprising a plurality of spaced elongate strips extending across the plate assemblies; and a plurality of elongate adhesive strips for bonding the assemblies together, said adhesive strips extending across the plate assemblies parallel to the spacer means and defining together with the spacer means parallel channels for receiving the liquid crystal material. Preferably one of the plates is provided with a colour filter comprising elongate regions of coloured pixels and elongated non-pixel regions, and the strips of said spacer means and said adhesive are locate at the non-pixel regions.

Preferably the regions of coloured pixels are arranged in a repeating pattern of parallel lines of red, green and blue pixels, and the strips of said spacer means and said adhesive are located between each line of coloured pixels.

The regions of coloured pixels may be arranged in a repeating pattern of groups comprising parallel lines of red, green and blue pixels, and the strips of said spacer means and said adhesive are located at the non-pixel regions between each group. Preferably the spacer means is a polyimide. Preferably the adhesive strips are a polymer such as a polyimide.

Preferably the adhesive strips are located intermediate the spacer strips.

According to a further aspect of the invention there is provided a method of manufacturing an LCD device comprising the steps of forming two plate assemblies each comprising an optically transparent plate with optically transparent layers constituting a barrier layer, an aUgnment layer and an array of electrodes, providing on at least one of the assemblies spacer means in the form of a plurality of spaced elongate strips extending across the assembly or assemblies, and providing on at least one of the assemblies a plurality of elongate adhesive strips extending across the assembly or assemblies parallel to the spacer strips offering the assemblies up to each other so as to define substantially parallel channels between the spacing strips and the adhesive strips for receiving liquid crystal material, heating the assembly to soften the adhesive strips and thereby effect a bond between the assemblies, filling the channels with Liquid Crystal material and sealing said channels.

Preferably the step of providing the spacer means comprises the steps of coating the surface of said at least one of the assemblies with an photosensitive coating, masking part of the coating to define elongate strips, exposing the masked coating to light and etching away the unmasked coating to leave spacer strips of the photosensitive material.

Preferably the step of providing the adhesive strips comprises the steps of coating the surface of said at least one of the assemblies with a an photosensitive coating, masking part of the coating to define elongate strips and etching away the unmasked coating to leave adhesive strips of the photosensitive material.

Preferably the coatings are polymers such as polyimides and the polymer of the adhesive strips has a lower melting point than that of the spacer strips. Preferably the spacer strips are baked at a temperature of 230°C for sufficient time to imidize the polyimide and the adhesive coating is baked at a temperature of 100°C. The present invention will now be described, by way of example only, and with reference to the accompanying drawings in which

Figure 1 illustrates one embodiment of the invention.

Figures 2 to 4 illustrates, schematically, various different layouts of spacers and adhesive strips of a LCD device as shown in Figure 1. Figure 5 illustrates an enlarged cross sectional view of one of a cell of the LCD device of Figure 1 showing the spacers and adhesive strips.

In Fig 1 a DFLCD device, part of which is shown diagrammatically edge-on, comprises first and second optically transparent parallel plates 1 and 2, for example of glass, having a ferroelectric liquid crystal material 3 sandwiched between them. Selected portions of the liquid crystal material can be addressed in known manner by means of a grid of parallel transparent electrodes 4, for example of indium tin oxide, positioned on the inner surface of the plate 1, and a similar but orthogonal grid of parallel transparent electrodes 5 positioned on the inner surface of the plate 2. The electrodes 4 and 5 are separated from the material 3 in known manner by barrier layers 6 and alignment layers 7 respectively. (An assembly including the above-mentioned components is known, for example, from a paper "An 8.5" Digital Ferroelectric Video Rate Colour Display" by P W H Surguy et al presented to the 1990 Eurodisplay conference and published in "Eurodisplay '90" at page 146 et seq). The assembly is positioned between crossed polariser plates 8 and 9 respectively. Optical transmission of selected areas of the resulting optical modulation device can be varied in known manner by addressing the corresponding portions of the liquid crystal material 3 by means of the relevant ones of the electrodes 4 and 5.

One of the plates 1, 2 is provided with a red-green-blue colour filter 2(a) formed by depositing suitably coloured materials to define coloured pixel areas 2(b) with non- pixel gaps 2(c) between each colour. The red-green-blue colours are deposited as lines of width of approximately 100 to 200μ dependant on display size and resolution.

A plurality of spacer members 10, each approximately 10 to 20μ wide and projecting 1.3μ to 1.5μ, are provided on one of plate assemblies 1, 4, 6, 7 or 2, 5, 6, 7. The spacers 10 are provided for maintaining spacing between these assemblies and are generally fabricated beneath the alignment layer. One of both assemblies 1, 4, 6, 7 or 2, 5, 6, 7 are also provided with elongate strips of a photo-definable adhesive 12 positioned between the spacers 10 and are generally fabricated on top of the alignment layer. These spacer members 10 and adhesive strips 12 are provided at regions 2(c) between the colour bands of the filter (a) where there are no colour pixels, and form parallel walls of approximately 10 - 20 μ width. The walls which extend perpendicular to the plane of the paper and thereby define parallel channels, also extending perpendicular to the plane of the paper (and parallel to the plate assemblies 1, 4, 6, 7 and 2, 5, 6, 7), containing the liquid crystal material 3. In the example shown, the width of the channels is typically 100 - 200μ and spans one colour pixel 2(b) of the red-green-blue colour 4>ands of the filter 2(a), but if desired the channels may be formed between groups of red-green-blue coloured pixel (ie. spaced at 600μ). The spacers 10 are located in the gaps between the indium-tin-oxide electrodes and are lOμ to 20μ wide and are made from transparent polyimide material know as BREWER T1059 (which was developed for use in colour filter technology). An adhesion promoter is spincoated onto the substrate and oven dried. The polyimide (Brewer T1059) is then spincoated onto the substrate and soft-baked before being patterned using standard photolithographic techniques. A photo-resist layer is applied to the polyimide and exposed. The exposed photo-resist and underlying polyimide is developed away. The remaining photo-resist layer is removed by a solvent from the soft- baked polyimide. The substrates are then hard-baked at 230°C for 3 hours to imidize the polyimide. The adhesive strips 12 are formed in a similar way to that used for making the spacers by using a photo-definable adhesive (such a material used was AZ5214E photo-resist). This material has a lower melting point than the Brewer T1059 polyimide so that when heated to achieve bonding it softens to achieve bonding without the spacers 10 softening.

The assemblies 1, 4, 6, 7, and 2, 5, 6, 7 are offered up to each other and heated to a temperature of 130° to cause the adhesive strips 12 of polyimide to melt slightly and effect a bond without melting the spacers 10.

The layout of spacer members 10 and adhesive strip 12 are shown diagrammatically in the plan view of Fig. 2, together with the channels 11 therebetween. As will be seen from Fig. 2 the spacer members 10 and adhesive strips 12 extend across the whole width of the device.

If desired, the spacers 10 and adhesive strips 12 need not be provided on only one of the assemblies 1, 4, 6, 7 or 2, 5, 6, 7, but could be provided on both assemblies 1, 4, 6, 7 and 2, 5, 6, 7 by forming them alternately on each assembly 1, 4, 6, 7 or 2, 5, 6, 7. If desired the spacers 10 may be provided on one of the assemblies 1,4,6,7,7 Or 2,5,7 and the adhesive strips provided on the other asemblie as shown in Figure 4

Referring to Figure 3 alternate spacers 10 are provided on assembly 1, 4, 6, 7 (shown shaded) and the other spacers 10 on assembly 2, 5, 6, 7, (shown unshaded). Similarly alternate strips of adhesive 12 are provided on assembly 1, 4, 6, 7 (shown shaded) and the other strips of adhesive 12 are provided on assembly 2, 5, 6, 7 (shown unshaded). Each the spacers 10 and adhesive strips 12 extend for a short distance across the width of the LCD device but are aligned with each other so that they form a continuous wall extending across the LCD device when fully assembled. If desired small gaps 13 could be left between some or all of the spacers 10 and the adhesive strips 12 to allow each channel containing the liquid crystal material to communicate with adjacent channels.

Referring to Figure 5, there is shown a cross sectional view of one of the channels containing Liquid Crystal material 14. As will be seen, the process of etching away of the polyimide to form a spacer leaves a slightly tapered side wall to each spacer 10 (exaggerated in the drawing) whereas the softening or melting of the polyimide adhesive layers tends to cause the side walls of the adhesive strip 12 to bulge (exaggerated in the drawing). This is believed to cause a beneficial effect on the preferred alignment of the liquid crystals because the bulge of the adhesive extends along one edge of each channel and complements the chevron orientation of the liquid crystals. The tapered sides of the spacers also compliment the orientation of the liquid crystal.

Although the embodiments described above illustrate the invention applied to the manufacture of a DFLCD colour display device, it is to be understood that the invention may be applied to monochromatic ferroelectric LCDs, Twisted Nematic (TN) LCDs, or to Super Twisted Nematic (STN) LCDs.

Claims

1. A LCD device comprising two spaced optically transparent plate assemblies each of which comprises a plate, barrier layer alignment layer and array of electrodes; a spacer means for maintaining a space between the plate assemblies for receiving a liquid crystal material, said spacer means comprising a plurality of spaced elongate strips extending across the plate assemblies; and a plurality of elongate adhesive strips for bonding the assemblies together, said adhesive strips extending across the plate assemblies parallel to the spacer means and defining together with the spacer means parallel channels for receiving the liquid crystal material.

2. A LCD device according the claim 1 wherein one of the plates is provided with a colour filter comprising elongate regions of coloured pixels and elongated non-pixel regions, and the strips of said spacer means and said adhesive are locate at the non-pixel regions.

3. A LCD device according to claim 2 wherein the regions of coloured pixels are arranged in a repeating pattern of parallel lines of red, green and blue pixels, and the strips of said spacer means and said adhesive are located between each line of coloured pixels.

4. A LCD device according to claim 2 wherein the regions of coloured pixels are arranged in a repeating pattern of groups comprising parallel lines of red, green and blue pixels, and the strips of said spacer means and said adhesive are located at the non-pixel regions between each group.

5. A LCD device according to any one of claims 1 to 4 wherein the spacer means is a polyimide.

6. A LCD device according to any one of claims 1 to 5 wherein the adhesive strips are a polymer.

7. A LCD device according to claim 6 wehrein the polymer is a polyimide.

8. A LCD device according to any one of the preceding claims wherein the adhesive strips are located intermediate the spacer strips.

9. A method of manufacturing an LCD device comprising the steps of forming two plate assemblies each comprising an optically transparent plate with optically transparent layers constituting a barrier layer, an alignment layer and an array of electrodes, providing on at least one of the assemblies spacer means in the form of a plurality of spaced elongate strips extending across the assembly or assemblies, and providing on at least one of the assemblies a plurality of elongate adhesive strips extending across the assembly or assemblies parallel to the spacer strips offering the assemblies up to each other so as to define substantially parallel channels between the spacing strips and the adhesive strips for receiving liquid crystal material, heating the assembly to soften the adhesive strips and thereby effect a bond between the assemblies, filling the channels with Liquid Crystal material and sealing said channels.

10. A method according to claim 9 wherein the step of providing the spacer means comprises the steps of coating the surface of said at least one of the assemblies with an photosensitive coating, masking part of the coating to define elongate strips, exposing the masked coating to light and etching away the unmasked coating to leave spacer strips of the photosensitive material.

11. A method according to claim 9 or claim 10 wherein the step of providing the adhesive strips comprises the steps of coating the surface of said at least one of the assemblies with a an photosensitive coating, masking part of the coating to define elongate strips and etching away the unmasked coating to leave adhesive strips of the photosensitive material.

12. A method according to claim 11 wherein the coatings are polymers and the polymer of the adhesive strips has a lower melting point than that of the spacer strips.

13. A method according to claim 11 wherein the polymers are polyimides.

14 A method according to claim 13 wherein the spacer strips are baked at a temperature of 230┬░C for sufficient time to imidize the polyimide.

15. A method according to claim 11 wherein the adhesive coating is baked at a temperature of 100┬░C.

16. A LCD device substantially as herein described with reference to the accompanying drawings.

17. A method of manufacturing a LCD device substantially as herein described with reference to the accompanying drawings.