GB2339320A

GB2339320A - Volume black matrix; multilayer liquid crystal device

Info

Publication number: GB2339320A
Application number: GB9814774A
Authority: GB
Inventors: Henning Molsen; Jonathan Harrold; David Ezra
Original assignee: Sharp Corp
Current assignee: Sharp Corp
Priority date: 1998-07-09
Filing date: 1998-07-09
Publication date: 2000-01-19
Also published as: GB9814774D0

Description

M&C Folio No P50059GS 1 2339320 A Multilayer Liquid Crystal Display Device

The present invention relates to a multilayer liquid crystal display device. It also relates to a substrate suitable for use, inter alia, as an intermediate substrate in a multilayer liquid crystal display device.

Multilayer liquid crystal display devices (LCD) are disclosed in, for example, US-A-5 0 15 074. Figure 1 is a schematic sectional view of a multilayer liquid crystal display device of the type disclosed in US-A-5 015 074. 10 The LCD of Figure I is provided with a lower substrate 1, an intermediate substrate 2, and an upper substrate 3. A first liquid crystal layer 4 is disposed between the lower substrate 1 and the intermediate substrate 2, and a second liquid crystal layer 5 is disposed between the intermediate substrate 2 and the upper substrate 3.

Electrodes are disposed on each side of both of the liquid crystal layers 4, 5, to enable an electric field to be applied across each of the liquid crystal layers. The device illustrated in Figure I is an active matrix device, and has a first set of pixel electrodes 6 arranged on the lower substrate 1. Each of the lower pixel electrodes 6 is controlled by an associated thin film transistor (TFT) 7. A transparent common electrode 8 is disposed on the under side of the intermediate substrate 2, and an electric field is applied to the lower liquid crystal layer 4 by applying a voltage between a selected pixel electrode 6 and the common electrode 8.

The upper substrate 3 is provided with an array of transparent pixel electrodes 9, each controlled by an associated TFT 10. A transparent upper common electrode 11 is disposed on the upper surface of the intermediate substrate 2.

The device illustrated in Figure I is a reflective LCD, and the lower pixel electrodes 6 are reflective electrodes. They consist of a reflective metallic layer deposited over a polymer layer 12 having a roughened upper surface. This means that the reflective electrodes 6 have a roughened surface, and so provide diffuse reflection of incoming M&C Folio No P50059GB 2 light. The upper pixel electrodes 9 and the upper and lower common electrodes 11, 8 are transparent and are made of, for example, indium. tin oxide (ITO).

The device shown in Figure 1 is completed by alignment layers 13, 14, 15, 16, disposed adjacent to the liquid crystal layers. A planarisation layer 17 is provided between the reflective electrode 6 and the lowermost alignment layer 13, to ensure that the alignment layer 13 is planer.

The liquid crystal layers 4, 5 are not continuous across the entire LCD. The lower liquid crystal layer 4 consists of a region of a first liquid crystal (LCl) separated from a region of a second liquid crystal (LC2) by a separating wall 18. The upper liquid crystal layer 5 consists of a region of the first liquid crystal (LC I) separated from a region of a third liquid crystal material CLO) by a separating wall 19. The separating wall 19 in the upper liquid crystal layer 5 is not disposed directly over the separating wall 18 in the lower liquid crystal layer 4. This means that light passing vertically downwards through the device may pass through LC I in the upper liquid crystal layer 5 and LC2 in the lower liquid crystal layer 4 (pixel A); through LO in the upper liquid crystal layer 5 and through LC2 in the lower liquid crystal layer 4 (pixel B); or through LO in the upper liquid crystal layer 5 and through LC 1 in the lower liquid crystal layer 4 (pixel Q.

The device shown in Figure 1 uses liquid crystal material in guest-host mode by adding dichroic dyes to a liquid crystal host. These materials can be switched between a state in which they transmit substa ritially all visible light incident on them and a state in which the dichroic.dyes absorb light in a particular wavelength range. Examples of suitable liquid crystal materials are available from Rolic AG, Switzerland. In one possible arrangement, liquid crystal material LC1 absorbs red and green light in its absorbent state and transmits blue light, material LC2 absorbs blue and green radiation in its absorbent state and transmits red light, and material LO absorbs blue and red light in its absorbent state but transmits green light. If all liquid crystal materials are in their transparent state, then none of the liquid crystal materials absorbs light and white light incident on the display will be reflected back by the display.

M&C Folio No P50059GB 3 If material LC 1 is put into its absorbent state, then it will transmit only blue light. Pixels A and C of the display will therefore appear blue, whereas pixel B will remain white (since materials LC2 and LC3 are both in their transmissive states). If materials LC I and LC2 are both put into their absorbent states, however, then pixel A will appear black. This is because only blue light is transmitted through LCl, but this is absorbed in material LC2. Pixel B will appear red, and pixel C will appear blue.

It is possible in principle to produce a device similar to that shown in Figure 1, but having three liquid crystal layers using the guest host mode and no in- plane walls. It would be necessary to incorporate a further intermediate substrate into the device and, for an active matrix device, this intermediate substrate would be required to carry pixel electrodes and TFTs for the middle liquid crystal layer.

The two layer LCD of Figure I is susceptible to the problem of optical cross-talk when the LCD is illuminated, or viewed, from oblique angles. Optical cross-talk arises as a consequence of light travelling from one pixel in the upper liquid crystal layer 5 to a second pixel, in the lower liquid crystal layer 4, which is not juxtaposed with the first pixel. An example of a light path which could give rise to cross-talk is illustrated in Figure 1 at "Y'.

US-A-5 015 074 teaches that the problem of cross-talk can be reduced by making the inter-mediate substrate thinner. However, there are practical limitations as to how thin the intermediate substrate may be. If the intermediate substrate is a glass substrate, then it is difficult to reduce its thickness below about 30 microns. If the intermediate substrate is a polymer substrate then it is, in principle, possible to reduce its thickness to around one micron. However, handling and processing a substrate having a thickness of one micron is difficult, particularly if the substrate is required to carry patterned features such as electrodes or switching elements (as would be the case in a three layer LCD).

M&C Folio No P50059GB 4 US-A-4 556 286 discloses further examples of a multi-layer LCD. This document does not, however, refer to the problem of overcoming optical cross-talk when the device is viewed obliquely.

UY, Patent Application Nos. 9801788.2, 9801789.0 and 9801795.5 disclose methods of manufacturing multi-layer LCDs in which adjacent liquid crystal layers are separated by a very thin membrane. The use of a thin membrane reduces optical cross-talk arising when the device is viewed at an oblique angle.

A first aspect of the present invention provides a transparent polymeric substrate, the substrate having an opaque region extending over a significant portion of the depth of the substrate, the depth being defined between a first surface of the substrate and a second surface of the substrate. By "substrate" is meant a component which is suitable for use as a substrate in a device such as an electro-optic or opto-optic device (if necessary after undergoing further processing such as the provision of, for example, electrodes, switching elements, or alignment layers for liquid crystal materials). The words "transparent" and "opaque" as used throughout the description of the invention and the claims should be interpreted with respect to the intended wavelength range of operation of the device into which the substrate is to be incorporated. Where a substrate is intended for use in the visible spectrum, for example, it will be sufficient if it is substantially non-absorbing over the visible spectrum transparent, and if the opaque region is substantially non-transmissive over the visible spectrum.

This substrate is particularly advantageous for use in a direct view multi-layer LCD such as that shown- in Figure 1. If it is positioned such that the opaque region of the substrate is located at the boundary between pixel A and pixel B of the LCD, then light will not be able to follow the path X shown in Figure 1 and optical cross-talk will be prevented.

The opaque region is acting as a three-dimensional black matrix or volume black matrix VBM. Since it extends over a significant part of the depth of the substrate it is far more effective at preventing optical cross- talk than a conventional two-dimensional black M&C Folio No P50059GB 5 matrix of the type disclosed in, for example, US-A-4 568 149. This discloses an LCD having a single liquid crystal layer disposed between two electrode plates. One electrode plate is provided with pixel electrodes, switching elements, gate lines and source lines, whilst a common electrode is disposed on the other electrode plate. A non- transmissive masking member is provided on at least one of the electrode plates, and is arranged so as to be over one of the gate lines. Spacer members, to keep the thickness of the liquid crystal layer constant, are provided over the non-transmissive member.

Another LCD with a 2-D black matrix is disclosed in US-A-4 824 213. This LCD has a matrix of pixels arranged in rows and columns, and a non-transmissive pattern is provided to mask the gaps between adjacent pixels. The non- transmissive pattern is intended to improve the contrast of the display by preventing light leakage through the gaps between the pixels.

US-A-4 552 437 also discloses an LCD having a black matrix provided on one of the substrates. This black matrix is provided so that the shapes of the pixels of the display are defined by the black matrix rather than by the shapes of the electrodes. The black matrix is applied to the electrode nearer to an observer of the LCD, so as to reduce parallax effects. However, since the black matrix is essentially two-dimensional (it has only a small thickness) and is disposed on the surface of the substrate it cannot satisfactorily eliminate parallax effects that would arise owing to oblique optical paths through the substrate.

In a substrate of the present invention, however, the opaque region is disposed within the substrate and extends over a significant part of the depth of the substrate. This means that the opaque region in a substrate of the present invention can block oblique light paths, such as that shown in Figure 1, much more effectively than a conventional 2-D black matrix.

The opaque region may extend from the first surface of the substrate significantly into the depth of the substrate towards the second surface of the substrate.

M&C Folio No P50059GB 6 The opaque region may extend over substantially the entire depth of the substrate, or it may extend over the entire depth of the substrate. This further improves the ability of the opaque region to block oblique light paths. 5 The opaque region may extend in a first direction, over substantially the entire width of the substrate in the first direction, the first direction not being parallel to the depth of the substrate. The opaque region may extend over the entire width of the substrate in the first direction. If used in the LCD of Figure 1, for example, such an opaque region would prevent light passing from pixel A to pixel B. A second aspect of the present invention provides a transparent substrate comprising first and second opaque regions, the first and second opaque regions each extending over a significant proportion of the depth of the substrate, the depth of the substrate being defined between a first surface of the substrate and a second surface of the substrate. The first and second opaque regions may each extend in a first direction, and be parallel to and spaced from one another. The two parallel opaque regions will act to collimate light.

Alternatively, the first opaque region may extend in a first direction and the second opaque region may extend in a second direction, the second direction being different from the first direction. The first opaque region may intersect the one opaque region.

Such a combination of opaque regions is capable of blocking oblique light paths that slant in two different directions (such as, for example, the path X in Figure 1 and a path slanting out of the plane of Figure 1).

A third aspect of the present invention provides a transparent substrate comprising a first set of opaque walls extending in a first direction and being spaced from one another, and a second set of opaque walls extending in a second direction and being spaced from one another, the second direction being crossed with the first direction, whereby the walls define a matrix of transparent regions in the substrate, each M&C Folio No P50059GB 7 transparent region extending from a first surface of the substrate to a second surface of the substrate and being separated from a neighbouring transparent region by one of the opaque walls. Each opaque wall may extend over a significant portion of the depth of the substrate, the depth of the substrate being defined between the first surface of the substrate and the second surface of the substrate. Each of the walls may extend significantly into the depth of the substrate from the first surface of the substrate to the second surface of the substrate. Each of the opaque walls may extend over substantially the entire depth of the substrate, or over the entire depth of the substrate. Such a substrate will collimate light effectively.

The substrate may be a glass substrate. Alternatively, the substrate may be a polymeric substrate. Thus, the most appropriate material can be chosen for the substrate dependent, for example, on whether additional processing of the substrate is required.

The present invention also provides an electro-optic display device comprising a first substrate, a first layer of an electro-optic material, an intermediate substrate, a second layer of an electro-optic material, and a second substrate; wherein the intermediate substrate is a substrate as specified above.

The present invention also provides an electro-optic display device comprising a first substrate, a first layer of an electro-optic material, an intermediate substrate, a second layer of an electro-optic material, and a second substrate; the device further comprising a first set of pixel electrodes defining pixels in the first layer of electro-optic material, and a second set of pixel electrodes defining pixels in the second electro-optic layer; wherein the intermediate substrate is a substrate having a matrix of transparent regions as specified above, each transparent region defined in the intermediate substrate being aligned with the pixels defined in the first and second layers of electro-optic material.

The display device may be a direct view display device.

The electro-optic layers may be layers of a liquid crystal material.

M&C Folio No P50059GB Preferred embodiments of the present invention will be described by way of illustrative examples with reference to the accompanying drawings, in which:- Figure 1 is a schematic sectional view of a conventional LCD; Figure 2 is a schematic plan view of a substrate with a Volume Black Matrix according to the present invention; Figure 3 is a schematic perspective view of the substrate shown in Figure 2; Figure 4 is a schematic illustration of a method of producing a substrate according to the present invention; Figures 5(a) to 5(d) are schematic views illustrating an alternative method of producing a substrate according to the present invention; Figures 6(a) to 6(d) are schematic views illustrating a further method of producing a substrate according to the present invention; 20 Figures 7(a) and 7(b) are schematic illustrations of another method of producing a substrate according to the present invention; Figure 8 is a schematic sectional view of a two layer LCD incorporating a substrate 25 according to the present invention; Figure 9 is a schematic sectional view of another two-layer LCD incorporating a substrate according to the present invention; and Figure 10 is a schematic sectional view of a three layer LCD incorporating two substrates according to the present invention.

M&C Folio No P50059GB 9 A substrate according to the present invention is schematically illustrated in Figures 2 and 3. The substrate is a transparent substrate, but is provided with a "Volume Black Matrix" ("VBM"). Thus, the substrate 20 consists of a plurality of transparent areas 21, with each transparent pixel area being separated from adjacent pixel areas by the VBM 22. In the example shown in Figures 2 and 3, the VBM 22 consists of a plurality of opaque "walls" 23,23'. In the example shown in Figures 2 and 3 the opaque walls 23,23' are arranged in a grid. The walls 23 are parallel to each another, the walls 23' are parallel to each other, and the walls 23 are perpendicular to the walls 23' so that the transparent pixel areas have a square cross section. 10 In the embodiment shown in Figure 3, the opaque walls 23 extend throughout the depth d of the substrate 20. Each opaque wall 23 also runs from one side edge of the substrate 20 to the opposite side edge.

Although the walls 23 extend throughout the entire depth d of the substrate 20 in the example shown in Figure 3, it would be possible to produce a substrate in which the opaque walls do not extend over the complete depth of the substrate. Such a substrate would prevent some optical cross-talk when used in a multi-layer LCD, although it would not be as effective in preventing cross-talk as a substrate in which the walls 23 extend throughout the complete depth of the substrate.

It will be appreciated that, if the substrate 20 shown in Figures 2 and 3 were used as the intermediate substrate 2 in the device shown in Figure 1, that the problem of optical cross-talk would be substantially eliminated. If the intermediate substrate 2 of the device shown in Figure I were provided with a VBM, with the VBM located such that an opaque wall 23 were located in the intermediate substrate at the boundary between pixel A and pixel B of the display, then light following the path X shown in Figure 1 would be absorbed by the opaque wall. Optical cross-talk between adjacent pixels of the device would therefore be substantially prevented.

Although Figures 2 and 3 show a VBM in the form of a grid of intersecting walls, the present invention is not limited to a VBM of this form. The precise form of the VBM M&C Folio No P50059GB 10 will, of course, depend on the structure of the device into which the VBM is to be incorporated. For example, if relative movement between an observer and a display occurs only in one direction then a VBM consisting just of the walls 23 and omitting the walls 23 ' could be used. Moreover, if a VBM does consist of a grid of intersecting walls 23,23' as shown in Figures 2 and 3 then the transparent pixel areas 21 do not need to be square, nor even rectangular.

One method of producing a substrate of the present invention is the method of thermally induced transfer of dye from a donor sheet to a receiver sheet (with the receiver sheet becoming the substrate). The method of thermally induced transfer of dye from a donor substrate is well known and is described in, for example, US-A-3 647 503, R. A. Hann et al, "The Society for Imaging Science and Technology", Seventh Annual Congress on Advances in Non-Impact Printing Technologies, Portland, Oregon, pages 237246 (1991), and N. Egashira et al "The Society for Imaging Science and Technology, Seventh Annual Congress on Advances in Non-Impact Printing Technologies, Portland, Oregon Pages 260-8 (1991).

Figure 4 is a schematic view of one method of producing a substrate with a VBM. This method uses a donor sheet 24, which consists of a layer 25 containing a dye coated on an inert carrier layer 26. The donor sheet 24 is placed on a polymeric receiver sheet 27, with the dye layer 25 adjacent to the receiver sheet 27. Selected areas of the donor sheet 24 are then heated using a focused laser beam 28 produced by a laser 29. Regions of the dye layer 25 on which the laser beam 28 is incident are heated, as laser light is absorbed by the dye. As a result of this localised heating of the dye layer, dye is selectively transferred from the heated regions of the dye layer 25 to the receiver sheet 27.

Although the layer 25 has been described as containing a dye, it could alternatively contain a mixture of two or more dyes, an ink, a mixture of two or more inks etc.

Moreover, in addition to at least one dye and/or at least one ink, the layer 25 may also be provided with a laser absorbing material to increase the amount of laser radiation absorbed by the layer 25. This will increase the amount of heat transferred to the dye M&C Folio No P50059GB I I layer 25 and so increase the amount of dye or ink transferred to the receiver substrate 27.

The dye is initially transferred to the surface of the receiver sheet 27. In order to create a VBM it is necessary for the dye to diffuse downwards through the receiver sheet, for substantially the whole thickness of the receiver sheet. In general, the temperature dependence of diffusion of a dye into the polymer receiver sheet 27 is governed by the Arrhenius rate equation:

D = Do exp (- ED/R In this equation D is the diffusion coefficient, Do is the frequency factor (that is, the theoretical diffusion constant at an infinitely high temperature), ED is the activation energy, R is the universal gas constant and T is the absolute temperature. At room temperature, the dye is effectively "frozen in" to the polymer, and the diffusion of the dye through the polymer is negligible. A typical value of D at room temperature is about 10-15 m2/s, whereas at 200'C D is 5 orders of magnitude greater at around 10-10 M 2/S.

It can therefore be seen that the receiving polymer sheet 27 must be heated in order to increase the rate of diffusion of the dye. However, the coefficient of diffusion of heat in the polymer receiver sheet 27 will be several orders of magnitude greater than the coefficient of diffusion for the dye. Therefore, diffusion of the dye in a globally heated polymer would be non-directional. In order to enhance diffusion in the vertical direction, the heating of the polymer receiver sheet 27 is localised, short and repetitive. This can be achieved for example, by pulsing the laser beam 28, and matching the wavelength of the laser beam 28 to the absorption wavelength of the dye layer 25.

In the method illustrated in Figure 4, if the laser beam 28 is repeatedly pulsed on to one location of the dye layer 25, then the volume of the receiver sheet 27 immediately below the point where the laser beam is incident on the dye layer 25 will be heated as a result M&C Folio No P50059GB 12 of thermal conduction from the dye layer 25 into the receiver sheet 27. The dye will therefore diffuse into the receiver sheet 27, and so give the VBM illustrated in Figure 4.

A complete VBM can be produced by scanning the laser beam 28 relative to the dye layer 25 in the desired pattern. This can be achieved by keeping the laser beam stationary and moving the donor sheet 24 (with the receiver sheet 27), by keeping the donor sheet stationary and scanning the laser beam 28 relative to the donor sheet 24, or by moving both the laser beam 28 and the donor sheet 24. It is possible to "Write" features having a lateral dimension of down to few microns using a focused laser beam.

A modification of the method of Figure 4 is illustrated in Figures 5(a) to 5(d). Figure 5(a) is similar to Figure 4, and shows a donor sheet 24, comprising a dye layer 25 disposed on a carrier layer 26, placed against a polymeric receiver substrate (or receiver sheet) 27. The dye layer 25 is disposed to be in contact with the receiver substrate 27.

Selected areas of the dye layer 25 are heated by irradiation with a focused laser beam 28.

In the method of Figures 5(a) to 5(d), the heat generated when the laser beam 28 is directed onto the dye layer 25 is not sufficient to cause the dye to diffuse throughout the depths of the receiver substrate 27. As is shown in Figure 5(b), although dye is transferred to the receiver substrate 27 as a result of the irradiation of the dye layer 25, the dye diffuses only a short distance into the receiver substrate 27.

The method of Figures 5(a) to 5(d) accordingly comprises a "post-write" heat treatment stage, which is illustrated in Figure 5(c). The receiver substrate 27 is heated in this stage, and this allows the dye to diffuse throughout the depth of the receiver substrate 27 to form a VBM 22. The receiver substrate 27 has thus been transformed into an intermediate substrate 20 having a VBM 22, for example such as the substrate illustrated in Figure 3.

The post-w-rite heat treatment can be carried out in any of the ways described below with reference to the method of Figures 6(a) to 6(d).

M&C Folio No P50059GB 13 An alternative method of producing a substrate 20 having a VBM 22 is illustrated in Figures 6(a) to 6(d). Figure 6(a) is similar to Figure 4, and shows a donor sheet 24 positioned such that its dye layer 25 is adjacent to a polymer receiver sheet 27. A focused laser beam 28 is directed onto the dye layer 25 by a laser scanning system 29.

The method of Figures 6(a) to 6(d) differs from the method of Figure 4 in that it uses a higher laser intensity. At higher laser power densities (: 106W/CM2) and short exposure times, the process of transfer of dye to the receiver substrate is by ablation - that is, there is an instantaneous (<lus) transfer of dye from the donor substrate 24 to the receiver substrate 27 without substantially heating the latter (Figure 6(b)). Consequently, dye is deposited only on the surface of the receiver substrate 27, since the rate of diffusion through the unheated receiver substrate is very slow. The method of laser ablation transfer is well known and discussed in G. R. Pinto "Dynamics of Laser Ablation Transfer Thermal Imaging: Fundamental Mechanisms Investigated by Secondary Ion Mass Spectroscopy Surface Analysis" Journal of Imaging Science and Technology Vol. 38, pages 565 - 570 (1994) and references therein.

Although the donor sheet 24 and the receiver sheet 27 are shown as being in contact in Figure 6(a), the ablation process doe's not require the donor substrate and the receiver substrate to be in intimate contact.

Since the dye transfer by ablation method does not result in the receiver substrate 27 being heated, the process illustrated in Figures 6(a) to 6(d) requires an additional heat treatment of the receiver substrate in order to allow the dye to diffuse into the polymer receiver substrate 27. This heat treatment can be performed simultaneously with the dye transfer step, for example if the receiver substrate 27 is disposed on a heated platen during the dye transfer step. Alternatively, this additional heat treatment can be a "postwrite" heat treatment which is carried out after the dye transfer has been accomplished.

A post-write heat treatment of the receiver substrate 27 could be achieved by locally heating the receiver substrate. This could be done by scanning a second light source M&C Folio No P50059GB 14 (for example a second laser beam) over the receiver substrate 27 so that areas of the receiver substrate illuminated by the second light beam were heated. Alternatively, the post-write heat treatment could be carried out by heating large areas of the receiver substrate, for example by heating the receiver substrate 27 using a hot plate or heated coil (not shown). This heating could be pulsed, and could be carried out from either or both sides of the receiver substrate.

In the method of Figures 6(a) to 6(d), the receiver substrate is heated over a relatively large area in step 6(c). This could be achieved by using, for example, a hot plate or heated coil (not shown). As a result ofthe post-write heat treatment of the receiver substrate 27, the dye transferred to the surface of the receiver substrate in step 6(a) is diffused into the receiver substrate 27 to form the VBM, as shown in Figure 6(d). Thus, the polymer receiver substrate 27 has been provided with a VBM 22 and now forms a substrate 20 according to the present invention.

If a post-write heat treatment of the polymer receiver sheet 27 is used in which large areas of the polymer sheet are heated, then it is necessary to bear in mind that heating a polymer film could lead to a dimensional change in the polymer film, owing to a rearrangement of the packing of the macromolecules of the polymer film. If, therefore, a large area post-wTite heat treatment is required, it is preferable to subject the polymer receiver substrate 27 to a preliminary heat treatment, at a similar temperature to the intended post-write heat treatment, before the dye transfer. As an alternative, the polymer receiver substrate 27 may be kept taut during the post-write heat treatment, so that it is prevented from shrinking during the heat treatment.

Although the methods described with reference to Figures 4, 5(a) to 5(d), and 6(a) to 6(d) use thermally induced dye transfer, a substrate of the present invention can be produced by other methods. A substrate can, in principle, be produced by any method which will transfer dye to the surface of the receiver substrate 27 followed by a post- write heat treatment. For example, dye could be transferred onto the surface of the receiver substrate 27 by any conventional printing process. However, other methods are in general less preferable than the method of thermally induced transfer of dye onto the M&C Folio No P50059GB 15 receiver substrate, since they have a lower resolution. Moreover, it is not currently possible to print onto a substrate having a thickness of a few microns.

A VBM can also be provided in a glass substrate. One method of doing this is illustrated in Figures 7(a) and 7(b).

Figure 7(a) shows a glass substrate 3 1, made of glass sensitive to UV radiation. An opaque mask 32, which is provided with a transparent pattern corresponding to the plan view of the desired VBM, is disposed over the glass substrate 3 1. The glass substrate 31 is then illuminated by UV light in Figure 7(a), and is subsequently heat treated, at a temperature of 400'C to 600T.

The steps of irradiating the glass substrate and the subsequent heat treatment create light absorbing agglomerates of silver atoms throughout the portions of the glass substrate that were illuminated. Although the silver agglomerates are themselves electrically conductive, they are dispersed within the glass substrate, and so are electrically insulated from one another. Hence, the VBM itself is non-conductive.

When a VBM is provided in a -lass substrate, the glass substrate can be further processed to provide it with, for example, driving means such as TFTs.

It is possible to use the method of radiating the receiver substrate through a mask with a polymeric substrate. Thus, in the methods of Figures 4, 5(a) to 5(d) and 6(a) to 6(d), the step of irradiating the receiver substrate could be carried out by disposing an opaque mask, provided by. a transparent pattern corresponding to the planned view of the desired VBM over the donor sheet, 24. The entire area of the donor sheet would then be irradiated, for example with IR radiation, so that the areas of the dye layer 25 corresponding to the transparent areas in the mask would be heated so as to transfer dye to the corresponding areas of the receiver substrate. If a post-write heat treatment is carried out, this would preferably be done by pulsing the irradiation source while irradiating the receiver substrate through the mask, since this would lead to a more vertical diffusion profile.

M&C Folio No P50059GB 16 In principle, a VBM could be produced in a glass substrate by irradiating the substrate with a focused light beam, and scanning the light beam relative to the substrate.

Examples of multi-layer LCD devices which comprise a substrate with an VBM according to the present invention will now be described.

Figure 8 shows a double layer LCD which uses a guest-host liquid crystal. This device is a reflective, active matrix device, and is generally similar to those described in US-A- 5015074.

This device has a lower substrate 33 on which are disposed pixel electrodes 34, switching elements (not shown) for the pixel electrodes, and an alignment layer 35. The device also has an upper substrate 36, which is provided with pixel electrodes 37, switching elements (not shown) for the pixel electrodes, and an alignment layer 38.

An intermediate substrate 39 which is provided with a VBM 40 according to the present invention is disposed between the upper and lower substrates. A first liquid crystal layer LC I is disposed between the upper substrate 36 and the intermediate substrate 39, and a second liquid crystal layer LC2 is disposed between the intermediate substrate 39 and the lower substrate 33. Support posts 41 are provided to space the upper substrate 36 from the intermediate substrate 39, and to space the lower substrate 33 from the intermediate substrate 39.

The upper and lower surfaces of the intermediate substrate are equipped with an alignment layer if necessary. The voltage is dropped across the two liquid crystal layers and the intermediate substrate.

Before the conductive coatings are applied to the upper and lower surfaces of the dividing substrate 39, the VBM 40 is formed by diffusing a dye, or dye mixture into the intermediate substrate 39 by a thermal transfer technique. The intermediate substrate, which acts as the receiving substrate during the thermal transfer process, is a polymer M&C Folio No P50059GB 17 film made of polyethyleneterephthalate (for example, Hostaphan by Hoechst or Melinex from ICI). The dye transfer sheet used in the thermal transfer process is similar to those described in Hann et al or Egashira et al, and includes an infra red absorbing dye.

The transfer of dye to the receiving substrate, and its subsequent diffusion, is achieved by locally heating the dye transfer sheet using a focused IR laser having an output power of around 2 tW, a focused beam diameter of around 54m, and a wavelength of 830m-n. The laser diode LT015NM produced by Sharp is a suitable laser source.

The receiving polymer substrate and the dye transfer sheet are kept in intimate contact during the thermal transfer process by means of a vacuum, as is known in the art.

The pattern required to form the VBM is generated by moving the laser beam with C) respect to the dye transfer sheet (donor substrate) and the receiver sheet. This can be done, for example, by moving the dye transfer sheet and receiver sheet simultaneously on an x-y translation stage, while keeping the laser beam stationary.

A post-write heat treatment, with preferred conditions of T > 1500C and duration t > I minute may be used, to improve the depth of the diffusion of dye into the receiving polymer sheet.

Once the VBM has been provided in the intermediate substrate 39, and the conductive coatings have been applied, the device shown in Figure 8 can be assembled in any conventional manner. It is important that the intermediate substrate is positioned so that the pixel areas in the intermediate substrate defined by the VBM correspond correctly with the pixels defined in the liquid crystal layers by the pixel electrodes 34, 37 provided on the lower and upper substrates 33, 36. As noted above, care has to be taken to prevent, or to allow for, shrinkage of the polymer substrate during the formation of the VBM. In some cases this problem can be tackled by providing markers on the substrates to be incorporated in the completed device, and using these markers to align the components during assembly of the device.

M&C Folio No P50059GB 18 As described above, an alternative way of carrying out the thermally induced dye transfer step when producing the intermediate substrate 39 for the device shown in Figure 8 is to irradiate the donor substrate through a mask. Irradiation with light through an opaque mask, on which is inscribed a transparent pattern that will form the VBM, can be used. If an additional heat treatment is carried out, this is preferably done by pulsing the IR source and irradiating the receiver substrate through the same mask, since this will lead to a more vertical diffusion profile.

Figure 9 shows a double layer, passively addressed, LCD. This is a reflective device, and could use a cholesteric liquid crystal.

In the device of Figure 9, stripe electrodes 43 are provided on the lower substrate 42. Stripe electrodes 47 are provided on the lower surface of the intermediate substrate 48; when the device is assembled, the stripe electrodes 47 on the dividing substrate are crossed with the stripe electrodes 43 on the lower substrate 42. Similarly, stripe electrodes 53 are provided on the supper substrate 54, and these are crossed with stripe electrodes 50 provided on the upper surface of the intermediate substrate 48. An alignment layer 44, 45, 51, 52 is disposed over each set of stripe electrodes 43, 47, 50, 53. A first liquid crystal layer LC I is disposed between the upper substrate 54 and the intermediate substrate 48, and a second liquid crystal layer LC2 is disposed between the intermediate substrate 48 and the lower substrate 42, and these are addressed by a passive matrix addressing method using the crossed stripe electrodes.

Before the stripe electrodes 47, 50 are deposited on the intermediate substrate, a VBM 49 is produced in the intermediate substrate 48.

In this embodiment, the intermediate substrate 48 is produced from a photo definable glass substrate such as Foturan (available from Schott, in thicknesses ranging from 0.1 to 2mm). As described above with reference to Figures 7(a) and 7(b), the glass substrate is illuminated with UV light having a wavelength in the range 280-340mm and an energy density per mm glass thickness of 2j/CM2 through a mask. The glass M&C Folio No P50059GB 19 substrate is then heated, according to the manufacturer's specifications at around 400600'C until an opaque pattern in the region that was illuminated has been formed.

The substrate may then be polished to eliminate any possible roughness generated 5 during the illumination and heating steps. The striped electrodes are then formed on I both sides of the dividing substrate so that they cover the transmissive areas of the substrate 48.

Figure 10 shows a three layer active matrix LCD. This is a reflective device, and uses a guest-host liquid crystal.

The LCD has a lower substrate 55 on which are disposed reflective pixel electrodes 56. TFTs 57 for the pixel electrodes 56 are also disposed on the lower substrate 55, and an alignment layer 58 is disposed over the TFTs 57 and pixel electrodes 56.

A lower intermediate substrate 59 is disposed over the lower substrate 55. A common electrode 60 and an alignment layer 61 are disposed on the lower surface of the lower intermediate substrate 59, and a liquid crystal layer LC2 is provided between the lower substrate and the lower intermediate substrate 59. The lower substrate 55 and the lower intermediate substrate 59 are kept an appropriate distance apart by spacers 62.

Pixel electrodes 63 and their associated TFTs 64 are disposed on the upper surface of the intermediate substrate 59. An alignment layer 65 is placed over the pixel electrodes 63 and the TFTs 64.

A second intermediate substrate 66 is provided over the lower intermediate substrate 59, and a liquid crystal layer LO is placed between the lower intermediate substrate 59 and the upper intermediate substrate 66. A common electrode 67 and an alignment layer 68 are disposed on the lower surface of the upper intermediate substrate and the second liquid crystal layer LO is controlled by means of the pixel electrodes 63 on the upper surface of the lower intermediate substrate 59 and the common electrode 67 on the lower surface of the upper intermediate substrate 66.

M&C Folio No P50059GB 20 A further common electrode 69 and another alignment layer 70 are disposed on the upper surface of the upper intermediate substrate 66.

The device is completed by an upper substrate 71, which is provided with pixel electrodes 72, their associated TFTs 73 and an alignment layer 74. A liquid crystal layer LC I is placed between the upper substrate 71 and the upper intermediate substrate 66, with the thickness of the liquid crystal layer being determined by spacers 62.

The lower intermediate substrate 59 is provided with a VBM 75, and the upper intermediate substrate is provided with a VBM 76. The four substrates 55, 59, 66, 71 are arranged such that the pixels defined in the three liquid crystal layers correspond, firstly with one another and, secondly, with the transparent pixel areas defined in the intermediate substrates 59, 66 by the VBMs 75, 76.

Although the present invention has been described with particular reference to a multilayer LCD, the present invention is not limited to use with a multilayer LCD. On the contrary, a substrate having a VBM can be incorporated in any electro-optic, or opto-optic, device in which its light collimating properties will be of use. For example, such a substrate could be incorporated into a back light for an LCD capable of displaying 3-D images, to reduce the diffuse emission from the LCD.

M&C Folio No P50059GB 21

Claims

CLAIMS:

1. A transparent polymeric substrate comprising an opaque region, the opaque region extending over a significant portion of the depth of the substrate, the depth of the substrate being defined between a first surface of the substrate and a second surface of the substrate.

2. A substrate as claimed in Claim 1, wherein the opaque region extends from the first surface of the substrate significantly into the depth of the substrate towards the second surface of the substrate.

3. A substrate as claimed in Claim I or 2 wherein the opaque region extends over substantially the entire depth of the substrate.

4. A substrate as claimed in Claim 1 or 2 wherein the opaque region extends over the entire depth of the substrate.

5. A substrate as claimed in Claim 1, 2, 3, or 4 wherein the opaque region extends in a first direction, over substantially the entire width of the substrate in the first direction, the first direction not being parallel to the depth of the substrate.

6. A substrate as claimed in Claim 5 wherein the opaque region extends over the entire width of the substrate in the first direction.

7. A transparent substrate comprising first and second opaque regions, the first and second opaque regions each extending over a significant proportion of the depth of the substrate, the depth of the substrate being defined between a first surface of the substrate and a second surface of the substrate.

8. A substrate as claimed in Claim 7, wherein the first and second opaque regions each extend in a first direction, and are parallel to and spaced from one another.

M&C Folio No P50059GB 22

9. A substrate as claimed in Claim 7 wherein the first opaque region extends in a first direction and the second opaque region extends in a second direction, the second direction being different from the first direction.

10. A substrate as claimed in Claim 9 wherein the first opaque region intersects the second opaque region.

11. A transparent substrate comprising: a first set of opaque walls extending in a first direction and being spaced from one another, and a second set of opaque walls extending in a second direction and being spaced from one another, the second direction being crossed with the first direction, whereby the walls define a matrix of transparent regions in the substrate, each transparent region extending from a first surface of the substrate to a second surface of the substrate and being separated from a neighbouring transparent region-by one of the opaque walls. 15

12. A substrate as claimed in Claim 11 wherein each opaque wall extends over a significant portion of the depth of the substrate, the depth of the substrate being defined between the first surface of the substrate and the second surface of the substrate.

13. A substrate as claimed in Claim 12 wherein each of the walls extends significantly into the depth of the substrate from the first surface of the substrate towards the second surface of the substrate.

14. A substrate as claimed in Claim 11, 12 or 13 wherein each of the opaque walls extends over substantially the entire depth of the substrate.

15. A substrate as claimed in Claim 11, 12 or 13 wherein each of the opaque walls extends over the entire depth of the substrate.

16. A substrate as claimed in any of claims 7 to 15 wherein the substrate is a glass substrate.

M&C Folio No P50059GB 23

17. A substrate as claimed in any of claims 7 to 15 wherein the substrate is a polymeric substrate.

18. A transparent substrate substantially as described herein with reference to Figures 2 and 3 of the accompanying figures.

19. An electro-optic display device comprising a first substrate, a first layer of an electro-optic material, an intermediate substrate, a second layer of an electro-optic material, and a second substrate; wherein the intermediate substrate is a substrate as defined in any preceding claim.

20. An electro-optic display device comprising a first substrate, a first layer of an electro-optic material, an intermediate substrate, a second layer of an electro-optic material, and a second substrate; the device further comprising a first set of pixel electrodes defining pixels in the first layer of electro-optic material, and a second set of pixel electrodes deffining p-i-Y-els in the second electro-optic layer; wherein the intermediate substrate is a substrate as defined in any of claims 11 to 17, each transparent region defined in the intermediate substrate being aligned with the pixels defined in the first and second layers of electro-optic material.

21. An electro-optic display device as claimed in Claim 19 or 20 where the device is a direct view display device.

22. An electro-optic display device as claimed in any of Claims 19 to 21 wherein the electro-optic layers are layers of a liquid crystal material.

23. A liquid crystal display device substantially as described herein with reference to any one of Figures 8 to 10 of the accompanying drawings.