GB2118210A - Reflectors for passive display devices - Google Patents

Abstract

An electrically non-conductive substrate (11) bearing thereon, preferably via an intermediate thin dielectric layer (12), a non-specularly reflective metal layer (13) itself covered by a thin layer (14) of a solid light transparent material having a refractive index of less than 1.4 is used as an internal non-specular reflector in display devices of the liquid crystal or electrochromic type. Such devices are generally operated in the reflective mode, in which the ambient lighting is transmitted through the device and reflected back out to the observer by a suitable reflective layer. Recently it has been suggested that there should be used an internal non-specular (Lambertian) metal reflector, but this type of reflector has a poor reflectivity in contact with most liquids, including liquid crystals, due to the so-called "window" effect. The provision of layer (14) removes this defect. <IMAGE>

Description

SPECIFICATION Reflectors This invention concerns reflectors, and relates in particular to non-specular reflectors for use internally of passive display devices.

Passive display devices, particularly of the liquid crystal or electrochromic types, are generally operated in the reflective mode, in which the ambient lighting is transmitted through the device and reflected back out to the observer by a suitable reflective layer. It was in the past common practice to place a non-specular (nonmirror-like) reflector, for example of a layer of matt-finished aluminium, immediately behind the backplate (and thus externally) of the device, though in some cases (e.g., in dynamic scattering liquid crystal display devices) a specular metal layer (a metallic mirror) was provided inside the display, in contact with the liquid crystal material, to form both the reflective surface and the back electrode. Both the above systems have disadvantages.The use of external reflectors causes shadowing or double image effects to be produced by oblique illumination, due to the separation of the reflector from the active layer of the device, while the use of internal mirror reflectors is only suitable for narrow angle viewing in dynamic-scattering liquid crystal displays (and not for the most-commonly-used twisted-nematic or dye-phase-change devices).

More recently it has been suggested that there should be used an internal non-specular (Lambertian) metal reflector. Such a non-specular reflector can be produced, for example, by forming a reflective metal layer on a substrate whose surface has previously been roughened (by rubbing with a fine abrasive, say, or by chemical etching). However, while very effective in air, this type of reflector has a poor reflectivity in contact with most liquids, including liquid crystals, due to the so-called "window" effect. This is dependent on the refractive index of the liquid, but for liquids with a refractive index in the region of 1.5 it is believed that internal reflection effects within the irregular reflective surface result in a significantly high absorption of the ambient light.

The invention seeks to produce a non-specular reflective surface, suitable for use within a passive display device operated in reflective mode, which will wholly or substantially maintain its reflective power even when in contact with a medium of average refractive index of the order of 1.5, thus avoiding the "window" effect.

In one aspect, therefore, this invention provides, for use as a non-specular reflector within a display device, an electrically non-conductive substrate bearing thereon a non-specularly reflective metal layer itself covered by a thin layer of a solid light transparent material having a refractive index of less than 1.4.

The invention provides a non-specular reflector.

By "non-specular" is meant non-mirror-like, or diffuse (Lambertian); light reflected off a truly nonspecular surface is generally of equal intensity in all possible directions. Such a reflector is desirable for a reflective mode display device in order that the display may be seen with equal ease from anywhere in front of the device.

The reflector of the invention may be for use with almost any sort of display device, but is primarily intended for use with reflective-mode liquid crystal or electrochromic displays. A typical such display device has, as its essential components: a frontplate; a backplate spaced from and sealingly mounted on the frontplate to define a cell; a liquid crystal or electrochromic material retained between the two plates, thus filling the cell; two electrode arrangements, one mounted adjacent (usually on) the cell-internal surface of each plate; and a light-reflective layer mounted to the rear of the cell so that light passing through the cell from front to back is reflected back through the cell and out at the front. The invention concerns such devices wherein the reflector is mounted internally of the cell rather than externally.

The inventive reflector is (in part) a nonspecularly reflective metal layer carried upon an electrically non-conductive substrate. The substrate may be of any suitable non-conductive, preferably rigid, material capable of withstanding the chemical, electrochemical and physical conditions within the device, but is conveniently a glass. However, where the device is a cell as described hereinbefore, then while the substrate may be a separate entity within the cell it is very preferably constituted by the backplate of the cell, the metal layer then being upon the inside surface thereof. In such a case the substrate is naturally formed of whatever material is used for the backplate, and examples of such materials are clear glass, opal glass, silica and a ceramic, or an enamelled metal.Opal glass and ceramics are convenient if it is desirable for the background of the device, around the edges of the metal reflector layer, also to be non-specularly reflective.

The non-conductive substrate bears a nonspecularly reflective metal layer - that is to say, a metal layer that diffusely reflects light incident upon it. Non-specular/diffuse reflectivity of a surface can usually be guaranteed if the surface is rough rather than smooth, though a facetted surface (one which has a myriad of small smooth areas with different orientations) rather than a truly rough surface (one in which there are no such smooth areas) may well be acceptable as a nonspecular surface provided the reflective facets are smail enough and sufficiently randomly orientated.

"Rough" non-specular layers can be achieved in a number'of ways, of which perhaps the simplest is merely to roughen (by abrasion, say) a smooth, specular, surface. It presently seems preferable, however, instead to roughen the surface of the underlying substrate before the metal layer is formed thereon; when placed in position, the metal layer (provided it is relatively thin) is itself rough yet reflective. In either case the degree of roughness is important though not, within reason, crucial. A very coarse surface will reflect light patchily rather than diffusely, while a very fine surface will be so nearly smooth and mirror-like as not to be non-specular at all.Looking at the surface on the microscopic level, and imagining it as a series of mountains and valleys, then, taking as an indicative parameter the average distance from the tip of any one peak to the tip of any adjacent peak, values of from 20 ,*4m down to 0.1 ,ttm provide generally acceptable results, while values in the range 5 to 1 ,um seem to give very satisfactory results. In one particular example the substrate was roughened in two stages: first it was coarsely ground with abrasive material of particle size 8 to 12 ,um (though it could equally have been tumbled, or air-blasted); then it was finely ground (lapped) with abrasive material of particle size 1 to 3 ,um until all signs of the original coarse grinding were removed.This gave a fine "matt" surface with the desired characteristics.

While, in the just-described preferred method, the metal layer can be formed directly upon the roughened substrate, nevertheless it has proven very advantageous (in terms of the increased proportion -- about twice as much - of light reflected) to provide an intermediate layer of 3 dielectric material having a thickness of from 0.1 to 1 ,um, especially from 0.3 to 0.5 ,*4m. There is some indication that a material with a relatively high refractive index provides slightly better results than one with a relatively low refractive index, possibly because of some matching effect with the high refractive index of the metal.

However, good results can be obtained using silicon oxide (either the dioxide, silica, or the monoxide), and this sort of layer can conveniently be applied by in vacuo electron beam evaporation.

It is not clear why the intermediate layer should have such a significant effect, but it seems not unreasonable to hypothesize that the intermediate layer results in the combination being microscopically smoother though macroscopically still quite irregular -- it smooths out the peak-totrough depth variations in the roughened substrate surface, so causing the metal layer's reflective surface itself to be smoother in the same way, and considerably reducing the possibility of multiple reflections, and the associated multiple absorptions, "within" the metal layer's reflective surface.

In the preferred method of making the nonspecularly reflective metal layer it is formed, either directly or (most preferably) via an intermediate layer. upon the roughened substrate. The metal layer is a thin one - having a thickness of from 0.1 to 2.0 ,um, advantageously from 0.2 to 1.0 m - and may be formed in situ by any convenient technique. A typical such technique is vacuum deposition (a source of the metal is evaporated, and then allowed to condense onto the substrate surface); another useable technique is R.F. sputtering.

The metal from which the non-specularly reflective metal layer is formed may be any suitable such metal capable of withstanding the chemical, eiectrochemical and physical conditions within the display device. Examples of such suitable metals are silver, chromium and (especially) aluminium.

Once provided upon the substrate, the nonspecularly reflective metal layer is itself covered by a thin layer of a solid light-transparent material having a refractive index of less than 1.4: this, it is presently believed, is a critical factor resulting in the significantly improved properties of the inventive reflector as compared with prior art reflectors.

It is not, at present, clear why the presence of the transparent cover layer should have the observed effect (a non-specular metal layer can have a reflectivity of 80% in air; with the transparent cover layer it can exhibit up to 70% reflectivity used in a liquid crystal cell, while without the cover layer it would have less than 5% reflectivity). Nevertheiess is seems reasonable to surmise that the cover layer having a refractive index lower than that of the average display system medium (liquid crystal materials generally have a refractive index around 1 .5) somehow matches the reflective properties of the metal surface to the transmissive properties of the display medium, and reduces the tendency for multiple reflections (and the associated multiple absorptions) "within" the rough metal layer reflective surface.

The cover layer is thin, solid, light-transparent and with a refractive index of less than 1.4. It is also preferably colourless. The layer's thickness appears not to be crucial provided it is small, and layers of from 0.1 to 1 ,um, especially from 0.4 to 0.6 ym, seem perfectly satisfactory. Available solid, light-transparent and preferably colourless materials are somewhat limited; very acceptable results have been obtained using a synthetic cryolite (sodium fluoaluminate, Na3AIFe; refractive index 1.36) and magnesium fluoride (refractive index 1.38).

Formation of the cover layer may be by any convenient method. Nevertheless, a particularly suitable method is in vacuo electron beam evaporation. Other useable methods are in vacuo thermal evaporation or sputtering.

The reflector of the invention has so far been described solely as a reflector for use within a display device. Where, however, the display device is such a device requiring electrodes (as in a liquid crystal or electrochromic cell, where electrodes are required to generate the currents or fields necessary to change the optical characteristics of the cell over some defined area), and where, moreover, the device is a field-effect device (rather than, say, one requiring the passage of current therethrough), then the reflector, being of conductive metal upon a non-conductive substrate, may very conveniently also be used as one of the electrodes. In such a case the metal layer may be in a patterned, discontinuous form (so that different areas of the cell may be "activated" in correspondence therewith). A patterned reflector/electrode may be prepared by initiaily forming the metal layer only in those areas where it is required. Preferably, however, the layer is initially prepared as a continuous sheet covering the total area, and is then subsequently removed in part (conveniently by a chemical etching process, perhaps following the application of a suitable resist in the appropriate pattern) to give the required pattern of discontinuous metal layer areas (upon which the cover layer is then formed).

The preferred non-specular reflectors of the invention can be expected to have a reflectivity of 60 to 70% compared to an efficient Lambertian reflector (such as an MgO- or BaSO4-coated surface) when in contact with a liquid crystal film.

By contrast, a simple reflector prepared by aluminising a glass plate ground with 9 ,um abrasive which has a reflectivity of about 80% in air, has a reflectivity of only about 5% in contact with a liquid crystal film! It has hereinbefore been stated that, when forming the non-specular reflective surface by depositing a reflective metal layer upon the roughened surface of a suitable substrate, it is very advantageous to provide an intermediate layer of a dielectric material.This concept is, it is believed, both novel and inventive in its own right - and accordingly, in another aspect the invention provides, for use as a non-specular reflector within a display device, an electrically non-conductive substrate having a roughened surface bearing thereon a thin layer (referred to herein as the intermediate layer) of a solid dielectric material itself covered by a reflective metal layer.

The various features of this aspect ofthe invention -- the type of substrate, the degree of surface roughening, the thinness and material of the intermediate layer, the reflective metal, and so on - have aiready been discussed in some detail hereinbefore, and no further comment need be made now.

The invention extends, of course, to a display device, particularly such a device employing a field-effect liquid crystal or electrochromic cell, whenever using a reflector in accordance with the invention.

An embodiment of the invention will now be described, though only by way of illustration, with reference to the accompanying drawing in which: Figure 1 shows an exaggerated diagrammatic perspective part cut-away view through a reflector according to the invention useable both as a reflector and as an electrode for, say, a liquid crystal cell; and Figure 2 shows an even more exaggerated diagrammatic edge-on view of the reflector of Figure 1.

The drawings are not to scale.

The reflector-cum-electrode of Figures 1 and 2 comprises a non-conductive substrate (11) bearing a first layer (12), a second, patterned, layer (13) and a third layer (14).

The substrate 11 will be the backplate of the liquid crystal cell in which the reflector is to be used. When so positioned, the righthand (as viewed) surface is, and is referred to hereinafter as, the external surface, while the lefthand (as viewed) surface is, and is referred to hereinafter as, the internal surface (the same convention is used hereinafter for the surfaces of the three other layers). The internal surface (1 5) of the substrate has been roughened (shown cross-hatched in Figure 1), and has a fine matt finnish.

Formed on the substrate's internal surface is an (intermediate) layer 12 of silica. The internal surface (16) of this silica layer is also "rough", as depicted by the heavy stippling, though generally less so than the substrate's internal surface 15.

Upon the internal surface 1 6 of the silica layer 12 is a metal layer 13. The metal layer is discontinuous -- it was originally formed as a continuous layer, but was then etched away (as 17) to form a reticulated pattern of smaller metalled areas (as 18). The internal surface (19) of the metal layer is non-specularly reflective (it is "smoothly" rough, as depicted by the light stippling).

Finally, upon the internal surface 19 of the metal layer 13 (and upon the internal surface 16 of the silica layer 12, where that surface has been revealed by removal of the metal layer therefrom) there is a cover layer 14 of - in this case magnesium fluoride.

The following Example and Test Results are now given, though only by way of illustration, to show details of various embodiments of the invention.

EXAMPLE Preparation of a Reflector/Electrode Stage 1: Preparation of the Substrate Surface A 1.5 mm thick glass plate, of a size to suit the backplate of the type of display required, was surface ground on a lapping machine with 9 ym alumina abrasive (Pennwalt-S. S. White No. 5w).

The surface was then washed well in water containing a little detergent, followed by plain water.

The plate was then lapped with a 1 ym alumina abrasive (Linde type C) using a damp Selvyt cloth to carry the abrasive material. The lapping was continued until no traces of the initial 9 X4m grinding remained, and the surface was washed well in water, followed by alcohol and acetone, and dried in an oven at 1 200C.

Stage 2: Formation of the Intermediate Silica Layer The plate was then placed in a vacuum evaporator assembly, and cleaned by ion discharge at 2 x 10-4 Torr (using air or nitrogen) at 2.5 Kv, 100 mA discharge current, for 1 5 minutes. It was then coated with silica (SiO2) by electron-beam evaporation at about 10-5 Torr, using 3 Kv at 40 mA beam current. A layer about 0.3 um thick was deposited, as indicated by a calibrated crystal monitor device.

Stage 3: Formation of the Reflective/Electrode Metal Layer The silica-coated plate was then further coated with aluminium by evaporation from a hot filament at a pressure of about 10-6 Torr. A layer about 0.5 ym thick was deposited.

Stage 4: Formation of the Cover Layer The plate was finally coated with about 0.5 ,um of magnesium fluoride, again using electron beam evaporation at 1 of6 Torr, with 3 Kv at 40 mA beam current.

The reflective film prepared in this way gave about 70% reflectivity when in contact with a typical liquid crystal material (the cyanobiphenyl mixture E7 from BDH Chemicals Ltd.).

Test Results Various reflectors were made using some or all of stages 1 to 4 (or comparable stages) described above. Some of these were in accordance with the invention, some not. All were tested to ascertain their non-specular reflectivity when in contact with a liquid crystal material (BDH's E7), and the results are shown below.

The Test The test itself was as follows:- A large plate of microscope glass was marked off into a matrix of smaller areas and used to prepare a corresponding matrix of cell "backplates". The processes used were those given in the Example hereinbefore, but not all stages of the Example were applied to each area.

Thus, half the plate was only coarse ground (using the 9 ,um abrasive), while the other half was then fine ground (using the 1 jum abrasive). Of each of these halves, half (i.e. a quarter of the total plate) was given a silica intermediate layer (as in Stage 2) and half was not. The plate was then metallized (with aluminium) and covered, in sideby-side stripes parallel to the abrasive demarcation line, with various cover layers, some in accordance with the invention and some not (in each half one strip was left without a cover layer).

Finally, two strips parallel to the intermediate layer demarcation line were contacted with the liquid crystal material BDH E7, one in each half (and in each half one strip was left uncontacted).

The resultant matrix of "backplates" is depicted in Figure 3 of the accompanying drawings: in the appropriate areas are written the reflectivity results obtained upon examination of the areas through a microscope with a digital photometer (the percentages are made on the basis of a comparison with a Lambertian magnesium-oxide on-giass surface taken to reflect 100%).

The Results The results are apparent from Figure 3, and could be said to speak for themselves. However, it is worth pointing out that the "conventional" reflective surfaces (no intermediate or cover layers) show a very significant reduction in reflectivity when contacted with liquid crystal material, that the use of either an intermediate layer or a cover layer results in a substantial improvement, that the use of cover layers of too high a refractive index (SiO2, n = 1.46; ZnS, n - 2) give poor results compared with the use of layers with a lower refractive index (MgF2, 1.38), and finally that the use of both intermediate and cover layers gives excellent results (especially upon the fine-ground surface).

Claims (16)

1. For use as a non-specular reflector within a display device, an electrically non-conductive substrate bearing thereon a non-specularly reflective metal layer itself covered by a thin layer of a solid light transparent material having a refractive index of less than 1.4.

2. The reflector as claimed in claim 1, wherein the substrate is a glass.

3. A reflector as claimed in either of the preceding claims, wherein the non-specularly reflective metal layer has a rough surface the degree of roughness of which is such that the average projection-to-projection distance is from 5 to 1 ,um.

4. A reflector as claimed in any of the preceding claims, wherein the non-specularly reflective metal layer is the combination of a rough surface on the underlying substrate and a thin metal layer borne thereby.

5. A reflector as claimed in claim 3, wherein between the metal layer and the rough substrate surface there is a thin intermediate layer of a dielectric material.

6. A reflector as claimed in claim 5, wherein the intermediate layer is from 0.3 to 0.5 ,um thick.

7. A reflector as claimed in either of claims 5 and 6, wherein the dielectric material is silicon oxide.

8. A reflector as claimed in any of the preceding claims, wherein the metal layer is from 0.2 to 1.0 ym thick.

9. A reflector as claimed in any of the preceding claims, wherein the metal layer is aluminium.

10. A reflector as claimed in any of the preceding claims, wherein the cover layer is from 0.4 to 0.6 jum thick.

11. A reflector as claimed in any of the preceding claims, wherein the cover layer is cryolite or magnesium fluoride.

12. A reflector as claimed in any of the preceding clairns, wherein the metal layer is in a patterned, discontinuous form.

1 3. A non-specular reflector as claimed in any of the preceding claims and substantially as described hereinbefore.

14. For use as a non-specular reflector within a display device, an electrically non-conductive substrate having a rough surface bearing thereon a thin layer of a solid dielectric material itself covered by a reflective metal layer.

1 5. A reflector as claimed in claim 14, wherein the type of substrate, the degree of surface roughness, the thinness and material of the intermediate layer, and the reflective metal are as defined in any of the preceding claims.

16. A reflector as claimed in either of claims 14 and 1 5 and substantially as described hereinbefore.

1 7. A reflective-mode liquid crystal or electrochromic display device comprising: a frontplate; a backplate spaced from and sealingly mounted on the frontplate to define a cell; a liquid crystal or electrochromic material retained between the two plates, thus filling the cell; and two electrode arrangements, one mounted adjacent the cell-internal surface of each plate; wherein the rear electrode arrangement is also a light-reflective layer, and together with the cell backplate constitutes a non-specular reflector as claimed in any of the preceding claims.