GB2184746A - Optical filter for liquid crystal display devices - Google Patents

Abstract

An optical filter is made by depositing upon a transparent substrate 8 a layer of aluminium, anodising the whole thickness of the aluminium to form a porous anodic oxide layer 7, and causing a dye to enter the pores over at least a portion of the anodic oxide layer. A matrix-addressed liquid crystal display cell comprises a coloured layer mosaic 7B, 7G, 7R, arranged to colour the image displayed by the display, wherein the coloured layer mosaic comprises a porous anodised structure 7 whose pores contain a dye. <IMAGE>

Description

SPECIFICATION Opticalfiítermanufacture This invention relates to the manufacture of optical filters, and finds particular but not exclusive application in the manufacture of colour matrixfilters for use in liquid crystal display devices.

According to the present invention there is provided a method of making a filter including the steps of depositing upon a transparent substrate a layer of aluminium, of anodising the whole thickness of the deposited aluminium to form a porous anodic oxide layer, of causing a dye to enter the pores over at least a portion ofthe anodic oxide layer.

It is well known that al uminium sheet can be coloured by anodising its surface to a depth of typically not more than about 25 microns using an electrolyte, such as sulphuric acid, which produces a porous anodic film, filling the poresthatresultfrom anodisation with pigment or dye, and then sealing the surfaceto trap the colourant retained within the pores.

There is however, at least one reason for believing that this process would not be applicable for the conver- sion into anodic oxide ofthe whole thickness of an aluminium film deposited upon an electrically insulating substrate. It is clearthat, once the anodic conversion process has penetrated in places right through the whole thickness of the layer, there is a high probability of non-uniformities ofthe layer, or of its processing, resulting in the formation of macroscopic islands of metal which, on account of their island status, have no metallic connection with the anode.It might be supposed that even when such islands are formed, their isolation from the anode would mean that no further electric current could flow through them, and hence that no further anodicconversion could take place in these regions,whichwould therefore remain indefinitely as macroscopic islands of unconverted aluminium metal.

Unexpectedly we have found that in practice no particular difficulty is encountered in anodising the entire thickness of a layer of aluminium measuring 90mm by 1 00mm, supported upon an electrically insulating substrate of the same size, the largest area currently tested. In particular by making anodic contact with titanium jigging at points nearthe four corners of an aluminium film it has been possible regularly to anodise the entire thickness of aluminium layers of this size which have been deposited by evaporation, and which therefore from geometrical considerations will II have a greater layer of thickness in the centre of the sheet than atthe corners where the contacts are made. (References to the anodisation of the entire thickness areto be understood as meaning thatthe anodisation has proceeded without leaving macroscopic islands of metal separated from the anode contact area(s), but are notto be understood as necessarily i::mplyingthe absence of any microscopic scateistart.S,of;rae.taL), There follows a description of the manufacture of a liquid crystal display cell provided with a colourfilter mosaic made by a method embodying the invention in a preferredform.The description referstothe accompanying drawing which depicts a schematic cross-section of a portion of the cell in the liquid crystal cell, a thin layer 1 of liquid crystal medium is contained in a hermetically sealed enclo sureformed by a transparent glass sheet 2 and a thinnertransparent plastics sheet 3 secured together with a perimeter seal 4.The glass sheet 2 carried on its inward facing surface a set of parallel transparent indium tin oxide (ITO) conductive tracks 5which extend in a direction at rightanglestothatof a similar set of tracks 6, not necessarily ofthe same pitch, formed on the plastics sheet3. In this particular instance the tracks 5 on the glass sheet 2 are 318 microns wide and are arranged on a pitch of 330 microns, while the tracks 6 on the plastics sheet 3 are 138 microns wide and are arranged on a pitch of 150 microns. Each area of intersection of a track 5 with a track 6 forms a pixel of the display, these pixels being addressed on a matrix basis via their electrode tracks 5 and 6.The liquid crystal layer 1 is in this case a positive dielectric anistropy smectic A material which can be electrically switched between a first stable state in which it is homeotropically aligned and hence visually clear, and a second stable state in which it relaxes from a dynamic scattering condition into afocal conic one which is optically scattering. The cell is obliquely lit, butisviewed against black background (not shown) so that pixels appear black when in theirclear state, and only redirect lighttowardsthe observer when in their scattering state. Alternatively, the cell incorporates a black guest pleochroic dye mixture, in which casethe illumination is direct rather than oblique.In this instance the black dye will prevent light from reaching theobserverthrough a pixel set into its scattering state, but will only weakly attenuate the light when the pixel is in the homeotropically aligned state.

The pixels are arranged in colourtriads, each comprising a red pixel, a green pixel and a blue pixel.

in order to provide a facility for generating multicoloured displays. The requisite colours are formed by colourfilter stripes 7R, 7G and 7B that are in registry with the electrode tracks 6. These colour filter stripes are formed in an anodised aluminium layer7 supported upon a transparent glass substrate 8 to which the plastics sheet 3 of the cell has been laminated.

Deposited upon a transparent glass sheet, which is to form a substrate 8 of the cell assembly, is an adherent layer of aluminium subsequently to be converted to the anodic oxide layer 7. Typically the thickness ofthis layer lies in the range from 1 micron to 12 microns. With athickness of only 1 microns the amount of dye that can be incorporated into the pores afteranodisation is restricted tothe extent that a desaturation of colour is noticeable and so, for applications where reasonably saturated colours are required, it is generally preferred to use a greater thickness in the region of Sto 12 microns. The aluminium is deposited conventionally, for instance by evaporation or by sputtering.

In accordance with normal practice for producing a porous anodised film, a relatively concentrated acid solution is employed forthe anodising electrolyte, this consisting ofsulphuric acid (65% concentrated sul phuric acid: 35% distilled water by volume) admixed with glycerine in the ratio 15 volumes dilute acid to 1 volume of glycerine. Following the teaching concern ing thejigging of aluminium for anodisation, contained in Chapter 7 of the book 'Anodic Oxidation of Aluminium and its Alloys' by V. F. Henley(published by Pergamon Press 1982), the aluminium coated glass plate is mounted in a titanium metal jig provided with titanium clips that make line contact with the alumi nium layer over a short line-contact region near each corner ofthe plate.Afterthe plate has been mounted in thejig, and theassembly has been immersed in the electrolyte, it has been found that a 5 micron thickness aluminium layer on a glass plate measuring90mm x 1 OOmm can be completely anodised to a transparent film (except forthe region immediately underthe four contacts) in a bath ofthe electrolyte maintained at 22"C using a tension of 12 volts to provide a current density of about 1.2 Amps/dm2, the anodisation being complete in a period of about30 minutes.

When the glass substrate 8, with its now fully anodised aluminium layer7, has been removed from the electrolyte and the jig, and has been rinsed, standard photolithographic procedures are used in a three-stage process to form red, green and blue stripes 7R, 7G and 7B in the layer 7, these stripes being 138 microns wide and at a pitch-of 150 microns. In the first part ofthisthree-stage processthe anodised layer is coated with adhesion promoter and UV-sensitive negative photoresist,which is baked for 1 minute at 90into cure it before exposing itto UV light th rough a mask (not shown) provided with a set of parallel opaque bars 138 microns wide at a pitch of 450 microns.The exposed photo resist is developed by dissolving awaythe regions not polymerised by the UV light, and then red dye is applied. This dye passes through the windows ofthe developed photoresist and enters the pores ofthe underlying portions of the anodised layer7 exposed by those windows, and in thisway the red stripes 7R a reformed. Next excess dye is wiped off and the photoresist is stripped ready for a repeat of the processing cycle which formedthe second stage ofthe three-stage process. In this second stage green dye is used instead of red, and the mask is indexed 150 microns so thatthe green stripes 7G are formed alongside the red ones.The process is then repeated forthethirdtime, in this instance using blue dye, and with the mask indexed a further 150 microns so thatthe blue stripes 7B are formed between the red and green ones. After this the plate is immersed in a Sgm/litre aqueous solution of sulphuric sealing salts at 95"C for45 minutes. The particular adhesion promoter employed is one marketed by Dupont under the designation VM 651.The particular photoresist, inks and sealing salts employed are materials marketed by Via Mega Electronics of Saffron Walden, Essex underthe designation Gedakopforthe dying of anodised aluminium.

Instead of using the one maskfor each ofthe three exposures, and indexing it across the appropriate amount between exposures, twill be apparent that three distinct masks can be employed, each having essentially the same pattern of bars, the bars of each mask being offset with respect to those of each of the othertwo masks.

From considerations of parallax it its clearly desirable to have the coloured stripes 7R, 7G and 7B as close as possibletotheliquid crystal layer 1, and hence in principle the plastics sheet 3 can be dispensed with, and instead arrangements madeto deposit the ITO tracks 6 directly upon the anodised layer 7. Drawbacks ofthis approach include the risk of degradation ofthe liquid crystal layer 1 by contamination with material leached out of the anodised layer7, the risk of degradation ofthe dyes stripes by contamination with material ofthe liquid crystal layer, and the difficulties involved in providing an TTO deposition and etching process which does not invoive processing conditions liable to degradethe dyes. Accordingly, in this instance it has been preferred to employ the intervening plastics layer 3.

This layer 3 is a thin layer of clear polyester sheet which is available in 100 micronthickform ready coated with ITO. Thinner sheeting is also available in uncoated form in a wide range of thicknesses covering the range from 1 micron to 100 microns, and these could be coated to provideafilmaffordingreduced parallax and henceawiderviewing angle for the display. For ease of handling both in coating with ITO and insubsequentlamination itwill generally be preferred to use a thickness of at least 20 microns.The ITO layer on the polyester sheeting 3 may be selectively etched to definethe electrode tracks 6 before the sheeting is laminated to the dyes anodised layer 7 supported by the glass substrate 8, in which case the lamination needs to be carried out with the dyed stripes 7R, 7G and 7B accurately registered with the electrode tracks 6. Alternatively, provided that the lamination can withstand subsequent processing involved in selectively etching the ITO, it is preferred to laminate first and etch afterwards. A convenient way of performing this lamination is to screen print a clear adhesive, such asthevarnish marketed by Sericol of Parsons Green Lane, London underthe designatiqn 'Polyscreen Varnish', on to the anodised layer 7, andthen to cover this with the polyester sheet 3.Any trapped air is removed by placing the assembly in a vacuum bag, evacuation ofwhich also servesto bed the polyester firmly down on to the anodised layer. The Polyscreen varnish may be cured after removal of the assemblyfrom the vacuum bag by heating in nitrogen at a pressure of about 80 kPa and a temperature of about 100 Cfora period of between two to three hours.

If the ITO layer has not already been patternedto definethetracks6,thispatterning is nowcarried out using conventional photolithographic practice. The laminated structure is then assembled with the perimeter seal-4 and the other ITO electrode sheet 2 and bonded together in conventional mannertoform an envelope for the liquid crystal layer 1,this medium being introduced intothe envelope by anystandard method such asvacuum filling through a gap in the perimeter seal, or by a vacuum through4lowfilling method employing holes (not shown) machined through sheet 1 in diagonally opposite corners ofthe area enclosed by the perimeter seal.Finally, the filling aperture or apertures are sealed offto provide a hermetic enclosure.

Theforegoing specific dOscription has related to the manufacture of a colour filter rncrsaisfor use in a matric addressed liquid crystal display cell of lightscattering type. It should be apparent however,that such colou rfilter mosaic wil I also find applical:íon in other types of liquid crystal cell including for instance of the light-valve type. In certain types of application the absence of any dye between adjacenttessera of the colour mosaic may lead to an undesirably great desaturation of colour displayed by the cell.In these circumstances it will be desirableto setthe individual tessera in a black matrix. Oneway of achieving this is to coverthe anodic oxide layer2 before the application ofthefirst photolithographic mask with a special protective layer(notshown) such as the proprietary coating applied by D.D. Produkte of Vertrieb, Luzern.

The dyes are chosen in relation to the material of this protective layer so that they dissolve it and thereby allow penetration ofthe dye into the pores of the underlying anodic oxide. The protective layer is designedto be left intact when the photoresist is stripped after application of the dye, and thus, after the stripping of the last photoresist mask, this protective layer remains in place exceptforthose regions where it has been penetrated by the dye. The anodic layer is then subjected to afirst sealing treatment, known as the colour seal. The material ofthe protective layer acts as a mask so that only the dyed regions are sealed by this sealing treatment. Then a black dye is applied.

This black dye dissolves the remaining areas of black dye and is thus able to penetrate into the pores of the regions of anodic oxide thereby exposed, but it does not penetrate the regions which have already been dyed because those regions have been sealed by the preceding colour seal step. Thus a black matrix is created. Excess dye is removed and the anodic layer is subjected to a second sealing treatment to seal in the black dye. This second sealing treatment involves the use of a different formulation, and unlike the first sealing treatment, effects full sealing of both the coloured areas and the black areas.

CLAMS 1. A method of making a filter including thesteps of depositing upon a transparent substrate a layer of aluminium, of anodising the whole thickness ofthe deposited aluminium to form a porous anodic oxide layer, of causing a dye to enterthe pores over at least a portion oftheanodic oxide layer.

2. Amethodasclaimedinclaim 1,whereinthe step of causing thedye to enterthe pores issucceeded by the step of sealing the surface of the anodic oxide layer over at least a portion ofthe region where dye has been caused to enter its pores.

3. A method as claimed in claim 1 or 2, wherein by means of selective masking dyesof di,ffereratcolour are caused to enter the pores of di,ff,e?entregions ofthe aoodieoxde layer..

4. A metodl as claimed in claim 3, wherein each dye is caused to enterthe pores through regions in a protective layer exposed by windows in an overlying mask layer, which protective layer is dissolved away bythe dye in those exposed regions, and wherein the surface ofthe anodic oxide is sealed in two stages, in the first stage ofwhich the residual areas of the protective layer act as a mask preventing the sealing of the undyed regions of the anodic layer, after which said residual areas are dissolved by the application of a black dye before performance ofthe second sealing stage.

5. A method of making a filter comprising a dye-incorporating anodic oxide of aluminium layer supported upon a transparent substrate, which method is substantially as hereinbefore described with reference to the accompanying drawing.

6. Afilter made by the method claimed in any

Claims (1)

1. preceding claim.

   7. Filter as claimed in claim 6 in whose anodic oxide layerthere is formed of regions of different colour.

   8. A matric addressed liquid crystal cell incorporating a filter as claimed in claim 6,thetessera of whose mosaic registerwith the individual pixels, or individual groups of pixels, ofthe liquid crystal cell.

   9. Amatrix-addressed liquid crystal display cell comprising a coloured layer mosaic arranged to colourthe image displayed by the display, wherein the coloured layer mosaic comprises a porous anodised structure whose pores contain a dye.