CA1121489A - Lcds (liquid crystal displays) controlled by mims (metal-insulator-metal) devices - Google Patents

Abstract

LCDs (LIQUID CRYSTAL DISPLAYS) CONTROLLED BY MIMs (METAL-INSULATOR-METAL) DEVICES Abstract of the Disclosure A matrix multiplexed liquid crystal (LC) display cell has switch devices at matrix crosspoints to provide a turn-on threshold for the LC material. The switch devices are thin film Metal-Insulator-Metal devices (MIM's). The cell is operated at low current so that the MIM's which are deposited on glass do not degrade rapidly in use. - i -

Description

This invention relates to liquid crystal (LC) display cells, specifically to such display cells matrix multiplexed to a high level.
In a matrix multiplexed addressing scheme for an LC
display cell, a series of scan pulses Vs is, for example, applied sequentially to each of a series of row conductors, (scan lines), while a series of data pulses Vd is applied to selected ones of a series of column conductors, (data lines). To turn on a LC picture element (pel) at a selected row and column intersection, the difference between Vs and Vd applied to the selected row and column respectively, is made great enough to alter the liquid crystal molecular orientation,andthus the cell optical transmissivity, in a manner known in the art.
Several factors combine to limit the number of lines that can be multiplexed in a LC display cell.
Firstly, at the instant a pel is selected, other, non-selected pels in the selected column also experience a pulse Vd. For one address period the RMS value of a.c. voltage experienced by these pels is insufficient to turn them on, but if N pels in a column are switched on and ~` off in a single field scan, the off pel will experi~ence Vd for N address periods. This may be enough to turn the pel onO It can be shown that the ratio of RMS voltage seen by an on pel to that seen by an off pel is:-VRMS (Vs + Vd)2 + Vd2 (N-l) FMS ~ (Vs ~ Vd)2 + Vd2 (N-l) As N increases, the ratio becomes smaller and, since liquid crystals do not have a sharp threshold separating on and off, the contrast ratio between on and off pels becomes poorer. At a certain number of row conductors the contrast ratio becomes unacceptable.
The problem is compounded as the angle from which the cell - 1 - ~q~

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is viewed deviates from an optimum value. Also, since the LC electro-optic response is temperature dependent, then iF the LC is to be off at Voff at high temperature, and on at Von at low temperature, the difference between Voff and Von must be greater than For constant temperature operation.
For the above reasons, prior art limits multiplexing to about q lines (or 8 lines for temperature compensated display cells).
A suggestion for solving this problem proposes placing a switch in series with each liquid crystal pel at the intersections of the scan and data line, such that pulses Vd do not activate the switch nor the pel controlled by it whereas a selection pulse Vs + Vd does activate the switch whereupon the LC experiences voltage. Such a switch should be symmetrical with respect to zero voltage since, for the purpose of preventing irreversibile electro-chemical degradation of the liquid crystal, net DC
bias should be avoided.
In its broadest aspect the invention proposes the use of a thin film metal-insulator-metal (MIM) device as the switch. MIM devices function by tunnelling or traPdepth modulation. In the form~r, carriers pass through the thin insulator by field enhanced quantum mechanical tunnelling.
In the latter, carriers are released from traps in the insulator as the field developed between the metal layers diminshes the potential barriers to current flow. Such devices are known which exhibit, in a switching regime, an increase of from 500 to 10,000 times the original current passed for a doubling of voltage. This turn-on is sufficiently sharp to increase the number of multiplexed lines, compared to the number permitted when no switch is used, by at least a factor of 8. If on the other hand, the number of multiplexed lines is maintained, then using MIM switches provides a greatly increased viewing angle, contrast ratio and permitted temperature range.
Thin film MIM's may have insulators such as aluminum oxide, . . : : : ~
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tantalum pentoxide, silicon nitride and silicon dioxide. The thickness of the dielectric layer determines the conduction process. Below 50-lOOA electron tunnelling is possible; from 100 to lOOOA trap depth modulation conduction processes dominate. The metal of the MIM may be any material which forms an ohmic, or weakly blocking, contact.
Embodimentsof the invention will now be described with reference to the accompanying drawings, in which:-Figure 1 shows in schematic form a matrix multiplexedaddressing scheme for a LC displayj Figure 2 is a part perspective, part sectional view, not to scale, of part of a liquid crystal display cell using one form of MIM
and Figure 3 is a view similar to Figure 2 but showing one plate of a display cell using an alternative form of MIM.
In the conventional matrix multiplexed addressing scheme for an LC display cell, as shown at bottom left in Figure 1, a series of scan pulses Vs is, in use, applied sequentially to each row of a series of row conductors 10 called scan lines, while a series of data pulses Vd is applied to selected ones of a series of column conductors 12 called data 20 lines. If an "on" pulse is desired at a LC pel 14 at a selected row and column intersection, the difference between Vs and Vd applied to the selected row and column respectively is made great enough to turn on the LC pel in a manner known in the art.
As previously explained, since LC's do not have a sharp threshold separating on and off, then a pel may turn on even though not specifically addressed because it experiences data pulses Vd driving other pels in the same column.

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~Zla~89 As shown at top right ;n Figure 1, the invention proposes the forming of a thin film MIM device 16 in series with each LC pel 14.
Referring to Figure 2, the LC cell comprises a pair of glass plates 18, 20 with a layer of twisted nematic LC 22 sealed between them. The inner surfaces of the plates 18, 20 are treated in a manner known in the art so as to properly orientate the LC molecules. As is well known, by applying a voltage across selected regions of the LC layer, the LC can be caused to undergo localized molecular reorientation with consequent alteration in optical transmissivity through the cell.
A switch is sited adjacent the position of each pel 14, the pels being defined by a row-column array of indium tin oxide transparent electrodes 24 on the inside surface of plate 18 and by a corresponding array of transparent electrodes (not shown) on the inside surface of plate 20.
Although not illustrated,switches can be series connected to each pel electrode,each pel thus having an associated thin film fabricated MIM device on each oF
the plates 18 and 20.
To fabricate the MIM's on the inside of the plate 18, a thin film 26 of tantalum is deposited by sputtering. The layer is thermally oxidized at 465C for 16 hours to protect the glass from following etch steps.
20 A second layer of tantalum is sputter deposited and photodefined into row conductors 10 which are from 2 to 25 miles wide and run the breadth of the plate. The conductors, which function as one side of the metal-insulator-metal (MIM) devices may be locally reduced in width to 0.5 mils at the MIM
active areas. The conductors 10 are anodized in a weak citrlc acid bath at 30-60V anodizing voltage to produce surface layers 28 of tantalum pentoxide which act as the insulators of the MIM devices. Cross conductors are then deposited in distinct gold bars 30 from 0.5 to 5.0 mils wide over the tantalum pentoxide lines, each bar overlying and electrically contacting a . .

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respective electrode 24. The active area of the MIM's typically 1.0 mil2, are formed at the regions of intersection of layers 28 and bars 30. Each MIM is thus series connected between one of the electrodes 24 and a tantalum conductor 10. The cell is fabricated by sealing the nematic liquid crystal layer 22 between the glass plates 18 and 20. The electrodes 24 common to a particular column on the glass plate 20 are electrically connected either by thin film conducting leads 32(or alternatively formed as continuous stripes) ~hich enable pulses to be selectively applied to LC pels 14 by applying data and scanning pulses Vd and Vs to the appropriate row conductors 10 on plate 18 and column conductors 32 on plate 20.
Other examples of MIM devices have insulators of tantalum oxy-nitride, aluminum oxide (A1203), silicon nitride (Si3N4) silicon dioxide (SiO2) silicon oxynitride and silicon monoxide (SiO). Another example of metallization is aluminum.
Although called a metal-insulator-metal device, the important performance characteristic of the device is that it should be prepared as a thin film device and that it should function as a switch by virtue of the field enhanced quantum mechanical tunnelling or trap depth modulation mentioned previously. Thus in an alternative embodiment of the invention, the "metal" at one face of the MIM is indium tin oxide which has the advantage of being inherently transparent and so does not significantly attenuate light transmitted through it. To take advantage of this property, another embodiment (not shown) uses a single thin film indium tin oxide region to function both as the liquid crystal electrode and one "metal" layer of the MIM. Other materials used in MIM devices, for example, NiCr, which are effectively transparent by virtue of being of the order only of a few hundred A, can also be used as combined electrode and metallization.
The particular thin film technique ~sputtering, vacuum ~LZla~

evaporation, anodization etc.) used in the formation of MIM layers is chosen to be compatible with the material being formed and the glass substrate material.
In an alternative embodiment, the fabrication order is reversed, the cross conductors 30 being deposited and anodized, and then the ion conductors being deposited subsequently. Although, for convenience, the insulator layer is obtained by anodization, it can also be formed in a separate deposition step.
A further embodiment is shown in Figure 3. On the inside of a piece of flat glass 18 which is to function as one side of the liquid crystal cell, a thin film layer 26 of etch protectant is formed as previously described. A thin film of tantalum is then formed over the etch protectant and then photo etched to produce distinct regions 34. The tantalum regions are ànodized to Form surface layers 36 of tantalum pentoxide. A thin film layer of gold is next formed on the substrate. From this layer, two gold pads 38a and 38b are photodefined on each region 34 to produce a structure equivalent to a pair of back-to-back MIM's. Leads 40a and 40b are integrally formed with the gold pads. Each of the leads 40a extends between one of the regions 38a and the electrode 24 of the pel which the MIM controls. Each ; 20 of the leads 40b interconnects the pads 38b on those MIM's in the same column as one another. An advantage of this embodiment is that the switch characteristics do not depend on current polarity since the device is symmetrical.
An advantage of using gold at one side of an MIM device is that it permits low resistivity connections with drive circuitry.
In an alternative process for fabricating this embodiment, the leads 40a and 40b are of tantalum and are formed simultaneously with regions 34. The leads are protected from anodi~ation by coating with photo-resist which is su~sequently removed.

- , ' ~lZ~ 39 As described with reference to the Figure 1 embodiment, the number of process steps in the manufacture of the Figure 3 cell is reduced if the pads 38, leads 40 and electrodes 24 are Formed at the same time, as a partially transparent thin film of indium tin oxide.
Using MIM switches at matrix crosspoints, high level multiplexing (100-1000 lines) of a matrix addressed array of liquid crystal display picture elements can be obtained without the pr;or art problems of narrow viewing angle, low contrast ratio between off and on element, and greatly limited operating temperature ranges. MIM switches may be used both in transmissive and reflective displays since a MIM switch can be made on the transparent sides of the cell and is not so big as to obstruct the picture elements.
Since the thin film MIM devices are very much less than 10 microns, in thickness, i.e. of the order of 1 micron, their presence on the transparent plates flanking the LC material does not prevent the use of a correspondingly thin layer of LC material as would thick film devices.
In turn, and assuming the resistivity of the LC material is very high, of the order of 101 ohm-cm., then the charge through the MIM device is limited by the LC resistance. Coupled with the fact that MIM devices used show their switching characteristics at very low currents, of the order of lO~A it will be appreciated that the MIM devices can be operated in a very low current regime which reduces the chance of their failing through excess heat dissipation. In the intended application to a large area (e.g. 9" x 9") high pel density (e.g. pel area oF less than 25 mil square), fabrication oF the MIM devices offers significant cost benefits over thin film transistor switches since fabrication techniques for the latter are more complex and are characterized by poor yield. In addition the Fabrication techniques proposed are vastly preferred to silicon IC tecnhiques, again, because of cost and ~z~

further because by using known techniques accurately planar glass surfaces can be achieved which ensure little var;ation in LC cell thickness.

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SUPPPLEMENTARY DISCLOSURE

For practical operation in a MIM switch controlled LCD, the capacitance of the MIM switch must be substantially less than the capacitance of the pel which it controls. Several embodiments for achieving this capacitance differential are illustrated in the following drawings in which:-Figure 4 shows in schematic torm a LC pel controlled by aMIM switch configuration relatively independent of current polarity;
Figure 5 shows in schematic Form an alternative MIM switch configuration for obtaining relative independence from current polarity;
lQ Figure 6 shows in circui~ schematic form a MIM switch controlled LC pel;
Figure 7 shows in circuit schematic form an alternative to the MIM switch controlled LC pel of Figure 6;
Figure 8 shows a production process sequence for making a practical display embodying the MIM switch controlled pel of Figure 7; and Figure 9 shows in cross-section a buried MIM switch embodiment of the invention.
To achieve long lifetime from LC displays the LC should not experience any net d.c. bias. For this reason the polarity of the drive pulse is usually periodically reversed. In the MIM switch controlled LCD
however the voltage pulse experienced by a LC pel is modified by the electrical response of the MIM switch. If the I-V characteristic of the individual MIM switches are not symmetric with respect to polarity then non-polar configurations of pairs of MIM switches may be fabricated instead at each pel site.

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Two non-polar configurations of MIM switch pairs are shown schematically in Figures 4 and 5, the diode representations of the MIM
switches 16 being indicative of the asymmetry in the I-V characteristic of each MIM. In the Figure 4 embodiment the I Y characteristic derives frorn the less conductive branch, while in the Figure 5 embodiment the I-V
characteristic derives from the more conductive branch. Yarious fabrication sequences can be readily derived from the previous description and one example for the Figure 4 embodiment, is described in detail with reference to Figure 3. In the Figure 4 and 5 embodiments the switch characteristics tend not to depend on current polarity since the configuration of switches is symmetrical. In both cases, current experiences both a MIM switch and its inverted structure MIM switch.
The deposition order described for the Figure 2 and 3 embodiments was as follows:-1. Etch stop deposi~ion - Ta20s on soda glass substrate.

2. Ta deposited, photodefined and anodized.

3. NiCr:Au crossover layer deposited and photodefined.

4. Transparent pel electrode deposited.
Processing however is not confined to this particular order.
If dry etching techniques are used, no etch stop layer need be deposited.
Other possible processing sequences are listed briefly below.
A. Ta deposition - anodi7ation - crossover deposition - pel electrode deposition.
B. Pel electrode deposition - Ta deposition - anodization crossover deposition.
C. Ta deposition - pel electrode deposition - anodization - crossover deposition.

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Also, as indicated previousl~, although currently it is ~ound convenient to deposit MIM switch and addressing leads in an operation distinct from pel electrode deposition, the LC pel and its series connected MIM switch can have their common terminal deposited as a homogeneous transparent conducting layer, thereby reducing the number of processing steps. I-Y characteristics of the MIM switches can be adjusted by performing the Ta sputter deposition in a nitrogen atmosphere of appropriate concentration.
Although not illustrated, MIM switches can be series connected to both electrodes of each pel, each pel thus having an associated thin film fabricated MIM switch on each of the plates 18 and 20. Alternatively there may be some advantage in fabricating a display with some MIM switches on one glass plate and other MIM switches on the other glass plate. When using opaque MIM switches for example~ such an embodiment has the advantage of maximizing the area of each plate which can be devoted to pel electrode formation. Moreover from a production viewpoint the two glass plates flanking the LC can be processed i 20 identically.
Considering operation of the LCD illustrated in Figure 2, a circuit equivalent of a liquid crystal pel 14 and its series connected MIM
switch 16 is shown in Figure 6. The liquid crystal pel 14 is represented as a resistance Rp in parallel with a capacitance Cp and the MIM
switch is represented as a variable resistance Rs in parallel with a capacitance Cs~ The cell is driven by a voltage V=Vs~Yp where Ys is the voll;age across the switch and Yp is the voltage across the pel.

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In time division multiplexing, the addressing per~od for each of N scan lines is T/N, where T is the total frame time. During this period (T/N <<for large N) selected pels are addressed with the Full Ys + ~d voltage. For the remainder of the frame, the "relaxation period", they see only the -Vd data pulses applied to the other lines.
The LC pel capacitance, Cp, must be charged through the MIM non-linear resistance Rs. In the voltage regime under consideration, Rs is designed to be small for the selected pels and large for unselected pels.
During the relaxation period the LC pel begins to discharge both through its own internal resistance Rp and to the now parasitic capacitance Cs f the MIM. In order to maintain as high an RMS voltage on the selected LC pel as possible, Rp must be large and Cs Cp. If Cc is not small compared to Cp, the voltage division effect of the capacitors greatly reduces the effective voltage on the LC.
For flat plate capacitors of which the pel and the MIM
switch are more-or-less examples:
C is proportional to A/ Q`
where A jS the area and Qis the spacing of the plates. The twisted nematic LC thickness is typically 10 microns and the thin film tantalum pentoxide insulator is typically 0.05 microns. Since the dielectric constants are similar, then if (A/Q)P jS to be very much greater than (A/Q)S~ then AP/10 jS ~ AS/0-05-or Ap >>200 AS.
In order that the switch operates before its series connected pel and, more importantly, before unselected pels in the same column as the selected pel, the area of the MIM switch must be very much smaller than the area of the LC pel. This area is typically 1/1600 of the pel area.

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As an alternative to regulating the relative areas of the pel and its controlling MIM switch, extra capacitance CE can be introduced in parallel with the LC capacitance Cp as shown schematically in Figure 7. This arrangement is preferable particularly when the size of MIM switches needed to preserve the capacitance relationship discussed previously is approaching the limits of photolithographic resolution.
Following, several production sequences for practically realizing the Figure 7 embodiment are described.
In a first method the dielectic of capacitor CE is Ta20s which has a high capacitance density so ensuring that capacitors CE occupy small areas of substrate and so do not materially reduce the substrate area available for pel electrodes.
In addition to a MIM Ta line 46, a second CE line 48 is deposited parallel to each MIM line, (Figure 8a). Using a mask, unshaded areas including regions 50, 52 are anodized, (Figure 8b). If anodized separately, the CE oxide 52 may be made of thickness different from that of the MIM oxide 50. ~he CE tantalum interconnects are then etched away to leave the anodized oxide region 52 and a contact pad 54 (Figure 8c). Additional dielectric is then deposited at regions 56 followed by top contact metallization regions 58 for the MIM switch and regions 60 connected to leads 62 for the capacitor CE. The CE is connected into the pel circuit on subsequent deposition of the pel ; electrode 24 (Figure 8e) and, after LCD packaging, by externally connecting the leads 62 to column electrodes 32 on the opposite plate 20 of the LCD. The dielectric region 56 is not reguired if capacitive crosstalk or leakagle is small.

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Although not specifically illustrated, the process step of anodizing through a mask can be avoided by depositing a layer of dielectric to function as the CE insulator at the same time as crossover isolation regions are deposited. Readily available dielectrics are of relatively low dielectric constant so large capacitors are required which may detract from the space available for pel electrodes.
In contrast to the two previous embodiments a large storage capacitor CE can be buried under the pel electrode 24. However~
particularly for twisted nematic LC displays, the capacitor CE must be transparent which somewhat limits the materials which can be used.
An advantage of the LCD-MIM switch combinations described previously is that the MIM switches are laterally offset from the LC pels 14 which they control. This means that the part of the LC layer 22 which undergoes a transmissivity change is located directly between the two glass plates 18 and 20 each of which is covered by a thin layer of oxidized tantalum and a thin fllm pel electrode 24. Thus the layer of liquid crystal 22 is nearly as flat as the surfaces of the underlying glass. Flatness if important is contrast variations are not to exist over the area of the pel. Such flatness is more difficult to achieve if the MIM switch and liquid crystal pel are superimposed as shown in Figure 9.
As an alternative to the previous embodiments in which the MI~ switch is offset from the pel electrode, the MIM switch 16 can be buried under the pel electrode 24. As illustrated in cross-section in Figure 9, anodized tantalum regions 64 are overlain by interconnect metallization having contact pads 68. To keep the MIM switch area small compared to LC pel area, a pel electrode 70 is deposited through a via in a thick dielectric layer 72 to provide a top contact 74 for the MIM switch , .. . . .

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16. A wholly transparent display can be obtained by using In: Sn: O or Cd: Sn: O instead of tantalum as bottom layer 76 and then depositing the MIM dielectric 64.
Fabrication of the MIM switches using thin film technology is considered to be advantageous since deposition of thin films permits the use of a thin twisted nematic LC which has a very low switching voltage. The threshold voltages of the MI'M switches can be made correspondingly low which contributes to the overall display panel, although having many densely located pels, being a relatively low power device which can be driven using CMOS logic.
The embodiments described have been in terms of a transmissive display using a twisted nematic LC extending between crossed polarizing glass plates.
However the display can be made reflective by siting a reflecting sheet on the far side of the LC from an illuminating source.
The glass plates confining the LC are made polarizing by the deposition of a thin polarizing film thereon.
Although twisted nematic LC's are preferred for the reasons discussed above, many liquids which can be sti~ulated to produce an electrical field related change in optical characteristics can be used in a MIM switch controlled matrix display. Thus, for example, the twisted nematic LC could be replaced by a dyed nematic phase transition LC in which crossed polarizers are unnecessary. Other guest-host LC's which function on the basis of anisotropic absorption of light by dichroic dyes will be familiar to those in the LCD display art. In addition, the liquid layer need not be a LC at all but could be an electrophoretic medium ; operating by field related movement of colloidal pigment particles.

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Claims (33)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:-

1. A liquid crystal display cell comprising a pair of plates flanking a liquid crystal material, the display cell having a plurality of picture elements, each picture element defined by a pair of opposed electrodes on the inside faces of the respective plates with means for applying a voltage between the opposed electrodes of each element, an electrode of each element being series connected to a respective switch element, the switch elements being thin film metal-insulator-metal devices (MIM) formed on the inside face of one of the plates.

2. A liquid crystal display cell as claimed in claim 1, the picture elements and said switch elements being arranged in rows and colunms, first lead means electrically connecting the metal at one side of each MIM to its series connected picture element, second lead means electrically connecting the metal at the other side of the MIM's in rows and third lead means electrically connecting the electrodes on the other plate in columns.

3. A liquid crystal display cell as claimed in claim 2, in which the metal at said one side of each MIM is formed as a single homogeneous substantially transparent layer with an electrode of its series connected picture element.

4. A liquid crystal display cell as claimed in claim 1 or 2 in which the insulator of said MIM device is one of the group consisting of tantalum pentoxide, aluminum oxide, silicon nitride, silicon dioxide, silicon oxynitride and silicon monoxide.

5. A liquid crystal display cell as claimed in claim 1 or 2 in which the metal of at least one side of said MIM device is one of the group consisting of tantalum, aluminum, gold, indium tin oxide, and NiCr.

6. A liquid crystal display cell as claimed in claim 1 or 2, in which the MIM is a tantalum - tantalum pentoxide - gold device.

7. A liquid crystal display cell as claimed in claim 1 or 2 in which the MIM is an aluminum - aluminum oxide-gold device.

8. A liquid crystal display cell as claimed in claim 1 in which the picture elements and said switch elements are arranged in rows and columns, the metal at one side of each MIM being in the form of first and second distinct regions, first lead means for electrically connecting each of said first regions to a series connected picture element, second lead means for electrically connecting said second regions together in rows and third lead means electrically connecting the electrodes on the other plate in columns.

9. A method of preparing a glass substrate for a matrix multiplexed liquid crystal display cell comprising depositing a row-column array of substantially transparent picture element electrodes, depositing a series of row conductors adjacent respective rows of electrodes, the material of said row conductors being suitable for use in a MIM device, forming insulator material suitable for use in a MIM device over at least a plurality of regions of said row conductors associated with respective picture elements, and depositing a series of cross conductors over each of the regions to extend to their respective associated picture elements, the material of said cross conductors being suitable for use in a MIM device wherein said materials are deposited using thin film techniques.

10. A method as claimed in claim 9 in which the order of deposition of said materials is reversed.

11. A method as claimed in claim 9 -or 10 in which the cross conductors are formed integrally with their respective associated picture element electrodes.

12. A method as claimed in claim 9 in which said insulator material is formed by anodization of said row conductors.

13. A method as claimed in claim 9 in which said glass substrate is initially coated with an etch protectant of thermally oxidized, sputter deposited tantalum.

14. A method as claimed in claim 13 in which said row conductors are sputter deposited, photoetched lines of tantalum.

15. A method as claimed in claim 9 in which said substantially transparent material is photoetched, vapour-deposited nichrome.

16. A method as claimed in claim 9 in which said substantially transparent material is indium tin oxide.

17. A method as claimed in claim 9 further comprising thin film forming on a second glass substrate an array of substantially transparent electrode regions, and lead means selectively electrically coupling said regions, and sealing the glass substrate together with nematic liquid crystal material therebetween and the regions on said first substrate opposed to corresponding regions on said second substrate to define picture elements therebetween.

CLAIMS SUPPORTED BY THE SUPPPLEMENTARY DISCLOSURE

18. A matrix multiplexed display comprising a pair of plates, at least one of the plates being transparent, the plates having sealed therebetween a layer of liquid characterized by electric field dependent optical characteristics, the display consequently characterized by electric field dependent optical transmissivity, a first plurality of electrodes on an inside surface of one plate and a second plurality of electrodes on an inside surface of the other plate, the display having a plurality of picture elements each element defined by a pair of electrodes one from said first plurality and one from said second plurality, each pel having a series connected switch comprising a thin insulating film flanked by thin film metal layers, the insulating film characterized by a gross change in resistivity at a predetermined voltage thereacross, the switches formed on the inside surface of at least one of the plates, the switches and the picture elements each having a capacitance associated therewith, the capacitance of the switches being substantially less than the capacitance of the picture elements.

19. A matrix multiplexed display as claimed in claim 18 in which each switch is substantially smaller in area than the pel controlled thereby.

20. A matrix multiplexed display as claimed in claim 19 in which each switch is laterally offset from the pel controlled thereby.

21. A matrix multiplexed display as claimed in claim 19 in which each pel and its series connected switch are superimposed, one picture element electrode having a part extending therefrom to provide a relatively small contact area with the switch insulating film.

22. A matrix multiplexed display as claimed in claim 18 in which on the inside of at least one of the plates, a thin film capacitor is formed in parallel with each picture element.

23. A matrix multiplexed display as claimed in claim 18 in which the liquid is a liquid crystal.

24. A matrix multiplexed display as claimed in claim 23 in which the liquid crystal is a twisted nematic liquid crystal and the plates confining the liquid crystal are cross polarizing plates.

25. A matrix multiplexed display as claimed in claim 23 in which the liquid crystal is in combination with a pleochroic dye.

26. A matrix multiplexed display as claimed in claim 18 in which said liquid is an electophoretic display medium.

27. A matrix multiplexed display as claimed in claim 18 in which said gross change in resistivity of said insulating layer results from a tunnelling mechanism.

28. A matrix multiplexed display as claimed in claim 18 in which said gross change in resistivity of the insulating layer results from a trap depth modulation mechanism.

29. A matrix multiplexed display as claimed in claim 18 in which the gross change is resistivity of said insulating layer results from the combination of a tunnelling mechanism and a trap depth modulation mechanism.

30. In a method of fabricating a display, the steps of:-depositing onto a transparent plate a row-column array of thin film metal regions, each region constituting a first terminal of a switch;
anodizing said metal regions thereby forming a dielectric layer on said metal regions;
forming a thin film conducting region over said anodized regions, the conducting regions forming a second terminal of respective switches;
forming a corresponding array of transparent electrodes on the plate;
forming a plurality of thin film leads parallel to the rows;
forming thin film interconnects between one terminal of each switch and the electrode adjacent thereto;
forming thin film interconnects between the other terminal of each switch and the row leads whereby to interconnect the switches in rows;
forming a corresponding array of transparent electrodes on a second transparent plate;
forming a plurality of thin film leads parallel to the columns on the second plate;
forming thin film interconnects between the electrodes and the column leads to interconnect the electrodes on said second plate in columns; and sealing between the plates a liquid having electic field related optical characteristics.

31. In a method of fabricating a display the steps of:-depositing onto one transparent plate a row-column array of thin film metal regions, each region constituting first terminals of a pair of switches;
anodizing said metal region;
forming spaced conducting regions on each anodized region, the conducting regions forming second terminals of the pair of switches;
forming a corresponding array of transparent electrodes on the plate;
forming a plurality of thin film leads parallel to the rows, forming thin film interconnects between the second terminal of one switch of each pair and the electrode adjacent thereto;
forming thin film interconnects between the second terminal of the other switch of each pair a lead adjacent thereto whereby to interconnect the switches in rows;
forming a corresponding array of transparent electrodes on a second transparent plate;
forming a plurality of thin film leads parallel to the column;
forming thin film interconnects between the electrodes and the column leads to interconnect the electrodes in columns on the second plate; and sealing between the plates a liquid having electric field related optical characteristics.

32. A method as claimed in claim 30 or 31 in which at least some of the conducting regions electrically connected to the second terminal of each switch are deposited simultaneously as a single homogeneous layer.

33. A method as claimed in claim 30 in which at least some of the conducting regions electrically connected to the first terminal of each switch are deposited simultaneously as a single homogenous layer.