GB2050031A - Liquid Crystal Displays Controlled via Metal-insulator- metal Devices - Google Patents

Abstract

A liquid crystal display cell comprises two substrates 18, 22 outside a liquid crystal layer 22, with picture elements defined by pairs of opposed electrodes on the inside faces of the substrates. Voltage is applied across each element via a switch element 28 formed by a thin film metal-insulator-metal device (MIM) on the inside face of one of the plates. The symmetrical non-linear resistance of the switch elements allows more elements to be multiplexed. <IMAGE>

Description

SPECIFICATION Liquid Crystal Displays (LCDs) Controlled by Metal-insulator-metal (MIMs) Devices This invention relates to a 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 V8 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 V8 and Vd applied to the selected row and column respectively, is made great enough to alter the liquid crystal molecular orientation, and thus 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 experience Vd for N address periods. This may be enough to turn the pel on. It can be shown that the ratio of RMS voltage seen by an on pel to that seen by an off pel is:

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 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 4 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 irreversible electrochemical 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 trap depth modulation. In the former, carriers pass through the thin insulator by field enhanced quantum mechanical tunnelling. In the latter, carriers are reieased from traps in the insulator as the field developed between the metal layers diminishes 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.

This film MIM's may have insulators such as aluminium oxide, tantalum pentoxide, silicon nitride and silicon dioxide. The thickness of the dielectric layer determines the conduction process. Below 50-1 oo,A electron tunnelling is possible; from 100 to 1 000A trap depth modulation conduction processes dominate. The metal of the MIM may be any material which forms an ohmic, or weakly blocking, contact.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows in schematic form a matrix multiplexed addressing scheme for an LC display: 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 V8 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 1 2 called data lines. If an "on " pulse is desired at a LC pel 14 at a selected row and column intersection, the difference between V8 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.

As shown at top right in Figure 1, the invention proposes the forming of a thin film MIM device 16 in series with each LC pei 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 the 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 1 8 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 4650C for 1 6 hours to protect the glass from following etch steps. A second layer of tantalum is sputter deposited and photodefined into row conductors 10 which are from 2 to 25 mils wide and run the breadth of the plate. The conductors, which function as one side of the metal-insulatormetal (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 citric acid bath at 30-60V anodizing voltage to produce surface layers 28 of tantalum pentoxide which acts 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 respective electrode 24. The active area of the MlM's typically 1.0 mils2, 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 1 8 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) which enable pulses to be selectively applied to LC pels 14 by applying data and scanning pulses Vd and V8 to the appropriate row conductors 10 on plate 1 8 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 tranmitted 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 of only of a few hundred A, can also be used as combined electrode and metallization.

The particular thin film technique (sputtering, vacuum 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 anodized 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 MlM's. Leads 4Oa 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 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 tantalum and are formed simultaneously with regions 34.

The leads are protected from anodization by coating with photo-resist which is subsequently removed.

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 (10010001ines) of a matrix addressed array of liquid crystal display picture elements can be obtained without the prior 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 tranparent 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 1010 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 of10yA 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 techniques, again, because of cost and further because by using known techniques accurately planar glass surfaces can be achieved which ensure little variation in LC cell thickness.

Claims (20)

Claims

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 columns, 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 or 2, in which the MIM is a tantalumtantalum pentoxide-gold device.

7. A liquid crystal display cell as claimed in claim 1 or 2 in which the MIM is an aluminumaluminum 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, said 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 1 in which the order of deposition of said materials is reversed.

11. A method as claimed in claim 1 or 2 in which the cross conductors are formed integrally with their respective associated picture element electrodes.

12. A method as claimed in claim 1 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.

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

1 6. 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 Jiquid crystal material therebetween and the regions on said first substrate opposed to corresponding regions on said second substrate to define picture elements therebetween.

18. A liquid crystal display cell substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

1 9. A liquid crystal display cell substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

20. A method of preparing a liquid crystal display cell, substantially as hereinbefore described.