GB2213987A - MIM devices and liquid crystal display devices incorporating such devices - Google Patents

MIM devices and liquid crystal display devices incorporating such devices Download PDF

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Publication number
GB2213987A
GB2213987A GB8729517A GB8729517A GB2213987A GB 2213987 A GB2213987 A GB 2213987A GB 8729517 A GB8729517 A GB 8729517A GB 8729517 A GB8729517 A GB 8729517A GB 2213987 A GB2213987 A GB 2213987A
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mim
devices
liquid crystal
characterised
silicon oxy
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GB8729517D0 (en )
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Jeremy Noel Sandoe
Ian Douglas French
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Philips Electronics UK Ltd
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Philips Electronics UK Ltd
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L45/00Solid state devices adapted for rectifying, amplifying, oscillating or switching without a potential-jump barrier or surface barrier, e.g. dielectric triodes; Ovshinsky-effect devices; Processes or apparatus peculiar to the manufacture or treatment thereof or of parts thereof
    • GPHYSICS
    • G02OPTICS
    • G02FDEVICES OR ARRANGEMENTS, THE OPTICAL OPERATION OF WHICH IS MODIFIED BY CHANGING THE OPTICAL PROPERTIES OF THE MEDIUM OF THE DEVICES OR ARRANGEMENTS FOR THE CONTROL OF THE INTENSITY, COLOUR, PHASE, POLARISATION OR DIRECTION OF LIGHT, e.g. SWITCHING, GATING, MODULATING OR DEMODULATING; TECHNIQUES OR PROCEDURES FOR THE OPERATION THEREOF; FREQUENCY-CHANGING; NON-LINEAR OPTICS; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating, or modulating; Non-linear optics
    • G02F1/01Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating, or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour
    • G02F1/13Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating, or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/136Liquid crystal cells structurally associated with a semi-conducting layer or substrate, e.g. cells forming part of an integrated circuit
    • G02F1/1362Active matrix addressed cells
    • G02F1/1365Active matrix addressed cells in which the switching element is a two-electrode device

Abstract

A MIM type device (24) suitable for use as a switching element for controlling an associated picture element (10) in an active matrix addressed display device having an array of liquid crystal picture elements (10) comprises two conductive layers (30, 34) and an intermediate layer of silicon oxy-nitride material (32) with a controlled funneling defect density leading to a non-linear resistive characteristic of the device in operation. The silicon oxy-nitride material is deposited using a plasma deposition process. <IMAGE>

Description

MIM DEVICES AND LIQUID CRYSTAL DISPLAY DEVICES INCORPORATING SUCH DEVICES This invention relates to a MIM (Metal-Insutator-Met#device comprising on a substrate a thin film insulative layer sandwiched between two conductive layers across which, in use, a voltage is applied, the device exhibiting a non-linear resistive characteristic in operation. The invention relates also to a liquid crystal display device incorporating MIM devices.

MIM devices, which can be regarded as a type of diode structure, have been used in active matrix addressed liquid crystal display devices as switching elements for controlling operation of the device's picture elements. These two terminal, non-linear devices offer the advantage over TFTs also used for such purposes that they are comparatively simple to fabricate.

The display device, which may be used for video, e.g. TV, display purposes, consists of a pair of transparent substrates, for example of glass, with liquid crystal material therebetween. One of the substrates carries a set of parallel address conductors constituting row conductors while the other substrate carries another set of parallel address conductors constituting column conductors which cross the one set of conductors substantially at right angles with individual picture elements being provided at the region of the intersections of the crossing row and column conductors.Each individual picture element comprises a picture element electrode carried on the one substrate, an opposing portion of one of the column conductors on the other substrate together with the liquid crystal material therebetween and is connected electrically in series with at least one MIM device between a respective row conductor and column conductor with the at least one MIM device being carried on the one substrate adjacent to its picture element electrode and connected between that electrode and the associated row conductor.

The MIM devices act as generally symmetrical switches to control operation of their associated picture elements. By virtue of their non-linear resistance behaviour, the devices in effect turn on in response to a sufficiently high applied field to allow video data signal voltages to be transferred to the picture elements to cause the desired display response. The switching behaviour of the MIM device results from a tunnelling effect in the thin film insulative layer and in this respect the voltage/resistance characteristic of the device is dependent on the nature and thickness of the insulative layer. The predominant mechanism in this behaviour appears to be the Poole Frenkel effect when considering insulative layer thicknesses in the region of a few tens of nanometres.Devices using such thicknesses of insulative layer have been found to offer more satisfactory performance for liquid crystal display device applications through their ability to provide the necessary on/off ratio in use at acceptable voltages.

On possible method of addressing the display device is accomplished by applying scanning voltage signals to the row voltage signals to the column conductors. The matrix array of picture elements are addressed on a row at a time basis to build up a display picture over one field.

In a known type of MIM structure, the insulative layer is formed as an anodised oxide surface layer on a metal layer constituting one of the conductive layers. As a typical example, one of the conductive layers may consist of tantalum which is anodised to form a thin film of insulative tantalum pentoxide on the surface and then covered by a conductive layer of nickel, chromium, tantalum, aluminium or other metal. The anodic oxidation of the metal layer is a reasonably convenient process and the thickness of the oxide layer obtained can be controlled by the applied voltage for oxidation.

Examples of such MIM devices, and their use in active matrix addressed liquid crystal display devices, are described in British Patent Specification No. 2091468A to which reference is invited for further details.

Although generally referred to as a Metal-Insulator-Metal device, conductive materials such as indium tin oxide (ITO) can be used as one or both of the "metal" layers.

In another known type of MIM structure used as an active element in a liquid crystal display device, the insulative layer consists of off-stoichiometric silicon nitride. The insulative layer, which is approximately 100 nanometres in thickness is deposited as a strip which extends over an edge portion of a photo-defined ITO film on a glass substrate serving as the picture element electrode. The other conductive layer of the structure consists of chromium which is sputtered over the insulative layer strip and photo-etched to define one of the set of address conductors with extensions overlying the edge portion of the ITO electrode and separated therefrom by the underlying silicon nitride layer.

This type of structure, which is also referred to as an MSI (Metal-Semi-Insulator) device, and a liquid crystal display device incorporating such structures are described in greater detail in the paper entitled "A New Active Diode Matrix LCD Using Off-Stoichiometric SiNx Layer" by M. Suzuki et al at pages 13 to 16 in "Proceedings of ITEJ", Eib 86-19, to which reference is invited for further information.

It is important to the successful operation of MIM devices that they have good insulation properties under low applied field conditions so as to provide a high resistance and that they become conductive at higher applied fields in a controlled manner to achieve characteristics similar to a forward biased diode. They need to have, therefore, appropriate non-linear characteristics suited to the operational criteria existing in a liquid crystal display device. These characteristics are dependent on the thickness of the insulative layer, as mentioned earlier, and are determined by the charge transfer mechanisms involved.

Conductivity at high applied fields is a result of defects in the insulative layer material. In the two above described types of structures, these defects result respectively from the inclusion of impurities in the anodised metal layer and from structural defects in the off-stoichiometric silicon nitride.

These known types of MIM device exhibit generally similar voltage/current relationships. It has been found that their characteristic voltage/log current curve when plotted graphically has a steep non-linear property at low voltages but tends to flatten off at higher voltages also likely to be experienced in operation when used in liquid crystal display devices. This phenomenon is considered to be an undesirable feature so far as their use in liquid crystal display devices is concerned and can lead to unsatisfactory performance.

It is an object of the present invention to provide a MIM structure suitable for use as an active element in an active matrix addressed liquid crystal display device which offers an improved performance.

According to a first aspect of the present invention, a MIM device comprising on a substrate a thin film insulative layer sandwiched between two conductive layers across which, in use, a voltage is applied, the device exhibiting a non-linear resistive characteristic in operation, is characterised in that the thin film insulative layer comprises silicon oxy-nitride.

Preferably the silicon oxy-nitride layer is plasma deposited.

The inclusion of oxygen in plasma deposited silicon nitride results in the creation of a controlled tunnelling defect density giving the MIM good electrical performance. The plasma deposition technique enables easy control of layer parameters such as thickness and produces a dielectric layer having a high degree of uniformity of the material's structure allowing electrical characteristics to be determined readily and reliably in a reproducible manner. In the case of a large number of devices being fabricated simultaneously over a large area, as is required in a matrix display device, substantially identical and unvarying operational performance of the devices is obtained. A high homogeneity of tunnelling sites can be obtained with the density of the sites being controlled and tailored to meet the desired electrical criteria in a convenient fashion.Moreover, this process leads to a highly conformal layer, the layer having a uniform thickness regardless of the structure of the surface onto which it is deposited.

In addition, plasma deposition is a comparatively low temperature process (typically less than 300 degrees celcius) and therefore causes no difficulties in fabricating the layer so far as the substrate and any prior deposited layers on the substrate are concerned.

Moreover, the silicon oxy-nitride material has a low dielectric constant, in the region of 6, compared with, for example, tantalum pentoxide which has a dielectric constant around 25, and therefore the MIM device exhibits a much lower capacitance effect in use than a MIM device using tantalum pentoxide. This is significant in the use of the device in an active element in a liquid crystal display device where it is necessary for optimum operation that the parasitic capacitance of the device is considerably less than the capacitance of the associated liquid crystal picture element.

It has been found that MIM devices in accordance with the invention have an adequately high resistance at low applied fields and show a substantially exponential relationship between flow current and applied voltage, almost symmetrically with respect to zero applied voltage, at higher fields rendering it ideal for display device applications. More particularly, when considering graphically voltage plotted against log current, an almost straight line is obtained over a significant voltage range with no tendency of flattening out at the higher voltages being shown. Current densities of more than one amp per square cm have been obtained under both DC and the kind of pulsed conditions required in an LCD for TV display purposes.Because of the generally symmetrical voltage/current characteristics of the device, the display device can readily be driven with the polarity of applied voltages reversed every field to avoid a net DC bias on the liquid crystal.

The conductive layers of the device may be of a metal such as, for example, chromium, tungsten, tantalum, nichrome, or titanium.

Tin oxide or indium tin oxide could also be used.

The thickness of the silicon oxy-nitride layer may be between 20 and 150 nanometres (200-1500 Angstroms) and preferably is between 40 and 80 nanometres (400-800 Angstroms). MIM devices having insulative layer thicknesses in this range have been found to offer a performance capability suitable particularly for the kind of driving conditions experienced in display devices used for TV display purposes. However, for other display purposes parameters such as this layer thickness could be varied to meet the particular drive conditions existing.

The thickness of each of the conductive layers is not critical but may typically be between 100 and 300 nanometres (1000-3000 Angstroms) in the case, for example, of chromium. These conductive layers may be deposited using a suitable process, for example, by a sputtering or evaporation technique.

According to another aspect of the present invention, there is provided an active matrix addressed liquid crystal display device having a matrix array of picture elements comprising opposing electrodes carried on facing surfaces of two spaced substrates with liquid crystal material therebetween, each picture element being connected to one of a plurality of address conductors on one substrate via a switching element, characterised in that the switching element comprises a MIM device in accordance with the first aspect of the present invention.

As with liquid crystal display devices incorporating known MIM structures as switching elements, the matrix array of MIM devices in the liquid crystal display device of the present invention are formed simultaneously on the one substrate using large scale photo-etching techniques to define areas of the various layers.

MIM devices and an active matrix addressed liquid crystal display device incorporating a plurality of such MIM devices as switching elements in accordance with the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic circuit diagram of part of the liquid crystal display device showing a few picture elements each connected in series with a respective non-linear switching element in the form of a MIM device between row and column address conductors; Figure 2 is a diagrammatic cross-section through a portion of the display device; and Figure 3 illustrates graphically the electrical performance of a typical one of the MIM devices of the display device, and more particularly the relationship between applied voltage, V, and the current density, J, in the device.

Referring to the figures, the liquid crystal display device has a row and column matrix array of individual picture elements 10, only a few of which are shown for simplicity. In practice there may be 100,000 or more elements.

Each element is defined by a pair of electrodes carried on the facing surfaces of two, spaced, glass substrates 12 and 14 with TN liquid crystal material 16 therebetween.

More particularly, the substrate 14 carries an array of individual, generally rectangular, picture element electrodes of transparent ITO, 18, arranged in rows and columns and defining the area of each picture element 10.

The substrate 12 carries a set of spaced, parallel, column address conductors 20 portions of which, where they overlie picture element electrodes 18, constitute the other electrodes of the elements.

The picture element electrodes 18 of all picture elements in the same row are connected to one of a set of parallel row address conductors 22 (Figure 1), extending at right angles to the column conductors 20, via associated series-connected two terminal non-linear switching elements in the form of MIM devices 24.

Although only one MIM device is shown for each picture element, two or more MIM devices could be associated with each picture element in known manner.

The individual picture elements 10 are addressed in conventional fashion using scanning signals applied to each row conductor 22 in turn and video data signals applied appropriately, in synchronism to the column conductors 20 to modulate light transmission through the picture elements in accordance with video information. The elements are energised on a row at a time basis so as to build up a display picture, e.g. a TV picture over one field.

The display device and its operation are similar to known display devices using MIM type non-linear switching elements.

Accordingly, the description of the device deliberately has been kept brief. For further information, reference is invited to the publications mentioned earlier whose disclosures in this respect are incorporated herein by reference.

The MIM devices comprise two conductive layers abutting an intermediate layer of insulative material and exhibit a switching characteristic by virtue of their non-linear current/voltage behaviour given by the intermediate insulative layer whereby they have a high resistance at low applied voltages and at higher applied voltages the device's resistance changes and drops significantly to allow adequate current to flow therethrough for the purpose of driving the picture elements.

The display device differs from known display devices, however, in the nature of the MIM devices employed. In accordance with the present invention, each MIM device comprises on the substrate 14, two conductive layers between which a dielectric thin film of silicon oxy-nitride is sandwiched. This material has a high defect density resulting from the inclusion of oxygen which creates a corresponding high density of tunnelling sites for quantum mechanical tunnelling effects giving the device its non-linear characteristic.

Referring particularly to Figure 2, each MIM device 24 comprises a lower conductive layer 30 of chromium arranged laterally of the subsequently defined ITO picture element electrode 18 with the electrode 18 extending over an edge portion of the layer 30 to provide electrical contact therewith. The layer 30 is deposited and defined into pads using any suitable techniques, for example sputtering and photo-etching.

A continuous passivation layer of, for example, silicon dioxide or silicon nitride, may be provided on the surface of the substrate 14 prior to deposition of the chromium pads 30 and the ITO electrodes 18.

The insulative layer of the MIM devices, here referenced at 32, is formed by depositing a layer of silicon oxy-nitride over the surface of the substrate with the chromium pads 30 using a plasma enhanced low pressure chemical vapour deposition process, and then defining the deposited layer into the required pattern by a photo-etching technique.

Plasma enhanced low pressure chemical vapour deposition processes are generally well known and have been well documented.

This technique has been used also for depositing silicon oxy-nitride material, although in contexts different to that contemplated herein, having been employed to produce layers of this material for use in the other fields. One example of such a process, and the equipment for carrying out this process, is given in European Patent Specification No. 0032024, details of which are incorporated herein by reference.

It will be appreciated, however, that for the purpose of providing the insulative layers of the MIM devices the process is controlled to form silicon oxy-nitride whose composition is such as to give the required non-linear resistive characteristics when used in the form of a thin film.

Briefly, the plasma deposition involves placing the substrate 14 with the pads 30 thereon in a reaction chamber where it is exposed to a plasma, or glow discharge, of silane, (SiH4) ammonia (NH3) and nitrous oxide (N2O) at a temperature around 300 degrees Celcius and a pressure of approximately 1 Torr resulting in the deposition on the substrate of silicon oxy-nitride. The silane, ammonia and nitrous oxide components are introduced into the reaction chamber in proportions suitable to produce a silicon-rich oxy-nitride material having the desired non-linear electrical characteristics.

The silicon oxy-nitride is deposited to a thickness of approximately 40 nanometres (400 Angstroms). Thereafter it is photo-etched into a pattern of strips extending columnwise portions of which strips overlying the pads 20 constitute the layers 32.

The second conductive layer, referenced 34, of the MIM device is then deposited as a layer of chromium which is subsequently defined by photo-etching to form strips overlying the silicon oxy-nitride strips. Besides serving as the second conductive layer of the MIM devices where they overlie the pads 30, the defined chromium strips 34 act also as the row address conductors 22.

The MIM devices may each be around approximately 10 mi crometres square.

A layer of ITO is then deposited and defined to form the individual picture element electrodes 18, the electrodes extending over an edge portion of their respective pads 30. The ITO electrodes could, however, be deposited prior to the chromium strips 34.

The exposed surface of the structure is then coated with a liquid crystal orientation layer 36 in known manner.

The column conductors 20 and an orientation layer 38 are fabricated on the substrate 12 conventionally.

The materials used for the two conductive layers of the MIM devices can differ from the particular example described above.

For example, tungsten, tantalum or nichrome may be used. The lower conductive layer 30 may be formed of ITO with a surface coating of chromium for protection. The upper conductive layer (and hence row conductors) may be of ITO.

The MIM devices using silicon oxy-nitride as the insulative layer formed in the above described manner exhibit desirable non-linear characteristics in-operation of the display device having an almost exponential relationship between current and applied voltage and being capable of operating at high level multiplexing with the required duty ratio for TV display purposes.

Because the silicon oxy-nitride has a low dielectric constant, their parasitic capacitance is very low compared with the capacitance of their associated picture element 10.

Figure 3 shows graphically the electrical characteristics of the MIM device where applied voltage, V, over the range of voltages occurring in use of the MIM device in the display device is plotted against the log of the current density (J) in amps per square cm through the device in an embodiment of the device in which the silicon oxy-nitride layer has a thickness of 40 nanometres and in which continuous, rather than pulsed, DC voltage is used.

The described MIM devices are suitable for the kind of driving conditions experienced for TV display purposes. By varying the flow rates of the components used in the plasma deposition process for the silicon oxy-nitride, and the thickness of the layer 32 obtained, the operational characteristics of the MIM devices may be altered to meet differences in performance requirements for different display purposes, for example to change the non-linearity curve to achieve desired contrast ratios and possibly more limited grey scale capability in operation.

Claims (8)

CLAIM(S)
1. A MIM device comprising on a substrate a thin film insulative layer sandwiched between two conductive layers across which, in use, a voltage is applied, the device exhibiting a non-linear resistive characteristic in operation, characterised in that the thin film insulative layer comprises silicon oxy-nitride.
2. A MIM device according to Claim 1, characterised in that the silicon oxy-nitride layer is deposited by a plasma deposition process.
3. A MIM device according to Claim 1 or Claim 2, characterised in that the silicon oxy-nitride layer has a thickness of between 20 and 150 nanometres.
4. A MIM device according to Claim 3, characterised in that the thickness of the silicon oxy-nitride layer is between 40 and 80 nanometres.
5. A MIM device according to any one of the preceding claims, characterised in that the conductive layers comprise chromium, tungsten, tantalum, nichrome, titanium, tin oxide or indium tin oxide.
6. A MIM device substantially as hereinbefore described with reference to, and as shown in, Figures 2 and 3 of the accompanying drawings.
7. A n active matrix addressed liqud crystal display device having a matrix array of picture elements comprising opposing electrodes carried on facing surfaces of two spaced substrates with liquid crystal material therebetween, each picture element being connected to one of a plurality of address conductors on one substrate via a switching element, characterised in that the switching element comprises a MIM device in accordance with any one of Claims 1 to 6.
8. An active matrix addressed liquid crystal display device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
GB8729517A 1987-12-18 1987-12-18 Mim devices & liquid crystal display devices inc such devices Withdrawn GB8729517D0 (en)

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GB8729517A GB8729517D0 (en) 1987-12-18 1987-12-18 Mim devices & liquid crystal display devices inc such devices

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GB8729517A GB8729517D0 (en) 1987-12-18 1987-12-18 Mim devices & liquid crystal display devices inc such devices

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GB2213987A true true GB2213987A (en) 1989-08-23

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0417851A2 (en) * 1989-09-15 1991-03-20 Philips Electronics Uk Limited Two-terminal non-linear devices and their method of fabrication
EP0430702A2 (en) * 1989-11-30 1991-06-05 Kabushiki Kaisha Toshiba Line material, electronic device using the line material and liquid crystal display
GB2244858A (en) * 1990-06-04 1991-12-11 Philips Electronic Associated MIM type devices for displays
US5466617A (en) * 1992-03-20 1995-11-14 U.S. Philips Corporation Manufacturing electronic devices comprising TFTs and MIMs

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4217601A (en) * 1979-02-15 1980-08-12 International Business Machines Corporation Non-volatile memory devices fabricated from graded or stepped energy band gap insulator MIM or MIS structure

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4217601A (en) * 1979-02-15 1980-08-12 International Business Machines Corporation Non-volatile memory devices fabricated from graded or stepped energy band gap insulator MIM or MIS structure

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0417851A2 (en) * 1989-09-15 1991-03-20 Philips Electronics Uk Limited Two-terminal non-linear devices and their method of fabrication
EP0417851A3 (en) * 1989-09-15 1991-09-25 Philips Electronic And Associated Industries Limited Two-terminal non-linear devices and their method of fabrication
EP0430702A2 (en) * 1989-11-30 1991-06-05 Kabushiki Kaisha Toshiba Line material, electronic device using the line material and liquid crystal display
EP0430702A3 (en) * 1989-11-30 1993-09-01 Kabushiki Kaisha Toshiba Line material, electronic device using the line material and liquid crystal display
US5428250A (en) * 1989-11-30 1995-06-27 Kabushiki Kaisha Toshiba Line material, electronic device using the line material and liquid crystal display
GB2244858A (en) * 1990-06-04 1991-12-11 Philips Electronic Associated MIM type devices for displays
US5466617A (en) * 1992-03-20 1995-11-14 U.S. Philips Corporation Manufacturing electronic devices comprising TFTs and MIMs

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