GB2244859A - MIM devices and active matrix displays incorporating such devices - Google Patents

Abstract

A MIM, two terminal non-linear, device (10) comprising two, spaced, conductive layers (12, 18) and an insulator layer extending therebetween further includes a layer of resistive material situated between the conductive layers for limiting current at high applied voltages and protecting the device from the effects of static charges. The device may comprise two insulator layers (14, 16) adjacent respective conductive layers (12, 18) with the resistive layer (15) extending between these insulator layers, or a single insulator layer (17) separated from the conductive layers (12, 18) by respective resistive layers (19, 20). Such devices can behave similarly to conventional devices allowing substantially symmetrical operation. An array of devices fabricated on a common substrate is used in an active matrix-addressed liquid crystal display device, the devices serving as switching elements connected in series between display elements and their associated address conductors. <IMAGE>

Description

DESCRIPTION MIM TYPE DEVICES, THEIR METHOD OF FABRICATION AND DISPLAY DEVICES INCORPORATING SUCH DEVICES This invention relates to MIM (Metal-Insulator-Metal) type devices and their fabrication. The invention relates also to display devices incorporating MIM type devices.

MIM devices generally comprise on a substrate a thin film insulating layer sandwiched between two conductive layers across which, in use, a voltage is applied, the device exhibiting a non-linear resistive characteristic in operation. Such devices can be regarded as a type of diode structure. They 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 advantages over TFTs also used for such purposes in that they are comparatively simple to fabricate and require fewer address lines, with no cross-overs, on their supporting substrate.

A typical MIM addressed display device consists of first and second glass substrates carrying respectively a set of row address conductors and a set of column address conductors with individual picture elements being provided at the region of the intersections of the crossing row and column conductors and comprising a picture element electrode on the first substrate, an opposing portion of one of the column conductors on the second substrate, together with the liquid crystal material therebetween, and is connected electrically in series with at least one MIM device between respective row and column conductors with the at least one MIM device also being carried on the first substrate adjacent to, and connected between, its respective electrode and row conductor.

The MIM devices act as bidirectional switches to control operation of their associated picture elements. By virtue of their non-linear resistance behaviour, the devices exhibit threshold characteristics and in operation are turned 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.

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 and the acronym should be construed accordingly. Moreover, the terms "insulator" and "insulating layer" as used herein are intended to be construed in the wider sense to include semi-insulators and non-stoichiometric materials known in the field of MIM devices.

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. 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 composition and thickness of the insulative layer and are determined by the charge transfer mechanisms involved. The switching behaviour of many MIM devices results from tunnelling or hopping of carriers in the thin film insulating layer and in this respect the voltage/resistance characteristic of the device is dependent on the magnitude of the electric field and thus the nature and thickness of the insulating layer.In some forms of MIM type devices the mechanism is controlled by the barrier between the metal and the (semi-) insulator. The thickness of the insulating layer may typically range between 50 and 10on, depending on the kind of material used and the mechanism involved.

In known types of MIM structures used in LC display devices, as described for example in US-A-4413883 and US-A-4683183, the insulating layer may comprise tantalum pentoxide formed by partly anodising a tantalum layer constituting one of the conductive layers. The insulative tantalum pentoxide is covered by a conductive layer of nickel, chromium, tantalum, aluminium or other metal. In alternative known types of MIM structures also described, materials such as silicon nitride, silicon dioxide, silicon oxynitride, silicon monoxide and zinc oxide are used for the insulator. Examples of display devices using MIM structures comprising non-stoichiometric insulator materials are described in EP-A-0182484.

MIM devices using insulative materials such as silicon oxynitride or silicon nitride are considered to offer superior characteristics in operation, particularly when used in the display device, by virtue of the lower dielectric constant of these materials compared with anodised tantalum for example.

While active matrix substrates of display devices employing MIM devices are generally simpler to construct than those using TFTs, it has been found that during the fabrication of display devices the MIM devices are susceptible to static discharge and can easily be damaged or destroyed by the effects of static electricity. Damage by static electricity is most likely to occur after the MIM devices have been fabricated on a substrate and during subsequent manufacturing or assembly processes in which the substrate is used. In the manufacture of a display device the substrate carrying the array of MIM devices is required to undergo various processing and assembly operations which can result in static charges being generated.It is known to provide in a finished display device means for protecting against static discharge during subsequent operation of the display device but such means do not provide protection during manufacture and assembly.

It is an object of the present invention to provide an improved MIM type device which is less susceptible to static discharge problems.

It is another object of the invention to provide such a MIM type device which is capable of being fabricated in arrays suitable for use in display devices.

It is a further object of the present invention to provide an active matrix addressed display device employing MIM type devices as switching elements which demonstrates higher immunity to the effects of static electricity.

According to one aspect of the present invention, a MIM type device comprising an insulator layer extending between first and second conductive layers is characterised in that the device further includes a layer of resistive material intermediate the first and second conductive layers.

By incorporating a resistive layer in the structure an effective series resistance is introduced which protects the device from static damage whilst allowing the device to behave substantially the same as a conventional MIM device in normal operation. Unlike conventional diode devices in which current tends to level off above a certain applied voltage, MIM type devices can allow very high current densities, if the normal range of operating voltages is exceeded and so, for example, static charges can lead to excessive currents causing damage.

The resistive layer serves as a current limiter at higher applied voltages, i.e. above the normal operating range. The resistive layer only has an appreciable effect when a high voltage is applied, that is the kind of voltage levels associated with static electricity. At comparatively low voltage levels around those normally used for driving MIM devices in active matrix addressed display devices and with which the MIM device is designed to operate, the resistive layer has practically no effect on the non-linear, switching, characteristic of the device.

The resistive layer may comprise amorphous silicon material. Such material can conveniently be deposited in a simple manner using commonly known techniques to the required thickness when forming the MIM type device structures. Moreover, this material can be selectively doped to tailor the resistivity of the layer for optimum performance. Alternatively, oxygen-doped polycrystalline silicon material or other suitable resistive materials, such as doped silicon carbide, could be employed for this layer.

In active matrix display devices, it is common to reverse the polarity of the applied voltages periodically. When used as a switching element in a display device, it is desirable therefore that the MIM type device exhibits a substantially symmetrical, non-linear, I-V characteristic for simplicity of driving. To this end, a number of different layer configurations may be used. In a first preferred embodiment, the insulator layer is provided adjacent the first conductive layer and followed in order by the resistive layer a further insulator layer and the second conductive layer. When compared with a conventional, three-layer, MIM device this embodiment can be regarded as having a resistive layer which divides, and separates, an insulator layer. For convenience, the two insulator layers are preferably formed of the same material.

Different insulator materials could be employed but this would likely affect the symmetry of the I-V characteristic.

In a second preferred embodiment the insulator layer provided between the two conductive layers is separated therefrom by respective layers of resistive material.

Other layer configurations may be used, although not necessarily giving symmetrical characteristics. For example a single insulator layer and a single resistive layer could be provided between the two conductive layers. Such a device would be expected to be generally unidirectional rather than bidirectional in nature and function in the manner of a standard diode.

The conductive and insulator layers may comprise materials commonly used in the art. For example, the first and second conductive layers may be of a metal such as chromium, tungsten, tantalum, nichrome, aluminium or titanium, or other conductive materials such as ITO or tin oxide. The insulator layers may be of various different materials such as silicon dioxide, silicon monoxide, silicon nitride, silicon carbide, non-stoichiometric, for example silicon-rich, mixtures of these materials, such as non-stoichiometric silicon nitride or silicon oxynitride, or tantalum oxy-nitride.

According to another aspect of the present invention there is provided an array of MIM type devices characterised in that the MIM type devices are in accordance with the first aspect of the invention and are carried on a common support. Such an array could be used as a component of a matrix display device with the MIM type devices being arranged in rows and the first and second conductive layers of the MIM type devices in a row respectively being connected electrically to a common address conductor and an associated one of a row of picture element electrodes also carried on the support.

According to a further aspect of the present invention, there is provided an active matrix addressed display device having a matrix array of picture elements comprising opposing electrodes carried on two spaced substrates with electro-optic display material, for example liquid crystal material, therebetween, each picture element being connected to one of a plurality of address conductors on one substrate via a switching element, which is characterised in that the switching elements comprises MIM type devices in accordance with the first aspect of the present invention. It is envisaged that electro-optic material other than liquid crystal material may be employed.

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

MIM type devices, their method of fabrication, and an active matrix addressed liquid crystal display device incorporating such 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: Figures 1 and 2 show schematic cross-sectional views through two embodiments of MIM type devices according to the present invention; Figures 3 and 4 show the equivalent circuits of the devices of Figures 1 and 2 respectively; Figure 5 is a schematic circuit diagram of part of a liquid crystal display device according to the invention showing a number of 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; and Figure 6 is a schematic cross-section through a portion of one form of display device.

It should be understood that the Figures are merely schematic and are not drawn to scale. In particular certain dimensions such as the thickness of layers or regions may have been exaggerated whilst other dimensions may have been reduced.

It should also be understood that the same reference numerals have been used throughout the Figures to indicate the same or similar parts.

Known MIM structures consist of first and second spaced conductive layers with an insulator layer sandwiched therebetween. The MIM type devices of Figures 1 and 2 similarly comprise first and second spaced conductive layers serving as device terminals and an intermediate insulator layer but in addition further include a layer of resistive material. The devices are intended to exhibit substantially symmetrical behaviour in operation and may comprise more than one resistive layer and the insulator layer may in effect be divided into separate layers in alternative configurations.The devices demonstrate in operation a switching characteristic by virtue of their non-linear current/voltage property given by their insulator layer(s) whereby they exhibit a high resistance at low applied voltage and at higher applied voltages the devices' resistance drops significantly to allow comparatively high current flow therethrough. Because of their symmetry the devices demonstrate substantially identical switching characteristics in response to reversed polarity voltages of corresponding magnitude being applied.

Referring to Figure 1, the MIM type device, generally referenced at 10, is formed on an insulative supporting substrate 11, for example of glass, and comprises superposed layers in parallel relationship consisting of, in order from the substrate, a first conductive layer 12 acting as one terminal, a first thin film insulator layer 14, a thin film layer of resistive material 15, a second thin film insulator layer 16 and a second conductive layer 18 acting as the second terminal. In this particular embodiment, the layers 12 and 18 are of metal such as tantalum or chromium, the layers 14 and 16 are of silicon nitride and the layer 15 is of hydrogenated amorphous silicon material, a-Si:H.

The equivalent circuit of the device of Figure 1 is illustrated in Figure 3. As will be seen, the device can be regarded as comprising two MIM elements connected in series via a resistance.

The device is fabricated by depositing sequentially the constituent layers in overlying relationship on the substrate 11. The metal layer 12 is formed by any suitable technique such as sputtering or evaporation and is photo-lithographically defined into a generally rectangular pad around 10 micrometres square with an integral extension (not visible) for connection purposes. The silicon nitride and a-Si layers 14, 15 and 16 are deposited using plasma enhanced low pressure chemical vapour deposition processes, although sputtering could perhaps be used.

Finally, the second metal layer is deposited by sputtering or evaporation. Unwanted portions of the layers 14, 15, 16 and 18 laterally of the pad 12 are selectively removed, the layer 18 being formed after defining of the underlying layers and provided at definition with an integral extension for connection purposes.

The thickness of the layers 12 and 18 is not critical but may typically be in the order of 100 and 300 nanometres (nm) respectively in the case of tantalum and chromium. The silicon nitride layers 14 and 16 are of similar thickness, around 30nm to 60nm. The insulator layers 14 and 16 may be regarded as equivalent to the single insulator layer in known devices but divided into two sub-layers spaced by the resistive layer. The composition and thickness of the insulator layers 14 and 16 are chosen, as with a single insulator layer device, to give the required non-linear resistance characteristic. In this embodiment the silicon nitride material of the layers 14 and 16 is silicon-rich, comprising approximately 75 per cent silicon, and is a semi-insulator. The thickness of the resistive layer 15 is selected primarily in dependence on its resistivity.In this example heavily doped a-Si material having a resistivity of around 1030hm cm is deposited to a thickness of approximately 10on.

The layers 14 and 16 may not be of identical thickness.

Because of the nature of deposition processes, interface effects between these layers and their adjacent metal layers 12 and 18 may result in slight differences in symmetry during operation and accordingly the layers 14 and 16 may be of slightly different thickness to counteract this.

Figure 2 shows another embodiment of MIM type device using a different configuration of layer structure. In this embodiment, a single silicon nitride insulator layer 17 (of similar composition to the layers 14 and 16) is situated between the first and second conductive layers 12 and 18 serving as respective terminals and separated therefrom by two resistive layers 19 and 20. The resistive layers are of approximately equal thickness, around 50nm, that is, approximately one half the thickness of the layer 15 in the previous embodiment and comprise doped, n-type, amorphous silicon material, (n a-Si:H), with a resistivity of approximately 103 ohms cm.The insulator layer 17 has a thickness of around 60nm to 120nm, that is, approximately equal to the combined thickness of the layers 14 and 16 in the previous embodiment. The layers are formed using processes similar to those used in the previous embodiment.

The equivalent circuit of this device is illustrated in Figure 4, and, as shown, can be regarded as a sing)# MIM element connected in series between two resistances.

In both the above-described embodiments, the resistivity of the material of the resistive layer(s) may be chosen to be different from the particular value specified but preferably is selected to be around 103 to 106 ohms cm.

In operation of the two above-described embodiments the resistive layer or layers limit current flow through the devices at high applied voltages to protect the devices from static damage by introducing an effective series resistance. The MIM type devices function similarly to conventional MIM devices in subsequent use, for example as switching elements in a display device. Providing the layer thicknesses are suitably chosen, it can be expected that the devices will act in much the same way, in terms of their electrical performance, as standard forms of MIM devices. The series resistance(s) given by the resistive layer(s) only have any appreciable effect when high voltages are present, as would be the case with static electricity.At normal voltage levels, that is voltages at which the devices are intended normally to be operated when used for example in a display device, typically between 5 and 20 volts, the devices behave substantially as if no resistive layer were present.

Although in the above embodiments particular materials have been described for the layers, it will be appreciated that other materials could be used. For example oxygen doped polycrystalline silicon or doped silicon carbide may be used for the resistive layer(s). Silicon dioxide, silicon oxy-nitride, tantalum pentoxide, zinc oxide, aluminium oxide and particularly non-stoichiometric mixtures of these materials may be used for the insulator layers. Furthermore, different metals or other conductive materials such as ITO, tungsten or nichrome can be used for the layer 12 or layer 18.

For use particularly in a display device, as will be described, large numbers of such MIM type devices are fabricated simultaneously on the substrate 11 from common layers using standard large scale deposition and photo-etching techniques. The devices are arranged in a row and column array and the devices of each row are connected to a respective common row conductor extending alongside the row of devices. This row conductor may be formed by deposition separately from the pads 12 of the devices and interconnected therewith, or, for simplicity, may be defined from the same deposited layer used to form the pads 12. The pads 12 could comprise portions of the row conductors.

A liquid crystal display device using such an array of MIM type devices will now be described with reference to Figures 5 and 6 which show respectively the circuit configuration of a part of the display device comprising a few, typical, picture elements and their associated MIM type devices, and a cross-section through a small portion of the device illustrating its construction.

The display device has a row and column matrix array of individual picture elements 22, only twelve of which are shown in Figure 5. In practice there may be more than 100,000 elements.

Each element 22 is defined by a pair of electrodes carried on the facing surfaces of two, spaced, glass substrates 11 and 23 with TN liquid crystal material 24 therebetween. The substrate 11 carries the array of MIM type devices 10. In addition, the substrate 11 carries an array of individual, generally rectangular, picture element electrodes 25 of transparent ITO arranged in rows and columns and defining individual picture elements 22.

The substrate 26 carries a set of spaced, parallel, column address conductors 27, portions of which, where they overlie picture element electrodes 25, constitute the other electrodes of the elements.

The picture element electrodes 25 of all picture elements in the same row are connected on the substrate 11 to one of a set of parallel row address conductors 28 (Figure 5), extending at right angles to the column conductors 27, via their associated, series-connected, MIM type devices 10. Although only one device is shown for each picture element, two or more devices could be used with each picture element in known manner.

The individual picture elements 22 are addressed in conventional fashion by applying scanning signals to each row conductor 28 in turn and video data signals appropriately, in synchronism, to the column conductors 27 (or vice versa) to modulate light transmission through the picture elements in accordance with video information. The elements are typically driven using an applied voltage of between 11 and 15 volts. They 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 foregoing description of the device deliberately has been kept brief. For further information, reference is invited to the specifications mentioned earlier whose disclosure in this respect is incorporated herein by reference.

Referring particularly to Figure 6, each MIM type device 10 is arranged laterally of its associated picture element. The pads 12 of the MIM devices 10 are formed as extensions projecting from, and integral with, the row conductors 28 by appropriate definition from a common layer. A layer of insulating material 30 such as silicon nitride is deposited completely over the devices 10, the row conductors 28, and the remaining exposed areas of the substrate 11. The ITO electrodes 25 are defined on this layer 30 and are formed with integral bridging strips 32 that extend over the associated MIM type device and contact the upper conductive layers 18 through windows etched in the layer 30, although in an alternative arrangement the upper conductive layers 18 may be constituted by portions of the strips 32 themselves.

The exposed surface of the structure is then coated with a liquid crystal orientation layer 36, for example of polyimide, in known manner. A similar orientation layer 38 is provided over the conductors 27 on the substrate 26.

It should be understood that the particular form of construction depicted in Figure 6 is provided by way of example only. Numerous alternative forms of construction, especially regarding the provision and interconnection of the MIM type devices, picture element electrodes and row conductors on the substrate 11, are possible, as will be apparent to persons skilled in the art. Moreover, a colour filter array may be provided on the substrate 26 for producing full colour displays.

Claims (11)

CLAIM(S)

1. A MIM type device comprising an insulator layer extending between first and second conductive layers, characterised in that the device further includes a layer of resistive material intermediate the first and second conductive layers.

2. A MIM type device according to Claim 1, characterised in that the device comprises first and second insulator layers adjacent respectively the first and second conductive layers and a resistive layer separating the two insulator layers.

3. A MIM type device according to Claim 1, characterised in that the insulator layer between the first and second conductive layers is separated therefrom by respective layers of resistive material.

4. A MIM type device according to any one of the preceding claims, characterised in that the resistive layer(s) comprise amorphous silicon material.

5. A MIM type device according to any one of Claims 1 to 3, characterised in that the resistive layer(s) comprise polycrystalline silicon material.

6. An array of MIM type devices characterised in that the devices comprise devices according to any one of the preceding claims and are carried on a common substrate.

7. An active matrix addressed display device having a matrix array of picture elements comprising opposing electrodes carried on two, spaced, substrates with electro-optical display material therebetween, each picture element being connected to one of a plurality of address conductors on one substrate via an associated switching element, characterised in that the switching elements comprise MIM type devices according to any one of Claims 1 to 5.

8. An active matrix addressed display device according to Claim 7, characterised in that the electro-optical display material comprises liquid crystal material.

9. A MIM type device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

10. An array of MIM type devices carried on a common substrate substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

11. An active matrix addressed display device substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.