ACTIVE MATRIX ARRAY DEVICES HAVING FLEXIBLE SUBSTRATES

This invention relates to the manufacture of electronic array devices, such as for example active matrix display devices, on flexible substrates, such as plastics substrates.

The most common form of active matrix display is an active matrix liquid crystal display (AMLCD). AMLCD devices are usually made on large glass substrates that are typically 0.7 mm thick. Two plates are needed for a cell, so that completed displays are just over 1.4 mm thick. Mobile phone manufacturers, and some laptop computer manufacturers, require thinner and lighter displays, and completed cells can be thinned in an HF (hydrofluoric acid) solution, typically to about 0.8 mm thick. Mobile phone manufacturers ideally want the displays to be even thinner, but it has been found that cells below 0.8 mm thick made by this method are too fragile. The HF thinning technique is not attractive because it is a wasteful process that uses hazardous chemicals that are difficult to dispose of safely and economically. There is also some yield loss during the etching process due to pitting of the glass. The attractiveness of light, rugged and thin plastic AMLCDs as an alternative has long been recognised. Recently, interest in plastic displays has increased even further, and there has been much research recently into AMLCDs electrophoretic displays and organic light emitting diode (OLED) displays on plastic substrates. The manufacturing directly onto plastic substrates or other flexible substrates such as metal foils is also attractive from a processing point of view as reel-to-reel processing can be used. In addition, flexible displays can be realized by processing a display on a flexible substrate layer which is supported by a non-flexible carrier and then either releasing the layer or transferring the layer onto a further flexible substrate. In addition to the benefits of size and weight, the use of flexible substrates enables new product designs to be implemented. For example, a display can be rolled around and mounted on a curved surface, or even be unrolled for use and rolled up for storage. For example, a display may be provided as a pull-out accessory of a device, which can then be retracted after use. In order to enable display device to be rolled in this manner, it is known to provide areas of weakness of the substrate in appropriate directions, for example as disclosed in US 2002/0139981 A1. The invention relates particularly to display devices which are intended to be deformed after manufacture in this way, for example between deployment and storage positions, or to create a curved or conformal display. The deformation of this type of device inevitably causes localised changes in device characteristics, and this results in degradation of the quality of the output image. This quality degradation can result when viewing the display when it is not perfectly flat, or it can result even when viewing the display in a flat position, but resulting from a permanent change in characteristics.

According to the invention, there is provided an active matrix array device, comprising: a substrate carrying an array of device elements arranged in rows and columns, each device element including a thin film transistor; a plurality of row conductors and a plurality of column conductors, wherein each row conductor comprises an elongate row line and a plurality of extensions extending from the row line, each extension having a portion defining the gate conductor of the thin film transistor of a respective device element, wherein the device is adapted to be deformable at least about an axis parallel with the row lines, and wherein the portion of each gate extension comprises an elongate gate conductor which extends in a direction which is non-perpendicular to the row line direction. The device of the invention arranges the gate conductor of a thin film transistor of each pixel to extend in a direction which is not perpendicular to an axis about which the display is deformed. This has been found to reduce the variation in TFT characteristics resulting from deformation of the device. Furthermore, by deforming the device about an axis parallel to the rows, the variation in characteristics of the row conductors is kept to a minimum. The row resistance should be kept to a minimum, as signals provided to the rows are typically for operating all of the TFT gates of the row, and RC time constants can be critical. The substrate preferably comprises a plastic or metal substrate. The device is typically a display device, or any other device which uses a sample and hold circuit, for example an active matrix liquid crystal display device, which includes a further flexible substrate mounted to the substrate carrying the array of pixels with liquid crystal material disposed between the substrates. The TFTs can be fabricated by any of the known technologies, such as a-Si and LTPS (low temperature poly-Si), organic TFTs, CMOS crystalline Si, GaAs etc. The portion of each extension may extend from the row line at an angle of 20 degrees - 70 degrees to the row line direction. This range of angles of inclination has been found sufficient to reduce the effects of bending on the TFT characteristics, but also enables the pixel layout to be substantially unaltered. The substrate can comprise weakened regions to promote preferential deformation of the substrate about the axis parallel with the row lines. In one embodiment, each extension can comprise a first portion extending perpendicularly to the row line direction, and the portion defining the gate conductor extending from an end of the first portion in a direction parallel to the row line direction. This defines an "L" shaped extension to the row line. This enables the gate conductor to run parallel with the row line, without significantly increasing the area required for the TFT. Each pixel may further comprise a second thin film transistor, and the gate conductor of the second thin film transistor can be defined by a second extension extending perpendicularly from the row. The extension for the first thin film transistor defining the gate conductor of the first TFT can then run parallel to the row line direction. In this way, a double TFT pixel structure has gate conductors for the two TFTs which are orthogonal, and this enables the performance of the device to be less sensitive to deformation in any direction. The device can then be adapted to be deformable at least about two axes, one parallel with the row lines and one parallel with the column lines. Alternatively, the device may be deformable about any axis, with no preferential deformation in any particular direction.

An example of the invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows a known pixel circuit for an active matrix display; Figure 2 shows a active matrix display device; Figure 3 shows a known pixel layout for an active matrix display; Figure 4 shows a first pixel layout for an active matrix display of the invention; Figure 5 shows a second pixel layout for an active matrix display of the invention; and Figure 6 shows a third pixel layout for an active matrix display of the invention.

The invention provides a flexible active matrix display device, in which the TFT for each pixel has a gate conductor which extends in a direction which is non-perpendicular to the axis about which the device is to be deformed. This has been found to reduce the effect of the deformation on the operating characteristics of the TFTs. Figure 1 shows a conventional pixel configuration for an active matrix liquid crystal display. This is an example of a class of circuits known as sample and hold circuits, and this example will be used in the description below to illustrate the invention. The display is arranged as an array of pixels in rows and columns. Each row of pixels shares a common row conductor 10, and each column of pixels shares a common column conductor 12. Each pixel comprises a thin film transistor 14 and a liquid crystal cell 16 arranged in series between the column conductor 12 and a common electrode 18. The transistor 14 is switched on and off by a signal provided on the row conductor 10. The row conductor 10 is thus connected to the gate 14a of each transistor 14 of the associated row of pixels. Each pixel additionally comprises a storage capacitor 20 which is connected at one end 22 to the next row electrode, to the preceding row electrode, or to a separate capacitor electrode. This capacitor 20 helps to store a drive voltage so that a signal is maintained across the liquid crystal cell 16 even after the transistor 14 has been turned off. In order to drive the liquid crystal cell 16 to a desired voltage to obtain a required gray level, an appropriate signal is provided on the column conductor 12 in synchronism with a row address pulse on the row conductor 10. This row address pulse turns on the thin film transistor 14, thereby allowing the column conductor 12 to charge the liquid crystal cell 16 to the desired voltage, and also to charge the storage capacitor 20 to the same voltage. At the end of the row address pulse, the transistor 14 is turned off, and the storage capacitor 20 helps to maintain a voltage across the cell 16 when other rows are being addressed. The storage capacitor 20 reduces the effect of liquid crystal leakage and reduces the percentage variation in the pixel capacitance caused by the voltage dependency of the liquid crystal cell capacitance. The rows are addressed sequentially so that all rows are addressed in one frame period, and refreshed in subsequent frame periods. As shown in Figure 2, the row address signals are provided by row driver circuitry 30, and the pixel drive signals are provided by column address circuitry 32, to the array 34 of display pixels. There is increasing interest in flexible displays, in which pixel circuits such as that shown in Figure 1 are provided on a flexible, polymer, substrate. This opens up a new range of possible applications for display devices, but also enables different processing techniques to be employed. Figure 2 shows schematically within the display area lines of weakness 35 which form part of the substrate and which promote deformation about their axes, so that the display can be curved or rolled about an axis parallel to the row direction. Deforming a fabricated display does, however, affect the pixel circuit performance. Figure 3 shows schematically the geometric layout of the known pixel circuit. The row conductor 10 has a perpendicular extension 11 , which defines the gate conductor for the TFT 14. For a bottom gate structure, a gate dielectric layer overlies the row metal, and the semiconductor body of the transistor is provided as an island over the gate dielectric layer. A higher metal layer defines the column conductor 12 and the source electrode (these are connected together) as well as the drain electrode. The pixel electrode 40 is connected to the drain, and in a reflective display it may be formed from the same metal layer, or it may be formed from a separate transparent layer such as ITO in a transmissive display. The invention is based on the recognition that different deformation directions have different influences on the pixel circuit characteristics. In particular, the deformation of the gate conductor and of the row conductor has most influence on the pixel circuit characteristics. The deformation of the gate conductor influences the TFT operating characteristics and in particular its leakage current, and the row resistance is particularly critical as the RC time constant of the row conductors influences the ability for all pixels in the row to be addressed within the available time. In the example of Figure 3, the display is to be deformed around an axis parallel to the row, as schematically shown by arrow 42. This means the display is curved from top to bottom. This deformation gives rise to mechanical stress particularly in deposited conductors which extend perpendicularly to the row direction, namely the gate conductor 11 and the column conductor 12. A change in resistance to the column conductor is less critical than a change in resistance for a row conductor, as the column conductor carries a signal only for a single pixel at a time. To reduce the sensitivity of circuitry on flexible substrates to mechanical stress, the invention provides a layout design for the gates of the transistors so that they lie in a direction, with respect to the rolling direction of the substrate, where they experience the reduced stress. The rolling direction is also chosen to reduce the effect of the deformation on selected conductor lines. Figure 4 shows a first embodiment of the invention. The device is again arranged to be deformed about the axis of the row conductors, in the rolling direction 42. As mentioned above, this orientation means that the deformation has the lowest effect on the row line resistance, which is desirable to keep the row line time constant as low as possible. The gate electrode 11 of the TFT is arranged at an angle to the perpendicular. This arrangement gives reduced stress generated by the gate metal on the semiconducting layers, so that variations to the TFT characteristics resulting from bending are reduced. The row conductor 10 thus comprises an elongate row line 10a and an extension 10b which defines the gate conductor 11 of the thin film transistor 14. The extension 10b extends in a direction which is non-perpendicular to the direction of the row line 10a. The extension 10b extends from the row line at an angle of 20 degrees - 70 degrees to the row line direction, and more preferably 30 - 60 degrees. These ranges of angles are sufficient to reduce the effects of bending on the TFT characteristics, but also enable the pixel layout to be substantially unaltered. Figure 5 shows a second embodiment, in which the extension forming the gate conductor is deposited in the form of an "L" shape. Thus, the extension 10b comprises a first portion 50 extending perpendicularly to the row line direction, and a portion 52 defining the gate conductor, which extends from the end of the first portion 50 in a direction parallel to the direction of the row line 10a. In this arrangement, the gate conductor extends in parallel to the row direction, to minimise the stress generated by the gate metal on the semiconducting layers. The above embodiments reduce the stress in both the TFTs and the row metal lines for displays which will be rollable in one direction. A further embodiment of the invention enables roiling or flexing of the display in any arbitrary direction (including rolling around an axis parallel with the row conductors). Figure 6 shows an embodiment in which a double TFT configuration is provided. A first TFT 14a is arranged in the same manner as shown in Figure 5 with the gate conductor extending parallel with the row. A second TFT 14b is arranged in conventional manner as shown in Figure 3, with the gate conductor extending perpendicular to the row direction. The two TFTs 14a, 14b have gate conductors orientated in mutually orthogonal directions, so that when the stress on one TFT is highest, that of the other TFT is lowest. In this way, at least one of the TFTs retains a low leakage current, so that the required sample and hold circuit operation will be implemented correctly irrespective of the direction in which the display is rolled/flexed. The display of Figure 6 can be designed to be rolled up in either the row or column directions, or else any direction of rolling may be allowed. When only one particular direction of rolling is permitted, other conductor lines may be selected to be arranged in parallel with the axis about which rolling takes place. For example, different pixel layouts enable the power lines to be in the row or column directions, and the particular pixel design can be selected so that as many of the conductor lines extend parallel with the rolling axis, namely in the row direction in the examples given above. Furthermore, there are more complicated pixel layouts with additional control lines, and these control lines may also then be designed to run in the same direction. The invention has been described above in connection with a layout for a standard pixel circuit for an active matrix LCD, but could also advantageously be applied to active matrix displays such as electrophoretic displays, electrowetting displays, electrochromic displays and MEMS based displays such as moving or rolling foil based displays, which make use of sample and hold circuits. However, a similar sample and hold circuit may form the basis of a data driver output stage, or the memory portion of an active matrix polymer LED display pixel circuit. In these circuits, it is particularly important that the leakage current of the TFT remains as low as possible. Leakage currents are known to increase when TFTs of known technology types are mechanically stressed. The detailed processes involved in the manufacture of a liquid crystal display or in the manufacture of other active matrix devices have not been described in detail. Furthermore, the different possible ways to make the substrate deform in a given manner have not been described in detail. The invention provides a modification to the layout of conductors on the substrate with reference to the deformation direction, and does not change the known processing steps used, or indeed the known drive schemes. These known processes and methods are not therefore described in detail. Various modifications will be apparent to those skilled in the art.