KR20070036081A - Active matrix array devices having flexible substrates - Google Patents

Abstract

An active matrix array device has an array of device elements in rows and columns. Each conductor 10 includes an extended row line 10a and a plurality of extensions 10b extending from the row lines, each extension 10b having a gate conductor (a thin film transistor) of a thin film transistor of each device element. 11) has one part defining. The device is compliant to at least deform about an axis parallel to the row line 10a, with portions of each gate extension extending in a direction that is not perpendicular to the row line direction. This non-vertical direction reduces the deformation of the TFT characteristics caused by the deformation of the device.

Description

ACTIVE MATRIX ARRAY DEVICES HAVING FLEXIBLE SUBSTRATES

The present invention relates to the manufacture of electrically arranged devices, for example active matrix devices, on flexible substrates such as plastic substrates.

The most common form of active matrix display is an active matrix liquid crystal display (AMLCD). AMLCD devices are typically fabricated on large glass substrates that are typically 0.7 mm thick. Two plates are required for one cell, and the finished display is only 1.4mm thick. Cell phone manufacturers and some notebook manufacturers require thinner and lighter displays, and the finished cell can be thinned, typically about 0.8 mm thick, in HF (hydrofluoric acid) solution. Mobile phone manufacturers ideally want a thinner display, but it has been found that cells less than 0.8 mm produced by this method are too fragile.

The HF thinning technology is not attractive because it is a pollution process that uses hazardous chemicals that are difficult to safely and economically discard. There is also a slight decrease in yield during the etching process due to pitting of the glass.

As an alternative, the appeal of lightweight, bumpy and thin plastic AMLCDs has long been recognized. Recently, interest in plastic displays has been further increased, and much research has recently been made on AMLCD electrophoretic displays and OLED displays on plastic substrates. Manufacturing directly on other flexible substrates, such as plastic substrates or metal foils, is also attractive from a process point of view, since a reel-to-reel process can be used. In addition, a flexible display can be implemented by treating the display on a flexible substrate supported by a non-flexible carrier and then removing the layer or transferring the layer on an additional flexible substrate layer.

In addition to size and weight advantages, the use of flexible substrates enables new product designs to be implemented. For example, the display may be rolled around and mounted on a curved surface, and may also be unfolded for use and rolled up for storage. For example, the display may be provided as a pull-out accessory of the device that can be unfolded for use. In order to enable the display to dry later in this way, it is known, for example, from US 2002/0139981 A1 to provide a weakened area of the substrate in a suitable direction.

The invention relates, in particular, to a display device intended to be deformed after manufacture in this way, for example between a spread and a storage position, or a display that is bent or folded in half.

Deformation of this kind of device inevitably leads to local deformation of device characteristics, which leads to poor quality of the output image. This degradation of image quality can be caused when viewing the display in a non-planar state, or can also be caused when viewing the display in a perfectly planar position, but results in a permanent change in properties.

According to the invention, as an active matrix array device,

A substrate comprising an array of device elements each including a thin film transistor and disposed in row and column directions;

Comprising a plurality of row conductors and a plurality of thermal conductors,

Each row conductor includes an extended row line and a plurality of extensions extending from said row line, each extension having a portion defining a gate conductor of a thin film transistor of each said device element, said device being said An active matrix array device is provided, compliant so as to be at least deformed about an axis parallel to the row line, wherein each portion of the gate extension includes an extended gate conductor that extends in a direction that is not perpendicular to the row line direction.

The device of the invention arranges the gate conductor of the thin film transistor of each pixel to extend in a direction that is not perpendicular to the axis in which the display is to be deformed. This is known to reduce the deformation of TFT characteristics caused by the deformation of the device. In addition, by deforming the device about an axis parallel to the row, the change in the properties of the row conductor is kept to a minimum. The row resistance should be kept to a minimum because the signal provided to the row is typically for the operation of all TFT gates in the row, and the RC time constant can be deterministic.

The substrate preferably comprises a plastic or metal substrate.

The device is typically further mounted to a display device or any other device using a sample and hold circuit, for example a substrate comprising a pixel array with a liquid crystal material disposed therebetween. An active matrix liquid crystal display device comprising a flexible substrate. The TFT can be manufactured by any conventional technique such as a-Si, LTPS (low temperature poly-silicon), organic TFT, COMS crystalline Si, GaAs and the like.

The portion of each extension extends in the row line direction at an angle of 20 to 70 degrees from the row line. This range of tilt angles is known to be sufficient to reduce the effect of bending on the TFT characteristics and also to keep the pixel layout largely unchanged.

The substrate may include a weakened area to promote desirable deformation of the substrate about an axis parallel to the row line.

In one embodiment, each extension may include a first portion extending perpendicular to the row line direction and a portion defining a gate conductor extending from an end of the first portion in a direction parallel to the direction of the row line. This defines an extension of the "L" shape in the row line. This makes it possible for the gate conductor to be arranged parallel to the row line without significantly increasing the area required for the TFT.

Each pixel may further include a second thin film transistor, and the gate conductor of the second thin film transistor may be defined by a second extension extending vertically from the row. The extension for the first thin film transistor defining the gate conductor of the first TFT can now be arranged parallel to the row line direction. In this way, the dual TFT pixel structure has gate conductors for two TFTs that are diagonal, which makes the device's performance less susceptible to deformation in any direction.

The device can now be adapted to be deformed about at least two axes, one parallel to the row line and the other parallel to the column line. Alternatively, the device may be deformed about any axis that does not have deflection deflected in any direction.

Examples of the present invention are now described in detail with reference to the accompanying drawings.

1 shows a conventional pixel circuit for an active matrix display.

2 illustrates an active matrix display device.

3 shows a conventional pixel layout for an active matrix display.

4 shows a first pixel layout for an active matrix display of the present invention.

5 illustrates a second pixel layout for an active matrix display of the present invention.

Figure 6 shows a third pixel layout for the active matrix display of the present invention.

The present invention provides a flexible active matrix display device having a gate conductor in which the TFT for each pixel extends in a direction that is not perpendicular to the axis in which the device is deformed. This has been found to reduce the effect of deformation on the operating characteristics of the TFT.

1 shows a conventional pixel configuration for an active matrix liquid crystal display. This is an example of a circuit type known as sample and hold circuits, which will be used in the description below to illustrate the present invention. The display is arranged in 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 includes a thin film transistor 14 and a liquid crystal cell 16 disposed in series between a thermal conductor 12 and a common electrode 18. Transistor 14 is switched on and off by a signal provided to row conductor 10. The row conductor 10 is thus connected to the gate 14a of each transistor 14 of the row of associated pixels. Each pixel further comprises a storage capacitor 20 with one end 22 connected to a next row electrode, a previous row electrode, or an individual capacitor electrode. This capacitor 20 helps to store the drive voltage to maintain the signal applied to the liquid crystal cell 16 even after the transistor 14 is turned off.

In order to drive the liquid crystal cell 16 to the desired voltage to obtain the required gray level, a suitable signal is provided to the column conductor 12 in synchronization with the row address pulses on the row conductor 10. This row address pulse turns the thin film transistor 14 on, causing the thermal conductor 12 to charge the liquid crystal cell 16 to a desired voltage and also to charge the storage capacitor 20 to the same voltage. At the end of the row address pulse, transistor 14 is turned off and storage capacitor 20 helps maintain a voltage across cell 16 when another row is addressed. The storage capacitor 20 reduces the leakage effect of the liquid crystal and reduces the percentage of change in pixel capacitance caused by the voltage dependence of the liquid crystal cell capacitance.

Rows are addressed sequentially so that all rows are addressed in one frame period and refreshed in subsequent frame periods.

As shown in FIG. 2, a row address signal by the row drive circuit 30 and a pixel drive signal by the column address circuit 32 are provided to the display pixel matrix 34.

There is a growing interest in flexible displays, in which pixel circuits such as those shown in FIG. 1 are provided on flexible polymer substrates. This not only opens up a new area of possible application for display devices, but also allows other processing techniques to be used. FIG. 2 shows a weakened-line 35 in the display that forms part of the substrate and facilitates deformation about that axis in which the display can be bent or rolled about an axis parallel to the row direction.

However, deforming the manufactured display affects the performance of the pixel circuit.

3 diagrammatically shows the geometrical layout of a conventional pixel circuit. The row conductor 10 has a vertical extension 11 that defines a gate conductor for the TFT 14. In the bottom gate structure, the gate dielectric layer is over the bottom metal layer, and the semiconductor body of the transistor is provided in island form over the gate dielectric layer. The top metal layer defines the thermal conductor 12 and the source electrode (they are connected to each other) as well as the drain electrode. The pixel electrode 40 is connected to a drain and in a reflective display it can be formed from the same metal layer or in a separate transparent layer such as ITO in a transmissive display.

The present invention is based on the fact that different deformation directions have different effects on the pixel circuit characteristics. In particular, deformation of the gate conductor and deformation of the row conductor have the greatest influence on the pixel circuit characteristics. The deformation of the gate conductor affects the TFT operating characteristics, in particular its leakage current, and the row resistance is particularly important because the RC time constant of the row conductor affects the ability of all pixels in the row to be addressed within the time available. . This means that the display will slide from top to bottom. In the example of FIG. 3, the display is deformed about an axis parallel to the row, as schematically shown by arrow 42. This deformation creates mechanical stress in the deposited conductors, ie gate conductors 11 and thermal conductors 12, which extend perpendicular to the row direction. The change in resistance at the thermal conductor is less critical than the change in resistance at the row conductor, because the thermal conductor only transmits signals to a single pixel at a time.

In order to reduce the sensitivity to the mechanical stress of the circuit on the flexible substrate, the present invention provides a layout design for the gate of the transistor such that the gate of the transistor is aligned in the direction to the curling direction of the substrate subjected to the reduced stress. The drying direction is also chosen to reduce the strain effect on the selected conductor line.

4 shows a first embodiment of the present invention.

The device is arranged to deform with respect to the axis of the row conductor in the direction of re-rolling 42. As mentioned above, this orientation means that the deformation has a minimal effect on the row line resistance, which is desirable to keep the row line time constant as small as possible.

The gate conductor 11 of the TFT is disposed at an angle with respect to the vertical. This arrangement provides the reduced stress produced by the gate metal on the semiconductor layer, reducing the change in TFT characteristics due to bending.

The row conductor 10 thus includes an extended row line 10a and an extension 10b that defines the gate conductor 11 of the thin film transistor 14. The expansion portion 10b extends in a direction not perpendicular to the direction of the row line 10a. The extension portion 10b extends in the row line direction at an angle of 20 to 70 degrees, more preferably at an angle of 30 to 60 degrees from the row line.

This range of angles is sufficient to reduce the effect of bending on the TFT characteristics, and also keeps the pixel layout largely unchanged.

5 shows a second embodiment in which the extension forming the gate conductor is deposited in the form of an "L" shape. Accordingly, the extension portion 10b includes a first portion 50 extending perpendicular to the row line direction and a gate conductor extending from an end portion of the first portion 50 in a direction parallel to the direction of the row line 10a. And a defining portion 52.

In this arrangement, the gate conductors extend parallel to the row direction to minimize the stress formed by the gate metal on the semiconductor layer.

This embodiment reduces the stress of both the TFT that can be rolled in one direction and the row metal line for the display.

A further embodiment of the invention allows for the curling or bending of the display in any direction (including rolling around an axis parallel to the row conductor).

6 shows one embodiment in which a dual TFT configuration is provided. The first TFT 14a is disposed in the same manner as shown in Fig. 5 with the gate conductors extending parallel to the row. The second TFT 14b is disposed in the same manner as shown in Fig. 3 in which the gate conductor extends perpendicular to the row direction. The two TFTs 14a and 14b have gate conductors oriented diagonally to each other so that when the stress on one TFT is the highest, the stress of the other TFT is minimal.

In this way, at least one of the TFTs maintains a low leakage current so that the required sample and hold circuit operation will be accurately implemented regardless of the direction in which the display is curled / bent.

The display of FIG. 6 can be designed to roll in a row or column direction, or any curling direction will be acceptable.

If only one particular direction of curl is allowed, the other conductor line will be chosen to be placed parallel to the axis where the curl occurs. For example, different pixel layouts may allow the power lines to be in the row or column direction, and a particular pixel design may be chosen such that most of the conductor lines extend parallel to the curl axis, for example, in the row direction of the example described above. have. In addition, there are more complicated pixel layouts with additional control lines, which can also be designed to be arranged in the same direction.

Although the invention has been described above in connection with a layout for a standard pixel circuit for an active matrix LCD, such as electrophoretic displays, electrowetting displays, electrochromic displays, moving or curled foil based displays using samples and hold ups, It can also be advantageously applied to active matrix displays such as MEMS based displays. However, similar sample and hold circuits will form the basis of the data drive output stage or the memory portion of the active matrix polymer LED display pixel circuit. In such a circuit, it is particularly important to keep the leakage current of the TFT as low as possible. Leakage current is known to increase when a TFT of the same kind as in the prior art is mechanically stressed.

Detailed processes associated with the manufacture of liquid crystal displays or other active matrix devices have not been described in detail. In addition, other possible ways of modifying the substrate in a given manner have not been described in detail. The present invention provides a modification to the layout of the conductors on the substrate with reference to the deformation direction and does not change the conventional used process steps or moreover the conventional driving method. Such conventional processes and methods are therefore not described in detail.

Various modifications will be apparent to those skilled in the art.

By utilizing the present invention, it is possible to minimize changes in the characteristics of the TFTs in the pixels caused in the manufacture of the flexible substrate.

Claims (20)

An active matrix array device, A substrate comprising an array of device elements each including a thin film transistor 14 and disposed in a row and column direction, A plurality of row conductors 10 and a plurality of thermal conductors 12, Each row conductor 10 includes an extended row line 10a and a plurality of extensions 10b extending from the row lines, each extension 10b having a thin film of each of the device elements. Having a portion defining a gate conductor 11 of a transistor, the device compliant to be deformed about at least an axis parallel to the row line 10a, and the portion of each extension 10b being in the row line direction And an expanded gate conductor extending in a direction that is not perpendicular to the. The active matrix array device of claim 1, wherein the substrate comprises a polymeric material. The active matrix array device of claim 1, wherein the substrate comprises a metal foil. 4. Active matrix array device according to any of the preceding claims, wherein each extension (10b) extends 20 degrees from the row line (10a) and 70 degrees in the row line direction. 5. The active matrix according to claim 1, wherein the substrate comprises a weakened area 35 for promoting a desired deformation of the substrate about an axis parallel to the row line 10a. Array device. 6. The extension part 10b according to any one of claims 1 to 5, wherein the extension part 10b extends from the end of the first part 50 and the first part 50 extending generally perpendicular to the direction of the row line 10a. And a portion (52) defining a gate conductor extending in a direction generally parallel to the direction of the row line (10a). 7. Active matrix array device according to any one of the preceding claims, wherein each device element comprises a second transistor (14b) in series with the thin film transistor (14a). 7. The thin film transistor (14) according to any one of claims 1 to 6, wherein the thin film transistor (14a) is a first thin film transistor, and each pixel further includes a second transistor (14b), and a gate of the second thin film transistor. Conductor is an active matrix array device defined by a second extension extending generally perpendicularly from said row line (10a). 10. The method of claim 8, wherein each of the extensions for the first thin film transistor 14a extends generally perpendicular to the direction of the row line 10a, and extends in a direction generally parallel to the direction of the row line from the first portion. And a portion defining a gate conductor. 10. The active matrix array device of claim 9, wherein the device is adapted to deform about at least two axes, one parallel to the row line (10a) and the other parallel to the thermal conductor (12). The active matrix array device according to claim 1, wherein the extension portion (10b) defining the gate conductor is elongated and has a width that is at least twice the width. 12. The active matrix array device of any one of the preceding claims, wherein each device element comprises sample and hold circuitry. 14. The active matrix array device of any of claims 1 to 13, comprising a display device, wherein the device element comprises a display pixel. 13. The active matrix array device of claim 1, comprising an active matrix plate for the display device for attaching to the display pixel array. 15. The active matrix array device of claim 14, comprising an active matrix plate for an electrophoretic display. 13. An active matrix array device according to any of the preceding claims, comprising a drive output stage. The active matrix array device of claim 1, comprising a memory circuit. 18. The active matrix array device of claim 17, wherein the memory circuitry is for an organic light emitting diode circuit (OLED). 14. The active matrix array device of claim 13, comprising an active matrix liquid crystal display device comprising an additional flexible substrate mounted to a substrate comprising a pixel array disposed between liquid crystal materials. 14. The active matrix array device of claim 13, wherein the display pixels comprise electrophoretic, MEMS, or electrochromic display pixels.

Images