JPH03175430A - Reflection type liquid crystal display device - Google Patents

Abstract

PURPOSE:To realize a large picture element area rate and low patterning accuracy by embedding a switching element and a signal conductor below a picture element electrode and providing a conductive shield layer among the picture element electrode, switching element, and signal conductor. CONSTITUTION:The switching element 106 and signal conductor 102 are embed ded below the picture element electrode 107 partially or entirely and the conduc tive layer is provided among the picture element electrode 107, switching element 106, and signal conductor 102 to shield them electrically. A thin film transistor 106 and the conductor 102 are embedded below the picture element electrode 107 and then parasitic capacity is generated between conductors of the gate 101 and drain 102 and the picture element electrode 107, but the parasitic capac ity is all connected to the potential of the shield by providing the electric shield layer 108, so a noise signal is prevented from being inputted directly from the conductors of the gate 101 and drain 102. Consequently, a high-brightness and high-contrast screen is obtained with the large picture element area rate and the requirements for the size accuracy are suppressed low.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal display device, and particularly to a liquid crystal display device for a liquid crystal projector that enlarges and projects an image.

[Conventional technology]

In recent years, development of liquid crystal display devices using active matrix substrates using thin film transistors and thin film diodes as switching elements has become active. These liquid crystal display devices are divided into two types, transmissive type and reflective type, depending on the display mode. Transmissive types are often used for television applications, as they can provide multi-color or full-color displays when combined with color filters, while reflective types are often used as display devices for personal computers such as word processors and laptops. Recently, enlarged projection type liquid crystal projectors using these liquid crystal display devices as optical modulators have been developed. In the case of a liquid crystal projector, full color display is possible even with a reflective liquid crystal display device. The first performance required of a liquid crystal display device for a liquid crystal projector is that it be small. This is because the size of the liquid crystal display device determines the size of optical systems such as lenses, prisms, mirrors, etc., which in turn determines the size of the entire device. Next, it is required to have high resolution while being small. This is because the larger the screen, the more noticeable the roughness of the pixels becomes. Therefore,
The pixel pitch must be extremely small.

FIG. 2 shows an example of a conventional reflective active matrix substrate using polycrystalline silicon thin film transistors. The figure shows one pixel, where 201 is a gate wiring made of aluminum, 202 is a gate electrode made of doped polycrystalline silicon, 203 is a drain wiring made of aluminum, and 204 is a doped wiring formed at the same time as 202. A drain wiring connection portion made of polycrystalline silicon, 205 a polycrystalline silicon film serving as an active layer of a thin film transistor, 206 a pixel electrode made of aluminum,
Reference numeral 207 denotes a matrix-shaped reflecting plate corresponding to a black matrix portion used in a transmission type liquid crystal display device, which is formed on the surface of a glass substrate that is an opposite substrate when viewed from the substrate on which the thin film transistor is formed. The line in the figure represents the gate length of the thin film transistor. The dimensions of each part are based on the gate length.

[Problem to be solved by the invention]

As mentioned earlier, the pixels need to be quite small.

On the other hand, switching elements such as thin film transistors cannot be made proportionally smaller. Therefore, the ratio of the area of the switching element to one pixel increases. Conversely, the proportion of pixel electrodes becomes smaller. This situation is shown in FIG. FIG. 3 shows changes in pixel area ratio in the case of a substrate with a diagonal of 3 inches to 5 inches, using the gate length in the configuration of FIG. 2 as a parameter and assuming the same total number of pixels. The pixel area ratio here refers to the ratio of the area of a portion that can effectively become a pixel out of the area required to constitute one pixel. From the perspective of image quality, the pixel area ratio is directly related to brightness and contrast, and of course the larger the ratio, the better. I would like it to be at least 50% or more. However, as shown in Figure 3, in the case of the smallest 3-inch substrate, the gate length is 2μ.
Even in the case of a 5-inch substrate, the thickness must be kept to 3 μm or less, which poses a major manufacturing problem.

The present invention solves the above-mentioned drawbacks of the prior art and provides a reflective liquid crystal display device which has a large pixel area ratio and requires low patterning accuracy.

[Means to solve the problem]

According to the present invention, at least switching elements are arranged in a matrix on an insulating substrate, each switching element is connected by a signal wiring, and one pixel electrode is electrically connected to each of the switching elements. In a reflective liquid crystal display device in which the liquid crystal is narrowed between an active matrix substrate arranged in the opposite direction and a counter substrate in which a counter electrode is provided on an insulating substrate, part or all of the switching elements and signal wiring are located below the pixel electrode. The pixel electrode, the switching element, and the signal wiring are partially or completely buried under the pixel electrode, and a conductive layer is provided between the pixel electrode, the switching element, and the signal wiring, and the pixel electrode and the switching element are buried. A reflective liquid crystal display device characterized in that elements and signal wiring are electrically shielded is obtained.

[Effect]

In the reflective liquid crystal display device of the present invention, in order to improve the pixel area ratio, switching elements such as thin film transistors and thin film diodes and part or all of the wiring are embedded under the pixel electrodes. By adopting this structure, the pixel area ratio is dramatically improved. In other words, the pixel electrode can be designed without worrying too much about the size of the active element or wiring. Additionally, there is more room in the size of the switching element. Figure 4 shows a vertical pixel pitch of 36 μm using the gap between adjacent pixel electrodes as a parameter.
, shows the change in pixel area ratio in the case of 53 μm horizontally (3-inch diagonal substrate). If you compare it with Figure 3, the difference is obvious. When compared with similar patterning accuracy,
For example, in the case of a conventional type with a 3 μm and 3 inch substrate, the pixel area ratio is only about 15%, but in the case of the present invention, the pixel area ratio is 80%.
% or more can be secured. However, simply adopting such a structure causes a new problem of parasitic capacitance. This will be explained based on the circuit configuration.

FIG. 5 shows an equivalent circuit for one pixel. Figure 5(a)
Figure 5(b) shows the basic configuration, and Figure 5(b) shows the equivalent circuit when the thin film transistor and wiring section are buried under the pixel electrode.
Figure (c) shows an equivalent circuit in which the thin film transistor and wiring portion are electrically separated from the pixel electrode by using a shield layer in the configuration of the present invention. In the figure, 501 is a thin film transistor, 502 is the equivalent capacitance of the liquid crystal, 503 is the parasitic capacitance between the gate and source, 504 is the parasitic capacitance between the drain and source, 505, 506, and 507 are the gate and drain J generated by the shield, respectively. source capacity.

As shown in Figure 5 (a), one pixel basically consists of a thin film transistor and a liquid crystal capacitor.If the thin film transistor and wiring are buried under the pixel electrode, the result will be as shown in Figure 5 (b). A parasitic capacitance is generated between the gate/drain wiring and the pixel electrode.As is clear from this equivalent circuit, the voltage of the pixel electrode is modulated by the scanning signal to the gate and the signal voltage to the drain. On the other hand, if an electrical shield layer is provided between the pixel electrode, the active element, and the wiring, the parasitic capacitance of all of these will be connected to the potential of the shield (as shown in the figure). Now let's set the shield to ground potential.)
This makes it possible to prevent noise signals from entering directly from the gate and drain wiring. Therefore, a low-noise liquid crystal display device can be realized without maintaining a large pixel area ratio.

〔Example〕

FIG. 1 shows an embodiment of the reflective liquid phase display device of the present invention. FIG. 1(a) is a plan view, and FIG. 1(b) is a cross-sectional view when FIG. 1(a) is cut along line AA'.
The number of pixels is shown. In the figure, 101 is a gate electrode and gate wiring made of aluminum, 102 is a drain wiring also made of aluminum, 103 is a train electrode that also serves as a drain connection part made of doped polycrystalline silicon, and 104 is formed at the same time as 103. 105 connection portion for double gated thin film transistor
is a source electrode 310 formed at the same time as 103 and 104.
6 is an active layer of a thin film transistor made of polycrystalline silicon, 107 is a pixel electrode made of aluminum, 108 is a shield layer also made of aluminum, 109 is a gate insulating film made of silicon dioxide, 110.111 is an interlayer made of silicon nitride film It is an insulating film. In this example, a double-gate thin film transistor with a so-called staggered structure is used. The purpose of using a double gate is to suppress leakage current, which is a problem with polycrystalline silicon thin film transistors. As can be seen from the figure, all parts of the thin film transistor and most of the gate/drain wiring are buried under the pixel electrode 107. The difference in pixel area ratio can be clearly seen by comparing with the conventional example shown in FIG. Even though there is overlap between the double positive electrodes, the shield layer 108 prevents direct capacitive coupling between the pixel electrode 107 and the thin film transistor or wiring 102, as shown in FIG. 1(b). Furthermore, the characteristic feature of this structure is that the shield layer 10
Reference numeral 8 also serves as a matrix reflector in the conventional example. This is naturally formed in a self-aligning manner, so there is no contrast reduction due to misalignment,
Moreover, it has the advantage that its width can be formed narrow.

The above explanation uses an example of an active matrix substrate that uses polycrystalline silicon as a thin film transistor, but other active elements include amorphous silicon thin film transistors, thin film diodes with silicon nitride film sandwiched between metal electrodes, and tantalum oxide films. Examples include thin film diodes.

〔Effect of the invention〕

As compared in FIGS. 3 and 4, the reflective liquid crystal display device of the present invention can obtain a pixel area ratio several times higher than that of the conventional device with the same dimensional accuracy. If the pixel area ratio improves, a screen with higher brightness and contrast can be obtained. Moreover, the layout of the active elements can be freely arranged, and the requirement for dimensional accuracy can be kept low. For example, in Figure 1, the thin film transistor is placed at the center below the pixel electrode, but whether it is placed horizontally or diagonally, the area of one pixel excluding the area of the wiring section and the required pattern spacing will be the same. This means that any layout can be created within the . Further, in the conventional type, the wiring width was an important factor in determining the pixel area ratio, but in the case of the present invention, it is hidden under the shield, so a considerable margin can be provided. Wider line width makes it more resistant to defects such as wire breakage and improves yield.

As described above, according to the present invention, a liquid crystal projector with high brightness and little change in gradation and contrast due to noise and a reflective liquid crystal display device with high contrast can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of a reflective liquid crystal display device of the present invention, FIG. 2 is a diagram showing an example of a conventional reflective liquid crystal display device, and FIG. Figure 4 shows the change in pixel area ratio for a 3-inch to 5-inch substrate. Figure 4 shows the change in pixel area ratio when the pixel pitch is 36 μm vertically and 53 μm horizontally, using the gap between adjacent pixel electrodes as a parameter. FIG. 5, which is a diagram showing changes, is a diagram showing an equivalent circuit of one pixel. In the figure, 101...gate electrode and gate wiring, 1
02...Drain wiring, 103...Drain electrode that also serves as a drain connection part, 104...Connection part for making the thin film transistor into a double gate, 105...Source electrode, 106...Activity of the thin film transistor layer, 107
...Pixel electrode, 108...Shield layer, 109...
- Gate insulating film, 110.111... Interlayer insulating film.

Claims (1)

[Claims] An active matrix in which at least switching elements are arranged in a matrix on an insulating substrate, each switching element is connected by signal wiring, and one pixel electrode is electrically connected to each of the switching elements. In a reflective liquid crystal display device in which a liquid crystal is narrowed between a substrate and a counter substrate having a counter electrode provided on an insulating substrate, part or all of the switching element and the signal wiring are buried under the pixel electrode, and the pixel A reflective liquid crystal display device, characterized in that a conductive shield layer is provided between an electrode and the switching element and signal wiring.