JPH08271928A - Liquid crystal display device - Google Patents

Abstract

PURPOSE: To provide a liquid crystal display device which can prevent photo conduction of a switching element by preventing the leakage of light to a switching element even when incident light is strong. CONSTITUTION: In a liquid crystal display device in which a first electrode 2 for display connected electrically to electrodes of plural switching elements 1 formed on a substrate 10 is provided at the upper part of the switching element 1, a capacitor 3 for storing electric charges electrically connected to the first electrode 2 for display is formed under the first electrode 2 for display, a light reflecting film 15 is formed at the upper part of the first electrode 2 for display, a transparent second electrode 4 for display is formed above this light reflecting film, and liquid crystal 5 is hermetically sealed between this second electrode 4 for display and the light reflecting film, this device is constituted so that an insulating shielding film 20 is formed between the first electrode 2 for display and the switching element 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal display device, and more particularly to a matrix liquid crystal display device used in a projector or the like.

[0002]

2. Description of the Related Art Generally, a molecular arrangement maintains a certain degree of order like a solid, but on the other hand, it has fluidity like a liquid and easily changes its arrangement with respect to an electric field to change its optical properties. A liquid crystal display device is known as a device using the emerging liquid crystal. This liquid crystal display device encloses liquid crystal between a common electrode and an individually controllable pixel electrode which is arranged so as to face the common electrode, and selectively applies a data signal to the pixel electrode so that the pixel electrode It is designed to change the optical characteristics of liquid crystals.

This type of liquid crystal display device is generally roughly classified into a transmissive display device and a reflective display device, and the transmissive display device has a relatively simple optical system configuration, and thus the cost is reduced. On the other hand, there is a merit that the display panel is downsized, but when the display panel is downsized, the area ratio occupied by the switch transistor and the wiring for selecting the pixel electrode is increased, the aperture ratio is decreased, and the brightness of the image is decreased to be dark. On the other hand, for example, Japanese Patent Publication No. 57-39422 and Japanese Patent Laid-Open No.
In the reflective display device as disclosed in Japanese Patent Laid-Open No. 338721/1999, the switch transistor and the wiring are arranged under the reflective pixel electrode. Therefore, even if the display panel is downsized, the aperture ratio does not decrease and a bright image is obtained. You can Therefore, in the enlargement projection type liquid crystal display device, a small size and high density reflective display panel is suitable.

A reflective liquid crystal display device using a MOS transistor will be described as an example of a conventional liquid crystal display device. FIG. 4 is a circuit diagram showing one pixel of a general liquid crystal display device. In the figure, reference numeral 1 denotes a switching element 1 composed of, for example, a MOS transistor, and the source of this element 1 is a pixel electrode 2 and a pixel electrode. A charge storage capacitor 3 having one terminal 2 and one terminal in common is connected. Liquid crystal 5 is sealed between the pixel electrode 2 and the common electrode 4. A gate line Xi for transmitting a selection signal is connected to the gate electrode 6, and a signal line Y for transmitting a video signal or the like to a source or a drain which is not connected to the pixel electrode 2.
j is connected. The operation of this device is such that, for example, when a selection signal is applied to the gate electrode 6 via the gate line Xi, the switching element 1 including a MOS transistor is used.
Is turned on, and the video signal of the signal line Yj is applied to the pixel electrode 2 at the same time as charging the capacitor 3 through the element 1. Here, even if the signal of the gate line Xi becomes zero and is not selected, the electric potential of the pixel electrode 2 is held by the electric charge stored in the capacitor 3.

During this time, a potential difference between the pixel electrode 2 and the common electrode 4 is applied to the liquid crystal 5, and this voltage changes the light transmittance of the liquid crystal. Therefore, by controlling this potential difference, an electric signal is transmitted. It can be converted into a modulated optical signal. Assuming that light for projection, for example, deflected read light, is incident on the pixel electrode 2 from the common electrode 4 side, the incident light passes through the common electrode 4 and the liquid crystal 5, and then the light reflection film described later. It is reflected by and passes through the liquid crystal 5 and the common electrode 4 again. At this time, the reflected light passes through the liquid crystal 5 to realize a modulated optical signal. An image can be formed by arranging such unit pixels in a matrix and scanning signals in the vertical and horizontal directions. In this scanning method, for example, the switching elements are simultaneously turned on in the X direction along the gate line Xi, the video signal is written in the capacitor, and the Y direction is sequentially scanned.

FIG. 5 is a sectional view showing a unit pixel when the pixel unit shown in FIG. 4 is integrated as a matrix structure. Switching element 1 composed of MOS type transistor
Is composed of a gate electrode 6, a drain 7, and a source 8. The gate electrode 6 is provided via a gate oxide film 9,
For example, the gate lines Xi in the X direction are made of polycrystalline silicon (see FIG. 4). Further, the drain 7 is connected to the signal line Yj extending in the Y direction (see FIG. 4).

Next to the source 8 is a single crystal silicon substrate 1
A capacitor 3 for charge storage is formed by sandwiching, for example, a SiO ₂ insulating film 12 between 0 and the capacitor electrode 11, and the capacitor electrode 11 is connected to the source 8. The pixel electrode 2 is the switching element 1 described above.
And above the capacitor 3, for example, the SiO ₂ insulating film 13
The pixel electrode 2 and the source 8 are electrically connected via an opening 14 provided in the thickness direction of the insulating film 13 for electrical connection. Furthermore,
Immediately above the pixel electrode 2, a dielectric light reflecting film 15 formed by laminating at least two insulating thin films such as SiO ₂ is formed over the entire surface of the substrate including the pixel electrode 2, and on this upper surface. The alignment film 16 is also formed over the entire surface.

A transparent glass 17 having a transparent common electrode 4 adhered on the surface (the lower surface in the drawing) is provided so as to face the surface of the silicon substrate 10 formed in this way, with a slight gap therebetween. The alignment film 18 is also formed on the entire lower surface of the common electrode 4. Then, a liquid crystal panel is formed by enclosing the liquid crystal 5 between the common electrode 4 and the light reflection film 15, whereby a reflection type liquid crystal display device is obtained. Then, the incident light 18 coming from above the transparent glass 17 passes through the liquid crystal 5 and then is reflected by the surface of the light reflection film 15 to go out as modulated light 19.

[0009]

By the way, when the gate selection signal is released, the charge storage capacitor 3 is in a floating state, and the charges stored in the capacitor 3 are applied with the next gate selection signal. Up to the above, although the leakage gradually occurs through the off resistance of the switching element 1 and the resistance component of the liquid crystal cell portion, this leakage amount does not pose a problem so much. However, the problem here is that the incident light 18 passes through the light reflecting film 15 and illuminates the switching element 1. That is, most of the incident light 18 is reflected by the light reflecting film 15, but a part of the incident light 18A passes through the light reflecting film 15 without being reflected by the light reflecting film 15 and illuminates the switching element 1. It will be. Particularly, when the amount of incident light becomes strong, the amount of transmitted light of the light reflection film 15 also becomes considerably large.

The incident light 18A thus transmitted is MOS
When the switching element 1 composed of a transistor is irradiated, photocarriers are generated in this portion. Of the generated carriers, holes flow into the substrate 10 and do not affect the characteristics. However, when the substrate 10 is a P-type semiconductor and the source 8 is an N-type semiconductor, the generated electrons flow into the source 8. The charge of the capacitor 3 leaks, which lowers the source voltage and changes the voltage of the pixel electrode 2 to change, so that the charge corresponding to the video signal cannot be charged. That is, there has been a problem that photoconduction occurs and the charge of the capacitor 3 leaks, and in the worst case, even an image cannot be displayed. On the other hand, it is possible to further increase the thickness of the light-reflecting film to reduce the amount of transmitted light, or to add an insulating light-shielding film to reduce the amount of transmitted light. Is a voltage required to drive the liquid crystal, that is, a voltage corresponding to the impedance of the light reflection film 15 or the light shielding film 15 is added as a necessary voltage between the common electrode 4 which is the counter electrode and the pixel electrode 2. Therefore, the required voltage is increasing. In addition, when the device was actually prototyped, the addition of the light-shielding film spreads the electric field between the pixel electrode and the common electrode because the impedance of the light-shielding film is smaller than the impedance of the liquid crystal, resulting in a display image. There was a problem that the resolution of was reduced.

The present invention focuses on the above problems,
It was devised to solve this effectively, and its purpose is to prevent light from leaking to the switching element even when the incident light is strong to prevent photoconduction of the switching element and to increase the driving voltage of the switching element. An object of the present invention is to provide a liquid crystal display device capable of preventing deterioration of display quality.

[0012]

In order to solve the above-mentioned problems, the present invention provides a first display electrode electrically connected to electrodes of a plurality of switching elements formed on a substrate as described above. A charge storage capacitor, which is provided on the switching element and is electrically connected to the first display electrode, is formed under the first display electrode, and is formed on the first display electrode. A liquid crystal display device in which a light reflecting film is formed, a transparent second display electrode is formed above the light reflecting film, and liquid crystal is enclosed between the second display electrode and the light reflecting film. In order to shield the switching element from light, an insulating light-shielding film is formed between the first display electrode and the switching element.

[0013]

The present invention is constructed as described above,
Since the insulating light-shielding film is provided between the first display electrode and the switching element, even if a part of the incident light incident from the second display electrode side passes through the light reflecting film, the lower layer thereunder. This light is blocked by the insulative light-shielding film provided in the above, and therefore the switching element is not irradiated with the transmitted light, so that the occurrence of photoconduction can be suppressed. Such a light shielding film may be provided between the first display electrode and the switching element, between the light reflection film and the first display electrode, or only between the adjacent first display electrodes. Further, as the material of the light-shielding film, for example, a cadmium tellurium (CdTe) semiconductor or a germanium (Ge) oxide film can be used.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the liquid crystal display device according to the present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 is a partial cross-sectional view showing an example of one pixel of the liquid crystal display device according to the present invention.
Incidentally, the same parts as those of the conventional device shown in FIG. The switching element 1 formed of a MOS transistor is composed of a drain 7 formed with a predetermined gap, a source 8 and a gate electrode 6 formed between them via a gate oxide film 9. Is formed on the single crystal silicon substrate 10, for example. The gate electrode 6 is wired as a gate line Xi in the X direction by, for example, polycrystalline silicon, and the drain 7 is connected to a signal line Yj extending in the Y direction (see FIG. 3).

Next to the source 8 is a single crystal silicon substrate 1
0 between the surface of 0 and the capacitor electrode 11 such as SiO ₂
A capacitor 3 for charge storage is formed by sandwiching the insulating film 12, and the capacitor electrode 11 is connected to the source 8. The pixel electrode 2 serving as the first display electrode is formed above the switching element 1 and the capacitor 3, and the pixel electrode 2 of the present invention is provided between the switching element 1 and the capacitor 3 and the pixel electrode 2. A characteristic insulating light-shielding film 20 is formed.
Specifically, the upper part of the switching element 1 and the side part of the capacitor 3 are covered with a SiO ₂ insulating film 13, and the insulating film 13, the capacitor electrode 11 and the pixel electrode 2 are covered.
And the light shielding film 20 is formed so as to cover the entire surface of the pixel matrix. The pixel electrode 2 and the source 8 cover the switching element 1 with SiO 2.
_{2 The} insulating film 13 and the light-shielding film 20 are formed along the thickness direction, and are electrically connected through an opening 14 for electrical connection.

The light-shielding film 20 is for substantially completely absorbing incident light transmitted through a light-reflecting film, which will be described later, and its material is visible light such as cadmium tellurium (CdTe) semiconductor and germanium oxide. Use a material that can absorb infrared rays. For example, depending on the material used, set the thickness to 1
The thickness is set to about μm, and the transmitted light is almost completely absorbed.

Directly above the pixel electrode 2, a light reflecting film 15 made of a dielectric, which is formed by laminating at least two insulating thin films such as SiO ₂ , is formed over the entire surface of the substrate including the pixel electrode. An alignment film 16 for aligning the liquid crystal is formed on the upper surface.

The surface of the silicon substrate 10 thus formed is opposed to the surface (the lower surface in the drawing) of, for example, ITO (Indium Tin) with a slight gap therebetween.
A transparent glass 17 having a transparent common electrode 4 as a second display electrode made of oxide) is provided. The common electrode 4 is connected to the common electrodes of all other pixels (not shown). On the lower surface side of the common electrode 4, an alignment film 18 for aligning the liquid crystal is formed over the entire surface. Then, the liquid crystal 5 is hermetically sealed between the alignment film 18 and the alignment film 16 on the pixel electrode 2 side, thereby forming a liquid crystal cell. Such unit pixels (liquid crystal cells) are arranged in a matrix in the vertical and horizontal directions as described above to form a liquid crystal panel.

Next, the operation of this embodiment configured as described above will be described. First, when a selection voltage is applied to the gate electrode 6 via the gate line Xi, a channel is formed between the drain 7 and the source 8 and conduction is established between the drain 7 and the source 8, and therefore the source 8 is connected. The pixel electrode 2 and the charge storage capacitor 3 that are being charged are charged to the potential of the video signal of the signal line Yj. Then, when the voltage of the gate electrode 6 becomes the voltage at the time of non-selection, the channel is disconnected and the source 8 and the pixel electrode 2 are in a floating state until the next gate selection voltage is applied. At this time, the pixel electrode 2 is The electric potential of the storage capacitor previously stored in the capacitor 3 is supplied.

Here, when the incident light 18 for projection, for example, the deflected S wave is incident from the common electrode 4 side toward the pixel electrode 2 side, this light passes through the common electrode 4 and the liquid crystal 5 and is emitted. The liquid crystal 5 is reflected by the reflection film 15 and becomes P-polarized light again.
And passes through the common electrode 4 and exits as modulated light 19. In this case, most of the incident light 18 is reflected by the light reflecting film 15, but a part of the light passes through the light reflecting film 15 and penetrates to the lower part in the figure. . In particular, when the incident light is strong, the amount of light that passes through the light reflecting film 15 also increases, which adversely affects the operation of the switching element 1 located below. However, in this embodiment, since the insulating light-shielding film 20 is formed between the pixel electrode 2 and the switching element 1 over substantially the entire surface of the pixel matrix (excluding the openings 14), light reflection is caused. Part of incident light 18A transmitted through the film 15
Is almost completely absorbed by the light shielding film 20 and does not reach the switching element 1.

Therefore, since the switching element 1 formed of the MOS transistor does not cause photoconduction, the charge of the charge storage capacitor 3 is not leaked, and the potential of the pixel electrode 2 is appropriate for the video signal. The charge can be maintained, an image of good quality can be obtained, and the occurrence of malfunction due to leakage can be prevented. By providing the light-shielding film between the pixel electrode and the switching element, it is necessary to drive the liquid crystal as in the case where the light-shielding film is provided between the pixel electrode and the counter electrode (second display electrode). The problem that the voltage rises does not occur.

By the way, since a metal such as aluminum is generally used for the pixel electrode, a good light shielding property may be obtained by the pixel electrode itself. In this case, the light transmitted through the gaps between the pixel electrodes (portions between the individual pixel electrodes arranged in a matrix) greatly affects the occurrence of photoconduction. For example, in the liquid crystal display device of FIG. 1, a light-shielding film may be further formed in the gap between adjacent pixel electrodes, or the light-shielding film in the gap may be thickened. Further, for example, as shown in FIGS. 2 and 3, the light-shielding film 20 is formed only in the gap portion between the adjacent pixel electrodes, or the light-shielding film is provided between the pixel electrode and the transistor and the light-shielding film 20 in the gap portion is formed. The thickness may be greater than the thickness of other portions (thickness of the light shielding film below the pixel electrode). In this case, for example, the thickness of the light shielding film in the gap portion is set to 1 μm and the thickness of the pixel electrode portion is set to about 0,5 μm.

In the above embodiment, the case where the MOS transistor is used as the switching element has been described as an example. However, the present invention is not limited to this, and a TFT (Thin Film Tr) formed by laminating thin films on a glass substrate, for example
Of course, an anistor) may be used.

[0024]

As described above, the liquid crystal display device according to the present invention can exhibit the following excellent effects. Since the light shielding film is formed between the first display electrode and the switching element to prevent the incident light from reaching the switching element, it is possible to prevent the photoconduction from occurring. Therefore, it is possible to prevent the electric charge of the pixel electrode from leaking, hold this potential at an appropriate value, realize an image of good quality, and prevent the occurrence of malfunction. . Further, by providing the light-shielding film between the pixel electrode and the switching element, it is possible to drive the liquid crystal as in the case where the light-shielding film is provided between the pixel electrode and the counter electrode (second display electrode). It is possible to solve the problem that the required voltage rises and the problem that the display quality deteriorates.

[Brief description of drawings]

FIG. 1 is a partial cross-sectional view showing an example of one pixel of a liquid crystal display device according to the present invention.

FIG. 2 is a partial cross-sectional view showing another embodiment of the device of the present invention, in which the light-shielding film is provided only between the adjacent pixel electrodes.

FIG. 3 shows still another embodiment of the device of the present invention, in which the light-shielding film is between the pixel electrode and the light-reflecting layer, and the film thickness between the pixel electrodes is thicker than the film thickness of other portions. FIG.

FIG. 4 is a circuit diagram showing one pixel of a general liquid crystal display device.

FIG. 5 is a partial cross-sectional view showing an example of one pixel of a conventional liquid crystal display device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Switching element, 2 ... Pixel electrode (first display electrode), 3 ... Charge storage capacitor, 4 ... Common electrode (second display electrode), 5 ... Liquid crystal, 7 ... Drain (electrode),
8 ... Source (electrode), 10 ... Silicon substrate (substrate), 1
5 ... Light reflection film, 18 ... Incident light, 18A ... Part of incident light,
19 ... Modulated light, 20 ... Insulating light-shielding film, Xi ... Gate line, Yj ... Signal line.

Claims (4)

[Claims] 1. A first display electrode electrically connected to electrodes of a plurality of switching elements formed on a substrate is provided above the switching element, and below the first display electrode, A charge storage capacitor electrically connected to the first display electrode is formed, a light reflecting film is formed on the first display electrode, and a transparent second film is formed above the light reflecting film. A liquid crystal display device in which a liquid crystal is enclosed between the second display electrode and the light reflection film, and the first display is provided in order to shield the switching element from light. A liquid crystal display device, characterized in that an insulating light-shielding film is formed between the electrode for use and the switching element. 2. A plurality of first display electrodes electrically connected to electrodes of a plurality of switching elements formed on a substrate are formed, and the first display electrodes are formed under the first display electrodes. A charge storage capacitor electrically connected to the second display electrode, a light reflection film is formed on the first display electrode, and a transparent second display film is formed above the light reflection film. In a liquid crystal display device in which an electrode is formed and liquid crystal is enclosed between the second display electrode and the light reflecting film, a first liquid crystal display device is provided between the switching element and the first display electrode and adjacent to each other. An insulating light-shielding film is formed between the display electrodes, and the thickness of the light-shielding film between the adjacent first display electrodes is thicker than the thickness of the light-shielding film below the first display electrode. A liquid crystal display device characterized by the above. 3. A plurality of first display electrodes electrically connected to electrodes of a plurality of switching elements formed on a substrate are formed, and the first display electrodes are formed under the first display electrodes. A charge storage capacitor electrically connected to the second display electrode, a light reflection film is formed on the first display electrode, and a transparent second display film is formed above the light reflection film. In a liquid crystal display device in which an electrode is formed and a liquid crystal is enclosed between the second display electrode and the light reflection film, an insulating light-shielding film is formed only between the adjacent first display electrodes. A liquid crystal display device characterized by being configured as follows. 4. The liquid crystal display device according to claim 1, wherein the light shielding film is an oxide of cadmium tellurium or germanium.