# Patent

## Title
Manufacturing method for liquid crystal display device

## Abstract

A LCD device having a large pixel holding capacitance includes opposedly facing first and second substrates, and liquid crystal between them. The first substrate includes a video signal line, a pixel electrode, a thin film transistor having a first electrode connected to the video signal line and a second electrode connected to the pixel electrode, a first silicon nitride film formed above the second electrode, an organic insulation film above the first silicon nitride film, a capacitance electrode above the organic insulation film, and a second silicon nitride film above the capacitance electrode and below the pixel electrode. A contact hole etched in both the first and second silicon nitride films connects the second electrode and the pixel electrode to each other. A holding capacitance is formed by the pixel electrode, the second silicon nitride film and the capacitance electrode.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Continuation of U.S. application Ser. No. 13/067,281 filed May 20, 2011, which is a Continuation of U.S. application Ser. No. 12/662,961 filed May 13, 2010, which is a Continuation of U.S. application Ser. No. 11/802,385 filed May 22, 2007. Priority is claimed based on U.S. application Ser. No. 13/067,281 filed May 20, 2011, which claims the priority of U.S. application Ser. No. 12/662,961 filed on May 13, 2010, which claims the priority of U.S. application Ser. No. 11/802,385 filed May 22, 2007, which claims the priority date of Japanese Patent Japanese application 2006-160428 filed on Jun. 9, 2006, the content of which is hereby incorporated by reference into this application.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a technique which is effectively applicable to a liquid crystal display device which includes a silicon nitride film.
2. Description of Related Art
An active matrix liquid crystal display device forms a holding capacitance in the inside of a pixel for holding a video signal written in the pixel for a relatively long time.
The inventors of the present invention filed a patent application (see following patent document 1) on one method for forming such a holding capacitance.
FIG. 7 is a cross-sectional view of an essential part showing one example of the holding capacitance described in patent document 1.
As shown in FIG. 7, in an IPS (In-Plane Switching) type liquid crystal display device described in patent document 1, an interlayer insulation film IN2 and an organic insulation film PAS are formed to cover a source electrode SD2 of a thin film transistor in order from below. A contact hole CH3A is formed in the interlayer insulation film IN2 and the organic insulation film PAS in a penetrating manner. A counter electrode CT and a reflection film RAL are formed on the organic insulation film PAS in order from below. Further, an interlayer insulation film IN3A is formed to cover the organic insulation film PAS, the counter electrode CT and the reflection film RAL. The interlayer insulation film IN3A is a coated insulation film or an insulation film formed by a CVD method and is formed also in the inside of the contact hole CH3A. Further, in the interlayer insulation film IN3A, a contact hole CH3B is formed more inside than the contact hole CH3A. A pixel electrode PX is formed on the interlayer insulation film IN3A. Further, the pixel electrode PX is connected with the source electrode SD2 of the thin film transistor via the contact hole CH3B and a video signal is applied to the pixel electrode PX via the thin film transistor. Liquid crystal not shown in the drawing is driven by an electric field generated between the pixel electrode PX and the counter electrode CT thus performing a display of an image.
Here, the holding capacitance is formed by the counter electrode CT (including the reflection film RAL), the interlayer insulation film IN3A and the pixel electrode PX.
Patent Document 1: Japanese patent application no. 2005-312165
SUMMARY OF THE INVENTION
However, in the liquid crystal display device shown in FIG. 7, when the coated insulation film is used as the interlayer insulation film IN3A, the interlayer insulation film IN3A exhibits a dielectric constant which is not so high thus giving rise to a drawback that the holding capacitance cannot be increased.
Further, although patent document 1 describes a technique which forms the interlayer insulation film IN3A by the CVD method in place of the coated insulation film, patent document 1 fails to describe a material of the film.
Further, FIG. 7 shows the structure which exposes the source electrode SD2 through the contact hole CH3A in forming the reflection film RAL by patterning. Accordingly, there exists a possibility that the source electrode SD2 is damaged by etching which is performed for patterning the reflection film RAL using an etchant or an etching gas.
Drawbacks other than the above-mentioned drawbacks will become apparent from the description of the whole specification or drawings.
In the present invention, as an interlayer insulation film above an organic insulation film, a silicon nitride film which is formed at a low temperature is used. Further, in forming a contact hole in the interlayer insulation film formed of the silicon nitride film, it is desirable to collectively etch the interlayer insulation film together with other interlayer insulation film arranged below the interlayer insulation film.
The present invention may adopt the following constitutions, for example.
(1) A liquid crystal display device including a first substrate including a video signal line, a pixel electrode, a thin film transistor having a first electrode thereof connected to the video signal line and a second electrode thereof connected to the pixel electrode, a first silicon nitride film formed above the second electrode, an organic insulation film formed above the first silicon nitride film, a capacitance electrode formed above the organic insulation film, and a second silicon nitride film formed above the capacitance electrode and below the pixel electrode, a second substrate arranged to face the first substrate in an opposed manner, and liquid crystal sandwiched between the first substrate and the second substrate, wherein the second silicon nitride film is a film which is formed at a temperature lower than a forming temperature of the first silicon nitride film, the first silicon nitride film and the second silicon nitride film form a contact hole therein by etching both of the first silicon nitride film and the second silicon nitride film collectively by dry etching, the second electrode and the pixel electrode are connected to each other via the contact hole, a potential different from a potential applied to the pixel electrode is applied to the capacitance electrode, and a holding capacitance is formed by the pixel electrode, the second silicon nitride film and the capacitance electrode.
(2) In the constitution (1), the capacitance electrode may have at least a portion thereof formed of a reflection film.
(3) In the constitution (2), the second electrode may be made of a material which is etched by an etchant or an etching gas used in patterning the reflection film.
(4) In the constitution (2) or (3), the second electrode may include the same material as the reflection film.
(5) In any one of the constitutions (2) to (4), the organic insulation film may have a surface unevenness on a portion thereof corresponding to the reflection film, and the reflection film may have a surface unevenness which reflects the surface unevenness of the organic insulation film.
(6) In the constitution (5), a height of the surface unevenness of the organic insulation film between a crest and a valley may be 0.3 μm or less.
(7) In any one of the constitutions (1) to (6), in the contact hole, a lower surface of the second silicon nitride film may be brought into contact with an upper surface of the first silicon nitride film at least at one portion of the first silicon nitride film.
(8) In any one of the constitutions (1) to (7), in the contact hole, a lower surface of the second silicon nitride film may be brought into contact with an upper surface of the first silicon nitride film over the whole circumference of the contact hole.
(9) In any one of the constitutions (1) to (8), in the contact hole, an end portion of a lower surface of the second silicon nitride film may be substantially aligned with an end portion of an upper surface of the first silicon nitride film.
(10) In any one of the constitutions (1) to (8), in the contact hole, an end portion of a lower surface of the second silicon nitride film may be retracted from an end portion of an upper surface of the first silicon nitride film.
(11) In any one of the constitutions (1) to (10), in the contact hole, the organic insulation film may not be exposed from the second silicon nitride film.
(12) In any one of the constitutions (1) to (11), the capacitance electrode may have at least a portion thereof formed of a transparent conductive film.
(13) In any one of the constitutions (1) to (12), the pixel electrodes may be formed of a transparent conductive film.
(14) In any one of the constitutions (1) to (13), the second silicon nitride film may be formed of a film which exhibits an etching rate in the vicinity of a surface thereof faster than the etching rate in other portion thereof.
(15) In the constitution (14), a film thickness of a portion of the second silicon nitride film arranged in the vicinity of the surface of the second silicon nitride film which exhibits the etching rate faster than the etching rate of other portion of the second silicon nitride film may be set to a value not less than 5% and not more than 30% of a film thickness of the second silicon nitride film.
(16) In any one of the constitutions (1) to (15), the second silicon nitride film may be a film formed by a plasma CVD method.
(17) In any one of the constitutions (1) to (16), the capacitance electrode may also function as a counter electrode and the liquid crystal may be driven by an electric field generated between the pixel electrode and the counter electrode.
(18) In any one of the constitutions (1) to (16), the second substrate may include a counter electrode and the liquid crystal may be driven by an electric field generated between the pixel electrode and the counter electrode.
(19) A liquid crystal display device including a first substrate including a video signal line, a pixel electrode, a thin film transistor having a first electrode thereof connected to the video signal line and a second electrode thereof connected to the pixel electrode, an organic insulation film formed above the second electrode, a capacitance electrode formed above the organic insulation film, and a silicon nitride film formed above the capacitance electrode and below the pixel electrode, a second substrate arranged to face the first substrate in an opposed manner, and liquid crystal sandwiched between the first substrate and the second substrate, wherein the silicon nitride film is a film which is formed at a temperature lower than a heat-resistant temperature of the organic insulation film after the formation of the organic insulation film, and a potential different from a potential applied to the pixel electrode is applied to the capacitance electrode, and a holding capacitance is formed by the pixel electrode, the silicon nitride film and the capacitance electrode.
(20) In the constitution (19), the silicon nitride film may be a film formed by a plasma CVD method.
(21) A liquid crystal display device including a first substrate including a video signal line, a pixel electrode, a thin film transistor having a first electrode thereof connected to the video signal line and a second electrode thereof connected to the pixel electrode, an organic insulation film formed above the thin film transistor, a reflection film formed above the organic insulation film, a silicon nitride film formed above the reflection film and below the pixel electrode, a second substrate arranged to face the first substrate in an opposed manner, and liquid crystal sandwiched between the first substrate and the second substrate, wherein the organic insulation film has a surface unevenness on a portion thereof corresponding to the reflection film, the reflection film has a surface unevenness which reflects the surface unevenness of the organic insulation film, the silicon nitride film is formed at a temperature lower than a heat-resistant temperature of the organic insulation film after the formation of the organic insulation film, and a height of the surface unevenness of the organic insulation film between a crest and a valley is 0.3 μm or less.
(22) In the constitution (21), the silicon nitride film may be a film formed by a plasma CVD method.
Here, the above-mentioned constitutions are exemplified only as examples and the present invention can be suitably modified without departing from a technical concept of the present invention. Further, examples of the constitution of the present invention besides the above-mentioned constitutions will become apparent from the description of the whole specification or drawings.
To explain typical advantageous effects of the present invention, they are as follows.
According to the present invention, it is possible to form a holding capacitance which exhibits a large capacitance.
Further, the present invention can prevent damages from being applied to the source electrode.
Further, according to the present invention, the process can be simplified.
Further, according to the present invention, it is possible to ensure the electric connection in the contact hole.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view for explaining a liquid crystal display device of an embodiment 1 of the present invention and also is a cross-sectional view taken along a line A-A′ in FIG. 2;
FIG. 2 is a plan view for explaining the liquid crystal display device of the embodiment 1 of the present invention;
FIG. 3 is a view for explaining the liquid crystal display device of the embodiment 1 of the present invention, and also is a cross-sectional view taken along a line B-B′ in FIG. 2;
FIG. 4A to FIG. 4F are views for explaining a manufacturing method of the liquid crystal display device of the embodiment 1 of the present invention;
FIG. 5 is a view for explaining a liquid crystal display device of an embodiment 2 of the present invention, and also is a view which corresponds to FIG. 1;
FIG. 6A to FIG. 6C are views for explaining a liquid crystal display device of an embodiment 3 of the present invention, and also are cross-sectional views showing a contact hole portion in an enlarged manner; and
FIG. 7 is a cross-sectional view of an essential part of a liquid crystal display device for showing one example of a holding capacitance described in patent document 1.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are explained in conjunction with drawings.
[Embodiment 1]
In an embodiment 1, the explanation is made with respect to one example in which the present invention is applied to an IPS liquid crystal display device.
FIG. 1 is a view for explaining a liquid crystal display device of the embodiment 1 of the present invention and also is a cross-sectional view taken along a line A-A′ in FIG. 2. FIG. 2 is a plan view for explaining the liquid crystal display device of the embodiment 1 of the present invention. FIG. 3 is a view for explaining the liquid crystal display device of the embodiment 1 of the present invention and also is a cross-sectional view taken along a line B-B′ in FIG. 2.
As shown in FIG. 1 to FIG. 3, liquid crystal LC is sandwiched between a transparent insulation substrate (first substrate) SUB 1 made of a glass or the like and a transparent insulation counter substrate (second substrate) SUB2 made of a glass or the like.
The liquid crystal display device according to the present invention is an active matrix type liquid crystal display device in which a plurality of pixels are arranged in a matrix array. The substrate SUB1 includes a plurality of scanning signal lines GL and a plurality of video signal lines DL which intersect the plurality of scanning signal lines GL. In the vicinity of each intersection, a thin film transistor is arranged as a switching element of the pixel. In FIG. 2, one pixel out of a plurality of pixels which are arranged in a matrix array is shown.
On the substrate SUB1, in order from below, a base film UC, a semiconductor film PS made of poly-silicon or the like, a gate insulation film GI, gate electrodes GT of the thin film transistors, and an interlayer insulation film IN1 are formed. On the interlayer insulation film IN1, drain electrodes (first electrodes) SD1 of the thin film transistors and source electrodes (second electrodes) SD2 of the thin film transistors are formed. Here, there may be a case that SD1 may be used to refer to the source electrodes. In such a case, SD2 may be used to refer to the drain electrodes. In this specification, SD1 is used to refer to the drain electrodes.
The gate electrodes GT are integrally formed with the scanning signal line GL. One portion of the video signal line DL also functions as the drain electrode SD1 thus providing the structure in which the video signal line DL and the drain electrode SD1 are connected with each other. The drain electrode SD1 is connected to the drain region of the thin film transistor via the contact hole CH1 which is formed in the gate insulation film GI and the interlayer insulation film IN1. The source electrode SD2 is connected to a source region of the thin film transistor via a contact hole CH2 formed in the gate insulation film GI and the interlayer insulation film IN1.
On the drain electrodes SD1 and the source electrodes SD2, an interlayer insulation film IN2 is formed. On the interlayer insulation film IN2, an organic insulation film PAS is formed. On the organic insulation film PAS, counter electrodes CT and a reflection film RAL are formed. On the counter electrodes CT and the reflection film RAL, an interlayer insulation film IN3 is formed. On the interlayer insulation film IN3, pixel electrodes PX are formed. The pixel electrodes are connected to the source electrodes SD2 of the thin film transistors via contact holes CH3 which are formed in the interlayer insulation film IN2 and the interlayer insulation film IN3.
On the pixel electrodes PX, an orientation film ORI1 is formed. Further, on a side of the substrate SUB1 opposite to the liquid crystal LC, a polarizer POL1 is arranged. Here, the orientation film ORI1 and the polarizing plate POL1 are omitted from FIG. 1.
On the counter substrate SUB2, a black matrix BM, color filters CF, an overcoat film OC, an orientation film ORI2 are formed. Further, a polarizer POL2 is arranged on a side of the counter substrate SUB2 opposite to the liquid crystal LC.
A retardation plate or a coated retardation layer may be arranged on at least one of the substrate SUB1 and the counter substrate SUB2 when necessary.
In this embodiment, the pixel electrode PX includes a linear portion and is formed into a comb-teeth shape. The counter electrode CT is formed in a planar shape. Here, the liquid crystal LC is driven by an electric field which is generated between the pixel electrode PX and the counter electrode CT to perform a display.
The pixel electrode PX and the counter electrode CT are formed of a transparent conductive film such as an ITO film, for example. The reflection film RAL is formed in one region within one pixel. Due to such a constitution, it is possible to perform a transflective (partially transmissive) display which includes a transmissive region and a reflection region in one pixel. Accordingly, it is possible to perform a transmissive display by making use of light from a backlight not shown in the drawing under a dark environment, while it is possible to perform a reflective display by making use of an external light under a bright environment. The reflection film RAL has, for example, the two-layered structure which has a lower layer thereof made of molybdenum tungsten alloy (MoW) and an upper layer thereof made of an aluminum silicon alloy (AlSi) containing 1% of Si, and the reflection film RAL is connected to the counter electrode CT. When it is necessary to change a thickness of the liquid crystal LC in the transmissive region and a thickness of the liquid crystal in the reflection region, for example, it is preferable to form a stepped-portion forming layer not shown in the drawing on the counter substrate SUB2.
A common potential different from a potential applied to the pixel electrodes PX is applied to the counter electrode CT (and the reflection film RAL). Accordingly, the holding capacitance is formed by the counter electrode CT (and the reflection film RAL), the pixel electrodes PX and the interlayer insulation film IN3. That is, the counter electrode CT (and the reflection film RAL) also functions as a capacitance electrode. Here, when a silicon nitride film is used as the interlayer insulation film IN3, compared to a case in which a coated insulation film IN3A is used as the interlayer insulation film IN3 which is explained in conjunction with FIG. 7, or a case in which a silicon oxide film is used as the interlayer insulation film IN3, a high dielectric constant can be obtained. Accordingly, it is possible to increase the holding capacitance. Further, by forming the pixel electrodes PX and the counter electrode CT using a transparent conductive film, it is possible to form a transparent holding capacitance and hence, a numerical aperture at the time of performing a transmissive display can be increased.
The interlayer insulation film IN2 may preferably be a silicon nitride film which is formed by a plasma CVD method. It is desirable that the interlayer insulation film IN2 is formed at a high temperature to obtain a dense film. The organic insulation film PAS may preferably be formed using a photosensitive acrylic resin or the like, for example. With the use of the organic insulation film PAS, it is possible to increase the flatness compared a case in which an inorganic insulation film is used as the interlayer insulation film IN2. Further, the organic insulation film PAS having a large thickness can be easily formed and hence, it is possible to decrease the parasitic capacitance. Further, by making use of a halftone exposure when necessary, the surface unevenness may be partially formed on the organic insulation film PAS easily.
The interlayer insulation film IN3 is formed above the organic insulation film PAS. The organic insulation film PAS generally has comparatively low heat resistance and hence, in this embodiment, the interlayer insulation film IN3 is formed by a plasma CVD method at a temperature lower than a forming temperature of the interlayer insulation film IN2. To make the dielectric constant high, a silicon nitride film is adopted as the interlayer insulation film IN3. The interlayer insulation film IN3 is formed at a low temperature and hence, the interlayer insulation film IN3 is not as dense as the interlayer insulation film IN2. However, due to the provision of the dense interlayer insulation film IN2, there arises no problem in practical use in the protection of the thin film transistor.

## Claims

1. A display device comprising:
a first substrate including a video signal line, a first electrode, a second electrode, a thin film transistor having a third electrode thereof connected to the video signal line and a fourth electrode thereof connected to the first electrode, a first silicon nitride film formed above the fourth electrode, an organic insulation film formed above the first silicon nitride film, a second substrate arranged to face the first substrate in an opposed manner,
wherein the second electrode is formed above the organic insulation film,
a second silicon nitride film is formed above the second electrode and below the first electrode,
the organic insulation film has a first contact hole,
the first silicon nitride film and the second silicon nitride film form a second contact hole therein which penetrates both of the first silicon nitride film and the second silicon nitride film,
the first electrode and the fourth electrode are connected to each other via the first contact hole and the second contact hole,
the second silicon nitride film is not in contact with the fourth electrode,
wherein the first contact hole has a side wall which consists of the organic insulation film, and
at least a part of the side wall is in contact with the second silicon nitride film.

2. A display device according to claim 1, wherein the whole of the side wall is in contact with the second silicon nitride film.

3. A display device according to claim 1, wherein the second silicon nitride film has a lower surface,
at least apart of the lower surface is in contact with an upper surface of the first silicon nitride film.

4. A display device according to claim 3, wherein the part of the lower surface is in contact with the upper surface over the whole circumference of the first contact hole.

5. A display device according to claim 3, wherein an end portion of the lower surface is substantially aligned with an end portion of the upper surface.

6. A display device according to claim 3, wherein an end portion of the lower surface is retracted from an end portion of the upper surface.

7. A display device according to claim 1, wherein the second contact hole therein which penetrates both of the first silicon nitride film and the second silicon nitride film collectively.

8. A display device according to claim 1, wherein the first silicon nitride film is more dense than the second silicon nitride film.

9. A display device according to claim 1, wherein the first silicon nitride film has higher crystallinity than the second silicon nitride film.

10. A display device according to claim 1, wherein a molecular density of the first silicon nitride film is larger than a molecular density of the second silicon nitride film.

11. A display device according to claim 1, wherein the device further comprises a plurality of pixels, each of the pixels having a pixel electrode,
the first electrode is the pixel electrode.

12. A display device according to claim 11, wherein the second electrode is a counter electrode.

13. A display device according to claim 1, wherein a potential different from a potential applied to the first electrode is applied to the second electrode,
a holding capacitance is formed by the first electrode, the second silicon nitride film and the second electrode.

14. A liquid crystal display device according to claim 12, wherein a potential different from a potential applied to the pixel electrode is applied to the counter electrode,
a holding capacitance is formed by the pixel electrode, the second silicon nitride film and the counter electrode.