JPH0728090A - Liquid crystal display device and its production - Google Patents

Abstract

PURPOSE:To reduce the thickness of the insulating film of a holding capacitance and to lower parasitic resistance so as to reduce the area of the holding capacitance and to improve frequency characteristics by constituting a MIM capacitance specified in a lower electrode, insulating film and upper electrode as the holding capacitance of a signal line driving circuit. CONSTITUTION:The MIM type holding capacitance in a part where a thin-film transistor(TFT) 3 for video signal writing and the holding capacitance 4 is integrally formed is composed of the lower electrode 41 metallized by doping with the same impurity as the impurity of the semiconductor active layer 31 of the TFT 3, the same insulating film 43 as the gate insulating film 34 of the TFT 3 and the upper electrode 42 of the same layer as the layer of the gate 35 of the TFT 3. Namely, the lower electrode 41 is formed to have an area larger than the area of the upper electrode 42. The source 33 and drain 32 of the TFT are simultaneously doped to make the part 41a of the lower electrode 41 lower in resistance than the central part. Therefore, the parasitic resistance of the lower electrode 41 is lowered and the frequency characteristics are improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an active matrix type liquid crystal display device and a method of manufacturing the same, and more particularly to a signal line drive circuit integrated with a drive circuit.

[0002]

2. Description of the Related Art A liquid crystal display device for displaying characters and figures is a matrix type liquid crystal display in which a large number of regularly arranged address wiring electrodes and data wiring electrodes are crossed and each crossed section is a pixel. The device is heavily used. Further, in the case of aiming at a higher-capacity and higher-definition display, an active matrix type liquid crystal display device having a driving switching element at an intersection of a gate line and a source line corresponding to an address wiring electrode and a data wiring electrode is used. ing. As such a switching element, a thin film transistor is used because it has excellent high-speed response and is suitable for full-color display.

Further, as shown in FIG. 3, a conventional signal line scanning drive circuit for sequentially driving thin film transistors includes a video signal sample and hold circuit in which a video signal writing thin film transistor and a sample and hold capacitor are integrated. Are formed on the substrate. That is, the thin film transistor portion is, for example, a semiconductor active layer made of polycrystalline silicon.
31, a source electrode 32 on both sides thereof, a drain electrode 33, a gate insulating film 34, a gate electrode 35, an interlayer insulating film layer 44, a display signal line 2 (gate line) and a signal line 7 (source line). . In addition, the sample and hold capacitor part is
41, the insulating film 43 and the upper electrode 42 are composed of MIM type capacitors.

The lower electrode 41 of the sample and hold capacitor portion is made of the same material as the drain electrode 35 of the thin film transistor portion, the insulating film 43 is made of the same material as the interlayer insulating film layer 44, and the upper electrode 42 is made of the same material as the display signal line 2. It is composed of a capacitor formed by.

[0005]

In the MIM type capacitor of the sample hold circuit integrated with the driving circuit as shown in FIG. 3, the interlayer insulating film layer 44 of the thin film transistor needs to have a constant film thickness. . Therefore, since the MIM type capacitor insulating film 43 having the same thickness as this is too thick,
A large area is required to form the hold capacitor.
This not only hinders the miniaturization of the liquid crystal display device, but also causes a problem that the yield is likely to decrease due to defects such as pinholes.

As an improvement to this problem, Japanese Patent Laid-Open No. 62-
As disclosed in Japanese Patent No. 178296, a metal-oxide film-semiconductor layer (MO layer having the same structure as that of a thin film transistor for holding capacitance) is formed.
Proposals have been made to reduce the area of the capacitance portion and improve the yield by using the S) type capacitance.

However, in the case of this MOS type capacitor,
One of the electrodes of the MOS type capacitor serves as a semiconductor layer, which naturally has a large sheet resistance. That is, since the parasitic resistance is large and the capacitor is connected in series with the capacitor, the frequency characteristic is poor, the impedance becomes high in a high frequency region, and the charging of the voltage to the capacitor portion becomes insufficient. Therefore, in operation, a predetermined voltage is applied while the thin film transistor for writing the video signal is on, but the thin film transistor is turned off before the capacitor is sufficiently charged, and the desired voltage is held. There is a problem that the capacity function cannot be fulfilled.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a small-sized and high-performance liquid crystal display device integrated with a drive circuit, which secures a small area of a hold capacitor and frequency characteristics. .

[0009]

According to the present invention, a large number of gate lines and source lines regularly arranged on a substrate, and switching thin film transistors provided at intersections of the gate lines and source lines are provided. In a liquid crystal display device including at least a signal line scanning drive circuit and a gate line scanning drive circuit for sequentially driving thin film transistors, the signal line drive circuit includes a source electrode, a drain electrode, a semiconductor active layer, a gate insulating layer, and a gate electrode. At least a sample hold circuit including a thin film transistor for writing a video signal and a MIM type capacitor including a lower electrode, an insulating film and an upper electrode is provided. And an insulating film of the MIM-type capacitor is a thin film transistor. Is a liquid crystal display device made of a gate insulating layer material of a transistor, the upper electrode of the MIM type capacitor is made of a gate electrode material of the thin film transistor, and the signal line driving circuit is a source electrode, a drain electrode, a semiconductor active layer, a gate. A thin film transistor for video signal writing, which includes at least an insulating layer and a gate electrode, and a sample hold circuit including a MIM-type capacitor including a lower electrode, an insulating film, and an upper electrode. Forming a lower electrode at the same time, forming a gate insulating layer of the thin film transistor and an insulating film of the MIM type capacitor at the same time, and forming a gate electrode of the thin film transistor and an upper electrode of the MIM type capacitor at the same time. The process of film formation and the source and drain parts of the thin film transistor Objects to be doped is a manufacturing method of a liquid crystal display device including at least a step of forming source and drain electrodes.

[0010]

According to the present invention, not only a large holding capacitance can be secured in a small area, but also the sheet resistance of the lower electrode of the MIM type capacitance can be lowered, so that the parasitic resistance is small and the frequency characteristic is improved. Can be done.

Since the magnitude of the capacitance is proportional to the area of the insulating film and inversely proportional to the thickness thereof, the insulating film may be made as thin as possible in order to obtain a large capacitance in a small area. Of course, there must be no pinholes. In the case of an active matrix type liquid crystal display device integrated with a drive circuit, it is considered good to use the gate insulating film material of the thin film transistor of the switching element also as the insulating film of the hold capacitor. However, this method has the following problems It is difficult to convert.

That is, when a hold capacitor is formed by simultaneously forming an insulating film of a hold capacitor when forming a gate insulating film of a thin film transistor, a MOS capacitor having a semiconductor layer of the thin film transistor as a lower electrode of the hold capacitor should be formed. become. The sheet resistance of this semiconductor layer can be changed by the impurity concentration and film thickness at the time of film formation, the applied voltage, etc., but when used as an active layer of a thin film transistor, the sheet resistance becomes several tens kΩ or more. On the other hand, the hold capacitance is about several pF, and the frequency characteristic is several MHz.
The above is necessary. Therefore, it is practically impossible to satisfy the requirement for the hold capacitor while satisfying the requirement for the active layer of the thin film transistor.

However, the resistance value of the semiconductor layer can be lowered depending on the impurity concentration during film formation. FIG. 4 shows the relationship between the impurity concentration and the resistivity when the thin film silicon is doped with impurities and activated at 700 degrees. The film thickness of the semiconductor layer is about 0.1 μm, but as is clear from FIG. 4, the sheet resistance changes from several hundred MΩ to several tens MΩ. Although it depends on the activation rate, the sheet resistance value can be largely changed by changing the impurity concentration even with the same film thickness.

Therefore, when the semiconductor active layer of the thin film transistor is formed, the lower electrode preparation layer of the holding capacitor is formed at the same time, and impurities are implanted in the lower electrode preparation layer to sufficiently reduce the sheet resistance to form the lower electrode. Then, by forming an insulating film of a hold capacitor at the same time as forming the gate insulating film of the thin film transistor on the lower electrode, it is possible to form a hold capacitor having a large area and a small parasitic resistance with a small area.

When the gate insulating film of the thin film transistor is formed by oxidation by the thermal oxidation method, the oxidation rate will vary depending on the concentration, so it is necessary to pay sufficient attention to the setting conditions. However, this is not necessary when forming in a low temperature process such as the CVD method.

If the sheet resistance of the lower electrode of the hold capacitor cannot be reduced sufficiently, it is necessary to further reduce the parasitic resistance. That is, for example, it is also effective to form the area of the lower electrode of the hold capacitor larger than the area of the upper electrode. By doing this, a portion having a larger area than the upper electrode of the lower electrode, that is, the exposed portion of the lower electrode,
Since impurities are implanted in the same way as the source and drain electrodes of the thin film transistor, the sheet resistance of this portion is lowered, and the parasitic resistance of the entire hold capacitance is also lowered.

Further, dividing the holding capacitance and connecting them in parallel is very effective in increasing the manufacturing yield. Since the hold capacitor portion requires a predetermined area, it is difficult to form an insulating film without pinholes, and it is inevitable that the manufacturing yield is reduced accordingly. Therefore, if the hold capacitor is divided and connected in parallel and a defect occurs due to the pinhole in the insulating film,
By disconnecting this defective hold capacitance part,
It is possible to prevent a decrease in manufacturing yield.

[0018]

Embodiments of the present invention will be described in detail below with reference to FIG. FIG. 1A shows a schematic configuration including a signal line scanning drive circuit and a thin film transistor as a switching element of an active matrix liquid crystal display device, and FIG. 1B shows a video signal writing thin film transistor of a signal line scanning drive circuit and an MIM. The schematic configurations of the mold sample-hold capacitors are shown below.

In FIG. 1A, the display signal line 2 is connected to the drain electrode of the video signal writing thin film transistor 3 to transmit the display signal. The output terminal of the scanning circuit 1 is connected to the gate electrode of the thin film transistor 3 for writing the video signal, and the output pulse from the scanning circuit 1 turns on the thin film transistor 3 for a certain period to sample the display signal and connect it to the source electrode of the thin film transistor 3. The captured signal line 7 is loaded. The hold capacitor 4 is connected to the source electrode of the thin film transistor 3 in parallel with the signal line 7. A constant potential is supplied to the hold capacitor 4 from the constant potential line 5.

The signal line 7 is connected to the source electrode of the thin film transistor 8 of the liquid crystal cell 9,
Is connected to the gate line of each pixel, and a signal is written in the liquid crystal cell of each pixel. 8 is the capacity of each liquid crystal cell.

FIG. 1B shows a schematic structure of a portion in which the video signal writing thin film transistor 3 and the hold capacitor 4 of FIG. 1A are integrally formed. Figure 1
In (B), the MIM type sample and hold capacitance is Si
In the semiconductor layer made of a thin film of, for example, a lower electrode 41 doped with P to reduce its resistance, an insulating film 43 made of SiO2 formed by a CVD method, and a polycrystalline Si doped with P as an impurity.
The upper electrode 42 is made of a thin film.

The lower electrode 41 of the hold capacitor is formed simultaneously with the semiconductor active layer 31 of the switching thin film transistor of the sample hold circuit. Further, one end of the lower electrode 41 of the hold capacitor is connected to one end of the source electrode of the thin film transistor. Further, the insulating film 43 of the hold capacitor is formed simultaneously with the gate insulating film 34 of the thin film transistor, and the upper electrode 42 of the hold capacitor is formed simultaneously with the gate electrode 35 of the thin film transistor.

After the upper electrode 42 of the hold capacitor is formed, impurities are injected into the source 33 and the drain 32 of the thin film transistor, the interlayer insulating film 44 is formed, the contact holes of the source electrode and the drain electrode are formed, and the display signal line is formed. 2. The electrode wiring of the signal line 7 is formed by a usual method.

Here, the lower electrode 41 of the hold capacitor has the same problem as the MOS type capacitor when the P doping amount is too small. Therefore, depending on the activation rate of impurities, 5 × 10 ¹⁷
/ Cm ³ or more is required, preferably 5 × 10 ¹⁸ / cm
It should be ³ or more.

The area of the lower electrode 41 is larger than that of the upper electrode 42. That is, the circumference of the lower electrode 41 is formed larger than the upper electrode 42 by about 2 μm.
Therefore, the protruding portion of about 2 μm is simultaneously doped in the process of doping the source 33 and the drain 32 of the thin film transistor, and the cross-hatched portion 41 a of FIG. 1B is lower than the central portion of the lower electrode 41. Has been made resistant. With such a configuration, the parasitic resistance of the lower electrode 41 of the hold capacitor can be further reduced, and the frequency characteristic can be improved accordingly.

The gate insulating film 34 of the thin film transistor and the insulating film 43 of the holding capacitor are formed by the CVD in the above embodiment.
Although the film is formed by the method, the following precautions are necessary when forming the film by forming the thermal oxide film of the semiconductor layer. That is, since impurities such as P are implanted into the portion forming the hold capacitor, it is necessary to pay attention to the thermal oxidation rate thereof as compared with the active layer portion of the thin film transistor. That is, as shown in FIG. 5, from the characteristics of the doping amount and the oxidation rate when P is doped and thermally oxidized, the thermal oxidation rate is very high when doping of 1 × 10 ²⁰ / cm ³ or more is performed. You can see.

Therefore, when the insulating film 43 of the hold capacitor is formed by the thermal oxidation method, it should be noted that the thermal oxidation rate greatly changes corresponding to the impurity concentration, and the capacitance value also changes.
It is necessary to appropriately select the sheet resistance value and capacitance value of the insulating film doped with impurities.

Regarding the examination of the sheet resistance value and the capacitance value of the insulating film as described above, the area of the lower electrode 41 is formed larger than the area of the upper electrode 42 as in the above embodiment, and the sheet resistance of the protruding portion is formed. Although the configuration for reducing the is also effective, the configuration as shown in FIG. 2 is also effective. That is, the thickness of the lower electrode 41 of the hold capacitor may be made larger than the thickness of the active layer 31 of the thin film transistor to reduce the sheet resistance value of the lower electrode 41 of the hold capacitor. In addition, in FIG.
The same components as in (B) are indicated by the same numbers. Although various methods can be selected as such a forming method, for example, only the lower electrode portion of the holding capacitor can be formed into two layers, or can be formed by two types of masks and etching.

Further, by disposing a plurality of such holding capacitors and connecting them in parallel (not shown), not only the manufacturing yield is improved but also the capacitance value can be increased without increasing the parasitic resistance. it can. Therefore, the hold capacitance can be increased without lowering the frequency characteristics and the manufacturing yield.

[0030]

As described above, according to the present invention, the lower electrode is doped with the same impurities as the semiconductor active layer of the thin film transistor as a hold capacitance of the signal line drive circuit integrated with the drive circuit of the active matrix type liquid crystal display device. And a metallized layer, the insulating film is the same insulating film as the gate insulating film of the thin film transistor, and the upper electrode is the same layer as the gate electrode of the thin film transistor. In addition, since the parasitic resistance can be reduced, the area of the hold capacitor can be reduced and the frequency characteristic can be improved.

[Brief description of drawings]

1A and 1B show an embodiment of the present invention, in which FIG. 1A shows a schematic structure including a signal line scan driver circuit of an active matrix liquid crystal display device and a thin film transistor as a switching element, and FIG. 8A and 8B show schematic configurations of a thin film transistor for writing a video signal and a MIM type sample hold capacitor of a signal line scanning drive circuit, respectively.

FIG. 2 is a schematic configuration diagram showing another embodiment of the present invention, corresponding to FIG.

FIG. 3 is a schematic configuration diagram corresponding to the conventional FIG. 1 (B).

[Fig. 4] Thin film silicon is doped with impurities and 70
FIG. 4 is a characteristic diagram showing the relationship between the impurity concentration and the resistivity when activated at 0 degrees.

FIG. 5 is a characteristic diagram showing a relationship between a doping amount and an oxidation rate when a semiconductor layer is doped with P and thermally oxidized.

[Explanation of symbols]

 2 ... Display signal line 5 ... Constant potential line 7 ... Signal line 31 ... Thin film transistor semiconductor active layer 32 ... Thin film transistor drain electrode 33 ... Thin film transistor source electrode 34 ... Thin film transistor gate insulating film 35 ... Thin film transistor gate electrode 41 ... Hold capacitance Lower electrode 42 ... Hold capacitor upper electrode 43 ... Hold capacitor insulating film 44 ... Interlayer insulating film

Claims (2)

[Claims] 1. A plurality of gate lines and source lines regularly arranged on a substrate, switching thin film transistors provided at intersections of the gate lines and source lines, and a signal for sequentially driving the thin film transistors. In a liquid crystal display device including at least a line scan drive circuit and a gate line scan drive circuit, the signal line drive circuit includes a source electrode, a drain electrode, a semiconductor active layer, a gate insulating layer, and a thin film transistor for writing a video signal. And a sample hold circuit including a lower electrode-insulating film-upper electrode and a MIM-type capacitor. The lower electrode of the MIM-type capacitor includes a metallized layer obtained by doping a semiconductor active layer material of the thin film transistor with impurities. The insulating film of the MIM type capacitor is made of the gate insulating layer material of the thin film transistor. The electrode on the MIM type capacitance liquid crystal display device, comprising the gate electrode material of the thin film transistor. 2. A plurality of gate lines and source lines regularly arranged on a substrate, switching thin film transistors provided at intersections of the gate lines and source lines, and a signal for sequentially driving the thin film transistors. In a method of manufacturing a liquid crystal display device including at least a line scan drive circuit and a gate line scan drive circuit, the signal line drive circuit includes a video signal including at least a source electrode, a drain electrode, a semiconductor active layer, a gate insulating layer, and a gate electrode. M composed of a thin film transistor for writing, and a lower electrode-insulating film-upper electrode
At least a sample hold circuit including an IM type capacitor is provided, and the semiconductor active layer of the thin film transistor and the M
Forming a lower electrode of the IM type capacitor at the same time, forming a gate insulating layer of the thin film transistor and an insulating film of the MIM type capacitor at the same time, and forming a gate electrode of the thin film transistor and the MIM type A method of manufacturing a liquid crystal display device, comprising at least a step of simultaneously forming a film of an upper electrode of a capacitor and a step of doping a source and a drain portion of the thin film transistor with impurities to form a source and a drain electrode. .