JP2001083549A - Liquid crystal display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve picture quality by composing a display part of plural pixels, providing each pixel with at least one thin film transistor, and driving the display part by a driving circuit part. SOLUTION: External signals such as a microprocessor are inputted to a control circuit. Moreover, display data (character data) from a memory storing display information, a character generator, or the like are controlled for management by a control circuit, and displayed on a display part through a scanning side driving circuit and a data side driving circuit. Then, a driving circuit of a display device is divided into a scanning circuit 8 and a signal circuit, and the signal circuit is comprised of a multiplexer 9, a partition matrix switch 10, a fast shift register 11 externally attached to a substrate 4. The pixels of a display part 5 consist of TFTs 20 as active elements, and capacitors 19 comprising a liquid crystal as a display medium and pixel electrodes, and are arranged in a matrix form.

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 an active matrix type display device using a liquid crystal or the like.

[0002]

In recent years, such as the display using a liquid crystal display, for driving the liquid crystal of each pixel, the thin film transistors for each pixel (T hin F ilm T ransistor: short T
Active matrix to form a FT) (A ctive M atri
x : AMX) system is used for short. Since a glass substrate is usually used as the display substrate, the process temperature for manufacturing the display is about 6 ° C.
Limited to 40 ° C or less. For this reason, TFT
The active layer is made of polycrystalline silicon (PolycrystallineSilicon:
When it is formed in Poly-Si for short, low pressure CVD (L
The deposition temperature of Poly-Si by the PCVD method or the like is also restricted. Attempts have been made to increase the crystallinity of Poly-Si, increase the particle size of Poly-Si, and increase the carrier movement of the TFT by raising the deposition temperature of the film to near the maximum process temperature under this restriction. For this example,
Nikkei Electronics 1984.9.10 P211 (deposition temperature 600 ° C), 33rd Proceedings of the Japan Society for Materials Science (spring 1986) P544 (deposition temperature 610 ° C) Japan Display Tech
Digest (1986) 3, 5 (deposition temperature: 630 ° C.). Judging these Poly-Si films from the deposition temperature (J. Electrochem. Soc, 127, 686 (198)
0), 131, 676 (1984))
It can be seen that 10 ° is the main orientation.

[0003]

The conventional AMX type display device has several problems because the carrier mobility is still insufficient. The first problem is that it takes a long time for the address time of the peripheral drive circuit formed on the same substrate as the display section. For this reason, the number of pixels of the display unit could not be increased much, and the image quality was not always satisfactory. The second problem is that since the size of the TFT in the display unit cannot be reduced so much, the aperture ratio does not increase and the image quality is not sufficient from this.

An object of the present invention is to provide a high-performance and high-quality liquid crystal display device.

[0005]

According to the liquid crystal display device of the present invention, a plurality of thin film semiconductor elements arranged in a matrix on a glass substrate and each of the plurality of thin film semiconductor elements are connected to each other. It has a plurality of pixel electrodes. Usually, on a glass substrate, an active layer of a thin film semiconductor device is # 1.
A polycrystalline silicon film having a main orientation of 11 ° is formed.

First, Poly with {111} as the main orientation
The reason why -SiTFT has a higher carrier mobility than Poly-SiTFT of other orientations will be described. Figure 2 shows Poly-S
The depletion layer formed near the grain boundary of i and the band configuration are shown. FIG. 2A shows Poly-Si of {111} orientation, and FIG.
(b) shows Poly-Si of another orientation. The surface charge density at the interface between single crystal silicon and oxide film is {100},
It is known that the crystal orientation increases in the order of {110} and {111} (Appl. Phys. Lett. 8, 31 (1966)).
reference). This concept of charge density applies not only to the interface between the Poly-Si surface and the gate oxide film, but also to the crystal grain boundaries because oxygen in Poly-Si segregates at the crystal grain boundaries. Therefore, the direction perpendicular to the substrate (the vertical direction in FIG. 2)
Then, the depletion layer formed near the grain boundary is {100} oriented,
The orientation becomes relatively wide in the order of 10 ° orientation and {111} orientation.
Conversely, in the carrier traveling direction (the horizontal direction in FIG. 2), the depletion layer formed near the grain boundary has {100} orientation, {11}.
The orientation becomes relatively narrow in the order of 0 orientation and {111} orientation. Therefore, the potential barrier generated at the grain boundary becomes relatively lower in the order of {100} orientation, {110} orientation, and {111} orientation in the carrier traveling direction. The mobility of poly-Si carriers is determined by the height of a potential barrier generated near a grain boundary. Therefore, Poly-Si TFT with {111} as the main orientation
Indicates that the mobility of carriers is relatively higher than that of Poly-Si TFTs of other orientations.

As described above, the carrier mobility is large.
Using a Poly-Si TFT with 11 ° as the main orientation,
When the display portion is formed, the number of pixels of the display portion can be increased.

In addition, Poly with {111} as the main orientation
The aperture ratio can be increased by using the -Si TFT for the active matrix of the display unit.

[0009]

FIG. 6 shows a basic structure of a liquid crystal display (LCD).

A signal from an external device such as a microprocessor (not shown) is input to the control circuit 104. Display data (character data) from a memory 105 for storing display information, a character generator (not shown), and the like are managed and controlled by a control circuit 104, and are passed through a scanning drive circuit 103 and a data drive circuit 102 to a display unit. Displayed at 101. [Embodiment 1] A description will be given of a point that the number of pixels of the display unit 5 can be increased by forming a peripheral driving circuit using a Poly-Si TFT having a large carrier mobility of {111} having a main orientation as described above. As shown in FIG. 1, the driving circuit of the display device is generally divided into a scanning circuit 8 and a signal circuit, and the signal circuit includes a multiplexer 9, a divided matrix switch 10, and a high-speed shift register 11 externally attached to the substrate 4.
Consists of The pixels of the display unit include a TFT 20 as an active element,
A capacitor 1 composed of a liquid crystal as a display medium and a pixel electrode
9 and are arranged in a matrix as shown. The number of pixels of the display unit 5 is mainly determined by the characteristics of the multiplexer 9 and the divided matrix switch 10 of the signal circuit. Write time T per pixel viewed from the signal side
ad is

[0011]

(Equation 1)

## EQU1 ## Here, f _F is the frame frequency (normally 60 Hz), and N is the number of lines on the signal side.
Tad is the TFT characteristic of the signal circuit and is usually about 10 μse
c. Here, Poly− with {111} as the main orientation
When a circuit is formed using a SiTFT, Tad can be reduced to 1 μsec or less because the carrier mobility is large and the ON characteristics are excellent. Therefore, it is possible to increase the number N of lines on the signal side by one digit or more. The condition of the scanning circuit 8 is not as severe as that of the signal circuit, but the clock frequency fcp is a measure of the circuit characteristics.

[0013]

[Expression 2] fcp = f _F × M (Expression 2) Here, M is the number of scanning lines. fcp
Is usually about 10 KHz, but if a Poly-Si TFT having {111} as the main orientation is used, fcp can be changed to MHz.
It is possible to raise to the order. Therefore, the number of lines on the scanning side can be increased by two digits or more. As described above, the number of pixels M × N in the display unit can be increased by three digits or more compared to the conventional method. [Embodiment 2] Poly with {111} as the main orientation
The point that the aperture ratio can be increased by using the -Si TFT for the active matrix of the display portion will be described. The aperture ratio indicates a region where the liquid crystal can be driven by the transparent electrode in the display unit, and is an indicator of the image quality of the display device. The reason why the aperture ratio cannot be increased beyond a certain value is that TFT and Al
This is because an electrode exists on each pixel. The gate width W and gate length L of a TFT are usually 50 μm and 10 μm, respectively.
m. The value of L is determined because the minimum dimension of the process is about 10 μm, and then the gate width is determined in order to obtain a sufficient ON current of the TFT. {11
1) If a poly-Si TFT is used for each pixel, the gate width can be reduced to 20 μm or less while the process dimensions remain unchanged. Decreasing the gate width reduces not only the area of the gate region but also the area of the source and drain regions. Therefore, the aperture ratio is reduced from about 65% to about 75
% Can be increased. Accordingly, the image quality of the display unit is improved. The reduction in the size of the TFT also leads to an improvement in the yield. [Embodiment 3] In this embodiment, a structure and a manufacturing method of a Poly-Si TFT having {111} as a main orientation will be described.

FIG. 3 shows a cross-sectional structure of a Poly-Si TFT having a main orientation of {111} forming the display device shown in FIG. The substrate 4 is a glass plate having a strain temperature of about 640 ° C.

The substrate 4 is kept at 550.degree.
The film 12 is deposited by a low pressure CVD method using a monosilane gas diluted to% as a raw material. The film thickness is 1500 °.
Next, a heat treatment is performed in N ₂ at 600 ° C. for 24 hours. After heat treatment, {111} oriented Poly-Si TFT film 12
Is formed. After the photo-etching step, an SiO ₂ gate insulating film 19 is deposited at 1500 ° by a normal pressure CVD method. Next, a gate electrode is formed. After the photo-etching step, the source and drain regions 13 and 14 are implanted. The conditions are as follows: phosphorus (P) is supplied at a voltage of 30 KeV for 5
A dose of × 10 ¹⁵ cm ^-2 is implanted. Then, Phospho Silicate Glass (PSG) 16 was added to 48
Deposit 5000 ° C at 0 ° C. Further, in N ₂ , 600
A heat treatment is performed for 20 hours at a temperature of ° C. to activate the implant region. After the contact photo-etching step, the Al electrode 17 is sputtered at 6000 °. After the photo-etching process, the transparent electrode ITO (Indium Tit
an Oxyde) is sputtered at 1000 °. After the photo-etching step, the liquid crystal is sealed between the color filter and another glass substrate provided with a polarizing film to complete a display device. The channel width and the channel length of the TFT in the display section of this embodiment are 20 μm and 10 μm, respectively. The number of lines in the matrix of the display unit 5 is 600 × 1980.
The aperture ratio is 75%. In the present embodiment, the TFT 20 for driving the liquid crystal 19 serving as the capacitor in FIG. 1 has been described as an example. However, it is needless to say that such a TFT may be used for a peripheral circuit, for example, the scanning circuit 8. [Embodiment 4] FIG. 4 shows another embodiment of the present invention. In this embodiment, since all the driving circuits such as the scanning circuit 8 and the signal circuit 18 are externally mounted on the substrate 4, the display unit 5 is formed by using a Poly-Si TFT having {111} as a main orientation.
Of the active matrix was formed. This allows
As in the case of Example 2, the aperture ratio could be increased from 65% of the prior art to 75%. [Embodiment 5] FIG. 5 shows another embodiment of the present invention. In this embodiment, Poly-SiTF having {111} as a main orientation
By using T, the high-speed shift register 11 of the signal circuit which has been externally attached to a portion other than the substrate 4 can be incorporated in the same substrate as the display unit 5.

As a result, in the present embodiment, the number of connection terminals could be reduced from the conventional 177 to 38.

Conventionally, the mobility of a TFT is low (small).
Therefore, the high-speed shift register 11 is provided on the display unit 5 on the same substrate.
It is difficult to provide them on the same substrate. [Embodiment 6] This embodiment shows a case where {111} -oriented Poly-Si is used only in a peripheral driving circuit. This includes Poly
It is necessary to apply the LPCVD layer twice by changing the Si deposition temperature. First, an L-shaped quartz plate is used as a mask at a peripheral circuit forming position, and a 60
An LPCVD film is deposited at 1500 ° C. under the conditions of 0 ° C. and 0.6 Torr. Next, a rectangular quartz plate is placed at the TFT forming position of the display section to serve as a mask, and 550 is placed at the peripheral circuit forming position.
An LPCVD film is deposited at 1500 ° C. under the conditions of 0.6 ° C. and 0.6 Torr. Subsequently, when heat treatment is performed at 600 ° C. for 24 hours, the film deposited at 550 ° C. becomes a Poly-Si film having {111} as a main orientation, and the film deposited at 600 ° C. has {111} as a main orientation. To be a Poly-Si film. Subsequent processes are the same as described above. A display in which only such peripheral circuits are oriented {111} has the following features. That is, since the peripheral circuits requiring high-speed operation have the {111} orientation, they can be driven with their dimensions kept small.

Since the TFTs in the matrix portion are {111} oriented, the dimensions cannot be reduced and the aperture ratio cannot be increased, but the off-power supply can be reduced, and the peripheral circuit has a corresponding amount. An operation margin for driving the display portion can be increased.

In addition to the effects described in the above embodiments, the carrier having a high orientation of {111} has a high mobility.
If a Poly-Si TFT is used, it is possible to mix circuits that could not be mixed with a conventional low-mobility Poly-Si on the same substrate, so that the size of the device can be reduced.

In the above-mentioned drawings, the parts denoted by the same reference numerals are parts that perform the same function.

[0021]

According to the present invention, the image quality of a liquid crystal display device can be improved.

[Brief description of the drawings]

FIG. 1 is a plan view of a display device according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing a relationship between orientation of polycrystalline silicon and a band structure.

FIG. 3 is a cross-sectional configuration diagram of a TFT according to one embodiment of the present invention.

FIG. 4 is a plan view of a display device according to another embodiment of the present invention.

FIG. 5 is a plan view of a display device according to still another embodiment of the present invention.

FIG. 6 is a diagram showing an overall configuration of a display device of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Crystal grain, 2 ... Grain boundary, 3 ... Depletion layer, 4 ... Substrate, 5
... Display part, 6 ... Maximum energy position of valence band, 7 ... Maximum energy position of conduction band, 8 ... Scanning circuit, 9 ... Multiplexer, 10 ... Matrix switch, 11 ... High-speed shift register, 12 ... {111} Polycrystalline silicon film to be oriented, 13 ... source, 14 ... drain, 15 ...
Gate electrode, 16 ... phosphor glass, 17 ... Al electrode, 18
.., A signal circuit, 101, a display unit, 102, a data-side driving circuit, 103, a scanning-side driving circuit, 104, a control circuit, and 105, a memory.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01L 29/78 620 (72) Inventor Takaya Suzuki 4026 Kuji-cho, Hitachi City, Hitachi, Ibaraki Prefecture Hitachi, Ltd. Hitachi, Ltd. In the laboratory (72) Inventor Kenji Miyata 4026 Kuji-cho, Hitachi City, Ibaraki Prefecture Hitachi Research Laboratory, Hitachi, Ltd.

Claims (4)

[Claims] 1. A liquid crystal display device having a display portion and a drive circuit portion on a glass substrate, wherein a clock frequency of the drive circuit portion is 1 MHz or more, and the display portion is constituted by a plurality of pixels. A liquid crystal display device in which a pixel includes at least one thin film transistor and drives the display unit with the drive circuit unit. 2. The liquid crystal display device according to claim 1, wherein the drive circuit section has a plurality of thin film transistors, and an active layer of the thin film transistors in the drive circuit section is polycrystalline silicon having {111} as a main orientation. . 3. The display section includes: a plurality of scanning wirings; a plurality of signal wirings intersecting the plurality of scanning wirings; and a thin film transistor formed corresponding to each intersection of these wirings. 3. The liquid crystal display device according to claim 1, comprising a pixel electrode connected to said thin film transistor. 4. The liquid crystal display device according to claim 3, wherein the glass substrate has a strain temperature of about 640 ° C.