JPH10154818A - Liquid crystal indicating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the quality of picture by a method wherein a polycrystalline silicon film, having a plurality of picture element electrodes connected to a plurail of thin film semiconductor elements and also having specific orientation, is formed on a glass substrate as the active layer of the thin film semiconductor element. SOLUTION: A driving circuit is divided into a scanning circuit 8 and a signal circuit, and the signal circuit is composed of a multiplexer 9, a split matrix switch 10, and a high speed shift register 11 which is externally attached to a substrate 4. The picture element of an indication part 5 is composed of a TFT 20 which is an active element, liquid crystal which is an indication medium, and a capacitor 19 consisting of a pixel electrode, and the pixel is arranged in matrix form. The number of pixel of the indication part 5 is determined by the characteristics of the multiplexer 9 of the signal circuit and a split matrix switch 10, and when a circumference driving circuit is formed using a polysilicon TFT having the orientation 111} of large carrier mobility, the number of pixelxs of the display part 5 and be increase.

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 TFT) active matrix to form the (A cti
ve M atri x: short AMX) method is used. Since a glass substrate is usually used as the display substrate, the process temperature for manufacturing the display is limited to about 640 ° C. or less. For this reason, among other things, the active layer of the TFT must be polycrystalline silicon (Polycrystalline silicon).
Silicon: When formed in Poly-Si), the deposition temperature of Poly-Si by low pressure CVD (LPCVD) or the like is also restricted. By raising the film deposition temperature to near the maximum process temperature under this constraint, Poly-
Increase the crystallinity of Si, increase the particle size of Poly-Si,
Attempts have been made to increase FT carrier movement. An example of this is Nikkei Electronics 1984.9.10.
P211 (Deposition temperature: 600 ° C), Proceedings of the 33rd Society for Response Science (Spring, 1986) P544 (Deposition temperature: 610 ° C) Japa
n Display Tech Digest (1986) 3, 5 (deposition temperature: 630 ° C.). Judging these Poly-Si films from the deposition temperature (J. Electrochem. Soc, 1
27,686 (1980), 131,676 (1984)
It is understood that {110} is the main orientation in each case.

[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.

[0006] First, P with {111} as the main orientation
The reason why the poly-Si TFT has a higher carrier mobility than the Poly-Si TFT having another orientation will be described. Figure 2 is Poly
The depletion layer formed near the grain boundary of -Si and the band configuration are shown. FIG. 2A shows Poly-Si of {111} orientation, and FIG. 2B shows Poly-Si of another orientation. The surface charge density at the interface between the single crystal silicon and the oxide film is {100},
It is known that the crystal orientation increases in the order of {110} and {111} (Appl. Phys. Lett. 8, 31 (196)
6)). This concept of the 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, in the direction perpendicular to the substrate (vertical direction in FIG. 2), the depletion layer formed near the grain boundary has {100} orientation,
The orientation becomes relatively wide in the order of {110} orientation and {111} orientation. Conversely, the traveling direction of the carrier (left-right direction in FIG. 2)
Then, the depletion layer formed near the grain boundary is {100} oriented,
The orientation becomes relatively narrow in the order of 10 ° 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, a Poly-Si TFT having {111} as the main orientation
Shows that the mobility of carriers is relatively higher than that of Poly-Si TFTs of other orientations. As described above, P <b> 1, in which {111} having a large carrier mobility is the main orientation,
When a display portion is formed using an oly-Si TFT, the number of pixels in the display portion can be increased.

Further, Pol having {111} as a main orientation
The aperture ratio can be increased by using the y-Si TFT for the active matrix of the display unit.

[0008]

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] Poly-SiT having a main orientation of {111} in which carrier mobility is large as described above
The point that the number of pixels of the display unit 5 can be increased by forming a peripheral driver circuit using FT will be described. As shown in FIG. 1, the driving circuit of the display device is generally divided into a scanning circuit 8 and a signal circuit. 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. The pixels of the display unit are TF, which is an active element.
T20 and a capacitor 19 composed of a liquid crystal as a display medium and a pixel electrode are arranged in a matrix as shown in the figure. The number of pixels of the display unit 5 mainly depends on the multiplexer 9 of the signal circuit and the divided matrix switch 1.
It is determined by the characteristic of 0. The writing time Tad per pixel viewed from the signal side 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 by using -Si TFT, the carrier mobility is large and the ON characteristics are excellent.
It can be reduced to the following. 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] Next, the point that the aperture ratio can be increased by using a Poly-Si TFT having {111} as a main orientation for an active matrix of a 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 the TFT and the Al electrode exist on each pixel. The gate width W and the gate length L of the TFT are usually 50 μm and 10 μm, respectively. 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. If a {111} oriented 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. Accordingly, the aperture ratio can be increased from about 65% to about 75%.
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, {111}
The structure and the manufacturing method of the Poly-Si TFT having the 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 ° C., and a film 12 is deposited by a low pressure CVD method using a monosilane gas diluted to 20% with helium as a raw material. The film thickness is 1500 °. Then N
₂ , heat treatment is performed at 600 ° C. for 24 hours. After the heat treatment, a {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 condition is that phosphorus (P) is applied at a voltage of 30 KeV and 5 × 10 ¹⁵ cm ^−2.
Of the dose. Then, the Phospho glass (Phospho Sil)
icate Glass (abbreviated PSG) 16 at 480 ° C for 500
Deposit 0 °. Further, under N ₂ at 600 ° C.
A heat treatment for 0 hour is performed to activate the implantation region.
After the photo-etching process for the contact, the Al electrode 1
7 is sputtered at 6000 °. After the photo-etching step, ITO (indium titanium oxide), which is a transparent electrode, is sputtered at 1000 °. After the photo-etching process,
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 the present embodiment, the scanning circuit 8 and the signal circuit 1
Since all the driving circuits such as 8 are externally mounted on the substrate 4, only the active matrix of the display section 5 is formed using a Poly-Si TFT having {111} as a main orientation.
As a result, the aperture ratio could be increased from 65% of the prior art to 75% as in Example 2.

[Embodiment 5] FIG. 5 shows another embodiment of the present invention. In the present embodiment, P
By using the oly-Si TFT, 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 section 5. As a result, in this embodiment, the number of connection terminals is increased from the conventional 177 to 38.
Could be reduced to books.

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 the {111} orientation Poly-Si is used only in the peripheral driving circuit. This is done by changing the Poly-Si deposition temperature and performing LPCVD twice.
It is necessary to add layers. First, an L-shaped quartz plate is used as a mask at a peripheral circuit forming position, and an LPCVD film is deposited at a temperature of 600 ° C. and 0.6 Torr at 1500 ° at a TFT forming position of a display portion. Next, a rectangular quartz plate is placed at the TFT forming position of the display section as a mask, and an LPCVD film is formed on the peripheral circuit forming position at 550 ° C. and 0.6 Torr under a condition of 0.6 Torr.
Deposit 00 °. Subsequently, when heat treatment is performed at 600 ° C. for 24 hours, the film deposited at 550 ° C. becomes a Poly-Si film having a main orientation of {111}, and the film deposited at 600 ° C. has a main orientation of {111}. Poly-Si film is obtained. 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 have {111} orientation, the dimensions cannot be reduced and the aperture ratio cannot be increased. However, it is possible to reduce the off power supply, 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 embodiment, if a Poly-Si TFT having high carrier mobility having {111} as the main orientation is used, the conventional Poly-Si TFT having low mobility on the same substrate can be used. Since the circuits that could not be mixed can be mixed, the size of the apparatus can be reduced.

In the above figures, the parts denoted by the same reference numerals are the parts that perform the same function.

[0024]

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.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kenji Miyata 4026 Kuji-cho, Hitachi City, Ibaraki Pref. Hitachi Research Laboratory, Hitachi, Ltd.

Claims (2)

[Claims] 1. A liquid crystal display device having a pair of substrates and a liquid crystal layer disposed between the pair of substrates, wherein one of the pair of substrates is a normal glass substrate, and the normal glass substrate is It has a plurality of thin-film semiconductor elements arranged in a matrix and a plurality of pixel electrodes connected to each of the plurality of thin-film semiconductor elements. The normal glass substrate has an active layer of the plurality of thin-film semiconductor elements. Liquid crystal display device in which a polycrystalline silicon film having {111} as a main orientation is formed. 2. The liquid crystal display device according to claim 1, wherein the strain temperature of the normal glass substrate is about 640 ° C.