JPH0772777B2 - Liquid crystal display - Google Patents

Description

TECHNICAL FIELD The present invention relates to a liquid crystal display device including a thin film transistor.

[Prior Art] Conventionally, a display panel using an active matrix has attracted attention as a method capable of realizing a large-sized panel having a large number of dots because its matrix size can be made extremely large as compared with the dynamic method.

In particular, a light-receiving element such as a liquid crystal has a limited drive duty in a dynamic system, and an active matrix application is considered for a television display and the like.

FIG. 1 shows one cell of a conventional active matrix.

The address line X is input to the gate of the transistor 2, and the transistor is turned on to store the signal on the data line Y in the holding capacitor 3 as electric charge. Until the data is written again, it is held by this capacitor 3 and simultaneously drives the liquid crystal 4.

Where VC is the common electrode signal. Since the liquid crystal leaks very little, it is sufficient for holding charges for a short time. The manufacturing method of transistors and capacitors here is exactly the same as the process of normal IC.

FIG. 2 is an example in which the cell of FIG. 1 is produced by a silicon gate process.

Transistor 10 and capacitor on a single liquid crystal silicon wafer
11 are built. Address line X and capacitor upper electrode 11
Is made of poly liquid crystal silicon (polysilicon), and the data line Y and the liquid crystal drive electrode 13 are made of Al.
The substrate and Al are connected to each other by 8, 9 and the polysilicon and Al are connected to each other.

However, such a structure has a problem that the process is complicated and the yield is difficult to control. Therefore, a display body having a thin film transistor formed on a transparent substrate has been considered.
When such a thin film transistor is used, amorphous silicon, polysilicon, etc. have been used as a semiconductor thin film.

[Problems and Objectives to be Solved by the Invention] When such a thin film transistor is formed on a glass substrate, there is a drawback that the semiconductor layer of the thin film transistor or the gate electrode has poor adhesion to the glass substrate and is easily peeled off. It was Further, there is also a drawback that impurities from the glass substrate diffuse into the channel portion of the thin film transistor, which disturbs the characteristics of the transistor.

[Means for Solving Problems] In the present invention, a liquid crystal is sandwiched between a pair of glass substrates, and a plurality of pixel electrodes are arranged in a matrix on one of the pair of glass substrates. In a liquid crystal display device in which a thin film transistor is connected to each pixel electrode, a first insulating film having good adhesion is formed on the one glass substrate, and the thin film transistor is formed on the upper surface and the side surface of the gate electrode. A second insulating film is formed, a channel region made of a silicon thin film is formed on the upper surface of the second insulating film, and the channel region is an intrinsic inverted stagger type thin film transistor not doped with impurities; The thin film transistor is formed on the first insulating film.

[Example] FIG. 3 shows a matrix cell used in the present invention.

The only difference from FIG. 1 is that a GND wiring for the capacitor is newly provided, and the basic circuit is the same. In this case, the GND potential means a constant bias, and the bias level does not matter.

FIG. 4 shows an example of the cell structure.

(A) is a plan view, and the address line 26 serves as the gate of the channel 28 of the transistor having the data line 25, the drive electrode and the electrode 29 of the capacitor as the source / drain. or
The GND line 27 is formed at the same time as the address line 26 and forms a capacitance with the electrode 29. Figure 4 (B) is AB of (A).
It shows a cross section along a line.

Explaining one example of the manufacturing process, polysilicon is used as a silicon thin film on a high melting point glass substrate 31 such as quartz.
Grow 3000 Å, then implant P ions on the entire surface to N
Use as a mold. However, in some cases to improve adhesion,
Thin SiO ₂ may be formed in advance.

Further, by photo etching, the gate 26 and the capacitor electrode
After forming 27, a SiO ₂ film 30 of about 1500 Å is grown by thermal oxidation as a gate insulating film and a dielectric film of a capacitor. After that, a second layer of polysilicon is applied to form a pattern by photo-etching, and then P ions are implanted again in a region other than the channel portion 28 by a resist mask to form a liquid crystal that also serves as a source / drain electrode, a data line wiring portion, and a capacitor electrode. Drive electrodes are formed.

Locally on this channel portion 28 or evenly over the entire substrate,
Laser is applied to melt polysilicon in a short time.
By solidifying and growing grains, it is possible to improve the performance such as threshold value and conductance. This is so-called laser annealing.

In the structure shown in FIG. 4, an electrode that transmits light is used, but an Al electrode may be further provided thereon.

FIG. 5 shows a simple cross section of a liquid crystal display device using the matrix substrate of the present invention.

A liquid crystal body 38 is sandwiched between a quartz substrate 35 on which a polysilicon electrode 37 is placed and a glass 36 on which a common electrode 39 made of a Nesa film is placed.
After sandwiching with polarizing plates 32 and 33, a reflector 34 is placed on the lower side.
Turn on. In this way, most of the light incident from above passes through the polysilicon electrode 37, is reflected by the reflector 34, and is sensed by the human eye. This method can use normal FE type liquid crystal, so it has high contrast and wide vision at the same time.

FIG. 6 shows a cross section of a cell formed on a normal glass substrate as another example of the present invention. A polysilicon film is formed on the glass substrate 40 by a low temperature film forming method such as sputtering or plasma CVD, and P ions or B ions are implanted on the entire surface. Next, the gate 43 and the capacitor electrode 42 are formed by photoetching. Further, the insulating film 44 is formed. Again, SiO ₂ or the like grown by low temperature is used.

Further, the second layer polysilicon, which also serves as the source / drain of the transistor and the capacitor and the drive electrode, is formed at a low temperature.

The polysilicon is either undoped or implanted with a sufficient amount of B ions to enhance the darkness.

After that, the laser beam is locally or entirely irradiated and annealed. A part of the laser beam is absorbed by the polysilicon of the first layer, but it is transmitted through the glass substrate 40. Therefore, activation of the ion-implanted impurities in the first-layer polysilicon,
When the grain growth of the second layer of polysilicon (especially the channel portion 48) is performed with an appropriate beam energy for an appropriate time (pulse interval for pulse laser, scanning speed for CW laser), the glass substrate Can be annealed within a range where there is almost no effect on.

The feature of this method is that the laser annealing can significantly reduce the influence on the glass substrate with respect to the conventional thermal annealing, so that it is possible to use the glass at a low cost.
The laser annealing is to simultaneously activate the impurities and grow the polysilicon grains in the channel portion to improve the characteristics (particularly the mobility) of the transistor.

Then, Al is applied and photoetching is performed to form source / drain electrodes 46 and 47. Since Al and polysilicon are difficult to make contact with each other as they are, they may be heat-treated or irradiated with a weak laser beam.

[Effect] According to the present invention, liquid crystal is sandwiched between a pair of glass substrates, a plurality of pixel electrodes are arranged in a matrix on one glass substrate of the pair of glass substrates, and a thin film transistor is provided on each pixel electrode. In the liquid crystal display device, the first insulating film having good adhesion is formed on the one glass substrate, and the thin film transistor has the second insulating film formed on the upper surface and the side surface of the gate electrode. A channel region made of a silicon thin film is formed on the upper surface of the second insulating film, and the channel region is an intrinsic inverted stagger type thin film transistor not doped with impurities, and the thin film transistor is the first thin film transistor. Since it is formed on the insulating film, when forming the thin film transistor on the glass substrate, the adhesion between the glass substrate and the thin film transistor It has the effect of being extremely improved.

Further, it is possible to prevent impurities from the glass substrate from diffusing into the intrinsic channel region of the thin film transistor,
This has the effect of stabilizing the characteristics of the thin film transistor.

Furthermore, since impurities from the glass substrate can be prevented from diffusing into the gate insulating film of the thin film transistor, leakage between the gate and the source and between the gate and drain and dielectric breakdown can be prevented, and the threshold voltage Vth
Can be effectively prevented from shifting.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a cell used in a conventional active matrix. FIG. 2 is a plan view of a cell using bulk silicon. FIG. 3 is a cell diagram of the present invention. 4 (A) and 4 (B) are a plan view and a sectional view of the implementation example. FIG. 5 is a sectional view when the substrate of the present invention is mounted on a panel. FIG. 6 is a diagram showing another embodiment of the present invention. 7,8,9 …… Contact hole 10 …… Polysilicon gate 11 …… Polysilicon upper electrode of capacitor 3 ………… Al driving electrode 26,27 …… First layer polysilicon 25,29 …… 2 Layer polysilicon 28 …… Channel 31 …… Quartz substrate 32,33 …… Polarizer 34 …… Reflector 35,36 …… Transparent substrate 39 …… Nesa film 37 …… Polysilicon drive electrode 38 …… Liquid crystal body 40 …… Glass substrate 42,43 …… First layer polysilicon 45 …… Second layer polysilicon 46,47 …… Al 48 …… Channel

Claims (1)

[Claims] 1. A liquid crystal is sandwiched between a pair of glass substrates, a plurality of pixel electrodes are arranged in a matrix on one of the pair of glass substrates, and a thin film transistor is connected to each of the pixel electrodes. In the liquid crystal display device having the above structure, a first insulating film having good adhesion is formed on the one glass substrate, and the thin film transistor has a second electrode on a top surface and a side surface of a gate electrode.
An insulating film is formed, a channel region made of a silicon thin film is formed on the upper surface of the second insulating film, and the channel region is an intrinsic inverted stagger type thin film transistor which is not doped with impurities. A liquid crystal display device, wherein the thin film transistor is formed on the first insulating film.