JPH0968725A - Polysilicon thin-film transistor and liquid crystal display device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To embody high-performance polysilicon thin-film transistors(TFTs) which are improved in the yield in process and operate stably over a long period of time and a liquid crystal display device. SOLUTION: This polysilicon TFT is constituted by providing one main surface of a glass substrate 1 with silicon nitride films 2, polysilicon layers 3, gate insulating films 4, gate wiring layers 5, interlayer insulating films 6 and source-drain wiring layers 7. The silicon nitride films 2 are formed in the same patterns as the patterns of the polysilicon layers 3 on the glass substrate 1 and the polysilicon layers 3 are directly laminated on these silicon nitride films 2. As a result, the intrusion of alkaline ions from the glass substrate 1 to the polysilicon layers 3, etc., is surely prevented by the silicon nitride films 2. Since the silicon nitride films 2 are formed to the same patterns as the patterns of the polysilicon layers 3, the troubles in the process, such as distortion of the substrate 1 and the etching abnormality at the edges of the polysilicon layers, by providing undercoat layers heretofore even to the parts not overlapping on the polysilicon layers are eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polysilicon thin film transistor and a liquid crystal display device.

[0002]

^2. Description of the Related Art Since a polysilicon thin film transistor has a high carrier mobility of several tens to several hundreds of cm ² / Vs, it can be used as a pixel switching element and a driving circuit element in an active matrix driving type liquid crystal display device. it can. FIG. 3 shows a structure of a conventional top-gate structure polysilicon thin film transistor.
As shown in the figure, this polysilicon thin film transistor has an undercoat layer 22 on the main surface of the glass substrate 21.
A polysilicon layer 23, a gate insulating film 24, a gate wiring layer 25 forming a gate electrode and a gate wiring, an interlayer insulating film 26, a source / drain wiring layer 27 forming a source / drain electrode and a source / drain wiring are provided. Consists of The polysilicon layer 23 is a source / drain region 23 in which impurity ions are implanted on both sides of the channel region 23a.
b is arranged.

Regarding the method of manufacturing this polysilicon thin film transistor, as a method of forming the polysilicon layer 23, a laser annealing method or a solid phase growth method using heat is known. As a method of forming the source / drain regions 23b in the polysilicon layer 23, an ion implantation method + laser activation method or thermal activation method is known. Among them, the method using a laser is a low-temperature process, and is therefore suitable for mass-producing polysilicon thin film transistors using an inexpensive glass substrate.

Next, a method of manufacturing a polysilicon thin film transistor using the laser annealing method and the laser activation method will be described with reference to FIG.

(1) On the glass substrate 21, the SiO ₂ film as the undercoat layer 22 and the amorphous silicon film 2
After 3'is sequentially formed, it is annealed by an excimer laser to crystallize the amorphous silicon film 23 'to form polysilicon. (2) After the polysilicon film 23 is patterned into an island shape by the photolithography method, S is formed as the gate insulating film 24 on the glass substrate 21 including the surface of the polysilicon island 23.
i0 ₂ film is formed, further patterning the gate wiring layer 25 is formed thereon by photolithography. (3) Impurity ions are injected into the polysilicon layer 23 using the resist (not shown) left on the gate wiring layer 25 as a mask (ion injection with mass separation or ion doping without mass separation) to form a channel region 23a. Source / drain regions 23b are formed on both sides. (4) After removing the resist, an interlayer insulating film (SiO ₂ film) 26 is formed on the gate insulating film 24 and the gate wiring layer 25 by CVD.
It is formed by the method. (5) The polysilicon layer 23 is irradiated with laser light and heated to activate the channel region 23a and the source / drain regions 24b. However, the order of the processing steps (4) and (5) can be reversed. (6) The source of the gate insulating film 24 and the interlayer insulating film 26
A contact hole communicating with the drain region 23b is opened by photolithography, and the aluminum film is patterned to form the source / drain wiring layer 27.

As described above, a polysilicon thin film transistor is completed.

However, in the structure of this polysilicon thin film transistor, when the SiO ₂ film is used as the undercoat layer 22, the following problems occur.

When an inexpensive glass substrate 21 is used (for example, Corning 7059, 1737, NA45, 35)
Etc.), it is not possible to sufficiently suppress the mixing of impurities from the glass substrate 21 into the respective film layers thereon, especially the mixing of alkali ions such as Na +. Alkali ions mixed in the polysilicon layer 23 and the gate insulating film 24 bring about a variation in threshold voltage when the transistor is operated for a long time, which is a major factor of degrading image quality. In addition, the polysilicon layer 23
The undercoat layer 22 that is not covered with is a polysilicon layer 23 that absorbs laser light in the laser activation process.
Exists on the substrate in a state of being subjected to heat conduction from the substrate and damaging in the subsequent etching process, and the glass substrate 21 is distorted by the stress of this portion, and particularly 300 mm × 400 m
When a large-area substrate having a size of m or more is used, there is a practical problem. Further, the undercoat layer 22 has poor adhesion to the gate insulating film (SiO ₂ film) 24, and etching abnormality is likely to occur at the edge of the polysilicon layer 23. In addition, the mixing of the alkali ions into the insulating film forming the auxiliary capacitance element causes a leak current of the auxiliary capacitance element to increase due to ionic conduction, which causes deterioration of the retention characteristic of the auxiliary capacitance element. This deterioration of the storage characteristic of the auxiliary capacitance directly leads to a decrease in the contrast ratio on the liquid crystal display panel, which causes a serious problem in practical use.

A silicon nitride film (SiN film) may be used as the undercoat layer. The undercoat layer made of this SiN film sufficiently functions as a blocking layer for alkali ions. However, also in this case, the undercoat layer in the portion not covered with the polysilicon layer is damaged by heat conduction from the polysilicon layer in the laser activation step and the subsequent etching step, and the stress in that portion also causes damage. It distorts the glass substrate. Or Si
The N-film-like rack is generated and the film is peeled off. The undercoat layer of SiN film is also poor adhesion between the gate insulating film (Si0 ₂ film), also prone to etching abnormality at the edge of the polysilicon island further. Further, in the undercoat layer made of the SiN film, microcracks are easily generated in the portion not covered with the polysilicon layer in the laser activation process.

As another undercoat layer, a method of stacking a SiN film and a SiO ₂ film in order from the substrate side can be considered. However, even with this method, since there is an undercoat layer in a region not covered by the polysilicon layer, it is difficult to solve the above-mentioned problems in the process due to this.

[0011]

As described above, in the conventional polysilicon thin film transistor, the undercoat layer of the portion not covered with the polysilicon layer has a distortion of the substrate, an etching abnormality at the edge of the polysilicon island, and the like. Was a factor that caused the problem. Further, in the case of using SiO ₂ as the undercoat layer, the mixing of alkali ions from the glass substrate into the polysilicon layer or the like cannot be sufficiently prevented, and the operating characteristics of the transistor fluctuate over time and the image quality deteriorates. There was a problem.

The present invention is intended to solve such a problem, and an object thereof is to provide a high-performance polysilicon thin film transistor and a liquid crystal display device which can improve the yield and can be stably operated for a long time. .

[0013]

In order to achieve the above-mentioned object, a polysilicon thin film transistor and a liquid crystal display device of the present invention have a polysilicon layer provided on an insulating substrate via a silicon nitride film. The nitride film and the polysilicon layer are formed as seed layers in the same pattern.

By thus forming the seed layer of the silicon nitride film and the polysilicon layer in the same pattern, the mixing of alkali ions from the substrate into the polysilicon layer or the like can be sufficiently blocked by the silicon nitride film, and the conventional polysilicon film can be used. By providing the undercoat layer even in a portion that does not overlap with the silicon layer, process defects such as substrate distortion and etching abnormality at the edge of the polysilicon layer can be eliminated. As a result, it is possible to improve the yield and to realize a high-performance polysilicon thin film transistor and a liquid crystal display device which operate stably for a long time.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view showing the structure of a polysilicon thin film transistor which is an embodiment of the present invention.

As shown in the figure, this polysilicon thin film transistor has a silicon nitride film 2, a polysilicon layer 3, a gate insulating film 4, a gate wiring layer 5, and a gate wiring layer 5 on one main surface of an insulating substrate such as a glass substrate 1. The interlayer insulation film 6 and the source / drain wiring layer 7 are provided. The polysilicon layer 3 has source / drain regions 3b in which impurity ions are implanted, which are arranged on both sides of the channel region 3a. The silicon nitride film 2 is formed on the glass substrate 1 in the same pattern as the polysilicon layer 3, and the polysilicon layer 3 is laminated on the silicon nitride film 2. One main surface of the glass substrate 1 including the surface of the polysilicon layer 3 is covered with a gate insulating film 4, and a gate wiring layer 5 forming a gate electrode and a gate wiring is patterned on the gate insulating film 4, and The surface of the gate insulating film 4 including the surface of the gate wiring layer 5 is covered with an interlayer insulating film 6. Then, the gate insulating film 4 and the interlayer insulating film 6 are opened at predetermined positions, and the source / drain wiring layer 7 is provided therein to form the polysilicon thin film transistor of this embodiment.

The thickness of the silicon nitride film 2 is preferably 50 nm or more. If it is 50 nm or more, it is possible to sufficiently block the mixing of alkali ions from the glass substrate 1 into the polysilicon layer 3 with the silicon nitride film 2.

As a method of patterning the silicon nitride film 2 and the polysilicon layer 3 into the same shape, CF ₄₀ is used.
Examples include a method of simultaneously patterning two layers by dry etching using microwave discharge using a _two- system gas.

As described above, in this polysilicon thin film transistor, the polysilicon layer 3 is provided on the glass substrate 1 via the silicon nitride film 2 having a film thickness of 50 nm or more. The mixing of alkali ions into the silicon nitride film 2 can be surely blocked, and since the silicon nitride film 2 is formed in the same pattern as the polysilicon layer 3, an undercoat layer is provided even in a portion which does not overlap the conventional polysilicon layer. Therefore, process defects such as distortion of the substrate and abnormal etching at the edge of the polysilicon layer can be eliminated. As a result, the yield can be improved, and a high-performance polysilicon thin film transistor that operates stably for a long time can be realized. Further, the polysilicon layer 3 is directly formed on the silicon nitride film 2.
Since the layers are laminated, it is expected that the hydrogen diffusion from the silicon nitride film 2 to the polysilicon layer 3 will improve the characteristics and stability.

A SiO ₂ film may be formed as an undercoat layer between the glass substrate 1 and the silicon nitride film 2. By adopting this structure, the alkali ion blocking performance can be further enhanced.

FIG. 2 is a sectional view showing the structure of an active matrix driving type liquid crystal display device using the polysilicon thin film transistor as a pixel switching element and a driving circuit element.

As shown in the figure, this liquid crystal display device is
R, G, B color filter layer 1 on glass substrate 11
2, black matrix layer 13, overcoat layer 1
4, the transparent substrate 15, the counter substrate 17 provided with the alignment film 16, and the above-mentioned polysilicon thin film transistor on the glass substrate 1, and the transparent electrode 18, the protective film 19,
The liquid crystal layer 19 is sandwiched between the TFT array substrate 10 and the alignment film 20 and the sealing material 8 is provided around the liquid crystal layer 19.
It is configured by sealing with.

In this liquid crystal display device, the auxiliary capacitance element in the pixel portion is composed of a polysilicon layer 43, an insulating film 44, a metal wiring layer 45 and the like. The polysilicon layer 43 is laminated on the glass substrate 1 via the silicon nitride film 42. The silicon nitride film 42 is formed by the polysilicon layer 43.
Are formed in the same pattern as. Here, the polysilicon layer 43, the insulating film 44, and the metal wiring layer 4 of the auxiliary capacitance element portion
5 and the silicon nitride film 42 are formed as the same layer as the polysilicon layer 3, the gate insulating film 4, the gate wiring layer 5, and the silicon nitride film 2, which form a thin film transistor (n-type TFT, P-type TFT), It was patterned at the same time.

With this structure, the silicon nitride film 42 prevents the alkaline ions from mixing from the glass substrate 1 into the insulating film 44 of the auxiliary capacitance element, so that the characteristics of the auxiliary capacitance element can be kept stable and the auxiliary capacitance can be maintained. By forming the silicon nitride film 42 of the element portion in the same pattern as the polysilicon layer 43, it is possible to eliminate process defects such as substrate distortion and etching abnormality at the edge of the polysilicon layer.
The yield can be improved.

[0026]

As described above, according to the present invention, since the silicon nitride film and the polysilicon layer in the polysilicon thin film transistor are formed as seed layers in the same pattern,
Alkali ions from the substrate into the polysilicon layer can be sufficiently blocked by the silicon nitride film, and since the undercoat layer has been provided up to the part that does not overlap with the conventional polysilicon layer, substrate distortion and polysilicon layer edge It is possible to eliminate process defects such as etching abnormalities. As a result, it is possible to improve the yield and to realize a high-performance polysilicon thin film transistor and a liquid crystal display device which operate stably for a long time.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a configuration of a polysilicon thin film transistor which is an embodiment of the present invention.

FIG. 2 is a sectional view showing a configuration of a liquid crystal display device which is an embodiment of the present invention.

FIG. 3 is a sectional view showing the structure of a conventional polysilicon thin film transistor.

FIG. 4 is a diagram showing a method of manufacturing a conventional polysilicon thin film transistor.

[Explanation of symbols]

 1, 11 ...... Glass substrate 2 ...... Silicon nitride film 3 ...... Polysilicon layer 3a ...... Channel region 3b ...... Source / drain region 4 ...... Gate insulating film 5 ...... Gate wiring layer 6 ...... Interlayer insulation film 7 ...... Source / drain wiring layer 10 ...... Signal electrode substrate 15, 18 ・ ・ ・ Transparent electrode 16, 20 ・ ・ ・ Alignment film 17 ・ ・ ・ Scan electrode substrate 19 ・ ・ ・ Liquid crystal layer 42 ・ ・ ・ Silicon nitride film 43 in auxiliary capacitance element section 43 ・ ・ ・... Polysilicon layer of auxiliary capacity element section 44 ... Insulating film of auxiliary capacity element section 4 ... Metal wiring layer of auxiliary capacity element section

Claims (4)

[Claims] 1. An insulating substrate, a polysilicon layer provided on the insulating substrate via a silicon nitride film, the polysilicon layer including respective regions of a channel, a source and a drain, and the polysilicon layer. In a polysilicon thin film transistor including a gate insulating film formed on the gate insulating film and a gate electrode formed on the gate insulating film, the silicon nitride film and the polysilicon layer are seed layers formed in the same pattern. A polysilicon thin film transistor characterized by the above. 2. The polysilicon thin film transistor according to claim 1, wherein the silicon nitride film has a film thickness of 50 nm or more. 3. A pair of insulating substrates having a transparent electrode and an alignment film on one main surface are arranged to face each other with a liquid crystal layer interposed therebetween.
Further, one of the insulating substrates is a polysilicon layer provided on the insulating substrate via a silicon nitride film and including regions of a channel, a source and a drain, and a gate formed on the polysilicon layer. A liquid crystal display device comprising a polysilicon thin film transistor having an insulating film and a gate electrode formed on the gate insulating film, wherein the silicon nitride film and the polysilicon layer are seed layers formed in the same pattern. Liquid crystal display device characterized by. 4. The liquid crystal display device according to claim 3, wherein the one insulating substrate provided with the polysilicon thin film transistor has a polysilicon layer provided on the insulating substrate via a silicon nitride film, A liquid crystal display, further comprising an auxiliary capacitance element including an insulating film and a metal wiring layer, and the silicon nitride film and the polysilicon layer are formed in the same pattern so as to overlap each other in a plane. apparatus.