JP2000243970A - Thin film transistor, manufacture thereof, liquid crystal display device using the same and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-performance thin film transistor causing no problem in a driving circuit during display on a large screen and a liquid crystal display device using it, when a thin film transistor using polycrystalline silicon is used as a driving circuit part in the liquid crystal display device. SOLUTION: In a thin film transistor having a polycrystalline silicon thin film formed on a transparent insulating substrate 11 as an active region, crystal grain of the polycrystalline silicon thin film are grown anisotropically in a specific direction in the surface so as to make silicon crystal grains in a slender shape, and make the gate length direction of the thin film transistor approximately in parallel with the longitudinal direction of the crystal grains so that mobility of carriers in the active region (channel) of the TFT is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a thin film transistor (TFT) and a liquid crystal display (LCD), and more particularly to a thin film transistor using a polycrystalline silicon thin film as an active layer constituting a source region and a drain region.

[0002]

2. Description of the Related Art In recent years, a liquid crystal display device capable of higher-speed display has been demanded.
As one means, the silicon thin film constituting the active layer such as the gate region, the source region, and the drain region of the thin film transistor for display switching which controls the transmission of light through the liquid crystal layer is made of polycrystalline silicon instead of amorphous silicon. A method has been proposed. This intends to utilize the fact that the mobility of carriers in a polycrystalline silicon thin film is higher in principle than the mobility of carriers in an amorphous silicon thin film.

On the other hand, in the case of a conventional liquid crystal display device having a thin film transistor using an amorphous silicon thin film as an active layer of a transistor, a driver circuit for performing signal processing at the time of liquid crystal display requires a separately manufactured single crystal silicon. Was configured by externally attaching a semiconductor chip having an active layer as the active layer, but it is possible to form a polycrystalline silicon thin film having high carrier mobility as described above on an insulating substrate (specifically, a glass substrate). Then, the above driver circuit can also be formed by a thin film transistor having a polycrystalline silicon thin film formed on an insulating substrate as an active layer.

In this case, unlike a liquid crystal display device using a thin film transistor in which an amorphous silicon thin film is used as an active layer, since there is no step of externally attaching a semiconductor chip, an externally connecting step itself becomes unnecessary, and Since the occurrence of problems such as poor connection can be prevented, a liquid crystal display device can be manufactured at low cost.

Therefore, a method of manufacturing a liquid crystal display device using a polycrystalline silicon thin film as an active layer will be described below with reference to the drawings. Here, only the step of forming a thin film transistor after forming a polycrystalline silicon thin film on a transparent insulating substrate such as a glass substrate will be described.

FIG. 3 is a cross-sectional view of a conventional thin film transistor when a polycrystalline silicon thin film is used as an active layer of the thin film transistor. In FIG. 3, reference numeral 31 denotes a transparent insulating material such as glass. The substrate 32 has a buffer layer 3 having a function of preventing diffusion of an element having an adverse effect on an active layer formed of a silicon thin film such as an alkali metal contained in the insulating substrate 31 when heat treatment is performed.
3 is an amorphous silicon thin film, 34 is a polycrystalline silicon thin film,
Reference numeral 35 denotes a gate insulating film made of, for example, a laminated film of SiO ₂ and Si ₃ N ₄ , reference numeral 36 denotes a gate electrode, reference numeral 37 denotes a source region which is a part of an active region of a transistor, and reference numeral 38 denotes a part of an active region of the transistor. A drain region, 39 is a contact hole, 310 is a source electrode, and 311 is a drain electrode.

A conventional thin film transistor using a polycrystalline silicon thin film as an active layer of a thin film transistor is formed as follows.

First, as shown in FIG. 3A, an amorphous silicon thin film 3 is formed on an insulating substrate 31 with a buffer layer 32 interposed therebetween.
3 is deposited.

Next, a heat treatment is performed on the amorphous silicon thin film 33 to perform a polycrystallization treatment. Specifically, an excimer laser is irradiated on the amorphous silicon thin film 33,
The polycrystalline silicon thin film 34 is obtained by instantaneously melting and cooling. Thereafter, unnecessary portions of the polycrystalline silicon thin film 34 are removed, and a gate insulating film 35 and a gate electrode 36 are formed thereon.
Are sequentially formed. In this state, in order to form a source region and a drain region of the thin film transistor, impurities that determine the conductivity type of the polycrystalline silicon thin film 34 are introduced into the polycrystalline silicon thin film 34 using at least the gate electrode 36 as a mask (FIG. 3B). ).

Thereafter, as shown in FIG. 3C, the source region 37 and the drain region 38 are formed by irradiating the polycrystalline silicon thin film 34 with an excimer laser again to activate the impurities by heat treatment. . Finally,
As shown in FIG. 3D, a source electrode 310 and a drain electrode 311 are formed by forming a contact hole 39 and burying a metal.

[0011]

However, even in a conventional thin film transistor using a polycrystalline silicon thin film as an active layer formed through a process as shown in FIG. There are insufficient points.

In view of the above problems, the present invention provides a liquid crystal display device using a thin film transistor using polycrystalline silicon, which does not cause a problem when displaying a large screen, and a liquid crystal display device using the same. And a method for producing the same.

[0013]

In order to achieve the above object, the present invention provides a thin film transistor having a polycrystalline silicon thin film formed on a transparent insulating substrate as an active region, wherein the polycrystalline silicon thin film is A crystal grain is anisotropically grown in a plane parallel or perpendicular to the edge of the array substrate, and the gate length direction of the thin film transistor is substantially parallel to the longitudinal direction of the crystal grain.

At this time, if 0.5 to 2 crystal grains of the polycrystalline silicon thin film are contained per 1 μm of gate length, the field effect mobility of all the thin film transistors becomes 300 cm.
² / Vs or more is convenient.

Further, it is possible to provide a thin film transistor having higher performance in which the length of the crystal grain of the polycrystalline silicon thin film in the longitudinal direction is longer than the gate length.

On the other hand, in order to achieve the above object, the present invention provides a step of forming an amorphous silicon thin film on a transparent insulating substrate, and a step of subjecting the amorphous silicon thin film to a heat treatment so that the crystal grains become in-plane. Forming a polycrystalline silicon thin film anisotropically grown in a specific direction, and forming a gate electrode on said polycrystalline silicon thin film via a gate insulating film. A method of manufacturing a thin film transistor, wherein the gate electrode is formed such that a gate length direction is substantially parallel to a longitudinal direction of the crystal grain.

At this time, in the step of heat-treating the amorphous silicon thin film to form a polycrystalline silicon thin film, a belt-like laser beam having a uniform longitudinal intensity and an intensity distribution in the transverse direction is shaped, and When the operation of irradiating the laser beam by moving the substrate or the laser beam in a direction substantially perpendicular to the longitudinal direction is repeated, it is convenient to manufacture the thin film transistor in the gate length direction substantially in parallel with the longitudinal direction of the crystal grains.

Further, when the substrate or the laser beam is moved in a direction substantially perpendicular to the longitudinal direction of the laser beam and in a direction in which the strength of the laser beam in the short direction is increased, the electric field effect movement characteristics are further improved. A thin film transistor can be provided.

Further, it is convenient to manufacture a thin film transistor array by forming a gate bus line at the same time as forming a gate electrode.

According to another aspect of the present invention, there is provided a thin film transistor in which a plurality of thin film transistors each having an active layer of a polycrystalline silicon thin film in which crystal grains are anisotropically grown in a specific direction in a plane are formed. 1 insulating substrate, a second insulating substrate placed opposite to the first insulating substrate, and inserted between the first insulating substrate and the second insulating substrate. A liquid crystal display device having a liquid crystal layer, wherein a gate length direction of the thin film transistor and a longitudinal direction of the crystal grains are substantially parallel.

Here, a more compact liquid crystal display device can be provided by forming a driver circuit for performing a liquid crystal display operation using thin film transistors.

When the driver circuit is used on the data drive side, a high-performance liquid crystal display device having excellent driving characteristics can be provided.

Further, when a shift register is included in the driver circuit, a more compact liquid crystal display device can be provided.

Further, when a thin film transistor is used as a liquid crystal switch in a pixel portion, a liquid crystal display device with good contrast can be provided.

According to another aspect of the present invention, there is provided a thin film transistor in which a polycrystalline silicon thin film in which crystal grains are anisotropically grown in a specific direction in a plane is formed. An insulating substrate; a second insulating substrate placed opposite to the first insulating substrate;
A method for manufacturing a liquid crystal display device, comprising: an insulating substrate according to (1) and a liquid crystal layer interposed between the second insulating substrates, wherein at least the first insulating substrate is formed on a transparent insulating substrate by an amorphous method. Forming a polycrystalline silicon thin film, heat-treating the amorphous silicon thin film to form a polycrystalline silicon thin film in which crystal grains grow anisotropically in a specific direction in a plane, Forming a gate electrode with a gate insulating film interposed therebetween.
A glass substrate on which a color filter is formed as a second insulating substrate by hand using an FT) array manufacturing process;
Alternatively, there is provided a liquid crystal display device comprising a step of using an insulating substrate made of a transparent glass substrate, combining the first insulating substrate and the second insulating substrate, bonding them together with leaving a gap, and injecting liquid crystal into the gap. A manufacturing method is provided.

At this time, in the step of heat-treating the amorphous silicon thin film to form a polycrystalline silicon thin film, a band-like laser beam having a uniform longitudinal intensity and an intensity distribution in the lateral direction is shaped. By repeating the operation of irradiating the laser beam by moving the substrate or the laser beam in a direction substantially perpendicular to the longitudinal direction, a method for manufacturing a high-performance liquid crystal display device having excellent display characteristics can be provided.

When the substrate or the laser beam is moved in a direction substantially perpendicular to the longitudinal direction of the laser beam and in a direction in which the intensity of the laser beam in the lateral direction increases, the gate length direction of the thin film transistor and the crystal A liquid crystal display device in which the longitudinal directions of the grains are substantially parallel can be manufactured.

Furthermore, if a gate bus line is formed at the same time as the formation of the gate electrode, a liquid crystal display device can be manufactured efficiently.

Further, when at least the gate length direction of the thin film transistor group forming the source driver and the longitudinal direction of the crystal grains are substantially parallel, a higher-speed liquid crystal display device can be manufactured.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a thin film transistor and a liquid crystal display according to an embodiment of the present invention will be described with reference to the drawings. The liquid crystal display device of the present invention basically includes a conventional liquid crystal display device (a first insulating substrate on which a thin film transistor array having a polycrystalline silicon thin film as an active layer is formed, except for a thin film transistor portion, (A liquid crystal layer is inserted between a first insulating substrate and a second insulating substrate formed opposite to the first insulating substrate). Therefore, description will be made focusing on a thin film transistor portion.

The inventor of the present invention has proposed an amorphous silicon thin film in order to obtain a thin film transistor capable of obtaining sufficient characteristics even in a large-screen liquid crystal display device using a polycrystalline silicon thin film as an active layer of a transistor. It has been found that it is extremely effective to control the direction in which a thin film transistor is formed together with the method of excimer laser annealing when polycrystallizing by heating.

Embodiment 1 Hereinafter, a thin film transistor and a liquid crystal display according to Embodiment 1 of the present invention will be described with reference to the drawings.

First, FIG. 1 is a plan view of a transparent insulating substrate on the array side on which a thin film transistor group for a pixel switch and a thin film transistor group for a driver circuit of a liquid crystal display are formed in the liquid crystal display device of the present invention.

As shown in FIG. 1A, reference numeral 11 denotes a transparent insulating substrate made of glass or the like, and a pixel portion 12, a gate drive circuit (gate driver) portion 13,
A thin film transistor group using a polycrystalline silicon thin film as an active layer of a transistor is formed in each of the source drive circuit (source driver) sections 14.

Here, the thin film transistor array formed in the pixel section 12 is for a pixel switch, and the thin film transistors formed in the gate drive circuit section 13 and the source drive circuit section 14 are thin film transistors for a driver circuit. . In this embodiment, the driver circuit includes a shift register.

In the liquid crystal display device according to the present embodiment, thin film transistors are formed in all regions of the pixel portion 12, the gate drive circuit portion 13, and the source drive circuit portion 14 by the following method.

The basic manufacturing process itself is described in FIG.
However, in the present embodiment, the step of polycrystallizing a previously formed amorphous silicon thin film by heat treatment is different from the above-described conventional one.

In this embodiment, the shape of the excimer laser beam is a band. In detail, a band-shaped laser beam having energy uniform in the longitudinal direction and intensity distribution in the lateral direction is shaped, and while irradiating this laser beam, the substrate or the laser beam is moved in a direction substantially orthogonal to the longitudinal direction. By heating (usually moving the substrate) by irradiation, the amorphous silicon thin film is polycrystallized.

More specifically, S
An iO ₂ layer is formed, and plasma CV is applied through the SiO ₂ layer.
The amorphous silicon layer having a thickness of about 10
Formed at 0 nm. Next, the thin film transistor to be formed is set so that the gate length direction and the moving direction of the substrate are substantially parallel (in FIG. 1B, each direction is set so that the end of the array substrate and the beam longitudinal direction lie substantially perpendicularly). ) And in a direction substantially perpendicular to the longitudinal direction of the laser beam, and
The substrate or the laser beam is moved while irradiating the laser beam in a direction in which the lateral intensity of the laser beam increases. Usually, the amorphous silicon thin film is continuously crystallized by irradiating an excimer laser beam every time the substrate side is moved with respect to the excimer laser (step-and-repeat method).

FIG. 1B shows a specific excimer laser irradiation method. In this figure, reference numeral 15 denotes the beam shape of the excimer laser, and arrow 16 denotes the relative scanning direction of the excimer laser.

At this time, when the above-described heat treatment is performed using an excimer laser having an intensity distribution in the short direction, the crystal grains (also called domains or grains) of the polycrystallized silicon thin film have a circular shape. Instead, the crystal grains have an elongated elliptical shape in the scanning direction, and the crystal grains have a shape in which a longitudinal direction and a lateral direction are clearly present.

For example, using a KrF (or XeCl) excimer laser, the thickness of amorphous silicon (a-Si) as a precursor is set to 100 nm.
Substrate temperature 500 ° C, laser irradiation energy 330
After setting the scanning movement pitch at 1 micron / shot by setting mJ / cm ² , and confirming with an optical microscope, it has a longitudinal direction in the scanning direction, a grain size in the longitudinal direction of 3 to 5 microns, and a lateral direction of 0.5. A thin film composed of silicon microcrystals of ~ 2 microns was obtained.

The preferable condition at this time is that the film thickness is 3
0 nm to 200 nm, the higher the substrate temperature, the better, but the preferred range when using glass for the substrate is 300 ° C. to 600 ° C.
Laser irradiation energy of 280 to 420 mJ / cm ²
Met.

The laser irradiation power distribution in the lateral direction of the above-mentioned shaped laser beam may have an inclination of about 3 to 10 mW / cm ² per 10 microns.

Next, using the polycrystalline silicon thin film formed as described above, a gate electrode of each TFT is formed thereon via a gate insulating film. This step is also described in FIG.
In the present embodiment, the thin film transistor is formed such that the gate length direction of the thin film transistor and the longitudinal direction of the crystal grains are substantially parallel to each other. Thereafter, the same steps as those shown in FIG. 3 were performed to obtain the polycrystalline silicon thin film transistor shown in FIG.

Here, S indicates a source electrode 21, D indicates a drain electrode 22, G indicates a gate electrode 23, and the crystallized polysilicon thin film is indicated by 24. In this figure, a small elliptical shape indicates a grain 25 of a silicon crystal grown anisotropically.

According to the thin film transistor of the present embodiment formed as described above, the gate length direction of the thin film transistor and the longitudinal direction of the crystal grains of the polycrystalline silicon formed to be elongated are substantially parallel. Grain boundaries of crystal grains existing in the channel region of the thin film transistor can be minimized. Accordingly, the barrier of carrier movement when the thin film transistor is operated is reduced (close to a single crystal microscopically), and high field effect mobility (specifically, about 480 cm ² / Vs) is obtained. A thin film transistor group having the same was obtained.

Embodiment 2 First, as shown in FIG. 4, a first electrode group 41 mounted in a matrix and a first embodiment for driving the electrodes
Having the transistor group 42 formed by the method of
A color filter group 44 and a second electrode 45 mounted on the array substrate 43 and facing the first electrode group.
Are formed directly on the surface of the second substrate 46 having any of the above, or after forming an arbitrary thin film thereon, the alignment films 47 and 47 'having an alignment effect on the liquid crystal are formed thereon. Next, the first and second color filter substrates 43 and 46 were aligned so that the electrodes faced each other, and were fixed using a spacer 48 and an adhesive 49 with a gap of about 5 μm. afterwards,
The TN liquid crystal (ZLI479) is provided on the first and second substrates.
2: Merck's product) 410 and then the polarizing plate 41
1 and 412 were combined to complete a display element.

Such a device includes a backlight 413
By driving each transistor using a video signal while illuminating the entire surface, an image could be displayed in the direction of arrow A.

Further, even if the entire liquid crystal screen was enlarged from 3.8 inches to 13 inches, delays in signal processing and the like could be minimized, and high-speed display could be performed.

[0051]

As described above, according to the present invention, the shape of the microcrystals constituting the polycrystalline silicon thin film obtained by annealing the amorphous silicon thin film with an excimer laser is elongated, By making the direction of the gate length of the TFT to be manufactured substantially in parallel, a high-performance TFT array having high field-effect mobility can be manufactured, and sufficient characteristics can be obtained even in consideration of application to a large-screen liquid crystal display device. Have,
A thin film transistor group for a driving circuit can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic plan view of an array substrate on which a group of driving thin film transistors of a liquid crystal display device according to an embodiment of the present invention is formed.

FIG. 2 is a conceptual plan view of a TFT showing a relationship between a gate length direction and a crystal grain of the thin film transistor according to the embodiment of the present invention.

FIG. 3 is a conceptual cross-sectional view of a manufacturing process of a thin film transistor using a conventional polycrystalline silicon as an active layer of a transistor.

FIG. 4 is a conceptual diagram of a sectional structure of the liquid crystal display device of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Insulating substrate 12 Pixel part 13 Gate drive circuit part 14 Source drive circuit part 15 Excimer laser beam 21 Source electrode 22 Drain electrode 23 Gate electrode 24 Polysilicon thin film 25 Silicon crystal grain (elongated polycrystalline silicon microcrystal) 31 Insulating substrate 32 Buffer layer Reference Signs List 33 amorphous silicon thin film 34 polycrystalline silicon thin film 35 gate insulating film 36 gate electrode 37 source region 38 drain region 39 contact hole 310 source electrode 311 drain electrode 41 first transparent electrode group 42 transistor group 43 first array substrate 44 Color filter group 45 Second electrode 46 Second color filter substrate 47, 47 'Orientation film 48 Spacer 49 Adhesive 410 Liquid crystal 411, 412 Polarizing plate 413 Backlight

 ────────────────────────────────────────────────── ─── Continued from the front page F term (reference) DA02 DB03 FA00 JA01 JA10 5F110 AA01 BB02 CC01 DD02 DD13 GG02 GG13 GG14 GG16 GG45 PP03 PP24

Claims (17)

[Claims] 1. A thin film transistor having a polycrystalline silicon thin film formed on a transparent insulating substrate as an active region, wherein crystal grains of the polycrystalline silicon thin film are anisotropically grown in a specific direction in a plane. And a gate length direction of the thin film transistor is substantially parallel to a longitudinal direction of the crystal grain. 2. The polycrystalline silicon thin film has a gate length of 1
2. The thin film transistor according to claim 1, wherein said thin film transistor is contained in an amount of 0.5 to 2 per micron. 3. The thin film transistor according to claim 1, wherein the length of the crystal grain of the polycrystalline silicon thin film in the longitudinal direction is longer than the gate length. 4. A step of forming an amorphous silicon thin film on a transparent insulating substrate, and heat-treating the amorphous silicon thin film so that polycrystals in which crystal grains grow anisotropically in a specific direction in a plane. A method of manufacturing a thin film transistor, comprising: forming a silicon thin film; and forming a gate electrode on the polycrystalline silicon thin film via a gate insulating film, wherein a gate length direction and a longitudinal direction of the crystal grains are different. A method of manufacturing a thin film transistor, wherein the gate electrode is formed so as to be substantially parallel. 5. In a step of forming a polycrystalline silicon thin film by heat-treating an amorphous silicon thin film, a belt-like laser beam having a uniform longitudinal intensity and an intensity distribution in a lateral direction is shaped, 5. The method according to claim 4, wherein the operation of irradiating the laser beam by moving the substrate or the laser beam in a direction substantially perpendicular to the direction is repeated. 6. The laser beam according to claim 5, wherein the substrate or the laser beam is moved in a direction substantially perpendicular to the longitudinal direction of the laser beam and in a direction in which the intensity of the laser beam in the lateral direction increases. Method for manufacturing a thin film transistor. 7. The method of manufacturing a thin film transistor according to claim 4, wherein a gate bus line is formed at the same time as forming the gate electrode. 8. A first insulating substrate on which a plurality of thin film transistors each having an active layer of a polycrystalline silicon thin film in which crystal grains are anisotropically grown in a specific direction in a plane; A liquid crystal display device comprising: a second insulating substrate placed opposite to a conductive substrate; and a liquid crystal layer inserted between the first insulating substrate and the second insulating substrate.
A liquid crystal display device, wherein a gate length direction of the thin film transistor and a longitudinal direction of the crystal grains are substantially parallel. 9. The liquid crystal display device according to claim 8, wherein a driver circuit for performing a liquid crystal display operation is constituted by the thin film transistor. 10. The liquid crystal display device according to claim 9, wherein a driver circuit is used on a data drive side. 11. The liquid crystal display device according to claim 10, wherein the driver circuit includes a shift register. 12. The liquid crystal display device according to claim 8, wherein the thin film transistor is used as a liquid crystal switch. 13. A first insulating substrate on which a thin-film transistor having a polycrystalline silicon thin film in which crystal grains are anisotropically grown in a specific direction in a plane as an active layer is formed;
Manufacturing of a liquid crystal display device comprising: a second insulating substrate placed opposite to the first insulating substrate; and a liquid crystal layer inserted between the first insulating substrate and the second insulating substrate. A method comprising: forming an amorphous silicon thin film on at least a first insulating substrate on a transparent insulating substrate; Forming a polycrystalline silicon thin film grown anisotropically in the direction, and interposing a gate insulating film so that a gate length direction of a gate electrode is substantially parallel to a longitudinal direction of crystal grains of the polycrystalline silicon thin film. Forming a thin film transistor array including a step of forming a gate electrode, and using a glass substrate on which a color filter is formed or a transparent glass substrate as a second insulating substrate; Method of manufacturing a liquid crystal display device characterized by having a bonded leaving a gap by combining an insulating substrate and the second insulating substrate, injecting liquid crystal into the gap step. 14. A step of heat-treating an amorphous silicon thin film to form a polycrystalline silicon thin film, wherein a strip-shaped laser beam having a uniform longitudinal intensity and an intensity distribution in a transverse direction is shaped, 14. The method according to claim 13, wherein the operation of moving the substrate or the laser beam in a direction substantially perpendicular to the direction and irradiating the laser beam is repeated. 15. The method according to claim 14, wherein the substrate or the laser beam is moved in a direction substantially perpendicular to the longitudinal direction of the laser beam and in a direction in which the intensity of the laser beam in the lateral direction increases. Method for manufacturing a liquid crystal display device. 16. The method according to claim 15, wherein a gate bus line is formed simultaneously with the formation of the gate electrode. 17. The semiconductor device according to claim 13, wherein at least a gate length direction of a thin film transistor group forming a source driver and a longitudinal direction of said crystal grains are substantially parallel.
17. The method for manufacturing a liquid crystal display device according to item 16.