JPH06112486A - Thin film transistor, manufacture thereof, and liquid crystal display substrate - Google Patents

Abstract

PURPOSE:To restrain a leakage current from flowing out of a semiconductor layer and to make a thin film transistor have a low OFF-state current and a small parasitic capacitance by a method wherein the upside of one part of an insulating layer over a gate electrode is set lower than that of the other part of the insulating layer. CONSTITUTION:An insulating layer 33 is formed covering the upside of a substrate 31 and a gate electrode 32, and a recess 35 slightly larger in width than the gate electrode 32 is formed on the upside of the insulating layer 33 over the gate electrode 32. The inner side face 35a of the recess 35 is a slope inclined by an angle of theta, one part of the insulating layer 33 under the base of the recess 35 is set smaller in thickness than the other part. By this setup, even if light from a backlight is incident on the periphery of the gate electrode 32, the incident light is reflected by the sloped inner side face 35a toward a substrate 31 side, so that backlight is prevented from entering a semiconductor layer 36 above the gate electrode 32.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film transistor having a structure in which characteristic deterioration due to light such as a backlight is suppressed, a substrate for a liquid crystal display device including the thin film transistor, and a method of manufacturing the thin film transistor.

[0002]

2. Description of the Related Art FIG. 11 shows a constitutional example of an equivalent circuit of an active matrix liquid crystal display device using thin film transistors as switching elements. In FIG.
, Gn and a large number of signal electrode lines S1, S2, ..., Sm are arranged in a matrix, and each scanning electrode line G is connected to the scanning circuit 1 and each signal electrode line is formed. Each S is connected to the signal supply circuit 2, a thin film transistor (switch element) 3 is provided in the vicinity of the intersection of each line, and the drain of this thin film transistor 3 is connected to the capacitance section 4 serving as a capacitor and the liquid crystal element 5 to form a circuit. Is configured.

In the circuit shown in FIG. 11, one scan electrode line G is obtained by sequentially scanning the scan electrode lines G1, G2, ..., Gn.
All of the above thin film transistors 3 are turned on all at once, and in synchronization with this scanning, the capacitor portion connected to the thin film transistor 3 in the on state from the signal supply circuit 2 through the signal electrode lines S1, S2, ..., Sm. The signal charge is stored in the capacitance section 4 corresponding to the liquid crystal element 5 to be displayed. The accumulated signal charges continue to excite the corresponding liquid crystal element 5 until the next scan even when the thin film transistor 3 is turned off, so that the liquid crystal element 5 is controlled by the control signal and displayed. Become. That is, by performing such driving, each liquid crystal element 5 is statically driven even if the external driving circuits 1 and 2 are time-divisionally driven.

12 and 13 show an example of the structure of the conventional active matrix liquid crystal display device shown in the equivalent circuit of FIG. 11 in which parts such as the scanning electrode lines G and the signal electrode lines S are provided on the substrate. It is shown. 12 and 1
In the active matrix display device shown in FIG. 3, the scanning electrode lines G and the signal electrode lines S are wired in a matrix form on the transparent substrate 6 made of glass or the like at the intersecting portions with the insulating layer 9 interposed therebetween. Further, the thin film transistor 3 is provided near the intersection of the scanning electrode line G and the signal electrode line S.

The thin film transistor 3 shown in FIG. 12 and FIG.
Has the most general structure. An insulating layer 9 is provided on a gate electrode 8 provided by being drawn from a scanning electrode line G, and a semiconductor layer 10 made of amorphous silicon (a-Si) is provided on the insulating layer 9. And a drain electrode 11 and a source electrode 12 made of a conductor such as aluminum are provided on the semiconductor layer 10 so as to face each other. The uppermost layer of the semiconductor layer 10 is a semiconductor layer 10a such as ion-doped amorphous silicon, and the thin film transistor 3 shown in FIG. 13 has a structure generally called a channel etch type.

The drain electrode 11 is formed of an insulating layer 9
The source electrode 12 is connected to the pixel electrode 15 formed on the substrate 6 through a contact hole 13 formed in the substrate 6, and the source electrode 12 is connected to the signal electrode line S. Also,
A channel portion 14 is formed by the semiconductor layer 10 on the lower side between the drain electrode 11 and the source electrode 12 which face each other. Then, the insulating layer 9 and the drain electrode 11
A passivation layer 16 is provided so as to cover the source electrode 12 and the like, an alignment film 17 is formed on the passivation layer 16, and a transparent substrate 19 having an alignment film 18 is provided above the alignment film 17. Alignment film 1 provided
A liquid crystal 20 is sealed between 7 and 18 to constitute an active matrix liquid crystal display device, and the pixel electrode 1
When an electric field 5 is applied to the molecules of the liquid crystal 20, the alignment of the liquid crystal molecules can be controlled. In FIG. 13, the reference numeral 22 is a black mask attached to the bottom surface side of the substrate 19.

[0007]

FIG. 14 is an enlarged view of a main part of a channel-etch type thin film transistor 3'having the same structure as the thin film transistor 3 shown in FIG. In the thin film transistor 3 ′ shown in FIG.
The same components as those of the thin film transistor 3 shown in FIG. As shown in FIG. 14, the insulating layer 9 formed so as to cover the upper surface of the substrate 6 and the gate electrode 8 is
The upper surface of the substrate 6 is formed in a flat shape with a certain thickness, but the insulating layer 9 laminated on the gate electrode 8 is raised one step higher than the other portions to form a convex portion 9a. An inclined surface 9b is formed on the peripheral portion of the portion 9. Therefore, in the case where a backlight is provided on the back surface side of the substrate 6 of the liquid crystal display device, when the light emitted from the backlight passes through the substrate 6, a part of the light is inclined as shown by an arrow a in FIG. The reflected light may be reflected by the surface 9b, and the reflected light may be incident on the gate electrode 8 and reflected by the upper surface of the gate electrode 8 and eventually reach the semiconductor layer 10 of the channel portion 14. Then, the conductivity of the semiconductor layer 10 that receives light increases, and photocurrent flows.
When the thin film transistor 3 is driven, the gate electrode 8
There was a problem that a leak current had flowed while the circuit should have been turned off. When this leak current occurs, the off-current when driving the liquid crystal increases, which may adversely affect the light transmission characteristics of the liquid crystal.

Next, problems that occur when the liquid crystal is driven by an alternating current will be described. If a charge of the same polarity is continuously applied to the liquid crystal 20, the ionic component of the alignment films 17 and 18 in contact with the liquid crystal is concentrated on one side due to the direct current component, and an electric field is generated due to the adsorbed charge, so that the display is burned. For this reason, the fact that the liquid crystal has the same light transmission characteristics even when the polarities of the voltages applied to the pixel electrodes 15 are reversed is used to drive the liquid crystal in an alternating current to solve the problem of burn-in.

However, when the liquid crystal is driven by an alternating current, a parasitic capacitance occurs, the gate voltage jumps into the pixel electrode, and a dynamic voltage shift of the potential of the pixel electrode 15 occurs. The parasitic capacitance that causes the voltage shift is because the insulating layer 9 formed in a part of the active matrix liquid crystal display device becomes a capacitance. This is the same as the substrate 6 in the structure of the actual thin film transistors 3 and 3 ′ shown in FIGS.
Since the scanning electrode line G and the pixel electrode 15 are formed on the insulating layer 9 to cover the scanning electrode line G and the pixel electrode 15 and various films are formed on the insulating layer 9, thin-film transistors 3 and 3 ′ are formed. This is because the insulating layer portion between these portions and the scanning electrode line G forms a capacitance, which becomes a parasitic capacitance, which is structurally unavoidable in the current active matrix liquid crystal display device. It is a thing.

Considering the parasitic capacitance in the structure shown in FIG. 14, the gate electrode 8 and the drain electrode 11 are considered.
The insulating layer 9 provided between the gate electrode 8 and the gate electrode becomes a parasitic capacitance. However, in the structure shown in FIG. 14, since the distance between the end of the gate electrode 8 and the base 11a of the drain electrode 11 is short,
There is a problem that the gate-drain capacitance (C _GD ) becomes relatively large and the parasitic capacitance becomes large. Further, when the gate-drain capacitance as described above occurs, there is a problem that switching noise occurs when driving the liquid crystal.

On the other hand, since the problem of switching noise due to the generation of parasitic capacitance and the generation of off-current during liquid crystal driving as described above are serious problems in a liquid crystal display device, a storage capacitor described below is used. By incorporating the above into a liquid crystal display device, the problems of the above-mentioned parasitic capacitance and switching noise were solved. In the conventional liquid crystal display device,
Even if the capacitance value or the like changes a little due to the above-mentioned factors, in the equivalent circuit shown in FIG. 11, if the capacitance of the capacitance section 4 is sufficiently large, the effect is reduced, and as shown in FIG. 16 is connected to the scanning electrode line G, or as shown in FIG.
A single independent electrode line 25 made of Al or the like is provided in parallel with the capacitor, and the capacitance section 4 is connected to the independent electrode line 25,
By adopting this as a storage capacitor, a structure is adopted which cancels out the above-mentioned adverse effects.

In the active matrix liquid crystal display device having the above structure, if the independent electrode lines 25 are made of the same metal material as the gate electrode lines G, such as Al, they are simultaneously formed on the substrate 6 when the gate electrode lines G are formed. Independent electrode wire 25
However, although the number of steps is not increased unnecessarily, the independent electrode line 25 becomes an opaque portion and blocks the light incident on the pixel electrode 15a, which reduces the aperture ratio of the liquid crystal display device. There is. In order to avoid a decrease in aperture ratio, the independent electrode line 25 is formed of a transparent conductive material such as ITO (Indium Tin Oxide), and in order to provide the independent electrode line 25, it is combined with the process of forming the pixel electrode 15. The ITO film formation process is required twice, which increases the number of processes and lowers the yield. From the above, it is preferable that the storage capacitance is not present from the viewpoint of aperture ratio and yield. Therefore, it is desired to reduce the off-current and the gate-drain capacitance (C _GD ) when driving the liquid crystal.

The present invention has been made in view of the above circumstances, and has a structure in which light from a backlight or the like does not enter the semiconductor layer, which can suppress the leak current of the semiconductor layer, reduce the off current of the thin film transistor, and reduce switching noise. An object of the present invention is to provide a thin film transistor capable of reducing the parasitic capacitance that causes the above, a substrate for a liquid crystal display device including the thin film transistor, and a method for manufacturing the thin film transistor.

[0014]

In order to solve the above-mentioned problems, a gate electrode is formed on a transparent substrate, and an insulating layer is formed so as to cover the upper surface of the transparent substrate and the gate electrode. A semiconductor layer is formed on the insulating layer, a source electrode and a drain electrode are formed on the semiconductor layer in an adjacent state, and the upper surface of the insulating layer above the gate electrode is a portion of the insulating layer where the gate electrode is not formed. It is formed to be lower than the upper surface.

According to a second aspect of the present invention, in order to solve the above problems, the upper surface of the insulating layer above the gate electrode of the first aspect is formed lower than the upper surface of the insulating layer in the portion where the gate electrode is not formed. As a result, a recess is formed on the upper surface side of the insulating layer above the gate electrode, and the inner side surface of this recess is inclined.

In order to solve the above-mentioned problems, the invention according to claim 3 forms a gate electrode on a transparent substrate, forms an insulating layer covering the upper surface of the transparent substrate and the gate electrode, and forms a semiconductor layer on the insulating layer. And a source electrode and a drain electrode are formed adjacent to each other on the semiconductor layer, and an upper surface of the insulating layer above the gate electrode is formed lower than an upper surface of the insulating layer in a portion where the gate electrode is not formed. Of the opposite ends of the drain electrode and the source electrode, at least the end of the drain electrode is located outside the upper region of the gate electrode.

In order to solve the above-mentioned problems, the invention according to a fourth aspect is such that the scanning electrode lines and the signal electrode lines are wired in a matrix on a transparent substrate and are partitioned by the scanning electrode lines and the signal electrode lines. In a liquid crystal display device substrate provided with a pixel electrode and a thin film transistor in a portion, a gate electrode connected to the scanning electrode line is formed on the substrate,
A semiconductor layer is formed on the gate electrode via an insulating layer, and an upper surface of the insulating layer above the gate electrode is formed lower than an upper surface of the insulating layer in a portion where the gate electrode is not formed.

According to a fifth aspect of the present invention, there is provided a liquid crystal display substrate according to the fourth aspect, wherein:
The end portion of the electrode connected to the pixel electrode, which is opposite to the end portion of the source electrode, is located outside the upper region of the gate electrode.

According to a sixth aspect of the present invention, in order to solve the above problems, a gate electrode is formed on a transparent substrate, an insulating layer is formed to cover the upper surface of the transparent substrate and the gate electrode, and a semiconductor layer is formed on the insulating layer. And a source electrode and a drain electrode are formed adjacent to each other on the semiconductor layer, and the upper surface of the insulating layer above the gate electrode is formed lower than the upper surface of the insulating layer where the gate electrode is not formed. A method of manufacturing a thin film transistor comprising the steps of: forming a gate electrode on a substrate; forming an insulating layer on the upper surface of the substrate and covering the gate electrode to form an insulating layer; A portion of the insulating layer is covered with a mask, and the upper surface of the insulating layer above the gate electrode is etched from the upper surface of the insulating layer where the gate electrode is not formed. A recess in the insulating layer of the gate electrode upper is etched to be lower, in which subsequently, to form the semiconductor layer and the source electrode and the drain electrode on the insulating layer after removing the resist.

According to a seventh aspect of the invention, in order to solve the above problems, a gate electrode is formed on a transparent substrate, an insulating layer is formed so as to cover the upper surface of the transparent substrate and the gate electrode, and a semiconductor layer is formed on the insulating layer. And a source electrode and a drain electrode are formed adjacent to each other on the semiconductor layer, and the upper surface of the insulating layer above the gate electrode is formed lower than the upper surface of the insulating layer where the gate electrode is not formed. A method of manufacturing a thin film transistor comprising the steps of forming a gate electrode on a substrate, covering the top surface of the substrate and the gate electrode to form an insulating layer, and applying a negative voltage to the gate electrode. Plasma etching is performed by concentrating plasma ions on the convex portion of the insulating layer above the gate electrode to perform plasma etching, and the gate electrode is formed on the upper surface of the insulating layer above the gate electrode. Plasma etching is performed until it becomes lower than the upper surface of the insulating layer in the non-etched portion to form a recess in the insulating layer above the gate electrode, and then the semiconductor layer, the source electrode, and the drain electrode are formed on the insulating layer. .

In order to solve the above-mentioned problems, a gate electrode is formed on a transparent substrate, an insulating layer is formed so as to cover the upper surface of the transparent substrate and the gate electrode, and a semiconductor layer is formed on the insulating layer. And a source electrode and a drain electrode are formed adjacent to each other on the semiconductor layer, and the upper surface of the insulating layer above the gate electrode is formed lower than the upper surface of the insulating layer where the gate electrode is not formed. A method of manufacturing a thin film transistor comprising the steps of: forming a gate electrode on a substrate; forming an insulating layer by covering the upper surface of the substrate and the gate electrode; and projecting an insulating layer formed on the gate electrode. Part of the photosensitive resin and the other part of the photosensitive resin, and the desired part of the photosensitive resin is irradiated with light, and after the light irradiation is developed, the photosensitive resin above the convex portion is removed to remove the photosensitive resin above the gate electrode. Form a recess in the insulating layer And is subsequently to form a semiconductor layer and a source electrode and a drain electrode on the insulating layer.

According to a ninth aspect of the invention, in order to solve the above problems, a gate electrode is formed on a transparent substrate, an insulating layer is formed so as to cover the upper surface of the transparent substrate and the gate electrode, and a semiconductor layer is formed on the insulating layer. And a source electrode and a drain electrode are formed adjacent to each other on the semiconductor layer, and the upper surface of the insulating layer above the gate electrode is formed lower than the upper surface of the insulating layer where the gate electrode is not formed. A method of manufacturing a thin film transistor comprising the steps of: forming a gate electrode on a substrate; forming an insulating layer by covering the upper surface of the substrate and the gate electrode; and projecting an insulating layer formed on the gate electrode. The photochemical vapor deposition method is carried out while irradiating light to the portion except the portion, and the insulating layer is laminated on the portion other than the convex portion above the gate electrode until it becomes higher than the convex portion, and the insulating layer is formed above the gate electrode. Forming a recess , In which subsequently forming the semiconductor layer and the source electrode and the drain electrode on the insulating layer.

[0023]

Since the upper surface of the insulating layer above the gate electrode is formed lower than the upper surface of the insulating layer in other portions, and the distance between the upper surface of the gate electrode and the upper surface of the insulating layer above it is short, the transparent substrate Even if a part of the light such as the backlight that has passed through is incident on the peripheral side of the gate electrode, it is less likely that the light will be incident on the semiconductor layer above the gate electrode. Further, when a recess is formed in the insulating layer above the gate electrode and the inner side surface of the recess is inclined, the inclined inner side surface reflects the incident light from the back surface side of the substrate to the outer side of the gate electrode. Therefore, there is less risk that light will be incident on the semiconductor layer side above the gate electrode. Therefore, a photocurrent is not generated in the semiconductor layer, and the off-state current of the thin film transistor is lower than that in the conventional structure. Therefore, when the thin film transistor is driven, the ratio of on-current to off-current, that is, the on-off ratio is improved.

Further, the distance between the base of the drain electrode and the peripheral edge of the gate electrode becomes longer than in the case of the conventional structure, and the thickness of the insulating layer sandwiched therebetween increases, so that the gate electrode
The drain capacitance decreases. Therefore, switching noise is reduced. Further, the conventional structure having a storage capacitor to reduce apparent switching noise can eliminate the need for the storage capacitor. Therefore, a special film-forming process for forming the storage capacitor is not required, and the film-forming process can be simplified, so that the yield at the time of manufacturing is improved, and a special light-shielding film is formed for forming the storage capacitor. Since it is unnecessary, the aperture ratio of the liquid crystal display device is improved when the thin film transistor is used as the substrate for the liquid crystal display device.

Further, by providing the structure in which the end of the drain electrode is provided outside the upper region of the gate electrode, there is no overlap between the drain electrode and the gate electrode. Drain (pixel side) electrode, which reduces the gate-drain capacitance.

On the other hand, when forming a recess in the upper part of the insulating layer above the gate electrode of the thin film transistor, after forming the gate insulating layer, the insulating layer is covered in the same manner as in the prior art, and the convex part formed on this insulating layer is photo-processed. If it is selectively removed by etching using a mask, it is possible to accurately and surely form a recess of a desired size in the insulating layer on the gate electrode. It is possible to prevent light from being incident on the semiconductor layer above the gate electrode by being reflected to the other side, and thus it is possible to manufacture a thin film transistor having an excellent structure in which leakage current and switching noise can be reduced.

Next, if the insulating layer is coated on the gate electrode and then the insulating layer is plasma-etched with a negative bias applied to the gate electrode, positive charges can be concentrated near the gate electrode. The insulating layer above the gate electrode can be mainly etched, and a recess is formed above the insulating layer. Thereby, it is possible to prevent the light from being reflected to the outside of the gate electrode by the inner surface of the recess and causing the light to be incident on the semiconductor layer above the gate electrode,
It is possible to manufacture a thin film transistor having an excellent structure that can reduce leakage current and switching noise. In addition, after forming an insulating layer on the gate electrode, a photosensitive resin is coated, the portion other than the upper region of the gate electrode is photo-cured, and the uncured portion is developed and removed. A recess is formed in the. As a result, it is possible to prevent light from being reflected to the outside of the gate electrode by the inner surface of the recess and to make the light incident on the semiconductor layer above the gate electrode, and to reduce leakage current and switching noise. A thin film transistor having a structure can be manufactured.

Furthermore, if the insulating layer is formed on the gate electrode and then the insulating layer is selectively formed on a portion other than the upper portion of the gate electrode by photo-CVD, an insulating layer having a recess is formed. As a result, it is possible to prevent light from being reflected to the outside of the gate electrode by the inner surface of the recess and to make the light incident on the semiconductor layer above the gate electrode, and to reduce leakage current and switching noise. A thin film transistor having a structure can be manufactured.

[0029]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of a thin film transistor according to the present invention.
Includes a gate electrode 32 formed on a transparent substrate 31 such as glass (for example, Corning # 7059 glass having a refractive index of 1.530), and an insulating layer 33 (which covers the upper surface of the substrate 31 and the gate electrode 32). For example, a silicon nitride film having a refractive index of about 1.85) is formed. And
On the upper surface of the insulating layer 33 above the gate electrode 32, a recess 35 slightly larger than the width of the gate electrode 32 is formed. The inner side surface 35a of the recess 35 is an inclined surface having an inclination angle θ, and the thickness of the insulating layer 33 on the bottom surface of the recess 35 is
It is formed thinner than the insulating layer 33 in other portions.
This inclination angle θ is not particularly limited, but is 10
It is preferable that the number of degrees is equal to or higher than that because the light of the backlight does not reach the channel portion.

A semiconductor layer 36 of amorphous silicon or the like is formed to cover the insulating layer 33 and the recess 35, and a source electrode 37 and a drain electrode 38 are formed above the gate electrode 32 so as to face each other. A semiconductor layer 39 such as amorphous silicon that is ion-implanted between the source electrode 37 and the semiconductor layer 36, and a semiconductor layer 40 such as amorphous silicon that is ion-implanted between the drain electrode 38 and the semiconductor layer 36, respectively. A channel portion 41 is formed by the semiconductor layer 36 formed between the source electrode 37 and the drain electrode 38.

Although not shown in FIG. 1, the thin film transistor 30 of this structure is incorporated in the liquid crystal display device instead of the thin film transistor 3 of the liquid crystal display device shown in FIGS. 12 and 13. In the same manner as the conventional thin film transistor 3, it is used for driving liquid crystal. That is, in order to perform liquid crystal display, a signal is applied to the desired scanning electrode line G and signal electrode line S to charge the specific pixel electrode 15 by the specific thin film transistor 30 and apply an electric field to the molecules of the liquid crystal 20. By doing so, it is possible to display and hide.

In the thin film transistor 30 having the above structure, the recess 35 is formed in the insulating layer 33 above the gate electrode 32, and the insulating layer 33 in the recess 35 is thinly formed.
Moreover, since the inner side surface 35a of the concave portion 35 is inclined at the inclination angle θ, even if the light from the backlight is incident on the periphery of the gate electrode 32 as shown by the arrow b in FIG. There is no possibility that the light of the backlight is reflected by the substrate 31 side and enters the semiconductor layer 36 above the gate electrode 32. Therefore, the semiconductor layer 3 of the channel portion 41
6, no photocurrent is generated, and the thin film transistor 30
14 is lower than that of the conventional structure shown in FIG. 14, the ratio of the on-current to the off-current, that is, the on / off ratio when driving the thin film transistor 30 is improved. Further, since the distance between the base portion 38a of the drain electrode 38 and the peripheral portion of the gate electrode 32 is longer than in the case of the conventional structure shown in FIG. 14, the gate-drain capacitance (parasitic capacitance: C _GD ) is reduced. Decrease. Therefore, switching noise is reduced.

FIG. 2 shows a second embodiment of the thin film transistor according to the present invention. The thin film transistor 5 of this example is shown in FIG.
0, the same components as those of the thin film transistor 30 of the first embodiment are designated by the same reference numerals, and the description of those components will be omitted. The thin film transistor 50 of this example differs from the thin film transistor 30 of the first embodiment in that the drain electrode 38 'and the semiconductor layer 40' are different.
However, the point is that the gate electrode 32 is not provided above the gate electrode 32, but is provided further laterally away from the upper region. As a result, the channel portion 41 'is formed wider than the channel portion 41 of the thin film transistor 30 of the first embodiment, and the light receiving portion 41a made of the semiconductor layer 36 is formed above the right side portion of the gate electrode 32 in FIG. .

With this structure, since the overlapping portion between the drain electrode 38 'and the gate electrode 32 is eliminated, the light receiving portion 41a irradiated with the backlight can be substantially used as the drain (pixel side) electrode. This makes it possible to reduce the gate-drain capacitance (parasitic capacitance: C _GD ). Further, in the structure shown in FIG. 2, since the concave portion 35 is formed on the upper surface of the insulating layer 33 as in the first embodiment, the leak current due to the photocurrent can be suppressed as in the second embodiment. The storage capacity can be eliminated. Therefore, also in the present structure, it is possible to provide a thin film transistor having a high on / off ratio and a high aperture ratio as in the structure of the first embodiment, and at the same time, improve the manufacturing yield.

FIG. 3 shows a third embodiment of the thin film transistor according to the present invention. The thin film transistor 6 of this embodiment is shown in FIG.
0, the same components as those of the thin film transistor 30 of the first embodiment are designated by the same reference numerals, and the description thereof will be omitted. The thin film transistor 60 of this example is different from the thin film transistor 30 of the first embodiment in that the drain electrode 38 ′ and the semiconductor layer 40 ′ are not in the upper region of the gate electrode 32, but are further separated laterally from this upper region. And the source electrode 37 '.
The semiconductor layer 39 ′ is provided not in the upper region of the gate electrode 32, but further apart from the upper region in the lateral direction. Due to these, the channel portion 41 '' is formed wider than the channel portion 41 of the thin film transistor 30 of the first embodiment, and above the right side portion of the gate electrode 32 of FIG.
The light receiving portion 41a is formed, and the light receiving portion 41b is formed above the left side portion of the gate electrode 32.

In the thin film transistor 60 having the structure shown in FIG. 3, the same effect as that of the thin film transistor 50 having the structure shown in FIG. 2 can be obtained.

Next, the thin film transistor 5 having the structure shown in FIG.
The relationship between the equivalent circuit of 0, the ON resistance and the OFF resistance, and the ON current and the OFF current will be described. In the case of the thin film transistor 50 in which the light receiving portion 41a can be used as the drain electrode as shown in FIG. 2, the equivalent circuit of this structure is
As shown in FIG. 4, the resistance Rp of the light receiving portion 41a is connected in series to the thin film transistor 50. The resistance Rp of the light receiving portion 41a is the thin film transistor 5
Since it is larger than the _ON resistance R _ON of 0, the effective ON resistance R ′ _ON is restricted to a minimum value by Rp. That is, the on-current I _ON is restricted by Rp.
The on current I _ON it is enough to be charged within a time determined total capacitance C = CLC + Cs of the pixel portion is minimum. Further, CLC represents the capacitance of the pixel portion with the liquid crystal sandwiched therebetween.

Here, in the thin film transistors 3, 3'of the conventional structure shown in FIG. 14, the storage capacitance Cs cannot be determined because I _OFF is large, and the total capacitance C of the pixel portion is large, so that the ON current is large. Since I _ON also needed to be large, R
p could not be added. That is, in the structure of FIG. 14, even if the structure shown in FIG.
The light receiving portion 41a cannot be used as the drain electrode 38 ', and the structure shown in FIG. 14 necessarily requires an overlapping portion between the drain electrode 11 and the gate electrode 8.
The capacitance between the gate and the drain (parasitic capacitance: C _GD ) becomes large, which causes a vicious circle in which a large storage capacitance Cs is required.

On the other hand, in the structure shown in FIG. 2, the gate-drain capacitance (parasitic capacitance: C _GD ) and the off current I _OFF.
Since both can be reduced at the same time, the storage capacity C
As a result, the yield and the aperture ratio can be improved as described above.

The above results will be illustrated and explained in an easy-to-understand manner as follows based on FIGS. 5 and 6. Figure 5
14 shows respective values in the ON region and the OFF region regarding the relationship between the drain current and the gate voltage in the thin film transistor 3 ′ having the conventional structure shown in FIG. 14 in which the recess 35 is not formed in the insulating layer 33. . Further, FIG. 5 also shows the relationship in the ON region in the case where the drain electrode 11 is provided so as to be displaced from the region above the gate electrode 8 in the thin film transistor 3 ′ having the conventional structure shown in FIG. Generally, in a liquid crystal display device, it is said that the ratio of on-current to off-current needs to be about 6 digits. However, as is clear from FIG. 5, the gate electrode 8 is tentatively compared with the conventional structure shown in FIG. Drain electrode 1 from the upper region
1 is removed, that is, the generated resistance R of the light receiving portion
In the structure in which p is connected in series to the thin film transistor 3 ', the on-current is reduced by one digit, so that the ratio of the on-current to the off-current cannot be substantially reduced to six digits. It is clear that the structure in which the drain electrode 38 'is displaced as shown cannot be realized.

Next, FIG. 6 shows respective drain currents in the case of the structure of the thin film transistor 30 shown in FIG. 1 in which the recess 35 is formed in the insulating layer 33 and the structure of the thin film transistor 50 shown in FIG. 2 in which the light receiving portion 41a is provided. The relationship between the gate voltage and the gate voltage is shown in the ON region and the OFF region. As is clear from FIG. 6, the recess 3 is formed in the insulating layer 33.
5 is formed to suppress the generation of leak current due to photocurrent,
The drain current in the off region is reduced by two to three digits as compared with the case of FIG. Then, the structure shown in FIG.
In the case of no Rp without a, the ratio of on-current to off-current can be 8 to 9 digits, which is not a problem, and by providing the structure shown in FIG. 2, that is, the light receiving portion 41a, Even if the gate current is reduced by about one digit, the ratio of the on-current to the off-current can be maintained at 7 to 8 digits, which can be realized without any problem.

From the above, by adopting the structure of the present invention, the capacitance between the gate and the drain (parasitic capacitance: C _GD ) is drastically reduced. Therefore, in the past, a storage capacitor was provided to reduce apparent switching noise. What had the structure,
It became clear that the storage capacity could be eliminated.

Next, a method of manufacturing the thin film transistor having the above structure will be described. FIG. 7 is for explaining a first example of a method of manufacturing a thin film transistor according to the present invention. First, as shown in FIG.
The gate electrode 32 and the insulating layer 33 are formed thereon by a film forming method equivalent to the conventional method. In this state, a convex portion 33a made of an insulating layer is formed on the insulating layer 33 on the gate electrode 32. Next, only the convex portion 33a is etched and thinned by using the photolithography technique. Here, when a transparent substrate is used as the substrate 31, the entire upper surface of the insulating layer 33 is coated with a negative type photoresist, and when exposed from the back surface side of the substrate 31, the gate electrode 32 is made of a light-shielding conductive material. Therefore, only the photoresist in the area other than the gate electrode 32 is exposed. When development is performed thereafter, only the photoresist in the unexposed region in the upper region of the gate electrode 32 is developed and disappears as shown in FIG. 7B, so that the exposed photoresist 70 remains.

Then, by wet etching or dry etching, only the insulating layer 33 above the gate electrode 32 can be thinned, and the recess 35 can be formed as shown in FIG. 7C. Then, by removing the remaining photoresist 70, the insulating layer 33 having the recess 35 as shown in FIG. 7D can be obtained. Figure 7
After the insulating layer 33 shown in (d) is formed, the semiconductor layer 3 is then formed using a conventional film forming method that has been conventionally used.
By forming 6, 39, 40 and forming the source electrode 37 and the drain electrode 38 as shown in FIG. 1, the thin film transistor 30 having the structure shown in FIG. 1 can be obtained.

In the above-mentioned example, when it is desired to form the insulating layer 33 thickly in the portion other than the channel portion even above the gate electrode 32, in addition to the exposure from the back surface side of the substrate 31, the channel It suffices to expose from the front surface side of the substrate 31 using a photomask that is not exposed to light.
In addition, if the gate electrode 32 other than the channel portion is formed of a transparent conductive material, the insulating layer 33 having the desired concave portion 35 can be formed only by exposure from the back surface side of the substrate 31.

FIG. 8 is for explaining a second example of the method of manufacturing a thin film transistor according to the present invention.
First, as shown in FIG. 8A, the gate electrode 32 and the insulating layer 33 are formed on the substrate 31 by a film forming method equivalent to the conventional method. In this state, the convex portion 33a is formed on the insulating layer 33 on the gate electrode 32. Next, the gate electrode 32
As shown in FIG. 8A, plasma etching is performed with the power supply 72 connected and a negative bias current applied to the gate electrode 32. According to the plasma etching, the positive charge particles in the atmosphere can be selectively concentrated in the vicinity of the gate electrode 32, so that the insulating layer 33 above the gate electrode 32 is selectively etched.
As shown in (b), a desired insulating layer 33 having a recess 35 is obtained. At this time, if the strength of the bias current applied to the gate electrode 32 is adjusted, the recess 35 to be formed is formed.
It is possible to adjust the depth of. After the insulating layer 33 shown in FIG. 8B is formed, the semiconductor layers 36, 39, 40 are then formed by using a conventional film forming method that has been conventionally used.
Then, the source electrode 37 and the drain electrode 38 are formed as shown in FIG. 1, whereby the thin film transistor 30 having the structure shown in FIG. 1 can be obtained.

FIG. 9 is for explaining a third example of the method of manufacturing a thin film transistor according to the present invention.
First, as shown in FIG. 9A, the gate electrode 32 and the insulating layer 33 are formed on the substrate 31 by a film forming method equivalent to the conventional method. In this state, the convex portion 33a is formed on the insulating layer 33 on the gate electrode 32. Next, the insulating layer 33 is coated with a photocurable photosensitive resin layer 74, and the back surface of the substrate 31 is exposed. Here, if the gate electrode 32 is formed of a light-shielding conductive material, the photosensitive resin layer 74 is formed.
Of the upper part of the gate electrode 32 is the uncured layer 74a.
And the other portion becomes the cured layer 74b.

If development processing is performed thereafter, the uncured layer 74a
Since only the insulating layer 33 can be removed, the resin insulating layer 75 can be laminated on the portion of the insulating layer 33 excluding the upper region of the gate electrode 32 as shown in FIG. An insulating layer having a recess 76 formed from is obtained. At this time, if a highly reliable insulating film such as polyimide is used as the photosensitive resin, the photosensitive resin layer can be fully utilized as a part of the insulating layer. After the resin insulating layer 75 shown in FIG. 9C is formed, the semiconductor layers 36, 39 and 40 are then formed using a conventional film forming method which has been conventionally performed, and the source electrode 3 is formed.
By forming 7 and the drain electrode 38, the thin film transistor 70 having the structure shown in FIG. 9D can be obtained.

It is also possible to form the recess 76 by covering the insulating layer 33 with a photomask from the upper surface side at the time of the exposure, exposing from the upper surface side with the use of this photomask, and developing.

FIG. 10 is for explaining a third example of the method of manufacturing a thin film transistor according to the present invention. First, as shown in FIG. 10A, the gate electrode 32 and the insulating layer 33 are formed on the substrate 31 by the film forming method equivalent to the conventional method. In this state, the insulating layer 3 on the gate electrode 32
A convex portion 33 a is formed on the upper portion of 3. Next, the insulating layer 3
3 is used to grow a thin film while irradiating light, and the insulating layers 81 and 81 shown in FIG. 10B are formed while irradiating light to a portion other than the upper region of the gate electrode 32. At this time, the light may be selectively emitted from the upper side of the substrate 31 or may be emitted from the back side of the substrate 31.
In the case of irradiating light from the back surface side of the substrate 31, if the gate electrode 32 is formed of a light-shielding conductive material, the light is shielded by the gate electrode 32 without selective light irradiation, so that the surrounding area is Light can be selectively emitted.
After the insulating layer 81 made of the resin shown in FIG. 10B is formed, the semiconductor layers 36, 39 and 40 are then formed by using a conventional film forming method which has been conventionally performed, and the source electrode 37.
By forming the drain electrode 38 and the drain electrode 38, the thin film transistor 80 having the structure shown in FIG. 10C can be obtained.

[0051]

As described above, according to the present invention, the upper surface of the insulating layer above the gate electrode is formed lower than the upper surface of the insulating layer in other portions, so that the transparent substrate can pass through. Even if light such as a backlight is incident around the gate electrode and is reflected by the semiconductor layer in that portion, the possibility that the light is incident on the semiconductor layer above the gate electrode is reduced. Therefore, unnecessary photocurrent is not generated in the semiconductor layer, and the off-state current of the thin film transistor is lower than that in the conventional structure, so that the ratio of the on-state current and the off-state current when driving the thin film transistor, that is, the on-off ratio is improved. Also,
Since the distance between the base portion of the drain electrode and the peripheral portion of the gate electrode is longer than that in the thin film transistor having the conventional structure in which the convex portion is formed on the insulating layer, the gate-drain capacitance is reduced. Therefore, switching noise is reduced. Further, although the conventional thin film transistor has a structure in which a storage capacitor is provided to reduce apparent switching noise, the storage capacitor can be eliminated. Therefore, the yield at the time of manufacturing is improved, and the aperture ratio of the liquid crystal display device is improved when the thin film transistor is used as the substrate for the liquid crystal display device.

Further, when a recess is formed in the insulating layer above the gate electrode and the inner side surface of the recess is inclined, the inclined inner side surface allows incident light from the back side of the substrate to the outside of the gate electrode. Since the light is reflected by the light, the light does not enter the semiconductor layer above the gate electrode. Therefore, unnecessary photocurrent is not generated in the semiconductor layer, and the off-state current of the thin film transistor is lower than that in the conventional structure, so that the on / off ratio is improved when the thin film transistor is driven. Further, the distance between the base of the drain electrode and the peripheral edge of the gate electrode is longer than in the case of the conventional structure, so that the gate-drain capacitance is reduced. Therefore, switching noise is reduced. Further, the conventional structure having a storage capacitor to reduce apparent switching noise can eliminate the need for the storage capacitor. Therefore, the yield is improved and the aperture ratio of the liquid crystal display device is improved when the thin film transistor is used as the substrate for the liquid crystal display device.

Further, by adopting a structure in which the end of the drain electrode is provided outside the upper region of the gate electrode, the overlapping portion of the drain electrode and the gate electrode is eliminated, so that the portion irradiated by the backlight is substantially formed. Drain (pixel side) electrode, thereby reducing the gate-drain capacitance. Further, in this structure, as in the case of the structure described above, in which the concave portion is formed on the upper surface of the insulating layer, the leak current due to the photocurrent can be suppressed similarly, and the storage capacitor can be eliminated. Therefore, also in this structure, it is possible to provide a thin film transistor having a high on / off ratio and a high aperture ratio or a substrate for a liquid crystal display device as in the case of the structure of the previous example, and also improve the manufacturing yield.

On the other hand, in order to form the concave portion on the insulating layer above the gate electrode of the thin film transistor, after forming the gate insulating layer, the insulating layer is formed in the same manner as in the prior art, and the convex portion formed on the insulating layer is formed. If the portion is selectively removed by etching using a photomask, a concave portion can be formed in the insulating layer on the gate electrode, and thus, with the structure as described above, leakage current and switching noise can be prevented. A small number of thin film transistors can be manufactured.

Next, after the gate electrode is covered with the insulating layer, if the insulating layer is plasma-etched with a negative bias applied to the gate electrode, positive charges can be concentrated in the vicinity of the gate electrode. The insulating layer above the gate electrode can be mainly used for etching, and a recess can be formed above the insulating layer above the gate electrode. In addition, after forming an insulating layer on the gate electrode, a photosensitive resin is coated, the portion other than the upper region of the gate electrode is photo-cured, and the uncured portion is developed and removed. A recess can be formed in Further, if the insulating layer is formed on the gate electrode and then the insulating layer is selectively formed on the portion other than the upper portion of the gate electrode by photo-CVD, the insulating layer having the recess can be formed. From the above, by carrying out the method of the present invention, it is possible to manufacture a thin film transistor having the above-described structure and having less leakage current and switching noise.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of a thin film transistor according to the present invention.

FIG. 2 is a sectional view showing a second embodiment of the thin film transistor according to the present invention.

FIG. 3 is a sectional view showing a third embodiment of the thin film transistor according to the present invention.

FIG. 4 is a circuit diagram showing an equivalent circuit of a first embodiment of the thin film transistor according to the present invention.

FIG. 5 is a diagram showing a relationship between a drain current and a gate current of a conventional thin film transistor.

FIG. 6 is a diagram showing the relationship between the drain current and the gate current in the first and second embodiments of the thin film transistor according to the present invention.

FIG. 7 is an explanatory view showing a first example of a method of manufacturing a thin film transistor according to the present invention.

FIG. 8 is an explanatory view showing a second example of the method of manufacturing a thin film transistor according to the present invention.

FIG. 9 is an explanatory diagram showing a third example of a method of manufacturing a thin film transistor according to the present invention.

FIG. 10 is an explanatory view showing a fourth example of the method of manufacturing a thin film transistor according to the present invention.

FIG. 11 is a circuit diagram showing an example of an equivalent circuit of a conventional active matrix liquid crystal display device.

FIG. 12 is a plan view showing a main part of a structural example of an active matrix liquid crystal display device previously proposed by the present inventors.

13 is a sectional view taken along the line AA of FIG.

14 is an enlarged view showing a part of a thin film transistor of the active matrix liquid crystal display device shown in FIG.

FIG. 15 is a circuit diagram showing an example of an equivalent circuit of a conventionally known active matrix liquid crystal display device.

FIG. 16 is a circuit diagram showing an equivalent circuit of an active matrix liquid crystal display device provided with an independent electrode line, which has been previously proposed by the present inventors.

[Explanation of symbols]

 G scan electrode line, S signal electrode line, 30, 50, 60, thin film transistor, 70, 80, thin film transistor, 31 substrate, 32 gate electrode, 33 insulating layer, 35 recess, 35a inclined surface, 36 semiconductor layer, 37 source electrode, 38, 38 ', drain electrode, 39, 40, 40', semiconductor layer, 41, 41 ', 41' ', channel portion, 75, 81, insulating layer,

Claims (9)

[Claims] 1. A gate electrode is formed on a transparent substrate, and an insulating layer is formed so as to cover the upper surface of the transparent substrate and the gate electrode.
The semiconductor layer is formed on the insulating layer, the source electrode and the drain electrode are formed on the semiconductor layer so as to be adjacent to each other, and the upper surface of the insulating layer above the gate electrode is a portion where the gate electrode is not formed. A thin film transistor formed lower than the upper surface of the insulating layer. 2. The upper surface of the insulating layer above the gate electrode according to claim 1 is formed lower than the upper surface of the insulating layer in a portion where the gate electrode is not formed, and thus the upper surface of the insulating layer above the gate electrode. A thin film transistor, characterized in that a concave portion is formed on the side and an inner side surface of the concave portion is inclined. 3. A gate electrode is formed on a transparent substrate, and an insulating layer is formed to cover the upper surface of the transparent substrate and the gate electrode.
A semiconductor layer is formed on the insulating layer, a source electrode and a drain electrode are formed on the semiconductor layer in an adjacent state, and the upper surface of the insulating layer above the gate electrode is a portion of the insulating layer where the gate electrode is not formed. A thin film transistor which is formed to be lower than an upper surface, and at least an end portion of a drain electrode among end portions of the drain electrode and the source electrode facing each other is located outside an upper region of the gate electrode. 4. A liquid crystal display in which scanning electrode lines and signal electrode lines are wired in a matrix on a transparent substrate, and pixel electrodes and thin film transistors are provided in a portion partitioned by the scanning electrode lines and the signal electrode lines. In the device substrate, a gate electrode connected to the scan electrode line is formed on the substrate, a semiconductor layer is formed on the gate electrode via an insulating layer, and the upper surface of the insulating layer above the gate electrode is the gate electrode. A substrate for a liquid crystal display device, which is formed so as to be lower than an upper surface of an insulating layer in a portion where no is formed. 5. The liquid crystal display device substrate according to claim 4, wherein an end of the drain electrode connected to the pixel electrode, the end of the drain electrode facing the end of the source electrode is located outside the upper region of the gate electrode. A substrate for a liquid crystal display device characterized by the above. 6. A gate electrode is formed on a transparent substrate, and an insulating layer is formed so as to cover the upper surface of the transparent substrate and the gate electrode,
A semiconductor layer is formed on the insulating layer, a source electrode and a drain electrode are formed on the semiconductor layer in an adjacent state, and the upper surface of the insulating layer above the gate electrode is a portion of the insulating layer where the gate electrode is not formed. A method of manufacturing a thin film transistor that is formed to be lower than the upper surface, in which a gate electrode is formed on a substrate, an insulating layer is formed on the upper surface of the substrate and the gate electrode to cover them, and an insulating layer is formed above the gate electrode A portion of the formed insulating layer other than the convex portion is covered with a resist, and etching is performed mainly on the convex portion of the insulating layer, and the upper surface of the insulating layer above the gate electrode is the upper surface of the insulating layer where the gate electrode is not formed. To form a recess in the insulating layer above the gate electrode,
After that, the resist is removed, and then the semiconductor layer, the source electrode, and the drain electrode are formed on the insulating layer. 7. A gate electrode is formed on a transparent substrate, and an insulating layer is formed to cover the upper surface of the transparent substrate and the gate electrode,
A semiconductor layer is formed on the insulating layer, a source electrode and a drain electrode are formed on the semiconductor layer in an adjacent state, and the upper surface of the insulating layer above the gate electrode is a portion of the insulating layer where the gate electrode is not formed. A method for manufacturing a thin film transistor that is formed to be lower than the upper surface, in which a gate electrode is formed on a substrate, an insulating layer is formed on the upper surface of the substrate and the gate electrode to cover them, and a negative electrode is formed on the gate electrode. Plasma etching is performed with the voltage applied to concentrate the plasma ions on the convex portions of the insulating layer above the gate electrode to perform plasma etching. Plasma etching is performed until it becomes lower than the upper surface of the insulating layer to form a recess in the insulating layer above the gate electrode, and then the semiconductor layer and the source are formed on the insulating layer. A method of manufacturing a thin film transistor, which comprises forming an electrode and a drain electrode. 8. A gate electrode is formed on a transparent substrate, and an insulating layer is formed so as to cover the upper surface of the transparent substrate and the gate electrode.
A semiconductor layer is formed on the insulating layer, a source electrode and a drain electrode are formed on the semiconductor layer in an adjacent state, and the upper surface of the insulating layer above the gate electrode is a portion of the insulating layer where the gate electrode is not formed. A method of manufacturing a thin film transistor that is formed to be lower than the upper surface, in which a gate electrode is formed on a substrate, an insulating layer is formed by covering the upper surface of the substrate and the gate electrode, and then formed on the gate electrode. The photosensitive resin is applied to the convex portion and the other portion of the insulating layer, and the desired portion of the photosensitive resin is irradiated with light, and after the light irradiation is developed, the photosensitive resin above the convex portion is removed. Then, a recess is formed in the insulating layer above the gate electrode, and then the semiconductor layer, the source electrode, and the drain electrode are formed on the insulating layer. 9. A gate electrode is formed on a transparent substrate, and an insulating layer is formed so as to cover the upper surface of the transparent substrate and the gate electrode.
A semiconductor layer is formed on the insulating layer, a source electrode and a drain electrode are formed on the semiconductor layer in an adjacent state, and the upper surface of the insulating layer above the gate electrode is a portion of the insulating layer where the gate electrode is not formed. A method of manufacturing a thin film transistor that is formed to be lower than the upper surface, in which a gate electrode is formed on a substrate, an insulating layer is formed by covering the upper surface of the substrate and the gate electrode, and then formed on the gate electrode. The photochemical vapor deposition method is performed while irradiating light on the portion of the insulating layer excluding the convex portion, and the insulating layer is laminated on the portion other than the convex portion above the gate electrode until it becomes higher than the convex portion, and the gate is formed. A method of manufacturing a semiconductor transistor, comprising forming a recess of an insulating layer above an electrode and then forming a semiconductor layer, a source electrode and a drain electrode on the insulating layer.