JPH1070283A - Thin film transistor and fabrication thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a thin film transistor in which an inexpensive substrate can be employed by forming a light absorbing layer of a conductive material having high melting point on a first passivation layer, forming a second passivation layer thereon and then forming a silicon channel region of a thin film transistor further thereon. SOLUTION: An inexpensive hard glass substrate 1 is entirely coated with a lower passivation layer 12 of a silicon oxide or a silicon nitride and a light absorbing layer 13 of a material having high melting point is formed thereon in a small size region. The thin film transistor comprises an upper passivation layer 14 covering the layer 13, a source layer 15 and a drain layer 16 formed directly above the light absorbing layer 13 on the passivation layer 14 while being spaced apart from each other, and a channel layer 17 having an overlapping margin between them. Since a first passivation layer is formed on a substrate, warping of the substrate can be suppressed.

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 (TFT) having a channel portion of silicon and a method of manufacturing the same, and more particularly, to a thin film transistor structure in which an inexpensive glass substrate can be used and a channel portion useful for adopting a low temperature process. The present invention relates to a solid phase growth technique.

[0002]

2. Description of the Related Art Conventionally, as shown in FIG. 5, an inexpensive hard glass substrate 1 is used for a structure of a polycrystalline silicon thin film transistor having a staggered structure which is applied to a low-temperature process or the like. A phosphorus-doped source film 3 and a drain film 4 formed on the passivation film 2 at a distance from each other, and the source film 3 and the drain film 4
And a channel film 5 which is an undoped polycrystalline silicon film having an overlap margin between the channel film 5 and a MOS (M
IS) A thin silicon oxide film 6 serving as a gate insulating film to be formed and a gate electrode 7 of N-type high-concentration polycrystalline silicon
A silicon oxide film 8 serving as an interlayer insulating film covering the gate electrode 7 and the like; an aluminum source electrode 9 in conductive contact with the source film 3 and the drain film 4 via a contact hole; and a pixel electrode serving as a transparent electrode (ITO). (Drain electrode) 10.

A thin film transistor (TFT) having such a structure
6A, a passivation film 2 of a silicon oxide film is entirely coated on a hard glass substrate 1 as shown in FIG. 6A, and a low-pressure CVD method or an ion implantation method is performed thereon. To form a source film 3 and a drain film 4 by patterning. Next, as shown in FIG. 6 (B), a polycrystalline silicon film is entirely coated on the source film 3 and the drain film 4 and is patterned to form an undoped channel film 5. In order to increase the capacity, the entire substrate is heated in a heating furnace, and the polycrystalline silicon of the channel film 5 is recrystallized (solid phase growth) to form polycrystalline silicon having a large grain size.

[0004]

However, the solid-phase growth process has the following problems.

In the solid phase growth step, the entire substrate is heated at a high temperature of about 600 ° C. for several tens of hours in order to obtain an appropriate grain size. However, an inexpensive hard glass substrate 1 having a low dislocation temperature is used as a substrate material. Since the substrate 1 is used, the substrate 1 itself is easily softened, and the substrate after being removed from the furnace is deformed such as distortion and expansion and contraction. For this reason, microfabrication after the solid phase growth step is extremely difficult, and cannot be practically used at all. In other words, when an inexpensive hard glass substrate 1 is used in a low-temperature process, the grain size of polycrystalline silicon can be increased by solid-phase growth of the channel film 5 and the hard glass substrate 1 is deformed. Always accompany.

In addition to the difficulty of microfabrication due to shape deformation, softening of the hard glass substrate 1 during solid phase growth causes impurities to enter the channel film 5 from the hard glass substrate 1 via the passivation film 2. Although the grain size is increased by the solid phase growth, the intrusion of the impurities rather causes the deterioration of the transistor characteristics.

Therefore, an object of the present invention is to maintain the substrate at a low temperature to the extent that it does not soften, and to heat the channel film so that the polycrystalline silicon is optimally grown in solid phase. An object of the present invention is to provide a thin film transistor in which an inexpensive substrate can be used, a channel film has a large grain size, and transistor characteristics are improved by improving the film structure and employing a short-time indirect local heating method. .

[0008]

Means for Solving the Problems To solve the above-mentioned problems, the means taken by the present invention is to provide at least a channel portion on a transparent substrate (such as a film structure applied also in a normal low-temperature process or a high-temperature process). In a thin film transistor structure using polycrystalline silicon as the channel film, a channel portion is not directly formed on a transparent substrate via a passivation film, but a light absorbing film sandwiched between upper and lower passivation films is inserted in a region immediately below the channel portion. It was established.
This light-absorbing film is rich in selective light absorption in a predetermined wavelength range so as to be compatible with lamp annealing during the manufacturing process, and acts as a heat source as a heat storage film during the annealing step, and itself melts and deforms. It is formed of a refractory high melting point material (heat resistant material). Although this light absorbing film mainly serves as an indirect heat source for promoting the solid phase growth of the channel portion in the manufacturing process, it can be formed using a conductive material so as to serve as a back gate electrode which is a functional element in the operation of the transistor. good.

In addition to using the light absorbing film itself as a back gate electrode, for example, a structure having a source wiring or a drain wiring formed of the same material in the process of forming the light absorbing film may be employed. good.

In order to obtain such a structure, a method of manufacturing a thin film transistor according to the present invention is to cover a transparent substrate with a lower passivation film and then selectively absorb light in a predetermined wavelength region in a region where a channel portion is to be formed on the film. After forming at least a light absorbing film using a high melting point material having a good property, the light absorbing film is covered with an upper passivation film, and a sandwich structure of the light absorbing film sandwiched between the upper and lower passivation films is provided. Next, after a required channel portion using polycrystalline silicon is formed directly or indirectly on the light absorbing film, a lamp radius is applied with a light source (for example, a halogen lamp) adapted to the above-described predetermined wavelength region, and the channel is formed. The portion is solid-phase grown to increase the grain size of the polycrystalline silicon.

[0011]

As described above, by adopting a structure in which the light absorbing film sandwiched between the upper and lower passivation films is interposed between the transparent substrate and the channel portion, light irradiation can be performed on the entire substrate by a halogen lamp or the like. The irradiation light passes through the transparent substrate itself, but is efficiently absorbed by the light absorbing film, so that the temperature of the light absorbing film rises, and this increases the temperature,
This light absorbing film itself becomes a local heat source in the vicinity thereof, and heats the channel portion by heat conduction or heat radiation. Therefore, solid phase growth of polycrystalline silicon in the channel portion is promoted, and polycrystalline silicon having a large grain size is formed. Therefore, the ON current capacity is increased due to the characteristics of the transistor. In this lamp annealing step, the transparent substrate itself mainly receives conduction heat from the light absorbing film which is a local heat source, but the transparent substrate has a considerably larger heat capacity than the light absorbing film, and the light absorbing film is transparent. Since the transparent substrate is formed in a small size on one side of the substrate, the transparent substrate functions as a heat sink and does not reach a high temperature. Therefore, even if an inexpensive hard glass substrate is used, no substrate deformation occurs, and the obstacle to fine processing of the upper layer is eliminated. The thing to note here is that
The temperature of the light absorbing film is equal to or higher than the dislocation point of the transparent substrate (for example, 700
８００800 ° C.). This benefit contributes to optimization of the solid phase growth temperature without deformation of the transparent substrate,
A transistor with an increased on-current capacity is realized. Further, the lamp annealing time can be shortened (for example, 1 to 2 hours) as compared with the case of the conventional thermal annealing using a heating furnace, and the throughput can be increased.

The light absorbing film and the upper and lower passivation films prevent intrusion of impurities generated from the transparent substrate immediately below the light absorbing film into the channel portion during lamp annealing. Of course, it is desirable that the thickness of the lower passivation film is sufficiently large. However, since only the portion immediately below the light absorbing film is heated by heat conduction, even if the lower passivation film is thin, the light absorbing film itself does not penetrate impurities. Function as a barrier for On the other hand, the upper passivation film prevents impurities from the light-absorbing film from entering the channel portion. However, since the light-absorbing film is made of a high-melting-point material, the amount of evaporated impurities is small. The thickness may be relatively thin.

The light absorbing film is not only meaningful in the above manufacturing process, but also can be used as a back gate electrode. In this case, the number of steps can be reduced. In addition, in the step of forming the light absorbing film, a source wiring or a drain wiring can be simultaneously formed separately, which also leads to a reduction in the number of steps.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1A is a longitudinal sectional view showing a structure of a thin film transistor according to an embodiment applied to a low-temperature process of the present invention, and FIG. 1B is a plan view of the same structure.

The structure of this polycrystalline silicon thin film transistor uses an inexpensive hard glass substrate 1, a lower passivation film 12 of a silicon oxide film or a silicon nitride film which covers the entire surface thereof, and a small size region on the film 12. A light-absorbing film 13 of a high melting point material such as tungsten, molybdenum, titanium, silicide, silicon, and the like, an upper passivation film 14 covering the film 13, A phosphorus-doped source film 15 and a drain film 16 formed separately on the upper passivation film 14 and an undoped polycrystalline silicon film having a margin between the source film 15 and the drain film 16. A channel film 17 and a thin silicon oxide serving as a gate insulating film to form a MOS portion on the channel film 17 The film 18 and the N-type high-concentration gate electrode 19 of polycrystalline silicon, a thick silicon oxide film 20 as an interlayer insulating film covering the gate electrode 19, etc., the source film 1
An aluminum source electrode 21 and a pixel electrode (drain electrode) 22 as a transparent electrode (ITO), which are in conductive contact with the drain electrode 5 and the drain film 16 via a contact hole;
It is provided with.

As shown in FIG. 1B, the light absorbing film 13 is patterned so as to include the source film 15 and the drain film 16 and the channel film 17 within its plane occupied area. The width of 17 is smaller than those of the source film 15 and the drain film 16.

In the process of obtaining a channel film 17 in this thin film transistor, first, as shown in FIG. 2A, a hard glass substrate 1 is prepared, and a lower passivation film 1 such as a silicon oxide film or a silicon nitride film is formed thereon.
2, a high melting point material film (for example, tungsten, molybdenum, titanium, silicide, silicon) having a thickness of about 2000 to 3000 ° is formed thereon by sputtering or the like, and then the film is patterned. The light absorbing film 13 is obtained. Next, as shown in FIG. 2B, an upper passivation film 14 having a thickness of about 500 to 1000 ° is coated on the light absorbing film 14 by CVD. Next, as shown in FIG. 2C, a phosphorus-doped polycrystalline silicon film is coated on the passivation film 14 immediately above the light absorption film 14 by low-pressure CVD or ion implantation, and then the source is formed by patterning. A film 15 and a drain film 16 are formed. Next, as shown in FIG.
A polycrystalline silicon film is entirely covered on the source film 15 and the drain film 16 and is patterned to form an undoped channel film 5. At this point, the grain size of the polycrystalline silicon of the channel film 5 is relatively small, but the entire substrate is subjected to lamp annealing (center wavelength 1.1 μm) by light irradiation using a halogen lamp as a light source. The irradiation light passes through the transparent hard glass substrate 1, but the irradiation light hitting the region of the light absorbing film 13 is efficiently absorbed therein. As a result, the temperature of the light absorbing film 13 rises and becomes high temperature, so that the light absorbing film 13 itself becomes a local heat source to the surroundings, and heats the channel film 17 by heat conduction or heat radiation. When the channel film 17 is heated, the polycrystalline silicon grows in a solid phase. The temperature of this solid phase growth is the channel film 1
7, the temperature of the channel film 17 also depends on the illuminance and time of light irradiation. In this example, light irradiation was performed intermittently so that the light absorbing film 13 did not become extremely high temperature and maintained a certain steady temperature.
Further, the temperature of the channel film 17 during the solid phase growth is set to 700 to 80.
It could be maintained at 0 ° C.

This temperature is a temperature exceeding the dislocation point of the hard glass substrate 1. Moreover, the annealing time could be reduced to 1-2 hours. By this lamp annealing step, polycrystalline silicon having a large grain size is obtained,
Although the on-current capacity can be increased and the throughput can be increased by shortening the solid phase growth process, the greatest advantage is that the inexpensive hard glass substrate 1 is not deformed, so that the fine workability and the flatness of the TFT can be improved. Sex is not spoiled. Since the light absorption film 13 is heated as a local heat source by the light irradiation and indirectly heats the surroundings, the hard glass substrate 1 itself is not directly heated, but rather functions as a heat sink. This is because the temperature can be suppressed and maintained at a temperature lower than the glass transition point.

The light absorbing film 13 also functions as an impurity barrier film in the annealing step. In the annealing step, the hard glass substrate 1 under the light absorbing film 13 is heated, and the channel film 17 may be contaminated by the back diffusion of the impurity. However, the light absorbing film 13 prevents the intrusion of the impurity due to the back diffusion. Although the diffusion of impurities from the light absorbing film 13 itself is conceivable, it is not a problem as far as the evaporation impurity itself is very small because of the high melting point material and the diffusion is prevented by the upper passivation film 14.

After this lamp annealing step, an upper layer thin film is formed by a normal process, and a thin film structure as shown in FIG. 1A is obtained. An oxide film can also be formed.

FIG. 3 is a longitudinal sectional view showing the structure of a thin film transistor according to a second embodiment of the present invention. In FIG. 3, the same portions as those shown in FIG. 1 (A) are denoted by the same reference numerals, and description thereof will be omitted.

In this embodiment, the source wirings 1 spaced apart from the light absorbing film 13a on the lower passivation film 12
3b and a drain wiring 13c. The source wiring 13b is in conductive contact with the upper source film 15, and the drain wiring 13c is in conductive contact with the upper drain film 16, respectively. The source wiring 13b and the drain wiring 13C are formed of the light absorbing film 13
are formed simultaneously in the step of forming a. Therefore, the source wiring 13b and the drain wiring 13c are
Although it is made of the same material as a, the material is a conductive material. According to this embodiment, although the material of the light absorbing film 13a has conductivity and the degree of freedom in selecting the film material is slightly reduced,
There is an advantage that the number of steps is reduced as compared with the first embodiment. Of course,
In the lamp annealing step of the channel film 17, the source wiring 13b and the drain wiring 13C are also
It effectively functions as a local heat source similar to 3a.

FIG. 4 is a longitudinal sectional view showing the structure of a thin film transistor according to a third embodiment of the present invention. In FIG. 4, the same portions as those shown in FIG. 1 (A) are denoted by the same reference numerals, and description thereof will be omitted.

Although the structure of this embodiment is almost the same as that of the first embodiment, the light absorbing film 23 is used as a back gate electrode. Therefore, the channel film 17
The upper passivation film 14 immediately below the gate electrode functions as a gate insulating film. According to such a structure, doubling of the on-electrode capacitance is realized.

Although each of the above embodiments shows the structure of a thin film transistor suitable for a low-temperature process, a high-temperature thin film transistor having a structure in which a part of a polycrystalline silicon film is formed as an undoped channel portion and both sides thereof are formed as a source portion and a drain portion. A light absorbing film may be provided in a thin film transistor structure suitable for a process.

[0027]

As described above, the present invention is characterized in that a light absorbing film which can be a local heat source in lamp annealing is provided near the channel portion of polycrystalline silicon. To play.

In the lamp annealing step, the polycrystalline silicon in the channel portion can be solid-phase grown above the dislocation point of the substrate while maintaining the substrate temperature below the dislocation point, so that the on-current capacity can be increased without deformation of the substrate. Can be increased. In other words, an inexpensive glass substrate having a low dislocation point can be used.

It is not a total heating method in which the entire substrate is heated in a heating furnace for a long time, but a local heating method, and a solid phase growth at a temperature higher than the dislocation point of the substrate is also possible. Throughput is significantly improved.

The light absorbing film also functions as a protection film for penetration of impurities generated from within the substrate into the channel during solid phase growth, so that the quality of transistor characteristics can be improved.

Further, the structure in which the source wiring or the drain wiring is formed on the lower passivation film and is separated from the light absorbing film on the lower passivation film by using the same material as that of the above-described structure, as compared with the method of manufacturing a thin film transistor having the above structure. The number of steps can be reduced.

Further, in the structure in which the light absorbing film is formed of a conductive material as the back gate electrode, the on-current capacity can be doubled.

[Brief description of the drawings]

FIG. 1A is a longitudinal sectional view showing a structure of a thin film transistor according to an example applied to a low-temperature process of the present invention,
(B) is a plan view of the same structure.

FIGS. 2A to 2D are longitudinal sectional views for explaining main processes in the embodiment.

FIG. 3 is a longitudinal sectional view illustrating a structure of a thin film transistor according to a second embodiment of the present invention.

FIG. 4 is a longitudinal sectional view illustrating a structure of a thin film transistor according to a third embodiment of the present invention.

FIG. 5 is a longitudinal sectional view showing a structure of a thin film transistor applied to a conventional low-temperature process.

FIGS. 6A and 6B are longitudinal sectional views for explaining steps up to obtaining a channel film in the conventional structure.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Hard glass substrate 12 ... Lower passivation film 13, 13a, 23 ... Light absorption film 14 ... Upper passivation film 15 ... Source film 16 ... Drain film 17 ... Channel film 18 ... Silicon oxide film 19 ... Gate electrode 20 ... Interlayer insulating film Silicon oxide film 21: Source electrode 22: Pixel electrode (drain electrode) 13b: Source wiring 13: Drain wiring

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[Procedure amendment]

[Submission date] September 1, 1997

[Procedure amendment 1]

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[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a thin film transistor (TFT) having a silicon channel portion and a method of manufacturing the same.

[Procedure amendment 3]

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[Correction target item name] 0008

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[0008]

SUMMARY OF THE INVENTION The present invention comprises a step of forming a first passivation film on a transparent substrate, and a step of forming a light absorbing film made of a conductive and high melting point material on the first passivation film. Forming a second passivation film on the light absorbing film; and forming a channel region of a thin film transistor made of silicon on the second passivation film.

[Procedure amendment 4]

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A thin film transistor according to the present invention comprises: a first passivation film formed on a transparent substrate; a light absorbing film made of a conductive and high melting point material formed on the first passivation film; And a channel region of a thin film transistor made of silicon formed on the second passivation film.

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[0027]

As described above, according to the structure of the present invention, the following remarkable effects can be obtained.

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(A) The light absorbing film can prevent light from the back surface of the substrate from entering the channel, and can reduce a leak current at the time of off.

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(B) Since the light absorbing film is formed on the first passivation film formed on the substrate, the light absorbing film can be formed with good adhesion.

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(C) Since the thin film transistor is formed after the formation of the light absorbing film, there are various heat treatment steps for forming the thin film of the thin film transistor. However,
Even in such a heat treatment step, since the first passivation film is formed on the substrate, the occurrence of the warpage of the substrate can be suppressed, and thus the problem of misalignment between the light absorbing film and the thin film transistor can be suppressed. it can.

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Claims (4)

[Claims] 1. A thin film transistor having at least a channel portion made of polycrystalline silicon in a thin film structure on a transparent substrate, wherein the thin film transistor has a predetermined wavelength region sandwiched between upper and lower passivation films in a region immediately below the channel portion on the transparent substrate. A thin film transistor having a light-absorbing film made of a material having a high melting point and high selectivity. 2. The thin film transistor according to claim 1, wherein said light absorbing film is formed of a conductive material as a back gate electrode. 3. The semiconductor device according to claim 1, further comprising a source wiring or a drain wiring which is separated from the light absorbing film on the lower passivation film and formed of the same material. The thin film transistor as described in the above. 4. A method for manufacturing a thin film transistor in which at least a channel portion in a thin film structure on a transparent substrate is made of polycrystalline silicon, comprising: a step of coating a lower passivation film on the transparent substrate; A step of forming at least a light-absorbing film with a high-melting-point material rich in selective light-absorbing property in a predetermined wavelength region in a region to be formed; a step of covering the light-absorbing film with an upper passivation film; A method of manufacturing a thin film transistor, comprising: a step of forming a required channel portion using polycrystalline silicon; and thereafter, a step of performing lamp annealing with a light source suitable for the predetermined wavelength region.