JP2861989B2 - Thin film transistor and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[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.

[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]

According to the present invention, in a method of manufacturing a thin film transistor for forming a thin film transistor on a transparent substrate, a step of forming a light absorbing film made of a conductive and high melting point material on the transparent substrate is provided. Forming a passivation film on the light absorbing film, forming silicon to be a channel region of the thin film transistor on the passivation film, and forming a gate electrode on the silicon via a gate insulating film Wherein the light absorbing film is formed of the same material and in the same step as a source wiring or a drain wiring which is separated from the light absorbing film and connected to the thin film transistor.

A thin film transistor according to the present invention has a light absorbing film formed of a conductive and high melting point material formed on a transparent substrate, a passivation film formed on the light absorbing film, and a light absorbing film formed on the light absorbing film. The semiconductor device includes silicon serving as a channel region of the formed thin film transistor, and a gate electrode formed on the silicon with a gate insulating film interposed therebetween. The light absorbing film is separated from the light absorbing film and connected to the thin film transistor. And the same material as the source wiring or the drain wiring to be formed.

[0010]

[0011]

[0012]

[0013]

[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 the structure of a thin film transistor according to a first embodiment of the present invention.
(B) 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, and a layer directly above the light-absorbing 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 this 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. Here, 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, so that the light absorbing film 13 itself serves as a local heat source for its 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 the present embodiment, light irradiation was performed intermittently so that the light absorbing film 13 did not become extremely high in 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
It could be maintained at 800 ° 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 the thin film transistor according to the embodiment of the present invention. In FIG. 3, the same portions as those shown in FIG. 1A are denoted by the same reference numerals, and the 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 a structure of a thin film transistor according to a second reference example 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.

Each of the above-mentioned reference examples and embodiments shows the structure of a thin film transistor suitable for a low-temperature process. A part of the polycrystalline silicon film is used as an undoped channel part, and both sides thereof are used as a source part and a drain part. A light absorbing film may be provided even in a thin film transistor structure suitable for a high-temperature process of the structure.

[0027]

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

(A) The light absorbing film can prevent light from the back surface of the substrate from entering the channel.

(B) The light absorbing film can be formed of the same material and in the same step as the source wiring or the drain wiring of the thin film transistor. Therefore, the light absorption film can be formed without increasing the number of steps. Further, the number of layers to be stacked can be reduced.

[0030]

[0031]

[0032]

[Brief description of the drawings]

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

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

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

FIG. 4 is a longitudinal sectional view showing a structure of a thin film transistor according to a second reference example 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

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01L 29/78 627F 627G 21/88 S

Claims (2)

(57) [Claims] 1. A method of manufacturing a thin film transistor for forming a thin film transistor on a transparent substrate, comprising: forming a light absorbing film having conductivity and made of a high melting point material on the transparent substrate; Forming a passivation film, forming silicon to be a channel region of the thin film transistor on the passivation film, and forming a gate electrode on the silicon via a gate insulating film; The method of manufacturing a thin film transistor, wherein the absorbing film is formed in the same step with the same material as a source wiring or a drain wiring connected to the thin film transistor and separated from the light absorbing film. 2. A light-absorbing film having a conductivity and made of a high melting point material formed on a transparent substrate, a passivation film formed on the light-absorbing film, and a thin film transistor formed on the passivation film. And a gate electrode formed on the silicon via a gate insulating film, wherein the light absorbing film is separated from the light absorbing film and connected to the thin film transistor. Alternatively, the thin film transistor is formed using the same material as the drain wiring in the same step.