JP2001210836A - Method of manufacturing thin film transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new method of manufacturing thin film transistor. SOLUTION: In this method, a thin film transistor is manufactured in such a way that a base protective film is formed on a substrate having an insulating surface, and a channel area, a source area, and a drain area are formed on the protective film. Then a gate insulating film composed of the same material as that of the protective film is formed on the protective film, channel forming area, source area, and drain area and a gate electrode is formed on the insulating film. The source area and drain area contain micro crystals.

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 (hereinafter also referred to as TFT) using a non-single-crystal semiconductor thin film and a method for manufacturing the same. Particularly, the present invention relates to a high reliability applicable to a liquid crystal display, an image sensor and the like. The present invention relates to a method for manufacturing a thin film transistor.

[0002]

2. Description of the Related Art Recently, a thin film transistor using a non-single-crystal semiconductor thin film manufactured by a chemical vapor deposition method or the like has attracted attention. Since this thin film transistor is formed on an insulating substrate by the chemical vapor deposition method as described above, its fabrication atmosphere temperature can be formed at a low temperature of about 500 ° C. at the highest, and it is inexpensive soda glass and borosilicate glass. Can be used as a substrate.

This thin film transistor is a field effect type and has the same function as a so-called MOSFET, but can be formed on an inexpensive insulating substrate at a low temperature as described above. Further, the maximum area to be manufactured is limited only by the size of an apparatus for forming a thin film semiconductor, and has an advantage that a transistor can be easily manufactured on a large-area substrate. For this reason, it is extremely promising as a switching element of a liquid crystal display having a matrix structure having a large number of pixels and a switching element of a one-dimensional or two-dimensional image sensor.

In order to fabricate the thin film transistor, photolithography, which is an established technique, can be applied, so-called fine processing can be performed, and integration can be achieved like an IC or the like. FIG. 2 schematically shows a typical structure of this conventionally known TFT.

[0005] (20) is an insulating substrate made of glass,
(21) is a thin film semiconductor made of a non-single-crystal semiconductor, (22), (23)
Is the source / drain region, (24) and (25) are the source / drain electrodes, (26) is the gate insulating film, and (27) is the gate electrode. The thin film transistor configured as above adjusts the current flowing between the source / drain (22) and (23) by applying a voltage to the gate electrode (27).

At this time, the response speed of the thin film transistor is given by the following equation. S = μ · V / L ² where L is the channel length, μ is the carrier mobility, and V is the gate voltage.

[0007]

The non-single-crystal semiconductor layer used in the thin film transistor contains a large amount of crystal grain boundaries in the semiconductor layer, and the carrier mobility is higher than that of the single-crystal semiconductor due to these. Is very small, and the response speed of the transistor is very slow, as can be seen from the above equation. Especially when an amorphous silicon semiconductor is used, its mobility is about 0.1 to 1 (cm ² / V · Sec).
) And almost no operation as a TFT.

In order to solve such a problem, it is known that the channel length is shortened and the carrier mobility is increased as is apparent from the above equation, and various improvements have been made.

Particularly, when the channel length L is shortened, the response speed is affected by its square, which is a very effective means. However, when forming elements on a large-area substrate, which is a feature of TFTs, it is necessary to use photolithography technology to reduce the distance between the source and drain (which roughly corresponds to the channel length) to 10 μm or less. Obviously, it is difficult in terms of accuracy, yield, production cost, etc., and no effective means has yet been established to reduce the TFT channel length.

On the other hand, as a method of increasing the mobility (μ) possessed by the semiconductor layer itself, a single crystal semiconductor or a polycrystalline semiconductor is used as a semiconductor layer used for a TFT,
2. Description of the Related Art An active layer portion of an FT is made of a single crystal semiconductor or a polycrystalline semiconductor.

In the former method, it is necessary to increase the temperature for forming the semiconductor layer. On the other hand, in the latter method, the temperature is partially increased to make the active layer portion of the TFT a single-crystal semiconductor or a polycrystalline semiconductor. is necessary.

For example, (1) In an amorphous semiconductor thin film transistor, the film formation temperature of amorphous silicon is about 250 ° C., and the temperature of the subsequent thermal annealing step needs to be about 400 ° C. at the maximum. (2) In the thermally recrystallized polycrystalline semiconductor thin film transistor, the film forming temperature of polycrystalline silicon by a low pressure CVD method and the temperature required for the recrystallization step by heat are 500 to 6
50 ° C. (3) CV necessary for forming a semiconductor layer in a thin film transistor in which only the active layer is polycrystallized
Although the temperature of D is about 250 ° C. to 450 ° C., the temperature exceeds 600 ° C. in the step of recrystallization of the active layer using the CW laser.

As described above, there is an unavoidable heat treatment step in the manufacturing process of the thin film transistor.

On the other hand, the TFT is formed on a substrate made of soda glass or the like. In particular, in the staggered type and the coplanar type, an active layer having a surface conductive channel of a carrier is in direct contact with the glass substrate.

In the TFT manufacturing process, since there is an unavoidable heat treatment process as described above, alkali impurities such as sodium and potassium, metals and the like existing in the glass substrate are diffused outside to form an active layer and a TFT. Invades the semiconductor layer. As a result, the TFT deteriorates device characteristics such as a decrease in mobility and a change in threshold value, and adversely affects long-term reliability.

Further, the operation of the TFT causes the TFT itself to generate heat, whereby the temperature of the glass substrate rises, and similarly, impurities are diffused from the substrate to affect the TFT.

An object of the present invention is to solve the above-mentioned problems, and an object of the present invention is to provide a method for manufacturing a TFT having good element characteristics and high long-term reliability.

[0018]

In order to solve the above problems, the present invention provides an insulating film which can be used as a gate insulating film of a TFT element on a glass substrate by a CVD method or a sputtering method before forming the TFT element. This is characterized in that a film made of the same material as that described above is provided as a base protective film, and a TFT element is formed on the base protective film.

That is, the glass substrate is covered with an insulating film that can be used as a gate insulating film, for example, a silicon oxide film. This prevents the further diffusion of impurities and improves the characteristics of TFT elements and long-term reliability. Hereinafter, the present invention will be described with reference to examples.

[0020]

[Embodiment 1] FIGS. 1A to 1C show schematic steps of manufacturing a planar thin film transistor corresponding to Embodiment 1. FIG.

First, soda glass was used as the glass substrate (1), and silicon oxide (2) was formed on the entire surface of the soda glass (1) by a known sputtering method.
It was formed to a thickness of 0 nm. The conditions for the preparation are shown below. Sputtering gas Oxygen 100% Reaction pressure 0.5Pa RF power 400W Substrate temperature 150 ° C Deposition rate 5nm / min

Next, an I-type non-single-crystal silicon semiconductor film (3) was formed thereon with a thickness of about 100 nm by a known plasma CVD method. The conditions for the preparation are shown below. A substrate temperature of 300 ° C. The reaction pressure 0.05 Torr Rf power (13.56MH _z) 80W using gas SiH ₄

Thereafter, a predetermined etching process is performed to
The state shown in (A) was obtained. Thereafter, the active layer was subjected to laser annealing using an excimer laser to polycrystallize the active layer. The conditions are shown below. Laser energy density 200mJ / cm ² Number of irradiation shots 50

On this, as a low-resistance non-single-crystal semiconductor layer,
A non-single-crystal silicon film (4) having an N-type conductivity is formed.
The preparation conditions at this time were as follows. Substrate temperature 220 ° C. The reaction pressure 0.05 Torr Rf power (13.56MH _z) 120W using gas SiH ₄ + PH ₃ thickness 1500Å

This N-type non-single-crystal silicon film (4) is formed by introducing a large amount of H ₂ gas at the time of its formation, increasing the Rf power, and microcrystallizing to reduce the electric resistance. Is also good.

Next, using a known photolithography technique, this non-single-crystal silicon film (4) is
The channel formation region (7) was patterned except for (4) to obtain the state shown in FIG.

Thereafter, in order to activate the channel formation region (7), hydrogen plasma treatment was performed under the following conditions to activate the channel region. Substrate temperature 250 ° C RF power 100W Processing time 60 minutes

Thereafter, a gate oxide film (5) is formed to a thickness of 100 nm using the same material and the same forming method as the base protective film (2), and then contact holes in the source / drain regions are formed by a known etching method. And an aluminum electrode on it
By forming (6), the state shown in FIG. 1C was obtained, and the thin film transistor was completed.

In the case of this embodiment, the source / drain electrodes
Under (6), there are a gate insulating film (5) and a base protective film (2). Since these are formed by the same material and the same forming method, there is no difference in the thermal expansion of these films generated by heat treatment in the thin film transistor manufacturing process or heat generated during the operation of the thin film transistor, and disconnection of a metal electrode such as aluminum present thereon. Or, it was excellent in long-term reliability without peeling.

[Embodiment 2] FIG. 3 is a schematic view showing a manufacturing method of this embodiment. First, a silicon oxide film was formed on a soda glass substrate (1) by a known sputtering method under the same manufacturing conditions as in Example 1.

Next, a molybdenum metal (10) is formed to a thickness of 200 nm on the base protective film (2), and then a non-single-crystal having a P-type conductivity is formed thereon as a low-resistance non-single-crystal semiconductor layer. A silicon film (8) is formed. The manufacturing conditions at this time were as follows. Substrate temperature 230 ° C. The reaction pressure 0.05 Torr Rf power (13.56MH _z) 150W using gas SiH ₄ + B ₂ H ₆ thickness 200Å

In this case, the film thickness was set to 200 ° and only for the purpose of making ohmic contact with an I-type semiconductor layer to be formed in a later step.

Next, these were etched into a predetermined pattern to obtain the state shown in FIG. Then, an I-type non-single-crystal silicon semiconductor film (3) is formed thereon by a known sputtering method.
It was formed to a thickness of 200 nm. The conditions for the preparation are shown below. A substrate temperature of 250 ° C. The reaction pressure 0.2 Pa Rf power (13.56MH _z) 80W Gas used Ar

Next, as in the first embodiment, the I-type semiconductor layer (3) was polycrystallized and activated by hydrogen plasma treatment to obtain the state shown in FIG. 3 (B).

Further, a gate insulating film (5) was formed to a thickness of 100 nm in the same manner as in Example 1 by a sputtering method, and then a gate electrode (9) was formed of molybdenum metal to form a predetermined pattern. Thus, the thin film transistor shown in FIG. 3C was completed.

In this embodiment, since the metal electrode is provided below the low-resistance semiconductor layer, the wiring resistance is very small. In particular, when a TFT is used as a switching element of a large-area liquid crystal device, since the wiring resistance is small, a large amount of TFT can be responded at a high speed without the waveform of a drive signal being rounded. Further, the present invention is naturally applicable to thin film transistors having other device structures.

[0037]

According to the structure of the present invention, it is possible to prevent impurities existing in the low-temperature glass as the substrate from invading the active layer of the thin film transistor and the element itself.
A thin film transistor having high transconductance and high field effect mobility can be provided.

Further, it is possible to suppress impurities diffused from the substrate due to heat generated during device operation, to suppress deterioration of the electrical characteristics of the thin film transistor, and to realize a thin film transistor having good long-term stability and reliability. did it.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a manufacturing process of a TFT according to one embodiment of the present invention.

FIG. 2 is a cross-sectional structural view of a conventional TFT.

FIG. 3 is a schematic view showing a manufacturing process of a TFT according to one embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Substrate 2 Underlayer protective film 3 Active layer 4 Source / drain region 5 Gate insulating film 6 Gate and source / drain electrode 7 Channel formation region 8 Source / drain region 9 Gate electrode 10 Source / drain electrode 20 Substrate 21 Active layers 22, 23 Source / drain region 24, 25 Source / drain electrode 26 Gate insulating film 27 Gate electrode

Claims (9)

[Claims] 1. A base protective film is formed on a substrate having an insulating surface, and a channel region, a source region, and a drain region are formed on the base protective film, and the base protective film, a channel formation region, a source region, and a drain are formed. A thin film transistor in which a gate insulating film made of the same material as the base protective film is formed over a region, and a gate electrode is formed over the gate insulating film, wherein the source region and the drain region have microcrystals; A method for manufacturing a thin film transistor. 2. A base protective film is formed on a substrate having an insulating surface, a channel region, a source region, and a drain region are formed on the base protective film, and the base protective film, a channel formation region, a source region, and a drain are formed. A thin film transistor in which a gate insulating film made of the same material as the base protective film is formed on a region, and a gate electrode is formed on the gate insulating film, wherein the source region and the drain region are made of polycrystalline silicon; Method for manufacturing a thin film transistor. 3. A base protective film made of silicon oxide is formed on a substrate having an insulating surface; a channel region, a source region, and a drain region are formed on the base protective film; In a thin film transistor in which a gate insulating film made of silicon oxide is formed over a source region and a drain region, and a gate electrode is formed over the gate insulating film, the source region and the drain region have microcrystals. Method for manufacturing a thin film transistor. 4. A base protective film made of silicon oxide is formed on a substrate having an insulating surface; a channel region, a source region, and a drain region are formed on the base protective film; In a thin film transistor in which a gate insulating film made of silicon oxide is formed over a source region and a drain region, and a gate electrode is formed over the gate insulating film, the source region and the drain region are made of polycrystalline silicon. Method for manufacturing thin film transistor. 5. A source region having a base protective film formed on a substrate having an insulating surface, an active layer formed on the base protective film, crystallizing the active layer, and a source region having microcrystals on the active layer. Forming a drain region; forming a gate insulating film made of the same material as the base protective film on the base protective film, the active layer, the source region and the drain region; forming a gate electrode on the gate insulating film A method for manufacturing a thin film transistor. 6. An under protective film made of silicon oxide is formed on a substrate having an insulating surface, an active layer is formed on the under protective film, the active layer is crystallized, and microcrystals are formed on the active layer. Forming a source region and a drain region, a gate insulating film made of silicon oxide on the base protective film, the active layer, the source region and the drain region, and forming a gate electrode on the gate insulating film. A method for manufacturing a thin film transistor, comprising: 7. A base protective film is formed on a substrate having an insulating surface, and a polycrystalline semiconductor layer including a channel formation region and a microcrystalline semiconductor layer including a source region and a drain region are formed on the base protective film. Forming a gate insulating film made of the same material as the base protective film and the base protective film covering the channel region; and forming a gate electrode on the gate insulating film. 8. A base protective film made of silicon oxide is formed on a substrate having an insulating surface, and a polycrystalline semiconductor layer including a channel formation region and a microcrystal including a source region and a drain region are formed on the base protective film. Forming a gate insulating film made of silicon oxide covering the base protective film and the channel region; and forming a gate electrode on the gate insulating film. 9. The method for manufacturing a thin film transistor according to claim 1, wherein the base protective film and the gate insulating film are formed by the same method.