JP2805035B2 - Thin film transistor - Google Patents

Description

The present invention relates to a thin film transistor (hereinafter also referred to as a TFT) using a non-single-crystal semiconductor thin film and a method of manufacturing the same, and particularly to a liquid crystal display and an image sensor. The present invention relates to a thin film transistor having high reliability that can be applied.

[Background Art] In recent years, 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 is called a so-called thin film transistor.
Although it has the same function as a MOSFET, it can be formed at a low temperature on an inexpensive insulating substrate as described above, and the maximum area to be manufactured is limited only to the dimensions of a device for forming a thin film semiconductor, This has an advantage that a transistor can be easily manufactured over a large-area substrate. For this reason, it is very promising as a switching element of a liquid crystal display having a matrix structure having a large number of pixels or a one-dimensional or two-dimensional image sensor.

In addition, photolithography, which is an established technique, can be applied to fabricate this thin film transistor, so-called fine processing can be performed, and integration can be achieved in the same manner as ICs and the like.

FIG. 2 schematically shows a typical structure of this conventionally known TFT.

(20) is an insulating substrate made of glass, (21) is a thin film semiconductor made of a non-single-crystal semiconductor, (22) and (23) are source / drain regions, (24) and (25) are source / drain electrodes , (26) is the gate insulating film and (27) is the gate electrode.

By applying a voltage to the gate electrode (27), the thin-film transistor having the above-described structure allows the source / drain (2
Adjusts the current flowing between 2) and (23).

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.

The non-single-crystal semiconductor layer used for this thin film transistor contains a large amount of crystal grain boundaries and the like in the semiconductor layer, and the cause thereof is very small carrier mobility as compared with a single-crystal semiconductor. Thus, the problem that the response speed of the transistor is very slow occurs. Especially when using amorphous silicon semiconductor, its mobility is about
It was on the order of 0.1 to 1 (cm ² / V · Sec) and hardly operated 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 clear from the above equation, and various improvements have been made.

In particular, when the channel length L is shortened, the response speed is affected by the square, which is a very effective means.

However, when forming an element on a large-area substrate, which is a feature of TFT, using photolithography technology to reduce the distance between the source and drain (corresponding to the approximate channel length) to 10 μm or less requires processing accuracy, Obviously, it is difficult in terms of yield, production cost, etc., and no effective means has yet been established as a means to shorten the TFT channel length.

On the other hand, to increase the mobility (μ) of the semiconductor layer itself, a single crystal semiconductor or a polycrystalline semiconductor is used as a semiconductor layer used for the TFT, or a single crystal semiconductor or a polycrystal Semiconductors are being used.

In the former method, it is necessary to increase the temperature when forming the semiconductor layer. On the other hand, in the latter method, the temperature is raised partially 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 a thermally recrystallized polycrystalline semiconductor thin film transistor, the film formation temperature of polycrystalline silicon by a low pressure CVD method and the temperature required for the recrystallization step by heat are 500 to 650 ° C.

(3) In a thin film transistor in which only the active layer is polycrystallized, the CVD temperature required to form a semiconductor layer is 250 ° C.
Although it is about 450 ° C., the temperature exceeds 600 ° C. in the recrystallization step of the active layer by the CW laser.

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

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

In the TFT manufacturing process, there is an unavoidable heat treatment process as described above, so sodium existing in the glass substrate,
Alkali impurities such as potassium and metals diffuse to the outside and enter the active layer and the semiconductor layer forming the TFT. 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.

In addition, the TFT itself generates heat due to the operation of the TFT, whereby the temperature of the glass substrate rises, and similarly, impurities are diffused from the substrate to affect the TFT.

[Effect of the Invention] The present invention solves the above-mentioned problems, and an object of the present invention is to provide a TFT structure with good element characteristics and high long-term reliability.

[Constitution of the Invention] The present invention solves the above-mentioned problem by forming a TFT on a glass substrate by a CVD method or a sputtering method before forming a TFT element.
A film made of the same material as the insulating film that can be used for the gate insulating film of the FT element is provided as a base protective film, and is formed on the base protective film
It is characterized by forming a TFT element.

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 diffusion of GaN and improves TFT device characteristics and long-term reliability.

 Hereinafter, the present invention will be described with reference to examples.

Example 1 FIG. 1 shows a schematic manufacturing process of a planar thin film transistor corresponding to Example 1.

First, soda glass was used as the glass substrate (1),
On the soda glass (1), 300 nm of silicon oxide (2) is used as an underlayer protective film on the entire surface by a known sputtering method. Sputter gas Oxygen 100% Reaction pressure 0.5 Pa RF power 400 W Substrate temperature 150 ° C. Film formation rate 5 nm / min On these, an I-type non-single-crystal silicon semiconductor film (3)
Was formed to 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 followed to obtain a state shown in FIG. 1 performs a predetermined etching process (A).

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: 200 mJ / cm ² Number of irradiation shots: 50 A non-single-crystal silicon film (4) having N-type conductivity is formed thereon as a low-resistance non-single-crystal semiconductor layer. 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 non-single crystal silicon film of the N type (4), H ₂ during its formation
A gas in which a large amount of gas is introduced to increase the Rf power to cause microcrystallization and reduce the electric resistance may be used.

Next, using a known photolithography technique, the non-single-crystal silicon film (4) is patterned into a channel forming region (7) while leaving the source / drain region (4) to obtain a state shown in FIG. 1 (B). Was.

Thereafter, a hydrogen plasma treatment was performed under the following conditions to activate the channel formation region (7), thereby activating the channel region.

Substrate temperature 250 ° C RF power 100W Processing time 60 minutes After this, 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 undercoating film (2), and then contacts the source and drain regions Holes were formed by a known etching method, and an aluminum electrode (6) was formed thereon to obtain the state shown in FIG. 1C, thereby completing a thin film transistor.

In the case of the present embodiment, a gate insulating film (5) and a base protective film (2) exist below the source and drain electrodes (6).

Since these are formed by the same material and by the same forming method, there is no difference in thermal expansion of these films generated by heat treatment in the thin film transistor manufacturing process or heat generation 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.

Example 2 FIG. 3 shows a schematic diagram of a manufacturing method of this example.

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 underlying protective film (2), and then a non-single-crystal silicon film (8) having a P-type conductivity is formed thereon as a low-resistance non-single-crystal semiconductor layer. ) Is formed. The manufacturing conditions at this time were as follows.

Purpose 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 this case the thickness of ohmic contact between the I-type semiconductor layer to produce in the process after a 200Å And only.

Next, these were etched into a predetermined pattern to obtain the state of FIG. 3 (A).

Next, an I-type non-single-crystal silicon semiconductor film (3) is formed thereon.
Was formed to a thickness of 200 nm by a known sputtering method. 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 using gas Ar Next, in the same manner as in Example 1 The I-type semiconductor layer (3)
Activated by polycrystallization and hydrogen plasma treatment
The state shown in FIG.

Further, a gate insulating film (5) is formed by a sputtering method.
Was formed to a thickness of 100 nm in the same manner as in Example 1, and then a gate electrode (9) was formed from molybdenum metal to form a predetermined pattern.

Thus, the thin film transistor shown in FIG. 3C was completed.

In the case of this embodiment, since the metal electrode is provided under the low-resistance semiconductor layer, the wiring resistance is very small. In particular, TF is used as a switching element for large-area liquid crystal devices.
When T is used, since the wiring resistance is small, the waveform of the drive signal is not rounded, and a large number of TFTs can respond at high speed.

In addition, the present invention is naturally applicable to thin film transistors having the various device structures.

[Effect] According to the configuration of the present invention, a thin film transistor having high transconductance and high field effect mobility can be prevented from being present in the active layer of the thin film transistor and even the element itself by impurities present in the low-temperature glass as the substrate. Could be provided.

Further, impurities diffused from the substrate due to heat generated during device operation can be suppressed, and a thin-film transistor having good long-term stability and reliability can be realized by suppressing a fire of electrical characteristics of the thin-film transistor.

[Brief description of the drawings]

1 (A) to 1 (C) and 3 (A) to 3 (C) are schematic views showing the steps of manufacturing a TFT according to an embodiment of the present invention. FIG. 2 shows a cross-sectional structure of a conventional TFT. DESCRIPTION OF SYMBOLS 1 ... Substrate 2 ... Underlying 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

 ──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-63-172470 (JP, A) JP-A-61-183970 (JP, A) JP-A-59-108360 (JP, A) JP-A-62 152171 (JP, A) JP-A-63-301518 (JP, A) JP-A-62-286282 (JP, A) JP-A-62-254466 (JP, A)

Claims (2)

(57) [Claims] 1. A base protective film made of the same material as a gate insulating film on a glass substrate, and at least a channel region, a source region, and a drain region on the base protective film.
In a thin film transistor provided with a gate insulating film and a gate electrode, the source region and the drain region include microcrystals. 2. A base protective film on a glass substrate; a semiconductor layer having a channel region, a source region and a drain region on the base protective film; and a gate made of the same material as the base protective film on the semiconductor layer. A thin film transistor, comprising: an insulating film; and a gate electrode over the gate insulating film, wherein the source region and the drain region include microcrystals.