JPH08195492A - Formation of polycrystalline film, and manufacture of film transistor - Google Patents

Abstract

PURPOSE: To form a polycrystalline film excellent in equality by making a drive film contain fine crystals which become the nuclei of crystals for polycrystallization. CONSTITUTION: A fine crystalline silicon film 2 is made, by plasma CVD method using silane and hydrogen as material gas, on a substrate 1 covered with an SiO2 film as a buffer layer for preventing the diffusion of impurities within the glass substrate, and subsequently an amorphous silicon film 3 is made using only the silane as material gas. And,it is finely processed into the shape of an island by photolithography and etching, and then crystallization is performed by irradiating it with an excimer laser beam 4. At this time, the fine crystal layer 2 becomes a seed crystal, and the growth of the crystal grains is performed, so a polycrystalline silicon layer 5 having crystal grains equal all over the surface is obtained. Using this polycrystalline film, a film transistor excellent in equality without dispersion can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a polycrystalline thin film used for various purposes, a liquid crystal display device (hereinafter abbreviated as LCD) for driving liquid crystal, an image reading sensor, a RAM (Random Access). The present invention relates to a method of manufacturing a thin film transistor used as a load of memory.

[0002]

2. Description of the Related Art A polycrystalline silicon thin film transistor which is being developed for a liquid crystal display device and a manufacturing method thereof will be described below as an example with reference to the drawings.

In the field of liquid crystal display using a thin film transistor in recent years, a polycrystalline silicon thin film transistor (hereinafter referred to as poly-Si) which can be formed at a relatively low temperature (generally below 600 ° C.) can use an inexpensive glass substrate instead of an expensive quartz substrate. T
FT) is attracting attention. One of the methods of forming polycrystalline silicon at low temperature is a method of locally melting and crystallizing amorphous silicon by using laser annealing. The drawback of this method is that since the laser is a pulsed laser, the crystallinity deteriorates at the overlapping portion of each pulse, and the transistor characteristics at the overlapping portion of the pulse are poor. To solve this, for example, Extended Abstracts of the 1991 International C
onference on Solid State Devices and Materials, Yo
Kohama, 1991, pp623-625 describes a method of heating a substrate during irradiation with an excimer laser. Here, as a conventional method for manufacturing a poly-Si TFT, the above-mentioned Ex
tended Abstracts of the 1991 International Confere
nce on Solid State Devices and Materials, Yokoham
a, 1991, pp623-625 will be briefly described as a conventional example.

FIG. 7 is a cross-sectional view of a conventional thin film transistor, which will be described below with reference to this drawing. First, after depositing an amorphous silicon layer on the entire surface of the glass substrate 15, the substrate is heated to 400 ° C. and irradiated with an excimer laser, and the amorphous silicon layer on the substrate is locally heated and melted to be crystallized. Then, polycrystalline silicon is obtained. Then, a desired island-shaped patterned polycrystalline silicon thin film 16 is obtained by photolithography and etching. Next, the gate insulating layer 17
Then, a SiO ₂ layer is formed by the AP-CVD method. Next, impurities serving as donors or acceptors are introduced by using the gate electrode 18 formed on the source region 1
9 and the drain region 20 are formed. Subsequently, the interlayer insulating layer 21 is formed, and the source region 19 and the drain region 20 are formed.
Source electrode 2 through contact hole 22 reaching
3 and the drain electrode 24 to form poly-S
i TFT is being manufactured.

Further, since a poly-Si TFT has a larger field effect mobility than a transistor using amorphous silicon as a semiconductor layer, P-channel and N-channel transistors can be formed by selectively using boron or phosphorus as impurities. Since it can be selectively formed, a CMOS circuit can be formed and a drive circuit for a pixel transistor can be formed on the same substrate (not shown).

In this case, the poly-Si TFT is heated by heating the substrate.
The variation in mobility is suppressed within ± 10%.

[0007]

[Problems to be Solved by the Invention]
Even when the ly-Si TFT was manufactured, the inventors of the present invention have found that it is insufficient. That is, even if a liquid crystal display (hereinafter abbreviated as LCD) is manufactured by using the mobility variation of ± 10%, the low mobility portion appears as streaky unevenness on the image, and the display quality is reduced. Has a problem of being extremely low.

In view of the above points, an object of the present invention is to provide a method for forming a polycrystalline thin film having excellent uniformity and a method for manufacturing a thin film transistor in which variations in mobility are suppressed to a smaller extent. .

[0009]

As a result of studies by the inventors, it has been found that the maximum value of the variation in mobility at which unevenness does not appear on an image is ± 5%. On the other hand, FIG. 8 is a schematic diagram showing the energy distribution of one shot of the excimer laser beam, and it can be seen that the energy is low at the beam edge in the AA ′ cross section. In addition, (Fig.
In (a), step-and-repeat irradiation often used in excimer laser annealing is schematically shown.
The mobility on the cross section BB ′ when a thin film transistor is formed by this irradiation method is schematically shown in FIG. 9B. The mobility varies greatly because the crystallinity varies in the overlapping portions of the laser shots. The cause of this variation is that there is a low-energy portion (FIGS. 8A and 8B) in the peripheral portion of one laser shot, and that portion does not grow into a sufficiently large crystal grain but stays in a small crystal grain. Was found to be the main cause. Therefore, as a result of careful examination by the inventors, if microcrystals exist at a certain density in the amorphous material that is the precursor of laser annealing, the crystal grains grow with the microcrystals as crystal nuclei. It has been found that it is extremely effective in suppressing the variation in the characteristics, that is, the variation in the characteristics such as the mobility.

In order to solve the above problems, therefore, the method for forming a polycrystalline thin film of the present invention is characterized in that microcrystals are previously deposited and contained in the precursor amorphous. A thin film transistor is manufactured by using, as an active semiconductor layer, a polycrystalline semiconductor having a polycrystalline thin film obtained by the above forming method.

[0011]

The present invention can provide a polycrystalline thin film having excellent uniformity due to the above constitution, and by manufacturing a thin film transistor using this polycrystalline thin film, a thin film transistor with less variation and good uniformity can be obtained. it can.

[0012]

【Example】

(Embodiment 1) An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a process sectional view for explaining a method for manufacturing a polycrystalline thin film according to a first embodiment of the present invention, and the manufacturing method will be described below in sequence. Although not shown in the figure, the substrate 1 (7 manufactured by Corning Incorporated) coated with a SiO ₂ film as a buffer layer for preventing diffusion of impurities in the glass substrate was used.
059 glass), for example, a microcrystalline silicon thin film 2 having a film thickness of 5 nm is formed by a plasma CVD method using silane (SiH ₄ ) and hydrogen (H ₂ ) as a source gas, and then only silane is used as a source gas. An amorphous silicon thin film 3 having a thickness of 50 nm is formed (FIG. 1A). Then, after finely processing into islands by ordinary photolithography and etching, X with a wavelength of 308 nm and a pulse width of 45 nsec.
eCl excimer laser light 4 200-500 mJ /
Crystallization is performed by irradiating while repeating step and repeat at an energy density of cm ² . At this time, since the microcrystalline layer 2 serves as a seed crystal to grow crystal grains, a polycrystalline silicon layer 5 having uniform crystal grains on the entire surface is obtained.

(Second Embodiment) FIG. 2 shows a second embodiment of the present invention.
Process cross-section for explaining a method for manufacturing a polycrystalline thin film by
It is a figure, and it demonstrates using this figure. In particular in the figure
Although it was not, there was a barrier to prevent the diffusion of impurities in the glass substrate.
SiO as a buffer layer₂Glass substrate with film 1
(Corning 7059 glass)
(Si₂H ₆) Is used as a source gas by the CVD method.
An amorphous silicon (a-Si) thin film 3 having a thickness of 80 nm
Form at 450 ° C. Then, at 480 ℃, 5 nm
A crystalline silicon thin film 2 is deposited (FIG. 2 (a)). next,
Excimer laser light 4 was irradiated in the same manner as in Example 1 to give multiple bonds.
A crystalline silicon thin film 5 is obtained (FIG. 2B). In this example
In the case of Example 1, the film thickness of a-Si is 80 nm.
As compared with the thickness of 30 nm, the crystallization is on the a-Si surface side.
Since it often happens from a.
-Deposited on Si surface.

(Embodiment 3) FIG. 3 is a process sectional view for explaining a method of manufacturing a polycrystalline thin film according to a third embodiment of the present invention, which will be described with reference to this drawing. Although not explicitly shown in the figure, a glass substrate 1 coated with a SiO ₂ film as a buffer layer for preventing diffusion of impurities in the glass substrate 1
Plasma CV using, for example, silane (hereinafter referred to as SiH ₄ ) as a source gas on (Corning 7059 glass)
Amorphous silicon (a-Si) having a film thickness of 20 nm by the D method
The thin film 3 is formed at 300 ° C. Subsequently, a 5 nm microcrystalline silicon thin film 2 is deposited using silicon fluoride (hereinafter referred to as SiF ₄ ) as a source gas. Furthermore, silane (hereinafter S
Amorphous silicon (a-Si) thin film 6 having a thickness of 20 nm is deposited using iH ₄ ) as a source gas (FIG. 3).
(A)). Next, the excimer laser beam 4 is irradiated in the same manner as in Example 1 to obtain a polycrystalline silicon thin film (FIG. 3).
(B)). In this embodiment, microcrystalline silicon serving as a seed crystal is sandwiched with a-Si.

Although silicon is used as the thin film material to be polycrystallized in the first to third embodiments, it is not limited to silicon. For example, germanium or silicon-germanium alloy (SiGe) may be used as a semiconductor material. Good. Further, plasma CVD and thermal CVD have been exemplified as the deposition method of the amorphous material, but it goes without saying that other deposition methods such as ECR-CVD, remote plasma CVD, and sputtering method may be used. It goes without saying that the substrate is not limited to the 7059 glass substrate manufactured by Corning, and any other glass substrate or an insulating substrate such as quartz or sapphire may be used.

(Embodiment 4) FIGS. 4A to 4C are process sectional views for explaining a method of manufacturing a thin film transistor according to a fourth embodiment of the present invention, and the manufacturing method will be described below step by step. Although not shown in the figure, for example, silane (SiH ₄ ) and hydrogen (on a glass substrate 7059 glass manufactured by Corning Co., Ltd.) coated with a SiO ₂ film as a buffer layer for preventing diffusion of impurities in the glass substrate (SiH ₄ ) and hydrogen ( Microcrystalline silicon 2 having a film thickness of 5 nm is formed by a plasma CVD method using H ₂ ) as a source gas, and subsequently, an amorphous silicon thin film 3 having a thickness of 50 nm is formed using only silane as a source gas (FIG. 1).
(A)). Then, after finely processing into islands by ordinary photolithography and etching, XeCl excimer laser light 4 having a wavelength of 308 nm and a pulse width of 45 nsec is used.
Is irradiated while repeating step-and-repeat at an energy density of 200 to 500 mJ / cm ² to perform crystallization. At this time, since the microcrystalline thin film 2 serves as a seed crystal to grow crystal grains, a polycrystalline silicon thin film 5 having uniform crystal grains on the entire surface is obtained (FIG. 1 (b)). Next, for example, SiO ₂ is deposited on the entire surface as the gate insulating layer 7 by the AP-CVD method. Then, chromium (Cr) is deposited by, for example, a sputtering method, and Cr is patterned by photolithography and etching to form the gate electrode 8. Then, the gate electrode 8 in this state is used as a mask at the time of doping to form a source / drain region, and an ion doping method (or a bucket type ion doping method; For example Extended Abstrac
ts of the 22nd (1990) International Conference on
Solid State Devices and Materials, p. 971 or
p.1197) to form a source region 9 and a drain region 10 (FIG. 1 (c)).
300-600 ° C to activate the introduced impurities
Heat treatment is performed to some extent. Then, after that, an insulating film SiO ₂ film 11 is formed for interlayer insulation by, for example, the AP-CVD method, a contact hole 12 reaching the source region 9 and the drain region 10 is formed therein, and a source electrode 13 and a drain electrode 14 are formed. For example, aluminum (A
1) is deposited by sputtering, and then patterned by photolithography etching to obtain poly-S
The iTFT is completed (FIG. 1 (d)). It is desirable to add a hydrogenation step because the characteristics are further improved by compensating the dangling bonds at the grain boundaries of the polycrystal with hydrogen. In this case, the variation in TFT mobility is 4
%was gotten.

(Embodiment 5) FIG. 5 is a process sectional view for explaining a method of manufacturing a thin film transistor according to a fifth embodiment of the present invention. The manufacturing method will be described below in sequence. Although not specifically shown in the figure, for example, disilane (Si ₂ H ₆ ) was formed on a glass substrate 1 (7059 glass manufactured by Corning Co., Ltd.) coated with a SiO ₂ film as a buffer layer for preventing diffusion of impurities in the glass substrate. C used as a source gas
Amorphous silicon (a-S
i) The thin film 3 is formed at 450 ° C. Continuously 480 ℃
To deposit a 5 nm microcrystalline silicon thin film 2 (see FIG. 5).
(A)). After that, the excimer laser beam 4 is irradiated to obtain polycrystalline silicon (FIG. 5B), and a thin film transistor is completed by the same process as in Example 4 (FIG. 5B).
(D)). In this example, since the film thickness of a-Si was set to 80 nm and made 30 nm thicker than that in the case of Example 4, crystallization often occurs from the a-Si surface side. -Deposited on Si surface. In this case, the variation in TFT mobility was 4.5%.

(Embodiment 6) FIG. 6 is a process sectional view for explaining a method for manufacturing a thin film transistor according to a sixth embodiment of the present invention, and the manufacturing method will be described below step by step. Although not specifically shown in the figure, for example, silane (hereinafter SiH ₄ ) was formed on the glass substrate 1 (7059 glass manufactured by Corning Co., Ltd.) coated with a SiO ₂ film as a buffer layer for preventing diffusion of impurities in the glass substrate. An amorphous silicon (a-Si) thin film 3 having a film thickness of 20 nm is formed at 300 ° C. by the plasma CVD method used as a source gas. Continued
A 5 nm microcrystalline silicon thin film 2 is deposited using silicon fluoride (hereinafter SiF ₄ ) as a source gas. Furthermore, silane (hereinafter referred to as SiH ₄ ) is used as a source gas to obtain a film thickness of
An amorphous silicon (a-Si) thin film 6 having a thickness of nm is deposited (FIG. 6A). After that, the polycrystalline silicon 5 is obtained by irradiating the excimer laser light 4 (FIG. 6B), and the thin film transistor is completed by the same process as in Examples 4 and 5 (FIG. 6D). In this embodiment, microcrystalline silicon serving as a seed crystal is sandwiched with a-Si.
In this case, the variation in TFT mobility was 4 to 5%.

Although polycrystalline silicon was used as the semiconductor material in the above Examples 4 to 6, other semiconductor materials such as germanium (Ge) and silicon-germanium alloy (S) were used.
iGe) or the like may be used. Further, although Cr is used as the material of the gate electrode 8 and Al is used as the material of the source electrode 13 and the drain electrode 14, aluminum (Al), tantalum (Ta), molybdenum (Mo), chromium (C) is used.
r), metals such as titanium (Ti) or alloys thereof, or poly-Si or poly-Si containing a large amount of impurities
It may be a transparent conductive layer such as Ge alloy or ITO. Also, it is possible to adopt an LDD structure in order to improve the off characteristics. It is also possible to selectively form P-channel and N-channel transistors by selectively using boron or arsenic as an acceptor as an impurity and phosphorus or aluminum as a donor to build a CMOS circuit on a substrate. Needless to say.

[0021]

As described above, according to the present invention, the precursor thin film includes a microcrystalline layer that serves as a crystal nucleus of polycrystallization, and this microcrystalline layer serves as a seed crystal to grow crystal grains. Because
A polycrystalline thin film having a uniform crystal grain size and high uniformity can be manufactured. In addition, the thin film transistor manufactured using the polycrystalline thin film thus formed is a thin film transistor having excellent uniformity, and its practical effect is great.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view in each main step for explaining a method for manufacturing a polycrystalline thin film according to a first embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view for each main step for explaining the method for manufacturing a polycrystalline thin film according to the second embodiment of the present invention.

FIG. 3 is a schematic cross-sectional view for each main step for explaining the method for manufacturing a polycrystalline thin film according to the third embodiment of the present invention.

FIG. 4 is a schematic cross-sectional view for each main step for explaining a method of manufacturing a thin film transistor according to a fourth embodiment of the present invention.

FIG. 5 is a schematic cross-sectional view for each main step for explaining a method of manufacturing a thin film transistor according to a fifth embodiment of the present invention.

FIG. 6 is a schematic cross-sectional view for each main step for explaining a method of manufacturing a thin film transistor according to a sixth embodiment of the present invention.

FIG. 7 is a schematic cross-sectional view of a conventional thin film transistor.

8A and 8B are a schematic plan view of one shot of an excimer laser beam and a schematic view showing the energy intensity of the AA 'cross section.

9A and 9B are a schematic diagram showing a step-and-repeat irradiation method using an excimer laser and a schematic diagram showing variations in the mobility of a transistor in a BB ′ cross section when irradiation is performed by the step-and-repeat method.

[Explanation of symbols]

 1 Glass Substrate 2 Microcrystalline Silicon Layer 3 Amorphous Silicon Layer 4 Excimer Laser Light 5 Polycrystalline Silicon Device 6 Amorphous Silicon Layer 7 Gate Insulating Layer 8 Gate Electrode 9 Source Region 10 Drain Region 11 Interlayer Insulating Layer 12 Contact Hole 13 source electrode 14 drain electrode

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location H01L 21/268 Z 27/12 R (72) Inventor Tetsuya Kawamura 1006 Kadoma, Kadoma, Osaka Prefecture Matsushita Electric Appliance Industry Co., Ltd. (72) Inventor Yutaka Miyata 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (7)

[Claims] 1. A method for forming a polycrystalline thin film, comprising the steps of forming a precursor thin film containing amorphous as a main component on an insulating substrate and forming a polycrystal by laser annealing the precursor thin film. The method for forming a polycrystalline thin film, wherein the precursor thin film contains fine crystals that become crystal nuclei for polycrystallization. 2. The precursor thin film is formed by depositing a microcrystalline layer and an amorphous layer on an insulating substrate to obtain an amorphous layer / microcrystalline layer / insulating substrate amorphous layer / microcrystalline layer / non-crystalline layer. The method for forming a polycrystalline thin film according to claim 1, wherein any one of a crystalline layer / insulating substrate microcrystalline layer / amorphous layer / insulating substrate is used. 3. The method for forming a polycrystalline thin film according to claim 2, wherein the microcrystalline layer is deposited by using a CVD method. 4. The method for forming a polycrystalline thin film according to claim 3, wherein the CVD method is any one of plasma CVD method, remote plasma CVD method ECR-CVD method, LP-CVD method and thermal CVD method. . 5. The method for forming a polycrystalline thin film according to claim 4, wherein the laser annealing uses an excimer laser. 6. The method for forming a polycrystalline thin film according to claim 5, wherein the polycrystalline thin film is a semiconductor thin film containing silicon or germanium as a main component. 7. A method of manufacturing a thin film transistor, which uses a polycrystalline semiconductor for an active semiconductor layer, wherein the method of forming a polycrystalline thin film according to claim 1 is used.