JP2001057435A - Fabrication of semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a silicon film excellent in crystallinity by irradiating a silicon film with luminous energy having output varying abruptly and melting the silicon film, and then controlling solidification artificially by providing a spatial difference in the cooling rate. SOLUTION: An aluminum nitride film is formed by reactive sputtering using a glass substrate 101. The aluminum nitride film is then etched selectively to form aluminum nitride regions 102, 103. Subsequently, a silicon oxide film 104 is formed by plasma CVD and a silicon oxide film 107 is converted into a polysilicon film by low pressure CVD. It is then patterned and etched to form gate electrodes 108, 109 for N type and P type TFTs. Thereafter, a silicon oxide film is formed as an interlayer insulator 112 on the entire surface. Finally, the interlayer insulator 112 is etched to make a contact hole in the source-drain of the TFT and interconnections 113-115 of chromium or titanium nitride are formed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for manufacturing a semiconductor device using a semiconductor having crystallinity.

[0002]

2. Description of the Related Art Thin film transistors (hereinafter referred to as TFTs) and thin film diodes (TFs) using a crystalline silicon thin film semiconductor are used.
D) is known. The TFT or TFD is formed by forming a thin film semiconductor on an insulating substrate or on an insulating surface provided on a semiconductor substrate and using the thin film semiconductor. This TFT is used for various integrated circuits, and is particularly used for a switching element provided in each pixel of an active matrix type liquid crystal display device, a driver element formed in a peripheral circuit portion, and a three-dimensional integrated circuit. It is considered to be used as an element.

As a method of obtaining a crystalline silicon film used in such an element, a method of heating an amorphous silicon film at a temperature of 600 ° C. or higher is known. In this method, a silicon film in an amorphous state (amorphous silicon film) is solid-phase grown and converted into a crystalline silicon film. In addition, there is also known a method in which a silicon film is melted by irradiating a laser beam or an equivalent strong light thereto, and the silicon film is cooled and solidified to obtain a crystalline silicon film.

[0004] However, the silicon film obtained by these methods is polycrystalline, and its properties are inferior to those of a single crystal due to the existence of grain boundaries. In addition, since the locations where the grain boundaries are generated cannot be controlled, the characteristics of the obtained TFTs vary greatly.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and controls the progress of crystallization.
An object is to obtain a sufficiently crystalline silicon film by controlling a place where a grain boundary is generated.

[0006]

According to the present invention, a silicon film is irradiated by irradiating a pulsed laser beam or a strong light equivalent thereto (coherent light or non-coherent light), that is, light energy whose output fluctuates rapidly. The solidification is artificially controlled by causing a spatial difference in the cooling rate of the silicon film after the light energy is cut off and the light energy is cut off. That is, the silicon film solidifies and crystallizes at an early stage in a portion where the cooling rate is high, but a state where the silicon film is not crystallized in a portion where the cooling rate is low is created. When such a state exists, the previously crystallized portion becomes a nucleus, and crystal growth spreads from a region having a high cooling rate to a region having a small cooling rate. As a result,
The obtained crystalline silicon has extremely good crystallinity, and depending on the conditions, a substantially single crystal silicon film having no grain boundaries can be obtained in a range of 10 μm to 1 mm square.

In the present invention, in order to provide such a distribution of the cooling rate, a film of a material having high thermal conductivity such as aluminum nitride, diamond, and boron nitride may be selectively formed. These materials may be in a crystalline state or an amorphous state. A silicon film is formed directly or indirectly on such a high thermal conductive film and further above or below. When these high thermal conductive films are formed on the silicon film, they can be removed after the crystallization step is completed, so that the degree of freedom in device configuration is increased. In particular, when diamond is selected as the high thermal conductive material, it is preferable because it can be easily oxidized and removed by oxygen plasma treatment or the like.

In the present invention, the crystallinity of the silicon film is not particularly limited.
The material may be in an amorphous, microcrystalline, or polycrystalline state, but may be selected in consideration of absorption of laser light or the like. However, when the silicon film is not completely melted by irradiation with laser light or the like, the remainder of the crystalline silicon film becomes a nucleus, and crystal growth different from that intended in the present invention occurs. Attention is necessary because there is a possibility. In order to obtain a high crystalline silicon film more effectively in the present invention, it is preferable to heat the coating to a temperature of 400 ° C. or more during irradiation with laser light or the like.
Such heating is convenient for the progress of crystal growth because the overall cooling rate after laser irradiation is reduced.

The substrate may be thin. Since the substrate also has a function as a heat sink, heat given to the silicon film by irradiation with laser light or the like is immediately absorbed by the substrate. The thicker the substrate, the greater the heat capacity, which tends to be strong. Conversely, dissipation of heat is prevented when the substrate is thin and the periphery of the substrate is surrounded by a heat-insulating material, which also lowers the overall cooling rate after laser irradiation, thus increasing the progress of crystal growth. Above is convenient. If there is a mechanical problem if the entire substrate is made thinner, the same effect can be obtained even if only the necessary places are made thinner. For example, TFT
In such a method, holes are formed in the substrate only in the regions where the holes are formed. The above-described method of heating the silicon film and the method of thinning the substrate are more effective when used in combination.

In the case where a TFT is manufactured using the silicon film having high crystallinity obtained in this manner, a single crystal or a substantially single crystal state portion may be used as a TFT channel formation region. Good. The substantially single crystal state is not a perfect crystal state, and it is necessary to neutralize dangling bonds existing in a crystal region by adding hydrogen or a halogen element. A state in which only 1% or less of the crystal orientation other than the main crystal orientation is recognized by structural analysis means such as scattering spectroscopy (a state in which the crystal orientation is strongly oriented)
Say Such a single crystal or substantially single crystal state cannot be obtained in the region where the high thermal conductive material film is formed, so that the channel forming region is necessarily a region other than the region where the high thermal conductive material film is formed. Is provided.

In the present invention, in the cooling process after laser irradiation, crystal nuclei are always generated from a predetermined location, and crystal growth is stably generated from that portion. Therefore, this crystallization process is extremely reproducible. Therefore, the fluctuation of the crystallinity due to the fluctuation of the energy of the laser is sufficiently small, so that the obtained thin film semiconductor device becomes extremely high.

[0012]

[Embodiment 1] FIG. 1 shows this embodiment. This embodiment relates to a process for manufacturing a TFT on a glass substrate. In this embodiment, Corning 7059 glass is used as the substrate 101, and its thickness is 1.1 mm.
Alternatively, it was 30 μm. Next, a thickness of 200 to 50 by a sputtering method, particularly a reactive sputtering method.
An aluminum nitride film having a thickness of 00 °, preferably 300 to 1000 ° was formed. The ratio of nitrogen to aluminum in the aluminum nitride film was preferably 0.8 to 1.2. Then, the obtained aluminum nitride film was selectively etched to form aluminum nitride regions 102 and 103. (Fig. 1 (A))

Thereafter, a silicon oxide film 104 having a thickness of 500 to 5000 Å, preferably 500 to 3000 Å is formed by a plasma CVD method. In order to improve heat conduction, the silicon oxide film 104 was preferably as thin as possible. Then, the amorphous silicon film is continuously formed by plasma CVD.
It was formed to a thickness of 100 to 1000 °, preferably 400 to 800 °, for example, 500 ° by the LPCVD method. Here, the amorphous silicon film 105 was formed to a thickness of 1000 ° by a plasma CVD method.

Further, a KrF excimer laser (wavelength: 248 nm, pulse width: 30 nsec) was irradiated in a vacuum. Energy density of laser irradiation is 200-450
mJ / cm ² , the number of shots was 2 to 5 shots per one place. The substrate temperature was room temperature or 500 ° C. Other lasers include an ultraviolet laser such as a XeCl excimer laser (wavelength 308 nm) and an ArF excimer laser (wavelength 193 nm), a visible light laser such as a ruby laser, and Nd: YAG.
An infrared laser such as a laser may be used. However, in each case, the laser is required to be a pulsed laser.

FIG. 3 is a conceptual diagram showing a temperature change of the silicon film immediately after the laser irradiation under such conditions. Naturally, the intention of the present invention is that the temperature drops more rapidly on aluminum nitride than on other regions (on silicon oxide). In addition, when the substrate is kept at room temperature (T _s =
(At room temperature), the heat transfer was large, and the temperature decreased more rapidly than when the substrate was kept at 500 ° C. (T _s = 500 ° C.). Reflecting the above discussion, the duration of the state above the freezing point temperature shown in FIG. 3 is shorter on aluminum nitride than on silicon oxide, and when the substrate is kept at room temperature, It is shorter than the state heated to ° C.
As a result, when the substrate temperature was room temperature, no crystallization was observed from the aluminum nitride region to the surroundings as shown by the arrow in FIG. On the other hand, when the substrate temperature is 500 ° C., FIG.
The crystal growth indicated by the arrow was observed.

The heat flow at this time will be described.
When the region irradiated with the laser beam cannot cover the region surrounded by the aluminum nitride regions 102 and 103,
Alternatively, when the aluminum nitride films 102 and 103 are separated from each other (FIG. 6A), the thermal energy given to the silicon film is conducted in all directions, but is not irradiated with the laser through the aluminum nitride film. Because the speed of heat diffusion to the area is large, mainly along the laser irradiation area,
It moves one-dimensionally toward the aluminum nitride region.

The solid arrows in FIG. 6A indicate the flow of heat, while the thick lines indicate that the flow of heat is large.
The flow towards the aluminum nitride region is shown. Heat flows from the laser irradiation area in other directions, but the amount is small. Reflecting such a heat flow, the direction of crystallization is opposite to the main flow of heat as shown by the dotted arrow in the figure. Then, the crystals grown from both collide with almost the center of the laser irradiation region, and a portion where crystallization distortion is accumulated is generated. (FIG. 6
(A))

On the other hand, when the laser is applied to the entire area surrounded by the aluminum nitride (FIG. 6B), the heat flows two-dimensionally, and as a result, the heat flows in the opposite direction to the heat flow. Crystallization proceeds toward the center of the region surrounded by aluminum. In this case, crystal distortion is accumulated at the center. (FIG. 6 (B)) In any case, such a portion of the crystal having large distortion is T
It is not suitable for use as an FT channel formation region.

The same was observed when the thickness of the substrate was changed. FIG. 4 shows a substrate having a thickness of 1.1 mm and a thickness of 30 μm.
FIG. 3 is a conceptual diagram of a temperature change immediately after laser irradiation of a silicon film in the above method. Laser irradiation was performed at room temperature, and the substrate was fixed and supported at both ends without using a susceptor or the like to avoid heat conduction. The atmosphere was in a vacuum. As is clear from FIG. 4, even when the substrates have the same thickness, the temperature drops more rapidly on aluminum nitride than on other regions (on silicon oxide). In addition, those substrate is thin (t _s = 30μ
m) is, is gentle drop in temperature than towards the substrate is thick (t _s = 1.1mm). As a result, even with laser irradiation at room temperature, if the substrate is thinned to 30 μm and laser irradiation is performed in an adiabatic state, crystallization from the aluminum nitride region to the periphery as indicated by the arrow in FIG. Was observed over a length of 10-1000 μm.

If a thin substrate as described above is heated to a temperature of 400 ° C. or more in an adiabatic state with an infrared lamp or the like as the substrate heating means and laser irradiation is performed, a silicon film having better crystallinity can be obtained. Was completed. Thus, the silicon film was crystallized. (FIG. 1 (B)) After the above laser irradiation, the silicon film is patterned and etched to form an island-shaped silicon film region 106, and a gate oxide film having a thickness of 500 to 2000 Å, preferably 7
Plasma CVD of silicon oxide film 107 of 100 to 1500 °
A polycrystalline silicon film having a thickness of 2000-1 μm was formed by a low pressure CVD (LPCVD) method. And this is patterned and etched,
Gate electrodes 108 (for N-channel TFT) and 109
(For a P-channel TFT). The polycrystalline silicon film was doped with 1 ppm to 5 atomic% of phosphorus to increase conductivity. (Fig. 1 (C))

Thereafter, impurities (phosphorus and boron) were implanted in a self-aligned manner into the island-like silicon film of each TFT by ion doping (also called plasma doping) using the gate electrode as a mask. Phosphine (PH ₃ ) for doping phosphorus
And diborane (B ₂ H ₆ ) was used for doping with boron. The dose amount was 5 × 10 ^{14 to} 6 × 10 ¹⁵ cm ⁻² . The doping was alternately performed with one doping covered with a mask. Then, a KrF excimer laser (wavelength 248 nm, pulse width 30 ns)
Irradiation c) improved the crystallinity of the portion where the crystallinity was deteriorated by the introduction of the impurity region. The energy density of the laser is 150 to 400 mJ / cm ² , preferably 200 to 250 mJ / cm ² . Thus, the N-type impurity (phosphorus) region 110 and the P-type impurity (boron) region 11
1 was formed. The sheet resistance in these regions is 200-8
It was 00Ω / □.

In this step, instead of using laser light, a flash lamp is used for a short period of time.
強 1200 ° C. (temperature of the silicon monitor) and heat the sample, using strong light equivalent to so-called laser light such as so-called RTA (rapid thermal annealing) (RTP, also called rapid thermal process). You may. After that, a silicon oxide film having a thickness of 300
0-8000 ° was formed. Substrate temperature is 250-450
° C, for example, 350 ° C. The film was formed under the condition that a flat film could be obtained as much as possible. In order to obtain more flat surface, even if this silicon oxide film was mechanically polished, it was effective.

Then, the interlayer insulator 112 is etched to form contact holes in the source / drain of the TFT, and wirings 113 to 115 of chromium or titanium nitride are formed.
Was formed. Finally, at 300-400 ° C. in hydrogen.
Anneal for 1-2 hours to complete hydrogenation of silicon. Hydrogenation may be performed by implanting accelerated hydrogen ions by an ion doping method.
Thus, the TFT was completed.

The TFT manufactured in this embodiment has an N-channel mobility of typically 150 to 750 cm.
² / Vs, 100-450 cm ² with P-channel type
/ Vs. In addition, the sub-threshold characteristic value (S
Value) is 0.05 to 0.15 digits / V, and M
It was comparable to that of the OS transistor. In this embodiment, since the TFT is formed on an insulating substrate, when a circuit is actually formed, there is no parasitic capacitance with the substrate. Realized with lower power consumption.

Embodiment 2 FIG. 2 shows a manufacturing process of this embodiment. As a substrate, Corning 70 with a thickness of 1.1 mm
59 was used. First, a thickness of 1000 to 5000 １００, for example, 2000 Å is formed on the substrate 201 by a plasma CVD method.
A silicon oxide base film 202 was formed. Then, an intrinsic (I-type) amorphous silicon film 203 having a thickness of 100 to 1000 Å, for example, 500 Å was formed by a plasma CVD method. Then continuously 500-5000mm thick, for example 10
A polycrystalline diamond film of 00 ° was formed by a plasma CVD method. Then, the polycrystalline diamond film is selectively etched to form a polycrystalline diamond region 204.
And 205 were formed.

Then, similarly to the first embodiment, the silicon film 203 is irradiated with a KrF excimer laser (wavelength: 248 nm).
Was crystallized. Laser irradiation was performed in a vacuum, the energy density was 200 to 350 mJ / cm ² , and the number of shots was 5 shots per one place. The substrate temperature was set to 500 ° C. As a result, crystallization of the silicon film 203 progressed as shown by the arrow in FIG. (Figure 2
(A))

The same effect can be obtained by performing laser irradiation after patterning and etching the silicon film 203 to form island-shaped silicon film regions 206 and 207, as shown in FIG. 2B. Was. The polycrystalline diamond film sufficiently transmitted the laser light used, but in any case, the silicon film in the region below the polycrystalline diamond film was not sufficiently crystallized.

Thereafter, the polycrystalline diamond films 204 and 205 were oxidized by being left in oxygen plasma. Residues (mainly amorphous carbon) of the polycrystalline diamond film could be completely removed by washing with sulfuric acid and hydrogen peroxide. Then, the silicon film 203 is patterned and
By etching, island-shaped silicon film regions 206 and 207 were formed. Of the island-like regions, regions denoted by 213 and 214 are regions below the polycrystalline diamond film, and the crystallinity of the silicon film was extremely poor. There is no problem because it is used as an area other than the area.

After that, a silicon oxide film 208 having a thickness of 500 to 2000.degree., For example, 1200.degree. Is deposited by a plasma CVD method, and subsequently, a 3000 to 8000.degree. Containing 0.2% by weight of scandium). Then, the aluminum film is patterned to form a gate electrode 209 (for an N-channel TFT).
And 211 (for a P-channel type TFT).

Further, the surfaces of these aluminum electrodes were anodized to form oxide layers 210 and 212 on the surfaces. This anodization was performed in an ethylene glycol solution containing tartaric acid at 1 to 5%. The thickness of the obtained oxide layers 210 and 212 was about 2000 °. Since the oxides 210 and 212 have a thickness for forming an offset gate region in a later ion doping process, the length of the offset gate region can be determined in the anodic oxidation process. (Fig. 2 (C))

Next, the island-shaped silicon film region 206 is formed by ion doping (also called plasma doping).
A gate electrode portion (ie, the gate electrodes 209, 2
N-type impurities (phosphorus) and P-type impurities (boron) were added in a self-aligned manner using the mask 11 and the surrounding oxide layers 210 and 212 as a unit. The dose is 5 × 10 ¹⁴ to 8 × 10 ¹⁵ cm ⁻² , for example, 1 ×
It was 10 ¹⁵ cm ^-2 and boron was 2 × 10 ¹⁵ cm ^-2 . As a result, the N-type impurity region 215 and the P-type impurity region 21
6 was formed. As is apparent from the figure, the impurity region and the gate electrode are in an offset gate state separated from each other by the thickness of the anodic oxides 210 and 212. Such an offset gate state is particularly effective in reducing a leak current (also referred to as an off-state current) when a reverse voltage (a negative voltage in the case of an N-channel TFT) is applied to the gate electrode.

Thereafter, annealing was performed by laser light irradiation. As a laser beam, a KrF excimer laser (wavelength: 248 nm, pulse width: 20 nsec) was used. The laser light irradiation condition is that the energy density is 2
00 to 400 mJ / cm ² , for example, 250 mJ / cm ²
Then, 2 to 10 shots, for example, 2 shots were irradiated per one place. When irradiating this laser light, the substrate
The effect may be increased by heating to about 450 ° C. By this step, the regions 213 and 214 having insufficient crystallinity have become regions having sufficient crystallinity.
In fact, these regions were used as the source or drain of the TFT. (FIG. 2 (D))

Subsequently, a silicon oxide film 21 having a thickness of 6000.degree.
7 was formed as an interlayer insulator by a plasma CVD method. Then, a contact hole was formed in the interlayer insulator 217, and electrodes / wirings 218 to 220 of the TFT were formed using a metal material, for example, a multilayer film of titanium nitride and aluminum. Finally, in a hydrogen atmosphere of 1 atm.
Annealing was performed for 0 minutes to complete an active matrix pixel circuit having a TFT. (FIG. 2 (E))

[Embodiment 3] In this embodiment, a thin region is partially provided on a substrate, and a TFT is formed in that region. FIG. 5 schematically shows a cross section of the TFT of this embodiment. The substrate 501 having a thickness of 1.1 mm is partially, for example, 50 ×
An area 502 having a size of 200 μm and a thickness of 50 μm was provided. On such a region, the cooling rate of the silicon film was low due to the heat insulating effect, and the same effect as shown in FIG. 4 was obtained.

As a result, a sufficiently large crystal is formed in the region 50.
2 was obtained. However, in other regions, the crystallinity of the obtained crystalline silicon film was not so good because the substrate was thick. In this embodiment, an aluminum nitride film is first selectively formed on a substrate as in the first embodiment, and then a silicon oxide film and an amorphous silicon film are deposited as bases. Then, in this state, a KrF excimer laser (wavelength 248)
nm, and a pulse width of 30 nsec). The energy density of laser irradiation was 200 to 450 mJ / cm ² . The substrate temperature was room temperature.

As a result, irrespective of the laser irradiation at room temperature, the same lateral crystal growth as that of the other examples (that is, crystal growth from the aluminum nitride region to the periphery) was observed. Typical characteristics of a TFT manufactured using the obtained crystalline silicon film were 250 to 750 cm ² / Vs for an N-channel mobility.

[0037]

According to the present invention, a TFT in which at least the channel forming region is formed of a substantially single crystal silicon film can be manufactured. As a result, the characteristics of the TFT were dramatically improved. The present invention can be applied not only to TFTs but also to devices using other thin film semiconductors, and it goes without saying that the present invention is effective in improving these characteristics. In the embodiment, a thin film semiconductor device (here, TFT) on an insulating substrate is used.
However, it is needless to say that the present invention is also effective for a TFT formed on a single crystal semiconductor substrate.

In the conventional crystallization step after laser irradiation, since there was no such a stable crystal nucleus, the crystallinity of the silicon film obtained by the energy of the laser varied greatly. That is, if the energy is too small, not only the silicon film is insufficiently melted and crystal growth is insufficient, but also if the energy is too large, the silicon film is completely melted and crystal nuclei exist even in the cooling process. Instead, it became supercooled and became amorphous. For this reason, the optimum laser energy is in a very narrow range that satisfies the condition that the film is sufficiently melted and a portion serving as a crystal nucleus remains.

On the other hand, according to the present invention, in a cooling process after irradiation with laser light or the like, a place where a crystal nucleus is first generated (a selectively formed high thermal conductive material film) is specified. In the cooling process, crystal growth proceeded from the nucleus. For this reason, a silicon film of the same degree of crystallinity could be stably obtained in an extremely wide energy range in which the film was sufficiently melted and the film was not evaporated.

In the conventional laser crystallization, the characteristics of the obtained thin film semiconductor device are not stable, and the process is not mass-produced. This is because the optimum laser energy is narrow as described above. According to the present invention, the range of the optimum laser energy is sufficiently widened, and the characteristics of the obtained semiconductor device are also stabilized. For this reason, laser crystallization has become a semiconductor process with high productivity. Thus, the present invention is an industrially useful invention.

[Brief description of the drawings]

FIG. 1 shows the steps of Example 1.

FIG. 2 shows a process of Example 2.

FIG. 3 shows a conceptual diagram of a temperature change of a silicon film after laser irradiation.

FIG. 4 shows a conceptual diagram of a temperature change of a silicon film after laser irradiation.

FIG. 5 shows a schematic view of a cross section of a TFT of Example 3.

6 shows the flow of heat and the direction of crystallization during laser irradiation in Example 1. FIG.

[Explanation of symbols]

 101 glass substrate 102 103 aluminum nitride film 104 silicon oxide film 105 silicon film 106 island silicon film region 107 gate insulating film (silicon oxide film) ) 108, 109 Gate electrode (polycrystalline silicon) 110: N-type impurity region 111: P-type impurity region 112: Interlayer insulator (silicon oxide) 113-114 Electrode

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Yasuhiko Takemura 398 Hase, Atsugi-shi, Kanagawa Inside Semiconductor Energy Laboratory Co., Ltd.

Claims (7)

[Claims] 1. A method according to claim 1, wherein a film made of aluminum nitride, boron nitride or diamond is selectively formed on the insulating surface, an amorphous or crystalline silicon film is formed on the film, Intense light equivalent to laser light is directly applied to the silicon film, and the silicon film is crystallized from a region where the film is formed, and a thin-film semiconductor device is formed using the crystallized silicon film. Of manufacturing a semiconductor device. 2. An amorphous or crystalline silicon film is formed on an insulating surface, and a film made of aluminum nitride, boron nitride or diamond is selectively formed on the silicon film. A semiconductor comprising: directly irradiating the silicon film with intense light equivalent to the laser light; crystallizing the silicon film from a region where the coating is formed; and forming a thin-film semiconductor device using the silicon film. Method for manufacturing the device. 3. An amorphous or crystalline silicon film is formed on an insulating surface, and a film made of aluminum nitride, boron nitride or diamond is selectively formed on the silicon film. Directly irradiating the silicon film with strong light equivalent to the laser light, crystallizing the silicon film from the region where the film is formed, removing the film entirely, and using the silicon film to form a thin film semiconductor device A method for manufacturing a semiconductor device, which is formed. 4. The silicon film according to claim 1, wherein the silicon film is directly irradiated with pulsed laser light or intense light equivalent to the laser light, and the silicon film is heated to 400 ° C. or higher. A method for manufacturing a semiconductor device, comprising: 5. The method according to claim 1, wherein the thin film semiconductor device is a thin film transistor. 6. The method for manufacturing a semiconductor device according to claim 5, wherein the thickness of the substrate in a portion where the channel formation region of the thin film transistor is provided is smaller than the thickness of the substrate in the other portion. 7. The method for manufacturing a semiconductor device according to claim 5, wherein the channel formation region of the thin film transistor is single crystal or substantially single crystal.