JPH10312962A - Formation of polycrystalline silicon thin film and polycrystalline silicon thin-film transistor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a polycrystalline silicon thin film of uniformity and high mobility. SOLUTION: A first silicon thin film 2 is formed on an insulated substrate, and this is turned into polycrystalline silicon by an excimer laser irradiation to make a first polycrystalline silicon thin film 4. Next, a second silicon thin film 5 is formed on this and is turned into polycrystalline silicon by the irradiation of the excimer laser 6 again. At this time, the melting depth of silicon by the second eximer laser irradiation is controlled to avoid completely melting the first polycrystalline silicon thin film, and recrystallization is performed by setting this a core, for obtaining a polycrystalline silicon thin film 8 reflecting the grain diameter of the first polycrystalline silicon thin film. Since energy density for irradiation can be low, compared with that of a conventional method, the projection/recess of grains and the roughness of a surface are small for obtaining a uniform film. In addition, as the grain diameter of the first polycrystalline silicon thin film can provide diameters larger than conventional ones even if energy density is low, a finally obtained grain diameters can be large to realize high mobility.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polycrystalline silicon thin film transistor used for an active matrix liquid crystal display, an image sensor and various integrated circuits, and a method for producing a polycrystalline silicon thin film particularly suitable for an active layer of the polycrystalline silicon thin film transistor.

[0002]

2. Description of the Related Art In recent years, with the development of multimedia, devices and devices for handling images have been actively developed. A polycrystalline silicon thin film transistor is a key device used for a pixel switch, a driving circuit, and the like of an active matrix type liquid crystal display, an image sensor, and the like, which are the core of these image input / output devices. In particular, since polycrystalline silicon thin film transistors have high mobility, they can be integrated up to the peripheral driving IC, so that IC and connection costs can be reduced, and there is no density limitation due to mounting, making them ideal for forming large-capacity, high-density devices. ing.

There are many methods for forming polycrystalline silicon, which is the central material of such polycrystalline silicon thin film transistors. Among them, those used for thin film transistors are:
High mobility (required value is 50-100 cm for drive circuit)
From the viewpoint of ( ² / V · s or more), methods using the solid phase growth method or the excimer laser annealing method have been concentrated.

In the solid phase growth method, an amorphous silicon thin film is annealed at a temperature of about 500 to 600 ° C. for a long time to polycrystallize it. When annealing is performed for a long time, micronuclei are generated in the amorphous silicon thin film several hours later, and the nuclei gradually grow and change into a polycrystalline silicon thin film. The crystallization speed depends on the properties of the amorphous silicon film and the like, but a complete polycrystalline silicon thin film is obtained in about 10 to 50 hours. This crystal grain size of the polycrystalline silicon thin film formed by hundreds a nm~ several microns, the mobility of a polycrystalline silicon thin film transistor n-type created in the polycrystalline silicon thin film is several tens to hundred cm ² / V
-It is s.

On the other hand, the excimer laser annealing method irradiates an amorphous silicon thin film with an excimer laser such as XeCl, KrF, or ArF, and instantaneously converts the thin film into a polycrystalline silicon thin film. FIG. 7 shows a polycrystallization process. As shown in FIG. 7A, an amorphous silicon thin film 15 is formed on a glass substrate 1 by a low pressure CVD method or a plasma CVD method, and is irradiated with an excimer laser beam 16 as shown in FIG. Thus, a polycrystalline silicon thin film 17 is formed as shown in FIG. An excimer laser is a pulse laser of ultraviolet light. Since it is ultraviolet light, energy is absorbed only on the surface of the amorphous silicon film. Moreover, since the pulse width is as short as 20 to 50 nanoseconds, the amorphous silicon film can be melted and recrystallized without damaging the glass serving as the substrate. The melting time is on the order of hundreds of nanoseconds. The polycrystalline silicon thin film thus formed has a crystal grain size of several tens n.
m to several hundred nm, and the mobility of the n-type polycrystalline silicon thin film transistor formed with this film is several tens to 300 cm ^2.
/ V · s. The reason why high mobility is exhibited even though the crystal grain size is smaller than that of the solid phase growth method is that there are few defects in the crystal and at the crystal grain boundaries.

As described above, according to the excimer laser annealing method, high mobility can be obtained relatively easily, but there is a problem in uniformity. To solve this problem, a cap annealing method has been developed. FIG. 8 shows the polycrystallization process. As shown in FIG. 8A, an amorphous silicon thin film 19 is formed on a glass substrate 1 by a low pressure CVD method, a plasma CVD method, or the like, and a cap film 20 made of a silicon dioxide or silicon nitride film is formed thereon.
To form Next, the excimer laser beam 16 is irradiated through the cap film 20 as shown in FIG. Thereafter, the cap film is removed, and a polycrystalline silicon thin film 22 is formed as shown in FIG. The polycrystalline silicon thin film thus formed is a two-layered film because nuclei are generated from both sides of the glass substrate and the cap film. Further, the crystal grain size is smaller than that of the conventional method.

FIG. 9 shows this state. With the cap film, the change with respect to the laser energy density is small. Therefore, even if the energy of the laser pulse varies, the crystal grain size does not vary much. In the case without the cap film, the crystal grain size changes greatly due to a small change in the energy density, and this directly appears as a change in the mobility of the polycrystalline silicon thin film transistor. As can be seen by comparing FIGS. 7 and 8, the surface roughness of the formed polycrystalline silicon film is different. In the case of the cap annealing method, a very flat film is obtained because the film is pressed by the cap film, but in the case without the cap film, the roughness becomes more noticeable as the grain size increases. In the application to thin film transistors, the flatter the better, the better the uniformity and the yield, and the cap annealing method is more advantageous.

The above is a description of a method using a single-layer silicon film. As a conventional example, a method using a two-layer silicon film is disclosed in JP-A-4-186722 and JP-A-4-186.
No. 723. This is formed by a process as shown in FIG. That is, after the first silicon film 24 is formed on the insulating substrate 23, the laser beam 25 is selectively irradiated to form only the crystallized region 26, and the second silicon film 27
Are laminated, and the crystallized portion is used as a seed for laser irradiation 2
8 or a silicon film 29, 3 having a large grain size by solid phase growth.
It is to get 0.

Although the laser used in this method is not specified in the specification, it has been disclosed in the embodiment of Japanese Patent Application Laid-Open No. 4-186722 that "the laser beam 3 is irradiated to cool the crystal region 4 around the crystal region 4 during cooling. The rearrangement of atoms progresses in a short time within a few seconds, and a large crystal grain having the crystal nucleus 4 as the central nucleus is formed. " I understand. That is, silicon must be melted for a sufficiently long time to grow a crystal in the second silicon film in the lateral direction by this method. Although there is a description that a few seconds is short here, it means that a much longer melting time is required as compared with the crystal melting time by the above-described excimer laser.

It is also considered that the laser irradiation on the first silicon film common to these two publications is based on the assumption that a continuous wave laser is used. This is because, in order for the present invention to be satisfied, the region to be irradiated with laser must be monocrystallized, but in the case of a pulsed laser such as an excimer laser, a single region that becomes a nucleus even if partial irradiation is performed This is because a silicon crystal cannot be obtained.

[0011]

As described above, there are three types of single-layer films and two types of high-mobility polycrystalline silicon thin films as two-layer films. There is a problem.

In a polycrystalline silicon thin film formed by a solid phase growth method, high mobility is relied only on an increase in crystal grain size.
If the grain size is significantly increased in order to obtain a desired mobility, there arises a problem that the characteristics of the polycrystalline silicon thin film transistor vary greatly. FIG. 10 shows this state. In the figure, (a) shows a case where the crystal grain size is large.
In the case where (b) is small, A, B, C, and D in the figure indicate regions of the active layer of the polycrystalline silicon thin film transistor. For example, when a mobility of 100 cm ² / V · s is obtained, the particle size must be increased to about 1 μm by the solid phase growth method. In the case of the large grain size shown in (a), the region A contains many crystal grain boundaries, whereas the region B contains only one grain boundary. Therefore, the performance of the polycrystalline silicon thin film transistors formed in the respective regions A and B is significantly different. Of course, B shows higher performance. On the other hand, when the particle size is small, as can be seen from C and D, characteristics with high uniformity can be obtained irrespective of where it is made. As described above, in the solid-phase growth method, uniformity is sacrificed when trying to increase the mobility, and mobility is sacrificed in order to increase the uniformity.

On the other hand, in the case of the excimer laser annealing method as well, the conventional method has a problem of uniformity due to the variation in the particle size as described above. There is no problem regarding the absolute value of the particle size unlike the solid phase growth method. Mobility of at least 100 cm ² / V · s can be obtained even with a particle size of about 300 to 500 nm. The cap annealing method avoids this, but in the case of the cap annealing method, the mobility is lower than that of the conventional method, and the energy density of the laser pulse is increased to obtain high mobility. There is a problem that it becomes necessary.

FIG. 11 shows an example of the mobility of a polycrystalline silicon thin film transistor formed by capless annealing and cap annealing. It can be seen that the mobility is lower in the case of the cap annealing method than in the conventional method. The change in mobility with respect to the laser energy density is smaller in the cap annealing method. This leads to higher uniformity of the cap annealing method. As described above, each method has advantages and disadvantages, and although sufficient characteristics including uniformity can be obtained up to about 50 cm ² / V · s, 150 to ² cm ² / V · s.
A method for obtaining a polycrystalline silicon thin film transistor having a thickness of at least 00 cm ² / V · s with good uniformity has not been established.

In the method using a two-layer film, the laser used is a continuous wave laser, and the melting time of silicon is on the order of seconds. Therefore, the underlying substrate is exposed to a temperature of 1000 ° C. or more. is there. In order to withstand such large thermal damage, the substrate must be made of a material having high heat resistance such as quartz. However, the quartz substrate is very expensive, and it is difficult to use the quartz substrate in a device such as a liquid crystal display to which the present invention is applied. By the way, the price of substrate is 2 digits to 3 for quartz and glass
There is a digit opening.

Further, in the method using a two-layer film, some optical means for selectively irradiating the first silicon film with laser is required. If it is attempted to control the particle size to about several hundred nanometers as described above, the size required for the nucleus should be about tens to hundreds of nanometers. It is almost impossible to make this using only a laser beam, and it is necessary to form a pattern using some optical system. Therefore, there is a problem that a very complicated system such as an optical mask and a control system for controlling the optical mask must be used.

An object of the present invention is to eliminate the drawbacks of the above conventional method and to provide a method of forming a polycrystalline silicon thin film for obtaining a polycrystalline silicon thin film transistor with high uniformity and high mobility.

[0018]

The present invention comprises a step of forming a first silicon thin film on a substrate having at least an insulating surface, and a step of polycrystallizing the first silicon thin film by excimer laser irradiation. Forming a second silicon thin film on the polycrystalline first silicon thin film;
And a step of polycrystallizing the second silicon thin film with the first silicon thin film as a nucleus. The first silicon thin film is previously polycrystallized by excimer laser irradiation, and the second silicon thin film is polycrystallized using this as a nucleus. The first silicon thin film is formed thinner than the active layer of the thin film transistor, and a desired particle size is easily obtained. Since the grain size of the second silicon thin film is determined by the crystal grain size of the first silicon thin film, a uniform grain size can be obtained in a wide range of crystal growth conditions.

The second silicon thin film may be crystallized by excimer laser irradiation or by a solid layer growth method. By forming a cap film on the second silicon film, a flat polycrystalline silicon thin film can be obtained.

A polycrystalline silicon thin film transistor using a polycrystalline silicon thin film formed by such a method as an active layer can realize high mobility and uniform characteristics.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1) FIG. 1 shows a first embodiment of the present invention.
This will be described in detail with reference to FIG. As a first step (a),
On a glass substrate 1, a first silicon thin film 2 thinner than a final polycrystalline silicon film thickness is formed. This film may take the form of either an amorphous silicon film or a polycrystalline silicon film. This is irradiated with excimer laser light 3 to transform it into a polycrystalline silicon thin film 4 (b). Next, a second silicon thin film 5, here an amorphous silicon film, is formed from above (c). The film thickness is obtained by subtracting the thickness of the first silicon thin film formed earlier from the total design value. An excimer laser beam 6 is further irradiated from above (d). (E) shows the relationship between the molten silicon 7 and the polycrystalline first silicon thin film. The melting does not reach the entire surface, and the molten silicon cools and the crystal grows using the underlying polycrystal as a seed. Therefore, the crystal grain size of the polycrystalline silicon thin film 8 finally formed is almost the same as the crystal grain size of the polycrystalline silicon thin film 4 obtained by polycrystallizing the first silicon thin film.

What is important here is the relationship between the energy density of the excimer laser and the thickness of the silicon thin film. As shown in FIG. 9 described above, for example, the growth state of the crystal grain size with respect to the energy density differs between a silicon thin film having a thickness of 30 nm and a silicon thin film having a thickness of 75 nm. In this example, an amorphous silicon film formed by a low pressure CVD method was used. It can be seen that the crystal becomes larger at a lower energy density because the temperature rises even with the same input energy as the thickness becomes smaller. Therefore, using a thin film requires a low energy density to obtain a desired crystal grain size. However, in order to obtain the performance as a polycrystalline silicon thin film transistor, a certain film thickness is required, for example, for reducing the resistance of the source / drain region, and it is not sufficient to be thin.

The present invention focuses on this point. As described above, the second silicon thin film is formed, and the crystal is formed while maintaining its size with the crystal grains of the first polycrystalline silicon thin film as nuclei. It is to grow. This is shown in FIG. The figure shows the dependence of the crystal grain size on the laser energy density of an amorphous silicon film formed by low-pressure CVD for the first layer of silicon thin film, and an amorphous silicon film formed by low-pressure CVD for the second layer. Is shown. The solid line shows an example of the first embodiment. It can be seen that the crystal grain size is already defined in the first polycrystalline silicon thin film and that it is not necessary to melt the entire film, so that a uniform grain size is obtained from the low energy density portion. If it is increased to 500 mJ / cm ² or more, the entire surface of the film up to the first polycrystalline silicon thin film starts to be melted, so that the particle diameter approaches the value obtained when a 75 nm amorphous silicon thin film is laser-annealed. Thus, in the present invention, it is possible to obtain a desired particle size with a low energy density. Further, a constant particle size can be obtained in a wide energy density range.

FIG. 6 shows an example in which the polycrystalline silicon thin film thus obtained is applied to a polycrystalline silicon thin film transistor. In the conventional method, the mobility is suppressed low on the low energy density side and the change on the high energy density side is large. On the other hand, in the method using the polycrystalline silicon thin film of the present invention, the whole from the low energy density side to the high energy density side Thus, a high mobility that hardly depends on the energy density is obtained. When the energy density is further increased, the film quality is deteriorated and the mobility is lowered.

(Embodiment 2) Next, a second embodiment of the present invention will be described in detail with reference to FIG. As a first step (a), a first silicon thin film 2 thinner than a final polycrystalline silicon thin film thickness is formed on a glass substrate 1. This is irradiated with excimer laser light 3 to transform it into a polycrystalline silicon thin film 4 (b). Next, a second silicon thin film 5, here an amorphous silicon film, is formed from above (c). The film thickness is obtained by subtracting the thickness of the first silicon thin film formed earlier from the total design value. Up to this point, the operation is the same as that of the first embodiment. A cap film 9 made of a silicon dioxide or silicon nitride film is formed thereon (d). Excimer laser light 6 is further irradiated from above (e). (F) shows the relationship between the molten silicon 10 and the polycrystalline first silicon film. The melting does not reach the entire surface, and the molten silicon cools and the crystal grows using the underlying polycrystal as a seed. Therefore, the crystal grain size of the polycrystalline silicon thin film 11 finally formed after the cap is removed becomes substantially the same as the crystal grain size of the polycrystalline silicon thin film 4 obtained by polycrystallizing the first silicon thin film (g). ).

The second embodiment differs from the first embodiment in the presence or absence of a cap. In the second embodiment, since the cap is used, the surface of the polycrystalline silicon thin film 11 is very flat. A major difference from the conventional cap annealing method is that the polycrystalline silicon thin film is a single-layer film. Conventionally, nuclei are generated from both surfaces of the glass substrate and the cap film. However, in the present invention, since there are crystal grains to be nuclei without waiting for the generation of nuclei, these grow dominantly. There is some nucleation from the cap film,
Almost all of them are affected by crystal growth from the underlayer, and a single-layer flat polycrystalline silicon thin film can be obtained.

FIG. 5 shows the relationship between the crystal grain size and the laser energy density obtained as a result. Dotted lines indicate the results of the cap annealing according to the second embodiment. In the case of cap annealing, the energy input due to reflection on the cap surface and the like becomes smaller, so that the rising slightly shifts to a higher energy density side. As for the upper limit, the flat region is spread on the higher energy density side than the loss due to the cap because the growth is suppressed by the cap. Although the crystal grain size is smaller than that of the conventional method having no cap as a whole, there is no great difference as in the conventional cap annealing method.

(Embodiment 3) Next, a third embodiment of the present invention will be described in detail with reference to FIG. As a first step (a), a first silicon thin film 2 thinner than a final polycrystalline silicon thin film thickness is formed on a glass substrate 1. This is irradiated with excimer laser light 3 to transform it into a polycrystalline silicon thin film 4 (b). Next, a second silicon thin film 5, here an amorphous silicon film, is formed from above (c). The film thickness is obtained by subtracting the thickness of the first silicon thin film formed earlier from the total design value. Up to this point, it is the same as in the first and second embodiments. Thereafter, this is annealed at 600 ° C. for several hours to several tens hours to obtain a polycrystalline silicon thin film 12 (d).

In the polycrystalline silicon thin film thus obtained, defects in the crystal and defects at the crystal grain boundaries are smaller than those obtained by the conventional solid phase growth method, reflecting the crystallinity of the underlying layer. . Therefore, in the conventional solid phase growth method, 100 c
In order to obtain a mobility of about m ² / V · s, a crystal grain size on the order of microns was required.
Similar results are obtained with m. Also, in this embodiment, the laser irradiation is only required for the first time, so that there is an advantage that the required output of the excimer laser can be reduced compared to the first and second embodiments.

(Embodiment 4) A fourth embodiment of the present invention will be described in detail with reference to FIG. As a first step (a), a first silicon thin film 2 thinner than a final polycrystalline silicon thin film thickness is formed on a glass substrate 1. This is irradiated with excimer laser light 3 to transform it into a polycrystalline silicon thin film 4 (b). Next, a second silicon thin film 5, here an amorphous silicon film 5, is formed from above (c). The film thickness is obtained by subtracting the thickness of the first silicon thin film formed earlier from the total design value. Up to this point, the configuration is the same as in the first, second, and third embodiments. A cap film 13 made of a silicon dioxide or silicon nitride film is formed thereon (d). Thereafter, this is annealed at 600 ° C. for several hours to several tens hours to obtain a polycrystalline silicon thin film 14 (e). The polycrystalline silicon thin film thus formed becomes a film having a flatter surface shape reflecting the characteristics of the cap annealing method.

The four embodiments have been described above. Which method is selected depends on the required mobility and threshold. The first embodiment is excellent for obtaining the highest mobility. However, when driving a polycrystalline silicon thin-film transistor at 5 V or 3.3 V, the surface shape poses a serious problem. In such an application, a polycrystalline silicon thin film transistor having a low threshold value is required, and the thickness of the gate oxide film is inevitably in the range of 40 nm to 10 nm.
However, in the first embodiment, the surface roughness may be generated up to about half of the film thickness. In such a thin insulating film, pinholes may be formed and the yield may be reduced, and the reliability may be reduced. . On the other hand, in the case of using the cap film as in the second and fourth embodiments, the change in the film thickness is the first.
1/10 or less of the embodiment. In the third and fourth embodiments of the solid-phase growth system, it is possible to minimize the laser output, which is advantageous in terms of apparatus cost and running cost as described later in the section of effects.

(Embodiment 5) A polycrystalline silicon thin film transistor using the polycrystalline silicon thin film prepared according to Embodiments 1 to 4 as an active layer was produced. A polycrystalline silicon thin film is formed by any of the methods of Embodiments 1 to 5, a gate insulating film is formed, and etching is performed while leaving a portion to be a thin film transistor in an island shape. After forming the gate electrode, impurities are implanted using the gate electrode as a mask. An interlayer insulating film is formed, and a contact electrode is taken out and completed. The mobility of the polycrystalline silicon thin film transistor of the present invention is excellent in uniformity and high as shown in FIG.

When the cap annealing of the second or fourth embodiment is performed, the cap film may be used as it is as the gate insulating film, or the cap film may be removed and a separate gate insulating film may be formed.

The thin film transistor is not limited to the above-mentioned planar type, but may be any of an inverted stagger type and a forward stagger type.

[0035]

The first effect relates to the fact that the laser energy density can be reduced. The low energy density means that if the same excimer laser is used, the area that can be irradiated at once increases, and the throughput improves. If the irradiation area is the same, a small excimer laser can be used, and the cost of the apparatus can be reduced. In addition, the smaller the excimer laser, the better the uniformity between the pulses and the lower the running cost. For example, 200
When comparing W and 100 W excimer lasers, the pulse uniformity is improved by a factor of three with the standard deviation of the beam energy.
The price will be about 2/3. In the case of an excimer laser, since the pulse life is only about 9 times the pulse life, the laser tube needs to be replaced in about six months to about one year, which can lead to a significant reduction in running cost.

In the combined use with solid phase growth, an excimer laser with a lower output can be used, and the above effect can be enhanced.

The second effect is that uniform and high transistor characteristics can be obtained. According to the present invention, since the uniform crystal grain size can be controlled without depending on the variation of the laser energy density, the polycrystalline silicon thin film transistor can be made uniform while maintaining high mobility.

A third effect is that a low threshold polycrystalline silicon thin film transistor having a thin gate insulating film can be realized. According to the present invention, it is possible to obtain a polycrystalline silicon thin film having a desired particle size while obtaining a flat surface by using cap annealing together. A flat surface was achieved by the conventional cap annealing method, which meant that a polycrystalline silicon thin film having a small grain size was produced at the same time. However, the present invention has made it possible to realize a polycrystalline silicon thin film having a desired particle size and a flat surface. As a result, even when a very thin insulating film is formed on the surface, the leakage current of the insulating film does not increase due to the unevenness of the surface.

Further, as compared with the conventional two-layer silicon film method, the substrate is not thermally damaged, so that an inexpensive glass substrate can be used, and laser irradiation on the first silicon film is batch irradiation. In addition, there is an effect that a mask or the like for performing partial irradiation is not required.

As described above, according to the polycrystalline silicon thin film forming method of the present invention, a uniform and high mobility polycrystalline silicon thin film can be obtained by excimer laser annealing at a low energy density. A great effect can be obtained for improving the performance of the applied device.

[Brief description of the drawings]

FIG. 1 shows a method for forming a polycrystalline silicon thin film according to the present invention.

FIG. 2 shows a method for forming a polycrystalline silicon thin film according to the present invention.

FIG. 3 shows a method for forming a polycrystalline silicon thin film according to the present invention.

FIG. 4 is a method of forming a polycrystalline silicon thin film according to the present invention.

FIG. 5 shows a change in crystal grain size with respect to laser energy density according to the method of the present invention.

FIG. 6 shows a comparison between the mobility of the polycrystalline silicon thin film transistor according to the present invention and the conventional method.

FIG. 7 shows a conventional method of forming a polycrystalline silicon thin film.

FIG. 8 shows a conventional method for forming a polycrystalline silicon thin film.

FIG. 9 shows a change in crystal grain size with respect to the energy density of a conventional laser.

FIG. 10: Effect of difference in crystal grain size on polycrystalline silicon thin film transistors

FIG. 11 shows the mobility of a polycrystalline silicon thin film transistor.

FIG. 12 shows a conventional method for forming a polycrystalline silicon thin film using a two-layer silicon film method.

[Explanation of symbols]

 Reference Signs List 1 glass substrate 2 first silicon thin film 3, 6 excimer laser beam 4 polycrystalline silicon thin film 5 second silicon thin film 7, 10 molten silicon 8, 11, 12, 14 polycrystalline silicon thin film 9, 13 cap film

Claims (6)

[Claims] 1. A step of forming a first silicon thin film on a substrate having at least an insulating surface; a step of polycrystallizing the first silicon thin film by excimer laser irradiation; Forming a second silicon thin film from above the silicon thin film; and polycrystallizing the second silicon thin film using the first silicon thin film as a nucleus. Forming method. 2. A step of forming a first silicon thin film on a substrate having at least an insulative surface; a step of polycrystallizing the first silicon thin film by excimer laser irradiation; Forming a second silicon thin film from above the silicon thin film; and irradiating the second silicon thin film with an excimer laser to polycrystallize the first silicon thin film as a nucleus. A method for forming a crystalline silicon thin film. 3. A step of forming a first silicon thin film on a substrate having at least an insulating surface; a step of polycrystallizing the first silicon thin film by excimer laser irradiation; Forming a second silicon thin film on the silicon thin film, forming an insulating film on the second silicon thin film, and further irradiating an excimer laser on the insulating film to obtain the second silicon thin film. A process of polycrystallizing the second silicon thin film using the first silicon thin film as a nucleus. 4. A step of forming a first silicon thin film on a substrate having at least an insulating surface; a step of polycrystallizing the first silicon thin film by excimer laser irradiation; Forming a second silicon thin film from above the silicon thin film, and annealing the second silicon thin film at a temperature of 500 ° C. or more to form the second silicon thin film with the first silicon thin film as a nucleus. And forming a polycrystalline silicon thin film. 5. A step of forming a first silicon thin film on a substrate having at least an insulating surface, a step of polycrystallizing the first silicon thin film by excimer laser irradiation, and a step of polycrystallizing the first silicon thin film. Forming a second silicon thin film from above the silicon thin film, forming an insulating film on the second silicon thin film, and annealing the second silicon thin film at a temperature of 500 ° C. or more. A step of polycrystallizing the second silicon thin film using the first silicon thin film as a nucleus. 6. A polycrystalline silicon thin film transistor having a polycrystalline silicon thin film formed by any one of claims 1 to 5 as an active layer.