JP2856533B2 - Method for manufacturing polycrystalline silicon thin film - Google Patents

Description

The present invention relates to a method for manufacturing a polycrystalline silicon thin film, and more particularly to a method for manufacturing a polycrystalline silicon thin film using a solid phase growth method.

(Prior Art) In recent years, the active speed of an active element such as a polycrystalline silicon thin film transistor using a polycrystalline silicon thin film has been greatly improved compared to an active element such as a thin film transistor using an amorphous silicon film. Attention has been focused on, and application to a contact sensor or a drive circuit of a liquid crystal display device is particularly expected.

The operating speed of an active element used in such a contact sensor or a drive circuit of a liquid crystal display device is required to be at least 8 MHz to 10 MHz or more.

In order to meet such requirements, high electron mobility is required, and therefore, a crystal grain size of at least 50 cm ² / V · S is required.

By the way, methods for manufacturing a polycrystalline silicon thin film have been studied in various fields, and are roughly classified into a liquid crystal growth method and a solid phase growth method.

The liquid crystal growth method includes, for example, physical thin film deposition such as electron beam evaporation and sputtering, or low-pressure CVD (Chemical Vapor).
Deposit), polycrystalline silicon or amorphous silicon obtained by chemical thin film deposition such as plasma CVD is irradiated with a laser beam or an electron beam above the melting point temperature, melted, and then recrystallized by cooling. Things.

The solid phase growth method uses amorphous silicon which is further ion-implanted into polycrystalline silicon or amorphous silicon obtained by physical thin film deposition or chemical thin film deposition as described above and has a melting point of 550 to 650 ° C or lower. Is maintained for several hours to obtain a polycrystalline silicon film having a large crystal grain size.

(Problems to be Solved by the Invention) In the above-mentioned liquid phase growth method, polycrystalline silicon or amorphous silicon is melted and then recrystallized, so that a polycrystalline silicon film having good crystallinity can be obtained. Incorporation of impurities increases due to the molten state.
For this reason, since it becomes a polycrystalline silicon film with low purity, when applied to an active element such as a thin film transistor, there is a disadvantage that a highly reliable operation cannot be obtained.

On the other hand, the solid phase growth method provides a good polycrystalline silicon film because no impurities are mixed into the film during the production, but a film having better crystallinity than the liquid phase growth method is obtained. There are drawbacks that could not be obtained.

The present invention has been made in view of the above-described problems, and provides a method for manufacturing a polycrystalline silicon film using a novel solid-phase growth method. An object of the present invention is to provide a method for manufacturing a polycrystalline silicon film which is optimal for an active element such as a thin film transistor.

[Structure of the Invention] (Means for Solving the Problems) The method for producing a polycrystalline silicon thin film of the present invention is a method for producing a polycrystalline silicon film by heating an amorphous silicon film and performing solid phase growth. A method for producing a thin film, comprising: a refractive index (n) and an extinction coefficient (k) of an amorphous silicon film.
However, at a wavelength of 250 nm, n = 1.88 ± 0.01, k = 3.41 ± 0.0
1, At a wavelength of 300 nm, n = 3.18 ± 0.01, k = 3.70 ± 0.01,
At a wavelength of 350 nm, n = 4.44 ± 0.01, k = 3.16 ± 0.01, at a wavelength of 400 nm, n = 5.00 ± 0.01, k = 2.22 ± 0.01, wavelength
At 450 nm, n = 5.07 ± 0.01, k = 1.47 ± 0.01, wavelength 55
At 0 nm, n = 4.80 ± 0.01, k = 0.63 ± 0.01, wavelength 600n
In m, n = 4.65 ± 0.01, k = 0.38 ± 0.01, wavelength 650 nm
At n = 4.52 ± 0.01, k = 0.17 ± 0.01, wavelength 700 nm, n = 4.43 ± 0.01, k = 0.03 ± 0.01, wavelength 750 nm, n = 4.35 ± 0.01, k = 0.01 ± 0.01, wavelength 800 nm, It is characterized in that n = 4.30 ± 0.01, k = 0.01 ± 0.01, and n = 4.25 ± 0.01, k = 0.01 ± 0.01 at a wavelength of 850 nm.

(Operation) The present inventors examined the correlation between amorphous silicon, which is a starting material in the solid-phase growth method, and a polycrystalline silicon thin film produced from the same from various directions. The facts became clear.

In other words, the smaller the voids of the amorphous silicon film as the starting material, that is, the higher the denseness, the larger the grain size of each crystal of the finally manufactured polycrystalline silicon thin film.

A film used as a starting material has a denser amorphous silicon, and the distance between nearest neighbor atoms of the amorphous silicon is narrower than that of the amorphous silicon having a lower density. For this reason, the bonding energy between the atoms of the amorphous silicon having a high density of the film is high,
The bonds between atoms are hard to be broken.

Therefore, in amorphous silicon having a high film density, self-diffusion during solid phase growth is difficult to be performed, and the frequency of crystal nuclei is lower than that in amorphous silicon having a low film density. .

Therefore, when amorphous silicon having a high density of a film is used as a starting material, the frequency of generation of crystal nuclei is low, and the generated crystal nuclei achieve sufficient crystal growth without hindering the growth of adjacent crystal nuclei. It becomes possible.

In addition, when amorphous silicon having a high density of a film is used as a starting material, a large number of silicon atoms exist within a small distance from a crystal nucleus, so that once a crystal nucleus is generated, crystal growth proceeds rapidly. It will be.

For this reason, the higher the density of the amorphous silicon film as the starting material, the larger the grain size of each crystal of the finally manufactured polycrystalline silicon thin film is obtained. The time required for growth does not change much compared to the conventional case.

In addition, the present inventors have found that the density of an amorphous silicon film as a starting material is clearly indicated by a refractive index (n) and an extinction coefficient (k) determined by a spectroscopic ellipsometer.

Further, when various experiments were further performed, the refractive index (n) for each wavelength of amorphous silicon, which is a starting material, was particularly measured.
It has been found that an extinction coefficient (k) within a predetermined range is an essential condition for obtaining a polycrystalline silicon thin film having good crystallinity.

It is considered that the density of the amorphous silicon film is determined by the void amount in the amorphous silicon film, and the void amount in this film is determined by a refractive index (n) and an extinction coefficient obtained by a spectroscopic ellipsometer. It is considered that this greatly affects (k).

The refractive index (n) and the extinction coefficient (k) in this specification
Is an automatic spectroscopic ellipsometer (manufactured by SOPRA; Multi-Optic
al-Spectrometric-Scanner Model ES-4G).

When the refractive index (n) is smaller than the above range with respect to the wavelength, it means that the amount of voids in the film is large, and a polycrystalline silicon thin film having good crystallinity cannot be obtained. Conversely, if the refractive index (n) is larger than the above range, a polycrystalline silicon thin film having good crystallinity can be obtained, but it is difficult to form such a starting material of amorphous silicon. That's why.

Similarly, if the extinction coefficient (k) is smaller than the above range, a polycrystalline silicon thin film having good crystallinity cannot be obtained, and conversely, amorphous silicon larger than the above range may be formed. Is technically difficult.

According to the present invention, as described above, solid-phase growth is performed using a dense amorphous silicon film having a refractive index (n) and an extinction coefficient (k) within a predetermined range for each wavelength as a starting material. A polycrystalline silicon thin film having good crystallinity can be obtained even if manufactured in the same manner as described above.

In addition, as a method for obtaining the dense amorphous silicon described above, it can be obtained by appropriately changing each condition of a conventionally known method, and in particular, reduced pressure CVD using silane gas or disilane gas, plasma CVD, or the like. A method of depositing a chemical thin film, or a method of implanting silicon ions, silicon hydride ions, silicon fluoride ions, or the like into polycrystalline silicon to make the polycrystalline silicon amorphous is preferable.

(Embodiment) Hereinafter, a method for manufacturing a polycrystalline silicon thin film according to an embodiment of the present invention will be described in detail.

(Specific example) Using disilane as a reaction gas by a low pressure CVD device,
An amorphous silicon film having a refractive index (n) and an extinction coefficient (k) for each wavelength as shown in Table 1 was formed on an insulating substrate under the conditions of a deposition temperature of 470 ° C. and a gas pressure of 0.8 torr by 0.2 μm. Deposited in film thickness.

Here, the measurement of the refractive index (n) and the extinction coefficient (k) for each wavelength was determined using an automatic spectroscopic ellipsometer (manufactured by SOPRA).

The distance between the nearest neighbors of the amorphous silicon film is calculated to be 2.38 angstroms from this denseness, which is very close to 2.35 angstroms, which is the distance between the nearest neighbors of the single crystal silicon film.

Then, the substrate was held at a substrate temperature of 600 ° C. for 24 hours, and was subjected to solid phase growth to obtain a polycrystalline silicon thin film. When the crystal grain size of the polycrystalline silicon thin film thus obtained was measured,
A polycrystalline silicon thin film with good crystallinity having a crystal grain size of 2.0 to 3.0 microns was obtained.

(Comparative Example) Using a low pressure CVD apparatus, silane was used as a reaction gas, and the refractive index (n) for each wavelength as shown in Table 1 on the insulating substrate under the conditions of a film forming temperature of 550 ° C. and a gas pressure of 0.4 torr. An amorphous silicon film having a decay coefficient (k) was deposited to a thickness of 0.2 microns.

Here, the measurement of the refractive index (n) and the extinction coefficient (k) for each wavelength is obtained by using an automatic spectroscopic ellipsometer as in the specific example described above.

When the distance between the nearest neighbors of the amorphous silicon film is calculated from the density, the distance is 2.43 Å, which is considerably larger than the specific example.

Then, the substrate temperature was maintained at 600 ° C. for 24 hours in the same manner as in the specific example, followed by solid phase growth to obtain a polycrystalline silicon thin film. When the crystal grain size of the polycrystalline silicon thin film thus obtained was measured, a polycrystalline silicon thin film having a crystal grain size of 0.1 μm was obtained.

As is clear from the above specific examples and comparative examples, by setting the refractive index (n) and the extinction coefficient (k) for each wavelength of amorphous silicon, which is a starting material for solid-phase growth, within a predetermined range, A polycrystalline silicon thin film having good crystallinity can be easily manufactured.

In addition to the specific examples described above, for example, a method of implanting silicon ions into a film under predetermined conditions after forming an amorphous silicon film or a polycrystalline silicon thin film by a low-pressure CVD apparatus for realizing the present invention. This is one of the easier methods.

[Effects of the Invention] As described above in detail, according to the method for producing a polycrystalline silicon thin film of the present invention, the disadvantage that it is difficult to obtain a polycrystalline silicon thin film having a large crystal grain size, which is a disadvantage of the solid phase growth method, is obtained. Thus, a polycrystalline silicon thin film as good as that obtained by the liquid phase growth method could be obtained.

In addition, a polycrystalline silicon thin film having high purity can be obtained without introducing impurities into the film as compared with the liquid phase growth method.

Claims (1)

(57) [Claims] 1. A method for manufacturing a polycrystalline silicon thin film, wherein a polycrystalline silicon film is manufactured by heating the amorphous silicon film to cause solid phase growth, wherein a refractive index (n) and extinction of the amorphous silicon film are reduced. Coefficient (k)
Is n = 1.88 ± 0.01, k = 3.41 ± 0.01 at a wavelength of 250 nm, n = 3.18 ± 0.01, k = 3.70 ± 0.01 at a wavelength of 300 nm, n = 4.44 ± 0.01, k = 3.16 ± 0.01 at a wavelength of 350 nm, wavelength At 400 nm, n = 5.00 ± 0.01, k = 2.22 ± 0.01, at 450 nm, n = 5.07 ± 0.01, k = 1.47 ± 0.01, at 500 nm, n = 4.96 ± 0.01, k = 0.97 ± 0.01, at 550 nm , N = 4.80 ± 0.01, k = 0.63 ± 0.01, at a wavelength of 600 nm, n = 4.65 ± 0.01, k = 0.38 ± 0.01, at a wavelength of 650 nm, n = 4.52 ± 0.01, k = 0.17 ± 0.01, at a wavelength of 700 nm, n = 4.43 ± 0.01, k = 0.03 ± 0.01, n = 4.35 ± 0.01, k = 0.01 ± 0.01 at a wavelength of 750 nm, n = 4.30 ± 0.01, k = 0.01 ± 0.01 at a wavelength of 800 nm, n = 4.25 at a wavelength of 850 nm A method for producing a polycrystalline silicon thin film, wherein ± 0.01, k = 0.01 ± 0.01.