JPH02202018A - Manufacture of polycrystalline silicon thin film - Google Patents

Abstract

PURPOSE:To obtain a polycrystalline silicon thin film of high quality at a low temperature on the substrate other than single crystal silicon by a method wherein polycrystalline silicon is grown on a substrate using silane as film- forming gas and also using reaction gas, containing fluoric gas but not containing hydrogen gas, as etching gas. CONSTITUTION:A substrate other than single crystal silicon is used as a substrate, silane is used as film-forming gas, and reaction gas containing fluoric gas and not containing hydrogen gas is used as etching gas. Accordingly, hydrogen is not used as diluting gas in the reaction gas, hydrogen is only brought into plasma by the dissolution of difluorosilane and the like which will be conjointly used as silane and etching gas, and the covering crystal surface with hydrogen when a silicon thin film is grown can be prevented. Moreover, the fluoric gas which has the reaction activity higher than that used in the past. Consequently, as the silicon thin film grown surface is exposed in a clean state, the growth of polycrystalline silicon can be conducted in an extremely uniform manner. As a result, a high quality polycrystalline silicon thin film having a large electron mobility can be obtained even when a substrate other than single crystal silicon is used.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a method for manufacturing polycrystalline silicon thin films that can be used for semiconductor devices such as thin film transistors and solar cells, and particularly relates to a method for forming polycrystalline silicon thin films on substrates such as glass at low temperatures. The present invention relates to a method for manufacturing a polycrystalline silicon thin film.

(Prior art) A polycrystalline silicon thin film is a state in which a large number of crystalline silicon particles of several hundred to several micrometers are aggregated.
Manufactured by VD method.

For example, in JP-A-58-172217, 400-9
Thermal CVD is carried out by supplying the hesilane compound to the reaction chamber at 00'C.
and deposit polycrystalline silicon f! on the substrate. It discloses forming an i-film.

However, the above method not only requires a large amount of thermal energy to heat the reaction gas with poor heat transfer efficiency to a high temperature, but also has the problem of poor thin film productivity because heating and cooling takes too much time and cost. , 400~700°
When temperature conditions around C were used, there was a drawback that the film forming rate was slow and impractical.

In addition, since polycrystalline silicon thin films are generally formed on substrates made of materials other than crystalline silicon, such as alumina or graphite, impurities such as aluminum and nickel contained in the substrate will be renewed if exposed to high temperatures of 700"C or higher. It is known that it diffuses into the silicon crystal formed in the silicon crystal and deteriorates the characteristics of the deposited thin film ([Latest LSI
Process Technology J, P2O3, Published in 1982, Written by Kazuo Maeda, Industrial Research Institute Mi. In addition, glass substrates have a low melting point, and it is difficult to use them as substrates at temperatures above 700'C.

Furthermore, recent research has shown that in the case of polycrystalline silicon produced by thermal CVD, there are many gaps between silicon particles due to the high temperature during production, which prevents them from adhering to each other, as well as dangling grain boundaries. It has been found that the bond deteriorates the electrical characteristics, and in order to correct this disadvantage, it is necessary to passivate the grain boundaries with hydrogen.

For this reason, the plasma CVD method, which can produce polycrystalline silicon thin films at low temperatures, has recently attracted attention (see JP-A-63-157872 and JP-A-63-175417).
In these methods, a large amount of hydrogen gas is used as a component of the reaction gas, so the obtained polycrystalline silicon contains hydrogen between silicon particles compared to the case of thermal CVD method, and the polycrystalline silicon obtained It has the advantage that there is no need to passivate it afterwards.

(Problems to be Solved by the Invention) However, the polycrystalline silicon rjIy4 obtained by the above method contains about 2.5 at. There was room for further improvement in terms of performance.

Therefore, the present inventors conducted further research on a method for manufacturing polycrystalline silicon thin films using the plasma CVD method, which has the advantage of lowering the substrate temperature.
In particular, when hydrogen is not used as a reaction gas, the grain size of polycrystalline silicon can be expanded to about 0.05 to 3 μm, thereby improving the physical properties of polycrystalline silicon thin films, and The inventors have discovered that particularly good results can be obtained by using silane as the etching gas and fluorine gas, which has a high etching ability, as the etching gas, and have arrived at the present invention.

Accordingly, a first object of the present invention is to provide an improved method for producing high quality polycrystalline silicon thin films at low temperatures.

A second object of the present invention is to provide a method for producing a high-performance polycrystalline silicon thin film with large crystal grain size by plasma CVD.

(Means for Solving the Problems) The above aspects of the present invention are a method of forming a polycrystalline silicon thin film on a heated substrate by turning a reactive gas containing a raw material having silicon atoms into plasma by plasma CVD. , a substrate other than single crystal silicon is used as the substrate, and polycrystalline silicon is formed on the substrate using silane as a film-forming gas and a reactive gas containing fluorine gas but not hydrogen gas as an etching gas. This was achieved by a method of manufacturing a polycrystalline silicon thin film, which is characterized by growing polycrystalline silicon thin films.

The manufacturing method of the present invention will be explained in detail below.

The substrate used in the present invention can be appropriately selected from substrates other than single crystal silicon, such as glass, ceramic, and metal.

When single crystal silicon is used as a substrate, if the crystallization conditions are selected as plasma CVD, an epitaxial film will be obtained, but the polycrystalline silicon thin film of the present invention will not be obtained.
Therefore, in the present invention, a polycrystalline silicon thin film is formed on the above-mentioned substrate other than single crystal silicon by plasma CVD.

In the present invention, at least one type of silane selected from the group consisting of 5iHn, 5izH*, St, and H is used as a silicon atom supply gas for forming polycrystalline silicon.

In addition, F2 is used as the etching gas, but F8
In addition to 5iH4-, ,X, , (x is an F or Cl atom, and n is an integer of 1 to 4, preferably an integer of 2 to 4) can also be used in combination. As an etching gas that can be used in combination, SiF is particularly preferred.

The amount of gas such as fluorosilane that can be used in combination is preferably 100% or less, particularly 5 to 80%, based on the silane in terms of volume.

The etching gas releases fluorine atoms that can etch the silicon surface under plasma conditions.

That is, the fluorine atoms in the plasma are considered to have the following functions: (1) exposing a constantly clean silicon growth surface, (2) removing impurities from the grown silicon thin film, and (2) preventing silane polymerization in the gas phase.

Since the present invention is for obtaining polycrystalline silicon, it is particularly necessary to strike a balance between crystal growth and etching. Such a balance can be easily adjusted by appropriately mixing the silane and etching gas, particularly fluorine gas.

The amount of fluorine gas used is 0.1 to 50 times that of silane, especially 0.1 to 50 times that of silane.
1 to 10 times (volume ratio) and even 0. It is preferable to set it as 1 to 1 times. If it is less than 0.1 times, the etching effect will be weak, and if it exceeds 50 times, the etching will be too strong to form a silicon crystal thin film.

In the present invention, since active fluorine gas is used as the etching gas, it is preferable to add at least one inert gas selected from helium, neon, and argon as a diluent gas to the reaction gas. In this case, the amount of inert gas added is 0.1 to 50% of fluorine gas,
000 times, preferably 1 to 1,000 times, more preferably 5 to 100 times. If it is 50,000 times or more, the concentration of silane, which is a film-forming gas, is too low and the growth of the silicon crystal thin film will be extremely slow. If it is less than 1 times, the fluorine concentration in the reaction gas will be high and corrosion of the equipment may become a problem.

Since the pressure in the chamber for generating plasma is related to the amount of energy possessed by atoms etc. that reach the substrate, the pressure needs to be between 0.1 Torr and 15 Torr, particularly 0.5T.

For a history of rr to 1 O Torr, it is preferable to set it to 1 to 5 Torr.

The main feature of the present invention, unlike conventional methods, is that hydrogen gas is not used in the reaction gas.

In the present invention, a reaction gas that satisfies the above conditions is
Power density 0.1-5 W/cd, preferably 1-3
The reaction gas was turned into plasma by discharging at W/c4, and about 10
0°C to 700°C preferably about 100'C to 600°C
C1 A polycrystalline silicon thin film is formed on the substrate other than silicon single crystal, which is maintained at a constant temperature, more preferably between about 200''C and 500C.

The discharge may be high frequency discharge, direct current discharge, microwave discharge, or the like.

When the substrate temperature is lower than 100° C., an amorphous phase appears, forming a microcrystalline structure and deteriorating the quality. If the substrate temperature is higher than 700° C., corrosion of the device will be significant, and the advantages of plasma CVD as a low temperature method cannot be utilized.

The power density varies depending on the type and pressure of the reactant gas, but
If the power density is lower than O, IW/c4, the pressure of the reaction gas must be sufficiently lowered, resulting in a slow film formation rate.
If it exceeds 5 W/cIa, it is not preferable because the quality of the thin film cannot be maintained at a high level.

In addition, in the present invention, not only can an intrinsic polycrystalline silicon thin film be produced, but also polycrystalline silicon can be formed by mixing a dopant gas of Group Ⅰ or Group V of the periodic table of elements into the reaction gas. The crystalline silicon thin film can be p-type or n-type.

Examples of the dopant gas in this case include hydrides such as diborane, phosphine, and arsine.

(Function) In the silicon thin film growth method of the present invention, hydrogen is not used as a diluent gas in the reaction gas, so hydrogen is only brought into the plasma by decomposition of silane and difluorosilane, which may be used together as an etching gas. Therefore, unlike the conventional method, hydrogen does not cover the crystal surface during the growth of a silicon thin film. Furthermore,
Fluorine gas, which has a higher reaction activity than in the conventional method, is used. For these reasons, in the present invention, the silicon thin film growth surface is always exposed in a clean state, preventing the formation of amorphous silicon or a distorted silicon film that forms grain boundaries, and thus preventing polycrystalline silicon growth. It is estimated that this is done extremely uniformly.

According to the method of the present invention, the silicon crystal particles forming the polycrystalline silicon thin film have a diameter of about 0.05 μm or more, particularly 0.1 μm.
As the crystal surface area in the thin film decreases, the amount of hydrogen for sealing the dangling bonds existing at the crystal interface becomes less than 2 at.
It is also easy to set the content to 2 atomic % or less. As described above, the silicon crystals of the polycrystalline silicon thin film obtained by the present invention have a large grain size and contain less hydrogen than the polycrystalline silicon thin film obtained by the conventional plasma CVD method. The physical properties of the crystalline silicon thin film are good. (Effects of the Invention) As detailed above, according to the present invention, even if a substrate other than single crystal silicon is used, a high quality polycrystalline silicon thin film with high electron mobility can be obtained at low temperature. AI
- Even if a substrate containing impurities such as SF e s Ni is used, the impurities in the substrate will not diffuse into the formed silicon thin film. Therefore, by using an inexpensive substrate that is not sufficiently purified, the thin film can be Not only can the manufacturing cost of semiconductor elements be significantly lowered, but also the use of thin films can be greatly expanded along with this cost reduction.

The present invention will be described in more detail below with reference to Examples, but the present invention is not limited thereto.

Example 1゜Fg:SiH4:He=1:2:
9 was supplied as a reaction gas at 11,005 CC, and the pressure of the reaction gas was adjusted to 1.2 Torr on a glass substrate heated to 0°C. A silicon thin film was formed to a thickness of Oam.

The particle size of the sample obtained as described above was measured by X-ray diffraction, and the average particle size was 1.000 Å or more. Furthermore, the electron mobility of this thin film is approximately 50 cd.
/volt-second, and the hydrogen content was less than 0.1 atomic percent.

Examples 2 to 6 Polycrystalline silicon thin films were obtained in the same manner as in Example 1 under the conditions shown in Table 1, and the same measurements were carried out. The results are shown in Table 1. Table 1 also lists the conditions and results of Example 1.

Comparative Examples 1 to 3 A silicon thin film was formed in the same manner as in Example 1 under the conditions shown in Table 2, with the reaction gas supply amount being 60 SCCM. The properties of the obtained silicon thin film are as shown in Table 3.

As is clear from the results in Table 3, polycrystalline silicon thin films were not obtained in Comparative Examples 1 and 2, and the properties of the polycrystalline silicon thin film obtained in Comparative Example 3 were different from those of the polycrystalline silicon thin film of the present invention. It was confirmed that it was inferior.

Claims (1)

[Claims] A method of forming a polycrystalline silicon thin film on a heated substrate by converting a reactive gas containing a raw material having silicon atoms into plasma by plasma CVD, in which a substrate other than single crystal silicon is used as the substrate, and the film forming property is A method for producing a polycrystalline silicon thin film, comprising growing polycrystalline silicon on the substrate using silane as a gas and a reaction gas containing fluorine gas and no hydrogen gas as an etching gas.