JPH05239649A - Formation of silicon oxide thin film - Google Patents

Abstract

PURPOSE:To realize the formation of a silicon oxide thin film with power saving and without increasing the temp. of a substrate by mixing steam or hydrogen in reaction gas of a plasma CVD method. CONSTITUTION:After a vacuum chamber 1 is evacuated to a specified vacuum, reaction gas is fed to the chamber from a pipe 7. The reaction gas is a mixture of gaseous organic silane compounds, oxidizing gas and steam or hydrogen. The ratios of steam and hydrogen to organic silane compounds in reaction gas are preferably 5-100 (by vol.) and 15-200 (by vol.) respectively. When high frequency power is impressed to a cathode 5 to generate plasma discharge, reaction gas is decomposed and oxidized to form a silicon oxide thin film on the surface of a substrate 4. Steam or hydrogen in the reaction gas is activated by plasma discharge to promote the decomposition of organic silane molecules. Since C or C-H does not remain in the thin film, a thin film with good electrical property is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a silicon oxide thin film, and more particularly to a method for forming a silicon oxide thin film suitable for a gate insulating film of a thin film transistor (TFT) represented by a drive circuit of a liquid crystal display device. ..

[0002]

2. Description of the Related Art Recently, as a driving circuit for a liquid crystal display element,
It has been proposed to use a polycrystalline silicon thin film transistor (Poly-SiTFT) that uses polycrystalline silicon having a high electron mobility, and has already been proposed.
It is being used in some LCD projectors. Since this polycrystalline silicon thin film transistor is manufactured by using the LSI process, a process temperature of 1000 ° C. or higher is applied when the gate insulating film is formed on the substrate by the thermal oxidation method, and therefore the substrate is heat resistant. Only some expensive quartz glass can be used.

On the other hand, since a large-screen liquid crystal display element is used in office automation equipment and consumer equipment, it is essential that it be inexpensive, and for this purpose, it is necessary to use an inexpensive glass substrate. However, since the glass substrate is inferior in heat resistance, when the glass substrate is used, the gate insulating film of the polycrystalline silicon thin film transistor must be formed by a low temperature process.

By the way, tetramethylsilane (TM
S), tetraethylsilane (TES), tetraethoxysilane (TEOS) and other organic silane compounds are used as a raw material for forming a silicon oxide thin film, a thermal CVD method and a plasma CVD method are typical, but they have a large area. The plasma CVD method is preferable in consideration of uniform film formation.

However, the silicon oxide thin film prepared by the plasma CVD method using the above-mentioned organic silane compound as a raw material has excellent step coverage, but has poor electrical characteristics and is unsuitable for a gate insulating film. This is because the precursor that contributes to film formation is an organic polymer containing a C—H bond, so that it exhibits fluidity on the substrate surface and the step coverage is improved, but a C or C—H bond is present in the thin film. This is because it includes.

Therefore, in order to obtain a silicon oxide thin film having excellent electrical characteristics, it is necessary to obtain a dense thin film containing no C or C—H bond, and for that purpose, a film is formed by the plasma CVD method. When increasing the substrate temperature,
It is necessary to increase the input power of high frequency power to completely decompose the organosilane compound gas in the gas phase.

However, as a method of increasing the substrate temperature, it is not possible to use an inexpensive glass substrate as described above, and a method of increasing the input power of high frequency power is CVD.
The load on the device increases, which is not preferable in consideration of yield and the like.

[0008]

Therefore, an object of the present invention is to increase the substrate temperature or increase the input power of high frequency power of a silicon oxide thin film having excellent electrical characteristics such as withstand voltage and leak current. It is to obtain a method that can be formed without doing.

[0009]

This problem is solved by mixing water vapor or hydrogen in the reaction gas when forming a silicon oxide thin film by the plasma CVD method using a reaction gas containing an organosilicon gas. It

The present invention will be described in detail below. Figure 1
Shows an example of a plasma CVD apparatus used in the present invention, and reference numeral 1 in the drawing is a vacuum chamber. The vacuum chamber 1 is connected to an exhaust system (not shown), and the inside of the vacuum chamber 1 is configured to have a predetermined degree of vacuum. A tray 3 also serving as an anode is provided in the vacuum chamber 1.
The substrate 4 is mounted on the substrate. The substrate 4 on the tray 3 is heated by the heater 2. A cathode 5 is provided in the vacuum chamber 1 so as to face the tray 3, and the cathode 5 is connected to a high frequency power source 6 provided outside the vacuum chamber 1. Further, a pipe 7 for supplying a reaction gas is provided in the vacuum chamber 1.

Next, an embodiment of the forming method of the present invention using this plasma CVD apparatus will be described. First, the vacuum tank 1
After evacuating the inside to a predetermined degree of vacuum, a reaction gas is supplied to the vacuum chamber 1 through the pipe 7. The reaction gas at this time, an organic silane compound gas such as tetramethylsilane, tetraethylsilane, tetraethoxysilane, and oxygen,
A mixed gas composed of an oxidizing gas such as ozone-containing oxygen, carbon monoxide, carbon dioxide, and nitric oxide, and water vapor (H ₂ O) or hydrogen is used.

By mixing water vapor or hydrogen into the reaction gas, a silicon oxide thin film having excellent electrical characteristics can be formed. The mixing amount of water vapor or hydrogen is preferably 5 to 100 times (volume ratio) for water vapor and 15 to 200 times (volume ratio) for hydrogen with respect to the organic silane compound in the reaction gas.

After the pressure in the vacuum chamber 1 becomes stable, high-frequency power from the high-frequency power source 6 is applied to the cathode 5 to start plasma discharge to decompose and oxidize the reaction gas, thereby oxidizing the surface of the substrate 4. A silicon thin film is formed. After the silicon oxide thin film having a predetermined thickness is formed, the discharge is stopped and the supply of the reaction gas is stopped.

In such a forming method, the electrical characteristics of the obtained silicon oxide thin film are excellent. This is because water vapor or hydrogen mixed in the reaction gas is decomposed and activated by plasma discharge, active hydrogen and oxygen accelerate decomposition of organic silane molecules, and C or C—H in the obtained silicon oxide thin film. It is presumed that no bond remains. Therefore, a high-quality silicon oxide thin film can be obtained even if the substrate temperature is set to 400 ° C. or lower or the high frequency input power is reduced.

FIG. 2 is a graph showing the dielectric strength of a silicon oxide thin film obtained by adding water vapor during film formation at a substrate temperature of 300 ° C. It can be seen from this graph that the dielectric strength increases with the addition of water vapor.

FIG. 3 is a graph showing the relationship between the high-frequency input power and the withstand voltage of the obtained silicon oxide thin film. In the figure, the black circles indicate the case where no water vapor is added, and the white circles indicate the case where 20 SCCM of water vapor is added. Indicates. From this, the high frequency input power is 2500 W (0.39 W / cm ² ) to 10
Even if it is reduced to 00 W (0.16 W / cm ² ), 6 MV /
It can be seen that a silicon oxide thin film having a withstand voltage of cm or more can be obtained. Furthermore, it has been confirmed that the same effect was obtained by using hydrogen instead of water vapor.

Further, in the above-mentioned embodiment, the reaction gas in which the organic silane compound gas, the oxidizing gas and the steam or hydrogen are mixed is supplied into the vacuum chamber, but these gases are individually supplied to the vacuum chamber. It can also be introduced in 1.
Further, the method of the present invention can be naturally used for forming a silicon oxide thin film (PSG thin film, BSG thin film) doped with phosphorus or boron.

[0018]

As described above, according to the method for forming a silicon oxide thin film of the present invention, when the silicon oxide thin film is formed by the plasma CVD method using the reaction gas containing the organic silicon gas, water vapor is contained in the reaction gas. Alternatively, since hydrogen is mixed, the dielectric strength, the leakage current, the leakage current, and the like can be increased without increasing the substrate temperature or the input power of the high frequency power.
A silicon oxide thin film having excellent electrical properties such as interface state density and fixed charge density can be manufactured. Therefore, an inexpensive glass substrate can be used as a substrate, and manufacturing facility cost and electric power cost can be reduced.

[Brief description of drawings]

FIG. 1 is a plasma CV used in an example of the present invention.
It is a schematic block diagram of a D device.

FIG. 2 is a graph showing the relationship between the amount of water vapor added and the withstand voltage.

FIG. 3 is a graph showing the relationship between high-frequency input power and withstand voltage.

[Explanation of symbols]

 1 Vacuum Tank 2 Heater 3 Tray 4 Substrate 5 Cathode 6 High Frequency Power Supply 7 Pipe

Claims (1)

[Claims] 1. A method for forming a silicon oxide thin film, which comprises mixing water vapor or hydrogen into the reaction gas when forming the silicon oxide thin film by a plasma CVD method using a reaction gas containing an organic silicon gas.