JPH05279049A - Production of synthetic quartz glass - Google Patents

Abstract

PURPOSE:To obtain quartz glass having excellent heat resistance by heat treating a porous body comprising vitreous SiO2 fine particles in an ammonia- contg. atmosphere and sintering in a specified temp. range at a specified temp. rising rate. CONSTITUTION:A gas phase synthesized quartz glass base body which is a porous body having about 12m<2>/g specific surface area is heat treated at about 1000 deg.C for 0.5-4 hours in an atmosphere containing about 80vol.% ammonia to obtain a porous body (nitrogen-doped base body) having the max. 5.2wt.% N. Then this body is sintered under reduced pressure (about 10Torr) at <=100 deg.C/h temp. rising rate in >1200C temp. range. In this process, before the pores of the base material are closed, decomposed gas (e.g. N2, H2) can be released to the outside of the base body so that the base material doped with high concn. N can easily be made dense. Thereby, the obtd. synthetic quartz glass has 0.15wt.% N content and >1100 deg.C strain point (max. temp. of heat resistance).

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention is used as a high-purity heat-resistant material such as a process tube, a liner tube, and a wafer boat used in a semiconductor manufacturing process, and a material for a quartz glass substrate of a thin film transistor using polycrystalline silicon. The present invention relates to a method for manufacturing synthetic quartz glass having enhanced heat resistance.

[0002]

2. Description of the Related Art In a semiconductor manufacturing process such as LSI which requires the use of high-purity materials and jigs, a furnace core tube (process tube, liner tube) or wafer boat used for heat treatment of a silicon wafer, or other High-purity quartz glass products are mainly used as jigs and tools. Quartz glass, which has a high degree of purity, is relatively easy to obtain, and is characterized by not being altered or deformed at a heat treatment temperature of about 1000 ° C. However, for example, in the step of heat-treating LSI in a temperature range of about 1200 ° C., viscous deformation of quartz glass poses a problem and therefore cannot be used, and application of a high-purity silicon carbide heat-resistant material has been attempted. .

However, in recent years, as the degree of integration of LSIs has increased, contamination of silicon wafers due to trace impurities contained in a furnace core tube using a silicon carbide-based heat resistant material and a wafer support member has become a problem. Along with the demand for higher purity of heat-resistant materials, the demand for higher heat resistance of quartz glass is also increasing.

In response to the above requirements for quartz glass,
In JP-A-63-236722, for the purpose of imparting high heat resistance and preventing the invasion of impurities, an impurity element as a nucleus for inducing cristobalite formation is introduced into the surface layer of the quartz glass material, and quartz is used. A method of coating the surface of a glass material with a cristobalite layer has been proposed. Quartz glass material obtained by this method, because the heat resistance of the cristobalite crystal layer formed on the surface is higher than that of quartz glass, the heat resistance of the entire material improves, but when the period of use is long, The region of the cristobalite layer expands and becomes brittle, which causes problems such as generation of dust and cracks in the material, and causes problems other than thermal deformation.

On the other hand, as a method for essentially improving the heat resistance of a quartz glass material, a nitrogen atom (N) is introduced (doped) into the material to form a stronger Si-N bond than the Si-O bond. The method is known. This method uses VAD (Vapor-p
hase Axial Deposition) method, OVD (Outside Vapor Dep)
osition) method, MCVD (Modified Chemical Vapor Depos
ition) method, etc. to form a glassy porous body of silicon dioxide (SiO ₂ ) by gas phase synthesis, place this porous body in a heating furnace and sinter in an atmosphere containing ammonia (NH ₃ ). This is a method of obtaining a dense nitrogen-doped quartz glass material by performing heat treatment in a temperature range (1200 ° C. or less) where densification does not occur, and then heating to a high temperature and sintering. However, with this method, the strain point that serves as an index of the heat resistant temperature (the temperature at which the viscosity coefficient of the material becomes 10 ^14.5 poise, hereinafter also called the heat resistant temperature)
(N) necessary to raise the temperature to over 1100 ° C
However, when sintering is attempted, a glass foaming phenomenon occurs due to the generation of nitrogen gas (N ₂ ) or hydrogen gas (H ₂ ) which is a decomposition product of NH ₃ during sintering, and the nitrogen gas is highly concentrated in the quartz glass. It is not possible to obtain a dense quartz glass doped with.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to solve the problem of glass foaming in obtaining the above nitrogen-doped quartz glass material and to contain a high concentration of N in the glass.
An object of the present invention is to provide a method for producing synthetic quartz glass, which has better heat resistance than conventional ones.

[0007]

[Problems to be Solved by the Invention] A method for producing ultra-high-purity synthetic quartz glass by vapor phase synthesis includes MCVD method and OVD method.
Method, which is a method applying a technique generally called soot method such as VAD method, has been developed and put into practical use for the purpose of manufacturing an optical fiber. A porous quartz glass body (hereinafter referred to as a quartz glass base material) obtained by vapor phase synthesis is a hydrolysis reaction in the oxidizing atmosphere such as a flame by vaporizing silicon chloride such as silicon tetrachloride [the following (1) formula ] Or an oxidation reaction [Equation (2) below] is carried out to deposit the glassy SiO ₂ fine particles that have been produced, and this is baked and solidified.

SiCl ₄ + 2H ₂ O → SiO ₂ + 4HCl (1) SiCl ₄ + O ₂ → SiO ₂ + 2Cl ₂ (2) The vapor-phase synthetic quartz glass base material produced in this manner is fine particles. Since it is separated from halogen-based gases such as hydrogen chloride (HCl) and chlorine (Cl ₂ ) during the deposition step, the metal halide produced by the corrosion of the metal container for synthesis by the halogen-based gas is quartz. It is possible to prevent impurities from being mixed into the glass base material, and it is possible to obtain an extremely high-purity vitreous porous body of SiO ₂ .

This quartz glass base material is a porous material having a specific surface area of about 12 m ² / g, which allows gas permeation and permeation, and various gas molecules are formed on the surface of the SiO ₂ fine particles constituting the base material. Can be fixed.

It is known that N is effective as a component (dopant) to be doped in order to improve the heat resistance of quartz glass. N ₂ and ammonia (NH ₃ ) are mainly used to dope N into the quartz glass base material, but NH ₃ is more easily fixed to the base material than N ₂ and N is doped at a high concentration. Suitable for NH ₃ reacts with the quartz glass base material as shown in equation (3) below and is fixed.

Si-OH + NH ₃ → Si-NH ₂ + H ₂ O (3) The sintering start temperature of this nitrogen-doped base material is about 100 ° C. higher than that of the undoped synthetic quartz glass base material. It is known to be high, about 1300 ° C.

However, in the process of sintering the nitrogen-doped base material, about half of the fixed N is desorbed as N ₂ or H ₂ , and this N ₂ or H ₂ is formed in the closed pores formed in the sintering process. Staying in place, causing thermal expansion to cause the foaming phenomenon,
Prevents densification of the base material.

The inventors of the present invention consider that the release of N from the nitrogen-doped base material and the accompanying generation of N ₂ and H ₂ occur particularly strongly at 1200 ° C. or higher. If the temperature rising rate from the maximum temperature to the maximum sintering temperature required for densification is set to 100 ° C / hr or less, the above-mentioned decomposition gas is discharged outside the base metal before the base metal pores are closed. It has been found that it becomes possible to escape, and the base material doped with a high concentration of N can be easily densified.

The gist of the present invention based on the above findings is that "a porous body composed of glassy SiO ₂ fine particles obtained by vapor phase synthesis is heat-treated in an atmosphere containing ammonia, and then heated to a temperature of 1200 ° C. or higher. Temperature rising rate in the region is 100 ℃ / hr
Sintered as follows, the N content in the quartz glass is 0.15 wt%
The above is the method for producing synthetic quartz glass ".

The synthetic quartz glass mentioned above means the MCVD method,
It is a quartz glass obtained by vapor phase synthesis by applying the OVD method, the VAD method, or the like.

[0016]

The various conditions specified in the present invention will be described below.

In the method for producing synthetic quartz glass according to the present invention, the N content in the glass is set to 0.15 wt% or more is 1100.
This is to obtain a quartz glass having a strain point of higher than 0 ° C, that is, a heat resistant temperature.

FIG. 1 is a graph showing the relationship between the N concentration in N-doped synthetic quartz glass and the heat resistance temperature (strain point). The heat resistance temperature (strain point) of synthetic quartz glass is N in the glass.
It increases linearly with increasing concentration. From this figure, 11
In order to obtain quartz glass having a heat resistant temperature of more than 00 ° C, it is sufficient to dope with about 0.15wt% of N, and in order to obtain a heat resistant temperature of more than 1200 ° C, doping of about 1.5wt% of N is necessary. I know there is.

To dope N into synthetic quartz glass,
The quartz glass base material obtained by vapor phase synthesis may be heat-treated in an atmosphere containing NH ₃ .

For vapor phase synthesis, as described above, MCVD is performed.
Method, OVD method, VAD method or the like can be applied.
Further, the heat treatment may be carried out by a known method, and the amount of N-doping into the quartz glass base material can be controlled in a wide range by adjusting the heat treatment time, as shown in Examples described later.

A nitrogen-doped quartz glass base material in which N is fixed on the surface of glassy SiO ₂ particles by acting NH ₃ at 1300 ° C.
If heated to the above temperature range, sintering proceeds and dense quartz glass is obtained.

However, as shown in FIG. 2, when the nitrogen-doped quartz glass base material is heated to 1290 ° C. or higher, it expands and does not become densified. This is because gas (N ₂ or H ₂ O) is generated during the heating process due to reactions such as the following equations (4) to (6), while at least the surface layer of the base metal undergoes sintering to form pores. Since it is in a closed state, it is presumed that the above-mentioned foaming phenomenon occurs.

Si ₃ N ₄ + 3SiO ₂ → 6SiO + 2N ₂ ...
(4)

[0024]

[Equation 1]

[0025]

[Equation 2]

In this case, if the rate of temperature rise in the high temperature range is made sufficiently slow, the rate of closed porosity at which pores are clogged due to sintering becomes very slow, so that (4)- It is possible to secure the time required for the gas generated by the reaction of equation (6) to diffuse outside the base material. In order to complete the gas diffusion earlier than the blockage of pores due to the densification of the base material, it is necessary to set the heating rate to 100 ° C / hr or less. Further, although it is shown that the densification is progressing in the region where the linear expansion velocity is negative in FIG.
At about 1200 ° C, the rate of closed porosity of the nitrogen-doped base material, that is, the rate of densification, is extremely low. Therefore, the rate of temperature increase during sintering is set to 100 ° C / hr or lower in the temperature range of 1200 ° C or higher.

According to the above method, a quartz glass base material containing a high concentration of nitrogen can be easily densified by heat treatment in an atmosphere containing NH _3, and a synthetic quartz glass containing a high concentration of nitrogen can be obtained. Obtainable.

[0028]

Example: The bulk density obtained by vapor phase synthesis by the VAD method is about 0.3 g.
/ cm ³ , specific surface area of about 12 m ² / g, average particle size of about 0.2 μm, silica glass base material in NH ₃ 80 vol% (balance: N ₂ ) atmosphere at 1000 ℃, treatment time 0.5 ~ After heat treatment for 4 hours, as shown in Fig. 3, the maximum N content was 5.2wt%.
A porous material (nitrogen-doped base material) containing was obtained.

Of these porous materials, the N content is 1.9 wt.
% Porous body under reduced pressure (about 10 ^-3 Torr) and maximum temperature of 1100 ~
Whether or not densification was performed was examined by changing the sintering temperature in the range of 1600 ° C.
The densification was determined by visually observing the presence or absence of translucency of the 5 mm thick sample. If it is densified, it has no translucency, and if it is not densified, it has translucency.

The results are shown in Table 1. In the table, ◯ indicates that the material is densified, and x indicates that the material is not densified. As is clear from this result, the maximum temperature in the sintering process is 1100.
In the case of ℃ or 1200 ℃, it did not densify even if kept for 24 hours, but the maximum sintering temperature was 1300 ℃, 1450 ℃ or 1600 ℃.
When set to ℃ (temperature rising rate in the temperature range of 1200 ℃ or higher: 50 ℃
/ hr), a dense glass was obtained.

The maximum sintering temperature is fixed at 1450 ° C.,
When the heating rate in the temperature range of 1200 ° C or higher is 150 ° C / hr or 120 ° C / hr, it is not densified and 100 ° C / hr
Alternatively, a dense quartz glass was obtained at 50 ° C / hr.

Of the quartz glasses obtained under the above respective conditions, the strain points of dense quartz glass are all about 1150 ° C.,
It has higher heat resistance than conventional quartz glass. In addition, impurity metal elements (Al, Fe, Ti, Ca, Mg, Cu, P, B, Na,
The concentrations of K and Li) were 1 ppm or less, and the purity was extremely high.

Among the above-mentioned porous materials, the maximum temperature of a porous material (nitrogen-doped base material) containing 3.1 wt% of N was also increased under reduced pressure.
1500 ℃, heating rate in the temperature range of 1200 ℃ or more 50 ℃ / h
When sintered as r, dense quartz glass was obtained,
The strain point was over 1200 ° C.

[0034]

[Table 1]

[0035]

According to the method of the present invention, a synthetic quartz glass containing a high concentration of N can be obtained. This synthetic quartz glass has higher heat resistance than conventional ones, and is suitable as a high-purity heat-resistant material such as process tubes and wafer boats used in semiconductor manufacturing processes, or as a material for quartz glass substrates of thin film transistors using polycrystalline silicon. Is.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the N concentration in synthetic quartz glass containing N and the heat resistance temperature (strain point).

FIG. 2 is a graph showing the relationship between the heat treatment temperature and the coefficient of linear expansion of a N-doped quartz glass base material.

FIG. 3 is a graph showing the relationship between the treatment time and the N concentration in the base material when the silica glass base material is heat-treated in an atmosphere containing NH ₃ .

Claims (1)

[Claims] 1. A porous body composed of glassy SiO ₂ fine particles obtained by vapor phase synthesis is heat-treated in an atmosphere containing ammonia, and then the temperature rising rate in a temperature range of 1200 ° C. or higher is 100 ° C./hr. A method for producing synthetic quartz glass, which comprises sintering as described below to make the N content in the quartz glass 0.15 wt% or more.