JPH11102906A - Silicon oxide film depositing method and oxide film depositing equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a protective oxide film having substantially the same high quality as a thermal oxide film as a protective film in the deposition of a gate oxide film formed of a silicon oxide film. SOLUTION: In the case of depositing a silicon oxide film on a silicon substrate surface using this method, first, a natural oxide film which is produced in cleaning a substrate is removed by acid washing and a protective oxide film is again formed on the substrate surface with an oxidizing agent. Next, when the substrate is introduced into a heat-treating region 12 in an oxide film-depositing equipment 10 for depositing a silicon oxide film on the substrate surface with the protective oxide film, the heat-treating region 12 is blocked from the outside with nitride flow curtains 32 and 34, a chlorine contained nitride gas is flowed into the heat-treating region 12 and the substrate is introduced into the heat-treating region 12, while the substrate is exposed to the chlorine contained nitride gas atmosphere. Subsequently, a water vapor contained oxidative gas is introduced into the heat-treating region 12, and a silicon thermal oxide film is deposited on the substrate. Next, the substrate is annealed by introducing the chlorine contained nitride gas into the heat-treating region 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for forming a silicon oxide film, and more particularly, to a silicon oxide film having a high dielectric breakdown voltage and a high TDDB evaluation formed as a gate oxide film. The present invention relates to a method for forming an oxide film and an apparatus for forming an oxide film most suitable for performing the method.

[0002]

2. Description of the Related Art A MOS type silicon semiconductor device comprises a metal electrode such as polysilicon, an insulating film made of a silicon oxide film provided below as a gate oxide film, and a silicon semiconductor substrate under the insulating film. ing. The transistor characteristics and reliability of a MOS type silicon semiconductor device greatly depend on the quality of a silicon oxide film as a gate oxide film. Therefore, a silicon oxide film is required to have a high breakdown voltage and a long-term reliability. I have.

A silicon oxide film as a gate oxide film is
Using a thermal oxidation furnace, the silicon substrate is thermally oxidized in an oxidizing atmosphere in the furnace, so that the silicon substrate is formed on the substrate surface. More specifically, after cleaning the silicon semiconductor substrate by RCA cleaning or the like to remove fine particles and metal impurities on the substrate surface, the oxide film generated in these cleaning processes is acid-cleaned with a hydrofluoric acid aqueous solution or the like. To expose the substrate surface. Next, the exposed substrate surface is again treated with an oxidant for 0.6.
A protective oxide film having a thickness of about 1.2 nm is formed. Subsequently, the substrate having the protective oxide film is introduced into an oxide film forming apparatus, and a gate oxide film is formed by thermal oxidation. What is RCA cleaning?
This is a washing method in which the substrate is washed with a mixed solution of an aqueous ammonia solution and an aqueous hydrogen peroxide solution, and then washed with a mixed solution of an aqueous hydrochloric acid solution and an aqueous hydrogen peroxide solution.

On the other hand, with the thinning of the gate oxide film corresponding to the miniaturization and high integration of silicon semiconductor devices, the silicon oxide film forming apparatus (oxidizing furnace) is shifting from a horizontal furnace to a vertical furnace. is there. This is because the vertical furnace can suppress the entrapment in the atmosphere at the time of entering the furnace and can reduce the formation of a silicon natural oxide film having a low film quality due to the entrainment in the atmosphere, as compared with the conventional horizontal furnace. However, even in a vertical furnace, a natural oxide film of about 2 nm is formed due to entrainment in the atmosphere. Therefore, the vicinity of the furnace port is replaced with nitrogen gas, and the formation of the natural oxide film is suppressed as much as possible by using a non-oxidizing atmosphere. May be added.

Here, a configuration of a conventional vertical oxide film forming apparatus will be described with reference to FIG. As shown in FIG. 5, a conventional vertical oxide film forming apparatus 40 has an electric heater 20 around it, a heat treatment chamber 12 for housing and heat-treating a substrate, and a preliminary chamber provided below the heat treatment chamber. 14 is provided. The heat treatment chamber 12 and the preliminary chamber 14 are partitioned by a partition wall 17 and communicate with each other by an opening 16 provided in the partition wall 17. The opening 16 is opened and closed by a shutter 18 that can be freely opened and closed. The heat treatment chamber 12 is a chamber for performing a heat treatment on a substrate to form an oxide film, and the preliminary chamber 14 is a chamber for temporarily storing a substrate to be introduced into the heat treatment chamber 12. A reaction gas inlet 22 for introducing a reaction gas is provided at the head of the heat treatment chamber 12, and an exhaust port connected to a suction device for exhausting gas from the heat treatment chamber 12 is provided at the bottom of the heat treatment chamber 12. 24 are provided. In the upper part of the heat treatment chamber 12, the introduced reaction gas is dispersed,
Dispersion plate 26 is provided so as to flow uniformly inside. The spare chamber 14 is also provided with a gas inlet 28 for introducing an inert gas or the like as necessary, and a gas outlet 30 for exhausting the inside of the spare chamber 14.

After filling the heat treatment chamber 12 with an inert gas or a nitrogen gas, a silicon semiconductor substrate is introduced, the atmosphere is switched to an oxidizing atmosphere, and the silicon semiconductor substrate is heat-treated to form a gate oxide film. Conventionally, when a gate oxide film is formed, a method of introducing high-purity steam into a heat treatment chamber 12 maintained at a high temperature to thermally oxidize a silicon semiconductor substrate has been used. In this oxidation method, as a high-purity water vapor to be used, a method is generally used in which hydrogen gas and oxygen gas are mixed at a high temperature and water vapor generated by burning is used, and this is used for hydrogen combustion oxidation or pyrogenic oxidation. That. The pyrogenic oxidation method can form a gate oxide film having higher electrical reliability than oxidation using high-purity oxygen, that is, dry oxidation.

Next, a conventional method for forming a silicon oxide film as a gate oxide film using the above-described vertical oxide film forming apparatus will be described with reference to FIGS. FIG.
(A), (b) and FIGS. 7 (c), (d) are schematic cross-sectional views showing the inside of an oxide film forming apparatus for each step when an oxide film is formed by a conventional method. is there. First, after forming an element isolation region in a silicon semiconductor substrate and performing RCA cleaning,
The natural oxide film on the substrate surface is removed with hydrofluoric acid to expose the silicon substrate surface, and then a protective oxide film is formed on the silicon substrate surface with an oxidizing agent to prepare a substrate on which a silicon thermal oxide film is formed. . Next, as shown in FIG. 6A, the prepared substrate W is introduced into the preliminary chamber 12, and the gas in the chamber is exhausted as necessary. On the other hand, nitrogen gas is introduced into the heat treatment chamber 12,
The temperature is raised to and maintained at 800 ° C. while partially evacuating. Next, as shown in FIG. 6B, the shutter 18 is opened, and the substrate W is transferred from the preliminary chamber 14 to the heat treatment chamber 12.
Next, as shown in FIG. 7C, while the shutter 18 is closed and a temperature of 800 ° C. is maintained, a mixed gas of water vapor and oxygen gas is introduced into the heat treatment chamber 12, and the substrate W is exhausted partially. Heat treatment is performed. Subsequently, as shown in FIG.
While maintaining the temperature at 800 ° C., a mixed gas of hydrochloric acid vapor and nitrogen gas is introduced into the heat treatment chamber 12, and the substrate W is subjected to hydrochloric acid annealing while partially exhausting the gas.

As described above, in the conventional oxide film forming apparatus,
When the substrate W is transferred into the heat treatment chamber 12, gas flows into the heat treatment chamber 12.
During the transfer, the heat treatment chamber 12 is in a nitrogen gas atmosphere and the halogen gas such as chlorine can be introduced basically when the shutter is closed. Otherwise, dangerous halogen gas flows out through the spare chamber to the outside.

[0009]

Incidentally, the quality of the gate oxide film is becoming more and more important because the gate oxide film is made thinner in accordance with the miniaturization and higher integration of silicon semiconductor devices. However, since the gate oxide film is composed of the protective oxide film and the silicon thermal oxide film formed thereon, if the thickness of the gate oxide film becomes as thin as 8 nm or less, the silicon substrate becomes thin. The quality and thickness of the protective oxide film before thermal oxidation and the method of forming the protective oxide film directly affect the quality of the gate oxide film. For example, a protective oxide film formed by a conventional gate oxide film forming method has the following problems. (1) It is difficult to form a uniform oxide film on the entire substrate surface with good reproducibility by controlling the concentration of the oxidizing agent. (2) Impurities in the oxidant chemical are easily taken into the oxide film. (3) The film density of the protective oxide film is 2.06 to 2.23 g / cm.
³ , which is smaller than the thermal oxide film. Therefore, it is difficult to improve the quality of the gate oxide film, and the reliability of the gate oxide film is affected.

Therefore, Japanese Patent Laid-Open No. 5-29293 discloses that
When forming a protective oxide film by cleaning the surface of the substrate with an oxidizing treatment liquid, the substrate is shielded from the air and a cleaning process is performed.
A method for forming a uniform protective oxide film has been proposed. JP-A-6-291112 discloses a method in which a natural (protective) oxide film is formed on a substrate surface with a hydrogen peroxide solution, and then the substrate is introduced into a furnace at 550 ° C. in a non-oxidizing atmosphere and heat-treated for a predetermined time. It is proposed that the quality of a protective oxide film can be improved to the same level as that of a thermal oxide film by applying the method to a substrate. However, Japanese Unexamined Patent Publication No.
The method disclosed in Japanese Patent No. 29293 can form a protective oxide film having a uniform film quality, but it is difficult to suppress the incorporation of impurities into the film. Further, Japanese Patent Application Laid-Open No. 6-29111
According to the method disclosed in Japanese Patent Application Laid-Open No. H08-158, it is difficult to suppress the incorporation of impurities into the film, and the furnace is subjected to an annealing process.
Since it is necessary to raise the temperature from 0 ° C. to, for example, 900 ° C. for the thermal oxidation treatment, considering the annealing time, one film forming step requires a considerably long time, and thus the productivity is low.

Ohmi et al. Have proposed a method of forming a protective oxide film by thermal oxidation. The method of Omi et al. Is a method in which a silicon semiconductor substrate that has been subjected to hydrogen treatment and terminated with hydrogen is subjected to dry oxidation at 300 ° C. in which terminal hydrogen is stably present, and an oxide film generated thereby is used as a protective film ( Tadahiro Omi: Ultra Clean ULSI technology (Baifukan),
p. 21). However, according to this method, the temperature of the heat treatment furnace must be increased from a low temperature at the time of forming the protective oxide film to a high temperature of the gate oxidation treatment. It is necessary to lower the temperature of the heat treatment furnace to a low temperature. Therefore, the time required for one oxide film formation step is 1.5 to 2 times longer than that of a normal method, and the components (quartz tube, heat insulating material, substrate There is a problem that the load on the dummy and the like is large.

As described above, in the conventional method of forming a silicon oxide film, it has been difficult to economically improve the quality of the protective oxide film to a satisfactory level. Accordingly, an object of the present invention is to provide a method for forming a silicon oxide film, wherein a protective oxide film having the same quality as a thermal oxide film is formed as a protective film when forming a gate oxide film made of a silicon oxide film. And an oxide film forming apparatus necessary for implementing the method.

[0013]

In order to achieve the above object, a method for forming a silicon oxide film according to the present invention is a method for forming a silicon oxide film on a substrate surface of a silicon semiconductor substrate, A natural oxide film generated when the substrate is washed is removed by acid cleaning, and then a protective oxide film is formed on the substrate surface again with an oxidizing agent. When the substrate is introduced into the heat treatment region to form a film, the heat treatment region is shut off from the outside by the inert gas airflow curtain, a non-oxidizing gas containing a halogen gas flows into the heat treatment region, and a non-oxidizing gas containing a halogen gas is introduced. A substrate introduction step of exposing the substrate to the heat treatment area while exposing the substrate to an oxidizing gas atmosphere, and then introducing an oxidizing gas containing water vapor into the heat treatment area to form a silicon thermal oxide film on the substrate And that step, then, it is characterized by a step of introducing a non-oxidizing gas containing a halogen gas into the heat treatment area subjected to a heat treatment to the substrate.

In the step of forming a protective oxide film, the type of the oxidizing agent is not limited as long as a protective oxide film can be formed on the substrate surface. Preferably, any one of ozone water, hydrogen peroxide solution, and sulfuric acid hydrogen peroxide solution is used. Is used as an oxidizing agent. Thereby, a high-quality protective oxide film can be formed. In the substrate introduction step, as a non-oxidizing gas containing a halogen gas,
A chlorine-containing non-oxidizing gas is used, and chlorine is contained in the chlorine-containing non-oxidizing gas to be introduced at a ratio of 0.02 to 10% by volume, more preferably 0.02 to 5% by volume. To have been.

The following effects can be expected by exposing the substrate to an inert gas atmosphere containing chlorine in the process of feeding the substrate into the heat treatment chamber. (1) The impurities in the protective oxide film are removed by chlorine. (2) Silicon dangling bond is
Termination is performed according to the following equation. Si ・ + Cl → SiC
l (3) Dehydration reaction proceeding according to the formula SiOH + HCl → SiCl + H ₂ O, and H ₂ generated by the dehydration reaction
O reoxidizes unreacted portions of silicon (Si-Si bonds) and unstable Si-O bond portions having weak bonds,
Stabilize. With the above effects, the quality of the protective oxide film is improved.
The reliability of the gate oxide film including the protective oxide film and the thermal oxide film thereon is improved.

An oxide film forming apparatus for suitably carrying out the method of the present invention comprises a heat treatment chamber for performing a heat treatment on a substrate to form a thermal oxide film, and an opening which is partitioned by a partition from the heat treatment chamber and which can be opened and closed by a shutter. A chamber that communicates with the heat treatment chamber via a heat treatment chamber, and a preliminary chamber that temporarily stores the substrate to be carried into the heat treatment chamber. A gas inlet and a gas outlet are provided, and when an inert gas flows out from the gas outlet while flowing in from the gas inlet, an inert gas airflow curtain is formed along the partition wall, and the substrate is transferred from the preliminary chamber to the heat treatment chamber. The heat treatment chamber is shut off from the preliminary chamber by an inert gas airflow curtain.

The inert gas is, for example, nitrogen, and it is not always necessary to use an expensive gas such as argon.
Further, the flow rate of the inert gas for forming the inert gas airflow curtain varies depending on the size of the heat treatment chamber, and is determined by experiments or the like.

In the conventional vertical heat treatment furnace, an inert gas atmosphere containing chlorine, which has the effects (1) to (3) described above, cannot be realized in the substrate introduction step. That is, when the substrate is introduced from the preliminary chamber into the thermal processing chamber, the chlorine gas flows from the thermal processing chamber into the preliminary chamber because the shutter that separates the thermal processing chamber and the preliminary chamber is open. Then, the interior of the room may be corroded by chlorine and moisture in the preparatory room, and in particular, the robot system for transporting the substrate may be corroded and destroyed, and there is also a danger that chlorine gas may escape from the apparatus. On the other hand, in the oxide film forming apparatus of the present invention, when the substrate is introduced from the preliminary chamber into the thermal processing chamber, it is possible to prevent the chlorine gas in the thermal processing chamber from flowing into the preliminary chamber by the inert gas flow curtain. Furthermore, entrainment of gas in the pre-chamber can be prevented by the inert gas flow curtain. Therefore, by using the oxide film forming apparatus of the present invention, the substrate is exposed to a non-oxidizing gas atmosphere containing a halogen gas, and the above-described effects (1) to (3) are obtained, and the substrate is formed using an oxidizing agent. The converted protective oxide film can be converted to a high-quality protective oxide film.

[0019]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Embodiment 1 This embodiment is an embodiment of an oxide film forming apparatus according to the present invention, and FIG. 1A is a schematic diagram showing the configuration of the oxide film forming apparatus of this embodiment. 1B is a longitudinal sectional view, and FIG.
FIG. 2 is a cross-sectional view of the oxide film forming apparatus of FIG. The oxide film forming apparatus 10 of the present embodiment is a vertical oxide film forming apparatus, and includes a heat treatment chamber 12 in an upper part and a preliminary chamber 14 in a lower part. The heat treatment chamber 12 is a chamber for performing a heat treatment on the substrate to form a thermal oxide film, and the preliminary chamber 14
This is a chamber for temporarily storing a substrate to be introduced into the heat treatment chamber 12. The heat treatment chamber 12 and the preliminary chamber 14 are partitioned by a partition wall 17 having an opening 16 for communicating with each other.
The opening 16 is opened by a shutter 18 which can be freely opened and closed.
It is closed, and the heat treatment chamber 12 and the preparatory chamber 14 are communicated and shut off. The shutter 18 is configured as a sliding door, and the door slides to both sides or one side to open an opening 16 between the heat treatment chamber 12 and the preliminary chamber 14, and slides toward the opening 16 to close. Preparatory room 12 and heat treatment room 1
The movement of the substrate between the two is performed via the opening 16 using an elevator (not shown).

In order to heat the heat treatment chamber 12, a heater 20 made of an electric heater is provided around the heat treatment chamber 12,
The reaction gas inlet 22 for introducing the reaction gas is connected to the heat treatment chamber 12.
On the head and also to exhaust gas from the heat treatment chamber,
An exhaust port 24 connected to a suction device is provided at the bottom of the heat treatment chamber 12, respectively. Further, a dispersion plate 26 for dispersing the introduced reaction gas and uniformly flowing the inside of the heat treatment chamber 12 is provided in an upper portion inside the heat treatment chamber 12. Further, the preliminary chamber 14 is provided with a gas inlet 28 for introducing an inert gas or the like as needed, and a gas outlet 30 for exhausting the interior of the preliminary chamber 14.

Further, in this embodiment, a nitrogen inlet 32 for inflowing nitrogen gas and a nitrogen exhaust port 34 for sucking nitrogen gas are provided in the heat treatment chamber wall near the upper side of the partition wall 17. By exhausting the gas from the nitrogen exhaust port 34 while allowing the gas to flow through the nitrogen inlet 32, a nitrogen gas curtain can be formed on the opening 30. Shutter 1
Due to the nitrogen current curtain formed on the opening 30 on
When transferring the substrate from the preliminary chamber 14 to the thermal processing chamber 12, the communication between the preliminary chamber 14 and the thermal processing chamber 12 is fluidly shut off so that the gas in the thermal processing chamber 12 does not flow into the preliminary chamber 14. be able to.

Embodiment 2 This embodiment is an embodiment in which an oxide film is formed by the oxide film forming method according to the present invention using the oxide film forming apparatus of Embodiment 1. FIGS. 2A, 2B, and 3 are schematic longitudinal sectional views showing the inside of the oxide film forming apparatus for each step of the present embodiment. (1) First, the substrate which has been subjected to trench element isolation by a normal method is sequentially washed with ammonia-hydrogen peroxide, sulfuric acid-hydrogen peroxide and hydrochloric acid-hydrogen peroxide to remove impurities and fine particles on the substrate surface. The natural oxide film formed on the substrate is removed by washing with a hydrofluoric acid aqueous solution to expose the silicon semiconductor substrate surface. Next, the substrate is immersed in a hydrogen peroxide solution maintained at a predetermined temperature for about one minute to form a 0.7 nm-thick chemical oxide film as a protective oxide film as shown in FIG. Thus, using the above-described oxide film forming apparatus 10,
This means that the substrate W on which the silicon oxide film is to be formed in the element formation region has been prepared.

(2) Next, the substrate W is fed into the preliminary chamber 14 of the oxide film forming apparatus 10 (see FIG. 2A). At this time, the heat treatment chamber 12 is heated by the heater 2 while partially evacuating while introducing nitrogen gas into the heat treatment chamber 12 from the reaction gas inlet 22.
By heating at 0, the temperature of the soaking area of the heat treatment chamber 12 is raised to 800 ° C. and maintained. (3) Subsequently, as shown in FIG.
Nitrogen gas is flowed at a flow rate of 2 to 20 liters / minute and is discharged from the nitrogen exhaust port 34 to form a nitrogen gas flow curtain on the opening 16. Further, a mixed gas of a nitrogen gas at a flow rate of 10 liter / min and an HCl gas at a flow rate of 10 ml / min is introduced into the heat treatment chamber 12 from the reaction gas inlet 22. Since the flow rate of the nitrogen gas for forming the nitrogen gas flow curtain varies depending on the dimensions of the heat treatment chamber 12, it is determined in advance by experiments or the like. Next, the shutter 18 is opened, the substrate is transferred from the preliminary chamber 14 to the heat treatment chamber 12 by an elevator (not shown), and heat treatment is performed in the heat treatment chamber 12 (FIG. 2).
(B)). By heating to 800 ° C. in a nitrogen atmosphere containing chlorine while passing through a nitrogen gas flow curtain, the chemical oxide film on the substrate is annealed with HCl, impurities in the chemical oxide film are removed, and dangling bonds are terminated. Then, the chemical oxide film is reoxidized by the dehydration reaction and H ₂ O generated by the dehydration reaction. It is assumed that the temperature of the substrate rises to about 300 ° C. while the substrate passes through the nitrogen gas flow curtain. As described above, by heating the substrate W in the nitrogen atmosphere containing chlorine, the quality of the chemical oxide film formed as the protective oxide film can be improved.

(4) After introducing the substrate W into the heat treatment chamber 12, the shutter 18 is closed and the flow of nitrogen from the nitrogen inlet 32 is stopped to eliminate the nitrogen gas curtain.
In place of the mixed gas of the nitrogen gas and the HCl gas, a nitrogen gas is introduced from the reaction gas inlet 22 for a while. Next, the reaction gas is switched to a pyrogenic gas until the total thickness of the thermal oxide film and the protective oxide film becomes 4 nm.
A thermal oxide film is formed. The formed thermal oxide film and protective oxide film function as a gate oxide film. Subsequently, a mixed gas of nitrogen gas and HCl gas is introduced instead of the pyrogenic gas, and HCl annealing is performed in a nitrogen atmosphere containing chlorine for 30 minutes.

When the gate oxide film formed by the method of Embodiment 2 was evaluated, the breakdown voltage, TDDB, and TDDB were compared with those of the conventional film forming method in which chlorine gas was not supplied during the introduction of the substrate into the heat treatment chamber 12. (Time Dependent Dielectric Br
eakdown) has been significantly improved in reliability.

[0026]

According to the method of the present invention, when a substrate is introduced into a heat treatment region for forming an oxide film on a substrate surface having a protective oxide film, the heat treatment region is shut off from the outside by a nitrogen gas curtain, Since the substrate is introduced into the heat treatment region while exposing the substrate to a non-oxidizing gas atmosphere containing a halogen gas, the film quality of the protective oxide film can be improved. Therefore, the film quality of the protective oxide film that forms a part of the gate oxide film together with the thermal oxide film is improved, so that a highly reliable gate oxide film can be realized. The oxide film forming apparatus according to the present invention,
A film forming apparatus suitable for performing the method of the present invention is realized.

[Brief description of the drawings]

FIG. 1A is a schematic longitudinal sectional view showing a configuration of an oxide film forming apparatus of an embodiment of the present invention, and FIG. 1B is a view of the oxide film forming apparatus of FIG. It is a cross-sectional view in II.

FIGS. 2A and 2B are schematic vertical cross-sectional views each showing a state in an oxide film forming apparatus for each process of the present embodiment.

FIG. 3 is a schematic vertical cross-sectional view showing the inside of the oxide film forming apparatus in the process of the present embodiment following FIG. 2 (b).

FIG. 4 is a schematic sectional view showing a section of a substrate to be introduced into an oxide film forming apparatus.

FIG. 5 is a schematic sectional view showing a configuration of a conventional vertical oxide film forming apparatus.

6 (a) and 6 (b) are schematic cross-sectional views of the inside of an oxide film forming apparatus for each step when an oxide film is formed by a conventional method. .

7 (c) and (d) respectively show FIG.
It is a typical apparatus sectional view showing a situation in an oxide film formation device for each process at the time of forming an oxide film by a conventional method following (b).

[Explanation of symbols]

Reference numeral 10: the oxide film forming apparatus of Embodiment 1; 12, a heat treatment chamber; 14, a preliminary chamber; 16, an opening; 17, a partition;
18 Shutter, 20 Heater, 22 Reactant gas inlet, 24 Exhaust port, 26 Dispersion plate, 28 Gas inlet, 30 Gas outlet, 32 Nitrogen inlet, 34
... Nitrogen exhaust port, 40... Conventional oxide film forming apparatus.

Claims (5)

[Claims] 1. A method of forming a silicon oxide film on a substrate surface of a silicon semiconductor substrate, wherein a natural oxide film generated when the substrate is cleaned is removed by acid cleaning, and then a protective oxide film is again formed by an oxidizing agent. Forming a silicon oxide film on the surface of the substrate having the protective oxide film, and introducing the substrate into the heat treatment region in order to form a silicon oxide film on the substrate surface having the protective oxide film. A non-oxidizing gas containing a halogen gas is flowed into the heat treatment region while the substrate is exposed to a non-oxidizing gas atmosphere containing a halogen gas, and then the substrate is introduced into the heat treatment region. Introducing a gas into the heat treatment region to form a silicon thermal oxide film on the substrate; and then introducing a non-oxidizing gas containing a halogen gas into the heat treatment region to heat the substrate. Method of forming a silicon oxide film, characterized in that a step of applying a sense. 2. The silicon oxide film according to claim 1, wherein in the protective oxide film forming step, any one of ozone water, hydrogen peroxide solution and sulfuric acid hydrogen peroxide solution is used as an oxidizing agent. Membrane method. 3. The method for forming a silicon oxide film according to claim 1, wherein in the substrate introducing step, a non-oxidizing gas containing chlorine is used as the non-oxidizing gas containing a halogen gas. . 4. The method according to claim 3, wherein the chlorine-containing non-oxidizing gas introduced in the substrate introducing step contains chlorine in a ratio of 0.02 to 10% by volume. A method for forming a silicon oxide film. 5. A heat treatment chamber for subjecting a substrate to heat treatment to form a thermal oxide film, and a chamber separated from the heat treatment chamber by a partition and communicating with the heat treatment chamber through an opening which can be opened and closed by a shutter. An oxide film forming apparatus comprising: a preliminary chamber for temporarily storing a substrate to be carried into a chamber; a gas inlet and a gas outlet for an inert gas near a partition wall of the heat treatment chamber; Inert gas flows out from the gas outlet while flowing in from the inlet, forms an inert gas airflow curtain along the partition walls, and when the substrate is transferred from the preliminary chamber to the heat treatment chamber, the heat treatment chamber is shut off from the preliminary chamber by the inert gas airflow curtain An oxide film forming apparatus characterized in that: