JPH09134919A - Method of forming silicon nitride film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a nitride film stably and with good reproducibility so as to facilitate the film control, by taking the film growth process on two stages of growing a silicon nitride film initially under first condition, and subsequently, forming a film under second condition different from it. SOLUTION: In a device using ONO gate structure such as a flash memory, etc., a lower electrode is made, and then a base silicon oxide film to constitute an ONO film is made. The formation of a silicon nitride film is performed by decompressed CVD method. That is, the formation of a nucleus can be performed without depending upon the condition of the surface of a silicon oxide film, by growing a film, setting the pressure higher that that under optimum pressure conditions on a CVD device. Hereby, incubation time can be fixed regardless of the surface condition of the silicon oxide film. Subsequently, film growth is performed under the pressure condition in normal film growth process, thus this is excellent in film controllability since the film growth rate is not so high and besides the base has a nucleus.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for forming a silicon nitride film. INDUSTRIAL APPLICABILITY The present invention can be utilized, for example, as formation of a silicon nitride film that can be applied when manufacturing various electronic materials (semiconductor devices and the like).

[0002]

2. Description of the Related Art In the fields of various electronic materials and the like, various requests have been made to improve their performance. For example, in the field of semiconductor devices, for example, in a non-volatile memory (NV memory) having a stacked gate structure, it is important to reduce the thickness of an insulating film between gates (for example, polysilicon gates) in connection with cell size reduction. It has become a challenge. From the viewpoint of charge retention, which is a characteristic peculiar to NV memory, in addition to low leakage current and long life, the properties required for this insulating film are currently ONO (Oxide-Ni).
It has been found that a tride-oxide) insulating film is optimal.

However, with respect to the formation of the ONO film, there remains a problem to be solved, particularly in the step of forming the silicon nitride film on the silicon oxide film. That is, the silicon nitride film (SiN) in the ONO insulating film
Is generally formed by the LP-CVD method,
Due to its structure, the base becomes a silicon oxide film when the silicon nitride film is formed. When the silicon nitride film is formed on the silicon oxide film as described above, it is difficult to control the thickness of the silicon nitride film in the conventional technique. Especially, film thickness 10n
It is difficult to control a thin film of about m.

In the process of depositing a silicon nitride film on the surface of a silicon oxide film, growth nuclei having a high silicon content are first formed (this portion is called a transition region), and thereafter,
It has been reported that a silicon nitride film Si ₃ N ₄ having a stoichiometric composition is formed (Nobuhiko Inoue et al., “Technology for forming high quality ultra-thin silicon nitride film for megabit DRAM”).
Oki Electric Research and Development October 1993 No. 160 Vol. 6
0, 51-54). As shown in FIG. 7 as a conventional structure diagram, a growth nucleus 2 called a transition region is formed on a silicon oxide film 1, and a silicon nitride film 3 having a stoichiometrically matched composition is formed thereon. It is. for that reason,
The relationship between the film formation time of the thin film silicon nitride film and the film thickness formed is as shown in FIG. 8, and there is a so-called incubation time during which the film formation does not proceed for a certain period at the beginning of the film formation time. Occur. During this incubation time, growth nuclei with a high silicon content (the one schematically shown by reference numeral 2 in FIG. 7) are formed, but the incubation time is unstable due to a slight difference in the oxide film surface state. . This is considered to be a major cause of difficulty in controlling the thin film during the formation of the silicon nitride film.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the prior art and provides a method for forming a silicon nitride film, which enables stable and reproducible formation of a silicon nitride film, and whose thin film control is easy. It is intended to be provided.

[0006]

In order to achieve the above object, the present invention employs the following technical means. That is, in the method for forming a silicon nitride film by depositing a silicon nitride film on a silicon oxide film, the step of initially forming the silicon nitride film under the first condition, and then the silicon nitride film under the first condition A method of forming a silicon nitride film is characterized by performing a two-step film forming process including a film forming process under a second condition different from the above.

In this case, the first condition and the second condition are
A method for forming a silicon nitride film having a structure in which any one of the film forming pressure, the film forming temperature, and the reaction gas flow rate for film forming is made different and the other conditions are made the same it can.

Further, according to the present invention, in the method of forming a silicon nitride film on a silicon oxide film by a low pressure CVD apparatus, a silicon nitride film is formed under a first pressure condition (eg, normal film formation) at the initial stage of film formation. Formation of a silicon nitride film characterized by including a step of forming a silicon nitride film under a higher pressure) and a step of subsequently forming a silicon nitride film under a second pressure condition (for example, a normal film forming condition) Let's do it.

In this case, a step of forming silicon nitride under the first pressure condition (for example, higher than the normal film forming condition) at the initial stage of film formation, and then a silicon nitride film under the second pressure condition (for example, higher pressure) A step of forming a film while continuously lowering the film forming pressure to a normal film forming condition), and then a second step of forming a silicon nitride film.
The method for forming a silicon nitride film may have a step of forming a film under the pressure condition (for example, a normal film forming condition).

In order to achieve the above object, the present invention takes the following technical means. That is, in the method for forming a silicon nitride film in which the silicon nitride film is formed on the silicon oxide film by the low pressure CVD apparatus, the silicon nitride film is formed under the first temperature condition (for example, lower than the normal film forming condition) at the initial stage of the film formation. A method of forming a silicon nitride film is characterized by including a step of forming a film and a step of subsequently forming a silicon nitride film under a second temperature condition (for example, a temperature of a normal film forming condition).

In this case, a step of forming silicon nitride under the first temperature condition (for example, lower than the normal film forming condition) in the initial stage of film formation, and then a silicon nitride film under the second temperature condition (for example, lower temperature) A process of continuously increasing the temperature to a normal film forming condition (film forming temperature) and then forming a second silicon nitride film.
The method for forming a silicon nitride film can be configured to include a step of forming a film under the temperature condition (for example, a normal film forming condition temperature).

[0012]

According to the method for forming a silicon nitride film of the present invention,
The lower silicon nitride film can be formed on the silicon oxide film under the condition that the lower silicon nitride film is initially formed so that the upper silicon nitride film is initially stably formed. Under this first condition, it is considered that a portion to be a growth nucleus is formed. As described above, in the present invention, it is not always necessary to form a uniform entire surface film in the first step, and silicon nitride may be formed in a state where an upper silicon nitride layer such as a growth nucleus is favorably grown. It is a thing. For example, in the first step, film formation is performed to some extent under conditions that are compatible with the underlying silicon oxide film (for example, conditions that have good adhesion), and then in the second step, for example, the optimum in the device. The main silicon nitride film can be formed under the conditions. In the second step, a stable silicon nitride film having good reproducibility and excellent thin film controllability is formed.

As a result, it is possible to form a stable and reproducible silicon nitride film with good thin film control.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below. First, a preferred embodiment of the present invention will be described. Further, specific examples of the present invention will be described with reference to the drawings. However, as a matter of course, the present invention is not limited to the examples described below.

In the first aspect of the present invention, in the method for forming a silicon nitride film by depositing a silicon nitride film on a silicon oxide film, a step of initially forming a silicon nitride film under the first condition, In this case, a silicon nitride film is formed under a second condition different from the first condition and a second film forming process. In this case, the first condition and the second condition are With regard to, the conditions may be such that any of various physical parameters and chemical parameters are changed. In the first step, the parameters may be selected and set under the condition that the silicon nitride film such as the growth nuclei on which the upper layer silicon nitride film is grown can be formed favorably. The second step can be performed under the condition that a standard or ideal silicon nitride film can be formed. Two or more parameters may be changed, but in terms of simplicity, it is preferable to change only one parameter and keep the other conditions the same. Although the number of steps may be more than two, it is convenient to limit the number of steps to two in terms of the process.

Typically, any one of the film forming pressure, the film forming temperature, and the reaction gas flow rate for film forming is changed,
The other conditions are preferably the same as the method for forming a silicon nitride film.

A second invention is a method of forming a silicon nitride film on a silicon oxide film by a low pressure CVD apparatus, wherein silicon nitride is formed under the first pressure condition at the initial stage of film formation. There is a step and a step of subsequently forming a silicon nitride film under a second pressure condition, but the second pressure condition of the subsequent step is a normal film forming condition, that is, a standard optimum condition for the apparatus. It is possible to preferably employ a mode in which the first pressure condition prior to that is set to a higher pressure than the normal film formation condition.

In this case, the step of forming silicon nitride under the first pressure condition (for example, higher than the normal film forming condition) at the initial stage of film formation, and then the silicon nitride film under the second pressure condition (for example, higher) It is possible to adopt a configuration including a step of forming a film while continuously lowering it to an optimum normal film forming condition), and a step of forming a silicon nitride film under a second pressure condition subsequently. .

According to a third aspect of the present invention, in the method for forming a silicon nitride film on a silicon oxide film by a low pressure CVD apparatus, the silicon nitride film is formed under the first temperature condition at the initial stage of film formation. And a step of subsequently forming a silicon nitride film under the second temperature condition. However, the second temperature condition of the subsequent step is the normal film forming condition temperature,
That is, it is possible to preferably employ a mode in which the film is formed by setting the standard optimum condition in the apparatus and setting the first temperature condition prior to the optimum temperature condition lower than the normal film forming condition.

In this case, a step of forming silicon nitride under the first temperature condition (for example, lower than the normal film forming condition) at the initial stage of film formation, and then a silicon nitride film under the second temperature condition (for example, It is possible to adopt a configuration including a step of forming a film while continuously increasing it to a film forming temperature of an optimum normal film forming condition) and a step of forming a silicon nitride film under the second temperature condition subsequently. .

Example 1 This example embodies the present invention in a method for manufacturing a semiconductor device. In particular, the present invention was applied to the manufacturing of a non-volatile (NV) memory to improve the method for forming a thin silicon nitride film on the surface of a silicon oxide film.

FIG. 1 shows a basic process flow chart for forming a silicon nitride film in this embodiment. FIG. 2 schematically shows a cross section of the structure of the silicon nitride film formation.

In this embodiment, in a method for forming a silicon nitride film on a silicon oxide film by a low pressure CVD apparatus, a silicon nitride film is first formed at the initial stage of film formation.
The process I including the step I of forming the film under the pressure condition of 1 and the step II of forming the silicon nitride film under the second pressure condition that is lower than the first pressure condition are performed. Here, the second pressure condition is a film forming pressure under normal film forming conditions, that is, an optimum pressure condition for forming a silicon nitride film for an apparatus used. However, when the ideal pressure cannot be applied due to various other conditions, the standard optimum condition is set as the most appropriate condition.

More specifically, in this embodiment, the following steps (a) to (c) are performed. In this embodiment, as shown below, the present invention is embodied in manufacturing a device having an ONO structure in a gate structure.

(A) In a device using an ONO gate structure such as a flash memory cell, a lower electrode is formed and a base silicon oxide film 1 forming an ONO film is formed. The method for forming this silicon oxide film is arbitrary. For example, even in a general thermal oxidation method, LP-
SiO ₂ may be formed by CVD. With respect to the silicon oxide film formed by any means, the silicon nitride film can be satisfactorily formed in the next step.

(B) The silicon nitride film is formed by the low pressure CVD method. Here, the present invention is applied to form a silicon nitride film in the following two steps. First, the first step is performed as follows.

First, regarding the optimum pressure condition for the CVD apparatus used for implementation (or, if the ideal optimum pressure condition is not always clear, a normal thin film forming process which is a preferable standard use condition based on the specifications of the CVD apparatus) The film formation is performed under a higher pressure than the film formation pressure condition (1). In this first step, nucleation is performed on the surface of the silicon oxide film, but in this embodiment, by increasing the film formation pressure in this way, the growth rate of the silicon nitride film is increased, so that the silicon oxide film is formed. Nucleation can be performed independently of the surface condition. This makes it possible to keep the incubation time constant regardless of the surface state of the silicon oxide film.

Specific film forming conditions used in this example are shown below.

The purpose of the process of the first step is to form nuclei on the surface of the silicon oxide film. The film thickness of the silicon nitride film here is extremely thin, and the surface of the silicon oxide film is processed. The level is Particularly, in this embodiment, the high pressure conditions as described above were used in order to form the nucleation densely. This nucleation portion is schematically indicated by reference numeral 2'in FIG.

(C) Subsequently, film formation is performed under the pressure condition of the conditions of the normal thin film formation process. The normal film forming conditions are the optimum conditions or standard conditions for the apparatus used as described above. The film formation in this step is
Generally, under the following conditions, the film forming rate is not so high, and since the nucleation is performed on the base, the thin film controllability is excellent. The film forming conditions are shown below.

By the above process, the silicon oxide film 1
When forming the thin silicon nitride film 3 on the surface, in the case of the present embodiment, the silicon nitride film with a film thickness of 8 nm can be formed with good controllability and good reproducibility. Also, since the transition region is eliminated, the electrical characteristics of the film are improved.

As described above, according to this embodiment, the thin silicon nitride film can be formed on the silicon oxide surface with good controllability. Therefore, by using this, the ONO film of the next-generation flash memory or the like can be thinned, and the practical advantage is great. In addition, in the present example, the above-described steps were continuously performed in the same chamber also for the purpose of preventing the formation of a natural oxide film.

Example 2 This example embodies the present invention in a method for manufacturing a semiconductor device. In particular, the present invention is applied to a method for forming a thin film silicon nitride film on the surface of a silicon oxide film and improved by applying the present invention to the manufacturing of a nonvolatile memory.

FIG. 3 shows a basic process flow chart for forming a silicon nitride film in this embodiment. For the cross section showing the formation of the silicon nitride film, refer to FIG. 2 schematically showing the cross section.

In this embodiment, in the method of forming a silicon nitride film on a silicon oxide film by a low pressure CVD apparatus, the first pressure condition (from the normal film forming condition And a step of forming a silicon nitride film under a second pressure condition that is lower than the first pressure condition (film forming pressure of normal film forming condition). Take steps.

In particular, in this embodiment, the step IA of forming silicon nitride under the first pressure condition (higher pressure than the normal film forming condition) at the initial stage of film formation, and then the silicon nitride film at the first pressure Step I ′ for forming a film while continuously lowering it to a second pressure condition (film forming pressure of normal film forming condition) lower than the condition
A step including A and a step IIA of subsequently forming a silicon nitride film under the second pressure condition (the film forming pressure of the normal film forming condition) is taken.

More specifically, in this embodiment, the following steps (a) to (d) are performed.

(A) ONO such as a flash memory cell
In a device using a gate structure, a lower electrode is formed and a base silicon oxide film of an ONO film is formed. This silicon oxide film can be formed by LP using a general thermal oxidation method.
-It may be formed by CVD. With respect to the silicon oxide film formed by any means, the silicon nitride film can be satisfactorily formed in the next step, as in the above example.

(B) The silicon nitride film is formed by the low pressure CVD method. In the present invention, the silicon nitride film formation is performed while continuously changing. First, step IA is performed as follows.

First, the film formation is performed at a higher film formation pressure than in the normal thin film formation process. As a result, the growth rate of the silicon nitride film can be increased by increasing the film forming pressure to form nuclei on the surface of the silicon oxide film, and the nucleation can be performed without depending on the state of the surface of the silicon oxide film. This makes it possible to keep the incubation time constant regardless of the surface state of the silicon oxide film.

Specific film forming conditions are shown below.

The purpose of this first step process is to form nuclei on the surface of the silicon oxide film. The film thickness of the silicon nitride film here is extremely thin, and the surface of the silicon oxide film is processed. The level is In this example, as in Example 1, the high pressure conditions as described above were used in order to form the nucleation densely.

(C) Then, step I'A is performed. Here, film formation is performed while continuously lowering the film formation pressure. The film forming rate here gradually decreases.

(D) Finally, step IIA is performed. In this step, a silicon nitride film is usually formed by a thin film forming process. In this step IIA, the film forming rate is low, and since the nuclei are formed on the base, the thin film controllability is excellent. The final film forming conditions are shown below.

With the above process, in the case of the thin silicon nitride film formed on the surface of the silicon oxide film, in the case of the present embodiment, the film thickness of 8 nm can be formed with good controllability and good reproducibility. Also, because the transition area disappears,
The electrical properties of the film are improved.

This embodiment also has the same effect as that of the first embodiment. In this example, as in Example 1, the above steps were continuously performed in the same chamber.

Example 3 This example embodies the present invention in a method for manufacturing a semiconductor device. Especially when manufacturing non-volatile memory,
The present invention is applied to improve the method of forming a thin silicon nitride film on the surface of a silicon oxide film.

FIG. 4 shows a basic process flow chart for forming a silicon nitride film in this embodiment. For the cross section showing the formation of the silicon nitride film, refer to FIG. 2 schematically showing the cross section.

In this embodiment, in the method for forming a silicon nitride film on a silicon oxide film by a low pressure CVD apparatus, the silicon nitride film is formed under the first temperature condition (from the normal film forming condition) at the initial stage of film formation. And a step IIB of forming a silicon nitride film under a second temperature condition (normal film forming condition temperature) higher than the first temperature condition. Take steps. Here, the temperature of the normal film forming condition is the optimum temperature condition for forming the silicon nitride film for the device used, but when the ideal temperature is unknown or cannot be applied due to various other conditions, The standard optimum conditions are set as the most appropriate conditions.

More specifically, in this embodiment, the following steps (a) to (c) are performed.

(A) ONO such as a flash memory cell
In a device using a gate structure, a lower electrode is formed and a base silicon oxide film of an ONO film is formed. This silicon oxide film can be formed by LP using a general thermal oxidation method.
-It may be formed by CVD. With respect to the silicon oxide film formed by any means, the silicon nitride film can be satisfactorily formed in the next step, as in the above example.

(B) The silicon nitride film is formed by the low pressure CVD method. In the present invention, the silicon nitride film is formed in the following two steps. First, the first step is performed as follows.

First, film formation is performed at a lower film formation temperature than in the normal thin film formation process. When the film formation temperature is lowered in the initial stage of film formation, nucleation is more densely formed on the surface of the silicon oxide film than in the case of the normal process (IEEE TR
AN SECTION ON DEVICES. VOL.
41, NO. 10, OCTOBER 1994, p17
47-1752).

Specific film forming conditions are shown below.

The film forming temperature in the process of the first step is low and the film forming rate is very slow. Therefore, dense nucleation can be performed on the surface of the silicon oxide film here.
The film thickness of the silicon nitride film here is extremely thin,
It is assumed that the surface of the silicon oxide film has been processed. Nucleation is the main purpose of this process, and even if the growth rate is slow, it can be said that it is advantageous for good nucleation.

(C) Subsequently, step IIB is involved. Here, the film is usually formed by a thin film forming process. In this second step, fine nucleation is already performed on the base in the first step, and therefore the thin film controllability is excellent. The film forming conditions are shown below.

By the above process, in the case of forming the thin silicon nitride film on the surface of the silicon oxide film, in the case of this embodiment, the film thickness of 8 nm can be formed with good controllability. Also, since the transition region is eliminated, the electrical characteristics of the film are improved.

According to this embodiment, the thin silicon nitride film can be formed on the silicon oxide surface with good controllability, and the ONO film for the next-generation flash memory or the like can be thinned. Further, since the transition region is eliminated, there is an advantage that the electric characteristics of the film are improved.

Embodiment 4 This embodiment embodies the present invention in a method for manufacturing a semiconductor device. In particular, the present invention is applied to a method for forming a thin film silicon nitride film on the surface of a silicon oxide film and improved by applying the present invention to the manufacturing of a nonvolatile memory.

FIG. 5 shows a basic process flow chart for forming a silicon nitride film in this embodiment. For the cross section showing the formation of the silicon nitride film, refer to FIG. 2 schematically showing the cross section.

In this embodiment, in a method for forming a silicon nitride film on a silicon oxide film by a low pressure CVD apparatus, in the initial stage of film formation of silicon nitride, the first temperature condition (from the normal film forming condition A step of forming a silicon nitride film under a second temperature condition (a temperature of a normal film forming condition) higher than the first temperature condition. To take.

In particular, in this embodiment, a step IC for forming silicon nitride under the first temperature condition (a temperature lower than the normal film forming condition) at the initial stage of film formation, and then a silicon nitride film at the first temperature are formed. Step I ′ for forming a film while continuously increasing the temperature to a second temperature condition (a film forming temperature of a normal film forming condition) higher than the condition
A process including C and a process IIC of subsequently forming a silicon nitride film under the second temperature condition (normal film forming condition temperature) is performed. Here, the temperature of the normal film forming condition is the optimum temperature condition for the silicon nitride film for the apparatus to be used, but it is the best when the ideal temperature is unknown or cannot be applied due to various other conditions. Use standard optimum conditions that are appropriate conditions.

More specifically, in this embodiment, the following steps (a) to (d) are performed.

(A) ONO such as a flash memory cell
In a device using a gate structure, a lower electrode is formed and a base silicon oxide film of an ONO film is formed. This silicon oxide film can be formed by LP using a general thermal oxidation method.
-It may be formed by CVD. With respect to the silicon oxide film formed by any means, the silicon nitride film can be satisfactorily formed in the next step, as in the above example.

(B) The silicon nitride film is formed by the low pressure CVD method. In the present invention, the silicon nitride film formation temperature is continuously changed. First, the process IC is performed as follows.

First, a film is formed at a film forming pressure lower than that in the normal thin film forming process. The purpose of this is to reduce the film formation temperature to form fine nuclei on the surface of the silicon oxide film, and to form nuclei independently of the state of the surface of the silicon oxide film.

Specific film forming conditions are shown below.

The purpose of this initial process is to form nuclei on the surface of the silicon oxide film. The film thickness of the silicon nitride film formed here is extremely thin, and the surface of the silicon oxide film is treated. . Preferably 0.3 nm in diameter
The growth nucleus is formed to some extent.

(C) Subsequently, the film formation is carried out while continuously raising the film formation temperature, and finally the film is formed by a normal thin film formation process. In the film formation here, the film formation rate gradually increases, and since nucleation has already been performed on the base,
Excellent thin film controllability. The final conditions are shown below.

By the above process, in the case of the present embodiment, it is possible to form a thin silicon nitride film on the surface of the silicon oxide film with a good controllability of 8 nm. In addition, since the transition region is eliminated, the electric characteristics of the film are improved.

Example 5 This example embodies the present invention in a method for manufacturing a semiconductor device. In particular, the present invention is applied to a method for forming a thin film silicon nitride film on the surface of a silicon oxide film and improved by applying the present invention to the manufacturing of a nonvolatile memory.

In the present embodiment, in the method of forming a silicon nitride film by depositing a silicon nitride film on a silicon oxide film, a step of initially forming a silicon nitride film under the first condition, followed by a silicon nitride film A two-stage film formation process including a film formation process under a second condition different from the first condition is performed, and the first condition and the second condition are the reaction gas flow rate for film formation. The conditions are different, and other conditions such as film forming pressure and film forming temperature are the same. In this example, the process ID and process IID shown in FIG. 6 were performed. Other specific configurations were the same as those in the above-mentioned respective embodiments. Also in this embodiment, the same operational effects as those in the above-mentioned respective embodiments were obtained.

[0073]

As described above, according to the method for forming a silicon nitride film of the present invention, the silicon nitride film can be stably formed, and the film thickness thereof can be easily and well controlled.

[Brief description of the drawings]

FIG. 1 is a diagram showing a basic process flow of forming a silicon nitride film in Example 1.

FIG. 2 is a sectional view schematically showing a structure for forming a silicon nitride film in an example.

FIG. 3 is a diagram showing a basic process flow for forming a silicon nitride film in Example 2.

FIG. 4 is a diagram showing a basic process flow for forming a silicon nitride film in Example 3.

FIG. 5 is a diagram showing a basic process flow for forming a silicon nitride film in Example 4.

FIG. 6 is a diagram showing a basic process flow for forming a silicon nitride film in Example 5.

FIG. 7 is a diagram showing a conventional technique.

FIG. 8 is a diagram showing a problem of the related art.

[Explanation of symbols]

 1 Silicon Oxide Film 2 Growth Nucleus (Transition Region) 2'Growth Nucleation Part 3 Silicon Nitride Film

Claims (6)

[Claims] 1. A method for forming a silicon nitride film by depositing a silicon nitride film on a silicon oxide film, comprising the steps of: first forming a silicon nitride film under a first condition; A method of forming a silicon nitride film, comprising a two-step film forming process including a film forming process under a second condition different from the first condition. 2. The first condition and the second condition are film forming pressure,
The formation of a silicon nitride film according to claim 1, wherein any one of the film forming temperature and the reaction gas flow rate for film forming is made different, and the other conditions are made the same. Method. 3. A method of forming a silicon nitride film on a silicon oxide film by a low pressure CVD apparatus, the method comprising: forming silicon nitride under a first pressure condition at an initial stage of film formation; And a step of forming a silicon nitride film under a second pressure condition which is lower than the first pressure condition. 4. A step of forming silicon nitride under a first pressure condition at the initial stage of film formation, and subsequently, continuously forming a silicon nitride film up to a second pressure condition lower than the first pressure condition. The method for forming a silicon nitride film according to claim 3, further comprising: a step of forming the silicon nitride film while lowering the film thickness; and a step of subsequently forming a silicon nitride film under the second pressure condition. 5. A method of forming a silicon nitride film on a silicon oxide film by a low pressure CVD apparatus, comprising: a step of forming silicon nitride at a first temperature condition at an initial stage of film formation; And a step of forming a silicon nitride film under a second temperature condition that is higher than the first temperature condition. 6. A step of forming a silicon nitride film under a first temperature condition in the initial stage of film formation, and subsequently, a silicon nitride film is continuously formed until a second temperature condition which is higher than the first temperature condition. The method for forming a silicon nitride film according to claim 5, further comprising: a step of forming the silicon nitride film while raising the temperature, and a step of subsequently forming the silicon nitride film under the second temperature condition.