JP2000208645A - Forming method for silicon group dielectric film and manufacture of nonvolatile semiconductor storage device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve elapsed dielectric breakdown characteristics by performing, after a pre-purge process where a nitrogen group gas is blasted for a specified period under depressurized environment, a specific chemical vapor-phase growth with the surface of a semiconductor layer for laminating a silicon nitride layer, and silicon oxide layer on the surface of a semiconductor layer. SOLUTION: A wafer is housed in a chamber after it is heated, and then the chamber is depressurized for nitrogen gas blast for a specified period. Then a lower-layer SiO2 film 3a is deposited by chemical vapor-phase growth. Then the chamber is opened to the atmosphere, the wafer is carried out and annealed for constant and tight film quality. Thus the lower-layer SiO2 film 3a of an ONO film 3 is formed. Then an Si3N3 film 3b is formed by depressurized CVD method, and further an upper-layer SiO2 film 3c is laminated by CVD method. Thus, the ONO film 3 with improved electrical resistance is formed, improving dielectric breakdown characteristics with respect to time passage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dielectric breakdown with time (T
The present invention relates to a method for forming a silicon-based dielectric film capable of improving DDB (Characteristic DDB) characteristics, and a method for manufacturing a nonvolatile semiconductor memory device using the same.

[0002]

2. Description of the Related Art SiO _{2 is} used as an insulating film for a nonvolatile semiconductor memory or an insulating film for forming a storage capacitor of a DRAM.
/ Si ₃ N ₄ / SiO ₂ three-layer insulating film (ONO
Membrane) is widely used. The ONO film has a structure in which a Si ₃ N ₄ film having a high dielectric constant is sandwiched between SiO ₂ films having high stress leak resistance or high dielectric breakdown characteristics.
The ONO film has a time-zero dielectric breakdown (TZDB; time-z) as compared with a SiO ₂ film having a thickness electrically equivalent to that (thickness converted in consideration of a difference in dielectric constant).
ero dielectric breakdown)
Characteristics and time-dependent dielectric breakdown (TDDB)
end dielectric breakdo
wn) It is known that all the characteristics are significantly improved.

[0003] The improvement of the dielectric breakdown characteristics by replacing the single-layer SiO ₂ film with a laminated film (ONO film) is achieved by using an ONO film and a Si film.
This is explained by the difference in the conduction mechanism of the O ₂ film. In the floating gate type flash memory shown in FIG. 7, the behavior of carriers will be described using an ONO film formed as a capacitor insulating film (inter-gate insulating film) 3 between the floating gate 1 and the control gate 2 as an example. .

In the floating type flash memory shown in FIG. 7, an n-type source / drain region 5 is formed on a surface of a p-type impurity region (p-well) 4 provided on a silicon substrate.
Are formed at predetermined intervals. Floating gate 1 made of, for example, polysilicon is formed via gate insulating film 6 so that a channel region is formed between source / drain regions 5. In the upper layer, the lower layer S
A capacitor insulating film 3 composed of an iO ₂ film 3a, a Si ₃ N ₄ film 3b, and an upper SiO ₂ film 3c is formed. A control gate 2 made of, for example, polysilicon or a laminated film of polysilicon and refractory metal silicide is formed above the capacitor insulating film 3. further,
A dielectric film 7 is formed so as to cover the floating gate 1, the control gate 2, and the like.

When charges are accumulated in the floating gate 1, the lower SiO ₂ film 3 a
Then, electrons are injected by Fowler-Nordheim tunneling. The electrons injected into the lower SiO ₂ film 3a flow into the Si ₃ N ₄ film 3b. On the other hand, holes are injected from the control gate 2 into the Si ₃ N ₄ film 3b by direct tunneling through the thin upper SiO ₂ film 3c. Si ₃ N ₄ holes injected into the film 3b is a Poole-Frenkel current Si ₃
It flows through the N ₄ film 3b, and the lower SiO ₂ film 3a / Si
_The lower SiO ₂ film 3 is formed at and near the interface of the ₃ N ₄ film 3b.
The electrons recombine with the electrons injected from the a side and are converted into electron currents.

As described above, in the ONO film, charge carriers are switched according to the position in the thickness direction in the film, so that high dielectric breakdown characteristics can be obtained. Compared to an ONO film with a SiO ₂ single-layer film having an equivalent reduced film thickness, the electrons injected into the film are accelerated so that the film thickness of SiO ₂ is small, so that the electrons are unlikely to have high energy. In addition, the electrons having high energy are recombined with holes injected from the opposite side in the Si ₃ N ₄ film 3b and are easily annihilated. Therefore, the ONO film is suitably used as a capacitor insulating film (inter-gate insulating film) of a floating gate type flash memory requiring high dielectric breakdown characteristics, or a storage capacity of a DRAM.

Conventionally, the production of the ONO film as described above has been performed by a combination of thermal oxidation of silicon and nitridation of the SiO ₂ surface. Specifically, first, dry thermal oxidation or pyrogenic oxidation is performed at a high temperature on the polysilicon surface to form a lower SiO ₂ film. Next, NO, N
_By heating in a nitrogen-based gas atmosphere such as ₂ O or NO ₂ , the surface of the lower SiO ₂ film is nitrided to form a Si ₃ N ₄ layer. Thereafter, an ONO film was formed by oxidizing the surface of the Si ₃ N ₄ layer.

In recent years, a lower layer and an upper layer SiO ₂ of an ONO film have been used.
Instead of a conventional thermal oxidation method, a CVD (chemi)
SiO ₂ film (hereinafter referred to as HTO film to be distinguished from a thermal oxide film) formed by a cal vapor deposition method;
xide film. ) (For example, a method for manufacturing a nonvolatile semiconductor memory device described in Japanese Patent Application Laid-Open No. 6-061499).

Since the HTO film is excellent in film thickness controllability, for example, when it is used as a capacitor insulating film of a flash memory, variation in capacitance between the control gate and the floating gate is suppressed. Therefore, H
The TO film is effective in preventing variation in the threshold voltage of the semiconductor memory device. Further, the HTO film has an advantage that the film can be formed at a lower temperature than the thermal oxide film. The HTO film is formed, for example, by a plasma CVD method.
Although it is possible to form a film at about ° C, in the case of a thermal oxide film, heating at about 900 to 1000 ° C is required. Due to heating during film formation, impurities may be re-diffused from the impurity diffusion layer formed on the base substrate. To reduce damage to the base or other semiconductor layers, H can be formed at a low temperature.
It is preferable to use a TO film.

A conventional method for forming an HTO film used as a lower layer or an upper SiO ₂ film of an ONO film will be described with reference to a flowchart of FIG. In advance,
A certain temperature (for example, 800
(° C). The chamber is opened to the atmosphere when loading and unloading wafers, but the range of temperature fluctuation in the chamber is about 780 to 820 ° C. As shown in FIG. 9 (A), a wafer is placed in a chamber and is
Flow ₂ O gas (prepurge). The flow rate of the N ₂ O gas is, for example, 200 sccm. If hydrogen is excessively introduced into the SiO ₂ film, the resistance to stress leak current decreases, but the incorporation of hydrogen can be prevented by pre-purging with N ₂ O gas.

Next, as shown in FIG. 9B, an HTO film is deposited by chemical vapor deposition (CVD). While the temperature in the chamber is maintained at 780 to 820 ° C., the pressure in the chamber is reduced using a vacuum pump, and the pressure in the chamber is set to, for example, 30 to 40 Pa. The gas introduced into the chamber during the film formation and the flow rate thereof are, for example, SiH ₂ Cl ₂ / N
₂ O = 100/200 sccm. After forming the HTO film, the inside of the chamber is opened to the atmosphere as shown in FIG. The inside of the chamber is returned from the high vacuum state to the atmospheric pressure in about 3 minutes. Thereafter, the wafer on which the HTO film has been formed is carried out of the chamber. Further, for example, at 900 ° C.,
The film quality is made uniform or densified by annealing for 0 minutes. Thereby, the film quality such as electrical resistance is improved. Through the above steps, the lower or upper layer S of the ONO film is formed.
An iO ₂ film is formed.

[0012]

However, according to the above-mentioned conventional method for forming an HTO film, the obtained SiO ₂
There is a problem that the dielectric breakdown characteristics of the film are inferior to the thermal oxide film. FIG. 1 shows the breakdown characteristics with time of the HTO film (c) and the thermal oxide film (d) formed according to the conventional process shown in the flowchart of FIG. FIG. 1 is a Weibull plot showing the correlation between the logarithm of the time until dielectric breakdown of the SiO ₂ film and the cumulative failure rate F. Here, the thermal oxide film (d) in FIG. 1 is a thermal oxide film formed on the silicon surface by pyrogenic oxidation, and is annealed at 900 ° C. for 10 minutes after the film formation like the HTO film. .

As an index of the long-term reliability of the SiO ₂ film, there is a time-dependent dielectric breakdown (TDDB) characteristic. The aging dielectric breakdown is
This is a phenomenon in which the silicon oxide film does not break down at the moment when current stress or voltage stress is applied, but dielectric breakdown occurs in the silicon oxide film after a certain time has elapsed after the stress is applied. In general, the time-dependent dielectric breakdown includes an initial breakdown region where dielectric breakdown occurs sporadically,
After a long period of time, all the samples in the population are divided into genuine fracture areas (wear areas) that rapidly break. The genuine dielectric breakdown characteristic is a dielectric breakdown characteristic under the condition that a sample having an initial failure or a random failure is removed, and is an index of the genuine breakdown voltage depending on the film quality of the silicon oxide film.

As shown in FIG. 1, the HTO film (c) has a shorter time to dielectric breakdown than the thermal oxide film (d), and is inferior in electric breakdown voltage. HT used as a capacitor insulating film in a nonvolatile semiconductor memory
When dielectric breakdown or leak current occurs in the O film, normal charge accumulation is not performed, and the storage retention characteristics of the nonvolatile semiconductor memory deteriorate. The present invention has been made in view of the above-described problems, and accordingly, the present invention provides a method of forming a silicon-based dielectric film capable of improving the time-dependent dielectric breakdown (TDDB) characteristics, and a nonvolatile method using the same. It is an object to provide a method for manufacturing a semiconductor memory device.

[0015]

In order to achieve the above object, a method of forming a silicon-based dielectric film according to the present invention comprises: a pre-purge step of blowing nitrogen-based gas under reduced pressure for a predetermined time on a surface of a semiconductor layer; Performing a first chemical vapor deposition (CVD) process on the surface of the semiconductor layer to form a first silicon oxide layer; and performing a second chemical vapor deposition process on the first silicon oxide layer to form a nitride film. Forming a silicon layer; and performing a third chemical vapor deposition on the silicon nitride layer to form a second
Forming a silicon oxide layer. In the method of forming a silicon-based dielectric film according to the present invention, preferably, the pre-purge step is performed in a state where the pre-purge step is heated to a temperature substantially equal to a temperature at which the first chemical vapor deposition is performed. The method for forming a silicon-based dielectric film of the present invention comprises:
Preferably, after the first silicon oxide layer is formed, a heat treatment is performed at a temperature higher than a temperature at which the first chemical vapor deposition is performed, to homogenize the film quality of the first silicon oxide layer. It is characterized by the following.

In the method of forming a silicon-based dielectric film according to the present invention, preferably, the time for performing the pre-purge step is such that the time-dependent dielectric breakdown characteristic of the first silicon oxide layer does not perform the pre-purge step. It is characterized in that it is set within a range that is improved in comparison. More preferably, the time for performing the pre-purge step is about 10 minutes. The method for forming a silicon-based dielectric film according to the present invention is preferably characterized in that the nitrogen-based gas is N ₂ O gas.

The method of forming a silicon-based dielectric film according to the present invention preferably includes a post-purge step of blowing nitrogen-based gas under reduced pressure for a predetermined time on the surface of the first silicon oxide layer. I do. In the method of forming a silicon-based dielectric film according to the present invention, preferably, the time for performing the post-purge step is such that the time-dependent dielectric breakdown characteristic of the first silicon oxide layer is shorter than when the pre-purge step is not performed. It is characterized in that it is set in a range that improves. More preferably, the nitrogen-based gas is N ₂ O gas.

The method of forming a silicon-based dielectric film according to the present invention preferably includes a post-purge step of blowing nitrogen-based gas under reduced pressure for a predetermined time on the surface of the second silicon oxide layer. I do. In the method of forming a silicon-based dielectric film according to the present invention, preferably, the time for performing the post-purge step is such that the time-dependent dielectric breakdown characteristics of the second silicon oxide layer is shorter than when the pre-purge step is not performed. It is characterized in that it is set in a range that improves.

Thus, the H film formed by the CVD method is formed.
Since the temporal breakdown characteristics of the TO film are improved, the electric breakdown voltage characteristics of the ONO film having the HTO film are improved. According to the method for forming a silicon-based dielectric film of the present invention, a dielectric film having a structure in which a Si ₃ N ₄ film having a high dielectric constant is sandwiched between HTO films having excellent film thickness controllability and high electric breakdown voltage characteristics Is formed.

Further, in order to achieve the above object, in the method of forming a silicon-based dielectric film according to the present invention, a first chemical vapor deposition (CVD) is performed on a semiconductor layer surface to form a first silicon oxide layer. A step of blowing a nitrogen-based gas under reduced pressure on the surface of the first silicon oxide layer for a predetermined time; and performing a second chemical vapor deposition on the first silicon oxide layer to form a silicon nitride layer. Forming a;
Performing a third chemical vapor deposition on the silicon nitride layer to form a second
Forming a silicon oxide layer. In the method of forming a silicon-based dielectric film according to the present invention, preferably, the post-purge step is performed in a state where the post-purge step is heated to a temperature at which the first chemical vapor deposition is performed. Preferably, in the method of forming a silicon-based dielectric film according to the present invention, after forming the first silicon oxide layer, a heat treatment is performed at a temperature higher than a temperature at which the first chemical vapor deposition is performed. The step of homogenizing the film quality of the silicon oxide layer.

In the method of forming a silicon-based dielectric film according to the present invention, preferably, the time during which the post-purge step is performed is such that the dielectric breakdown characteristics of the first silicon oxide layer with time indicate that the post-purge step is not performed. It is characterized in that it is set within a range that is improved as compared with the case. More preferably, the time for performing the post-purge step is about 10 minutes. In the method for forming a silicon-based dielectric film according to the present invention, preferably, the nitrogen-based gas is an N ₂ O gas.

The method for forming a silicon-based dielectric film according to the present invention preferably comprises a post-purge step of blowing nitrogen-based gas under reduced pressure for a predetermined time on the surface of the second silicon oxide layer. I do. In the method of forming a silicon-based dielectric film according to the present invention, preferably, the time for performing the post-purge step is such that the time-dependent dielectric breakdown characteristics of the second silicon oxide layer is shorter than when the pre-purge step is not performed. It is characterized in that it is set in a range that improves. Preferably, the nitrogen-based gas is N ₂ O gas.

Thus, the H film formed by the CVD method
Since the temporal breakdown characteristics of the TO film can be improved, the electric breakdown voltage characteristics of the ONO film on which the HTO film is formed can be improved. According to the method for forming a silicon-based dielectric film of the present invention, a dielectric film having a structure in which a Si ₃ N ₄ film having a high dielectric constant is sandwiched between HTO films having excellent film thickness controllability and high electric breakdown voltage characteristics Is formed.

In order to achieve the above object, a method of manufacturing a nonvolatile semiconductor memory device according to the present invention comprises a step of forming a gate insulating film on a semiconductor substrate, and a step of forming a first conductive film on the gate insulating film as a floating gate. Performing a first chemical vapor deposition (CVD) process on the surface of the semiconductor layer, a pre-purge process of blowing a nitrogen-based gas under reduced pressure on the surface of the first conductor layer for a predetermined time, Forming a first silicon oxide layer, performing a second chemical vapor deposition on the first silicon oxide layer to form a silicon nitride layer, and forming a third chemical Performing a vapor phase growth to form a second silicon oxide layer; forming a second conductor layer serving as a control gate on the second silicon oxide layer; Layer, the first Processing the capacitor insulating film comprising the first and second silicon oxide layers and the silicon nitride layer, the first conductor layer and the gate insulating film into a predetermined gate electrode pattern, and forming a source on the surface of the semiconductor substrate. Forming a region and a drain region, and using the lower part of the gate electrode as a channel formation region.

In the method of manufacturing a nonvolatile semiconductor memory device according to the present invention, preferably, the pre-purge step is performed in a state where the pre-purge step is heated to a temperature substantially equal to a temperature at which the first chemical vapor deposition is performed. I do. In the method for manufacturing a nonvolatile semiconductor memory device according to the present invention, preferably, the time for performing the pre-purge step is such that the time-dependent dielectric breakdown characteristic of the first silicon oxide layer is:
It is characterized in that it is set within a range that is improved as compared with the case where the pre-purge step is not performed. Preferably, the time for performing the pre-purge step is about 10 minutes. In the method for manufacturing a nonvolatile semiconductor memory device according to the present invention, preferably, the nitrogen-based gas is N ₂ O gas.

As a result, a capacitor insulating film having high dielectric breakdown characteristics can be formed in the nonvolatile semiconductor memory device. Therefore, the storage retention characteristics of the nonvolatile semiconductor memory device can be improved.

[0027]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the method for forming a silicon-based dielectric film and the method for manufacturing a nonvolatile semiconductor memory device according to the present invention will be described below with reference to the drawings. (Embodiment 1) A method for forming a silicon-based dielectric film according to the present embodiment will be described below with reference to the flowchart of FIG. FIG. 1A shows the time-dependent dielectric breakdown characteristics of the HTO film obtained by the method for forming a silicon-based dielectric film of the present embodiment.

The inside of the chamber of the CVD apparatus is previously heated to a certain temperature (for example, 800 ° C.). Loading wafers
The chamber is opened to the atmosphere at the time of unloading, but the range of temperature fluctuation in the chamber is about 780 to 820 ° C. FIG.
As shown in (A), after the wafer is placed in the chamber, the pressure in the chamber is reduced to, for example, 30 by using a vacuum pump.
４０40 Pa. 10 when the pressure inside the chamber is reduced
N ₂ O gas is flowed for a minute (prepurge). The flow rate of the N ₂ O gas is, for example, 200 sccm.

The data (a) in FIG. 1 is for the case where the N ₂ O gas flow rate is 200 sccm, but the N ₂ O gas flow rate can be changed as appropriate, for example, 200 to 400 sccm. In addition, as long as the dielectric breakdown characteristics of the HTO film to be formed are improved, the film forming gas may be a nitrogen-based gas other than the N ₂ O gas,
For example, it can be changed to NO or the like. According to the conventional film forming method, the pre-purge is performed under the atmospheric pressure to prevent the incorporation of hydrogen which causes a stress leak current. On the other hand, in the case of the film forming method of the present embodiment, a longer time pre-purge is performed under reduced pressure. Thereby, the electric breakdown voltage after film formation is improved.

Next, as shown in FIG. 2B, the temperature (780-820 ° C.) and the pressure (30-40
Chemical vapor deposition (CVD) while maintaining Pa) constant
To deposit an HTO film. The gas introduced into the chamber during the film formation and the flow rate thereof are, for example, SiH ₂ Cl ₂
/ N ₂ O = 100/200 sccm. SiH ₂ C
The l ₂ flow rate can be appropriately changed within a range of about 100 to 200 sccm, and the N ₂ O flow rate can be appropriately changed within a range of about 200 to 400 sccm. After the HTO film is formed, the inside of the chamber is opened to the atmosphere as shown in FIG. The inside of the chamber is returned from the high vacuum state to the atmospheric pressure in about 3 minutes. Then HT
The wafer having the O film formed thereon is carried out of the chamber. Further, for example, annealing is performed at 900 ° C. for 10 minutes,
Uniform or dense film quality.

By the above steps, the lower layer SiO of the ONO film is formed.
_Two films are formed. Subsequently, a Si ₃ N ₄ film is formed by, for example, a low pressure CVD method, and further, an upper layer H is formed by the CVD method.
By stacking the TO films, an ONO film with improved electric breakdown voltage can be formed.

The HTO film (FIG. 1 (a)) formed according to the above-mentioned method has a greatly improved dielectric breakdown characteristic over time as compared with the conventional HTO film (FIG. 1 (c)). The film forming method of the present embodiment is easy to carry out, merely adding a N ₂ O pre-purge step under reduced pressure to the conventional film forming method. According to the film forming method of the present embodiment, the dielectric breakdown characteristic is equal to or higher than that of a thermal oxide film formed by pyrogenic oxidation (FIG. 1D), which is considered to be excellent in electric breakdown voltage. Was obtained.

(Embodiment 2) A method for forming a silicon-based dielectric film according to this embodiment will be described below with reference to the flowchart of FIG. FIG. 4E shows the time-dependent dielectric breakdown characteristics of the HTO film obtained by the method for forming a silicon-based dielectric film of the present embodiment.

The inside of the chamber of the CVD apparatus is previously heated to a certain temperature (for example, 800 ° C.). Loading wafers
The chamber is opened to the atmosphere at the time of unloading, but the range of temperature fluctuation in the chamber is about 780 to 820 ° C. FIG.
As shown in (A), after the wafer is placed in the chamber, the pressure in the chamber is reduced to, for example, 30 by using a vacuum pump.
４０40 Pa. 10 when the pressure inside the chamber is reduced
N ₂ O gas is flowed for a minute (prepurge). The flow rate of the N ₂ O gas is, for example, 200 sccm.

The data in FIG. 4E shows that the N ₂ O gas flow rate is 2
Although the case is 00 sccm, the flow rate of the N ₂ O gas can be changed as appropriate, for example, 200 to 400 sccm. In addition, as long as the dielectric breakdown characteristics of the HTO film to be formed are improved, the film forming gas may be a nitrogen-based gas other than the N ₂ O gas,
For example, it can be changed to NO or the like. According to the conventional film forming method, pre-purge is performed under the atmospheric pressure to prevent the incorporation of hydrogen which causes a stress leak current. On the other hand, in the case of the film forming method of the present embodiment, a longer time pre-purge is performed under reduced pressure. Thereby, the electric breakdown voltage after film formation is improved.

Next, as shown in FIG. 3B, the temperature (780-820 ° C.) and pressure (30-40
Chemical vapor deposition (CVD) while maintaining Pa) constant
To deposit an HTO film. The gas introduced into the chamber during the film formation and the flow rate thereof are, for example, SiH ₂ Cl ₂ /
N ₂ O = 100/200 sccm. SiH ₂ Cl
₂ Flow rate is about 100-200 sccm, N ₂ O flow rate is 2
It can be changed appropriately within a range of about 00 to 400 sccm.

After the HTO film is formed, as shown in FIG. 3 (C), the temperature (780-820 ° C.) and the pressure (30-40 Pa) in the chamber are kept constant for 10 minutes with N _2. Flow O gas (post purge). The flow rate of the N ₂ O gas is, for example, 200 sccm. The data of FIG. 4E shows that the N ₂ O gas flow rate of the post-purge is 200 sccm.
In this case, the N ₂ O gas flow rate for the post-purge can be changed as appropriate, for example, from 200 to 400 sccm. In addition, as long as the dielectric breakdown characteristics of the HTO film to be formed are improved, the film forming gas may be a nitrogen-based gas other than the N ₂ O gas,
For example, it can be changed to NO or the like.

According to the conventional film forming method, as shown in FIG. 9, after the film forming step (B), the film is opened to the atmosphere without performing post-purging (C). On the other hand, in the case of the film forming method of the present embodiment, pre-purge is performed under reduced pressure after the CVD step (FIG. 3B) (FIG. 3C). By performing both the pre-purge under reduced pressure (FIG. 3A) and the post-purge before the CVD step, the electric breakdown voltage after the film formation is improved.

Thereafter, as shown in FIG. 3D, the inside of the chamber is opened to the atmosphere. The inside of the chamber is returned from the high vacuum state to the atmospheric pressure in about 3 minutes. Thereafter, the wafer on which the HTO film has been formed is carried out of the chamber. Further, annealing is performed, for example, at 900 ° C. for 10 minutes to make the film quality uniform or dense. Through the above steps, a lower SiO ₂ film of the ONO film is formed. Subsequently, an ONO film with improved electric breakdown voltage can be formed by forming a Si ₃ N ₄ film by, for example, a low-pressure CVD method, and further stacking an upper HTO film by the CVD method. After forming the upper HTO film, N ₂ O purge may be performed under reduced pressure.

According to the method described above, H 2 formed by performing both N ₂ O pre-purge and post-purge under reduced pressure.
The time-dependent dielectric breakdown characteristics of the TO film (FIG. 4E) are obtained when only the pre-purge is performed (Embodiment 1, FIG. 4A shows FIG. 1A).
And the same data). However, as compared with the conventional HTO film (FIG. 4 (c) shows the same data as FIG. 1 (c)), the time-dependent dielectric breakdown characteristics were improved.

(Embodiment 3) A method for forming a silicon-based dielectric film according to this embodiment will be described below with reference to the flowchart in FIG. FIG. 1B shows the time-dependent dielectric breakdown characteristics of the HTO film obtained by the method for forming a silicon-based dielectric film of the present embodiment. The inside of the chamber of the CVD apparatus is previously heated to a certain temperature (for example, 800 ° C.). The chamber is opened to the atmosphere when loading and unloading wafers, but the temperature fluctuation range in the chamber is 780 to 8
It is about 20 ° C. As shown in FIG. 5A, a wafer is placed in a chamber, and an N ₂ O gas is flowed under atmospheric pressure for about 2 minutes (prepurge). The flow rate of the N ₂ O gas is, for example, 200 s.
ccm. If hydrogen is excessively introduced into the SiO ₂ film, the resistance to stress leak current decreases, but the incorporation of hydrogen can be prevented by pre-purging with N ₂ O gas.

Next, as shown in FIG. 5B, an HTO film is deposited by chemical vapor deposition (CVD). While the temperature in the chamber is maintained at 780 to 820 ° C., the pressure in the chamber is reduced using a vacuum pump, and the pressure in the chamber is set to, for example, 30 to 40 Pa. The gas introduced into the chamber during the film formation and the flow rate thereof are, for example, SiH ₂ Cl ₂ / N
₂ O = 100/200 sccm. SiH ₂ Cl ₂
The flow rate is about 100 to 200 sccm, and the N ₂ O flow rate is 20
It can be changed appropriately within a range of about 0 to 400 sccm.

After forming the HTO film, as shown in FIG. 5C, while keeping the temperature (780-820 ° C.) and the pressure (30-40 Pa) in the chamber constant, N _{2 is applied for} 10 minutes. Flow O gas (post purge). The flow rate of the N ₂ O gas is, for example, 200 sccm. The data in FIG. 1B is for the case where the N ₂ O gas flow rate is 200 sccm,
The flow rate of the N ₂ O gas can be appropriately changed within a range of, for example, about 200 to 400 sccm. Also, the H
The post-purging gas may be changed to a nitrogen-based gas other than the N ₂ O gas, for example, NO, as long as the dielectric breakdown characteristics of the TO film are improved. According to the conventional film forming method, as shown in FIG. 9, after the film forming step (B), the film is opened to the atmosphere without performing post-purging (FIG. 9C). In contrast, in the case of the film forming method of the present embodiment, post-purging is performed under reduced pressure after CVD (FIG. 5C). Thereby, the electric breakdown voltage after film formation is improved.

Thereafter, as shown in FIG. 5D, the inside of the chamber is opened to the atmosphere. The inside of the chamber is returned from the high vacuum state to the atmospheric pressure in about 3 minutes. Thereafter, the wafer on which the HTO film has been formed is carried out of the chamber. Further, for example, 9
Annealing is performed at 00 ° C. for 10 minutes to make the film quality uniform or dense.

Through the above steps, the lower layer SiO of the ONO film
_Two films are formed. Subsequently, a Si ₃ N ₄ film is formed by, for example, a low pressure CVD method, and further, an upper layer H is formed by the CVD method.
By stacking the TO films, an ONO film with improved electric breakdown voltage can be formed. As shown in FIG. 1B, the HTO film formed by performing N ₂ O post-purging under reduced pressure according to the above-described method has a conventional HTO film (FIG. 1C). The dielectric breakdown characteristics with the lapse of time were improved as compared with the above.

The data of this embodiment (FIG. 1B)
Embodiment 1 (FIG. 1A, only pre-purge under reduced pressure)
Alternatively, when compared with the data of the second embodiment (FIG. 4E, both pre-purge and post-purge under reduced pressure), it can be seen that the characteristics in the region where the value on the vertical axis is small (the region where the cumulative failure rate is small) are different. In the case of the first or second embodiment, the slope of the plot is steep, the variation (standard deviation) is small, and it is a genuine dielectric breakdown region (wear region). On the other hand, in the present embodiment (FIG. 1B) in which the pre-purge under the reduced pressure is not performed, the slope of the plot is gentle in a region where the cumulative failure rate is small. This suggests that there is a difference between the pre-purge under reduced pressure and the post-purge in the effect of improving the film quality that contributes to the electric breakdown voltage characteristics. When pre-purge is performed under reduced pressure before film formation by the CVD method, initial failure or accidental failure is reduced, and in particular, initial dielectric breakdown characteristics can be improved.

Next, the correlation between the time for performing post-purging under reduced pressure and the dielectric breakdown characteristics in the method for forming a silicon-based dielectric film of the present invention will be described below with reference to FIGS. (Embodiment 4) FIG. 4, performs the N ₂ O pre-purge under reduced pressure for 10 minutes, when performed by changing the the N ₂ O pre-purge under reduced pressure time (0 min, 10 min, 30 min) time dependent dielectric breakdown of Changes in characteristics will be described. For comparison, an HTO film (no pre-purge and no post-purge under reduced pressure; the same data as in FIG. 1 (c)) according to the conventional method is also shown.

As shown in FIG. 4, when the pre-purge is performed under reduced pressure, the dielectric breakdown characteristics over time are greatly improved (FIG. 4).
(A), see Embodiment 1.) When a post-purge of about 10 minutes is performed on a sample that has been pre-purged under reduced pressure (FIG. 4E), the dielectric breakdown characteristics with time deteriorates slightly, but in the conventional case without pre-purge under reduced pressure (FIG. 4 (c)), the time-dependent dielectric breakdown characteristics were clearly improved (see Embodiment 2). In the case of a sample that was pre-purged under reduced pressure, the post-purge was performed for about 30 minutes (FIG. 4).
(F)), almost no difference was observed as compared with the case of 10 minutes post-purge (FIG. 4 (e)).

(Embodiment 5) FIG.
Changes in the dielectric breakdown characteristics over time when N ₂ O pre-purge under reduced pressure is performed for various times (10 minutes and 30 minutes) without performing ₂ O pre-purge are shown. The case of 0 minutes post-purging corresponds to the case (c) of the conventional example shown in FIG. 1 or FIG. In the case where there is no pre-purge under reduced pressure and only post-purge is performed for 10 minutes, the third embodiment shown in FIG.
Is the same data as Without pre-purge under reduced pressure,
When only post-purging under reduced pressure is performed for 10 minutes (b), the dielectric breakdown characteristics over time are improved as compared with the HTO film (c) according to the conventional method. When the post-purge is performed for 30 minutes without performing the pre-purge under reduced pressure (g), the HTO according to the conventional method is used.
The dielectric breakdown characteristics over time decreased to the same extent as the film (c).

In the case of the embodiment 4 in which the pre-purge is performed under reduced pressure, even if the post-purge is extended from 10 minutes (FIG. 4E) to 30 minutes (FIG. 4F), the dielectric breakdown characteristics are deteriorated. In contrast, in the case of Embodiment 5 in which the pre-purge was not performed under reduced pressure, if the post-purge time was longer than a certain time, the dielectric breakdown characteristics were likely to decrease, and the effect of improving the film quality was improved. Is reduced. As described above, in the dielectric film forming method of the present invention, the dielectric breakdown characteristics with time change depending on the post-purge time.
Therefore, as long as the electric breakdown voltage after film formation is improved,
The purge time is set as appropriate.

According to the method for forming a dielectric film of the present invention, H
Regarding the mechanism by which the time-dependent dielectric breakdown characteristics of the TO film improve,
Although it has not been clarified because a plurality of models can be considered, the possibility that an extremely thin SiN layer is formed on the surface of the base (specifically, polysilicon) of the HTO film due to thermal decomposition of N ₂ O can be mentioned. Can be Alternatively, regarding the effect of the post-purge, there is a possibility that N is bonded to dangling bonds on the surface of the HTO film and crystal defects are improved. Also, from the difference in the initial dielectric breakdown characteristics or the behavior when the post-purge time is changed, the pre-purge and the post-purge under reduced pressure can contribute differently to the improvement of the electric breakdown voltage of the HTO film. It is suggested.

Embodiment 6 A method for manufacturing a semiconductor memory device using the method for forming a silicon-based dielectric film of the present invention will be described below. According to the method of manufacturing a semiconductor memory device of the present invention, as shown in FIG. 7, a semiconductor device having the same structure as a conventional semiconductor memory device is manufactured. In FIG. 7, n-type source / drain regions 5 are formed at predetermined intervals on the surface of p-type impurity region (p-well) 4 provided on a silicon substrate. Floating gate 1 made of, for example, polysilicon is formed via gate insulating film 6 so that a channel region is formed between source / drain regions 5. On top of that, a lower SiO ₂ film 3
a, a capacitor insulating film 3 composed of a Si ₃ N ₄ film 3b and an upper SiO ₂ film 3c is formed. A control gate 2 made of, for example, polysilicon or a laminated film of polysilicon and refractory metal silicide is formed above the capacitor insulating film 3. Further, a dielectric film 7 is formed so as to cover the floating gate 1, the control gate 2, and the like.

In order to manufacture the semiconductor memory device having the above structure, first, as shown in FIG. 8A, for example, pyrogenic oxidation is performed on the surface of the p-type impurity region 4 provided on the silicon substrate. Then, a gate insulating film 6 is formed. A polysilicon layer 1 ′ for forming the floating gate 1 is formed thereon with a thickness of, for example, 100 nm.

Next, the lower layer SiO ₂ of the capacitor insulating film 3 is formed.
Before the film (HTO film) 3a is formed by the CVD method, an N ₂ O purge is performed on the surface of the polysilicon layer 1 ′ under reduced pressure for 10 minutes.
Do about a minute. Thereby, the electric breakdown voltage of the HTO film can be improved. Thereafter, a lower SiO ₂ film (HTO film) 3 a of the capacitor insulating film 3 is deposited by the CVD method. The CVD for forming the lower SiO ₂ film 3a is performed by, for example, SiH ₂ Cl ₂ / N ₂ O = 100/200 sccm.
Under the conditions described above, it is not necessary to replace N ₂ O introduced into the chamber by pre-purge. That is, the pre-purge step under reduced pressure, which is a feature of the present invention, can be easily performed.

The upper layer of the lower SiO ₂ film 3a is, for example, CV
An Si ₃ N ₄ film 3b is formed by the D method, and an upper SiO ₂ film 3c is further laminated thereon by, for example, a CVD method. After the formation of the lower SiO ₂ film 3a or the upper S
After the formation of the iO ₂ film 3c, N ₂ O post-purging may be performed under reduced pressure, for example, for about 10 minutes. Upper SiO ₂ film 3
On top of c, a polysilicon layer 2 'for forming the control gate 2 is formed with a thickness of, for example, 100 nm.

Next, as shown in FIG. 8B, using a resist (not shown) as a mask, a polysilicon layer 2 ′ for the control gate 2, a capacitor insulating film 3 having a three-layer structure, and a polysilicon for the floating gate 1. The silicon layer 1 'and the gate insulating film 6 are patterned. Next, FIG.
As shown in (1), an n-type impurity is ion-implanted into the p-type impurity region 4 on the substrate using the patterned gate electrode as a mask to form an n-type source / drain region 5.

Alternatively, using a patterned gate electrode as a mask, a relatively low concentration n-type impurity is ion-implanted to perform LDD (lightly doped drain).
After forming the (in) region, a sidewall may be provided on the gate electrode, and a relatively high concentration of n-type impurity may be introduced using the sidewall as a mask to form a source / drain region having an LDD structure. Thereafter, a dielectric film 7 is formed so as to cover the gate electrode, and the floating gate 1 is electrically isolated from the surroundings, whereby the nonvolatile semiconductor memory device shown in FIG. 7 is obtained.

According to the above-described method for manufacturing a nonvolatile semiconductor memory device of the present invention, a capacitor insulating film (ONO film) having high dielectric breakdown characteristics over time can be formed. Therefore, dielectric breakdown or leakage current of the capacitor insulating film is prevented, and charge accumulation is maintained. That is, the storage retention characteristics of the nonvolatile semiconductor memory device can be improved. Embodiments of the method for forming a silicon-based dielectric film and the method for manufacturing a nonvolatile semiconductor memory device according to the present invention are not limited to the above description. For example, the composition and flow rate of the gas to be purged can be appropriately changed. Others
Various changes can be made without departing from the spirit of the present invention.

[0059]

According to the method for forming a silicon-based dielectric film of the present invention, the time-dependent dielectric breakdown characteristics of a silicon oxide film (HTO film) formed by a CVD method can be improved. According to the method for manufacturing a nonvolatile semiconductor memory device of the present invention, the storage durability of the nonvolatile semiconductor memory device can be improved by improving the electrical resistance of the capacitor insulating film.

[Brief description of the drawings]

FIG. 1 compares the time-dependent dielectric breakdown characteristics of an HTO film formed by a method for forming a silicon-based dielectric film of the present invention with the time-dependent dielectric breakdown characteristics of an HTO film or a thermal oxide film formed by a conventional method. It is a Weibull plot.

FIG. 2 is a flowchart showing each step of a method for forming a silicon-based dielectric film according to Embodiment 1 of the present invention.

FIG. 3 is a flowchart showing each step of a method for forming a silicon-based dielectric film according to a second embodiment of the present invention.

FIG. 4 is a Weibull plot showing the dielectric breakdown characteristics over time when the pre-purge time under reduced pressure and the post-purge time are changed in the method for forming a silicon-based dielectric film of the present invention.

FIG. 5 is a flowchart showing each step of a method for forming a silicon-based dielectric film according to Embodiment 3 of the present invention.

FIG. 6 is a Weibull plot showing the dielectric breakdown characteristics over time when the post-purge time is changed without performing pre-purge under reduced pressure in the method of forming a silicon-based dielectric film of the present invention.

FIG. 7 is a sectional view of a nonvolatile semiconductor memory device manufactured by the present invention and a conventional method of manufacturing a nonvolatile semiconductor memory device.

FIGS. 8A to 8C are cross-sectional views illustrating manufacturing steps of a method for manufacturing a nonvolatile semiconductor memory device according to the present invention.

FIG. 9 is a flowchart showing each step of a conventional method for forming a silicon-based dielectric film.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Floating gate, 1 ', 2' ... Polysilicon layer, 2 ... Control gate, 3 ... Capacitor insulating film,
3a: lower SiO ₂ film, 3b: Si ₃ N ₄ film, 3c: upper SiO ₂ film, 4: p-type impurity region (p well), 5:
n-type source / drain regions, 6: gate insulating film, 7: dielectric film.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01L 27/115 F term (Reference) 5F001 AA25 AA43 AA63 AB08 AD17 AD61 AF06 AF07 AG03 AG21 5F058 BA11 BD02 BD04 BD10 BE10 BF04 BF24 BF29 BF30 BF55 BF60 BF62 BH20 BJ01 5F083 EP02 EP23 EP55 EP56 EP57 EP63 ER22 GA21 JA04 JA32 PR21

Claims (26)

[Claims] 1. A pre-purge step of blowing nitrogen-based gas under reduced pressure for a predetermined time on a surface of a semiconductor layer, and performing a first chemical vapor deposition (CVD) on the surface of the semiconductor layer to form a first silicon oxide layer. Forming a second chemical vapor deposition on the first silicon oxide layer to form a silicon nitride layer; performing a third chemical vapor deposition on the silicon nitride layer; Forming a second silicon oxide layer. 2. The method for forming a silicon-based dielectric film according to claim 1, wherein said pre-purge step is performed while being heated to a temperature substantially equal to a temperature at which said first chemical vapor deposition is performed. 3. A step of performing a heat treatment at a temperature higher than a temperature at which the first chemical vapor deposition is performed after forming the first silicon oxide layer to homogenize the film quality of the first silicon oxide layer. The method for forming a silicon-based dielectric film according to claim 1. 4. The time for performing the pre-purge step is set within a range in which the time-dependent dielectric breakdown characteristics of the first silicon oxide layer are improved as compared with a case where the pre-purge step is not performed. A method for forming a silicon-based dielectric film. 5. The method according to claim 4, wherein the time for performing the pre-purge step is about 10 minutes. 6. The method according to claim 1, wherein said nitrogen-based gas is N ₂ O gas. 7. The method for forming a silicon-based dielectric film according to claim 1, further comprising a post-purge step of blowing a nitrogen-based gas under reduced pressure for a predetermined time on the surface of the first silicon oxide layer. 8. The time for performing the post-purge step is set within a range in which the time-dependent dielectric breakdown characteristics of the first silicon oxide layer are improved as compared with the case where the pre-purge step is not performed. Of forming a silicon-based dielectric film. 9. The method according to claim 7, wherein said nitrogen-based gas is N ₂ O gas. 10. The method for forming a silicon-based dielectric film according to claim 1, further comprising a post-purge step of blowing a nitrogen-based gas under reduced pressure for a predetermined time on said second silicon oxide layer surface. 11. The time for performing the post-purge step is set within a range in which the time-dependent dielectric breakdown characteristics of the second silicon oxide layer are improved as compared with the case where the pre-purge step is not performed. Of forming a silicon-based dielectric film. 12. The method according to claim 10, wherein said nitrogen-based gas is N ₂ O gas. 13. A first chemical vapor deposition (C) method comprising the steps of:
VD) to form a first silicon oxide layer; a post-purge step of blowing a nitrogen-based gas under reduced pressure for a predetermined time on the surface of the first silicon oxide layer; Performing a second chemical vapor deposition on the silicon nitride layer to form a silicon nitride layer; and performing a third chemical vapor deposition on the silicon nitride layer to form a second silicon oxide layer. A method for forming a silicon-based dielectric film. 14. The method according to claim 13, wherein the post-purge step is performed while being heated to a temperature substantially equal to a temperature at which the first chemical vapor deposition is performed. 15. A step of performing a heat treatment at a temperature higher than a temperature at which the first chemical vapor deposition is performed after forming the first silicon oxide layer to homogenize the film quality of the first silicon oxide layer. 14. The method for forming a silicon-based dielectric film according to claim 13, comprising: 16. The time for performing the post-purge step is set within a range in which the time-dependent dielectric breakdown characteristics of the first silicon oxide layer are improved as compared with the case where the post-purge step is not performed. The method for forming a silicon-based dielectric film according to the above. 17. The time for performing the post-purge step is 10 minutes.
17. The method according to claim 16, which is performed on the order of minutes. 18. The method according to claim 13, wherein said nitrogen-based gas is N ₂ O gas. 19. The method for forming a silicon-based dielectric film according to claim 18, further comprising a post-purge step of blowing nitrogen-based gas under reduced pressure for a predetermined time on the surface of said second silicon oxide layer. 20. The time for performing the post-purge step is set within a range in which the time-dependent dielectric breakdown characteristic of the second silicon oxide layer is improved as compared with the case where the pre-purge step is not performed. Of forming a silicon-based dielectric film. 21. The method according to claim 19, wherein said nitrogen-based gas is N ₂ O gas. 22. A step of forming a gate insulating film on a semiconductor substrate; a step of forming a first conductive layer serving as a floating gate on the gate insulating film; A pre-purge step of blowing a nitrogen-based gas on the surface under reduced pressure for a predetermined time; a step of performing a first chemical vapor deposition (CVD) on the surface of the semiconductor layer to form a first silicon oxide layer; Performing a second chemical vapor deposition on the first silicon oxide layer to form a silicon nitride layer; and performing a third chemical vapor deposition on the silicon nitride layer to form a second silicon oxide layer. Forming a second conductive layer serving as a control gate on the second silicon oxide layer; and forming the second conductive layer, the first and second oxidation layers. A game comprising a silicon layer and the silicon nitride layer A non-volatile semiconductor comprising: a step of processing the inter-layer insulating film, the first conductor layer, and the gate insulating film into a predetermined gate electrode pattern; and a step of forming a source region and a drain region on the surface of the semiconductor substrate A method for manufacturing a storage device. 23. The method according to claim 22, wherein said pre-purge step is performed while being heated to a temperature substantially equal to a temperature at which said first chemical vapor deposition is performed. 24. The method according to claim 22, wherein the time for performing the pre-purge step is set within a range in which the time-dependent dielectric breakdown characteristics of the first silicon oxide layer are improved as compared with the case where the pre-purge step is not performed. A method for manufacturing a nonvolatile semiconductor memory device. 25. The method according to claim 24, wherein the time for performing the pre-purge step is about 10 minutes. 26. The method according to claim 22, wherein said nitrogen-based gas is N ₂ O gas.