JPH09153489A - Manufacture of semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxide film of high reliability. SOLUTION: First of all, a silicon substrate is introduced into a processing chamber 33 and shut off from an outside air. AHF gas is introduced into the processing chamber 33 so as to remove a natural oxide film formed on the surface of the substrate, keeping the silicon substrate at a temperature of 400 deg.C or below. Then, keeping the silicon substrate in a temperature range from a dew point to 400 deg.C, water vapor and oxygen gas are introduced into the processing chamber 33 so as to make it filled with oxidizing atmosphere. The temperature of the silicon substrate is set higher those in a second and a third process, and an oxide film is formed on the surface of the silicon substrate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for manufacturing a semiconductor device characterized by a method for forming an oxide film.

[0002]

2. Description of the Related Art Generally, a semiconductor element is formed through a process of forming and processing an insulating film such as an oxide film or a conductive film such as a polycrystalline silicon film on a silicon substrate. Of the films formed on the silicon substrate, the oxide film is a basic constituent element of a semiconductor device, as seen in, for example, a gate insulating film of a MOS type semiconductor device, and its quality depends on the performance of the entire device. Are deeply involved in.

The quality of the oxide film is most basically judged from the characteristics of the insulating film by the change in the characteristics when a voltage is applied, and is roughly classified into a dielectric breakdown due to a simple voltage application. It can be divided into two types: what to see, and evaluation of characteristic change and breakdown that occur when a voltage is continuously applied for a certain period of time. Of these characteristics, the one corresponding to the former is simply referred to as a dielectric strength characteristic, and the breakdown characteristic of the latter is referred to as a time-dependent dielectric breakdown life.

The former obtains information about remarkable defects that are relatively sparsely distributed in terms of location, while the latter obtains information about minute defects that are distributed more densely.

Corresponding to this, it is general that the former defect is called a random defect, the latter defect is called an intrinsic defect, and the corresponding dielectric breakdown life with time is called an intrinsic life. Further, the dielectric breakdown over time and the characteristic changes on the way to that in general may be collectively referred to as an intrinsic characteristic. As is clear from its properties, random defects are the types of properties that affect the so-called initial characteristics of the product, while, in general, the intrinsic characteristics are characteristics that also affect reliability.

The characteristics of these oxide films naturally depend on the forming method thereof, but in order to improve the quality thereof, both reduction of accidental defects and improvement of the intrinsic characteristics must be realized. Therefore, the forming method also needs to satisfy the conditions for realizing the improvement of these two characteristics.

Among them, what has been conventionally taken as a measure for reducing accidental defects is mainly to reduce pollution, particularly metal pollution, in a manufacturing process including oxide film formation. Specifically, except for the improvement of the equipment itself such as the material and structure of the heat treatment device for oxidation (abbreviated as oxidation furnace), the substrate is cleaned before oxidation (abbreviated as pretreatment), and then the oxide film is formed. The two main goals are to realize a clean oxidation process that prevents the substrate from being newly polluted during the process.

As the former pretreatment method, generally, a contaminating metal is eluted in a chemical solution with an acid such as hydrochloric acid (HCl), and then hydrogen peroxide (H ₂ O ₂ ) water or the like is used. This is to prevent the re-adsorption of the contaminated metal dissolved in the chemical solution on the substrate by thinly oxidizing the surface of the substrate with an oxidizing agent to make the surface hydrophilic.

On the other hand, in the latter oxidation process, the simplest method is to reduce the oxidation temperature in the sense that the amount of pollutant metal generated from the oxidation furnace itself is reduced. On the other hand, as a method for improving the intrinsic properties, particularly the intrinsic life, a substrate obtained by using deionized water (abbreviated as DIW) containing ozone of about 1 to 3 ppm (abbreviated as ozone water) is used. It has been known as a phenomenon that the intrinsic life of the oxide film on the substrate is improved by the treatment.

However, until now, its mechanism has not been understood and essential parameters have not been grasped, so that it has not been generally used. That is, since a general method for improving the intrinsic characteristics has not been known, it was not possible to form a higher quality oxide film.

However, with the miniaturization of the size of semiconductor elements and the diversification of applications, the quality of oxide films, particularly gate insulating films, has been reduced from the chance of random failure, which has been emphasized in the past. The demand for improvement has also increased.

[0012] For example, in the case of a tunnel insulating film of a flash memory, the improvement of the intrinsic characteristic has become as important as the reduction of the random failure due to the method of use and the function thereof.

On the other hand, in order to form a high-quality oxide film on a silicon substrate, prior to the formation of the oxide film, a so-called natural oxide film formed on the substrate surface in the atmosphere at normal pressure and normal temperature is once removed. After that, it is necessary to again form an oxide film on the same substrate under predetermined conditions such as temperature and atmosphere. However, the following technical problems exist in order to effectively carry out this.

That is, as a conventional method for removing the natural oxide film, the natural oxide film on the surface of the substrate is removed by immersing the substrate in a chemical solution containing diluted hydrofluoric acid or the like, and then the substrate is subjected to an electric furnace or the like. A method of forming an oxide film by arranging it in a processing chamber of the apparatus and exposing it to an oxidizing atmosphere at high temperature was adopted.

However, in this method, the substrate must be subjected to two kinds of treatments having different characteristics, that is, a wet treatment using a chemical solution and a treatment in a gas using an electric furnace or the like, and naturally the substrate treatment is performed between these treatments. Accompanied by transportation.

The transfer of the substrate itself involves problems such as generation of dust, and especially when connecting processes having different characteristics as in the above case, avoid the situation where the substrate is exposed to the air in the clean room. Therefore, it is unavoidable that the substrate is contaminated with organic substances or heavy metals.

In order to avoid such a situation, the following method has been proposed. That is, the removal of the natural oxide film on the surface of the substrate to the formation of the oxide film is performed in the same apparatus so as to eliminate the transportation of the substrate through the clean room atmosphere.

For this purpose, the method that is usually carried out is to remove the natural oxide film in the oxidation treatment chamber of the oxidation device or in the treatment chamber connected to it by a load lock, and then continue in the oxidation treatment chamber of the same device. To form an oxide film.

There are various methods for removing the natural oxide film. When the natural oxide film is removed in an oxidizing device, HF, NH ₄ F, and NF are used as etching gas.
Dry etching is performed by using a reactive gas containing fluorine such as ₃ , ClF ₃ or the like.

When the natural oxide film removal process is performed in the oxidation process chamber in this manner, there is no need to transfer the substrate, while
If the removal process of the natural oxide film is performed in a processing chamber different from the oxidation processing chamber, it is possible to avoid the problem that the substrate is exposed to the air in the clean room because the substrates are transferred between the processing chambers, although they are in the same device. This makes it possible to minimize the contamination of the substrate with organic substances, heavy metals and the like.

However, such a method has the following problems and is difficult to actually carry out. First
The problem is that the interface state density between the substrate and the oxide film formed by reoxidation increases. This problem occurs for the following reasons.

When the natural oxide film is removed using a reactive gas containing fluorine such as HF, NH ₄ F, NF ₃ and ClF ₃ , the surface of the substrate after the removal is free of Si atoms facing outward. The bond is in a state in which the bond is bonded to a fluorine atom (this is called "termination" by fluorine).

[0023] Since this Si-F chemical bonds of the binding energy increased stability, the silicon substrate continues even if the formation of the oxide film on the same surface to a high temperature in an oxidizing atmosphere, the SiO ₂ / Si interface Fluorine remains. Due to the residual fluorine, the interface state density increases due to the application of current stress, and the device characteristics deteriorate.

On the other hand, as described above, when the natural oxide film is removed in the solution using dilute hydrofluoric acid, the substrate is removed to remove the dilute hydrofluoric acid after the treatment with the dilute hydrofluoric acid solution. Immersion in deionized water (abbreviated as pure water) is always performed.

It is known that this treatment causes the Si-F bond to be hydrolyzed and replaced with Si-H or Si-O-H. That is, since fluorine on the surface of the substrate is removed by this reaction, there is no influence of fluorine in the step of forming the oxide film by the method including the removal treatment of the natural oxide film in the solution.

However, when the natural oxide film is removed by wet etching, as described above, it is unavoidable that the substrate is exposed to the air in the clean room, and the substrate is contaminated by organic substances or heavy metals. Cannot be avoided.

The second problem is related to the fact that the surface of the substrate from which the native oxide film has been removed is extremely active and thus susceptible to various contaminants. That is, if the surface of the active substrate is contaminated with an organic substance, when the temperature of the substrate is raised in the step of subsequently forming an oxide film on the surface, 450
The silicon in the substrate reacts with the carbon in the organic matter from around 0 ° C. to start forming SiC.

Since SiC is a thermally extremely stable compound and is hardly oxidized even when the substrate is oxidized in an oxidizing atmosphere, it acts as a mask for suppressing the oxidation of silicon.

Therefore, while the portion where SiC is not formed is selectively oxidized and the SiO ₂ / Si interface moves inward from the substrate surface, the portion where SiC is formed does not change. As a result, the SiC / Si interface stays at a fixed position below the substrate surface, so that an uneven shape is substantially generated on the substrate surface.

Usually, since SiC is formed on the surface of the substrate like islands within the surface of the same, the SiC portion itself is lifted off and removed as the oxidation of silicon around the substrate progresses. It rarely remains in the final SiO ₂ / Si structure.

However, the uneven shape of the surface of the substrate formed as described above has a characteristic of an extremely thin oxide film formed thereon, particularly, a thickness of less than 10 nm, for example, dielectric breakdown (TD) over time.
It is abbreviated as DB), which has an adverse effect such as shortening the life.

As to the suppression of organic contaminants that cause such characteristic deterioration, as described above, it is technically easy to reduce the amount in the processing chamber of the oxidation device to be smaller than that in a normal clean room atmosphere. However, as can be seen from the mechanism described here, the magnitude of the influence of organic substances on the characteristics depends extremely strongly on the surface state of the substrate which is the contaminated side.

That is, on the active surface immediately after the removal of the natural oxide film, the adsorbed organic matter is located at a position close to the silicon of the substrate, and SiC is likely to be formed by the subsequent thermal process, so that the characteristics are particularly affected. It will be remarkable.

In the conventional methods for removing the natural oxide film and forming the oxide film in the processing chamber of the oxidation device, it is necessary to prevent the adsorption of organic substances on the active surface immediately after the removal of the natural oxide film. Therefore, it was not possible to form a high quality oxide film, which should be achieved originally.

On the other hand, in the conventional method using the solution treatment, the natural oxide film on the surface of the substrate is removed by the dilute hydrofluoric acid treatment, and then an oxidizing agent such as pure water added with hydrogen peroxide solution or ozone is used. By performing treatment with strong chemicals,
Before the substrate surface was exposed to a clean room atmosphere, the surface of the substrate was covered with an oxide layer and the activity of the substrate was low, whereby the influence of organic contamination was suppressed as small as possible. However, as mentioned above,
There is a problem of being contaminated when exposed to a clean room atmosphere.

[0036]

As described above, with the miniaturization of the size of semiconductor elements and the diversification of applications, the quality of the oxide film is reduced in addition to the reduction of accidental defects, which has been conventionally regarded as important. There has also been an increasing demand for improved intrinsic properties. However, since a general method for improving the intrinsic characteristics has not been known, there is a problem that a high quality oxide film cannot be formed.

Further, prior to the formation of the oxide film, the natural oxide film was removed by using a reactive gas containing fluorine. However, the residual fluorine increases the interface state density and forms a high quality oxide film. There was a problem that I could not do it.

The present invention has been made in consideration of the above circumstances, and an object thereof is to provide a method of manufacturing a semiconductor device capable of forming a high quality oxide film.

[0039]

[Means for Solving the Problems]

[Outline] In order to achieve the above object, a method for manufacturing a semiconductor device according to the present invention (claim 1) includes a first step of removing a natural oxide film on a surface of a semiconductor substrate, and the removal of the natural oxide film. The surface of the formed semiconductor substrate has a low density and a stress of 1 × 10
The second step of forming an oxide layer of less than ⁸ N / m ^{2 and} a thickness of 1 nm or more, and the surface from which the native oxide film is removed by densifying the oxide layer by heat treatment in an oxidizing atmosphere. And a step of forming an oxide film having a higher density than that of the oxide layer of the second step.

Here, the oxide layer may be formed on the entire surface of the semiconductor substrate or selectively formed on the surface. Another method of manufacturing a semiconductor device according to the present invention (claim 2) is the method of manufacturing a semiconductor device (claim 1), wherein the water containing ozone, the gas containing ozone, or the gas containing oxygen radicals is used. A feature is that an oxide layer is formed.

A preferred mode of the method for manufacturing the semiconductor device (claims 1 and 2) is as follows. (1) Before forming the oxide layer, the contaminants containing C such as organic substances on the surface of the semiconductor substrate are removed from metals such as Cr and Cu. (2) F on the surface of the semiconductor substrate before forming the oxide layer
Metals such as e, Cr and Cu are removed. (3) The thickness of the finally formed oxide film is set to 10 nm or less. In the case of such a thin oxide film, the effect of the present invention becomes remarkable. (4) The first step to the fourth step are performed while the substrate is substantially shielded from the outside air.

Further, another method of manufacturing a semiconductor device according to the present invention (claim 3) comprises a first step of introducing a semiconductor substrate into a processing chamber to substantially isolate the semiconductor substrate from the outside air, A second step of removing a natural oxide film on the surface of the semiconductor substrate by introducing a gas containing fluorine into the processing chamber while keeping the temperature of the semiconductor substrate at 400 ° C. or lower; A third step of introducing an oxidizing gas containing water vapor into the processing chamber so as to form an oxidizing atmosphere in the processing chamber while maintaining the temperature at a temperature not lower than dew condensation and not higher than 400 ° C .; and the semiconductor substrate. And a fourth step of forming an oxide film on the surface of the semiconductor substrate by making the temperature of the semiconductor substrate higher than the temperature of the semiconductor substrate in the second and third steps.

Another method for manufacturing a semiconductor device according to the present invention (claim 4) is the same as the method for manufacturing a semiconductor device (claim 3), wherein the first, the second, the third, and the third are the same. The pressure in the processing chamber is reduced to 1 /
It is characterized in that it is maintained at 10 atmospheres or more.

A preferred mode of the method for manufacturing the semiconductor device (claims 3 and 4) is as follows. A gas containing oxygen radicals or ozone is used as the oxidizing gas introduced into the processing chamber in the third step.

The first step of the method for manufacturing a semiconductor device (claims 1 and 2) is the same as the first, second and third steps of the method for manufacturing a semiconductor device (claims 3 and 4). If the process is replaced by the process No. 3, a higher quality oxide film can be formed.

[Operation] The basic idea of the present invention (claims 1 and 2) is based on the discovery of two types of controlling factors for the intrinsic life of an oxide film. That is, first, the intrinsic life of an oxide film is as follows. First, the surface of the substrate before the oxide film is formed is contaminated. Second
In addition, it is dominated by the unevenness of the oxide film / substrate interface generated at the initial stage of thermal oxidation due to the compressive stress in the oxide film.

The mechanism of unevenness caused by the first organic contaminant is as follows. When forming an oxide film on a semiconductor substrate, first, the semiconductor substrate is cleaned by a wet process and then heat-treated in an oxidation furnace. Since the apparatuses used for both processes are usually different, the semiconductor substrate is transferred between the two apparatuses in a clean room atmosphere, and the substrate surface is inevitably exposed to the atmosphere in the clean room.

However, as is well known, the atmosphere in the clean room usually contains a considerable amount of organic substances. For this reason, the surface of the substrate after the pretreatment is inevitably affected by the organic contaminants before entering the oxidation step.

A part of the organic contaminants on the surface of the substrate is burnt and decomposed during the heat treatment for oxidation, but a part thereof reacts with the semiconductor substrate to form thermally stable SiC or the like. If such a reaction occurs locally unevenly, the formation of the essential oxide film is not performed uniformly, resulting in unevenness in the final oxide film / substrate interface.

At this time, the essence is that the adhered organic substances come into direct contact with the substrate, but in fact, R, which has been standardly performed as a pre-cleaning process before forming an oxide film, is actually performed.
An oxide film formed on a semiconductor substrate by a process such as CA cleaning (a chemical oxide film is distinguished from a natural oxide film formed in the atmosphere.
It is known that oxides are not necessarily formed uniformly on the surface of the substrate (for example, App).
l. Phys. Lett. 61, 102 (1992)).

As a result of exposing the surface in such a state to the atmosphere in the clean room, the attached organic matter may come into direct contact with the substrate depending on the location, resulting in the interface irregularity as described above. The above situation is schematically shown in FIG. In FIG. 1, 1 is a silicon substrate and 2 is a chemical substrate.
Oxide (initial oxide layer), 3 is an organic contaminant, and 4 is an oxide film.

On the other hand, the mechanism for generating unevenness at the initial stage of thermal oxidation due to the stress of the second oxide film is more basic. That is, the constituent atoms of the semiconductor substrate by thermal oxidation (abbreviated as substrate atoms)
When is taken into the oxide film, a compressive stress always acts on the oxide film due to volume expansion. Therefore,
The energy of the entire system including the substrate surface and the oxide film increases as much as the strain energy corresponding to this stress.

The most natural change for canceling this increase in energy is to form irregularities on the surface of the substrate where oxidation is progressing. This is because if the surface of the substrate has irregularities, the area of the oxide film formed thereon will effectively expand, and the strain energy will change in a decreasing direction.

As described above, the above system has a mechanism for spontaneously forming irregularities at the oxide film / substrate interface. In order for such a system change to actually occur, it is necessary to move the substrate atoms, but at a high temperature during thermal oxidation, such a change is a kind of stress due to an increase in both strain energy and thermal energy. It can actually happen as a migration. Of course, if the thermal oxide film on the substrate surface becomes thick, such movement of the substrate atoms will be suppressed due to the thermal stability of the oxide film.

Therefore, such a mechanism actually works only in the initial stage of thermal oxidation. The above situation is schematically shown in FIG. In addition, in FIG.
Is a silicon substrate, 12 is a thermal oxide film at the initial stage of oxidation, and 13 is a thermal oxide film having a predetermined thickness.

In addition to the above mechanism, more importantly, the unevenness of the oxide film / substrate interface, which is a problem, has a width of 10 nm or less and a height of about 0.1 nm, which has been a problem in many conventional discussions. Is not of a very densely distributed type, with a width of about 100 nm and a height of 1-2.
The distribution density is about 1 nm, depending on the forming conditions.
The point is that it is a relatively large one, about one per corner.

Even if the thickness of the oxide film is not thinner than the surroundings at the corresponding portion, such an interface unevenness is obtained in the case of a thin oxide film having a thickness of 10 nm or less.
An electric field concentration of about 10% is generated, and the leak current density flowing in the oxide film is locally increased by about one digit, resulting in a reduction in the intrinsic life of several tens of%.

In the conventional oxide film forming method, both of the above-mentioned two factors are affected, and the deterioration of the intrinsic life and therefore the deterioration of the intrinsic characteristics cannot be avoided. Therefore, in the present invention (claims 1 and 2), the intrinsic characteristics, especially the intrinsic life is effectively improved by removing the above two types of factors at the same time.

That is, in the present invention, instead of directly forming an oxide film, first, a low stress (less than 1 × 10 ⁸ N / m ² ) capable of effectively suppressing the movement of substrate atoms and a substrate surface Form a thick (1 nm or more) oxide layer that is not affected by organic contaminants in the atmosphere, and then oxidize the oxide layer or the exposed semiconductor substrate by heat treatment in an oxidizing atmosphere. Change the film to a certain density,
The formation of a thick oxide film improves the intrinsic properties, especially the intrinsic life. Thus, a high quality oxide film can be formed.

Next, the operation of the present invention (claims 3 and 4) will be described. Further, in the present invention, the method of removing the natural oxide film is a general method from the first step to the second step.

Here, if the temperature of the semiconductor substrate is too high,
Removal of natural oxide film by gas containing fluorine (etching)
In this case, the etching selection ratio between the natural oxide film and the semiconductor substrate cannot be secured, and the semiconductor substrate under the natural oxide film is etched, or the natural oxide film and another oxide film on the semiconductor substrate are separated. However, there is a problem in that the other oxide film is etched because the etching selection ratio cannot be secured.

Therefore, in the present invention, in order to avoid the above problem, the temperature of the semiconductor substrate is set to about 400 ° C. or lower. Further, most of the bonds of Si atoms on the surface of the substrate at the end of the second step are terminated by fluorine (F).

In the second step, when hydrogen fluoride (HF) gas is used as the gas containing fluorine, water vapor (H ₂ O) may be added depending on the conditions. In the next third step, when an oxidizing gas containing water vapor is introduced into the processing chamber, the water vapor causes the bonds of Si atoms on the surface of the substrate, which are terminated with fluorine, to be hydrogen (H) or hydroxyl (OH).
It changes to the state terminated by.

As a result, residual fluorine can be eliminated, so that the interface state density can be reduced. Furthermore, since the substrate surface becomes inactive, it becomes possible to prevent the adsorption of organic contaminants on the substrate surface after the natural oxide film is removed.

On the other hand, the introduction of the oxidizing gas creates an oxidizing atmosphere in the processing chamber, whereby a new oxide film is formed on the semiconductor substrate. Here, in order to effectively prevent the condensation of water vapor in the processing chamber, the temperature of the semiconductor substrate is set to 1
It is better to set it to about 50 ° C or higher. In addition, before a new oxide film is sufficiently formed, S may be formed on the semiconductor substrate due to organic contaminants.
In order to prevent the formation of iC and the like, the temperature of the semiconductor substrate must be about 400 ° C. or lower.

Further, for the same reason as above, the time until a new oxide film is formed after the removal of the natural oxide film must be minimized. It is important to quickly shift the state of etching the oxide film to the state of oxidizing the substrate surface.

For that purpose, for example, instead of once exhausting hydrogen fluoride and then introducing steam, it is preferable to push hydrogen fluoride out of the processing chamber by introducing steam.

In the third step, an oxidizing atmosphere containing water vapor is more effective than a normal oxygen atmosphere in order to sufficiently oxidize the substrate surface at a low temperature of about 400 ° C. or lower, but at an even lower temperature. In order to improve the oxidizing property, an oxidizing agent having a strong oxidizing power such as oxygen radicals or ozone may be introduced into the processing chamber at this stage.

In the third step, water vapor is indispensable as the atmosphere only in the first half part where the bonds of Si atoms on the surface of the substrate are changed to the state terminated by hydrogen or hydroxyl group. At the stage, the atmosphere need not necessarily contain water vapor as the oxidizing species.

The fourth step is a step of thickening the oxide film to a predetermined thickness. As the oxidation method, various methods such as wet oxidation in an atmosphere containing water vapor, dry oxidation using dry oxygen, and oxidation in an atmosphere diluted with an inert gas such as nitrogen or argon can be used. .

Through these steps, the total pressure of the gas phase for processing the semiconductor substrate is preferably about 1/10 atmospheric pressure or more. This is because as the total pressure of the processing atmosphere becomes lower, the mean free path of molecules in the atmosphere becomes longer, and contaminants generated far from the semiconductor substrate can easily reach the substrate surface. This is to prevent the semiconductor substrate from being easily affected by these contaminants.

When the total pressure is about 1/10 atmospheric pressure or more, the mean free path of the corresponding molecule becomes 1 mm or less and 10
The increase in contamination does not pose a problem for a semiconductor substrate having a size of about cm.

By the above first to fourth steps, the removal of the natural oxide film previously formed on the substrate surface without removing the increase of the interface state density and the adsorption of organic contaminants can be performed to remove the target oxide film. Since the formation can be performed in the vapor phase, it becomes possible to form a high-quality oxide film which has not been obtained in the past.

[0074]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) In this embodiment, after removing the natural oxide film on the surface of the silicon substrate, first, low density and low stress (1 × 10 ⁸ N / m 2
² or less), a thick oxide layer (more specifically, a chemical oxide layer) having a thickness of 1 nm or more is formed, and then the above oxide layer is subjected to thermal oxidation treatment to form an oxide film having characteristics similar to those of a thermal oxide film. It is an example of forming.

First, the natural oxide film on the surface of the silicon substrate is removed by a known method. Next, as pre-cleaning before forming an oxide film, when forming an oxide layer by chemical treatment on the substrate surface, by using ozone-added water as an oxidizing agent, while maintaining the characteristics of low density and low stress, for example, standard A thick oxide layer is formed as compared with the case where hydrogen peroxide solution is used as an oxidant as in the typical RCA cleaning.

Here, it is important that the thickness of the oxide layer is 1 nm or more. By forming an oxide layer having a sufficient thickness in this manner, the surface of the substrate is not affected by organic contaminants in the atmosphere.

In the present embodiment, by using ozone-added water obtained by adding 10 ppm of ozone to DIW,
An oxide layer having a thickness of about 1.2 nm is formed on the entire surface of the silicon substrate.

The natural oxide film generally means SiO _x formed on the surface of a substrate when the silicon substrate is left in the atmosphere or treated in a chemical solution containing an oxidizing agent (H ₂ O ₂ etc.).
It is a general term for the (1-2) films, but the detailed properties vary depending on the method of forming the film.

In particular, since there is a big difference in the forming method between the one left in the air and the one treated with the chemical solution, the former will be simply called a natural oxide film and the latter will be called a chemical oxide layer to distinguish them. . Therefore,
The oxide layer becomes a chemical oxide layer. The thickness of very thin oxide layers, such as native oxide films and chemical oxide layers, can be measured using X-ray photoelectron spectroscopy (XPS).

Chemical formed by ozone-added water
The oxide layer has a density lower than that of the thermal oxide film and is inferior in characteristics. Next, it is necessary to change this into a state similar to that of a thermal oxide film having excellent characteristics.

In order to do this, in this embodiment, the substrate temperature is raised while the atmosphere is kept oxidative. Specifically, the substrate after the ozone water treatment is heated from near room temperature to 600 ° C. in an atmosphere of O ₂ : N ₂ = 1: 9 and held for 30 minutes. In this step, the chemical oxide layer is thermally deteriorated to have both high density and high stress, and exhibits properties similar to those of a normal thermal oxide film.

Such changes in properties can be directly confirmed by measuring the respective physical quantities.
First, regarding the density, for example, “(Ohkubo e
t al. , 1995 Symposium on VL
As disclosed in SI Technology, p111 ", it can be measured using the grazing incidence X-ray reflection method (GIXR), which shows that the density of the chemical oxide is 2.3 in the thermal oxide film. It is about 5% smaller than.

On the other hand, with respect to the stress, a stress of about 1 × 10 ⁸ N / m ² or more can be measured by a method of observing the warp deformation of a usual substrate. In the case of an ordinary thermal oxide film, this stress always takes a value of 2 × 10 ⁸ N / m ² or more, so that it is possible to distinguish between the cases.

Through the steps described above, an oxide film having the same properties as the thermal oxide film with a thickness of about 1 nm or more is rapidly formed on the surface of the substrate. The resulting irregularities in the interface of the oxide film / substrate during thermal oxidation in the case of the conventional methods, a thin phase thickness of the oxidized film, an oxide film 10 ⁸ N
This is because it has a relatively large stress on the order of / cm ² and thus induces migration of substrate atoms near the substrate surface.

On the other hand, in the method of the present embodiment, at the initial stage of the formation of the oxide film, a chemical oxide layer that is relatively thick and difficult to deform is formed in a short time while the stress is small. Almost no migration occurs, and as a result, the generation of irregularities at the oxide film / substrate interface is suppressed.

After that, the silicon substrate is heated to a predetermined oxidation temperature to form an oxide film having a predetermined thickness. Here, oxidation was performed at 900 ° C. in the same oxidizing atmosphere as described above to form an oxide film having a thickness of about 7 nm.

As described above, according to the present embodiment, the oxide film is not directly formed, but a thick chemical oxide layer with low stress is formed, and then the oxide film is formed. Contamination of the substrate surface that causes deterioration,
Since it is possible to prevent the occurrence of irregularities at the oxide film / substrate interface, it becomes possible to form a high-quality oxide film. (Second Embodiment) The difference between this embodiment and the first embodiment is that in the gas phase containing ozone or oxygen radicals,
Forming the formation of the initial oxide layer.

Specifically, oxygen radicals are generated by discharge with an electric power of about 100 W in oxygen at a pressure of 5 to 10 Torr and the substrate surface is irradiated with the oxygen radicals, so that the thickness is about 1.1 to 1.2 nm. Forming an initial oxide layer.
By such a method, it is possible to form an initial oxide layer having a density and a stress smaller than those of the thermal oxide film. The steps after forming the initial oxide layer are the same as those in the first embodiment. (Third Embodiment) In this embodiment, the oxide film process of the first or second embodiment is continuously performed, that is, the semiconductor substrate is not exposed to the outside air during the oxide film process.

As hardware for carrying out such a continuous process, an apparatus of a type for performing a plurality of steps in a single processing chamber, or an apparatus of a type in which a plurality of processing chambers are connected in a load-lock system is used. Although possible, this embodiment will explain a case where the former device is used.

First, a silicon substrate is introduced into a processing chamber, and then anhydrous hydrogen fluoride (AHF) gas is introduced while keeping the temperature of the silicon substrate at about room temperature to remove the natural oxide film on the surface of the substrate. .

As the gas used for removing the natural oxide film, a mixed gas of AHF and water vapor can be used in addition to the AHF gas. Next, the inside of the processing chamber is once replaced with an inert gas such as nitrogen, then replaced with an oxidizing atmosphere containing oxygen, and the silicon substrate is gradually heated to about 200 ° C. to remove the fluorine remaining on the substrate surface.

Next, oxygen radicals are introduced into the processing chamber, the temperature of the semiconductor substrate is further raised to about 900 ° C., and an initial oxide layer having a thickness of about 1 to 1.2 nm is formed on the semiconductor substrate. Next, while keeping the temperature of the silicon substrate at the above temperature,
By replacing the inside of the processing chamber with a normal oxidizing atmosphere containing oxygen,
An oxide film having a predetermined thickness is formed on the surface of the substrate.

By thus forming the initial oxide layer, which is the initial oxide layer, in the vapor phase, the oxide film forming process can be continuously performed under the condition that the silicon substrate is shielded from the outside air. Like

As a result, the substrate is not exposed to the outside air during the steps from the cleaning of the surface of the substrate to the formation of the oxide film, so that it is not affected by contamination such as organic contaminants contained in the outside air. A clean oxide film process is possible. Therefore, the effect of contamination is removed in addition to the suppression of interface irregularities, and as a result, the reliability of the oxide film is further improved.

The intrinsic life of the oxide film formed according to the methods of the first to third embodiments increases with the film thickness of the initial oxide layer formed before the oxide film is formed. That is, FIG.
As shown in, the intrinsic life of a thin oxide film having a thickness of about 7 nm increases with the thickness of the initial oxide layer up to about 1 nm, and it is almost saturated beyond that.

Therefore, by setting the film thickness of the initial oxide layer formed before forming the oxide film to about 1 nm or more, the intrinsic life of the oxide film can be maximized. This corresponds to the fact that the unevenness at the interface between the oxide film and the semiconductor substrate was almost completely removed by forming the oxide film by the method of each of the above embodiments. FIG. 4 shows changes in the intrinsic life of an oxide film formed by the method of the present invention and the conventional method depending on the film thickness.

From this figure, it can be seen that the method of the present invention is effective in a thin film thickness region of less than 10 nm. The reason why the difference from the conventional method disappears when the thickness of the oxide film becomes thick is that the influence of the unevenness of the oxide film / substrate interface at the initial stage of oxidation disappears as the oxidation progresses.

An important result directly obtained by eliminating the unevenness of the oxide film / substrate interface as described above is the effect of improving the variation in the data writing characteristics of the nonvolatile memory using the oxide film as the tunnel insulating film. To be

The cell transistor of the non-volatile memory has, for example, the gate structure shown in FIG. 5, and by applying a high positive electric field to the control gate electrode 27, the cell transistor is formed in the floating gate electrode 25 on the tunnel oxide film 24. It aims to control the threshold voltage (V _th ) by accumulating electrons.

In FIG. 5, 21 is a p-type silicon substrate, 22 is an n-type source diffusion layer, 23 is an n-type drain diffusion layer, and 26 is a gate electrode insulating film. Therefore, the threshold voltage V _th at the time of electron accumulation is affected by the ease of current flow in the tunnel oxide film 24. If the characteristics of the tunnel oxide film 24 are different for each memory cell, the threshold voltage V _th The voltage V _th also shows the corresponding spread of distribution.

FIG. 6 shows the result of examining the V _th distribution of the threshold voltage of the cell transistor. From FIG. 6, the distribution of the threshold value V _{th in} writing data when the oxide film formed by the method of the present invention is used as the gate oxide film 24 is larger than that when the gate oxide film formed by the conventional method is used. It can be seen that the variation is small and the data writing characteristics are well aligned as a whole. This means that the characteristic distribution of the oxide film itself is uniform.

In the first to third embodiments, F
By adding a step of removing a metal such as e, Cr, or Cu that causes a random failure, a higher quality oxide film can be formed.

In this case, the above-mentioned process is performed before the structure which remains as the final oxide film is formed, that is, at least 1.
By performing before the step of forming the initial oxide layer having a thickness of about nm or more, it becomes possible to remarkably improve the intrinsic characteristics, particularly the intrinsic life, without impairing the effect of reducing accidental defects. (Fourth Embodiment) FIG. 7 is a schematic diagram showing a schematic structure of an oxide film forming apparatus used for forming an oxide film by the method of the present embodiment.

This oxide film forming apparatus is a vertical heat treatment apparatus of the hot wall type, and the heater 34 for heating the substrate can raise and lower the temperature of the silicon substrate 31 at a rate of about 50 to 100 ° C./minute. There is. The pressure inside the processing chamber 33 made of a quartz tube can be maintained at about 1 atm through an oxide film process by an exhaust system (not shown).

FIG. 8 is a diagram showing the relationship between the process time and the temperature of the silicon substrate 31 when forming an oxide film using the oxide film forming apparatus of FIG. In FIG. 8, the process of period I (substrate loading / purge) is the first of the present invention.
The process of period II (natural oxide film removal) corresponds to the second process of the present invention, and the process of period III (steam treatment / low temperature treatment) corresponds to the third process of the present invention. , And the process of period IV (heating) and the process of period V (oxidation)
Corresponds to the fourth step of the present invention. Further, the process of period VI indicates temperature lowering / purging, and the process of period VII indicates substrate unloading.

First, the temperature of the silicon substrate 21 is set to 200.degree.
After the silicon substrate 21 is brought into the processing chamber 33 in a state of being adjusted and is supported by the substrate supporting board 32, the processing chamber 33
To close the silicon substrate 21 from the outside air (period I).

In this state (substrate temperature 200 ° C.),
After replacing the inside of the processing chamber 33 with nitrogen gas, the AH
F gas is introduced into the processing chamber 33 to remove the natural oxide film on the substrate surface by etching (period II). A pipe heating heater 35 is provided in the AHF gas pipe.

Then, after the introduction of the AHF gas is stopped, steam (H ₂ O) and oxygen (O ₂ ) gases are introduced into the processing chamber 33 (period III). A pipe heating heater 35 is provided in the steam pipe, as in the case of the AHF gas pipe.

At this time, the water vapor is supplied after the concentration of the AHF gas in the processing chamber 33 is once lowered to some extent by nitrogen purging. During this time, the temperature of the semiconductor substrate 21 has risen to 300 ° C.

Since the processing chamber 33 has a finite volume, the AHF gas in the processing chamber 33 does not disappear immediately even by nitrogen purging or supply of water vapor, but it decreases over a period of about several minutes. During this period, the introduction of water vapor is already started, so that in the atmosphere in the processing chamber 33, the proportion of AHF gas decreases and the proportion of water vapor increases at the same time.

The reason why the oxygen gas is flowing together with the water vapor is
This is for increasing the total gas flow rate so that gas replacement can be performed quickly. This mixed gas is under the condition of etching the oxide film up to a certain point, but as the ratio of the AHF gas and the water vapor is reversed, the condition is rapidly changed to oxidize the silicon substrate 31, and the atmosphere in the processing chamber 33 is oxidized. It becomes a sexual atmosphere.

Further, the atmosphere containing water vapor simultaneously changes the silicon on the surface of the silicon substrate 31 which has been terminated by fluorine into a state terminated by hydrogen or a hydroxyl group. As a result, fluorine is efficiently removed from the surface of the substrate from which the natural oxide film has been removed, and the oxidation starts to progress again on the surface.

However, the temperature of the silicon substrate 31 is 300
The growth rate of the oxide film is limited due to the low temperature of ℃ and 1
The oxide film thickness formed within the processing time of about 0 minutes is 3 nm or less.

Then, the steam is switched to nitrogen gas to raise the temperature of the silicon substrate 31 to a predetermined oxidation temperature (period I
V). That is, in an atmosphere where oxygen is diluted with nitrogen,
The oxide film thickness formed before reaching a predetermined oxidation temperature is reduced.

When the temperature of the silicon substrate 31 reaches a predetermined temperature (1000 ° C. as shown in FIG. 8) which is higher than that of the process of the period II and the process of the period III, oxidation of a predetermined film thickness is performed in a predetermined oxidizing atmosphere. Form a film. In this embodiment,
As the oxidizing atmosphere, so-called wet oxidation in a mixed gas atmosphere of oxygen gas and water vapor is further diluted with nitrogen.

After the oxide film having a predetermined thickness is formed, the temperature of the silicon substrate 31 is lowered to about 500 ° C., and then the atmosphere in the processing chamber 33 is sufficiently purged (period V). After being discharged from 33 (period VI), the oxide film forming process is completed.

In order to compare the reliability of the oxide film formed according to the method of the present embodiment (the present invention) with that of the oxide film formed by the conventional method, the result of examining the TDDB life is shown in FIG.
It is. The time when 50% of the total is destroyed is shown.

As is apparent from the figure, the oxide film formed by the method of the present invention has a longer life than the film formed by the conventional method, especially when the film thickness is thinner. Correspondingly, the unevenness of the interface between the oxide film and the substrate formed at this time was observed by an atomic force microscope (AFM), and as a result, the maximum step difference in a 5 μm square visual field was It was confirmed that in the case of the method of the present invention, it was significantly improved to 1.1 nm, compared with 4.3 nm in FIG.

As described above, according to this embodiment, a highly reliable and high quality oxide film can be formed on the silicon substrate 31 even if hydrogen fluoride gas is used for removing the natural oxide film. .

Therefore, the reliability can be improved by using the oxide film thus formed as the gate oxide film of a normal MOS transistor. In addition, for example, the oxide film of the present embodiment is provided with EE to which a high electric field is applied.
If it is used as a tunnel oxide film of a PROM, the reliability can be improved and the number of times of data rewriting can be increased, and the performance can be improved.

The present embodiment can be variously modified as follows. For example, as mentioned in the action section, period II
Water vapor may be supplied into the processing chamber 33 in the step of. Further, in the step of the period III, a gas containing oxygen radicals or ozone can be used as the oxidizing gas, and if the function of removing the fluorine terminating the Si atoms on the substrate surface is satisfied, the water vapor It is also possible to limit the introduction of to only the first half of the same stage. As described above, various oxidation methods can be performed as necessary in the steps after the period IV.

As for the apparatus used, it is also possible to use a single-wafer type heating apparatus in addition to the batch type hot wall type apparatus described above. Due to the nature of the process,
The higher the temperature increase / decrease rate is, the better. RTP (Rapid Th
It is also conceivable to use a cold wall type device such as an ermal processor).

In this case, it is necessary that the inner wall of the processing chamber and the necessary parts of the piping are kept at a temperature at which water vapor does not condense, but it is not difficult to add such a function to a normal RTP apparatus. Of course, even the single wafer type can be used as it is if it is a hot wall type.

Further, after the oxide film is formed by the method of this embodiment, the semiconductor substrate 21 is not simply carried out of the processing chamber 23, but is connected to the same processing chamber 23 or to the processing chamber 23 by a load lock. In the processing chamber, a step of forming another film, for example, a polycrystalline semiconductor film for a gate electrode, on the oxide film may be continued.

The present invention is not limited to the above embodiment. For example, the natural oxide film removing step of the first to third embodiments may be replaced with that of the fourth embodiment. In addition, without departing from the gist of the present invention,
Various modifications can be made.

[0126]

As described in detail above, the present invention (claim 1,
According to claim 2), the improvement of the intrinsic characteristics makes it possible to obtain a high quality oxide film. Further, according to the present invention (Claims 3 and 4), even if a reactive gas containing fluorine is used for removing the natural oxide film, the generation of residual fluorine that causes an increase in the interface state density is prevented. Since this can be prevented, a high quality oxide film can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing how unevenness is generated at a substrate / oxide film interface due to attachment of contaminants such as organic substances.

FIG. 2 is a schematic diagram showing how unevenness occurs at the substrate / oxide film interface due to stress during initial oxidation.

FIG. 3 is a characteristic diagram showing the relationship between the intrinsic fracture life and the chemical oxide layer (initial oxide layer).

FIG. 4 is a characteristic diagram showing the relationship between intrinsic breakdown life and oxide film.

FIG. 5 is a sectional view of a cell transistor of a nonvolatile memory.

FIG. 6 is a diagram showing a threshold voltage distribution at the time of writing data in a cell transistor of a nonvolatile memory.

FIG. 7 is a schematic diagram showing a schematic configuration of an oxide film forming apparatus used in a fourth embodiment of the present invention.

FIG. 8 is a diagram showing a relationship between substrate temperature and process time.

FIG. 9 is a graph showing the dependence of the amount of passing charges on the oxide film, which shows that the TDDB life of the oxide film according to the present invention is longer than that of the conventional one.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Silicon substrate 2 ... Chemical oxide layer 3 ... Organic contaminants 4 ... Thermal oxide film 11 ... Silicon substrate 12 ... Thermal oxide film in the initial stage of oxidation 13 ... Oxide film of a predetermined thickness 21 ... P-type silicon substrate 22 ... N-type Source diffusion layer 23 ... N-type drain diffusion layer 24 ... Tunnel oxide film 25 ... Floating gate electrode 26 ... Gate electrode insulating film 27 ... Control gate electrode 31 ... Silicon substrate 32 ... Substrate supporting board 33 ... Processing chamber 34 ... Substrate heating Heater 35 ... Pipe heating heater

Claims (4)

[Claims] 1. A first step of removing a natural oxide film on a surface of a semiconductor substrate, and a low density and a stress of less than 1 × 10 ⁸ N / m ^{2 on} the surface of the semiconductor substrate from which the natural oxide film is removed. A second step of forming an oxide layer having a thickness of 1 nm or more; and a step of densifying the oxide layer by heat treatment in an oxidizing atmosphere to remove the native oxide film on the surface of the second step. And a step of forming an oxide film having a density higher than that of the oxide layer. 2. The method of manufacturing a semiconductor device according to claim 1, wherein the oxide layer is formed of water containing ozone, a gas containing ozone, or a gas containing oxygen radicals. 3. A first step of introducing a semiconductor substrate into a processing chamber to substantially shield the semiconductor substrate from outside air; and a temperature of the semiconductor substrate kept at 400 ° C. or lower,
A second step of introducing a gas containing fluorine into the processing chamber to remove a natural oxide film on the surface of the semiconductor substrate; and a temperature of the semiconductor substrate which is equal to or higher than a temperature at which dew condensation does not occur.
A third step of introducing an oxidizing gas containing water vapor into the processing chamber to maintain an oxidizing atmosphere in the processing chamber while maintaining the temperature at 0 ° C. or lower; And a fourth step of forming an oxide film on the surface of the semiconductor substrate by making the temperature of the semiconductor substrate higher than that in the third step. 4. The pressure in the processing chamber is reduced to 1/10 through all of the first, second, third and fourth steps.
4. The method for manufacturing a semiconductor device according to claim 3, wherein the atmospheric pressure is maintained.