JPH1174263A - Thermal oxide film formation of silicon carbide semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an SiO2 film which is 10% or more thicker than that conditions considered to have been the fastest oxidation velocity in a conventional method in an Si surface by setting a vapor partial pressure in a mixed gas of vapor and oxygen at a value in a specific range, when forming a thermal oxide film of a silicon carbide semiconductor device. SOLUTION: When a silicon oxide film is formed on a heated SiC surface by introducing vapor and oxygen, a vapor partial pressure p(H2 O)/[p(H2 O)+pO2 ] is controlled within the range of 0.1 to 0.9. Here, p(H2 O), p(O2 ) express the vapor pressures of vapor and oxygen, respectively. In a thermal oxide film formation method for forming an SiO2 film by pyrogenic oxidation for performing thermal oxidation by introducing hydrogen and oxygen, the flow ratio of hydrogen and oxygen is controlled in the range of 1:0.6 to 1:9.5. Accordingly, a partial pressure of vapor is in the range of 0.1 to 0.9 as oxidation atmosphere in a furmace.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming a silicon oxide film by thermal oxidation of a silicon carbide semiconductor device using silicon carbide.

[0002]

2. Description of the Related Art Silicon carbide (hereinafter referred to as SiC) has a wide band gap and a maximum insulating electric field which is one order of magnitude larger than that of silicon, so that it can be applied to next-generation power semiconductor devices and the like. It is a promising material. And 6
High quality single crystals of alpha phase such as H-SiC and 4H-SiC have been manufactured, and Schottky diodes and MOS field-effect transistors (MOSF
Semiconductor devices such as ET) and thyristors have been prototyped, and it has been confirmed from their characteristics that the characteristics are much better than those of conventional silicon.

[0003] Like silicon, SiC is used in an oxidizing atmosphere (eg, dry oxygen, water vapor, etc.) in a high temperature (10
When the substrate is exposed to (00 ° C. to 1200 ° C.), a silicon oxide film (hereinafter referred to as SiO ₂ film) grows on the surface. Moreover, it is known that an SiO ₂ film having a good insulating film-semiconductor interface can be obtained. Such physical properties are unique to compound semiconductors, and MOSFETs can be manufactured relatively easily by utilizing these properties, so that widespread application in the future is expected.

Various properties have been clarified for the growth of a SiO ₂ film on SiC by thermal oxidation. For example, FIG. 3 is a diagram showing the temperature dependence of the growth rate of a SiO ₂ film of SiC in a water vapor atmosphere by MRMelloch and JACooper (MRS Bulletin, March 1997, p.4).
2). For comparison, a silicon oxide film growth rate is also shown. K. Ueno and Y. Seki: "Silicon Carbide
and Related Materials 1995 "IOP publishing p.629
A. Golz, G. Horstmann, E. Stein von Kamienski and
H. Kurz: "Silicon Carbide and Related Materials 19
95 "Report on the growth of the SiO ₂ film on SiC also IOP publishing p.633 have been made.

As shown in FIG. 3, SiO ₂ on SiC
The growth rate of the film depends on the crystal orientation.
The silicon surface (hereinafter referred to as Si surface) has a feature that the growth rate is very low as compared with the (000-1) carbon surface (hereinafter referred to as C surface). From this, Si using the C plane
It is conceivable to prototype a C semiconductor device. However, actually, the C plane has a much higher interface state density at the interface between the SiO ₂ film and the SiC than the Si plane.
It was found that it was not suitable for an iC semiconductor device. Under such circumstances, recent development of SiC semiconductor devices requires S
The use of i-plane has become mainstream.

[0006]

The fact that the oxidation rate is low means that it is necessary to expose the substrate to a high temperature for a long time in order to obtain a thick oxide film. For example, from FIG.
It is estimated that about 17 hours are required to obtain an oxide film of m.

Since such a high-temperature and long-time heat treatment may cause various kinds of troubles such as introduction of defects into the semiconductor substrate, it is desirable that the oxidation time be as short as possible. Conventionally, a so-called wet oxidation method of heating pure water and bubbling oxygen has been generally used for thermal oxidation of a SiC semiconductor device. However, it is difficult to control the partial pressure of water vapor by the method, and There is a problem that water droplets are entrained during bubbling and contamination is likely to occur.

In view of such circumstances, an object of the present invention is to provide a method for forming a thermal oxide film of a silicon carbide semiconductor device which can form a clean and thick oxide film in a short time, especially on a Si surface.

[0009]

According to the present invention, there is provided a thermal oxide film forming method for growing a silicon oxide film by introducing water vapor and oxygen on a heated SiC surface. The partial pressure p (H ₂ O) / [p (H ₂ O) + p (O ₂ )] is controlled in the range of 0.1 to 0.9. Where p
(H ₂ O) and p (O ₂ ) indicate the vapor pressures of water vapor and oxygen, respectively.

Although the details of the mechanism explaining the effect of the partial pressure of water vapor are unknown, as shown in the experimental results described below, when the partial pressure of water vapor is controlled within the above range, the partial pressure becomes 1.0, that is, 100% of water vapor. %, The oxidation rate is increased, and a thick SiO ₂ film can be easily obtained. In particular, when the partial pressure of water vapor is controlled in the range of 0.1 to 0.4, the partial pressure becomes 1.
Oxidation rate is almost doubled as compared to the case of 0, that is, 100% of steam. Conversely, the oxidation time required to grow a SiO ₂ film of a certain thickness can be reduced by half.

In a thermal oxide film forming method for growing a SiO ₂ film by pyrogenic oxidation in which hydrogen and oxygen are introduced to thermally oxidize, a flow ratio of hydrogen to oxygen is set to 1: 0.
Control is performed in the range of 6-1 to 9.5. By doing so, the partial pressure of water vapor can be set in the range of 0.1 to 0.9 as the oxidizing atmosphere in the furnace.

In addition, the flow ratio of hydrogen to oxygen is 1: 2
If the control is performed in the range of 1: 9.5, the partial pressure of water vapor can be set in the range of 0.1 to 0.4 as the oxidizing atmosphere in the furnace. In a thermal oxide film forming method for growing a silicon oxide film by pyrogenic oxidation in which hydrogen and oxygen are introduced and thermally oxidized, a flow ratio of hydrogen to oxygen is set to about 1:
After forming most of the film thickness at 4.5, the remaining film thickness may be formed at a flow rate ratio of hydrogen to oxygen of approximately 1: 0.55.

The inventor has found that the interface state density can be reduced as the hydrogen flow rate is higher than the oxygen flow rate. Therefore, by doing so, the remaining film thickness is formed under the condition that an oxide film having a low interface state density can be formed while forming the majority of the film thickness under the condition of high oxidation rate and shortening the oxidation time. ,
A low interface state density can be realized.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a method for forming a thermal oxide film of a silicon carbide semiconductor device according to the present invention described below, pyrogenetic oxidation is performed. In this method, gaseous hydrogen and oxygen are introduced and reacted to form water, thereby obtaining a wet atmosphere. Since this method uses gas as a supply source, the level of contamination can be kept very low, and the partial pressure of water vapor can be finely controlled.

The experimental results will be described below with reference to the drawings, and the details of the thermal oxide film forming method of the present invention will be described. [Preliminary Experiment] A thermal oxidation experiment of silicon was performed in an atmosphere in which the partial pressure of water vapor was changed by changing the flow rates of hydrogen and oxygen to be introduced. For example, in pyrogenic oxidation, the flow ratio of hydrogen to oxygen is 1: 1.5, 1: 1,
1: 0.7, and the partial pressure of water vapor was 0.5, 0.
67, 0.83 atmosphere.

FIG. 4 is a diagram showing the partial pressure dependence of the thickness of the oxide film in the thermal oxidation of silicon. Thermal oxidation conditions are:
The temperature was set to 1000 ° C. for 160 minutes. The horizontal axis is the partial pressure of water vapor in the atmosphere. The total pressure is one atmosphere and the remainder of the water vapor is oxygen. From this figure, it is understood that the oxide film thickness increases as the ratio of water vapor increases. This indicates that the oxide film thickness increases as the atmosphere approaches from dry oxygen to wet oxygen. Therefore, the maximum oxidation rate is
It is expected that the partial pressure of water vapor will be obtained in an atmosphere of 1.0, that is, 100% water vapor, and it has been confirmed in another experiment.

[Experiment 1] It has been thought that the experimental result on silicon, in which the oxide film thickness increases as the water vapor partial pressure in the dry oxygen to wet oxygen atmosphere increases, will be the same in SiC. There was no experimental data to prove it. In order to confirm this, the inventor conducted an experiment in which the partial pressure of water vapor was changed by changing the flow ratio of hydrogen and oxygen by pyrogenic oxidation, as in the preliminary experiment in SiC, and found that silicon was completely different from silicon. It has been found that it has different characteristics. The experimental conditions were such that the hydrogen flow rate was 8 liters / min and the oxygen flow rate was varied. However, where the oxygen flow rate becomes too large, the hydrogen flow rate was set to 4 liter / min.

As a sample, p-type SiC having a carrier concentration of 1 × 10 ¹⁶ cm ^-3 and doped with Al (0001) Si was used. Thermal oxidation conditions are 1100 ° C. for 5 hours. FIG. 1 is a diagram showing the relationship between the partial pressure of water vapor and the SiO ₂ film thickness. The horizontal axis is the partial pressure of water vapor, p (H ₂ O) / [p
(H ₂ O) + p (O ₂ )]. It is believed that the hydrogen and oxygen at half the hydrogen flow rate react to form all water vapor, with the remaining oxygen remaining as a gas. The SiO ₂ film thickness was measured with an ellipsometer.

As can be seen from this figure, the SiO ₂ film thickness shows a peak when the partial pressure of water vapor is around 0.2. In the pyrogenic oxidation method in which hydrogen and oxygen are introduced, the condition of a partial pressure of water vapor of 1.0, that is, 100% of water vapor is dangerous and cannot be realized. However, by analogy with the tendency in the vicinity thereof, it is considered to be about 25 nm. Therefore, under the condition that the partial pressure of steam is about 0.2, the steam 100% or more is 1.5 times or more as compared with the case of the experimental maximum steam partial pressure of 0.95.
%, It is possible to obtain a thick oxide film which is almost twice as large as that in the case of%. And the partial pressure of water vapor is 0.1-0.
In the range of 9, compared with the case of the partial pressure of water vapor of 0.95,
It obtained 20% or more thick SiO ₂ film, in particular, in the range of the water vapor partial pressure from 0.1 to 0.4, 50% or more thick SiO ₂
It can be seen that a film is obtained.

The condition of low water vapor partial pressure is better for thick SiO 2
The mechanism and physical meaning of this phenomenon of growing _two films is under investigation and is unknown at this time. This result has other implications. Until now, formation of a SiO ₂ film on SiC has been generally performed by bubbling oxygen in boiling pure water and introducing wet oxygen into an electric furnace. This conventional method has been frequently used as a simple method since no special equipment is required. However, as can be seen from the above experimental results, it is difficult to control the partial pressure of steam with such a simple method,
Predict that it is difficult to precisely control the thickness of the iO ₂ film.
That is, when the partial pressure of water vapor changes, the growth rate of the SiO ₂ film changes, and as a result, the SiO ₂ film thickness also fluctuates. Therefore, a uniform and controlled thickness of SiO ₂
It can be seen that controlling the partial pressure of water vapor is extremely important for obtaining a film.

[Experiment 2] FIG. 2 is a graph showing the relationship between the oxidation time at different water vapor partial pressures and the SiO ₂ film thickness. This is an example where the oxidation temperature is 1100 ° C., and the horizontal axis is time. For example, in order to obtain a 50 nm SiO ₂ film, a water vapor partial pressure of 0.9 with a flow ratio of hydrogen to oxygen of 1: 0.55 is used.
5 atmosphere requires about 10 hours, but when the flow rate ratio between hydrogen and oxygen is 1: 3.5 and the partial pressure of water vapor is reduced to 0.25, the same oxidation time is required for about 5 hours and almost half the oxidation time. It can be seen that a film thickness can be obtained.

All experimental values are linear, about 15 n
m suggests a steep initial oxidation stage. This looks similar to what is seen during the initial oxidation stage in dry oxidation of silicon. [Experiment 3] The inventors have found that the higher the ratio of the hydrogen flow rate to the oxygen flow rate, the lower the interface state density. Therefore, it is rather unfavorable to form the SiO ₂ film under a condition of a low water vapor partial pressure in order to increase the oxidation rate, in terms of the interface state density.

For example, in FIG. 1, the interface state density of the SiO ₂ film formed under the condition that the steam partial pressure is 0.2 at which the oxidation rate is close to the maximum is smaller than that of the SiO ₂ film formed on the C plane of SiC. Although the difference is small, the water vapor partial pressure is twice or more that of the SiO ₂ film formed under the condition of 0.95. Therefore, after forming a SiO ₂ film for 4 hours under the condition that the water vapor partial pressure is 0.2, the water vapor partial pressure is set to 0.95, and the Si ₂ film is further formed for 1 hour.
An O ₂ film was formed. The thickness of the SiO ₂ film is 48 nm,
30 when oxidizing for 5 hours at a steam partial pressure of 0.95
It was much thicker than nm and the interface level density was equivalent to that of a SiO ₂ film with a water vapor partial pressure of 0.95.

As described above, after most of the film thickness is formed under the condition that the steam partial pressure is low, that is, the oxidation rate is high, the film is formed under the condition that the steam partial pressure is high, that is, the interface state density becomes low.
Further by forming a SiO ₂ film, while shortening the oxidation time, it is possible to realize a low SiO ₂ film interfacial凖位density. As a specific condition, the partial pressure of water vapor is approximately 0.2, that is, the flow ratio of hydrogen to oxygen is about 1: 4.5.
After forming a SiO ₂ film, the partial pressure of water vapor was reduced to approximately 0.
95, that is, the flow ratio of hydrogen to oxygen is preferably about 1: 0.55, and an SiO ₂ film is preferably formed.

The condition of a partial pressure of water vapor of 1.0, that is, 100% of water vapor is dangerous and cannot be realized by the pyrogenic oxidation method as described above, but can be realized by a bubbling method of boiling pure water. The subsequent oxidation may be performed by a bubbling method. In the above experiments, pyrogenetic oxidation was performed in which the control of the partial pressure ratio of water vapor was easy, but the essence of the present invention is to control the partial pressure of water vapor to perform thermal oxidation. Needless to say, pyrogenic oxidation is not necessarily required.

[0026]

As described above, according to the present invention, in the method of forming a thermal oxide film for a silicon carbide semiconductor device, the partial pressure of water vapor in a mixture of water vapor and oxygen is set to 0.1 to 1.0.
By setting the ratio to 0.9, the condition that the highest oxidation rate was conventionally considered to be obtained even on the Si surface was obtained.
A SiO ₂ film as thick as 10% or more and as large as 50% can be obtained. Conversely, to obtain a SiO ₂ film of a certain thickness, it can be formed in a short oxidation time. In addition, a low interface state density can be realized.

The SiO ₂ film is an important constituent element of a MOS semiconductor device in particular, and the fact that the present invention can provide a SiO ₂ film having a low interface state density in a short time requires the silicon carbide MOS semiconductor device. It is a place that contributes to the practical application of.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a relationship between an oxide film thickness on a Si surface of SiC and a partial pressure of water vapor.

FIG. 2 is a characteristic diagram showing a relationship between an oxidation time and an oxide film thickness on a Si surface of SiC.

FIG. 3 is a characteristic diagram showing a relationship between an oxidation time of SiC and silicon and an oxide film thickness.

FIG. 4 is a characteristic diagram showing a relationship between an oxide film thickness and a water vapor partial pressure in silicon.

Claims (6)

[Claims] 1. A method for forming a thermal oxide film by introducing water vapor and oxygen on a heated silicon carbide surface to grow a silicon oxide film, wherein the partial pressure of water vapor is 0.1 to 0.9. A method for forming a thermal oxide film for a silicon carbide semiconductor device, wherein the thermal oxide film is controlled within a range. 2. A thermal oxide film forming method for growing a silicon oxide film by introducing water vapor and oxygen on a heated silicon carbide surface, wherein the partial pressure of the water vapor is 0.1 to 0.4. A method for forming a thermal oxide film for a silicon carbide semiconductor device, wherein the thermal oxide film is controlled within a range. 3. A thermal oxide film forming method for growing a silicon oxide film by pyrogenic oxidation in which hydrogen and oxygen are introduced and thermally oxidized, wherein a flow ratio of hydrogen to oxygen is 1: 0.
2. The method for forming a thermal oxide film on a silicon carbide semiconductor device according to claim 1, wherein the control is performed in the range of 6-1 to 9.5. 4. A thermal oxide film forming method for growing a silicon oxide film by pyrogenic oxidation in which hydrogen and oxygen are introduced and thermally oxidized, wherein the flow ratio of hydrogen and oxygen is 1: 2 to 2.
3. The method according to claim 2, wherein the control is performed in a range of 1: 9.5.
A method for forming a thermal oxide film for a silicon carbide semiconductor device according to the above. 5. A thermal oxide film forming method for growing a silicon oxide film by introducing water vapor and oxygen on a heated silicon carbide surface, wherein an oxide film is formed under a condition of a low partial pressure of water vapor. Thereafter, a method of forming a thermal oxide film for a silicon carbide semiconductor device, further comprising forming an oxide film under a high partial pressure of water vapor. 6. A thermal oxide film forming method for growing a silicon oxide film by pyrogenic oxidation in which hydrogen and oxygen are introduced and thermally oxidized, wherein a flow ratio of hydrogen to oxygen is substantially 1: 1.
A thermal oxide film formation of a silicon carbide semiconductor device, wherein most of the film thickness is formed at 4.5 and the remaining film thickness is formed at a flow rate ratio of hydrogen to oxygen of about 1: 0.55. Method.