Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – By reaction with substrate
Reexamination Certificate
1998-07-02
2001-07-24
Lee, Eddie C. (Department: 2815)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
By reaction with substrate
C438S770000, C438S931000, C257S077000
Reexamination Certificate
active
06265326
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for forming a silicon dioxide film by thermal oxidation in the manufacture of a silicon carbide semiconductor device using silicon carbide as a semiconductor material.
BACKGROUND OF THE INVENTION
Silicon carbide (which will be referred to as SiC) has a wide band gap, and its maximum breakdown electric field is larger than that of silicon by one order of magnitude. Thus, SiC has been highly expected to be used as a material for power semiconductor devices in the next generation. As high-quality alpha-phase single crystals, such as 6H-SiC and 4H-SiC, have been increasingly manufactured, various semiconductor devices, such as Shottky diode, MOS field-effect transistor (MOSFET), and thyristor, using SiC as a semiconductor material have been fabricated and tested, and it has been confirmed that these devices exhibit far more excellent characteristics than known devices using silicon.
When SiC is exposed to an oxidizing atmosphere (for example, dry oxygen or water vapor) at a high temperature of 1000° C. to 1200° C., a silicon dioxide film (hereinafter referred to as SiO
2
film) is grown on the surface of SiC, as in the case of silicon. It is also known that the SiO
2
film thus formed provides a desirable interface between an insulating film and a semiconductor substrate. This characteristic is peculiar to SiC, and cannot be observed in other compound semiconductor materials. Since this characteristic may be advantageously utilized to relatively easily produce MOSFET, SiC is expected to be used in a wide range of applications in the future.
Various characteristics have been revealed with respect to growth of SiO
2
film on SiC by thermal oxidation. For example,
FIG. 3
is a graph showing the temperature dependency of the growth rate of SiO
2
film on SiC when it was exposed to an atmosphere composed of water vapor in a test conducted by M. R. Melloch and J. A. Copper (MRS Bulletin, March 1997, p.42). For comparison, the graph of
FIG. 3
also shows the grown rate of oxide film on silicon. Other reports relating to the growth of Si
2
film on SiC have been published in K. Undo and Y. Seki: “Silicon Carbide and Related Materials 1995” IOP publishing p.629, and A. Golz, G. Horstmann, E. Stein von Kamienski and H. Kurz: “Silicon Carbide and Related Materials 1995” IOP publishing p.633.
As shown in
FIG. 3
, the growth rate of SiO
2
film on SiC is dependent upon the crystal orientation. Namely, the growth rate in (0001) silicon face (hereinafter referred to as Si-face) is considerably smaller than the growth rate in (000-1) carbon face (hereinafter referred to as C-face). In view of this, it may be considered to use C-face as plane orientation when fabricating SiC semiconductor devices. In fact, the interface state density of the interface between SiO
2
film and SiC when C-face is used for plane orientation is far higher than that in the case where Si face is used for plane orientation, and thus it will be understood that the use of C-face for plane orientation is not appropriate, in particular, in the manufacture of MOS type SiC semiconductor devices. Under these circumstances, Si-face has been generally used in recent development of SiC semiconductor devices.
The small oxidation rate or growth rate as described above means that the SiC substrate needs to be exposed to a high temperature for a long time in order to provide a sufficiently thick oxide film. For example, it is estimated from
FIG. 3
that the Si-face of the SiC substrate needs to be exposed to a high temperature of 1100° C. for about 17 hours, in order to provide an oxide film having a thickness of 100 nm.
If the semiconductor substrate is subjected to heat treatment at a high temperature for such a long time, defects may arise in the semiconductor substrate, or other problems may occur during the heat treatment. For this reason, the oxidation time is desired to be reduced to be as short as possible.
Among known methods of thermal oxidation of SiC semiconductor devices, there has been generally employed a so-called wet oxidation method in which pure water is heated so as to cause bubbling of oxygen. In this method, however, it is difficult to control the partial pressure of water vapor, and drops of water are undesirably introduced into the resulting film, which tends to cause a problem of contamination.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method for forming a thermal oxide film of a silicon carbide semiconductor device, wherein a clean, thick oxide film can be formed in a short time, in particular, in Si-face of a silicon carbide substrate.
To accomplish the above object, the present invention provide a method for forming a thermal oxide film, in which water vapor and oxygen are introduced to grow a silicon dioxide film on a surface of SiC that is heated, wherein the partial pressure of water vapor represented by p(H
2
O) /[p(H
2
O)+p(O
2
)] is controlled to within the range of 0.1 to 0.9, where p(H
2
O) and p(O
2
) represent vapor pressures of water vapor and oxygen, respectively.
Although the mechanism as to how the thickness of the silicon dioxide film is influenced by the partial pressure of water vapor has not been known nor revealed, the oxidation rate or speed is increased if the partial pressure of water vapor is controlled to within the above range, as compared with the case where the partial pressure is controlled to 1.0, namely, where the oxidizing atmosphere consists solely of water vapor, whereby a SiO
2
film having a large thickness can be easily obtained.
If the partial pressure of water vapor is controlled to within the range of 0.1 to 0.4, in particular, the oxidation rate is almost doubled as compared with the case where the partial pressure is controlled to 1.0, namely, where the atmosphere consists solely of water vapor. In other words, the oxidation time required for growing a SiO
2
film having a certain thickness can be reduced to one half, compared to the oxidation time required for achieving the same thickness when the partial pressure is controlled to 1.0.
In a method for forming a thermal oxide film of a silicon carbide semiconductor device in which hydrogen and oxygen are introduced to grow a SiO
2
film on a heated SiC substrate by pyrogenic oxidation, the ratio of the flow rate of hydrogen to that of oxygen is controlled to within the range of 1:0.55 to 1:9.5. In this case, the oxidizing atmosphere in a furnace in which the surface of silicon carbide is oxidized has a partial pressure of water vapor which is controlled to within the range of 0.1 to 0.95.
If the flow rate of hydrogen to that of oxygen is controlled to within the range of 1:2 to 1:9.5, the partial pressure of water vapor contained in an oxidizing atmosphere in a furnace can be controlled to within the range of 0.1 to 0.4.
In a method for forming a thermal oxide film of a silicon carbide semiconductor device in which hydrogen and oxygen are introduced to grow a silicon dioxide film on a heated SiC substrate by pyrogenic oxidation, an oxide film providing a large proportion of the final thickness of the silicon dioxide film is initially formed while the ratio of the flow rate of hydrogen to that of oxygen is controlled to about 1:4.5, and an additional oxide film having the remaining thickness is then formed while the ratio of the flow rate of hydrogen to that of oxygen is controlled to about 1:0.55.
The inventor of the present invention have found that the interface state density can be reduced as the hydrogen flow rate is increased relative to the oxygen flow rate. In this case, a large portion of a silicon dioxide film having most of the final film thickness can be formed at a high oxidation rate within a reduced oxidation time, and the remaining portion of the silicon dioxide film can be formed under a condition that provides a low interface state density, whereby the interface state density of the resulting silicon dioxide film can be advantageously lowered.
REFERENCES:
patent: 5648282
Diaz José R.
Fuji Electric & Co., Ltd.
Lee Eddie C.
Rossi & Associates
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