JP2002175997A - Manufacturing method of silicon carbide semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form an impurity diffusion region in an SiC substrate at a low cost, without generating damages on the surface of the substrate. SOLUTION: After introducing nitrogen ions 4 into the surface of a p-type SiC substrate 1, the nitrogen atoms present in an impurity-doped layer 6 are made to diffuse toward the rear surface of the SiC substrate 1, by increasing the number of the interstitial silicon atoms of the SiC substrate 1 by its thermal oxidation, so as to form an n-type impurity diffusion layer 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a silicon carbide (hereinafter referred to as "silicon carbide").
A semiconductor substrate (hereinafter referred to as SiC substrate)
The present invention relates to a method for manufacturing a SiC semiconductor device using
The present invention relates to a technique for forming an impurity diffusion layer on an iC substrate.

[0002]

2. Description of the Related Art Conventionally, SiC in a SiC semiconductor device has been developed.
As a technique for locally doping impurities into a substrate, for example, a technique described in JP-A-8-316164 is known.

[0003] In the above-mentioned prior art, after a layer doped with impurities is formed on the surface of a SiC substrate, the SiC substrate is removed from the surface by 5%.
The substrate is irradiated with hydrogen ions or γ-rays from the substrate surface side while heating to 00 ° C. to 1500 ° C. to diffuse impurities.

[0004]

As described above, in the above-described conventional method of diffusing impurities into a SiC substrate, the following method is used.
The impurity is diffused by irradiating hydrogen ions or γ-rays while heating the iC substrate to a predetermined high temperature.

Therefore, when hydrogen ion irradiation is performed, the substrate surface is damaged by the ion irradiation. When irradiating with γ-rays, a γ-ray irradiator is required. However, this γ-ray irradiator is not widely used and is expensive, so that the cost increases.

An object of the present invention is to provide a method of manufacturing a SiC semiconductor device capable of forming an impurity diffusion region on a SiC substrate at low cost without damaging the substrate surface.

[0007]

SUMMARY OF THE INVENTION According to the present invention, an impurity diffusion constant is increased by increasing the number of interstitial silicon atoms of an SiC substrate into which an impurity is introduced. Is formed.

That is, in the method of manufacturing a SiC semiconductor device according to the first aspect of the present invention, the step of introducing an impurity into the surface of a silicon carbide semiconductor substrate and the number of interstitial silicon atoms of the semiconductor substrate are increased. Forming an impurity diffusion layer by diffusing the impurity toward the back surface of the semiconductor substrate.

According to a second aspect of the present invention, a step of introducing a second conductivity type impurity into the surface of a first conductivity type silicon carbide semiconductor substrate, and increasing the number of interstitial silicon atoms of the semiconductor substrate. Forming a second conductivity type impurity diffusion layer by diffusing the impurities toward the back surface of the semiconductor substrate.

According to a third aspect of the present invention, the number of the interstitial silicon atoms is increased by thermally oxidizing the semiconductor substrate in an oxidizing atmosphere.

According to a fourth aspect of the present invention, the thermal oxidation is performed in a steam atmosphere.

According to a fifth aspect of the present invention, the thermal oxidation is performed in a steam atmosphere having a partial pressure of steam of 0.2 or more when the total pressure of the oxidizing atmosphere is set to 1.

According to a sixth aspect of the present invention, the impurity is nitrogen or boron.

[0014]

According to the method for manufacturing a SiC semiconductor device of the first aspect of the present invention, by increasing the number of interstitial silicon atoms of the SiC substrate, the diffusion constant of the impurity is increased, and the deep impurity diffusion layer is formed. Can be formed. Therefore, unlike the conventional method, the surface of the SiC substrate is not damaged by hydrogen ion irradiation, and there is no need to perform γ-ray irradiation. A diffusion region can be formed.

According to a second aspect of the present invention, an impurity diffusion layer of the second conductivity type is formed on the surface of the first conductivity type SiC substrate.
It can be formed at low cost without damaging the surface of the C substrate.

According to the third aspect of the present invention, by thermally oxidizing the SiC substrate in an oxidizing atmosphere, the impurity diffusion region can be formed at low cost without damaging the surface of the SiC substrate.

According to a fourth aspect of the present invention, the thermal oxidation is performed in a water vapor atmosphere, whereby the diffusion constant of the impurity is significantly increased, and the impurity diffusion region can be formed at low cost without damaging the surface of the SiC substrate. Can be formed.

According to the fifth aspect of the present invention, the thermal oxidation is performed in a steam atmosphere having a partial pressure of steam of 0.2 or more when the total pressure of the oxidizing atmosphere is set to 1, so that the diffusion constant of impurities is remarkably increased. The impurity diffusion region can be formed at low cost without increasing the size and causing damage to the surface of the SiC substrate.

Further, according to claim 6 of the present invention, the impurities are:
By using nitrogen or boron, the covalent radius of both elements is smaller than the covalent radius of SiC of 0.97, so that the diffusion constant of the impurity can be effectively increased.

[0020]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIGS. 1A to 1D are process sectional views showing an embodiment of a method for manufacturing a SiC semiconductor device according to the present invention. In this embodiment, a method of increasing the number of interstitial silicon atoms in a SiC substrate will be described using an example of a planar pn junction diode that forms a pn junction by utilizing thermal oxidation.

First, as shown in FIG. 1A, a p-type SiC epitaxial growth film having a carrier concentration of 2 × 10 ¹⁶ cm ^-3 is formed on a p-type SiC single crystal substrate 1 having a carrier concentration of 1 × 10 ¹⁸ cm ^-3. 2 is formed by a thermal CVD method,
An ion implantation mask 3 made of a silicon oxide film (SiO ₂ film) is formed by a low-temperature CVD method. Next, while heating the SiC substrate 1 to 800 ° C.,
0, 60, 100, 150 keV, dose 8 × 10
¹⁴ , 1.4 × 10 ¹⁵ , 2.8 × 10 ¹⁵ , 3.4 ×
10 ¹⁵ cm ⁻² , total dose 8.4 × 10 ¹⁵ cm
^Under the condition of ⁻² , nitrogen ions 4 are locally implanted through the mask 3. Reference numeral 5 denotes the implanted nitrogen ions.

Next, after removing the mask 3 with a buffered hydrofluoric acid aqueous solution, a heat treatment is performed at 1700 ° C. for 1 minute in an Ar atmosphere to activate the implanted nitrogen 5.
As shown in (B), an impurity doping layer 6 is formed.

Then, in an oxidizing atmosphere containing water vapor,
Thermal oxidation is performed at 1100 ° C. for 420 minutes to diffuse nitrogen in the impurity-doped layer 6 and, as shown in FIG.
Form impurity diffusion layer 7 is formed. Reference numeral 8 denotes a thermal oxide film formed by the thermal oxidation. The atmosphere at this time is
When the total pressure of the oxidizing atmosphere is 1, the partial pressure of water vapor is 0.2
It was above.

FIG. 2 shows a SIMS (Secondary Ion Mass Spe
ctroscopy: secondary ion mass spectrometry)
The depth distribution of elementary atoms is shown. Nitrogen in FIG. 1 (A)
Immediately after ion implantation and at 1700 ° C. for 1 minute in FIG.
The distribution of nitrogen atoms after heat treatment during
As described above, there is no change before and after the heat treatment, and the junction depth is about 0.1 mm.
5 μm. On the other hand, 11 in FIG.
Contact after thermal oxidation in a steam atmosphere at 00 ° C for 420 minutes
As shown in FIG. 2B, the joint depth is about 1.1.
μm. The diffusion constant obtained from this diffusion length is
About 2.0 × 10 ^-13cm / s. This value is
Approximately two orders of magnitude higher than the reported nitrogen diffusion coefficient in SiC
It is a large value (Sov. Tech. Phys. Lett., Vol. 8 (198
2) pp.120).

Next, after forming a photoresist film (not shown) on the thermal oxide film 8 and patterning the photoresist film for forming an anode electrode, the thermal oxide film 8 is formed with a buffered hydrofluoric acid aqueous solution. As shown in FIG. 1D, a contact opening 9 is formed in the thermal oxide film 8 by wet etching, an anode electrode 10 made of Ni is formed on the n-type impurity diffusion layer 7 side, and an Al electrode is formed on the p-type SiC substrate 1 side. A cathode electrode 11 made of
Heat treatment was performed at 0 ° C. for 2 minutes to complete a planar pn junction diode. Here, Al reacts with the surface of the SiC substrate 1 to form an alloy layer, and an ohmic contact is obtained.

As described above, in the present embodiment, the step of introducing impurities (nitrogen ions 4) into the surface of SiC substrate 1 (FIG. 1A) and the step of introducing impurities (implanted nitrogen ions 5) into the surface
By increasing the number of interstitial silicon atoms of the SiC substrate 1 into which impurities are introduced, the impurity doping layer 6 (FIG.
Step (B) of diffusing the impurities toward the back surface of SiC substrate 1 to form impurity diffusion layer 7 (FIG. 1 (C))
It is provided with.

As described in the above embodiment, according to the present invention, when the impurity-doped layer formed on the SiC substrate is thermally oxidized, for example, in an oxidizing atmosphere, the number of interstitial silicon atoms in the SiC substrate increases. I do. Thereby, the diffusion constant of the impurity increases, and a deep impurity diffusion layer can be formed. The reason why the diffusion constant of impurities increases when thermal oxidation is performed in an oxidizing atmosphere is as described below.

FIG. 3 is a view for explaining the reason why the diffusion constant of impurities increases when thermal oxidation is performed in an oxidizing atmosphere in the present invention.

In FIG. 3, 12 is a Si atom and 13 is C
An atom, 14 is a self-interstitial atom, 15 is a vacancy, 16 is a dopant / self-interstitial atom pair, and 17 is a dopant / vacancy pair.

A point defect existing in the crystal (here, FIG. 3
It is known that the self-interstitial atoms 14 and vacancies 15) form a strain field in the vicinity thereof and generate a Coulomb field by capturing electrons (ReV. Mod. Phys., Vol. 61 (1989)). ) p
p.289).

For this reason, the point defect is paired with the dopant (impurity), and attempts to lower the energy of the entire system by relaxing the strain field or shielding the Coulomb field. That is, as shown in FIG.
A dopant smaller than the covalent radius of C attracts self-interstitial atoms 14 to form dopant-self-interstitial atom pairs 16 in order to lower the energy of the system. On the other hand, a dopant having a covalent radius larger than the covalent radius of SiC attracts vacancies 15 to form dopant / vacancy pairs 17 in order to reduce mechanical strain energy.

Usually, the dopant substituted at the lattice position cannot move to another lattice position because it is strongly covalently bonded to an adjacent atom (Si atom or C atom in the case of SiC). . However, when a point defect exists around the dopant, the point defect tries to form a dopant-point defect pair, that is, a dopant-self-interstitial atom pair 16 and a dopant-vacancy pair 17, so that the dopant and the point defect Are replaced, and the dopant goes out of the original lattice position. That is, the dopant is in a state where it can be diffused in the crystal. Therefore, the diffusion constant D of the dopant is qualitatively given by the following equation (1).

D / D _eq = f _i (C _i / C _ieq ) + (1−f _i ) (C _v / C _veq ) (1) In the above formula (1), f _i depends on the kind of the dopant. With the determined constant, the dopant / self-interstitial atom pairs 16 or the dopant / vacancy pairs 17 are formed, and the ratio at which the dopant is diffused is shown. That is, in the case of a dopant forming a pair with the self-interstitials 14, the value of f _i is almost close to 1, and in the case of a dopant forming a pair with the vacancy 15, the value of f _i is almost close to zero. Become. C _i is the self-interstitial atomic density, C _v is the vacancy density, D _eq is the diffusion constant in the thermal equilibrium state, C _ieq is the self-interstitial atomic density in the thermal equilibrium state, and C _veq is the vacancy density in the thermal equilibrium state. .

[0035] As apparent from Equation (1), using dopant and self-interstitial density C _i, once the vacancy density C _v, uniquely diffusion constant D is determined.

In the case of SiC, since the bond between Si and C is strong, even in a temperature region used for thermal diffusion of Si, the self-interstitial atom density C _i and the vacancy density C _v are small, so that the dopant diffusion Constant D is small. When thermal oxidation is performed on the impurity-doped layer formed on the surface of the SiC substrate, the oxide film /
Interstitial Si atoms are generated at the SiC substrate interface, and SiC
It diffuses into the substrate toward the back surface of the substrate. As a result, Equation (1) self-interstitial density C _i is increased in the diffusion constant D of the dopant is increased. Therefore, in order to efficiently increase the diffusion rate of the dopant by oxidation, it is necessary to increase the diffusion constant D of the dopant per unit time during the oxidation time, that is, it is generated at the oxide film / SiC substrate interface per unit time. Interstitial Si
It is important how to increase the amount of atoms (the above claims 1 and 2).

In the case of an ordinary oxidation method by thermal oxidation in a dry oxygen atmosphere, the amount of interstitial Si atoms generated at the oxide film / SiC substrate interface per unit time is small because the growth rate of the oxide film of SiC is low. It is difficult to efficiently increase the diffusion rate of the dopant.

The inventor of the present invention has found that, when the total pressure of the oxidizing atmosphere containing water vapor is set to 1, if the thermal oxidation is performed in an oxidizing atmosphere having a water vapor partial pressure of 0.2 or more, the oxidation reaction is remarkably accelerated, A large amount of interstitial Si at the film / SiC substrate interface
Since atoms are generated, the number of interstitial Si atoms in the SiC substrate rapidly increases, the diffusion constant D of the dopant increases remarkably as compared with the case where the above-mentioned normal thermal oxidation is performed, and the diffusion rate of Si decreases. It has been found that it is possible to increase the efficiency efficiently (above, regarding claims 3, 4 and 5).

In the present invention, when nitrogen or boron is used as the dopant, for example, the covalent radius of both elements is smaller than the covalent radius of SiC of 0.97. The value of _i is almost 1, and therefore the right side of the above equation (1) is only the first term. That is, only the self-interstitial atomic density C _i affects the diffusion constant D of the dopant, and it is possible to more effectively increase the diffusion rate of the dopant (above, regarding claim 6).

Further, according to the present invention, in the impurity doping layer formed on the surface of the SiC substrate, the impurity is diffused only in the oxidized portion. By performing oxidation with protection by a nitride film or the like, a deep impurity diffusion layer can be selectively formed.

Further, since the diffusion length of the dopant can be controlled by the oxidation time, the conventional ion implantation at a high acceleration voltage is unnecessary, and a pn junction of about 1 μm is formed at a low acceleration voltage of about 150 keV. It is possible.

Further, since the diffusion constant D is increased by utilizing only the oxidation reaction, there is no need to perform hydrogen ion irradiation or γ-ray irradiation on the impurity-doped layer. Further, since an expensive γ-ray irradiator is not required, an impurity diffusion region can be formed on the SiC substrate at low cost.

Although the present invention has been specifically described based on the embodiments, the present invention is not limited to the above-described embodiments, and it is needless to say that various modifications can be made without departing from the gist of the present invention. It is. For example, in the above embodiment, thermal oxidation was used to increase the number of interstitial silicon atoms of the semiconductor substrate, but silicon was introduced from the outside to increase the number of interstitial silicon atoms of the semiconductor substrate. Is also possible.

[Brief description of the drawings]

FIGS. 1A to 1D are process cross-sectional views illustrating an embodiment of a method for manufacturing a SiC semiconductor device of the present invention.

FIG. 2 is a diagram showing a distribution of nitrogen atoms in a depth direction analyzed by SIMS.

FIG. 3 is a diagram showing a dopant / point defect pair for explaining the reason why the diffusion constant of an impurity increases in the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... p-type SiC single crystal substrate, 2 ... p-type SiC epitaxial growth film, 3 ... ion implantation mask, 4 ... nitrogen ion,
5 ... implanted nitrogen ions, 6 ... impurity doping layer, 7 ... n
Type impurity diffusion layer, 8: thermal oxide film, 9: contact opening, 10: Ni anode electrode, 11: Al cathode electrode, 12: Si atom, 13: C atom, 14: self-interstitials, 15: vacancy , 16: dopant / self-interstitial atom pair, 17: dopant / vacancy pair.

Claims (6)

[Claims] A step of introducing an impurity into the surface of the silicon carbide semiconductor substrate; and increasing the number of interstitial silicon atoms of the semiconductor substrate to diffuse the impurity toward the back surface of the semiconductor substrate. Forming an impurity diffusion layer. A step of introducing an impurity of a second conductivity type into the surface of a silicon carbide semiconductor substrate of a first conductivity type; and increasing the number of interstitial silicon atoms of the semiconductor substrate to reduce the impurities to the semiconductor. Forming an impurity diffusion layer of the second conductivity type by diffusing toward the back surface of the substrate. 3. The method for manufacturing a silicon carbide semiconductor device according to claim 1, wherein the number of said interstitial silicon atoms is increased by thermally oxidizing said semiconductor substrate in an oxidizing atmosphere. 4. The method of manufacturing a silicon carbide semiconductor device according to claim 3, wherein said thermal oxidation is performed in a steam atmosphere. 5. The silicon carbide semiconductor device according to claim 4, wherein said thermal oxidation is performed in a steam atmosphere having a steam partial pressure of 0.2 or more when the total pressure of said oxidizing atmosphere is 1. Production method. 6. The method for manufacturing a silicon carbide semiconductor device according to claim 1, wherein said impurity is nitrogen or boron.