JP4267810B2 - Method for manufacturing silicon carbide semiconductor device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing an SiC semiconductor device using a silicon carbide (hereinafter referred to as SiC) semiconductor substrate (hereinafter referred to as SiC substrate), and more particularly to a technique for forming an impurity diffusion layer on the SiC substrate.
[0002]
[Prior art]
Conventionally, as a technique for locally doping impurities into a SiC substrate in a SiC semiconductor device, for example, a technique described in Japanese Patent Application Laid-Open No. 8-316164 is known.
[0003]
In the above prior art, after forming a layer doped with impurities on the surface of the SiC substrate, the impurities are diffused by irradiating hydrogen ions or γ rays from the substrate surface side while heating the SiC substrate to 500 ° C. to 1500 ° C. Is.
[0004]
[Problems to be solved by the invention]
As described above, in the conventional method of diffusing impurities into the SiC substrate, the impurities are diffused by irradiation with hydrogen ions or γ rays while heating the SiC substrate to a predetermined high temperature.
[0005]
Therefore, when hydrogen ion irradiation is performed, the substrate surface is damaged by the ion irradiation. In addition, in the case of irradiating γ rays, a γ ray irradiator is required, but this γ ray irradiator is not widely used and expensive, and therefore the cost increases.
[0006]
An object of the present invention is to provide a method of manufacturing a SiC semiconductor device capable of forming an impurity diffusion region in a SiC substrate at a low cost without causing damage to the substrate surface.
[0007]
[Means for Solving the Problems]
The present invention increases the diffusion constant of impurities by increasing the number of interstitial silicon atoms of the SiC substrate into which impurities are introduced, solves the above problems, and forms an impurity diffusion layer.
[0008]
That is, the method for manufacturing a SiC semiconductor device according to claim 1 of the present invention includes a step of accelerating and implanting impurity ions while heating the surface of a silicon carbide semiconductor substrate, and heating and activating the implanted impurities to produce impurities. A step of forming a doping layer; and the impurity doping layer is thermally oxidized in a water vapor atmosphere to increase the number of interstitial silicon atoms of the semiconductor substrate, whereby the impurities are directed toward the back surface of the semiconductor substrate. And a step of forming an impurity diffusion layer by diffusing.
[0009]
According to a second aspect of the present invention, the semiconductor substrate is of a first conductivity type, and the impurity is of a second conductivity type .
[0012]
According to a third aspect of the present invention, the thermal oxidation is performed in a steam atmosphere having a steam partial pressure of 0.2 or more when the total pressure of the oxidizing atmosphere is 1.
[0013]
According to a fourth aspect of the present invention, the impurity is nitrogen or boron.
[0014]
【The invention's effect】
According to the method of manufacturing an SiC semiconductor device of the first aspect of the present invention, by increasing the number of interstitial silicon atoms of the SiC substrate, it is possible to increase the impurity diffusion constant and form a deep impurity diffusion layer. . Therefore, the SiC substrate surface is not damaged by the hydrogen ion irradiation as in the conventional case, and it is not necessary to perform the γ-ray irradiation. Therefore, an expensive γ-ray irradiator is not required. A diffusion region can be formed. Further, by performing thermal oxidation in a water vapor atmosphere, the impurity diffusion constant is remarkably increased, and an impurity diffusion region can be formed at low cost without causing damage to the surface of the SiC substrate.
[0015]
According to claim 2 of the present invention, the second conductivity type impurity diffusion layer can be formed on the surface of the first conductivity type SiC substrate at a low cost without causing damage to the surface of the SiC substrate.
[0018]
According to claim 3 of the present invention, the thermal oxidation is performed in a water vapor atmosphere having a water vapor partial pressure of 0.2 or more when the total pressure of the oxidizing atmosphere is 1, so that the impurity diffusion constant is remarkably increased. Impurity diffusion regions can be formed at low cost without causing damage to the surface of the SiC substrate.
[0019]
Further, in claim 4 of the present invention, since the covalent bond radius of both elements is smaller than the covalent bond radius of SiC of 0.97 by using nitrogen or boron as the impurity, the impurity diffusion constant is effectively reduced. Can be increased.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0021]
1A to 1D are process cross-sectional views illustrating an embodiment of a method of manufacturing an SiC semiconductor device according to the present invention. In the present embodiment, a method of increasing the number of interstitial silicon atoms of a SiC substrate will be described using an example of a planar pn junction diode that forms a pn junction using thermal oxidation.
[0022]
First, as shown in FIG. 1 (A), a p-type SiC epitaxial growth film 2 having a carrier concentration of 2 × 10 ¹⁶ cm ⁻³ is heated on a p-type SiC single crystal substrate 1 having a carrier concentration of 1 × 10 ¹⁸ cm ^−3. After forming by the 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., the acceleration voltage is 30, 60, 100, 150 keV, the dose amount is 8 × 10 ¹⁴ , 1.4 × 10 ¹⁵ , 2.8 × 10 ¹⁵ , 3.4 × 10, respectively. Nitrogen ions 4 are locally implanted through the mask 3 under conditions of ¹⁵ cm ⁻² and a total dose of 8.4 × 10 ¹⁵ cm ⁻² . Reference numeral 5 denotes implanted nitrogen ions.
[0023]
Next, after removing the mask 3 with a buffered hydrofluoric acid aqueous solution, heat treatment is performed at 1700 ° C. for 1 minute in an Ar atmosphere to activate the implanted nitrogen 5, as shown in FIG. An impurity doping layer 6 is formed.
[0024]
Thereafter, thermal oxidation is performed at 1100 ° C. for 420 minutes in an oxidizing atmosphere containing water vapor to diffuse nitrogen in the impurity doping layer 6 to form an n-type impurity diffusion layer 7 as shown in FIG. . Reference numeral 8 denotes a thermal oxide film formed by this thermal oxidation. The atmosphere at this time was such that the partial pressure of water vapor was 0.2 or more when the total pressure of the oxidizing atmosphere was 1.
[0025]
FIG. 2 shows the depth distribution of nitrogen atoms analyzed by SIMS (Secondary Ion Mass Spectroscopy). The distribution of nitrogen atoms immediately after nitrogen ion implantation in FIG. 1 (A) and after heat treatment at 1700 ° C. for 1 minute in FIG. 1 (B) does not change before and after the heat treatment, as shown in FIG. The junction depth is about 0.5 μm. On the other hand, the junction depth after thermal oxidation in a steam atmosphere at 1100 ° C. for 420 minutes in FIG. 1C is about 1.1 μm as shown in FIG. 2B. ing. The diffusion constant determined from this diffusion length was about 2.0 × 10 ⁻¹³ cm / s. This value is about two orders of magnitude larger than the conventionally reported diffusion constant of nitrogen in SiC (Sov. Tech. Phys. Lett., Vol. 8 (1982) pp. 120).
[0026]
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 wet-etched with a buffered hydrofluoric acid aqueous solution. As shown in FIG. 1D, a contact opening 9 is formed in the thermal oxide film 8, an anode electrode 10 made of Ni on the n-type impurity diffusion layer 7 side, and a cathode made of Al on the p-type SiC substrate 1 side. The electrode 11 was formed, and heat treatment was performed at 1000 ° C. for 2 minutes in an Ar atmosphere 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.
[0027]
Thus, in the present embodiment, the step of introducing impurities (nitrogen ions 4) into the surface of SiC substrate 1 (FIG. 1A) and the SiC substrate 1 into which impurities (implanted nitrogen ions 5) are introduced into the surface. By increasing the number of interstitial silicon atoms, the impurity of the impurity doping layer 6 (FIG. 1B) is diffused toward the back surface of the SiC substrate 1 to form the impurity diffusion layer 7 (FIG. 1). (C)).
[0028]
As shown in the above embodiment, according to the present invention, when the impurity doping layer formed on the SiC substrate is thermally oxidized in a water vapor atmosphere, the number of interstitial silicon atoms in the SiC substrate increases. Thereby, the diffusion constant of impurities is increased, and a deep impurity diffusion layer can be formed. The reason why the diffusion constant of impurities increases when thermal oxidation is performed in a steam atmosphere is as follows.
[0029]
FIG. 3 is a diagram for explaining the reason why the impurity diffusion constant increases when thermal oxidation is performed in a water vapor atmosphere in the present invention.
[0030]
In FIG. 3, 12 is a Si atom, 13 is a C 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.
[0031]
It is known that point defects (here, self-interstitial atoms 14 and vacancies 15 in FIG. 3) present in the crystal form a strain field around the crystal and capture electrons to generate a Coulomb field. (ReV. Mod. Phys., Vol. 61 (1989) pp. 289).
[0032]
For this reason, point defects pair with dopants (impurities) and try to reduce the energy of the entire system by relaxing the strain field or shielding the Coulomb field. That is, as shown in FIG. 3, a dopant whose covalent bond radius is smaller than the covalent bond radius of SiC attracts the self-interstitial atoms 14 to form a dopant / self-interstitial atom pair 16 in order to reduce the energy of the system. To do. On the other hand, a dopant having a covalent bond radius larger than the covalent bond radius of SiC attracts the vacancy 15 to form a dopant / vacancy pair 17 in order to reduce mechanical strain energy.
[0033]
Usually, a dopant substituted at a lattice position is strongly covalently bonded to an adjacent atom (Si atom or C atom in the case of SiC), and therefore cannot move to another lattice position. However, when a point defect exists around the dopant, the point defect attempts to form a dopant / point defect pair, that is, a dopant / self-interstitial atom pair 16 and a dopant / vacancy pair 17. Are replaced, and the dopant deviates from the original lattice position. That is, the dopant is in a state where it can diffuse in the crystal. For this reason, the diffusion constant D of the dopant is qualitatively given by the following formula (1).
[0034]
D / D _eq = f _i (C _i / C _ieq ) + (1−f _i ) (C _v / C _veq ) (1)
In the above formula (1), f _i is a constant determined by the type of dopant, and indicates the ratio of the dopant that forms the dopant / self-interstitial atom pair 16 or the dopant / vacancy pair 17 and diffuses the dopant. That is, in the case of a dopant that forms a pair with the self-interstitial atom 14, the value of f _i is close to 1, and in the case of a dopant that forms a pair with the vacancy 15, the value of f _i is nearly zero. Become. Also, the C _i self-interstitial atom density, C _v is the pore density, D _eq is the diffusion constant in the thermal equilibrium state, C _IEQ self-interstitial density in thermal equilibrium, the C _Veq is vacancy density in the thermal equilibrium state .
[0035]
As is clear from the previous equation (1), once the dopant to be used, the self-interstitial atom density C _i and the vacancy density C _v are determined, the diffusion constant D is uniquely determined.
[0036]
In the case of SiC, since the bond between Si and C is strong, the self-interstitial density C _i and the vacancy density C _v are small even in the temperature region used for the thermal diffusion of Si, so that the diffusion constant D of the dopant is small. When the impurity doping layer formed on the surface of the SiC substrate is subjected to thermal oxidation, interstitial Si atoms are generated at the oxide film / SiC substrate interface along with the volume expansion when SiC is oxidized, and the substrate enters the SiC substrate. Diffuses toward the back. As a result, the self-interstitial atom density C _i in the formula (1) increases, and the diffusion constant D of the dopant increases. Therefore, in order to increase the diffusion rate of the dopant efficiently by oxidation, how to increase the diffusion constant D of the dopant in unit time during the oxidation time, that is, generated at the oxide film / SiC substrate interface per unit time. It is important how to increase the amount of interstitial Si atoms (in relation to claims 1 and 2 above).
[0037]
In the case of an oxidation method by normal thermal oxidation in a dry oxygen atmosphere, since the growth rate of the SiC oxide film is small, the amount of interstitial Si atoms generated at the oxide film / SiC substrate interface per unit time is small, and the dopant It is difficult to increase the diffusion rate efficiently.
[0038]
In particular, when the total pressure of an oxidizing atmosphere containing water vapor is set to 1, the present inventor significantly promotes the oxidation reaction when performing thermal oxidation in an oxidizing atmosphere having a water vapor partial pressure of 0.2 or more, and the oxide film / SiC Since a large amount of interstitial Si atoms are generated at the substrate interface, the number of interstitial Si atoms in the SiC substrate increases abruptly, and the diffusion constant D of the dopant is significantly higher than that in the case where the normal thermal oxidation is performed. It has been found that it is possible to increase the diffusion rate of Si efficiently (with respect to claims 3, 4 and 5 above).
[0039]
In the present invention, when using, for example, nitrogen or boron as a dopant, since the covalent radius of both elements is less than the covalent radius 0.97 SiC, the value of the constant f _i in Equation (1) Is approximately 1, and therefore, the right side of the previous equation (1) is only the first term. That is, only the self-interstitial atom density C _i affects the diffusion constant D of the dopant, and the diffusion rate of the dopant can be increased more effectively (in relation to claim 6 above).
[0040]
Further, according to the present invention, in the impurity doping layer formed on the surface of the SiC substrate, since the impurity diffuses only in the oxidized part, the part where the impurity is not desired to be diffused after the impurity doped layer is formed, such as a silicon nitride film, etc. It is possible to selectively form a deep impurity diffusion layer by performing oxidation while protecting the layer.
[0041]
Further, since the diffusion length of the dopant can be controlled by the oxidation time, conventional ion implantation with a high acceleration voltage is unnecessary, and a pn junction of about 1 μm can be formed with a low acceleration voltage of about 150 keV. It is.
[0042]
Furthermore, since the diffusion constant D is increased by using only the oxidation reaction, it is not necessary to irradiate the impurity doping layer with hydrogen ions or γ rays, so that the SiC substrate surface is not damaged and expensive. Since no γ-ray irradiator is required, the impurity diffusion region can be formed in the SiC substrate at a low cost.
[0043]
Although the present invention has been specifically described above 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 scope of the invention. For example, in the above embodiment, thermal oxidation is used to increase the number of interstitial silicon atoms in the semiconductor substrate. However, silicon is introduced from the outside to increase the number of interstitial silicon atoms in 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 of manufacturing an SiC semiconductor device according to the present invention.
FIG. 2 is a diagram showing a depth distribution of nitrogen atoms analyzed by SIMS.
FIG. 3 is a diagram showing dopant / point defect pairs for explaining the reason why the diffusion constant of impurities 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 ion, 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-interstitial atom, 15 ... vacancy, 16 ... dopant / self-lattice Interatomic pair, 17 ... dopant / vacancy pair.

Claims (4)

Accelerating and implanting impurity ions while heating the surface of the silicon carbide semiconductor substrate;
Heating and activating the implanted impurities to form an impurity doping layer;
The impurity doping layer is thermally oxidized in a water vapor atmosphere to increase the number of interstitial silicon atoms of the semiconductor substrate, thereby diffusing the impurities toward the back surface of the semiconductor substrate, thereby forming an impurity diffusion layer. And a step of forming the silicon carbide semiconductor device. The method for manufacturing a silicon carbide semiconductor device according to claim 1, wherein the semiconductor substrate is of a first conductivity type and the impurity is of a second conductivity type. The thermal oxidation method of manufacturing a silicon carbide semiconductor device according to claim 1, wherein the partial pressure of water vapor and carrying out in 0.2 above water vapor atmosphere when set to 1 and the total pressure of the oxidizing atmosphere. The impurity, method for manufacturing the silicon carbide semiconductor device according to any one of claims 1 to 3, characterized in that nitrogen or boron prime.

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