JP2013211500A - Deposition method to silicon carbide substrate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of improving uniformity of a doping concentration distribution in a process of performing epitaxial growth of a SiC film on a SiC substrate.SOLUTION: In the method of performing epitaxial growth of a conductive SiC film on a SiC substrate, by doping in addition to a first dopant an appropriate amount of a second dopant whose incorporated atom position is different from that of the first dopant, a concentration distribution of a main first dopant is compensated by a concentration distribution of a second dopant so that a doping concentration is uniformed.

Description

  The present invention relates to a film forming method on a silicon carbide substrate, and more particularly to a method of epitaxially growing a conductive silicon carbide film.

  A semiconductor using silicon carbide (hereinafter referred to as “SiC”) as a material is expected as a next-generation semiconductor element of silicon (hereinafter referred to as “Si”). The SiC semiconductor can reduce the resistance of the device in the on state to several hundredth compared with a semiconductor device using conventional Si as a material, and can be used in a higher temperature environment of 200 ° C. or higher. There are various advantages. This is due to the characteristics of the material itself that the band gap of SiC is about three times as large as that of Si, and that the electric field strength of dielectric breakdown is nearly one digit larger than that of Si.

  As devices using SiC, a Schottky barrier diode and a planar vertical MOSFET have been commercialized so far. Such a SiC device is usually composed of an epitaxial SiC film formed on a 4H-SiC substrate, and various device structures are further added to the epitaxial SiC film in order to provide a function as a semiconductor device. Therefore, the properties of the epitaxial SiC film to be formed, particularly the uniformity of the doping concentration within the film surface, are important requirements for improving the characteristics of the final semiconductor device and the yield rate.

  In an epitaxial growth process on a SiC substrate, hydrogen as a carrier gas, Si-based source gas (for example, monosilane or dichlorosilane), C-based source gas (for example, propane or methane), and a dopant (for example, nitrogen or n-type) A source gas (such as nitrogen, phosphine, trimethylaluminum, diborane, etc.) of phosphorus, p-type, etc.) is flowed into a reaction furnace consisting of a quartz tube, a heat insulating material and a graphite heating element, and 6 kPa to A susceptor that supports a substrate by controlling the atmosphere under reduced pressure of 12 kPa, induction heating the graphite heating element to 1550-1750 ° C. with a high-frequency heating coil wound around the outer periphery of the reactor, and heat radiation and heat transfer from the graphite heating element And an epitaxial thin film is grown on the substrate by a thermal CVD method.

  In the case of epitaxial growth on a SiC substrate, it is known that the doping concentration distribution is greatly influenced by the temperature distribution in the furnace (Non-Patent Document 1), and the temperature dependence of the dopant incorporation amount is the growth plane orientation, It is also known that it varies depending on the type of dopant and growth conditions (C / Si ratio) (Non-Patent Document 2). However, the doping concentration distribution of such an epitaxial film varies in characteristics such as reverse breakdown voltage and characteristic on-resistance of the device. The cause is always improved.

J. Nishio, M. Hasegawa, K Kojima, T. Ohno, Y. Ishida, T. Takahashi, T. Suzuki, T. Tanaka, K. Arai, Journal of Crystal Growth 258 (2003) 113-122 K. Kojima, S. Kuroda, H. Okumura, K. Arai. Microelectronic Engineering 83 (2006) 79-81

Therefore, it is conceivable to make the temperature distribution in the furnace uniform in order to improve the doping concentration distribution uniformly, but the temperature distribution in the furnace is unique to the apparatus and cannot be easily changed. A doping concentration distribution corresponding to the temperature dependence of the dopant uptake was obtained.
On the other hand, in order to reduce the influence of the temperature distribution in the furnace, a method has been proposed in which a wafer is placed on a tray and rotated in the furnace. However, even if this method is used, a temperature distribution is generated inside the rotating tray, so that the influence of the temperature distribution is unavoidable as the wafer diameter increases.
In addition, the growth conditions (C / Si ratio) can be adjusted to make some improvements, but it may be difficult to put into practical use because it affects other film properties such as surface roughness, surface defect density, and film thickness distribution. Many.

  The present invention has been made in view of the above situation, and an object of the present invention is to provide a method for improving the uniformity of the doping concentration distribution in the above-described SiC epitaxial growth process.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors, as a result, in addition to the main dopant (hereinafter referred to as “first dopant”), a dopant (hereinafter referred to as “the first dopant”) having a different atomic position. It was found that by doping an appropriate amount of "second dopant"), the concentration distribution of the first dopant can be compensated by the concentration distribution of the second dopant, and the doping concentration can be made uniform.

The present invention has been completed based on these findings, and according to the present invention, the following inventions are provided.
[1] In a method of epitaxially growing a conductive SiC film on a SiC substrate,
While flowing a reaction gas containing the first dopant which is the main dopant, the second dopant having a different atomic position from the first dopant is incorporated into the epitaxial film so that the amount of incorporation into the first dopant is A method for forming an epitaxial SiC film, characterized in that a doping concentration distribution is made uniform by flowing the dopant so as to be less than the same amount as that taken into the film.
[2] The combination of the first dopant and the second dopant is selected from a combination of nitrogen and aluminum, nitrogen and phosphorus, boron and aluminum, or boron and phosphorus. A method for forming the epitaxial SiC film as described.
[3] The epitaxial SiC film according to [1] or [2], wherein the amount of the first dopant taken up is 2.0 × 10 ¹⁵ cm ⁻³ or more and less than 5.0 × 10 ¹⁶ cm ⁻³ . Film forming method.
Method.
[4] A silicon carbide semiconductor device produced using the method according to any one of [1] to [3].

  According to the present invention, it is possible to make the doping concentration in the epitaxial SiC film uniform, suppress characteristic variations such as characteristic on-resistance and reverse breakdown voltage of a device formed using the same, and improve the yield.

The figure showing density | concentration distribution and doping concentration distribution in case the 1st dopant and 2nd dopant are different conductivity types. The figure showing concentration distribution and doping concentration distribution in case the 1st dopant and the 2nd dopant are the same conductivity types. Sectional drawing which shows schematic structure of the heating part of a horizontal type hot wall film-forming apparatus

  In the present invention, in addition to the main first dopant, an appropriate amount of a second dopant having a different atomic position to be incorporated is doped to compensate the concentration distribution of the first dopant with the concentration distribution of the second dopant. It is characterized by making the density uniform.

In conventional film formation, dopants other than the main conductivity type dopant (first dopant in the above case) (second dopant in the above case) are regarded as disturbance factors in controlling the concentration, and efforts to reduce as much as possible Has been made.
However, the present invention is characterized in that the uniformity of the concentration distribution is improved by controlling the second dopant and doping a certain amount.

In the method of the present invention, it is important that the first dopant and the second dopant have different atomic positions taken into SiC. That is, in SiC, for example, in nitrogen, which is an n-type dopant, and aluminum, which is a p-type dopant, nitrogen replaces a carbon site and aluminum replaces a silicon site due to a difference in atomic radius. It is known that both nitrogen, which is a dopant, and boron, which is a p-type dopant, substitute a carbon atom. In the present invention, in the case of dopants substituting the same atomic position, the atomic positions are brought together and cannot be controlled independently.
In the present invention, a specific combination of the first dopant and the second dopant is “nitrogen and aluminum”, “nitrogen and phosphorus”, “boron and aluminum”, or “boron and phosphorus”.

In the present invention, the selection of the first dopant and the second dopant needs to be determined by examining the temperature dependence of the amount of both dopants taken up.
That is, when the first dopant 1 and the second dopant 2 are of different conductivity types, as shown in FIG. 1, the temperature dependency of both is the same and the temperature dependency of the second dopant is higher than that of the first dopant. Must also be large. Further, when the first dopant and the second dopant have the same conductivity type, as shown in FIG. 2, it is necessary that the temperature dependence of the amounts of the first dopant and the second dopant taken in has a reverse tendency.
1 and 2, the horizontal axis indicates the position from the center (0) of one of the four substrates (wafers) arranged at equal intervals on the rotating circular tray in the apparatus of FIG. 3 to be described later. (Mm), + means a position on the outer peripheral side of the circular tray, and-means a position on the center side of the circular tray.

The temperature dependence of each of the above dopants varies under various conditions such as the apparatus used and the film formation conditions. Therefore, when epitaxially growing a SiC film having a desired conductivity type, each dopant under the various conditions is preliminarily grown. It is necessary to investigate the temperature dependence of.
As a result of investigation by the present inventors, the epitaxial growth on the C-plane substrate is 1600 to 1700 ° C., 6 to 10 kPa, and the C / Si ratio (the flow rate ratio of the Si-based source gas to the C-based source gas) is 1.0 to 1. In the range of 5, it was found that the temperature dependency of aluminum is small and the temperature dependency of nitrogen is large.
Therefore, as a result of simultaneously doping nitrogen when doping aluminum during growth, the uniformity of the p-type doping concentration was greatly improved as shown in the examples described later. FIG. 3 is a cross-sectional view showing a schematic configuration of the apparatus used. The apparatus will be described in Examples.
At this time, the nitrogen uptake must be less than the aluminum uptake. In addition, since the C surface is naturally easy to take in nitrogen, the controllable nitrogen uptake is preferably 2.0 × 10 ¹⁵ cm ⁻³ or more. Furthermore, when the nitrogen uptake amount is 5.0 × 10 ¹⁶ cm ⁻³ or more, there is a concern about a decrease in mobility due to impurity scattering, which is not desirable.

In addition, the temperature dependence of aluminum incorporation investigated this time was smaller than that of nitrogen. However, according to Non-Patent Document 1, when the C / Si ratio is lowered to 0.6, the temperature dependence of aluminum incorporation becomes larger. Is described. In the case of such film formation conditions, if nitrogen is used as dopant 1 and aluminum is used as dopant 2, the n-type doping concentration distribution can be improved by using the method of the present invention.
Again, the aluminum uptake must be less than the uptake of nitrogen. Further, the amount of aluminum taken in is preferably 2.0 × 10 ¹⁵ cm ⁻³ or more from the viewpoint of controllability. Furthermore, when the aluminum uptake amount is 5.0 × 10 ¹⁶ cm ⁻³ or more, there is a concern about a decrease in mobility due to impurity scattering, which is not desirable.

  In addition, if the board | substrate used for the method of this invention is a SiC substrate, it will not specifically limit, For example, a 4H-SiC board | substrate, a 6H-SiC board | substrate, etc. are used. Also. It is needless to say that the surface on which the epitaxial film is formed is not particularly limited and may be the C plane (000-1) or the Si plane (0001).

EXAMPLES Hereinafter, although this invention is demonstrated based on an Example, this invention is not limited to this Example.
In this example, an experiment was performed using the apparatus shown in FIG.
The apparatus shown in FIG. 3 is a cross-sectional view showing a schematic configuration of a heating unit of a general horizontal hot wall film forming apparatus as a SiC epitaxial film forming apparatus. The heating unit includes a reaction chamber composed of a quartz tube and a hot wall disposed inside through a heat insulating material (not shown), a susceptor installed in the reaction chamber, and a substrate (wafer) supported by the susceptor And a high-frequency heating coil wound around a quartz tube. In the apparatus used in the present embodiment, four wafers are arranged at equal intervals on a circular tray and rotated in a furnace.
When a reaction gas (arrow) for forming a SiC epitaxial film on a substrate (wafer) is introduced from an inflow port (not shown) and reaches a heating unit, the reaction gas is heated by a hot wall induction-heated by a high-frequency heating coil and reacted. An epitaxial SiC film is formed by a thermal CVD method on a wafer attached to a susceptor placed in the chamber space.

Using this apparatus, a film formation temperature of 1700 ° C., a film formation pressure of 10 kPa, a C / Si ratio of 1.2 (monosilane gas flow rate of 50 sccm, propane gas flow rate on the C surface (000-1) of a 4H—SiC substrate of Φ3 inch 20 sccm) and hydrogen gas flow rate of 100 slm, doping with nitrogen as an n-type dopant alone yielded a concentration distribution that monotonously increased from the center of the tray to the end, and a σ / mean of 6.4. %.
Similarly, when TMA (trimethylaluminum) is used to dope the p-type dopant alone, a concentration distribution that monotonously increases from the center of the tray to the edge is obtained, and σ / mean It was found to be 12.6%.

Therefore, when growing the p-type epitaxial film under the same conditions, 0.05 sccm of TMA (trimethylaluminum) was flowed in order to incorporate aluminum as the p-type first dopant. At this time, nitrogen gas was supplied at 0.02 sccm in order to simultaneously remove nitrogen as an n-type second dopant.
When the p-type doping concentration (Na-Nd) in the direction from the center to the edge of the tray was examined, it was 3.4% in terms of σ / mean (standard deviation / average), much larger than the distribution of the uptake of aluminum alone. It turns out that it is improving.

Claims (4)

In a method of epitaxially growing a conductive SiC film on a SiC substrate,
While flowing a reaction gas containing the first dopant which is the main dopant, the second dopant having a different atomic position from the first dopant is incorporated into the epitaxial film so that the amount of incorporation into the first dopant is A method for forming an epitaxial SiC film, characterized in that a doping concentration distribution is made uniform by flowing the dopant so as to be less than the same amount as that taken into the film.   2. The epitaxial according to claim 1, wherein a combination of the first dopant and the second dopant is selected from a combination of nitrogen and aluminum, nitrogen and phosphorus, boron and aluminum, or boron and phosphorus. A method of forming a SiC film. 3. The method of forming an epitaxial SiC film according to claim 1, wherein the amount of the first dopant taken up is 2.0 × 10 ¹⁵ cm ⁻³ or more and less than 5.0 × 10 ¹⁶ cm ⁻³ .
Method.   The silicon carbide semiconductor device created using the method of any one of Claims 1-3.

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