JP4247459B2 - Method for forming oxide film on silicon carbide semiconductor substrate - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming an oxide film on a semiconductor substrate made of silicon carbide (hereinafter referred to as SiC).
[0002]
[Prior art]
SiC is attracting attention as one of the next-generation semiconductor materials that can replace silicon. SiC is an excellent material in terms of physical properties, such as a large band gap, a large saturation drift velocity, a high thermal conductivity, and a dielectric breakdown electric field strength that is about one digit larger than silicon. Taking advantage of these features, silicon carbide is expected as a high temperature sensor, a high frequency device, a power device and the like. Devices that actually exceed the characteristics of silicon are being prototyped one after another.
[0003]
For power devices, the on-resistance is determined by the following equation:
[0004]
[Expression 1]
R _ON = 4BV _PP ² / ε _s μE _C ³
Here, R _ON is the on-resistance, BV _PP is the withstand voltage, ε _s is the dielectric constant of SiC, μ is the mobility, and E _C is the critical electric field strength of SiC.
[0005]
Comparing the silicon being used as SiC and conventional semiconductor material, the mobility of SiC 900 cm ² / Vs and the mobility 1350 cm ² / Vs is less than the silicon, the critical electric field intensity of the silicon 2 × 10 ⁵ V / It is 3 × 10 ⁶ V / cm or more an order of magnitude larger than cm. The relative dielectric constant of SiC is almost the same as that of 9.7 with respect to silicon of 11.9.
For this reason, the semiconductor element using SiC can reduce the on-resistance by about three digits compared to the silicon semiconductor element. Furthermore, since the band gap is large and the thermal conductivity is high, there is an advantage that thermal runaway is difficult and the cooling device can be downsized. This is the main reason why research as a next-generation power device is active.
[0006]
One of the power devices is a field effect transistor (hereinafter referred to as a MOSFET) having a metal-oxide-semiconductor gate structure. The MOSFET is a unipolar element through which only electrons flow, and has a feature that the switching speed is high because the gate signal is controlled by voltage. On the other hand, however, it is difficult to increase the on-resistance as compared to a bipolar transistor such as an insulated gate bipolar transistor (hereinafter referred to as IGBT) that uses conductivity modulation by minority carrier injection. Therefore, when silicon is used, IGBT is mainly adopted as a power device.
[0007]
However, when SiC is used, even the MOSFET can reduce the on-resistance as compared with the silicon IGBT. In addition, since SiC can be oxidized by the same process method as that of silicon, there is an advantage that the accumulation of silicon technology can be utilized. For this reason, MOSFETs using SiC are currently being actively researched.
[0008]
When SiC is used, the on-resistance can be lowered as compared with a silicon element having a similar structure for the above reason, but it is not yet at a sufficient level. This on-resistance is expressed as the series resistance of the substrate resistance, the contact resistance of the ohmic contact portion of the metal, the drift resistance of the epitaxial layer, and the channel resistance. Of these, most of the on-resistance is occupied by channel resistance.
[0009]
In order to reduce the channel resistance, the mobility of carriers flowing through the channel portion must be improved. A value of 100 to ²⁰⁰ cm ² / Vs is required to make a significant difference in characteristics with respect to an IGBT using silicon.
[0010]
[Problems to be solved by the invention]
However, only mobility of several tens of cm ² / Vs can be obtained in 4H-SiC only by using a process similar to that of silicon as the mobility at present. Considering that the mobility in the 4H—SiC crystal is 900 cm ² / Vs, this value is smaller by one digit or more.
It is known that the interface state density in the band gap of SiC increases rapidly as it approaches the conduction band edge [MKDas, BSUm, and JACooper, Jr., International Conference on Silicon Carbide and Related Materials, Raleigh NC, October pp. 11-15, (1999)]. These levels are thought to capture electrons and thus reduce the number of electrons that contribute to conduction, thus greatly reducing the apparent mobility. In addition, Coulomb scattering of conduction electrons due to negatively charged levels is considered to cause a decrease in mobility.
[0011]
The cause of this interface state is considered to be SiC carbon clusters remaining during oxidation [VVAfanas'ev, A. Stesmans and CI Harris, International Conference on Silicon Carbide and Related Materials, Stockholm, Sweden, September pp.857- 860, (1997)]. It is considered that carbon that is a constituent element of SiC is usually removed from the film in the form of CO, CO ₂ or the like by oxygen atoms during oxidation. However, it is considered that some carbon atoms remain and gradually gather at the oxide film / semiconductor interface where the diffusion rate increases because a large stress is generated. As a result, it is thought that carbon is densely clustered at the oxide film / semiconductor interface, and carbon clusters are actually detected at the oxide film / semiconductor interface [B. Hornetz, HJ. Michel and J. Halbritter, Joumal of Materials Reserch, Vol. 9, No. 12, Dec. (1994) pp. 3088-3093].
[0012]
Pensl et al. Reported that this carbon cluster forms a deep level around 2.77 eV. From these facts, it is considered that the high-density interface states near the conduction band are due to carbon clusters.
Various measures have been taken to remove carbon clusters. One of them is a method of performing annealing at about 1000 ° C. again in an atmosphere of hydrogen or argon at normal pressure after oxidation [S. Suzuki et.al., Materials Science Forum, Vols. 338-342, pp. 1073-1076, (1999)].
[0013]
It was confirmed that the interface state density in the vicinity of the midgap decreases by this method. This is considered to be because residual carbon was removed by annealing, and in the case of hydrogen annealing, dangling bonds of carbon clusters were terminated by hydrogen atoms. However, the mobility is not improved even when the interface state density near the gap is decreased [Seiji Suzuki, Shinsuke Harada, Junju Sakizaki, Ryoji Kosugi, Kenji Fukuda, Tomoyuki Tanaka, Kazuo Arai ,; SiC and related wide gap semiconductor study group 9th lecture proceedings, pp.86, (2000)].
[0014]
Although the interface state density has decreased due to re-annealing after oxidation, it is still not sufficient. To improve mobility, it is necessary to further reduce carbon clusters and to terminate dangling bonds of residual carbon more. It is thought that there is.
In view of such a problem, an object of the present invention is to provide an oxide film forming method which can avoid the influence of carbon clusters near the SiC surface and obtain a SiC device having good characteristics.
[0015]
[Means for Solving the Problems]
In order to solve the above problems, the present invention provides a thin oxide film forming process of 15 nm or less by thermal oxidation on a silicon carbide semiconductor substrate, and an annealing process in a gas atmosphere different from the oxide film forming process by thermal oxidation. repeating a plurality of times in this order, it decided to form a desired thickness of oxide film.
[0016]
Amount of residual carbon oxidation film in the thin phase, as compared with the case of performing the oxidation to a set thickness, not grown large carbon clusters, small. And the probability that the gas for decomposing and removing the carbon reaches the interface increases as the oxide film thickness decreases.
Therefore, if annealing at a stage where the oxide film thickness is small is repeated, residual carbon clusters existing at the interface can be easily removed, and carbon clusters can be removed at a lower temperature in a shorter time. It can be understood that even if the amount of residual carbon is the same, the decomposition reaction tends to proceed because the ratio of the surface area to the volume is larger in the case of many small clusters than in the case of a few large clusters. it can.
[0017]
If the carbon cluster removal process is diffusion-controlled, the shorter the thickness of the thin oxide film formed at one time, the shorter the heat treatment. If the thickness of the thin oxide film formed at one time is 15 nm or less when the film thickness required for the gate oxide film described later is 30 nm, the time required for removal can be reduced to almost 1/4 or less.
[0018]
The gas for the annealing step includes any of hydrogen, ammonia, nitric oxide, nitrous oxide, nitrogen, and argon.
Since such a gas reacts with carbon, carbon clusters can be removed quickly.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described based on examples.
[Example 1]
As a substrate to be oxidized, an epitaxial wafer was used in which a p-type epitaxial layer 2 was grown on a surface of the p-type 4H—SiC substrate 1 that was 8 ° off from the (0001) Si surface. The substrate 1 has a carrier concentration of 1 × 10 ¹⁸ / cm ³ , the p-type epitaxial layer 2 has a carrier concentration of 1 × 10 ¹⁶ / cm ³ and a thickness of 10 μm.
[0020]
RCA cleaning of this substrate was performed according to the following procedure.
First, in order to remove organic substances and noble metals, after treatment with sulfuric acid / hydrogen peroxide (H ₂ SO ₄ : H ₂ O ₂ = 4: 1, 120 to 150 ° C.) for 10 minutes, dilute HF is used to remove the natural oxide film. (0.5%, RT) Processing is performed. Thereafter, ammonium hydroxide (NHOH: H ₂ O ₂ : H ₂ O = 0.05: 1: 5, 80 to 90 ° C.) treatment is performed to remove particles present in the natural oxide film. Thereafter, in order to remove the metal present in the natural oxide film, a hydrochloric acid perwater treatment (HCl: H ₂ O ₂ : H ₂ O = 1: 1: 6, 80 to 90 ° C.) is performed. Finally, dilute HF treatment is performed again to remove the natural oxide film newly generated in these processes. During these treatments, rinse with pure water for about 5 minutes.
[0021]
Next, annealing was performed at 1000 ° C. for 30 minutes in a normal pressure hydrogen atmosphere.
The clean surface was then wet oxidized.
First, the temperature was raised from 700 ° C. to 1100 ° C. at a rate of 10 ° C./min in a nitrogen atmosphere. Thereafter, at 1100 ° C., the flow rate ratio of oxygen and hydrogen was maintained at 1: 1, and oxidation was performed for 1 hour to form an oxide film 3.
[0022]
Thereafter, in order to remove the carbon cluster, the atmosphere was changed to a one-to-one atmosphere of argon and hydrogen, and annealing was performed for 30 minutes.
This operation was performed a total of 5 times to form oxide films 3 to 7 having a thickness of 30 nm. Up to about 30 nm, the reaction rate is almost limited, and an oxide film of about 5 nm grows every time. The 30 nm oxide film has an appropriate thickness as a gate oxide film of the MOSFET.
[0023]
Thereafter, the temperature was lowered to 700 ° C. at a rate of 3 ° C./min in a nitrogen atmosphere.
In order to investigate whether or not the residual carbon at the oxide film / semiconductor interface subjected to such an oxidation treatment was removed, various observations were made together with a comparative example in which no special treatment was performed.
First, the oxide film was etched with dilute hydrofluoric acid to expose the surface of the silicon carbide semiconductor, and then observed with an atomic force microscope (AFM).
[0024]
In the comparative example, protrusions having a diameter of about 80 nm and a height of about 5 nm are observed at a density of several tens of pieces / μm ^2. However, such protrusions are not formed on the surface of silicon carbide when the method of the present invention is performed. I couldn't see it. Next, the distribution of carbon atoms in the depth direction from the surface of the oxide film was measured by secondary ion mass spectrometer (SIMS) analysis. Compared to the comparative example, when the method of the present invention was performed, the slope of the distribution of carbon atoms in the vicinity of the oxide film / silicon carbide semiconductor interface was steeper. This suggests the disappearance of carbon clusters.
[0025]
Further, the vicinity of the oxide film / semiconductor interface was directly observed with a transmission electron microscope (TEM). In the comparative example, carbon clusters were observed, but when the method of the present invention was performed, no carbon clusters were observed at the interface. Further, when a MOSFET was fabricated using this wafer, the carrier mobility was 150 cm ² / Vs, and the treatment according to the present invention was greatly improved from the conventional 50 cm ² / Vs.
[0026]
That is, it can be seen that the carbon cluster was sufficiently removed by the method of the present invention.
In the above-mentioned report by Suzuki et al., The effect of improving mobility was not observed even after hydrogen annealing, but the major difference from this case is the oxide film thickness. In the case of Suzuki et al., The thickness of the oxide film is 48 nm, hydrogen annealing is insufficient for that thickness, and it is considered that the carbon dangling bond was terminated only by hydrogen termination. Further, since the oxide film is grown thick, the carbon cluster grows, and the ratio of the surface area to the volume of the cluster decreases, so that it is considered that the removal by hydrogen was not performed effectively.
[0027]
In contrast, in the method of the present invention, hydrogen annealing is performed at a stage where the oxide film is thin, so that the nucleus of the carbon cluster at the oxide film interface is still small and the ratio of the surface area to the volume is large. , CH _x are removed. As the annealing gas, nitrogen, ammonia, argon, nitrous oxide, nitrogen oxide or the like can be used in addition to the above hydrogen.
[0028]
[ Reference example ]
The substrate, epitaxial layer, surface treatment method used, the first oxidation temperature rise, retention, annealing for carbon cluster removal, and cooling were the same as in Example 1.
At this time, the thickness of the oxide film 8 is 6 nm.
On this oxide film 8, a CVD oxide film 9 having a thickness of 24 nm was deposited by low pressure CVD. The film forming conditions were as follows. Monosilane (SiH ₄ ) and oxygen (O ₂ ) were used as gases, and the flow rate ratio was SiH ₄ : O ₂ = 1: 5. The pressure is 1 Pa, the film forming temperature is 300 ° C., and the time is 1 minute.
[0029]
Also in this example, the vicinity of the oxide film / semiconductor interface was directly observed with a transmission electron microscope (TEM), but no carbon cluster was observed at the interface. That is, it is considered that the carbon cluster was sufficiently removed by the method of the present invention.
The CVD oxide film 9 was not formed on the epitaxial film 2 of the silicon carbide substrate from the beginning, but the CVD oxide film 9 was formed after the thin thermal oxide film 8 was formed because of contamination of the silicon carbide substrate surface. This is for avoiding the generation of interface states due to incorporation into the MOS interface.
[0030]
As a method for depositing the CVD oxide film 9, a sputtering method or a plasma CVD method may be used in addition to the LPCVD method.
[0031]
【The invention's effect】
As described above, according to the present invention, a thin oxide film formation process of a thickness of 15 nm or less by thermal oxidation on a silicon carbide semiconductor substrate and an annealing process in a gas atmosphere different from the oxide film formation process are performed in this order. Just repeat several times, by an oxide film having a thickness of Tokoro stage, decomposition and removal of the oxide film / silicon carbide vicinity of the carbon clusters is performed reliably, occurrence of conventional seen interface state, the carrier It was shown that an SiC semiconductor device having good characteristics can be manufactured by preventing a decrease in mobility.
[0032]
Therefore, the present invention greatly contributes to the spread and development of silicon carbide semiconductor elements.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of an SiC semiconductor device by the method of the first embodiment of the present invention. FIG. 2 is a cross-sectional view of an SiC semiconductor device by the method of the reference embodiment of the present invention.
1 ・ ・ ・ p-type substrate
2 ... p-type shrimp layer
3 ... Oxide film by the first thermal oxidation
4 ... 2nd thermal oxidation film
S ... Oxide film by the third thermal oxidation
6 ... Oxide film by the fourth thermal oxidation
7 ... Fifth thermal oxidation film
8 ... Oxide film by the first thermal oxidation
9 ... CVD oxide film

Claims (2)

A process of forming a thin oxide film of 15 nm or less by thermal oxidation on a silicon carbide semiconductor substrate and an annealing process in a gas atmosphere different from the process of forming an oxide film by thermal oxidation are repeated a plurality of times in this order. A method for forming an oxide film on a silicon carbide semiconductor substrate, comprising forming an oxide film having a thickness. 2. The method for forming an oxide film on a silicon carbide semiconductor substrate according to claim 1 , wherein the annealing gas contains any one of hydrogen, ammonia, nitric oxide, nitrous oxide, nitrogen, and argon.

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