JP4997605B2 - Cold wall type heat treatment furnace and insulating film forming apparatus - Google Patents

Description

  The present invention relates to a cold wall type heat treatment furnace and an insulating film forming apparatus used for forming an insulating film.

  Silicon carbide (SiC) is a semiconductor material with excellent characteristics such as wide band gap, high thermal conductivity, high saturation electron drift velocity, and high breakdown electric field (Ec), and it is the next generation low-loss power that can replace silicon power devices It is attracting attention as a device element material. In the semiconductor device (semiconductor device) made of SiC, among the basic elements necessary for the inverter configuration, a rectifier element (diode) having a small capacity has already been commercialized, but a switching element has not been put into practical use. Development is urgently needed.

  As a switching element using SiC, a metal-oxide-semiconductor field-effect transistor (MOSFET) is expected. This is because SiC has a feature that an oxide film (insulating film) can be formed by a thermal oxidation process like silicon. On the other hand, in the case of an oxide film / SiC structure in which an oxide film is formed on SiC using a thermal oxidation process similar to that of silicon, a defect related to carbon is formed at the interface between the oxide film and SiC. It has been reported. This is a defect peculiar to an oxide film / SiC interface called a carbon cluster, which is considered to be generated during the formation of the thermal oxide film, and contributes to lowering the reliability of the device.

Due to the high interface state in the vicinity of the conductor due to the carbon cluster, the channel mobility of the SiC-MOSFET manufactured by the usual thermal oxidation method becomes extremely lower than the value expected from the electron mobility of the SiC bulk. And since the channel mobility in the present SiC-MOSFET is very small, on-resistance value (Ron) becomes extremely higher than the value theoretically expected from the physical property value. In particular, silicon carbide (4H—SiC) having a crystal structure called 4H has been attracting attention because its bulk electron mobility is as high as about 900 cm ² / Vs, but a channel of 4H—SiCMOSFET formed by a normal thermal oxidation method. The mobility is as low as about 5 to 10 cm ² / Vs. In order to improve this, it is necessary to reduce the high interface state due to the carbon cluster described above, and a heat treatment method in a nitrogen monoxide (NO) atmosphere has been proposed to reduce the interface state. .

  This heat treatment method in an NO atmosphere is a method in which an oxide film is formed on SiC by thermal oxidation in a dry oxygen or water vapor atmosphere, and then the generated carbon clusters are electrically inactivated by nitrogen atoms contained in NO gas. It is to remove. Regarding the heat treatment method of SiC in the NO gas atmosphere, the heat treatment time and temperature have already been investigated, and it has been reported that the effect becomes remarkable by the heat treatment at a high temperature for a long time.

However, the heat treatment in the NO atmosphere is performed using a normal hot wall furnace, and the NO gas itself is heated to a high temperature in the heat treatment in the hot wall furnace. For this reason, oxygen is generated by the reaction of 2NO → O ₂ + N ₂ , and formation of a thermal oxide film due to the oxygen cannot be ignored. Oxidation with oxygen forms carbon clusters at the oxide film / SiC interface, and as a result, it has been impossible to form an interface with excellent reduction in interface defects and excellent long-term reliability.
GYChung et al., Appl. Phys. Lett. 76, 1713 (2000)

  The present invention has been proposed in view of the above, and by performing heat treatment, interface defects between the semiconductor substrate side and the insulating film can be greatly reduced, and the reliability as a device is excellent. An object of the present invention is to provide a cold wall type heat treatment furnace and an insulating film forming apparatus capable of performing the above.

  In order to achieve the above object, according to the first aspect of the present invention, in a cold wall type heat treatment furnace used for forming an insulating film, a susceptor made of silicon carbide disposed in a reaction tube is heated by light irradiation, and the A feature is that an insulating film is formed by performing a heat treatment on a semiconductor substrate having a top layer made of silicon carbide.

  In the invention described in claim 2, in the invention described in claim 1, the light irradiation is light irradiation with a wavelength of 200 nm to a far infrared region.

  In the invention described in claim 3, in the invention described in claim 1 or 2, the heat treatment is performed at 1200 ° C. or more, and the temperature drop after the heat treatment is performed at a rate of 50 ° C./min or more. It is what I did.

  The invention according to claim 4 is the invention according to any one of claims 1 to 3, wherein a mixed gas of an inert gas and nitric oxide gas is supplied into the reaction tube. It is what I did.

  Furthermore, the invention according to claim 5 is the invention according to any one of claims 1 to 4, wherein the susceptor is made of a high-purity single crystal, polycrystal or sintered body made of silicon carbide. It shall be either.

  The invention described in claim 6 is the invention described in any one of claims 1 to 5, wherein the reaction tube is made of synthetic quartz.

  The invention described in claim 7 is the invention described in any one of claims 1 to 6, wherein the reaction tube is cooled at the outer peripheral wall by a cooling mechanism.

  According to an eighth aspect of the present invention, in an insulating film forming apparatus for forming an insulating film on a semiconductor substrate, a reaction tube in which a susceptor made of silicon carbide is disposed, and the reaction tube are heated by light irradiation from outside. Then, heat treatment is performed and the heat treatment temperature can be arbitrarily controlled, and a mixed gas of an inert gas and nitric oxide gas is supplied to the reaction tube to control the flow rate, and in the mixed gas A gas supply means capable of arbitrarily controlling the concentration of the nitrogen monoxide gas, a semiconductor substrate having a top layer of silicon carbide is placed on the susceptor, and an inert gas and nitrogen monoxide are placed in the reaction tube. A feature is that an insulating film is formed on the semiconductor substrate by supplying a mixed gas with the gas and performing heat treatment.

  The invention according to claim 9 is the invention according to claim 8, wherein the light irradiation by the heating means is light irradiation from a wavelength of 200 nm to a far infrared region.

  The invention according to claim 10 is the invention according to claim 8 or 9, wherein the heat treatment by the heating means is performed at 1200 ° C. or more, and the temperature drop after the heat treatment is performed at a rate of 50 ° C./min or more. Is to be done.

  The invention according to claim 11 is the invention according to any one of claims 8 to 10, wherein the gas supply means is 0.5% diluted with an inert gas in the reaction tube. A mixed gas containing 20% or less of nitric oxide gas is supplied.

  The invention according to claim 12 is the invention according to any one of claims 8 to 11, wherein the susceptor is made of a high-purity single crystal, polycrystal or sintered body made of silicon carbide. It shall be either.

  In this invention, the heat treatment is performed in the mixed gas atmosphere of the inert gas and the nitrogen monoxide gas containing the nitrogen monoxide gas diluted by the inert gas to 0.5% or more and 20% or less with respect to the insulating film. Compared with the case of 100% nitric oxide gas, the amount of oxygen atoms contained in the gas is reduced, so that the progress of oxidation by oxygen can be extremely reduced, and thereby the semiconductor substrate side and the insulating film Interfacial defects can be greatly reduced, and the reliability of the device can be improved.

  Further, since the uppermost layer of the semiconductor substrate is a silicon carbide layer and an insulating film is formed on the silicon carbide layer, the processing effect can be made remarkable, thereby further increasing the semiconductor substrate side and the insulating film. Interfacial defects can be reduced, and device reliability can be improved.

  Moreover, the decomposition of NO gas can be suppressed by using Ar as the inert gas, thereby further reducing the interface defects between the semiconductor substrate side and the insulating film, and the device has excellent reliability. Can be.

  In addition, after the heat treatment, by cooling at a temperature lowering rate of 50 ° C./min or more, it becomes possible to maintain the interface state between the SiC and the insulating film when the heat treatment is completed, thereby further insulating the semiconductor substrate side. Interface defects with the film can be reduced, and the reliability as a device can be improved.

  Furthermore, by performing the formation of the insulating film and the heat treatment in the NO atmosphere in the same apparatus, it is possible to avoid the introduction of impurities, thereby further reducing the interface defects between the semiconductor substrate side and the insulating film. And reliability as a device can be improved.

  In addition, since a cold wall type heat treatment furnace that is heated by light irradiation is used, it is possible to suppress a reaction in which nitrogen monoxide in the mixed gas is decomposed into oxygen and nitrogen by heat. As a result, the interface defects between the semiconductor substrate side and the insulating film can be further reduced, and the reliability of the device can be improved.

  In addition, since a detoxification facility for the nitrogen monoxide gas in the mixed gas is provided on the gas outlet side of the reaction tube, generation of harmful nitrided oxide can be prevented. Moreover, in this invention, since the density | concentration of nitric oxide gas is reduced significantly, the processing capacity of an abatement equipment can also be reduced according to it and it can reduce in size.

  Hereinafter, embodiments of the present invention will be described based on examples relating to interface characteristics of MOS capacitors fabricated using a silicon carbide substrate.

  FIG. 1 is a diagram schematically showing the configuration of an insulating film forming apparatus according to the present invention. The insulating film forming apparatus 1 is an apparatus for forming an insulating film on a semiconductor substrate, and includes a cold wall type heat treatment furnace 10, a gas supply unit 20, and a gas discharge unit 30, as shown in the figure. . The heat treatment furnace 10 has a reaction tube 11 made of quartz and an infrared lamp 12 provided along the outer periphery thereof, and the outer peripheral wall of the reaction tube 11 is cooled by a cooling gas supplied to a cooling mechanism. It is like that. A susceptor 13 made of a high-purity low-resistance SiC substrate (single crystal, polycrystal, sintered body, etc.) that sufficiently absorbs light in the infrared region is disposed in the approximate center of the reaction tube 11. A sample 14 is placed on the. Similar to the susceptor 13, the sample 14 is also made of a high-purity low-resistance SiC substrate that absorbs light in the infrared region.

The gas supply unit 20 is provided on the gas inlet side of the reaction tube 11. The gas supply unit 20 is pre-diluted with Ar gas so that nitric oxide (NO) is 20%, and is also pre-diluted with Ar gas so that NO is 2%. Ar gas cylinders and oxygen (O ₂ ) gas cylinders are arranged. The flow rate of gas from each of these cylinders is adjusted by mass flow controllers (MFC) 21, 22, and 23, and then mixed by a manifold 24, and then a heat treatment furnace. 10 is supplied.

  The gas discharge unit 30 is provided on the gas outlet side of the reaction tube 11, and a thermal decomposition type NO gas abatement device 31 and a vacuum exhaust system 32 are arranged.

In the insulating film forming apparatus 1 configured as described above, the reaction tube 11 is heated by light absorbed by the susceptor 13 or the sample 14 from a wavelength (200 nm) to a far infrared region where the NO gas flowing in the reaction tube 11 is not decomposed. Any wavelength may be used as long as it is. In this embodiment, heating is performed by irradiating light in the infrared region from the infrared lamp 12. As a result, even in a strong oxidizing gas atmosphere such as NO, N ₂ O, NO ₂ , and ozone, heating at a clean high temperature without contamination from the susceptor 13 is possible.

  A cold wall type heat treatment furnace includes a high-frequency heating furnace, but it is difficult to use a low-resistance SiC substrate as a susceptor in the high-frequency heating furnace.

  As is well known, quartz material has a very high infrared transmittance, so that the reaction tube 11 is hardly heated by infrared rays. In the present apparatus 1, a cooling mechanism is further added to the reaction tube 11 to minimize the temperature rise of the reaction tube 11 during the heat treatment. As a result, the temperature rise of the atmospheric gas can be extremely suppressed, and decomposition of the NO gas due to heat is suppressed.

  As described above, the exhaust side of the apparatus 1 is equipped with the NO gas abatement apparatus 31 and the vacuum exhaust system 32. In this insulating film forming apparatus 11, the heat treatment effect can be obtained even with a low NO concentration and a small flow rate, so that the processing capability of the abatement apparatus 31 can be reduced.

For the SiC substrate of the sample 14, an (0001) plane of n-type 4H—SiC with an epitaxial film was used. The impurity concentration of this SiC epitaxial film was 5 × 10 ¹⁵ cm ⁻² and the film thickness was 5 μm. Using this insulating film forming apparatus 1, a gate insulating film was formed on the epitaxial film of sample 14. This gate insulating film may be a thermal acid (nitriding) film, a deposited oxidation (nitriding) film, or a composite film thereof. In this embodiment, it is formed by thermal oxidation in a dry oxygen atmosphere at 1200 ° C. for 150 minutes. The gate oxide film thickness estimated from the storage capacity of the gate oxide film was 43 nm to 44 nm. After this thermal oxide film formation, heat treatment (POA: Post Oxidation Annealing) in an inert gas may be performed at the same temperature for a predetermined time. The inert gas may be any of Ar, N ₂ , He, etc., but N ₂ was used in this example. The heat treatment time was 30 minutes. After this POA, heat treatment according to the present invention, that is, heat treatment in an NO gas atmosphere diluted with Ar was performed. In this example, the heat treatment temperature and time were 1200 ° C. and 1 hour, respectively, and the NO concentration was 2%.

  After formation and heat treatment of the gate insulating film, a 500 mmφ Al electrode was formed on the gate insulating film using a metal mask to form a gate electrode. Al was also used for the back electrode. The interface characteristics of the fabricated MOS capacitor were evaluated by a high frequency-low frequency CV measurement method.

  FIG. 2 shows the high frequency-low frequency CV characteristics of the fabricated MOS capacitor. In the figure, (a) shows a case where only thermal oxidation is performed in a dry oxygen atmosphere, and (b) shows a case where heat treatment is further performed in a 2% NO atmosphere with respect to (a). A broken line represents a low-frequency CV characteristic, and a solid line represents a high-frequency CV characteristic.

In the above High-Low method, the interface state density _Dit is calculated by taking the difference between the low frequency measurement value and the high frequency measurement value. This is generally the interface state is obtained by utilizing the fact that not follow a high-frequency measuring differential capacitance in the CV characteristic of the high and low frequencies at the same gate voltage, thus representing the interface state density D _it. As can be seen from FIG. 2, in the CV characteristic of (b), the difference between the high frequency and the low frequency is smaller than that of (a). FIG. 3 shows the result of calculating the interface state density from the CV characteristics of FIG.

In FIG. 3, the horizontal axis indicates the energy level (Ec-E) from the conductor end. In FIG. 3, the value obtained by integrating the interface states in the energy level (Ec-E) range of 0.2 eV to 0.5 eV is (a) 3.5 × 10 ¹¹ cm ⁻² , (b). Was 1.1 × 10 ¹¹ cm ⁻² , and it was confirmed that the interface state decreased to 1/3 or less by performing 2% NO treatment. Further, as shown in (c), by setting the heat treatment time to 2 hours 30 minutes in a 2% NO atmosphere, the integrated value was similarly reduced to 7.2 × 10 ¹⁰ cm ⁻² . This was almost the same as the integral value 8.0 × 10 ¹⁰ cm ⁻² when heat treatment was performed for 1 hour in an atmosphere containing 5% NO shown in (d). That is, in the low concentration NO treatment, the total number of NO reaching the sample is considered to affect the treatment effect.

  From the above results, the lower limit of the NO concentration is that the treatment effect is rate-determined by the supply amount of NO when the NO concentration is low. Assuming that the time was 10 hours, it was set to 0.5%.

On the other hand, the upper limit was determined as follows. When SiC is oxidized with oxygen gas, if the oxygen partial pressure in the oxidizing atmosphere decreases, the oxidation rate decreases in proportion to the power of the pressure. Previous reports have shown that almost no oxide film is formed even at an oxidation temperature of 1200 ° C. at an oxygen partial pressure of about 50 Torr or less. Here, the partial pressure of 50 Torr is about 6.5% at (50/760 × 100)%. That is, if the partial pressure of oxygen molecules is lower than this, the progress of oxidation by oxygen is very small and can be ignored. Assuming that half of the NO introduced into the reactor is decomposed into oxygen, in order to reduce the amount of oxygen generated by the above-mentioned reaction of 2NO → O ₂ + N ₂ to 6.5% or less, The upper limit of NO concentration is about 26%. The above assumption corresponds to the decomposition of NO when NO gas is heated at 1175 ° C. for 200 seconds. If the heat treatment temperature is set high, the upper limit of the NO concentration becomes low. In order to perform an effective treatment, it is desirable to set the heat treatment temperature at a high temperature. However, considering the temperature commonly used with the quartz member, the maximum temperature is about 1300 ° C. is appropriate. Considering this, the upper limit of the NO concentration was set to 20%.

  From the above, by performing heat treatment at a NO concentration of 0.5% or more and 20% or less, it becomes possible to extremely reduce the progress of oxidation by oxygen compared to the case of 100%, and interface defects are reduced, and long-term reliability is achieved. It is possible to form a MOS (MIS) interface having excellent properties. In addition, it was confirmed that by actually performing the heat treatment at a NO concentration of 0.5% or more and 20% or less, interface defects are reduced and a MOS (MIS) interface having excellent long-term reliability can be formed. .

  In addition, after performing the heat treatment in NO, the interface temperature during the treatment was maintained by rapidly lowering the sample temperature, and it was confirmed that the interface defects were reduced and the reliability was improved as compared with the case of cooling. . This is particularly effective in a cold wall type heat treatment furnace by light irradiation heating, and high temperature and short time heat treatment reduces the thermal budget and brings about an efficient treatment effect. Further, the formation of the gate insulating film and the heat treatment are performed in the same apparatus, so that the mixing of impurities can be suppressed and the reliability of the semiconductor device can be improved.

In FIG. 1, the light irradiation method is such that the entire semiconductor sample is irradiated with light of the same wavelength. However, as shown in FIG. 4, a structure in which light of a different wavelength is irradiated may be used. . In FIG. 4, the ultraviolet irradiation unit 4 is disposed above the susceptor 13 to irradiate the sample surface with ultraviolet rays. And the light quantity of an ultraviolet-ray is increased using the elliptical condensing type reflective surface 4a. An Al film overcoated with MgF ₂ was used for the reflective surface 4a. Further, when accompanied by ultraviolet irradiation, synthetic quartz having a high ultraviolet transmittance was used as the quartz material of the reaction tube 11.

  Although the interface characteristics of the most basic MOS capacitor have been described in the above embodiments, the present invention is not limited to a horizontal MOS (MIS) FET, but also a vertical MOS (MIS) FET, an insulated gate bipolar transistor (IGBT). It is apparent that the present invention can be applied to all semiconductor devices having a gate insulating film forming process such as a MOS (MIS) thyristor.

  As described above, according to the present invention, by performing heat treatment of the gate insulating film at a predetermined time and temperature with an appropriate NO concentration using the insulating film forming apparatus 1, interface defects are reduced, and long-term reliability is achieved. An excellent MOS (MIS) type semiconductor device can be realized.

  Further, the insulating film forming apparatus of the semiconductor device includes a mechanism capable of arbitrarily setting the concentration of NO gas in the inert gas, a substrate heating mechanism by light irradiation, a cooling mechanism for the reaction tube 11, and a NO abatement apparatus 31. By forming an insulating film using this, an insulating film with low interface defects and high reliability can be formed.

  Further, as one step of forming the gate insulating film, heat treatment in an inert gas / NO mixed gas atmosphere of 0.5% or more and 20% or less diluted with an inert gas is performed using a cold wall type insulating film forming apparatus. Thus, an insulating film with low interface defects and high reliability is formed.

  Further, when at least the uppermost layer of the sample semiconductor substrate is made of silicon carbide, the treatment effect becomes remarkable, and an insulating film with particularly low interface defects and high reliability can be formed.

  In addition, when the inert gas is Ar, decomposition of the NO gas is suppressed, and an insulating film with low interface defects and high reliability can be formed.

  Further, by rapidly cooling the semiconductor device after the heat treatment, the interface state during the heat treatment can be maintained, and an insulating film with low interface defects and high reliability can be formed.

  Further, by performing the formation of the gate insulating film and the heat treatment in the NO atmosphere in the same apparatus, it is possible to avoid mixing of impurities and to form a highly reliable insulating film.

It is a figure which shows roughly the structure of the insulating film forming apparatus which concerns on this invention. It is a figure which shows the high frequency-low frequency CV characteristic of the produced MOS capacitor. When (a) is only subjected to thermal oxidation in a dry oxygen atmosphere, (b) is in a 2% NO atmosphere with respect to (a). This is a case where heat treatment is performed, and a broken line indicates a low-frequency CV characteristic and a solid line indicates a high-frequency CV characteristic. It is a figure which shows the interface state density characteristic computed using the High-low method of the produced MOS capacitor. It is a figure which shows roughly the structure of the insulating film formation apparatus which added the irradiation mechanism to the sample surface.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Insulating film formation apparatus 10 Heat processing furnace 11 Reaction tube 12 Infrared lamp 13 Susceptor 14 Sample 20 Gas supply part 21,22,23 Mass flow controller 24 Manifold 30 Gas discharge part 31 NO gas removal apparatus 32 Vacuum exhaust system 4 Ultraviolet irradiation unit 4a Reflective surface

Claims (10)

A reaction tube made of quartz in which a susceptor made of silicon carbide is disposed;
A heating means provided outside the reaction tube and capable of heating the semiconductor substrate disposed on the susceptor by light irradiation from a wavelength of 200 nm to a far-infrared region to perform a heat treatment at 1200 ° C. or higher;
A supply path for supplying a mixed gas containing nitrogen monoxide gas diluted with an inert gas to a reaction tube of 0.5% or more and 20% or less,
A cold wall type heat treatment furnace for forming an insulating film of a semiconductor substrate whose uppermost layer is silicon carbide . The cold wall type heat treatment furnace according to claim 1 , wherein the cold wall type heat treatment furnace is configured to be capable of lowering the temperature at a rate of 50 ° C / min or more after heat treatment. The susceptor, a high-purity single crystals of silicon carbide, either polycrystalline or sintered, cold-wall type heat treatment furnace according to claim 1 or 2. The cold wall heat treatment furnace according to any one of claims 1 to 3 , wherein the reaction tube is made of synthetic quartz. Furthermore, you provided a cooling mechanism for cooling the outer peripheral wall of the reaction tube, a cold wall type heat treatment furnace according to any one of claims 1 to 4. In an insulating film forming apparatus for forming an insulating film on a semiconductor substrate,
A reaction tube made of quartz in which a susceptor made of silicon carbide is disposed;
Provided outside of the reaction tube, arbitrarily controllable heating the heat-treatment temperature performs heating to 1200 ° C. or more heat treatment of the semiconductor substrate disposed on the susceptor by light irradiation from the wavelength 200nm to the far infrared region Means,
A mixed gas of an inert gas and nitric oxide gas is supplied to the reaction tube to control the flow rate thereof, and the concentration of the nitric oxide gas in the mixed gas is in the range of 0.5% to 20%. A controllable gas supply means,
A semiconductor substrate whose uppermost layer is silicon carbide is placed on the susceptor, and a heat treatment is performed by supplying a mixed gas of an inert gas and nitric oxide gas into the reaction tube to form an insulating film on the semiconductor substrate. ,
An insulating film forming apparatus. The insulating film forming apparatus according to claim 6 , wherein the temperature can be lowered at a rate of 50 ° C./min or higher after the heat treatment . The insulating film forming apparatus according to claim 7, further comprising a cooling mechanism that cools an outer peripheral wall of the reaction tube. The insulating film forming apparatus according to any one of claims 6 to 8, wherein the susceptor is one of a high-purity single crystal, polycrystal, and sintered body made of silicon carbide. The insulating film forming apparatus according to claim 6, wherein the reaction tube is made of synthetic quartz.

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