JP2007201343A - Manufacturing method of silicon carbide semiconductor element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a silicon carbide semiconductor element capable of efficiently removing or inactivating a carbon cluster remaining on an SiO<SB>2</SB>/SiC boundary even when forming an insulating film having a certain amount of thickness. <P>SOLUTION: In the manufacturing method of the silicon carbide semiconductor element in the process of forming the insulating film; the thickness of a thermally oxidized film is increased on the surface of a substrate by thermally treating the substrate for which a silicon carbide epitaxial film is formed under the atmosphere of an oxidized gas containing O<SB>2</SB>and/or H<SB>2</SB>O, and then the process of removing the carbon cluster existing on the SiO<SB>2</SB>/SiC boundary or the like by thermally treating the substrate under the atmosphere of a gas containing NO, N<SB>2</SB>O or NO<SB>2</SB>is repeated for two or more times. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a method for manufacturing a silicon carbide semiconductor device including a step of thermally insulating a substrate on which a silicon carbide epitaxial film is formed to form an insulating film of silicon dioxide on the surface of the silicon carbide epitaxial film. The present invention relates to an improvement in a technique for removing or inactivating carbon clusters remaining at an SiO ₂ / SiC interface during the formation of a thermal oxide film.

  Silicon carbide (SiC) has about 10 times the dielectric breakdown electric field strength and about 3 times the thermal conductivity as compared with silicon (Si), and has a relatively large electron mobility. It is expected as a semiconductor material that can realize dramatic performance improvement compared to power semiconductor elements.

  Recently, 4H-SiC and 6H-SiC single crystal substrates with a diameter of up to 3 inches have become commercially available, and various switching elements that greatly exceed the performance limit of Si have been reported one after another. Development is underway.

Since silicon carbide has Si atoms, an insulating film (SiO ₂ film) can be formed by a thermal oxidation process similar to that of silicon. As an element using this thermal oxidation process, MOSFE
A switching element having a gate insulating film such as T (metal / oxide / semiconductor field effect transistor) is expected. In recent years, high-purity epitaxially grown films can be obtained, and many MOS device prototypes have been reported.

However, the SiC-MOSFET in which the gate insulating film is formed by thermal oxidation has a large SiO ₂ / SiC interface state density that greatly affects the device characteristics. The current situation is inferior. There are attempts to form a gate insulating film by a method other than thermal oxidation, such as depositing a SiO ₂ film by CVD, but the method by thermal oxidation is considered promising, and further improvement of the characteristics of the thermal oxide film is required. ing.

When using a SiC-MOSFET in a power semiconductor device, in order to reduce the on-resistance and reduce the loss, it is a problem to lower the channel resistance by lowering the interface state density at the SiO ₂ / SiC interface. ing.

As a factor of the interface state at the SiO ₂ / SiC interface, the influence of carbon remaining at the interface has been pointed out, and it has been reported that defects related to carbon are formed at the SiO ₂ / SiC interface (non- Patent Documents 1 and 2). This residual carbon is a defect peculiar to the SiO ₂ / SiC interface called carbon cluster (amorphous carbon) and is considered to be generated during the formation of the thermal oxide film, which contributes to lowering the reliability of the device.

Oxidation occurs when silicon carbide is heated to a temperature of about 1100 ° C. in an oxidizing gas atmosphere containing oxygen. Oxidation of silicon carbide proceeds when the bonds of O atoms are combined with the bonds of Si atoms and C atoms are removed as oxides. However, some of the C atoms remain as clusters at the SiO ₂ / SiC interface.

  Due to the high interface state near the conductor due to this carbon cluster, the channel mobility of the SiC-MOSFET in which the gate insulating film is fabricated by the usual thermal oxidation method is higher than expected from the electron mobility of the SiC bulk. Lower. For this reason, the on-resistance value becomes higher than the value theoretically expected from the physical property values.

As a method for reducing the interface state due to such carbon clusters, heat treatment in an atmosphere of NO gas or N ₂ O gas has been proposed (Non-Patent Documents 3, 4, 5 and Patent Documents 1, 2). In the method described in these documents, after forming a thermal oxide film on the surface of the silicon carbide epitaxial film by thermal oxidation in a dry oxygen or water vapor atmosphere, carbon clusters generated at the SiO ₂ / SiC interface by thermal oxidation are obtained. It is removed or electrically inactivated by nitrogen atoms contained in NO gas or N ₂ O gas. Regarding annealing in these gas atmospheres, appropriate conditions for temperature, time, etc. have been investigated.
JP 2005-109396 A JP-A-2005-116893 Applied Phisics Letters Volume 77 No. 14 2000, pages 2186-2188 Phisyca Status Solidi (a) Volume 162 1997 321-337 Applied Phisics Letters Volume 76 No. 13 2000 1713-1715 IEEE Electron Device Letters Volume 22 No. 4 2001 176-178 Applied Physics Letters Volume 70 No. 15 1997 2028-2030

  However, in the method in the above document, thermal oxidation in a dry oxygen or water vapor atmosphere is performed at a time as a pre-process, thereby forming, for example, a thermal oxide film having a thickness of about 20 to 50 nm, followed by NO gas or the like as a final process. Heat treatment is performed in the atmosphere.

  For the purpose of improving the device characteristics, it is also known to heat-treat the silicon carbide substrate in various gas atmospheres such as hydrogen and inert gas after the formation of the thermal oxide film. The heat treatment is performed as a final step after forming the pre-step at a time.

However, in such a conventional method, since a thermal oxide film having a film thickness to be obtained as a target insulating film is formed in advance before the heat treatment with the NO gas or the like, an insulating film having a certain film thickness is produced. If necessary, heat treatment with the above-mentioned NO gas or the like may be required for a long time in order to remove or inactivate carbon clusters remaining at the SiO ₂ / SiC interface to a sufficient extent.

In other words, when the thermal oxide film is thickened to some extent, the efficiency of removing the carbon cluster in the deeper portion is reduced. When the thermal oxide film is further thickened, the carbon cluster is removed even if heat treatment is performed over time. It may not be achieved to a sufficient extent. The reason is that as the thermal oxide film becomes thicker, the diffusion of NO gas and the like from the surface of the thermal oxide film to the deep part is suppressed, and the gas does not easily reach the SiO ₂ / SiC interface where carbon clusters remain. Can be considered.

In the present invention, even when an insulating film having a certain thickness is formed, SiO ₂ / SiC
An object of the present invention is to provide a method for manufacturing a silicon carbide semiconductor element capable of efficiently removing or inactivating carbon clusters remaining at an interface.

As a result of diligent studies to achieve the above object, the present inventors have not formed a thermal oxide film having a film thickness to be obtained as an insulating film at a time before heat treatment with NO gas or the like but instead of NO gas or the like. The thermal oxidation process is performed in several steps while interposing the heat treatment process by, and by this, the NOx of the carbon cluster at the SiO ₂ / SiC interface etc. is increased by gradually increasing the thickness of the thermal oxide film The inventors have found that the removal or inactivation can be efficiently performed, and the present invention has been completed.

A method of manufacturing a silicon carbide semiconductor device according to the present invention includes a step of thermally oxidizing a substrate on which a silicon carbide epitaxial film is formed to form an insulating film of silicon dioxide on the surface of the silicon carbide epitaxial film. A method for manufacturing an element, comprising:
In the step of forming the insulating film, after increasing the film thickness of the thermal oxide film on the surface of the substrate by heat-treating the substrate in an oxidizing gas atmosphere containing O ₂ and / or H ₂ O The step of heat-treating the substrate in an atmosphere of a gas containing NO, N ₂ O or NO ₂ is repeated a plurality of times.

  The method for manufacturing a silicon carbide semiconductor element according to the above invention can be applied to a process of forming an insulating film such as a gate insulating film, a sacrificial oxide film, and a surface protective film, but is suitable for a process of forming a gate insulating film.

According to the present invention, even when an insulating film having a certain thickness is formed, SiO ₂ /
Carbon clusters remaining at the SiC interface can be efficiently removed or inactivated.
This improves device characteristics such as on-resistance.

  Hereinafter, the present invention will be described in detail. In the present invention, a silicon carbide single crystal substrate obtained by growing a silicon carbide epitaxial film from the surface is suitably used as a semiconductor substrate for forming an element. In addition, a substrate in which a silicon carbide epitaxial film is formed on the surface of another semiconductor single crystal substrate such as a silicon substrate may be used.

  As the silicon carbide single crystal substrate, a slice obtained by slicing a bulk crystal obtained by a sublimation method or a CVD method can be used. In the case of the sublimation method (modified Rayleigh method), for example, silicon carbide powder is put into a crucible, heated and vaporized at 2200 to 2400 ° C., and the surface of the seed crystal is typically at a rate of 0.8 to 1 mm / h. To grow bulk. The obtained ingot is sliced into a predetermined thickness so that a desired crystal plane appears. In order to suppress the propagation of basal plane dislocations to the epitaxial film, the surface of the cut wafer is processed by polishing using abrasive grains, hydrogen etching, chemical mechanical polishing (CMP), etc. It is preferable to smooth the mirror surface.

  A silicon carbide single crystal epitaxial film is grown from the surface of the silicon carbide single crystal substrate. Crystalline polymorphs (polytypes) exist in silicon carbide single crystals. For example, 4H—SiC (hexagonal quadruple periodic type), 6H—SiC (hexagonal hexaperiodic type), 2H—SiC (hexagonal crystal) Two-cycle type), 15R-SiC (rhombic fifteen-fold cycle type), and the like are used as the silicon carbide single crystal substrate. Examples of crystal planes for epitaxial growth include (0001) Si plane, (000-1) C plane, (11-20) plane, (01-10) plane, (03-38) plane, and the like.

  When epitaxially growing on the (0001) Si plane or the (000-1) C plane, the [01-10] direction, the [11-20] direction, or the intermediate direction between the [01-10] direction and the [11-20] direction For example, a substrate cut at an off angle of 1 to 12 ° in the off orientation is used, and silicon carbide is epitaxially grown from this crystal plane by a step flow growth technique.

  The epitaxial growth of the silicon carbide single crystal film is performed using a CVD method. Propane or the like is used as the C source gas, and silane or the like is used as the Si source gas. A mixed gas of these source gases, a carrier gas such as hydrogen, and a dopant gas is supplied to the surface of the silicon carbide single crystal substrate. As the dopant gas, nitrogen or the like is used when growing an n-type epitaxial film, and trimethylaluminum or the like is used when growing a p-type epitaxial film.

  Under these gas atmospheres, for example, silicon carbide is epitaxially grown at a growth rate of 2 to 20 μm / h under conditions of 1500 to 1600 ° C. and 40 to 80 Torr. Thereby, silicon carbide having the same crystal type as that of the silicon carbide single crystal substrate is step-flow grown.

  As a specific apparatus for performing epitaxial growth, a vertical hot wall furnace can be used. In the vertical hot wall furnace, a water-cooled double cylindrical tube made of quartz is installed. Inside the water-cooled double cylindrical tube, a cylindrical heat insulating material, a hot wall made of graphite, and a silicon carbide single crystal A wedge-shaped susceptor is provided to hold the substrate in the vertical direction. A high-frequency heating coil is installed around the outside of the water-cooled double cylindrical tube, the hot wall is induction-heated by the high-frequency heating coil, and the silicon carbide single crystal substrate held by the wedge-shaped susceptor is heated by radiant heat from the hot wall. To do. By supplying the reaction gas from below the water-cooled double cylindrical tube while heating the silicon carbide single crystal substrate, silicon carbide is epitaxially grown on the surface of the silicon carbide single crystal substrate.

  Using the silicon carbide single crystal substrate on which the epitaxial film is formed in this way, various semiconductor elements are manufactured through processes according to the element such as dopant ion implantation and annealing, formation of an insulating film, and electrode formation.

  Examples of the insulating film to which the present invention can be applied include a gate insulating film, a surface protective film (passivation film), and a sacrificial oxide film. Among these, specific examples of the semiconductor element having a gate insulating film include an insulated gate semiconductor element having a MOS structure such as a MOSFET, an IGBT (insulated gate bipolar transistor), or a MOS thyristor. The structure of the insulated gate transistor includes a planar type, a vertical type, and a trench gate type, and the present invention can be applied to any of these.

  Among these, an example of a method for manufacturing a MOSFET is as follows. First, as described above, an n-type SiC drift layer is formed on an n-type SiC single crystal substrate by an epitaxial growth method.

  Thereafter, a dopant such as aluminum or boron is ion-implanted into each part of the drift layer separated by a predetermined interval to form a pair of p-type base regions. Further, a dopant such as nitrogen or phosphorus is ion-implanted into these p-type base regions using a resist as a mask to form an n-type source region. After these ion implantations, the implanted ions are electrically activated by heat-treating the wafer at a high temperature.

Subsequently, a gate insulating film made of SiO _{2 is} formed on the entire surface of the wafer by thermal oxidation. Thereafter, a gate electrode is formed and patterned on the gate insulating film. The gate electrode is patterned in such a shape that a pair of base region and source region are located at both ends, and a drift layer exposed between the base regions is located in the center.

Further, the remaining portion of the gate insulating film on each source region is removed by lithography and etching techniques, and a source electrode is formed and patterned at a portion where the source region is exposed on the surface. A MOSFET electrode structure is obtained by forming a drain electrode on the back side of the substrate.

  The surface protective film to which the insulating film forming process of the present invention can be applied is a so-called passivation film for isolating and protecting the device from the external environment and mechanically and chemically protecting the surface of the semiconductor element. The formation of the surface protective film by the method can be applied to various unipolar elements and bipolar elements in addition to the above-described insulated gate semiconductor elements.

The sacrificial oxide film to which the insulating film forming process of the present invention can be applied is formed for the purpose of improving the morphology of the epitaxial film surface by forming the oxide film and removing it with hydrofluoric acid or the like. In this sacrificial oxide film forming process, in the conventional method, carbon clusters remain on each part of the surface, and this residual carbon may become an obstacle to appropriate oxidation growth during the subsequent formation of the insulating film by thermal oxidation. is there. It can also be a factor that increases the interface state density at the SiO ₂ / SiC interface. However, such a point can be improved by forming a sacrificial oxide film by the method of the present invention. In addition to the insulated gate semiconductor elements described above, the present invention can be applied to various unipolar elements and bipolar elements.

In the insulating film forming step of the present invention, SiC is applied in an atmosphere of an oxidizing gas such as dry O ₂ or water vapor.
A thermal oxidation step of increasing the thickness of the thermal oxide film on the surface of the SiC epitaxial film by heat-treating the substrate on which the epitaxial film is formed; and under an atmosphere of a gas containing NO, N ₂ O or NO ₂ Each of the carbon cluster reduction processes for heat-treating the substrate is alternately repeated a plurality of times.

As a result, even when the thermal oxide film is thick to some extent, multiple carbon cluster reduction processes are performed across multiple thermal oxide film formation processes that increase the film thickness in stages. The carbon clusters at the SiO ₂ / SiC interface can be efficiently removed or inactivated.

  The thermal oxidation step and the carbon cluster reduction step may be performed while switching and supplying the gas in the same reaction furnace, and preparing a reaction furnace for thermal oxidation and a heat treatment reaction furnace such as NO gas, Although the temperature may be once lowered after each step and the substrate is taken out from the furnace and introduced into another furnace, a method of alternately switching gases in the same furnace is preferable because the process can be shortened.

  As the reaction furnace for performing each of the above steps, for example, a cold wall type reaction furnace or hot wall that heats a substrate placed in a reaction tube through which a gas flows by resistance heating, heating by an infrared lamp, high frequency induction heating, or the like. Conventionally known structures such as a type reactor can be used.

In the reaction furnace, for example, a heat treatment gas such as O ₂ gas and NO gas, and a cylinder such as an inert gas are connected to the gas inlet side of the reaction tube together with a mass flow controller, a manifold, and the like. The gas required for the process is supplied to the reaction tube while adjusting the mixing ratio. An evacuation system or the like is installed on the gas outlet side of the reaction tube.

Hereinafter, an example of the insulating film forming process in the present invention using the above-described reaction furnace will be described. It should be noted that the SiC surface of the substrate on which the thermal oxide film is to be formed is previously cleaned according to an ordinary method such as RCA cleaning. When the substrate is introduced into the furnace and the thermal oxidation temperature is reached, the reaction furnace is changed from an inert gas atmosphere such as Ar or N ₂ to an oxidizing gas atmosphere such as an oxygen atmosphere containing water vapor or a dry oxygen atmosphere. This state is maintained for a predetermined time by switching. Thus, a thermal oxide film is formed on the SiC surface by a predetermined thickness.

After the thermal oxidation step is completed, the oxidizing gas atmosphere is switched to an inert gas atmosphere such as Ar or N ₂ , and the temperature is raised or lowered until reaching the heat treatment temperature in the next carbon cluster reduction step. However, when the heat treatment temperature is the same as the thermal oxidation temperature, switching to an inert gas atmosphere may be omitted.

  For example, when the thermal oxidation step and the carbon cluster reduction step are performed in different reactors, the temperature of the substrate can be moved between the two reactors, for example, once lowered to a temperature of 700 ° C. or lower, and the reaction for the thermal oxidation step. The substrate is taken out from the furnace and introduced into the reaction furnace for the carbon cluster reduction process.

Subsequently, when the inside of the reaction furnace reaches a predetermined temperature, the gas atmosphere is switched to an atmosphere of a heat treatment gas such as NO gas, and the carbon cluster reduction process is started. As a result, the carbon cluster at the SiO ₂ / SiC interface is removed or inactivated, and the interface state decreases. Note that the oxide film thickness may increase due to the oxynitriding of the MOS interface in an atmosphere of NO gas or the like.

  When the temperature is changed after the carbon cluster reduction process is completed, switch to the inert gas atmosphere again as described above to raise or lower the temperature, and switch to the oxidizing gas atmosphere when the temperature is stabilized. Then, the thermal oxidation process is performed again. When another reaction furnace is used for thermal oxidation, the substrate is taken out from the furnace after the temperature is lowered and introduced into the reaction furnace for thermal oxidation as in the case described above, and then the thermal oxidation process is performed again. Thereby, the film thickness of the thermal oxide film is further increased by a predetermined thickness.

  Subsequently, the carbon cluster reduction step is performed again by the method described above. In this way, each of the thermal oxidation process and the carbon cluster reduction process is alternately performed a plurality of times, thereby performing a plurality of heat treatments using NO gas or the like while the film thickness is thin until the final film thickness is obtained. This process is repeated to remove or inactivate the carbon clusters, and finally an insulating film having a desired film thickness is obtained.

The thermal oxidation in the thermal oxidation step may be performed in a dry atmosphere using dry O _{2 or} in a wet atmosphere. Generally, in wet oxidation, heated deionized water is bubbled with oxygen or an inert gas to flow vapor H ₂ O to the wafer, and H ₂ O is generated by reacting H ₂ and O _2. Any method may be used.

  The heating temperature of the substrate at the time of thermal oxidation is appropriately set according to a desired oxide film thickness and other conditions, and is, for example, in the range of 900 ° C to 1200 ° C. In general, the film quality of the thermal oxide film changes depending on the oxide film growth rate and the like, but the oxide film growth rate depends not only on the oxidation temperature but also on the oxidizing atmosphere, the amount of supplied oxygen, etc., so it is necessary to set the conditions appropriately.

The carbon cluster reduction step is performed under heating at an appropriate temperature in an atmosphere of NO, N ₂ O, or NO ₂ diluted with an inert gas such as Ar as necessary. The heating temperature of the substrate during the heat treatment is appropriately set according to the oxide film thickness, the type of atmospheric gas, the supply amount, and the like, and is, for example, in the range of 900 ° C to 1200 ° C. Although the heat processing time in one process is based on conditions, it is about 1 to 5 hours, for example.

  In the present invention, the above thermal oxidation step and carbon cluster reduction step are alternately repeated at least twice or more. For example, when a gate insulating film having a thickness of 20 nm to 50 nm is obtained, the thermal oxidation process is performed several times to increase the thickness of the thermal oxide film every 2 nm to 10 nm, and NO gas is used as a process in the meantime. It is possible to perform a heat treatment by, for example. The same applies when the insulating film is a surface protective film, a sacrificial oxide film, or the like.

  In addition, the amount of increase in film thickness can be changed by adjusting the conditions in each thermal oxidation process performed in stages, or the heat treatment conditions can be changed in accordance with the thermal oxide film thickness in each carbon cluster reduction process. Also good.

  The number of repetitions of the thermal oxidation step and the carbon cluster reduction step is, for example, NO gas etc. after forming a thermal oxide film at a time in the past in consideration of the interface state density, fixed charge, etc. obtained from CV characteristic measurement etc. Therefore, the effectiveness, the time required for the process, and the like compared with the heat treatment method may be determined appropriately. However, according to the method described above in which the thermal oxidation step and the carbon cluster reduction step are repeated while switching the gas in the same furnace, there is little increase in time due to the repetition of the step.

Claims (2)

A method for manufacturing a silicon carbide semiconductor element, comprising: thermally oxidizing a substrate on which a silicon carbide epitaxial film is formed to form a silicon dioxide insulating film on a surface of the silicon carbide epitaxial film;
In the step of forming the insulating film, the thickness of the thermal oxide film on the surface of the silicon carbide epitaxial film is increased by heat-treating the substrate in an oxidizing gas atmosphere containing O ₂ and / or H ₂ O. And a step of heat-treating the substrate in an atmosphere of a gas containing NO, N ₂ O or NO ₂ is repeated a plurality of times.   The method for manufacturing a silicon carbide semiconductor element according to claim 1, wherein the insulating film is a gate insulating film.