JP2014225692A - Semiconductor device and method of manufacturing semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor device capable of suppressing dielectric breakdown of a gate insulating film on a bottom surface of a gate trench, while suppressing increase in thickness of the gate insulating film on a side surface of the gate trench.SOLUTION: In a semiconductor device 1, an epitaxial layer 3 composed of SiC is formed so that a surface 31 is to be an Si plane. A gate trench 6 recessed from the surface 31 is formed in the epitaxial layer 3. A gate insulating film 9 is formed on a bottom surface 8 and a side surface 7 of the gate trench 6 so that a ratio of a thickness T2 of a part on the bottom surface 8 (an insulating film bottom part 11) to a thickness T1 of a part on the side surface 7 (an insulating film side part 10) is 0.3 to 1.0. A gate electrode 12 is embedded in the gate trench 6 through the gate insulating film 9.

Description

  The present invention relates to a semiconductor device using SiC and a manufacturing method thereof.

In recent years, the use of SiC (silicon carbide: silicon carbide) has been studied as a next-generation power device material that achieves high breakdown voltage and low on-resistance.
A trench gate structure is known as a structure for miniaturization of power devices and reduction of on-resistance. For example, power MOSFETs using a trench gate structure are becoming mainstream.

FIG. 11 is a schematic cross-sectional view of a semiconductor device having a conventional trench gate type VDMOSFET using SiC.
The semiconductor device 201 has a structure in which unit cells of trench gate type VDMOSFETs are arranged in a matrix.
The semiconductor device 201 includes an N ⁺ type SiC substrate 202 that forms the base of the semiconductor device 201. On the Si surface (silicon surface) of the SiC substrate 202, an N ⁻ type epitaxial layer 203 made of SiC (silicon carbide: silicon carbide) doped with N-type impurities at a lower concentration than the SiC substrate 202 is laminated. Has been. The base layer portion of the epitaxial layer 203 forms an N ⁻ -type drain region 204 that is maintained as it is after epitaxial growth. In the epitaxial layer 203, a P-type body region 205 is formed on the drain region 204 in contact with the drain region 204.

In the epitaxial layer 203, a gate trench 206 is formed by being dug down from the surface 217 (Si surface). The gate trench 206 penetrates the body region 205 in the layer thickness direction, and the deepest portion (bottom surface 216) reaches the drain region 204.
In the gate trench 206, the gate insulating film 207 made of SiO ₂ is formed over the entire inner surface of the gate trench 206 by thermally oxidizing the side surfaces 214 and the bottom surface 216 of the gate trench 206.

A gate electrode 208 is embedded in the gate trench 206 by filling the inside of the gate insulating film 207 with polysilicon doped with N-type impurities at a high concentration.
In the surface layer portion of the epitaxial layer 203, N ⁺ -type source regions 209 are formed on both sides of the gate trench 206 in the direction orthogonal to the gate width (left and right direction in FIG. 11). The source region 209 extends in the direction along the gate width along the gate trench 206, and the bottom thereof is in contact with the body region 205 from the surface 217 side of the epitaxial layer 203.

In addition, a P ⁺ -type body contact region 210 that penetrates from the surface 217 of the epitaxial layer 203 through the center of the source region 209 in the direction orthogonal to the gate width and is connected to the body region 205 is formed. .
On the epitaxial layer 203, an interlayer insulating film 211 made of SiO ₂ is stacked. A source wiring 212 is formed on the interlayer insulating film 211. The source wiring 212 is formed on the nickel silicide layer 218 and the nickel silicide layer 218 that are in contact with the source region 209 and the body contact region 210 through contact holes 213 formed in the interlayer insulating film 211 and the gate insulating film 207. And an aluminum layer 219.

Drain wiring 215 is formed on the back surface (carbon surface: C surface) of SiC substrate 202. Drain wiring 215 has a nickel silicide layer 220 in contact with SiC substrate 202 and an aluminum layer 221 formed on nickel silicide layer 220.
The gate electrode 208 is applied with a predetermined voltage (a voltage equal to or higher than the gate threshold voltage) in a state where a predetermined potential difference is generated between the source wiring 212 and the drain wiring 215 (between the source and drain). A channel is formed in the vicinity of the interface with the gate insulating film 207 in the body region 205 by the electric field from the electrode 208. Thereby, a current flows between the source wiring 212 and the drain wiring 215, and the VDMOSFET is turned on.

JP 2007-258465 A

Since the surface 217 of the epitaxial layer 203 is an Si surface, the bottom surface 216 of the gate trench 206 dug down from the surface 217 is an Si surface.
Therefore, when the gate insulating film 207 is formed by dry oxidation or wet oxidation, the ratio of the oxidation rate of the bottom surface 216 to the oxidation rate of the side surface 214 (the oxidation rate of the bottom surface 216 / the oxidation rate of the side surface 214) is 0.2 or Less than that. Therefore, in the gate insulating film 207, the thickness of the portion on the bottom surface 216 is smaller than the thickness of the portion on the side surface 214.

On the other hand, in the semiconductor device 201, when the VDMOSFET is turned off, a high potential difference is generated between the gate electrode 208 and the drain wiring 215 (between the gate and the drain), and the electric field is concentrated on the bottom surface 216 of the gate trench 206. As described above, in the gate insulating film 207 where the thickness of the portion on the bottom surface 216 is small, dielectric breakdown is likely to occur due to electric field concentration.
For this problem, a measure for increasing the thickness of the portion on the bottom surface 216 by increasing the oxidation time when forming the gate insulating film 207 is studied. However, since the oxidation of the side surface 214 proceeds in parallel with the oxidation of the bottom surface 216, the thickness of the portion on the side surface 214 becomes very large due to the difference in the oxidation rate.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor device and a method for manufacturing the same that can suppress the dielectric breakdown of the portion on the bottom surface of the gate trench while suppressing the increase in the thickness of the portion on the side surface of the gate trench in the gate insulating film. There is to do.

  In order to achieve the above object, the invention according to claim 1 is a first conductivity type semiconductor layer made of SiC and having a Si surface, a gate trench dug from the surface of the semiconductor layer, and the gate trench. A gate insulating film formed on the bottom and side surfaces of the substrate, wherein a ratio of a thickness of the portion on the bottom surface to a thickness of the portion on the side surface is 0.3 to 1.0, and the gate insulating film A semiconductor device including a gate electrode embedded in the gate trench.

According to this configuration, the gate trench is formed by digging down from the surface of the first conductivity type semiconductor layer made of SiC and having a Si surface. A gate insulating film is formed on the bottom and side surfaces of the gate trench. A gate electrode is embedded in the gate trench through a gate insulating film.
Thus, in this semiconductor device, a trench having a MOS (Metal Oxide Semiconductor) structure in which a gate electrode (Metal) is opposed to a semiconductor layer (Semiconductor) via a portion (Oxide) on a side surface of the gate trench in the gate insulating film. A gate type MOSFET is formed.

  In the MOSFET, the ratio of the thickness of the portion on the bottom surface to the thickness of the portion on the side surface in the gate insulating film is 0.3 to 1.0. Even if the thickness of the portion on the bottom surface is increased to such an extent that dielectric breakdown can be suppressed, the lower limit of (thickness of the portion on the bottom surface / thickness of the portion on the side surface) is 0.3. An excessive increase in the thickness of the portion can be suppressed. On the other hand, since the upper limit is 1.0, when the thickness of the portion on the bottom surface is designed to an appropriate size, the thickness of the portion on the side surface does not become excessively small. As a result, by appropriately designing the thickness of the portion on the bottom surface, it is possible to suppress the dielectric breakdown of the portion on the bottom surface while suppressing an increase in the thickness of the portion on the side surface.

  According to a second aspect of the present invention, in the semiconductor layer, a body region of a second conductivity type formed on a side of the gate trench and in contact with the gate insulating film on a side surface of the gate trench; 2. The semiconductor device according to claim 1, wherein the surface layer portion includes a first conductivity type source region formed adjacent to the gate trench, and the gate insulating film contains nitrogen.

  In this configuration, in the semiconductor layer, a second conductivity type body region that is in contact with the gate insulating film on the side surface of the gate trench is formed on the side of the gate trench. In the surface layer portion of the body region, a source region of the first conductivity type is formed adjacent to the gate trench. Therefore, in the trench gate type MOSFET in the semiconductor device, the portion in the vicinity of the interface with the gate insulating film in the body region is a channel portion where a channel is formed by the electric field from the gate electrode. In this semiconductor device, since the gate insulating film contains nitrogen, the channel mobility of the MOSFET can be improved.

The invention according to claim 3 is the semiconductor device according to claim 2, wherein the concentration of the second conductivity type impurity in the body region is 1e19 cm ⁻³ or less.
If the impurity concentration of the body region on the side of the gate trench exceeds 1e19 cm ⁻³ , when the bottom surface and side surface of the gate trench are oxidized, the trench side surface is oxidized at a relatively high oxidation rate with respect to the trench bottom surface. As a result, the portion on the side surface of the gate insulating film becomes very thick.

On the other hand, when the impurity concentration of the body region is 1e19 cm ⁻³ or less, the ratio of the oxidation rate of the trench side surface to the oxidation rate of the trench bottom surface is maintained at an appropriate level when oxidizing the bottom surface and side surface of the gate trench. be able to. As a result, an increase in the thickness of the portion on the side surface of the gate insulating film can be suppressed.
The invention according to claim 4 further includes an implantation layer formed by impurity implantation in a portion from the bottom surface of the gate trench to a middle portion in the thickness direction of the semiconductor layer in the semiconductor layer. The semiconductor device according to any one of Items 1 to 3.

By forming the implantation layer immediately below the bottom surface of the gate trench, after the formation of the implantation layer, when oxidizing the bottom surface and the side surface of the gate trench, the trench bottom surface is oxidized at a relatively high oxidation rate with respect to the trench side surface, The ratio of the thickness of the portion on the bottom surface to the thickness of the portion on the side surface in the gate insulating film can be 0.3 to 1.0.
The invention according to claim 5 is the semiconductor device according to claim 3, wherein the implantation layer is formed by implantation of the second conductivity type impurity.

If the implantation layer is formed by implantation of a second conductivity type impurity different from the conductivity type of the semiconductor layer, the energy barrier formed between the implantation layer and the semiconductor layer can be increased. Therefore, it is possible to make it difficult for the current to flow to the implantation layer. As a result, electric field concentration on the bottom surface of the gate trench can be suppressed.
The invention according to claim 6 is the semiconductor device according to any one of claims 1 to 5, wherein a thickness of a portion on the side surface of the gate trench in the gate insulating film is 2000 mm or less. is there.

If the thickness of the portion on the side surface of the gate trench exceeds 2000 mm, it is necessary to operate the semiconductor device with a high gate-on voltage (for example, about 20 V), and efficient transistor operation may not be performed.
On the other hand, if the thickness of the portion on the side surface of the gate trench is 2000 mm or less, the semiconductor device can be operated with an appropriate gate-on voltage, and an efficient transistor operation can be achieved.

Moreover, invention of Claim 7 is as described in any one of Claims 1-6 in which the edge part in the direction orthogonal to the gate width of the bottom part of the said gate trench is curving outward. It is a semiconductor device.
In this configuration, by curving the end portion of the bottom of the gate trench where the electric field tends to concentrate during turn-off, the electric field applied to the end portion can be distributed to portions other than the end portion. As a result, the dielectric breakdown of the portion on the bottom surface of the gate insulating film can be suppressed.

The invention according to claim 8 includes a source wiring formed on the semiconductor layer and in contact with the source region, and the source wiring has a polysilicon layer in a contact portion with the source region, The semiconductor device according to claim 2, further comprising a metal layer on the polysilicon layer.
For example, in the semiconductor device 201 shown in FIG. 11, in order to form the source wiring 212, first, the surface (source region 209 and body contact region) of the impurity-doped region (impurity region) in the epitaxial layer 203 is formed by sputtering. Ni is deposited on the surface 210. Next, Ni is silicided by reacting Si in the SiC with Ni by heat treatment at a high temperature (for example, about 1000 ° C.) in order to make Ni ohmic contact with the impurity region. Thereby, a nickel silicide layer 218 is formed. Thereafter, Al is deposited on the nickel silicide layer 218 by sputtering. Thereby, the aluminum layer 219 is formed, and the source wiring 212 is formed.

  However, when the nickel silicide layer 218 is formed, residual carbon (C) in SiC is precipitated in the vicinity of the surface of the nickel silicide layer 218 and the interface between the impurity regions in the nickel silicide layer 218 and carbon containing a large amount of C. A layer is formed. Since the carbon layer has poor adhesion to metal or SiC, delamination is likely to occur between the aluminum layer 219 and the nickel silicide layer 218 and between the nickel silicide layer 218 and the impurity region.

Therefore, in the configuration of claim 8, the source wiring contacted with the source region has a polysilicon layer in a contact portion with the source region and a metal layer on the polysilicon layer.
Polysilicon can form a good ohmic junction with a region doped with impurities in SiC (impurity region). Therefore, silicidation essential for the structure in which the metal layer is in direct contact with the source region can be omitted. Therefore, it is possible to prevent the generation of the carbon layer in the vicinity of the interface between the surface of the polysilicon layer and the source region in the polysilicon layer. As a result, layer peeling between the polysilicon layer and the metal layer and between the polysilicon layer and the source region can be suppressed. Therefore, the connection reliability of the source wiring can be improved.

The invention according to claim 9 is the semiconductor device according to claim 8, wherein an intermediate layer containing Ti is interposed between the polysilicon layer and the metal layer.
The material containing titanium has excellent adhesion to both the polysilicon material and the metal material. Therefore, in a semiconductor device having a configuration in which a layer containing titanium is interposed between the polysilicon layer and the metal layer, the adhesion between the polysilicon layer and the metal layer can be improved. As a result, the connection reliability of the source wiring can be further improved.

In the invention according to claim 10, the metal layer has a layer containing Al, and the intermediate layer has a structure in which a Ti layer and a TiN layer are laminated in this order from the polysilicon layer side. The semiconductor device according to claim 9.
Al can be used as an impurity for imparting conductivity to the polysilicon layer, but the resistance value of the polysilicon layer used as the source wiring is unstable unless mixed in the polysilicon layer in an appropriate amount. It may become.

  Therefore, in the structure of the tenth aspect, a TiN layer is interposed between the Al-containing layer and the polysilicon layer as a barrier layer for preventing diffusion of Al into the polysilicon layer. Thereby, since excess Al does not diffuse into the polysilicon layer, the impurity concentration of the polysilicon layer can be stabilized. As a result, the resistance value of the polysilicon layer can be stabilized.

  According to an eleventh aspect of the present invention, there is provided a step of forming a gate trench dug down from a surface of the first conductivity type semiconductor layer made of SiC and having a Si surface. Forming a gate insulating film on the bottom and side surfaces of the gate trench by oxidizing the bottom and side surfaces at a heat treatment temperature of 1200 ° C. or higher in a gas containing nitrogen and oxygen; and the gate insulating film And forming a gate electrode so as to fill the gate trench.

If the bottom and side surfaces of the gate trench are oxidized under the conditions (atmosphere gas and heat treatment temperature) in this method, the ratio of the thickness of the portion on the bottom surface to the thickness of the portion on the side surface in the gate insulating film is 0.3. -1.0.
In the step of forming the gate insulating film, as described in claim 12, it is preferable that the bottom surface and the side surface of the gate trench are oxidized in a gas containing at least N ₂ O. The N ₂ O gas is preferably supplied at a flow rate ratio of 30% or less with respect to the total flow rate of the supplied gas.

The step of forming the gate insulating film includes a step of charging the semiconductor layer into a resistance heating furnace, a gas containing nitrogen and oxygen is introduced into the resistance heating furnace, and an atmosphere containing nitrogen and oxygen And a step of controlling the heating temperature of the resistance heating furnace to 1200 ° C. or higher while maintaining the gas atmosphere.
For example, the following knowledge is known as a background art relating to heating of a semiconductor layer made of SiC (for example, Japanese Patent Application Laid-Open No. 2003-318388).

Specifically, as a semiconductor device employing SiC, for example, an SiC layer having an activated ion region in the surface layer portion, a gate oxide film formed on the surface of the SiC layer, and a gate oxide film are formed. A MOSFET having a MOS (Metal Oxide Semiconductor) structure composed of a gate electrode facing an ion region through a gate oxide film is known.
In order to manufacture such a MOS structure, for example, impurity ions are first implanted into the surface layer portion of the SiC layer. Next, the implanted ions are activated by heating the SiC layer in the resistance heating furnace. After the activation of ions, a gate oxide film is formed on the surface of the SiC layer by supplying an oxygen-containing gas in a CVD (Chemical Vapor Deposition) apparatus. Then, a gate electrode is formed on the gate oxide film by sputtering. Thereby, the layer structure (MOS structure) of the gate electrode (Metal) -gate oxide film (Oxide) -SiC layer (Semiconductor) is produced.

  In order to activate the ions in the SiC layer, it is necessary to perform an annealing treatment at a temperature of 1600 to 1700 ° C., for example. In the resistance heating furnace, the heating time up to a high temperature region becomes long, so that during the heating for ion activation, Si sublimates from the surface of the SiC layer, so-called Si loss occurs, and the surface of the SiC layer becomes rough. As a result, the interface between the SiC layer and the gate oxide film becomes uneven, and the channel mobility of the MOSFET decreases.

Therefore, a method of suppressing the surface roughness of the SiC layer by shortening the heating time to a high temperature region using a high frequency induction heating furnace and then forming a gate oxide film using a gate oxidation furnace is adopted. ing.
However, in such a method, two devices, a high-frequency induction heating furnace and a gate oxidation furnace, are separately required, which causes a problem that the device cost increases.

As another technique, it has been proposed to maintain the flatness of the surface of the SiC layer by forming a carbon film on the surface of the SiC layer prior to the activation of ions and preventing Si from being lost by this carbon film. .
The carbon film is formed, for example, by forming a film containing carbon on the surface of the SiC layer and heating the film containing carbon in a high-frequency induction heating furnace to evaporate elements other than carbon from the film.

However, the present inventors have conducted intensive research. As a result, the heating temperature for forming the carbon film may be about 1000 ° C., which is lower than the temperature for activating ions (1600 to 1700 ° C.). Therefore, it is necessary to control the heating temperature in two steps, but it has been found that it is difficult to precisely control the temperature of the high-frequency induction heating furnace.
Moreover, the carbon film becomes unnecessary after the activation of ions. This unnecessary carbon film is oxidized and removed by an oxidizing gas in an apparatus different from the high-frequency induction heating furnace. It is also considered to introduce an oxidizing gas into the high-frequency induction heating furnace and remove the carbon film following the activation of ions. However, since a carbon material is used for the heating element of the high-frequency induction heating furnace, When the gas is supplied, the carbon material is oxidized. For this reason, a carbon film removing apparatus is separately indispensable, and the problem that the apparatus cost is inevitable is also found.

In order to achieve the object of providing a method for manufacturing a semiconductor device capable of suppressing the roughness of the SiC layer surface by simple temperature control without increasing the device cost, the following invention has been made.
Specifically, the invention includes a step of forming an organic material film on the surface of the SiC layer in which ions are implanted in the surface layer portion, and the organic material film is formed in a resistance heating furnace after the formation of the organic material film. In the resistance heating furnace, the SiC layer on which the carbon film is formed is heated to activate ions in the SiC layer. The step of oxidizing, the step of oxidizing and removing the carbon film by introducing an oxygen-containing gas into the resistance heating furnace, and the removal of the carbon film followed by the oxygen-containing process in the resistance heating furnace. And oxidizing the surface of the SiC layer with a gas to form an oxide film.

  According to this manufacturing method, after the organic material film is formed, by heating the organic material film in a resistance heating furnace, the organic material film is transformed into a carbon film, and a carbon film is formed on the surface of the SiC layer. After the formation of the carbon film, the SiC layer is heated to activate ions in the SiC layer. Thereafter, the carbon film is oxidized and removed by introducing an oxygen-containing gas into the resistance heating furnace. After the removal of the carbon film, the surface of the SiC layer is oxidized by the oxygen-containing gas in the resistance heating furnace to form an oxide film.

  Prior to heating for ionic activity, a carbon film is formed on the surface of the SiC layer, so that Si escape from the surface of the SiC layer can be prevented when the SiC layer is heated. Therefore, the roughness of the SiC layer surface can be suppressed, and the flatness of the SiC layer surface can be maintained. As a result, since the interface between the SiC layer and the oxide film can be made smooth, the channel mobility of the semiconductor device can be improved.

  Furthermore, the step of heating the organic material film to transform it into a carbon film, the step of heating the SiC layer to activate ions, the step of oxidizing and removing the carbon film with an oxygen-containing gas, and oxidizing the surface of the SiC layer to oxidize Four processes including the process of forming a film can be continuously performed in one resistance heating furnace. Since an apparatus for removing the carbon film is not required separately, an increase in apparatus cost can be suppressed. In addition, since the resistance heating furnace is used, the heating temperature for forming the carbon film and the heating temperature for activating the ions can be precisely and easily controlled.

The oxygen-containing gas may be a gas containing oxygen and nitrogen. If the oxygen-containing gas for forming the oxide film is a gas containing oxygen and nitrogen, the channel mobility of the semiconductor device can be further improved.
As the gas containing oxygen and nitrogen, for example, a gas containing NO (nitrogen monoxide), N ₂ O (dinitrogen monoxide), or the like can be used.

The surface of the SiC layer is preferably a (0001) plane, that is, a Si plane.
As described above, the present inventors have made an invention using a resistance heating furnace as an invention relating to heating of a semiconductor layer made of SiC.
Therefore, the step of forming the gate insulating film includes a step of charging the semiconductor layer into a resistance heating furnace, and a gas containing nitrogen and oxygen is introduced into the resistance heating furnace, so that an atmosphere containing nitrogen and oxygen And the step of controlling the heating temperature of the resistance heating furnace to 1200 ° C. or higher while maintaining the gas atmosphere, in addition to the effects of the present invention, The effect by the utilized invention can be enjoyed.

FIG. 1 is a schematic cross-sectional view of a semiconductor device according to the first embodiment of the present invention. FIG. 2A is a schematic cross-sectional view for explaining the method for manufacturing the semiconductor device shown in FIG. FIG. 2B is a diagram showing a step subsequent to FIG. 2A. FIG. 2C is a diagram showing a step subsequent to FIG. 2B. FIG. 2D is a diagram showing a step subsequent to FIG. 2C. FIG. 2E is a diagram showing a step subsequent to FIG. 2D. FIG. 2F is a diagram showing a step subsequent to that in FIG. 2E. FIG. 2G is a diagram showing a step subsequent to FIG. 2F. 2H is a diagram showing a step subsequent to that in FIG. 2G. FIG. 2I is a diagram showing a step subsequent to that in FIG. 2H. FIG. 2J is a diagram showing a step subsequent to that in FIG. 2I. 2K is a diagram showing a step subsequent to that in FIG. 2J. FIG. 2L is a diagram showing a step subsequent to that in FIG. 2K. 2M is a diagram showing a step subsequent to FIG. 2L. FIG. 2N is a diagram showing a step subsequent to FIG. 2M. FIGS. 3A and 3B are schematic plan views of a semiconductor device according to the second embodiment of the present invention. FIG. 3A is an overall view, and FIG. Show. FIG. 4 is a schematic cross-sectional view of the semiconductor device according to the second embodiment of the present invention, and shows a cut surface taken along the cutting line IV-IV in FIG. FIG. 5A is a schematic cross-sectional view for illustrating the method for manufacturing the semiconductor device shown in FIG. FIG. 5B is a diagram showing a step subsequent to FIG. 5A. FIG. 5C is a diagram showing a step subsequent to FIG. 5B. FIG. 5D is a diagram showing a step subsequent to FIG. 5C. FIG. 5E is a diagram showing a step subsequent to that in FIG. 5D. FIG. 5F is a diagram showing a step subsequent to that in FIG. 5E. FIG. 5G is a diagram showing a step subsequent to that in FIG. 5F. FIG. 5H is a diagram showing a step subsequent to that in FIG. 5G. FIG. 5I is a diagram showing a step subsequent to that in FIG. 5H. FIG. 5J is a diagram showing a step subsequent to that in FIG. 5I. FIG. 5K is a diagram showing a step subsequent to that in FIG. 5J. FIG. 5L is a diagram showing a step subsequent to that in FIG. 5K. FIG. 5M is a diagram showing a step subsequent to that in FIG. 5L. FIG. 5N is a diagram showing a step subsequent to that in FIG. 5M. FIG. 5O is a diagram showing a step subsequent to that in FIG. 5N. FIG. 5P is a diagram showing a step subsequent to FIG. 5N. FIG. 5Q is a diagram showing a step subsequent to that in FIG. 5N. FIG. 5R is a diagram showing a step subsequent to that in FIG. 5N. FIG. 5S is a diagram showing a step subsequent to FIG. 5N. FIG. 5T is a diagram showing a step subsequent to that in FIG. 5N. FIG. 5U is a diagram showing a step subsequent to that in FIG. 5N. FIG. 6 is a graph showing a temperature change in the resistance heating furnace. FIG. 6 is a schematic cross-sectional view for explaining a modification of the semiconductor device shown in FIG. 4. FIG. 8 is a schematic cross-sectional view of a planar gate type semiconductor device. FIG. 9A is a schematic cross-sectional view illustrating a method for manufacturing the semiconductor device of FIG. FIG. 9B is a schematic cross-sectional view showing a step subsequent to FIG. 9A. FIG. 9C is a schematic cross-sectional view showing a step subsequent to FIG. 9B. FIG. 9D is a schematic cross-sectional view showing a step subsequent to FIG. 9C. FIG. 9E is a schematic cross-sectional view showing a step subsequent to FIG. 9D. FIG. 9F is a schematic cross-sectional view showing a step subsequent to FIG. 9E. FIG. 9G is a schematic cross-sectional view showing a step subsequent to FIG. 9F. FIG. 9H is a schematic cross-sectional view showing a step subsequent to FIG. 9G. FIG. 9I is a schematic cross-sectional view showing a step subsequent to FIG. 9H. FIG. 9J is a schematic cross-sectional view showing a step subsequent to FIG. 9I. FIG. 9K is a schematic cross-sectional view showing a step subsequent to FIG. 9J. FIG. 9L is a schematic sectional view showing a step subsequent to FIG. 9K. 10A, 10B, and 10C are graphs in which the thickness of the oxide film is plotted for each supply time of the oxidizing gas, where FIG. 10A is the graph of Example 1, and FIG. (C) shows the graph of the comparative example 2, respectively. FIG. 11 is a schematic cross-sectional view of a semiconductor device having a conventional trench gate type VDMOSFET using SiC.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a schematic cross-sectional view of a semiconductor device according to the first embodiment of the present invention.
The semiconductor device 1 has a structure in which unit cells of trench gate type VDMOSFETs are arranged in a matrix. In FIG. 1, some of the plurality of unit cells are shown.

The semiconductor device 1 includes an SiC substrate 2 that forms the base of the semiconductor device 1. SiC substrate 2 is doped with N-type impurities at a high concentration (for example, 1e18 to 1e21 cm ⁻³ ). The SiC substrate 2 has a front surface 21 (upper surface) as a Si surface and a rear surface (lower surface) as a C surface.
On the surface 21 of the SiC substrate 2, an N ⁻ -type epitaxial layer 3 made of SiC (silicon carbide: silicon carbide) doped with N-type impurities at a lower concentration than the SiC substrate 2 is laminated. Epitaxial layer 3 as a semiconductor layer is formed on SiC substrate 2 by so-called epitaxial growth. The epitaxial layer 3 formed on the surface 21 which is the Si surface is grown with the Si surface as the growth main surface. Therefore, the surface 31 of the epitaxial layer 3 formed by the growth is a Si surface like the surface 21 of the SiC substrate 2.

The portion on the C plane side (base layer portion) opposite to the portion on the Si plane side (surface layer portion) in the epitaxial layer 3 is an N ⁻ type drain region 4 in which the entire region is maintained as it is after epitaxial growth. There is no. The N type impurity concentration of the drain region 4 is, for example, 1e15 to 1e17 cm ⁻³ .
On the other hand, a P-type body region 5 is formed in the surface layer portion of the epitaxial layer 3. The body region 5 is in contact with the drain region 4 from the surface 31 side (Si surface side) of the epitaxial layer 3. The P-type impurity concentration of body region 5 is, for example, 1e16 to 1e19 cm ⁻³ .

  A gate trench 6 is dug from the surface 31 in the epitaxial layer 3. Although not shown in FIG. 1, a plurality of gate trenches 6 are formed at regular intervals, and they are parallel to each other in the same direction (a direction perpendicular to the plane of FIG. 1, hereinafter this direction is referred to as “gate width”). For example, a stripe structure.

Each gate trench 6 has a pair of planar side surfaces 7 facing each other with a space therebetween and orthogonal to the surface 31, and a bottom surface 8 having a portion parallel to the surface 31. The gate trench 6 penetrates the body region 5 in the layer thickness direction, and the deepest portion (bottom surface 8) reaches the drain region 4.
A gate insulating film 9 is formed on the inner surface of the gate trench 6 and the surface 31 of the epitaxial layer 3 so as to cover the entire inner surface (side surface 7 and bottom surface 8) of the gate trench 6. The gate insulating film 9 is made of an oxide film containing nitrogen, for example, a silicon nitride oxide film formed by thermal oxidation using a nitrogen-containing gas. The nitrogen content (nitrogen concentration) in the gate insulating film 9 is, for example, 0.1 to 10%.

Further, the gate insulating film 9, the thickness T ₂ of the portion on the bottom surface 8 (an insulating film base 11) is smaller than the thickness T ₁ of the portion on the side surface 7 (the insulating film side 10). Specifically, the ratio of the thickness T ₂ of the insulating film bottom 11 to the thickness T ₁ of the insulating film side 10 (thickness T ₁ of the _second thickness T _{2 /} insulating film side 10 of the insulating film bottom ₁₁₎ 0.3 to 1.0, preferably 0.5 to 1.0. Further, specific size of the both thicknesses are, for example, the thickness _{T 1} of the insulating film side 10 is 300～1000A, thickness _{T 2} of the insulating film bottom 11 is 150～500A.

The gate electrode 12 is buried in the gate trench 6 by filling the inside of the gate insulating film 9 with a polysilicon material doped with N-type impurities at a high concentration.
In the surface layer portion of the body region 5, N ⁺ -type source regions 13 are formed on both sides of the gate trench 6 in the direction orthogonal to the gate width (left-right direction in FIG. 1). The source region 13 is a region that is higher than the N-type impurity concentration of the drain region 4 and is doped with N-type impurities at a high concentration. The N type impurity concentration of the source region 13 is, for example, 1e18 to 1e21 cm ⁻³ . The source region 13 extends in a direction along the gate width at a position adjacent to the gate trench 6.

The epitaxial layer 3 is formed with a P ⁺ -type body contact region 14 connected from the surface 31 to the body region 5 through the central portion of the source region 13 in the direction orthogonal to the gate width. . The body contact region 14 is a region that is higher than the P-type impurity concentration of the body region 5 and is doped with a high concentration of P-type impurities. The P-type impurity concentration of body contact region 14 is, for example, 1e18 to 1e21 cm ⁻³ .

  That is, the gate trench 6 and the source region 13 are alternately provided in a direction orthogonal to the gate width, and each extend in a direction along the gate width. A boundary between adjacent unit cells is set on the source region 13 along the source region 13 in a direction orthogonal to the gate width. At least one body contact region 14 is provided across two unit cells adjacent in a direction orthogonal to the gate width. The boundary between unit cells adjacent in the direction along the gate width is set so that the gate electrode 12 included in each unit cell has a constant gate width.

An interlayer insulating film 15 made of SiO ₂ is laminated on the epitaxial layer 3. Contact holes 16 that expose the surfaces of the source region 13 and the body contact region 14 are formed in the interlayer insulating film 15 and the gate insulating film 9.
A source wiring 17 is formed on the interlayer insulating film 15. Source wiring 17 is in contact (electrically connected) to source region 13 and body contact region 14 through contact hole 16. The source wiring 17 has a polysilicon layer 18 at a contact portion with the source region 13 and the body contact region 14, and has a metal layer 20 on the polysilicon layer 18.

The polysilicon layer 18 is a doped layer formed using doped polysilicon doped with impurities. For example, the polysilicon layer 18 is a highly doped layer doped with impurities at a high concentration of 10 ¹⁹ to 10 ²¹ cm ^−3. Preferably there is. As the impurity for forming the polysilicon layer 18 as the doped layer (containing a high concentration doped layer), N-type impurities such as phosphorus (P) and A _{s (arsenic),} P-type impurity such as B (boron) Can be used. Further, the polysilicon layer 18 fills up the contact hole 16. The thickness of the polysilicon layer 18 varies depending on the depth of the contact hole 16, but is, for example, 5000 to 10000 mm.

The metal layer 20 is formed using, for example, aluminum (Al), gold (Au), silver (Ag), copper (Cu), alloys thereof, and metal materials containing them. The metal layer 20 is the outermost layer of the source wiring 17, and for example, a metal wire or the like is connected (bonded). Moreover, the thickness of the metal layer 20 is 1-5 micrometers, for example.
In the source wiring 17, an intermediate layer 19 containing titanium is interposed between the polysilicon layer 18 and the metal layer 20. The intermediate layer 19 is composed of a single layer containing titanium (Ti) or a plurality of layers having the layer. The layer containing titanium can be formed using titanium, titanium nitride, or the like. Moreover, the thickness of the intermediate layer 19 is, for example, 200 to 500 nm.

The source wiring 17 having the polysilicon layer 18, the intermediate layer 19 and the metal layer 20 as described above includes polysilicon (polysilicon layer 18), titanium (intermediate layer 19), titanium nitride (intermediate layer 19), and aluminum (metal). A layered structure (Po—Si / Ti / TiN / Al) in which the layers 20) are sequentially stacked is preferable.
A drain wiring 23 is formed on the back surface 22 of the SiC substrate 2. Drain wiring 23 is in contact (electrically connected) to SiC substrate 2. The drain wiring 23 has a polysilicon layer 24 at a contact portion with the SiC substrate 2, and has a metal layer 26 on the polysilicon layer 24.

The polysilicon layer 24 can be formed using the same material as that of the polysilicon layer 18 described above. The thickness of the polysilicon layer 24 is, for example, 1000 to 2000 mm.
The metal layer 26 can be formed using the same material as that of the metal layer 20 described above. The metal layer 26 is the outermost layer of the drain wiring 23, and is bonded to the die pad, for example, when the SiC substrate 2 is bonded to the die pad of the lead frame. Moreover, the thickness of the metal layer 26 is 0.5-1 micrometer, for example.

In the drain wiring 23, an intermediate layer 25 containing titanium is interposed between the polysilicon layer 24 and the metal layer 26. The intermediate layer 25 can be formed using the same material as that of the intermediate layer 19 described above.
A gate wiring 27 is contacted (electrically connected) to the gate electrode 12 through a contact hole (not shown) formed in the interlayer insulating film 15.

  By applying a predetermined voltage (a voltage equal to or higher than the gate threshold voltage) to the gate wiring 27 in a state where a predetermined potential difference is generated between the source wiring 17 and the drain wiring 23 (between the source and drain), the gate A channel is formed in the vicinity of the interface with the gate insulating film 9 in the body region 5 by the electric field from the electrode 12. Thereby, a current flows between the source wiring 17 and the drain wiring 23, and the VDMOSFET is turned on.

2A to 2N are schematic cross-sectional views for explaining a method of manufacturing the semiconductor device shown in FIG.
First, as shown in FIG. 2A, an epitaxial growth method such as a CVD (Chemical Vapor Deposition) method, an LPE (Liquid Phase Epitaxy) method, or an MBE (Molecular Beam Epitaxy) method is used. A SiC crystal is grown on the surface 21 (Si surface) of the SiC substrate 2 while doping impurities. Thereby, N ⁻ type epitaxial layer 3 is formed on SiC substrate 2. Subsequently, P-type impurities are implanted (implanted) into the epitaxial layer 3 from the surface 31 of the epitaxial layer 3. The implantation conditions at this time vary depending on the type of P-type impurity, but the acceleration energy is, for example, 200 to 400 keV.

Thereby, as shown in FIG. 2B, a region (P-type implantation region 28) in which the P-type impurity is implanted is formed in the surface layer portion of the epitaxial layer 3. With the formation of the P-type implant region 28, the drain region 4 that is separated from the P-type implant region 28 and maintains the state after the epitaxial growth is formed in the base layer portion of the epitaxial layer 3.
Next, as shown in FIG. 2C, a mask 29 made of SiO ₂ is formed on the epitaxial layer 3 by the CVD method. Subsequently, the mask 29 is etched through a photoresist (not shown) to be patterned into a pattern having an opening 30 in a region where the body contact region 14 is to be formed. After the opening 30 is formed, P-type impurities are implanted into the epitaxial layer 3 from the surface 31 of the epitaxial layer 3. The implantation conditions at this time vary depending on the type of P-type impurity, but the acceleration energy is, for example, 30 to 200 keV. As a result, a region where the P-type impurity is implanted at a high concentration (P ⁺ -type implantation region 32) is formed in the surface layer portion of the P-type implantation region 28. After the implantation of the P-type impurity, the mask 29 is removed.

Next, as shown in FIG. 2D, a mask 33 made of SiO ₂ is formed on the epitaxial layer 3 by a CVD (Chemical Vapor Deposition) method. Subsequently, the mask 33 is etched through a photoresist (not shown) to be patterned into a pattern having an opening 34 in a region where the source region 13 is to be formed. After the opening 34 is formed, N-type impurities are implanted (implanted) into the epitaxial layer 3 from the surface 31 of the epitaxial layer 3. The implantation conditions at this time vary depending on the type of N-type impurity, but for example, the acceleration energy is 30 to 200 keV. After the N-type impurity implantation, the mask 33 is removed. As a result, a region (N ⁺ -type implantation region 35) in which N-type impurities are implanted at a high concentration is formed in the surface layer portion of the P-type implantation region 28.

Next, as shown in FIG. 2E, the epitaxial layer 3 is heat-treated at 1400 to 2000 ° C., for example. Thereby, the implanted N-type and P-type impurities are activated to form body region 5 in the surface layer portion of epitaxial layer 3, and source region 13 and body contact region 14 are formed in the surface layer portion of body region 5. Is done.
Next, as shown in FIG. 2F, a mask 36 made of SiO ₂ is formed over the entire surface 31 of the epitaxial layer 3 by CVD, thermal oxidation, or the like. Note that the mask 36 can also be formed of SiN or the like by using a CVD method.

Next, as shown in FIG. 2G, the mask 36 is etched through a photoresist (not shown) to be patterned into a pattern having an opening 37 in a region where the gate trench 6 is to be formed.
Next, as shown in FIG. 2H, a mixed gas (SF 6 / O ₂ gas) containing SF 6 (sulfur hexafluoride) and O ₂ (oxygen) is incident on the surface 31 of the epitaxial layer 3 through the opening 37. . Thereby, the epitaxial layer 3 is dry-etched from the surface 31 (Si surface) to form a gate trench 6 having a bottom surface 8 having a portion parallel to the surface 31 (Si surface) and a side surface 7 orthogonal to the Si surface. Is done. After the formation of the gate trench 6, the mask 36 is removed.

Next, as shown in FIG. 2I, the SiC substrate 2 is carried into the diffusion furnace, and the nitrogen-containing gas is supplied in a state where the inside of the diffusion furnace is heated, whereby the inner surface (the side surface 7 and the bottom surface 8 of the gate trench 6). And the surface 31 of the epitaxial layer 3 is thermally oxidized. As the nitrogen-containing gas, for example, N ₂ O gas, NO gas, or the like can be used. Moreover, the heater temperature (heating temperature) of a diffusion furnace is 1200-1350 degreeC, for example, and the supply time (oxidation time) of nitrogen-containing gas is 3 to 5 hours, for example. Since the gate trench 6 is formed in the epitaxial layer 3 made of SiC, the inner surface of the gate trench 6 is oxidized at the oxidation rate of the bottom surface 8 having the Si surface and the oxidation rate of the side surface 7 which is a surface orthogonal to the Si surface. However, it proceeds under the condition that the relational expression: oxidation rate of bottom surface 8 / oxidation rate of side surface 7 <0. Thereby, the gate insulating film 9 is formed in which the thickness of the portion on the bottom surface 8 (insulating film bottom portion 11) is smaller than the thickness of the portion on the side surface 7 (insulating film side portion 10).

  Next, as shown in FIG. 2J, a doped polysilicon material is deposited on the epitaxial layer 3 by a CVD method. The deposited polysilicon material is etched back until the etch back surface is flush with the surface 31 of the epitaxial layer. As a result, the portion of the polysilicon material outside the gate trench 6 is removed, and the gate electrode 12 made of the polysilicon material remaining in the gate trench 6 is formed.

Next, as shown in FIG. 2K, an interlayer insulating film 15 made of SiO ₂ is laminated on the epitaxial layer 3 by the CVD method. Then, by patterning interlayer insulating film 15 and gate insulating film 9, contact hole 16 exposing source region 13 and body contact region 14 is formed in interlayer insulating film 15 and gate insulating film 9.
Next, as shown in FIG. 2L, a polysilicon material 38 is deposited by CVD until the contact hole 16 is filled.

Next, as shown in FIG. 2M, N-type or P-type impurities are implanted into the deposited polysilicon material 38. The implantation conditions at this time vary depending on the type of impurities, but the acceleration energy is, for example, 10 to 100 keV. Thereby, a polysilicon layer 18 doped with impurities at a high concentration is formed.
Next, as shown in FIG. 2N, titanium and titanium nitride are deposited in this order on the surface of the polysilicon layer 18 by a method such as sputtering or vapor deposition to form an intermediate layer 19. Subsequently, aluminum is deposited on the surface of the intermediate layer 19 by a method such as sputtering or vapor deposition to form the metal layer 20. The source wiring 17 is formed by patterning the metal layer 20, the intermediate layer 19 and the polysilicon layer 18 into a predetermined wiring pattern. Subsequently, a gate wiring 27 connected to the gate electrode 12 is formed. Thereafter, drain wiring 23 having polysilicon layer 24, intermediate layer 25, and metal layer 26 is formed on back surface 22 of SiC substrate 2 by the same method as that for source wiring 17.

Through the above steps, the semiconductor device 1 shown in FIG. 1 is obtained.
As described above, according to the semiconductor device 1, the gate trench 6 is formed by being dug down from the surface 31 (Si surface) of the epitaxial layer 3 made of SiC. Therefore, the oxidation of the inner surface of the gate trench 6 is based on the following equation: the oxidation rate of the bottom surface 8 having the Si surface and the oxidation rate of the side surface 7 orthogonal to the Si surface. Progress under conditions satisfying <0.

In the above manufacturing method, the oxidation of the inner surface of the gate trench 6 is different from the thermal oxidation (Dry oxidation) using oxygen gas or the thermal oxidation (Wet oxidation) using water vapor (H ₂ O) gas. It is performed by thermal oxidation using Therefore, the ratio of the oxidation rate of the bottom surface 8 to the oxidation rate of the side surface 7 (the oxidation rate of the bottom surface 8 / the oxidation rate of the side surface 7) should be larger than when the gate insulating film is formed by dry oxidation or wet oxidation. Can do.

Then, the gate insulating film 9 formed in this way is, the _second thickness T _{2 /} insulating film of the insulating film bottom ratio of the thickness T ₂ of the 11 (insulating film bottom 11 to the thickness T ₁ of the insulating film side 10 The thickness T ₁ ) of the side part 10 is in the range of 0.3 to 1.0.
Also increase the thickness T ₂ of the degree in the insulating film bottom portion 11 can be suppressed breakdown, having a lower limit of 0.3 (thickness T1 of the _second thickness T _{2 /} insulating film side 10 of the insulating film bottom 11) there, it is possible to suppress an excessive increase in the thickness T ₁ of the insulating film side 10. On the other hand, the upper limit is 1.0, when designing the thickness T ₂ of the insulating film bottom 11 to size, does not become excessively small thickness T ₁ of the insulating film side 10. As a result, by appropriately designing the thickness T ₂ of the insulating film bottom 11, it is possible to suppress the dielectric breakdown of the insulating film bottom 11 while suppressing the increase in the thickness T ₁ of the insulating film side 10. it can.

Further, since the gate insulating film 9 is made of a silicon nitride oxide film formed by thermal oxidation using a nitrogen-containing gas, the channel mobility of the VDMOSFET can be improved.
FIGS. 3A and 3B are schematic plan views of a semiconductor device according to the second embodiment of the present invention. FIG. 3A is an overall view, and FIG. Show.
This semiconductor device 41 is a trench gate type power VDMOSFET (individual element) using SiC, and has, for example, a square chip shape in plan view. The chip-like semiconductor device 41 has a length of about several millimeters in the left and right (up and down) directions on the paper surface of FIG.

  The semiconductor device 41 includes a SiC substrate 42 and a large number of unit cells 44 formed on the SiC substrate 42 and partitioned by gate trenches 43 having a lattice shape in plan view. That is, on the SiC substrate 42, the rectangular parallelepiped unit cells 44 arranged in the respective window portions of the lattice-like gate trench 43 are arranged in a matrix. Each unit cell 44 has, for example, a length of 10 μm or less in the horizontal (vertical) direction on the paper surface of FIG. 3B, and has a square shape in plan view that is dug from the surface side to the SiC substrate 42 side at the center. A source trench 45 is formed.

  A source pad 46 is formed on the surface of the semiconductor device 41. The source pad 46 has a substantially square shape in plan view with four corners curved outward, and is formed so as to cover almost the entire surface of the semiconductor device 41. In the source pad 46, a removal region 47 is formed in which a part thereof is removed in a substantially square shape in plan view, slightly to the left and right in the paper surface of FIG.

A gate pad 48 is disposed in the removal region 47. A gap is provided between the gate pad 48 and the source pad 46, which are insulated from each other.
FIG. 4 is a schematic cross-sectional view of the semiconductor device according to the second embodiment of the present invention, and shows a cut surface taken along the cutting line IV-IV in FIG.
A cross-sectional structure of the semiconductor device 41 will be described with reference to FIG. The semiconductor device 41 includes an N ⁺ type SiC substrate 42 (for example, the concentration is 1e18 to 1e21 cm ⁻³ ). The surface 49 (upper surface) of the SiC substrate 42 is an Si surface, and the rear surface 50 (lower surface) is a C surface.

On SiC substrate 42, an epitaxial layer 51 made of SiC having an N ⁻ type (for example, a concentration of 1e15 to 1e17 cm ⁻³ ) having a lower concentration than SiC substrate 42 is laminated. The epitaxial layer 51 as a semiconductor layer is formed on the SiC substrate 42 by so-called epitaxial growth. The epitaxial layer 51 formed on the surface 49 that is the Si surface is grown with the Si surface as the main growth surface. Therefore, the surface 52 of the epitaxial layer 51 formed by growth is a Si surface, like the surface 49 of the SiC substrate 42.

On the surface 52 side (Si surface side) of the epitaxial layer 51, a P-type body region 53 is formed in a well shape over a wide range, and its concentration is, for example, 1e16 to 1e19 cm ⁻³ . Further, in the epitaxial layer 51, a region closer to the SiC substrate 42 (C surface side) than the body region 53 is an N ⁻ -type drain region 54 (drift region) that is maintained in the state after the epitaxial growth. Yes.

In the body region 53, a source region 55 of N ⁺ type (for example, the concentration is 1e18 to 1e21 cm ⁻³ ) is formed almost all over the surface 52 side, and P is closer to the SiC substrate 42 side (downward) than the source region 55. ^{A +} type body (for example, a concentration of 1e18 to 1e21 cm ⁻³ ) is formed. A large number of body contact regions 56 are formed in a matrix.

  Further, the same number of source trenches 45 as the body contact regions 56 are formed so as to penetrate the individual body contact regions 56, and the lattice-like gate trenches are formed so as to surround each body contact region 56 in which the source trenches 45 are formed. 43 is formed. Thereby, a large number of unit cells 44 each functioning as a field effect transistor are formed in the epitaxial layer 51. That is, in the unit cell 44, the body contact region 56 is formed so as to surround the source trench 45, and the body region 53 is further formed so as to surround the body contact region 56. The opposite side of the body region 53 to the body contact region 56 side is exposed on the side surface of the gate trench 43. In the unit cell 44, the depth direction of the gate trench 43 is the gate length direction, and the circumferential direction of each unit cell 44 perpendicular to the gate length direction is the gate width direction.

  Both the source trench 45 and the gate trench 43 extend from the surface 52 of the epitaxial layer 51 through the body region 53 to the drain region 54, and in this embodiment, their depths are the same. The distance D1 between the side surface 59 of the source trench 45 and the side surface 57 of the gate trench 43 is, for example, 0.5 to 3 μm. When the distance D1 is within this range, an increase in resistance value (on resistance) when each unit cell 44 is turned on can be suppressed, and the electric field applied to the bottom of the gate trench 43 can be relaxed.

  In the gate trench 43, both end corners 61 in the direction perpendicular to the gate width at the bottom (opposite direction to the adjacent unit cell 44) are curved toward the drain region 54, and the side surface 57 and the bottom surface that face each other. 58 has a U-shaped cross section continuous with the curved surface. Further, like the gate trench 43, the source trench 45 has a U-shaped cross section in which a side surface 59 and a bottom surface 60 facing each other are continuous via a curved surface. Thereby, when the unit cell 44 is turned off, an electric field applied to the both end corners 61 at the bottom of the gate trench 43 can be dispersed to portions other than the both end corners 61. The dielectric breakdown of the portion (insulating film bottom 64) can be suppressed.

  In the drain region 54, an implantation layer formed by implantation of a P-type impurity (for example, B (boron), Al (aluminum), etc.) is formed in a portion from the bottom surface 58 of the gate trench 43 to the middle part in the thickness direction. As a result, an implant active layer 62 is formed. The implant active layer 62 is formed in a lattice shape overlapping with the gate trench 43 in plan view, and having a shape narrower than the distance between adjacent unit cells 44. In this embodiment, the depth of the implant active layer 62 is, for example, 0.1 to 0.5 μm.

The implant active layer 62 is a high resistance layer having a resistance value higher than that of the surrounding region (for example, the drain region 54) in the epitaxial layer 51. The resistance value is, for example, several tens k to several hundreds kΩ / □. is there. Further, the P-type impurity concentration of the implant active layer 62 is, for example, 1e16 to 1e21 cm ⁻³ .
A gate insulating film 63 is formed on the inner surface of the gate trench 43 so as to cover the entire area. The gate insulating film 63 is made of an oxide film containing nitrogen, for example, a silicon nitride oxide film formed by thermal oxidation using a gas containing nitrogen and oxygen. The nitrogen content (nitrogen concentration) in the gate insulating film 63 is, for example, 0.1 to 10%.

The gate insulating film 63, the thickness _{T 4} of the portion on the bottom surface 58 of the gate trench 43 (the insulating film bottom 64), the thickness _{T 3} of the portion on the side 57 of the gate trench 43 (insulating film side 65) less than the ratio of the thickness _{T 4} to the thickness _{T 3} (thickness _{T 4} / thickness _{T 3)} is a 0.3 to 1.0, preferably, is 0.5 to 1.0 . Further, specific size of the both thicknesses are, for example, a thickness _{T 3} is 300～1000A, thickness _{T 4} is 150～500A. If the thickness T ₃ is the range of the insulating film side 65, it is possible to operate the semiconductor device 41 with a suitable gate-on voltage, an efficient transistor operation can be achieved.

Then, the gate electrode 66 is embedded in the gate trench 43 by filling the inside of the gate insulating film 63 with a polysilicon material doped with N-type impurities at a high concentration.
On the epitaxial layer 51, an interlayer insulating film 67 made of SiO ₂ is laminated. Contact holes 68 are formed in the interlayer insulating film 67 and the gate insulating film 63 to expose the surfaces of the source trench 45 and the source region 55 of each unit cell 44.

  A source wiring 69 is formed on the interlayer insulating film 67. The source wiring 69 enters the source trenches 45 of all the unit cells 44 through the contact holes 68 at a time. In each unit cell 44, the drain region 54 and the body contact are sequentially formed from the bottom side of the source trench 45. The region 56 and the source region 55 are in contact. That is, the source wiring 69 is a wiring common to all the unit cells 44. An interlayer insulating film (not shown) is formed on the source wiring 69, and the source wiring 69 is connected to the source pad 46 (see FIG. 3A) via the interlayer insulating film (not shown). ) Is electrically connected. On the other hand, the gate pad 48 (see FIG. 3A) is electrically connected to the gate electrode 66 through a gate wiring (not shown) routed on the interlayer insulating film (not shown). ing.

The source wiring 69 has a polysilicon layer 70, an intermediate layer 71, and a metal layer 72 in order from the contact side with the epitaxial layer 51.
The polysilicon layer 70 is a doped layer formed using doped polysilicon doped with impurities. For example, the polysilicon layer 70 is a heavily doped layer doped with impurities at a high concentration of 1e19 to 1e21 cm ⁻³ . Impurities when forming the polysilicon layer 70 as a doped layer (including a highly doped layer) include N-type impurities such as N (nitrogen), P (phosphorus), As (arsenic), Al (aluminum), B P-type impurities such as (boron) can be used. The thickness of the polysilicon layer 70 is, for example, 5000 to 10,000 mm.

In this embodiment, the polysilicon layer 70 is formed so as to cover the entire surface of the unit cell 44 exposed in the contact hole 68, and the drain region 54, the body contact region 56, and the source in the source trench 45. It is in contact with the region 55.
By using polysilicon for the contact layer with the drain region 54, the body contact region 56, and the source region 55 in the source wiring 69, the source wiring 69 is connected to both the body contact region 56 and the source region 55, which are high-concentration impurity regions. Can be ohmic-bonded. On the other hand, for the low concentration drain region 54, the junction barrier is smaller than the diffusion potential of the body diode 73 (PN diode formed by the junction of the body region 53 and the drain region 54) inherent in the semiconductor device 41. A heterojunction junction can be formed.

  By the way, when a current flows through the body diode 73 included in the semiconductor device 41, holes (holes) moved from the body region 53 to the drain region 54 recombine with electrons in the drain region 54, and the binding energy generated at that time is generated. In some cases, defects of the SiC crystal in the epitaxial layer 51 may spread in the plane. Since this crystal defect has a high resistance value, if the crystal defect expands to the gate trench 43 side, the crystal defect may hinder normal transistor operation and increase the on-resistance.

  On the other hand, if a heterojunction junction is formed by the contact between the polysilicon layer 70 and the drain region 54 as in this embodiment, a reverse voltage is applied between the source and the drain, and a current flows through the body diode 73. Even when the current flows, the current can flow preferentially to the heterojunction junction side rather than the body diode 73 side. As a result, expansion of SiC crystal defects can be prevented, and an increase in on-resistance can be suppressed.

The intermediate layer 71 is laminated on the polysilicon layer 70, and is composed of a single layer containing Ti (titanium) or a plurality of layers having the layer. The layer containing Ti can be formed using Ti, TiN (titanium nitride), or the like. Moreover, the thickness of the intermediate | middle layer 71 is 200-500 nm, for example.
The metal layer 72 is laminated on the intermediate layer 71 and contains, for example, Al (aluminum), Au (gold), Ag (silver), Cu (copper), Mo (molybdenum), alloys thereof, and the like. It is formed using a metal material. The metal layer 72 is the outermost layer of the source wiring 69. Moreover, the thickness of the metal layer 72 is 1-5 micrometers, for example.

As a combination of the polysilicon layer 70, the intermediate layer 71, and the metal layer 72 as described above, specifically, Poly-Si (polysilicon layer 70), Ti (intermediate layer 71), TiN (intermediate layer 71) and A laminated structure (Poly-Si / Ti / TiN / Al) in which Al (metal layer 72) is sequentially laminated can be exemplified.
A drain electrode 74 is formed on the back surface 50 of the SiC substrate 42 so as to cover the entire area. The drain electrode 74 is a common electrode for all the unit cells 44. As the drain electrode 74, for example, a stacked structure (Ti / Al) in which Ti and Al are stacked in this order from the SiC substrate 42 side can be exemplified.

  A predetermined voltage (voltage higher than the gate threshold voltage) is applied to the gate pad 48 in a state where a predetermined potential difference is generated between the source pad 46 (source wiring 69) and the drain electrode 74 (between the source and drain). Thus, a channel is formed near the interface with the gate insulating film 63 in the body region 53 by the electric field from the gate electrode 66. Thereby, a current flows between the source wiring 69 and the drain electrode 74, and the VDMOSFET is turned on.

5A to 5U are schematic cross-sectional views for explaining a method of manufacturing the semiconductor device shown in FIG.
First, as shown in FIG. 5A, an epitaxial growth method such as CVD (Chemical Vapor Deposition), LPE (Liquid Phase Epitaxy), MBE (Molecular Beam Epitaxy) or the like is used. A SiC crystal is grown on the surface 49 (Si surface) of the SiC substrate 42 while doping impurities. Thereby, an N ⁻ type epitaxial layer 51 is formed on SiC substrate 42.

Subsequently, as shown in FIG. 5B, P-type impurities are implanted (implanted) from the surface 52 of the epitaxial layer 51 into the epitaxial layer 51. The implantation conditions at this time vary depending on the type of P-type impurity, but, for example, the acceleration energy is 200 to 3000 keV.
Next, as shown in FIG. 5C, a mask 75 made of SiO ₂ is formed on the epitaxial layer 51 by the CVD method. Subsequently, the mask 75 is etched through a photoresist (not shown) to be patterned into a pattern having an opening 76 in a region where the body contact region 56 is to be formed. After the opening 76 is formed, P-type impurities are implanted (implanted) into the epitaxial layer 51 from the surface 52 of the epitaxial layer 51. The implantation conditions at this time vary depending on the type of P-type impurity, but the acceleration energy is, for example, 30 to 400 keV. After the implantation of the P-type impurity, the mask 75 is removed.

Next, as shown in FIG. 5D, N-type impurities are implanted (implanted) from the surface 52 of the epitaxial layer 51 into the epitaxial layer 51. The implantation conditions at this time vary depending on the type of N-type impurity, but the acceleration energy is, for example, 30 to 400 keV.
Next, as shown in FIG. 5E, a mask 77 made of SiO ₂ is formed over the entire surface 52 of the epitaxial layer 51 by CVD, thermal oxidation, or the like. Note that the mask 77 can be formed of SiN or the like by using a CVD method. Subsequently, the mask 77 is etched through a photoresist (not shown) to be patterned into a pattern having an opening 78 in a region where the gate trench 43 and the source trench 45 are to be formed. After the opening 78 is formed, for example, a mixed gas containing SF6 (sulfur hexafluoride) and O ₂ (oxygen) (SF6 / O ₂ gas), a mixed gas containing SF6, O ₂ and HBr (hydrogen bromide) (SF6) / O ₂ / HBr gas) is incident on the surface 52 of the epitaxial layer 51 through the opening 78. Thereby, the epitaxial layer 51 is dry-etched from the surface 52 (Si surface), and the gate trench 43 and the source trench 45 are formed simultaneously. At the same time, a large number of unit cells 44 are formed in the epitaxial layer 51.

Next, as shown in FIG. 5F, the inner surface of the gate trench 43 and the inner surface of the source trench 45 are oxidized by a thermal oxidation method (Dry oxidation) using O ₂ gas. Thereby, the stopper film 79 is formed. Although the thickness of the stopper film 79 may not be uniform as a whole, FIGS. 5F to 5I show the case where the stopper film 79 has a uniform thickness for convenience.

Next, as shown in FIG. 5G, a polysilicon material which is a material different from the material (SiO ₂ ) of the mask 77 for forming the gate trench 43 and the source trench 45 is formed on the epitaxial layer 51 by a CVD method. The film 79 is deposited until the entire surface of the film 79 and the entire surface of the mask 77 are covered. Thereby, a protective mask 80 is formed on the stopper film 79 and the mask 77. The thickness of the protective mask 80 is controlled to be 0.1 to 0.5 μm, for example.

  Next, as shown in FIG. 5H, the protective mask 80 is etched back from above the epitaxial layer 51. The etch back is performed in a state where the portion of the protective mask 80 on the bottom surface 60 of the source trench 45 is masked, and is continued until the etching is stopped by the stopper film 79 and the mask 77. As a result, only a portion of the protective mask 80 on the bottom surface 58 of the gate trench 43 is removed, and portions of the protective mask 80 covering the side surface 57 of the gate trench 43 and the bottom surface 60 and side surface 59 of the source trench 45 remain.

Next, as shown in FIG. 5I, P-type impurities are implanted (implanted) into the epitaxial layer 51 from the bottom surface 58 of the gate trench 43 through the stopper film 79. The implantation conditions at this time vary depending on the type of P-type impurity, but the acceleration energy is, for example, 30 to 400 keV.
Next, as shown in FIG. 5J, the protective mask 80 is removed by wet etching, and then the mask 77 and the stopper film 79 are removed.

  Thereafter, as shown in FIG. 5K, an organic material film 81 is formed over the entire surface 52 of the epitaxial layer 51. The organic material film 81 is a material containing carbon (carbon), and for example, an organic material (eg, polyimide) used as a photoresist can be applied. Such an organic material film 81 is formed using, for example, a spin coater.

After the formation of the organic material film 81, the SiC substrate 42 is charged into the resistance heating furnace 82. The resistance heating furnace 82 is not particularly limited as long as it can secure airtightness in the resistance heating furnace 82 in which an object to be heated is set and can introduce various gases into the resistance heating furnace 82. The heating method may be either a direct heating method or an indirect heating method.
Then, with the SiC substrate 42 set in the resistance heating furnace 82, an inert gas (for example, N ₂ , Ar, etc.) is introduced into the resistance heating furnace 82, and the resistance heating furnace 82 controls the temperature rise. (First temperature increase control).

  In this first temperature rise control, as shown in FIG. 6, the heating temperature is controlled to rise from 100 ° C. to 1000 ° C. over 35 to 45 minutes, for example, and after the rise, for example, 5 to 10 minutes. The heating temperature is held at 1000 ° C. (first temperature holding). By this temperature rise and temperature holding, elements other than carbon in the organic material film 81 are evaporated, and the organic material film 81 is transformed into a carbon film 83 as shown in FIG. 5L. Therefore, the entire surface 52 of the epitaxial layer 51 is covered with the carbon film 83.

Subsequently, the resistance heating furnace 82 is further subjected to temperature rise control (second temperature rise control) while maintaining the inside of the resistance heating furnace 82 in an inert atmosphere.
In the second temperature increase control, as shown in FIG. 6, the heating temperature is controlled to rise from 1000 ° C. to 1600 ° C. over 30 to 60 minutes, for example. After the rise, for example, the heating temperature is held at 1600 ° C. (second temperature holding) for 5 to 10 minutes. By this temperature rise and temperature holding, ions of individual N-type impurities and P-type impurities implanted in the surface layer portion of the epitaxial layer 51 are activated, and as shown in FIG. Region 53, source region 55, and body contact region 56 are formed. In addition, a drain region 54 that maintains the state after epitaxial growth is formed in the base layer portion of the epitaxial layer 51.

Next, the temperature of the resistance heating furnace 82 is controlled to drop while the inside of the resistance heating furnace 82 is maintained in an inert atmosphere.
In the temperature drop control, as shown in FIG. 6, the heating temperature is limited (temperature drop restriction) so as to drop from 1600 ° C. to 1300 ° C. over 15 to 30 minutes, for example. After the temperature is lowered, a nitrogen / oxygen-containing gas is introduced into the resistance heating furnace 82 for 5 to 10 minutes, for example, while the heating temperature is maintained at 1300 ° C. (third temperature maintenance). By introducing the nitrogen / oxygen-containing gas, as shown in FIG. 5N, the carbon film 83 reacts with oxygen in the gas to be oxidized and removed. As the introduced nitrogen / oxygen-containing gas, a gas containing at least N ₂ O (dinitrogen monoxide) can be used, and NO (nitrogen monoxide) may be contained. Further, the N ₂ O gas is supplied at a flow rate ratio of 30% or less, preferably 1 to 30% with respect to the total flow rate of the introduced gas.

Thereafter, while introducing the nitrogen / oxygen-containing gas into the resistance heating furnace 82 at the same flow rate, for example, the heating temperature is held at 1300 ° C. (fourth temperature holding) for 200 to 240 minutes. As a result, the surface 52 of the epitaxial layer 51 is oxidized, and as shown in FIG. 5O, a silicon nitride oxide film (gate insulating film 63) covering the entire surface 52 is formed.
After the gate insulating film 63 is formed, an inert gas (for example, N 2, Ar, etc.) is again introduced into the resistance heating furnace 82, and the heating temperature is controlled to drop from 1300 ° C. to 300 ° C. After the temperature is lowered, SiC substrate 42 is taken out from resistance heating furnace 82.

Next, as shown in FIG. 5P, a doped polysilicon material 84 is deposited from above the epitaxial layer 51 by a CVD method. The deposition of the polysilicon material 84 is continued until at least the gate trench 43 and the source trench 45 are filled.
Thereafter, as shown in FIG. 5Q, the deposited polysilicon material 84 is etched back until the etch-back surface is flush with the surface 52 of the epitaxial layer 51.

Subsequently, as shown in FIG. 5R, only the polysilicon material 84 remaining in the source trench 45 is removed by dry etching. Thereby, the gate electrode 66 made of the polysilicon material 84 remaining in the gate trench 43 is formed.
Next, as shown in FIG. 5S, an interlayer insulating film 67 made of SiO ₂ is laminated on the epitaxial layer 51 by the CVD method.

Then, as shown in FIG. 5T, the interlayer insulating film 67 and the gate insulating film 63 are successively patterned, whereby a contact hole 68 is formed in the interlayer insulating film 67 and the gate insulating film 63.
Next, as shown in FIG. 5U, a polysilicon material is deposited by CVD until the contact holes 68 are filled. Thereafter, N-type or P-type impurities are implanted into the deposited polysilicon material. The implantation conditions at this time vary depending on the type of impurities, but the acceleration energy is, for example, 10 to 100 keV. Thereafter, for example, impurity diffusion is performed at 900 ° C. for 20 minutes. As a result, a polysilicon layer 70 doped with impurities at a high concentration is formed. Next, Ti and TiN are deposited in this order on the surface of the polysilicon layer 70 by a method such as sputtering or vapor deposition to form the intermediate layer 71. Subsequently, a metal such as Al is deposited on the surface of the intermediate layer 71 by a method such as sputtering or vapor deposition to form a metal layer 72. Thereby, the source wiring 69 is formed. Next, a drain electrode 74 is formed on the back surface 50 of the SiC substrate 42.

Thereafter, an interlayer insulating film (not shown), a source pad 46, a gate pad 48, and the like are formed, whereby the semiconductor device 41 shown in FIG. 4 is obtained.
As described above, according to the semiconductor device 41, the gate trench 43 is formed by being dug down from the surface 52 (Si surface) of the epitaxial layer 51 made of SiC, like the semiconductor device 1 of the first embodiment. Yes. Therefore, the oxidation of the inner surface of the gate trench 43 (see FIG. 5O) is based on the following equation: the oxidation rate of the bottom surface 58 having the Si surface and the oxidation rate of the side surface orthogonal to the Si surface. The process proceeds under the condition that the oxidation rate of 57 <0.

In the above manufacturing method, the oxidation of the inner surface of the gate trench 43 is not thermal oxidation (Dry oxidation) using O ₂ gas or thermal oxidation (Wet oxidation) using H ₂ O (water vapor) gas, but nitrogen. -It is performed by thermal oxidation using an oxygen-containing gas. Further, immediately below the bottom surface 58 of the gate trench 43, an implantation active layer 62 in which a P-type impurity is implanted is formed. Therefore, the ratio of the oxidation rate of the bottom surface 58 to the oxidation rate of the side surface 57 (the oxidation rate of the bottom surface 58 / the oxidation rate of the side surface 57) is increased as compared with the case where the gate insulating film 63 is formed by dry oxidation or wet oxidation. be able to.

The gate insulating film 63 thus formed has a ratio (thickness T ₄ / thickness T ₃ ) of the thickness T ₄ of the insulating film bottom 64 to the thickness T ₃ of the insulating film side 65. .3 to 1.0.
That is, even if the thickness T ₄ of the insulating film bottom 64 is increased to such an extent that dielectric breakdown can be suppressed, the lower limit of (thickness T ₄ / thickness T ₃ ) is 0.3. it is possible to suppress an excessive increase in the thickness T ₃ of the. On the other hand, since the upper limit is 1.0, when the thickness T ₄ of the insulating film bottom 64 is designed to an appropriate size, the thickness T ₃ of the insulating film side 65 does not become excessively small. As a result, by appropriately designing the thickness T ₄ of the insulating film bottom 64, it is possible to suppress the dielectric breakdown of the insulating film bottom 64 while suppressing the increase in the thickness T ₃ of the insulating film side 65. it can.

Further, since the gate insulating film 63 is made of a silicon nitride oxide film formed by thermal oxidation using a nitrogen-containing gas, the channel mobility of the VDMOSFET can be improved.
Further, since the implant active layer 62 is formed immediately below the gate trench 43, the energy barrier formed between the implant active layer 62 and the epitaxial layer 51 can be increased. Therefore, it is possible to make it difficult for current to flow to the implant active layer 62. As a result, electric field concentration on the bottom surface 58 of the gate trench 43 can be suppressed.

In addition, since the source trench 45 is formed at the center of each unit cell 44 surrounded by the gate trench 43, the equipotential lines in the vicinity of the corner portions 61 of the gate trench 43 can be suppressed. As a result, since the electric field applied to the both end corners 61 at the bottom of the gate trench 43 can be relaxed, the dielectric breakdown of the insulating film bottom 64 can be suppressed.
The source trench 45 may be deeper than the gate trench 43 as in the semiconductor device 85 shown in FIG. As a result, the electric field applied to the corner portions 61 at the bottom of the gate trench 43 can be further relaxed.

In the semiconductor device 41, since the source wiring 69 has the polysilicon layer 70 at the contact portion with the source region 55 and the body contact region 56, the source wiring 69 is connected to the body contact which is a high-concentration impurity region. An ohmic junction can be formed with respect to both the region 56 and the source region 55.
Therefore, when the semiconductor device 41 is manufactured, the step of forming the Ni layer on the surface 52 of the epitaxial layer 51 can be omitted, unlike the case where the layer made of only metal such as Al is directly contacted with the impurity region. The step of siliciding the Ni layer can be omitted. Therefore, generation of a carbon layer on the surface 52 of the epitaxial layer 51 can be prevented.

As a result, layer peeling between the source wiring 69 and the epitaxial layer 51 can be suppressed. Therefore, the connection reliability of the source wiring 69 can be improved.
Further, since the layer (polysilicon layer 70) that enters the source trench 45 and contacts the drain region 54, the body contact region 56, and the source region 55 is made of polysilicon having excellent coverage, the coverage of the source wiring 69 is improved. Can be made. As a result, the connection reliability of the source wiring 69 can be further improved.

  An intermediate layer 71 having a laminated structure of a Ti layer and a TiN layer is interposed between the polysilicon layer 70 and the metal layer 72. The material containing Ti has excellent adhesion to both the polysilicon material and the metal material. Therefore, the adhesion between the polysilicon layer 70 and the metal layer 72 can be improved. As a result, the connection reliability of the source wiring 69 can be further improved.

  Furthermore, when the metal layer 72 contains Al, the TiN layer can be used as a barrier layer for preventing the diffusion of Al from the metal layer 72 to the polysilicon layer 70. Diffusion to the layer 70 can be prevented. As a result, since the impurity concentration of the polysilicon layer 70 can be stabilized, the resistance value of the polysilicon layer 70 can be stabilized.

Next, an embodiment relating to an invention of a method of manufacturing an SiC semiconductor device using a resistance heating furnace will be described.
FIG. 7 is a schematic cross-sectional view of a planar gate type semiconductor device.
The semiconductor device 101 has a structure in which unit cells of a planar gate type VDMOSFET are arranged in a matrix. In FIG. 7, some of the plurality of unit cells are shown.

The semiconductor device 101 includes an N ⁺ type SiC substrate 102 that forms the base of the semiconductor device 101. On the surface 121 of the SiC substrate 102, an N ⁻ type epitaxial layer 103 made of SiC (silicon carbide: silicon carbide) doped with an N type impurity at a lower concentration than the SiC substrate 102 is laminated. The surface 131 of the epitaxial layer 103 is composed of, for example, a (0001) plane of SiC.

In the epitaxial layer 103, an N ⁻ -type drain region 104 that is maintained as it is after epitaxial growth is formed.
A P-type body region 105 is formed in the surface layer portion of the epitaxial layer 103. Although not shown in FIG. 7, a plurality of body regions 105 are formed at regular intervals, and extend in the same direction (direction perpendicular to the paper surface of FIG. 7) in parallel to each other. Arranged in a matrix (matrix). The drain region 104 is exposed between the adjacent body regions 105.

In the surface layer portion of the body region 105, an N ⁺ type source region 106 is formed at a distance from the periphery.
A gate insulating film 107 is formed on the surface 131 of the epitaxial layer 103 so as to straddle the drain region 104, the body region 105, and the source region 106. The gate insulating film 107 is made of SiO _2.

On the gate insulating film 107, a gate electrode 108 made of polysilicon doped with an N-type impurity at a high concentration is formed. The gate electrode 108 is opposed to the drain region 104, the body region 105, and the source region 106 with the gate insulating film 107 interposed therebetween.
On the epitaxial layer 103, an interlayer insulating film 109 made of SiO ₂ is laminated. A source wiring 111 is formed on the interlayer insulating film 109. The source wiring 111 is electrically connected to the body region 105 and the source region 106 through a contact hole 110 formed in the interlayer insulating film 109.

A gate wiring 112 is electrically connected to the gate electrode 108 through a contact hole (not shown) formed in the interlayer insulating film 109.
A drain electrode 113 is formed on the back surface of the SiC substrate 102.
When the potential of the gate electrode 108 is controlled while the source wiring 111 is grounded and an appropriate positive voltage is applied to the drain electrode 113, the interface between the body region 105 and the gate insulating film 107 is generated by the electric field from the gate electrode 108. A channel can be formed in the vicinity. Thereby, a current can flow between the source wiring 111 and the drain electrode 113.

8A to 8L are schematic cross-sectional views illustrating a method for manufacturing the semiconductor device of FIG.
First, as shown in FIG. 8A, an epitaxial layer 103 is formed on the surface 121 of the SiC substrate 102 by an epitaxial growth method. At this time, the main growth surface (surface 121) of the SiC substrate 102 is the (0001) plane. Since the surface 121 of the SiC substrate 102 is the (0001) plane, the epitaxial layer 103 formed by epitaxial growth on the SiC substrate 102 is also formed with the (0001) plane as the main surface. Therefore, surface 131 of epitaxial layer 103 parallel to surface 121 of SiC substrate 102 is a (0001) plane.

  Next, a photoresist 114 having an opening 115 is formed on the surface 131 of the epitaxial layer 103 by a known photolithography technique in a portion facing the region where the body region 105 is to be formed. Then, ions of P-type impurities (for example, boron ions) are incident on the surface 131 of the epitaxial layer 103 from above the photoresist 114. Thereby, as shown in FIG. 8B, the P-type impurity is implanted into the surface layer portion of the portion exposed from the opening 115 of the epitaxial layer 103.

  Subsequently, a photoresist 116 having an opening 117 in a portion facing the region where the source region 106 is to be formed is formed on the surface 131 of the epitaxial layer 103 by a known photolithography technique. Then, ions of an N-type impurity (for example, arsenic ions) are incident on the surface 131 of the epitaxial layer 103 from above the photoresist 116. As a result, as shown in FIG. 8C, the N-type impurity is implanted into the surface layer portion of the portion exposed from the opening 117 of the epitaxial layer 103 (on the surface 131 side from the P-type impurity implantation site).

  After the impurity ions are implanted into the surface layer portion of the epitaxial layer 103, an organic material film 118 is formed over the entire surface 131 of the epitaxial layer 103, as shown in FIG. 8D. The organic material film 118 is a material containing carbon (carbon), and for example, an organic material (eg, polyimide) used as a photoresist can be applied. Such an organic material film 118 is formed using, for example, a spin coater.

After the organic material film 118 is formed, the SiC substrate 102 is loaded into the resistance heating furnace 122. The resistance heating furnace 122 is not particularly limited as long as the resistance heating furnace 122 can secure airtightness in the resistance heating furnace 122 in which an object to be heated is set and can introduce various gases into the resistance heating furnace 122. The heating method may be either a direct heating method or an indirect heating method.
Then, with the SiC substrate 102 set in the resistance heating furnace 122, an inert gas (for example, N ₂ , Ar, etc.) is introduced into the resistance heating furnace 122, and the resistance heating furnace 122 controls the temperature rise. (First temperature increase control).

  In this first temperature rise control, as shown in FIG. 6, the heating temperature is controlled to rise from 100 ° C. to 1000 ° C. over 35 to 45 minutes, for example, and after the rise, for example, 5 to 10 minutes. The heating temperature is held at 1000 ° C. (first temperature holding). By this temperature rise and temperature holding, elements other than carbon in the organic material film 118 are evaporated, and the organic material film 118 is transformed into a carbon film 119 as shown in FIG. 8E. Therefore, the entire surface 131 of the epitaxial layer 103 is covered with the carbon film 119.

Subsequently, the resistance heating furnace 122 is further subjected to temperature increase control (second temperature increase control) while maintaining the inside of the resistance heating furnace 122 in an inert atmosphere.
In the second temperature increase control, as shown in FIG. 6, the heating temperature is controlled to rise from 1000 ° C. to 1600 ° C. over 30 to 60 minutes, for example. After the rise, for example, the heating temperature is held at 1600 ° C. (second temperature holding) for 5 to 10 minutes. By this temperature increase and temperature holding, ions of N-type impurities and P-type impurities implanted into the surface layer portion of the epitaxial layer 103 are activated, and as shown in FIG. A source region 106 is formed. Further, a drain region 104 that is separated from the body region 105 and maintains the state after epitaxial growth is formed in the base layer portion of the epitaxial layer 103.

Next, the temperature of the resistance heating furnace 122 is controlled to be lowered while the inside of the resistance heating furnace 122 is maintained in an inert atmosphere.
In the temperature drop control, as shown in FIG. 6, the heating temperature is limited (temperature drop restriction) so as to drop from 1600 ° C. to 1300 ° C. over 15 to 30 minutes, for example. After the temperature is lowered, an oxygen-containing gas is introduced into the resistance heating furnace 122, for example, for 5 to 10 minutes while the heating temperature is maintained at 1300 ° C. (third temperature maintenance). By introducing the oxygen-containing gas, as shown in FIG. 8G, the carbon film 119 reacts with oxygen in the oxygen-containing gas to be oxidized and removed. However, as the oxygen-containing gas introduced into the resistance heating furnace 122, it is preferable to use a gas containing oxygen and nitrogen, and specifically, NO (nitrogen monoxide), N ₂ O (dinitrogen monoxide). ) And the like can be used.

Thereafter, while introducing the oxygen-containing gas into the resistance heating furnace 122, for example, the heating temperature is held at 1300 ° C. (fourth temperature holding) for 200 to 240 minutes. As a result, the surface 131 of the epitaxial layer 103 is oxidized, and an oxide film 120 covering the entire surface 131 is formed as shown in FIG. 8H.
After the oxide film 120 is formed, an inert gas (for example, N ₂ , Ar, etc.) is reintroduced into the resistance heating furnace 122 and the heating temperature is controlled to drop from 1300 ° C. to 300 ° C. After the temperature is lowered, SiC substrate 102 is taken out from resistance heating furnace 122.

Next, a conductive material is deposited by sputtering. Then, the conductive material is patterned by a known photolithography and etching technique, and a gate electrode 108 is formed on the oxide film 120 as shown in FIG. 8I.
Thereafter, an interlayer insulating film 109 is laminated on the epitaxial layer 103 by a CVD (Chemical Vapor Deposition) method as shown in FIG. 8J.

Then, contact holes 110 are formed in the interlayer insulating film 109 and the oxide film 120 as shown in FIG. 8K by a known photolithography technique and etching technique. The remaining portion of the oxide film 120 becomes the gate insulating film 107.
Next, a conductive material is formed on the epitaxial layer 103 by sputtering. The conductive material is deposited (deposited) so as to fill the contact hole 110 and form a thin film on the interlayer insulating film 109. Then, the conductive material on the interlayer insulating film 109 is patterned by a known photolithography technique and etching technique. As a result, the source wiring 111 is formed as shown in FIG. 8L. In addition, a gate wiring 112 electrically connected to the gate electrode 108 is formed. Further, drain electrode 113 is formed on the back surface of SiC substrate 102.

Through the above steps, the semiconductor device 101 shown in FIG. 7 is obtained.
According to the above manufacturing method, after the organic material film 118 is formed, the organic material film 118 in the resistance heating furnace 122 is heated to be transformed into the carbon film 119 by the first temperature rise control of the resistance heating furnace 122. A carbon film 119 is formed on the surface 131 of the epitaxial layer 103.
After the formation of the carbon film 119, the epitaxial layer 103 is heated by the second temperature rise control of the resistance heating furnace 122 while the inside of the resistance heating furnace 122 is maintained in an inert atmosphere, and the N-type impurities in the epitaxial layer 103 and P-type impurity ions are activated.

  And temperature fall control (for example, temperature fall from 1600 degreeC to 1300 degreeC) is performed, maintaining the inside of the resistance heating furnace 122 in an inactive state. Thereafter, an oxygen-containing gas is introduced, for example, for 5 to 10 minutes with the heating temperature maintained at 1300 ° C. (third temperature maintained). Thereby, the carbon film 119 is oxidized and removed, and the surface 131 of the epitaxial layer 103 is exposed.

After the removal of the carbon film 119, the oxygen-containing gas is continuously introduced into the resistance heating furnace 122, and the resistance heating furnace 122 is maintained at a temperature (fourth temperature maintenance), whereby the exposed surface 131 is oxidized and oxidized. A film 120 is formed.
Prior to heating for ion activation (second temperature rise control), the carbon film 119 is formed on the surface 131 of the epitaxial layer 103, so that Si escape from the surface 131 is prevented when the epitaxial layer 103 is heated. be able to. Therefore, the surface 131 of the epitaxial layer 103 can be prevented from being roughened, and the flatness of the surface 131 can be maintained. As a result, the interface between the epitaxial layer 103 and the gate insulating film 107 can be smoothed, so that the channel mobility of the semiconductor device 101 can be improved.

  Further, the step of heating the organic material film 118 to transform it into the carbon film 119 (first temperature rise control), the step of heating the epitaxial layer 103 to activate ions (second temperature rise control), oxygen containing Four steps including a step of oxidizing and removing the carbon film 119 with a gas (temperature drop restriction control and third temperature holding) and a step of oxidizing the surface of the SiC layer to form an oxide film (fourth temperature holding) Two resistance heating furnaces 122 can be used continuously. Since an apparatus for removing the carbon film is not required separately, an increase in apparatus cost can be suppressed. In addition, since the resistance heating furnace 122 is used, the first temperature rise control, the second temperature rise control, the temperature fall restriction control, the third temperature hold, and the fourth temperature hold can be executed precisely and easily. it can.

Further, the surface 131 of the epitaxial layer 103 on which the oxide film 120 is formed is a (0001) plane, and the oxygen-containing gas introduced into the heating furnace is a gas containing oxygen and nitrogen.
For example, when the oxide film is formed by oxidizing the (0001) surface of the SiC layer with O ₂ gas, H ₂ O gas (water vapor) and N ₂ O gas, the channel mobility of the MOSFET including the SiC layer is: for example, each 1~5cm ² / V · s, a 5~15cm ² / V · s and 15~25cm ² / V · s, when the _{N 2} O gas is most excellent in channel mobility.

In the semiconductor device 101 of this embodiment, since the (0001) plane (surface 131) of the epitaxial layer 103 is oxidized with NO gas or N ₂ O gas to form the oxide film 120, the channel mobility of the semiconductor device 101 is increased. Can be further improved.
As mentioned above, although embodiment of this invention was described, this invention can also be implemented with another form.
For example, a configuration in which the conductivity type of each semiconductor portion of the semiconductor device 1 is inverted may be employed. That is, in the semiconductor devices 1, 41, and 85, the P-type portion may be N-type and the N-type portion may be P-type.

Further, the source wirings 17 and 69 and the drain wiring 23 (drain electrode 74) may have a laminated structure of a layer in which nickel (Ni) or titanium (Ti) is silicided and the metal layer described above.
In addition, various design changes can be made within the scope of matters described in the claims.

Next, although this invention is demonstrated based on an Example and a comparative example, this invention is not limited by the following Example.
Example 1 (N ₂ O oxidation)
First, an SiC crystal was grown on a Si surface of a wafer-like SiC substrate (manufactured by Cree) while doping an N-type impurity to form an epitaxial layer made of SiC. Next, a SiO ₂ mask having a predetermined pattern was formed on the surface (Si surface) of the epitaxial layer, and a trench was formed by making SF ₆ / O ₂ gas incident on the surface of the epitaxial layer through the SiO ₂ mask.

Next, the SiC substrate was carried into a diffusion furnace, and N ₂ O gas was supplied for 3 hours while the inside of the diffusion furnace was heated to 1275 ° C. As a result, the inner surface of the trench was oxidized to form an oxide film.
An oxide film was also formed by the same operation as described above when the N ₂ O gas supply time (oxidation time) was 8 hours and 12 hours.
Comparative Example 1 (Dry oxidation)
The same processes as in Example 1 were performed until the process of forming the trench. After forming the trench, the SiC substrate was carried into a diffusion furnace, and O ₂ gas was supplied for 4 hours in a state where the inside of the diffusion furnace was heated to 1150 ° C. As a result, the inner surface of the trench was oxidized to form an oxide film.

In addition, an oxide film when the O ₂ gas supply time (oxidation time) was 6 hours and 8 hours was formed by the same operation as described above.
Comparative Example 2 (Wet oxidation)
The same processes as in Example 1 were performed until the process of forming the trench. After the trench is formed, the SiC substrate is carried into a diffusion furnace, and water vapor (H ₂ O gas is supplied for 15 minutes while the inside of the diffusion furnace is heated to 1275 ° C. This oxidizes the inner surface of the trench to form an oxide film. did.

In addition, an oxide film when the supply time (oxidation time) of the H ₂ O gas was 25 minutes and 35 minutes was also formed by the same operation as described above.
1) Measurement of thickness of oxide film The thickness of each oxide film formed in Example 1 and Comparative Examples 1 and 2 was measured for each portion on the side surface of the trench and each portion on the bottom surface of the trench. The results are shown in FIGS. 10 (a) to 10 (c) (FIG. 10 (a): Example 1, FIG. 10 (b): Comparative Example 1, FIG. 10 (c): Comparative Example 2).
2) Thickness ratio of oxide film Using the thicknesses of the oxide films shown in FIGS. 10A to 10C, the ratio of the thickness of the portion on the bottom surface to the thickness of the portion on the side surface of the oxide film (Bottom / side) was calculated. The results are shown in FIGS. 10 (a) to (c).

According to FIG. 10A, the ratio of the thickness of the portion on the bottom surface to the thickness of the portion on the side surface of the oxide film (bottom / side surface) is about 0.54 (3 hours), 0 for each supply time. .46 (8 hours), 0.48 (12 hours).
Further, according to FIG. 10B, the ratio of the thickness of the portion on the bottom surface to the thickness of the portion on the side surface in the oxide film (bottom surface / side surface) is about 0.20 (4 hours) every supply time. , 0.20 (6 hours), 0.19 (8 hours).

  Further, according to FIG. 10C, the ratio of the thickness of the portion on the bottom surface to the thickness of the portion on the side surface of the oxide film (bottom surface / side surface) is about 0.23 (15 minutes) every supply time. , 0.21 (25 minutes), 0.22 (35 minutes).

DESCRIPTION OF SYMBOLS 1 Semiconductor device 3 Epitaxial layer 5 Body region 6 Gate trench 9 Gate insulating film 10 Insulating film side portion 11 Insulating film bottom portion 12 Gate electrode 13 Source region 18 Polysilicon layer 25 Intermediate layer 26 Metal layer 41 Semiconductor device 43 Gate trench 51 Epitaxial layer 53 Body region 55 Source region 61 Corner portion 62 Implant active layer 63 Gate insulating film 64 Insulating film bottom portion 65 Insulating film side portion 66 Gate electrode 69 Source wiring 70 Polysilicon layer 71 Intermediate layer 72 Metal layer 85 Semiconductor device

Claims (13)

A first conductivity type semiconductor layer made of SiC and having a Si surface as a surface;
A gate trench dug from the surface of the semiconductor layer;
A gate insulating film formed on the bottom and side surfaces of the gate trench, wherein the ratio of the thickness of the portion on the bottom surface to the thickness of the portion on the side surface is 0.3 to 1.0;
And a gate electrode embedded in the gate trench through the gate insulating film. A body region of a second conductivity type formed on a side of the gate trench in the semiconductor layer and in contact with the gate insulating film on a side surface of the gate trench;
A first conductivity type source region formed adjacent to the gate trench in a surface layer portion of the body region;
The semiconductor device according to claim 1, wherein the gate insulating film contains nitrogen. The semiconductor device according to claim 2, wherein a concentration of the second conductivity type impurity in the body region is 1e19 cm ⁻³ or less.   4. The device according to claim 1, further comprising an implantation layer formed by impurity implantation in a portion from the bottom surface of the gate trench in the semiconductor layer to a middle portion in the thickness direction of the semiconductor layer. The semiconductor device described.   The semiconductor device according to claim 4, wherein the implantation layer is formed by implantation of the second conductivity type impurity.   The semiconductor device according to claim 1, wherein a thickness of a portion on the side surface of the gate trench in the gate insulating film is 2000 mm or less.   The semiconductor device according to claim 1, wherein an end portion of the bottom portion of the gate trench in a direction orthogonal to the gate width is curved outward. A source line formed on the semiconductor layer and in contact with the source region;
The semiconductor device according to claim 2, wherein the source wiring has a polysilicon layer at a contact portion with the source region, and has a metal layer on the polysilicon layer.   The semiconductor device according to claim 8, wherein an intermediate layer containing Ti is interposed between the polysilicon layer and the metal layer. The metal layer has a layer containing Al;
The semiconductor device according to claim 9, wherein the intermediate layer has a structure in which a Ti layer and a TiN layer are laminated in this order from the polysilicon layer side. Forming a gate trench dug from the surface of the surface layer portion of the first conductivity type semiconductor layer made of SiC and having a Si surface.
Forming a gate insulating film on the bottom and side surfaces of the gate trench by oxidizing the bottom and side surfaces of the gate trench in a gas containing nitrogen and oxygen at a heat treatment temperature of 1200 ° C. or more;
Forming a gate electrode on the gate insulating film so as to fill the gate trench. The method of manufacturing a semiconductor device according to claim 11, wherein in the step of forming the gate insulating film, the bottom surface and the side surface of the gate trench are oxidized in a gas containing at least N ₂ O. The method for manufacturing a semiconductor device according to claim 12, wherein in the step of forming the gate insulating film, N ₂ O gas is supplied at a flow rate ratio of 30% or less with respect to a total flow rate of supplied gas.

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