Patent Publication Number: US-10777677-B2

Title: Insulated gate semiconductor device and method of manufacturing same

Description:
BACKGROUND OF THE INVENTION 
     Technical Field 
     The present invention relates to an insulated gate semiconductor device and a method of manufacturing the same, and particularly relates to an insulated gate semiconductor device having a trench gate structure and a method of manufacturing the same. 
     Background Art 
     A trench gate MOS field effect transistor (MOSFET) can be expected to reduce ON resistance due to narrowing the pitch of cells as compared to planar structures. However, in an insulated gate semiconductor device such as a SiC-MOSFET device that uses silicon carbide (SiC), there is a risk that the gate insulating film at the bottom of the trench will be destroyed by high voltages during reverse bias when attempting to make use of the high breakdown electric field strength of SiC. 
     Attempts have begun to try ion implantation (channeling ion implantation) that uses the phenomenon of channeling in the manufacturing of insulated gate semiconductor devices having SiC (see Patent Documents 1 to 3). However, in the inventions disclosed in Patent Documents 1 to 3, there has been no examination into using channeling ion implantation to solve problems that are unique to trench gate structures. 
     RELATED ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2015-216182 
     Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2013-219161 
     Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2014-49620 
     SUMMARY OF THE INVENTION 
     In view of the aforementioned problems, the present invention aims at providing an insulated gate semiconductor device and a method of manufacturing the same that can protect the gate insulating film at the bottom of the trench from high voltages during reverse bias. 
     Additional or separate features and advantages of the invention will be set forth in the descriptions that follow and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, in one aspect, the present disclosure provides an insulated gate semiconductor device, including: a drift layer of a first conductivity type made of a semiconductor material having a wider bandgap than silicon, a top surface of the semiconductor material having an off-angle; a base region of a second conductivity type made of the semiconductor material, disposed above the drift layer; a first high-impurity region of the first conductivity type, embedded in a top of the base region and having a higher impurity density than the drift layer, wherein a trench is provided penetrating the first high-impurity region and the base region; a gate insulating film disposed on side surfaces and a bottom surface of the trench; an embedded gate electrode embedded inside the trench, with the gate insulating film being interposed between the trench and the embedded gate electrode; a second high-impurity region of the first conductivity type made of the semiconductor material, disposed on a bottom surface side of the drift layer; a gate bottom protection region of the second conductivity type, embedded in the drift layer at a bottom of the trench; and a base bottom embedded region of the second conductivity type, embedded in the drift layer below the base region separately from the gate bottom protection region, and having a higher impurity density than the base region, wherein a cross-section of the base bottom embedded region has a trapezoid-shaped portion on at least a bottom side of the base bottom embedded region, an upper base and a lower base of the trapezoid are parallel, and a virtual straight line that connects a midpoint of the upper base and a midpoint of the lower base is tilted from a line normal to the top surface of the drift layer towards a direction of the off-angle by a prescribed tilt angle, and wherein a bottom surface of the base bottom embedded region is deeper than a bottom surface of the gate bottom protection region. 
     In another aspect, the present disclosure provides a method of manufacturing an insulated gate semiconductor device, the method including: forming a drift layer of a first conductivity type made of a semiconductor material having a wider bandgap than silicon, a top surface of the semiconductor material having an off-angle; forming a gate bottom protection region of a second conductivity type inside the drift layer by performing ion implantation in a direction perpendicular to a top surface of the drift layer; forming a base bottom embedded region of the second conductivity type inside the drift layer by performing ion implantation in a prescribed titled direction in which channeling of implanted impurities occurs in a direction that is tilted relative to a line normal to the top surface of the drift layer towards a direction of the off-angle, the base bottom embedded region being formed such that a position of the bottom surface of the base bottom embedded region being deeper than the gate bottom protection region in the drift layer; forming a base region of the second conductivity type made of the semiconductor material above the drift layer; forming a first high-impurity region of the first conductivity type in a top of the base region at a higher impurity density than the drift layer; forming a base contact region of the second conductivity type in the top of the base region above the base bottom embedded region; forming a trench penetrating the base region and reaching the gate bottom protection region; forming a gate insulating film on side walls and a bottom surface of the trench; and forming an embedded gate electrode in the trench, with the gate insulating film interposed between the trench and the embedded gate electrode. 
     The present invention makes it possible to provide an insulated gate semiconductor device and a method of manufacturing the same that can protect the gate insulating film at the bottom of the trench from high voltages during reverse bias by using channeling ion implantation. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross-sectional view of main parts of one example of an insulated gate semiconductor device in an embodiment of the present invention, and  FIG. 1B  is a cross-sectional view in which a portion of  FIG. 1A  has been enlarged. 
         FIG. 2  is a top view of one example of a SiC wafer forming a portion of the insulated gate semiconductor device in the embodiment. 
         FIG. 3  is a view in which a portion of the SiC wafer seen from the A-A direction in  FIG. 2  has been enlarged. 
         FIG. 4A  is a schematic view of a 4H-SiC crystal seen from the m-plane, and  FIG. 4B  is a schematic view of a 4H-SiC crystal seen from the Si-plane. 
         FIG. 5  is a cross-sectional view of a step for describing one example of a method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 6  is a cross-sectional view of a step after  FIG. 5  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 7  is a cross-sectional view of a step after  FIG. 6  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 8  is a cross-sectional view of a step after  FIG. 7  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 9  is a cross-sectional view of a step after  FIG. 8  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 10  is a cross-sectional view of a step after  FIG. 9  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 11  is a cross-sectional view of a step after  FIG. 10  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 12  is a cross-sectional view of a step after  FIG. 11  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 13  is a cross-sectional view of a step after  FIG. 12  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 14  is a cross-sectional view of a step after  FIG. 13  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 15  is a cross-sectional view of a step after  FIG. 14  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 16  is a cross-sectional view of a step after  FIG. 15  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 17  is a cross-sectional view of a step after  FIG. 16  for describing one example of the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 18  is a graph showing the relationship between implantation depth and impurity density when a tilt angle has been changed toward an off-angle direction during ion implantation in the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 19  is a graph showing the relationship between the tilt angle and impurity density during ion implantation in the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 20  is a graph showing the relationship between implantation depth and impurity density when the tilt angle has been changed in a direction orthogonal to the off-angle direction during ion implantation in the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 21  is a graph showing the relationship between implantation depth and impurity density when the temperature has been changed during ion implantation in the method of manufacturing the insulated gate semiconductor device in the embodiment. 
         FIG. 22  is a cross-sectional view of main parts of one example of an insulated gate semiconductor device in a first modification example of the embodiment. 
         FIG. 23  is a cross-sectional view of a step for describing one example of a method of manufacturing the insulated gate semiconductor device in the first modification example. 
         FIG. 24  is a cross-sectional view of main parts of one example of an insulated gate semiconductor device in a second modification example of the embodiment. 
         FIG. 25  is a cross-sectional view of a step for describing one example of a method of manufacturing the insulated gate semiconductor device in the second modification example. 
         FIG. 26  is a cross-sectional view of a step following the step of  FIG. 25  for describing one example of the method of manufacturing the insulated gate semiconductor device in the second modification example. 
         FIG. 27  is a cross-sectional view of main parts of one example of an insulated gate semiconductor device in another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the drawings. In the description below, portions in the drawings that are the same or similar will be assigned the same or similar reference characters. However, it should be noted that the drawings are schematic, and that the relation between the thickness and planar dimensions and the ratio of thicknesses of the respective layers differ in practice. Thus, specific thicknesses and dimensions should be determined in reference to the description below. Furthermore, there are naturally portions that differ in dimensional relations and ratios in their depiction among different drawings. 
     In the present specification, the “first main electrode region” means a semiconductor region serving as either the source region or drain region in an insulated gate FET (MISFET), an insulated gate static induction transistor (MISSIT), or the like. In an insulated gate bipolar transistor (IGBT), the “first main electrode region” means a semiconductor region serving as either the emitter region or collector region, and in an MIS gate static inductor thyristor (MIS gate SI thyristor), it means a semiconductor region serving as either the anode region or cathode region. The “second main electrode region” means a semiconductor region serving as whichever of the source region or drain region that is not the first main electrode region in the MISFET and MISSIT, means a region serving as whichever of the emitter region or collector region that is not the first main electrode region in the IGBT, or means a region serving as whichever of the anode region or cathode region that is not the first main electrode region in the MIS gate SI thyristor. In other words, if the “first main electrode region” is the source region, then “second main electrode region” means the drain region. If the “first main electrode region” is the emitter region, then “second main electrode region” means the collector region. If the “first main electrode region” is the anode region, then “second main electrode region” means the cathode region. 
     In the description of the embodiments below, the first conductivity type is illustratively described as n-type and the second conductivity type as p-type, but the conductivity types may be reversed, where the first conductivity type is p-type and the second conductivity type is n-type. In the present specification and the attached drawings, “+” or “−” attached to an “n” or “p” as superscript signifies a semiconductor region in which the impurity density is higher or lower, respectively, than a semiconductor region which is described without the “+” or “−”. Moreover, in the description below, even if it not explicitly limited as such, it is both technically and logically obvious that members or regions to which the limitation of “first conductivity type” and “second conductivity type” have been added signify members or regions made of a semiconductor material. Furthermore, when representing Miller indices in the present specification, “−” signifies a bar attached to the index immediately thereafter, and attaching a “−” before the index represents a negative index. 
     Furthermore, in the description below, the definition of “top” and “bottom” such as in “top surface” and “bottom surface” are merely expressions for the shown cross-sectional views, and if the orientation of the insulated gate semiconductor device were changed 90°, then “top” and “bottom” would be called “left” and “right,” and if changed 180°, “top” and “bottom” would be reversed and called “bottom” and “top,” for example. 
     &lt;Structure of Insulated Gate Semiconductor Device&gt; 
     As shown in  FIG. 1A , the insulated gate semiconductor device in an embodiment of the present invention is a MISFET including a first conductivity type (n −  type) drift layer  2 , and second conductivity type (p-type) base regions  7   a ,  7   b ,  7   c  arranged on the top surface side of the drift layer  2 . The tops of the base regions  7   a ,  7   b ,  7   c  are provided with n +  first main electrode regions (source regions; first high-impurity region)  8   a ,  8   b ,  8   c ,  8   d  with a higher impurity density than the drift layer  2 . Trenches  21   a  and  21   b  are disposed going through the source regions  8   a ,  8   b ,  8   c ,  8   d  and base regions  7   a ,  7   b ,  7   c , and gate insulating films  10   a ,  10   b  are disposed on the bottom surfaces and side surfaces of the trenches  21   a ,  21   b . Embedded gate electrodes  11   a ,  11   b  are embedded inside the trenches  21   a ,  21   b  with the gate insulating films  10   a ,  10   b  being interposed between the embedded gate electrodes and the trenches. The insulated gate semiconductor device in the embodiment of the present invention is a MISFET, and thus an n +  second main electrode region (drain region; second high-impurity region)  1  is disposed on the bottom surface side of the drift layer  2 . 
     For convenience,  FIG. 1A  shows a cross-sectional view of main components of two individual cell structures each containing the trench  21   a  or  21   b . However, the insulated gate semiconductor device in the embodiment of the present invention can pass a large current if a multi-channel structure is formed by periodically arraying a further plurality of these individual cell structures. 
     The tops of the base regions  7   a ,  7   b ,  7   c  are provided with p +  base contact regions  9   a ,  9   b ,  9   c  of a higher impurity density than the base regions  7   a ,  7   b ,  7   c , the p +  base contact regions being provided so as to contact the source regions  8   a ,  8   b ,  8   c ,  8   d . The top of the drift layer  2  is provided with an n +  current spreading layer (CSL)  3  of a higher impurity density than the drift layer  2 , the CSL being provided so as to be sandwiched between the bottom of the drift layer  2  and the base regions  7   a ,  7   b ,  7   c . The current spreading layer  3  is provided by introducing n-type impurities into the top of the drift layer  2 , and functions to reduce spreading resistance of the carrier. The current spreading layer  3  need not be provided, and instead the top surface of the drift layer  2  may contact the base regions  7   a ,  7   b ,  7   c  at the position where the top surface of the current spreading layer  3  would be. 
     P +  gate bottom protection regions  4   a ,  4   b  are arranged at the bottoms of the trenches  21   a ,  21   b  in order to protect the gate insulating films  10   a ,  10   b  at the bottoms of the trenches  21   a ,  21   b  from high voltages during reverse bias. Meanwhile, p +  base bottom embedded regions ( 5   a ,  6   a ), ( 5   b ,  6   b ), ( 5   c ,  6   c ) are respectively arranged on the bottom surface sides of the base regions  7   a ,  7   b ,  7   c  below the base contact regions  9   a ,  9   b ,  9   c . The cross-sectional shape of the base bottom embedded region ( 5   a ,  6   a ), ( 5   b ,  6   b ), ( 5   c ,  6   c ) has a trapezoidal shape at least on the respective bottom sides.  FIG. 1B  is a cross-sectional view in which portions of the p +  base bottom embedded regions  5   b ,  6   b  of  FIG. 1A  have been enlarged. 
     The base bottom embedded region ( 5   a ,  6   a ) has a shape that has, as a single unit, a trapezoid containing the first embedded region  5   a , which is made of a first trapezoid, and the second embedded region  6   a  disposed on the top surface of the first embedded region  5   a  and which is made of a second trapezoid contacting the bottom surface of the base region  7   a . The base bottom embedded region ( 5   b ,  6   b ) has a shape that has, as a single unit, a trapezoid containing the first embedded region  5   b , which is made of a first trapezoid, and the second embedded region  6   b  disposed on the top surface of the first embedded region  5   b  and which is made of a second trapezoid contacting the bottom surface of the base region  7   b . The base bottom embedded region ( 5   c ,  6   c ) has a shape that has, as a single unit, a trapezoid containing the first embedded region  5   c , which is made of a first trapezoid, and the second embedded region  6   c  disposed on the top surface of the first embedded region  5   c  and which is made of a second trapezoid contacting the bottom surface of the base region  7   c.    
     The top surfaces of the first embedded regions  5   a ,  5   b ,  5   c  are provided at the same depth as the top surfaces of the gate bottom protection regions  4   a ,  4   b . The thickness of the first embedded regions  5   a ,  5   b ,  5   c  and gate bottom protection regions  4   a ,  4   b  from the top surface to the bottom surface is approximately 0.2 μm to 1 μm, for example, but the bottom surfaces of the first embedded regions  5   a ,  5   b ,  5   c  are provided at deeper positions than the bottom surfaces of the gate bottom protection regions  4   a ,  4   b . A difference d between the bottom surfaces of the first embedded regions  5   a ,  5   b ,  5   c  and the bottom surfaces of the gate bottom protection regions  4   a ,  4   b  is approximately 0.1 μm to 0.5 μm, for example. If the difference d is 0.1 μm or greater, for example, then when a high voltage is applied during reverse bias, it is easier for the avalanche current to be pulled into the first embedded regions  5   a ,  5   b ,  5   c , which is preferable. The thickness of the second embedded regions  6   a ,  6   b ,  6   c  from the top surface to the bottom surface is approximately 0.2 μm to 1 μm. 
     In the embodiment, the drain region  1  is formed of a semiconductor substrate made of SiC (SiC substrate), and the drift layer  2  is formed of an epitaxial layer made of SiC (SiC layer). In addition to SiC, the drain region  1  and drift layer  2  can be various types of semiconductor material with a bandgap wider than the 1.1 eV bandgap of silicon, such as gallium nitride (GaN), diamond, or aluminum nitride (AlN). It has been reported that the bandgap at room temperature is 2.23 eV for 3C-SiC, 3.26 eV for 4H-SiC, 3.02 eV for 6H-SiC, 3.4 eV for GaN, 5.5 eV for diamond, and 6.2 eV for AN. A wide bandgap semiconductor with a bandgap of 2.0 eV or greater can be used for the drain region  1 , drift layer  2 , or the like, but for LEDs etc. a “wide bandgap” is usually defined as 2.5 eV or greater. In the present invention, the bandgap of a wide bandgap semiconductor will be described in reference to 2.23 eV, which is a bandgap at room temperature for 3C-SiC. 
     The trenches  21   a ,  21   b  going through the source regions  8   a ,  8   b ,  8   c ,  8   d  and base regions  7   a ,  7   b ,  7   c  in the depth direction have bottoms that reach the current spreading layer  3 . In  FIG. 1A , an example is shown in which the bottom surfaces of the trenches  21   a ,  21   b  are curved, but the bottom surfaces of the trenches  21   a ,  21   b  may be flat. In terms of a planar shape, the trenches  21   a ,  21   b  of each individual cell structure may be arrayed in a strip-like shape, or may have a rectangular planar shape, or a polygonal planar shape such as a hexagon shape. 
     In addition to a silicon oxide film (SiO 2  film), the gate insulating films  10   a ,  10   b  can be a single layer film of one of a silicon oxynitride (SiON) film, a strontium oxide (SrO) film having a greater dielectric constant than a SiO 2  film, a silicon nitride (Si 3 N 4 ) film, an aluminum oxide (Al 2 O 3 ) film, a magnesium oxide (MgO) film, an yttrium oxide (Y 2 O 3 ) film, a hafnium oxide (HfO 2 ) film, a zirconium oxide (ZrO 2 ) film, a tantalum oxide (Ta 2 O 5 ) film, or a bismuth oxide (Bi 2 O 3 ) film, or the gate insulating films can be a composite film in which a plurality of these are laminated, or the like. 
     The material of the embedded gate electrodes  11   a ,  11   b  can be a polysilicon layer (a doped polysilicon layer) to which a high concentration of impurities has been added, for example. A first main electrode (source electrode)  16  is disposed on the embedded gate electrodes  11   a ,  11   b  with interlayer insulating films  12   a ,  12   b  interposed therebetween, the first main electrode being separated from a gate surface electrode (not shown) positioned in the depth direction of the paper. The source electrode  16  electrically connects to the source regions  8   a ,  8   b ,  8   c ,  8   d  and the base contact regions  9   a ,  9   b ,  9   c.    
     First bottom barrier metal layers  13   a ,  13   b ,  13   c , second bottom barrier metal layers  14   a ,  14   b , and a top barrier metal layer  15  are included in layers below the source electrode  16 . The first bottom barrier metal layers  13   a ,  13   b ,  13   c  are arranged so as to be respectively metallurgically connected to the base contact regions  9   a ,  9   b ,  9   c . The second bottom barrier metal layers  14   a ,  14   b  are arranged so as to metallurgically contact the source regions  8   a ,  8   b ,  8   c ,  8   d  and cover the interlayer insulating films  12   a ,  12   b . The top barrier metal layer  15  is arranged so as to cover the first bottom barrier metal layers  13   a ,  13   b ,  13   c  and second bottom barrier metal layers  14   a ,  14   b . The source electrode  16  is arranged so as to cover the top barrier metal layer  15 . For example, the first bottom barrier metal layers  13   a ,  13   b ,  13   c  are formed of a nickel (Ni) film, the second bottom barrier metal layers  14   a ,  14   b  are formed of a titanium nitride (TiN) film, the top barrier metal layer  15  is formed of a laminate structure of titanium (Ti)/TiN/Ti, and the source electrode  16  is formed of an aluminum (Al) film. The gate surface electrode can be the same material as the source electrode  16 . 
     A second main electrode (drain electrode)  17  is disposed on the bottom surface side of the drift layer  2  so as to contact the drain region  1 . The drain electrode  17  can be a single layer film made of gold (Au), a metal film in which Al, nickel (Ni), and Au have been laminated in the stated order, or the like, and a metal plate such as molybdenum (Mo) or tungsten (W) may be further laminated on the bottommost layer thereof. 
     As shown in  FIG. 2 , the insulated gate semiconductor device in the embodiment is fabricated based on a semiconductor wafer  100  made of SiC. An orientation flat  101  showing the crystal orientation of the semiconductor wafer  100  is provided on the outer periphery of the semiconductor wafer  100 . The semiconductor wafer  100  is sliced along a direction serving as a surface tilted by a fixed offset angle (off-angle) θ 1  relative to a specific crystal orientation, and the position of the orientation flat  101  is determined by the crystal orientation. A notch may be provided in the outer periphery of the semiconductor wafer  100  instead of the orientation flat  101 . 
     As schematically shown in  FIG. 3 , the semiconductor wafer  100  has an off-angle θ 1  of approximately 4° to 8°, such as greater than 3° and less than or equal to 8°, for example, in the &lt;11-20&gt; direction about the &lt;0001&gt; (c-axis) direction. The off-angle θ 1  is an angle formed by a surface  102  of the semiconductor wafer  100  and the surface (substrate bottom surface) perpendicular to the c-axis, which is the (0001)-plane (Si-plane) or (000-1)-plane (C-plane). 
       FIG. 4A  schematically shows the crystal structure of 4H-SiC seen from the m-plane direction, which is the (1-100)-plane perpendicular to the (0001)-plane, and  FIG. 4B  schematically shows the crystal structure of 4H-SiC seen from the Si-plane. Both have sparse portions with no overlapping atoms. This is the same when viewed from the C-plane on the side opposite to the Si-plane. Thus, if ion implantation is performed from a direction parallel etc. to the c-axis, channeling will occur, in which the ions pass through the gaps in the crystal structure of the SiC and are deeply implanted. 
     For a wide bandgap semiconductor material such as SiC, it is difficult to set the projected range of the ions to be deep, as compared to Si, and if a heat treatment is performed after ion implantation, barely any of the implanted ions will be diffused. Meanwhile, in the semiconductor device in the embodiment shown in  FIG. 1A , the bottom surfaces of the first embedded regions  5   a ,  5   b ,  5   c  are formed at a deeper position than the bottom surfaces of the gate bottom protection regions  4   a ,  4   b , but an extremely high acceleration voltage is needed in order to implant ions at a deep position, which makes formation extremely difficult. 
     Thus, as shown in  FIG. 3 , the channeling direction is selected by adjusting an ion implantation angle (tilt angle) θ 2  such that an ion beam IB is tilted in a direction (hereinafter, “off-angle direction”) measured in the off-angle θ 1  relative to a line normal to the surface  102  of the semiconductor wafer  100 , thus performing ion implantation in a direction tilted relative to the surface  102  of the semiconductor  100  in order to intentionally generate a channeling phenomenon. The tilt angle θ 2  is an angle formed between the ion beam IB and a line normal to the surface  102  of the semiconductor wafer  100 . The tilt angle θ 2  can be adjusted by tilting the stage on which the semiconductor wafer  100  is mounted, for example. The channeling direction of the ion beam IB is preferably within ±5° measured in the off-angle direction relative to the c-axis, more preferably within ±3° measured in the off-angle direction relative to the c-axis, and most preferably parallel to the c-axis. 
     In the insulated gate semiconductor device in the embodiment, the tilt angle θ 2  is adjusted such that the channeling direction of the ion beam IB is tilted relative to a line normal to the top surface of the drift layer  2  (current spreading layer  3 ) toward the off-angle direction to select the channeling direction, and channeling ion implantation is performed in the selected channeling direction in order to form the first embedded regions  5   a ,  5   b ,  5   c  and second embedded regions  6   a ,  6   b ,  6   c . If, instead of tilted ion-implantation, a plurality of stages of ion implantation were performed in order to form a traditional box profile, the first embedded regions and second embedded regions would be trapezoids having parallel upper bases and lower bases, with the lower base being slightly shorter than the upper base by approximately 0.1 μm or less. In such a case, in terms of a line connecting the midpoint of the upper base to the midpoint of the lower base, the line would be perpendicular to the top surface of the drift layer  2 . With the channeling implantation, however, the cross sections of the first embedded regions  5   a ,  5   b ,  5   c  and second embedded regions  6   a ,  6   b ,  6   c  are each an approximate parallelogram having oblique sides substantially parallel to the channeling direction. Here, the left side surface of the first embedded regions  5   a ,  5   b ,  5   c  and the left side surface of the second embedded regions  6   a ,  6   b ,  6   c , which are the oblique sides of the approximate parallelograms, are tilted steeper than the right side surfaces, respectively. Namely, the first embedded regions  5   a ,  5   b ,  5   c  and second embedded regions  6   a ,  6   b ,  6   c  are trapezoids in which the upper base and lower base are parallel, and the line connecting the midpoint of the upper base and the midpoint of the lower base has a tilt angle that is tilted relative to a line normal to the top surface of the drift layer  2  toward the off-angle direction. As shown in  FIG. 1B , an upper base  5   b   1  and lower base  5   b   2  of the first embedded region  5   b  are parallel, and a line  5   b   3  connecting the midpoint of the upper base  5   b   1  and the midpoint of the lower base  5   b   2  has a tilt angle that is tilted toward the off-angle direction. In the same manner, an upper base  6   b   1  and lower base  6   b   2  of the second embedded region  6   b  are parallel, and a line  6   b   3  connecting the midpoint of the upper base  6   b   1  and the midpoint of the lower base  6   b   2  has a tilt angle that is tilted toward the off-angle direction. The tilt angle of the side surfaces of the first embedded regions  5   a ,  5   b ,  5   c  and second embedded regions  6   a ,  6   b ,  6   c  is an angle within ±5° measured in the off-angle direction relative to the c-axis of the crystal structure forming the drift layer  2 , for example. 
     During operation of the insulated gate semiconductor device in the embodiment, a positive voltage is applied to the drain electrode  17 , and when a positive voltage greater than or equal to a threshold value is applied to the embedded gate electrodes  11   a ,  11   b , an inversion layer (channel) is formed on the side of the base regions  7   a ,  7   b ,  7   c  closer to the embedded gate electrodes  11   a ,  11   b , thereby turning the device ON. While ON, current flows from the drain electrode  17  through the drain region  1 , drift layer  2 , the inversion layer of the base regions  7   a ,  7   b ,  7   c , and the base regions  8   a ,  8   b ,  8   d , to the source electrode  16 . Meanwhile, if the voltage applied to the embedded gate electrodes  11   a ,  11   b  is less than the threshold value, the inversion layer is not formed in the base regions  7   a ,  7   b ,  7   c ; thus, the device is OFF, and current does not flow from the drain electrode  17  to the source electrode  16 . 
     In the insulated gate semiconductor device according to the embodiment, the bottom surfaces of the first embedded regions  5   a ,  5   b ,  5   c  are provided at deeper positions than the bottom surfaces of the gate bottom protection regions  4   a ,  4   b , which makes it easier for avalanche current to be drawn to the first embedded regions  5   a ,  5   b ,  5   c  when a high voltage is applied during reverse bias, and this makes it possible to protect the gate insulating films  10   a ,  10   b  at the bottom of the trenches  21 ,  21   b  from high voltages during reverse bias. 
     &lt;Method of Manufacturing Insulated Gate Semiconductor Device&gt; 
     Next, one example of a method of manufacturing the insulated gate semiconductor device according to the embodiment of the present invention will be described in the case of a trench gate MISFET, with reference to  FIGS. 5-17 . The method of manufacturing the trench gate MISFET described below is one example, and various other types of methods of manufacturing can be performed, including the modification examples, within the scope set forth in the claims. 
     First, an n +  semiconductor substrate (SiC substrate) to which n-type impurities such as nitrogen (N) have been added is prepared. In the description below, the SiC substrate is a 4H-SiC substrate and has an off-angle of 4°. The n −  drift layer  2  is epitaxially grown on the top surface of the drain region  1 , with the n +  SiC substrate as the drain region  1 . The epitaxial growth also gives the top surface of the drift layer  2  an off-angle of 4°. 
     Next, multiple-stage ion implantation is performed from the top surface side of the drift layer  2  into the entire surface of the drift layer  2  with n-type impurity ions such as nitrogen (N). Thereafter, the implanted n-type impurity ions are activated by a heat treatment, and the n +  current spreading layer  3  is formed in the drift layer  2 , as shown in  FIG. 6 . Alternatively, the current spreading layer  3  may be epitaxially grown on the drift layer  2 , and the epitaxial growth also gives the top surface of the current spreading layer  3  an off-angle of 4°. Even in such a case, in this disclosure, the epitaxially grown current spreading layer  3  is regarded as constituting an upper portion of a drift layer; i.e., the current spreading layer  3  and the drift layer  2  that is under the current spreading layer  3  together constitute a newly re-defined drift layer  2 . Furthermore, the current spreading layer  3  need not necessarily be formed, and the following steps may be performed on the drift layer  2 . In this disclosure, therefore, the drift layer  2  may or may not include the current spreading layer  3  in its upper portion thereof. 
     Next, in order to form the first embedded regions  5   a ,  5   b ,  5   c  shown in  FIG. 1A , a photoresist film  31  is coated onto the current spreading layer  3 , and photolithography is used to pattern the photoresist film  31 . The patterned photoresist film  31  is used as an ion implantation mask to perform multiple-stage ion implantation into deep positions with p-type impurity ions such as Al, as shown in  FIG. 7 . At such time, the stage on which the SiC substrate is mounted is tilted to adjust the tilt angle θ 2 , and a channeling direction is selected in which the ion beam is tilted toward the off-angle direction relative to a line normal to the top surface of the current spreading layer  3  in order to perform the multiple-stage ion implantation. Thereafter, the photoresist film is removed through wet processing or the like. 
     Next, in order to form the gate bottom protection regions  4   a ,  4   b  shown in  FIG. 1A , a new photoresist film  32  is coated onto the current spreading layer  3 , and photolithography is used to pattern the photoresist film  32 . The patterned photoresist film  32  is used as an ion implantation mask, and multiple-stage ion implantation is performed perpendicular to the top surface of the current spreading layer  3  with p-type impurity ions such as Al at a shallower position than previously, as shown in  FIG. 8 . Thereafter, the photoresist film  32  is removed through wet processing or the like. 
     Next, in order to form the second embedded regions  6   a ,  6   b ,  6   c  shown in  FIG. 1A , a new photoresist film  33  is coated onto the current spreading layer  3 , and photolithography is used to pattern the photoresist film  33 . The patterned photoresist film  33  is used as an ion implantation mask to perform multiple-stage ion implantation with p-type impurity ions such as Al, as shown in  FIG. 9 . At such time, the stage on which the SiC substrate is mounted is tilted to adjust the tilt angle θ 2 , and a channeling direction is selected in which the ion beam is tilted toward the off-angle direction relative to a line normal to the top surface of the current spreading layer  3  in order to perform the multiple-stage ion implantation. 
     The channeling direction of the ion implantation during forming of the second embedded regions  6   a ,  6   b ,  6   c  may be the same direction as the channeling direction when forming the first embedded regions  5   a ,  5   b ,  5   c , or may differ within a range in which channeling is caused to occur. In order to form the second embedded regions  6   a ,  6   b ,  6   c  in shallower positions than the first embedded regions  5   a ,  5   b ,  5   c , the acceleration voltage of the ion implantation when forming the second embedded regions  6   a ,  6   b ,  6   c  is set lower than the acceleration voltage when forming the first embedded regions  5   a ,  5   b ,  5   c . The ion implantation when forming the second embedded regions  6   a ,  6   b ,  6   c  may be multiple-stage ion implantation from the direction perpendicular to the top surface of the current spreading layer  3 . Thereafter, the photoresist film  33  is removed through wet processing or the like. 
     Next, the implanted p-type impurity ions are activated by a heat treatment, and the p +  gate bottom protection regions  4   a ,  4   b  are selectively formed inside the current spreading layer  3 , as shown in  FIG. 10 . In the current spreading layer  3 , the p +  first embedded regions  5   a ,  5   b ,  5   c  are formed as parallelogram (first parallelogram) regions in positions deeper than the gate bottom protection regions  4   a ,  4   b . At the same time, the p +  second embedded regions  6   a ,  6   b ,  6   c  are selectively formed in the top of the current spreading layer  3  as parallelogram (second parallelogram) regions; thus, the base bottom embedded regions ( 5   a ,  6   a ), ( 5   b ,  6   b ), ( 5   c ,  6   c ) are formed in different-level patterns in units of parallelograms. 
     Next, as shown in  FIG. 11 , the p-type base region  7  is epitaxially grown on the top surface of the current spreading layer  3 . Next, a photoresist film (not shown) is coated onto the base region  7 , and photolithography is used to pattern the photoresist film. The patterned photoresist film is used as an ion implantation mask to perform multiple-stage ion implantation of n-type impurity ions such as N. Thereafter, the photoresist film is removed through wet processing or the like. Next, a new photoresist film (not shown) is coated onto the base region  7 , and photolithography is used to pattern the new photoresist film. The patterned new photoresist film is used as a new ion implantation mask to perform multiple-stage ion implantation of p-type impurity ions such as Al from the top surface side of the drift layer  2 . Thereafter, the implanted n-type impurity ions and p-type impurity ions are activated by performing a heat treatment, and the n +  source regions  8  and p +  base contact regions  9   a ,  9   b ,  9   c  are formed in the top surface of the base region  7 , as shown in  FIG. 12 . 
     Next, a photoresist film  34  is coated onto the n +  source regions  8  and p +  base contact regions  9   a ,  9   b ,  9   c , and photolithography is used to pattern the photoresist film  34 . The patterned photoresist film  34  is used as an etching mask for dry etching etc. such as reactive ion etching (RIE), and the trenches  21   a ,  21   b  are selectively formed going through the source regions  8   a ,  8   b ,  8   c ,  8   d  and base regions  7   a ,  7   b ,  7   c  to reach the top of the current spreading layer  3 , as shown in  FIG. 13 . Thereafter, the photoresist film  34  is removed through wet processing or the like. The trenches  21   a ,  21   b  may be formed by dry etching using an oxide film as an etching mask after an oxide film is formed on the source regions  8  and p +  base contact regions  9   a ,  9   b ,  9   c  and patterned via a photoresist film. 
     Next, as shown in  FIG. 14 , the gate insulating film  10  such as a SiO 2  film is formed on the bottom surfaces and side surfaces of the trenches  21   a ,  21   b  and the top surfaces of the source regions  8  and p +  base contact regions  9   a ,  9   b ,  9   c  via a thermal oxidation method, CVD, etc. Next, CVD etc. is used to deposit a polysilicon layer (doped polysilicon layer) to which a high concentration of impurities such as N have been added on the gate insulating film  10 . Thereafter, etch-back is performed on the polysilicon layer to embed the polysilicon layer inside the trenches  21   a ,  21   b  via the gate insulating film  10 . This results in formation of the embedded gate electrodes  11   a ,  11   b , which are made of the polysilicon layer, as shown in  FIG. 15 . 
     Next, CVD etc. is used to deposit the interlayer insulating film such as a SiO 2  film on the embedded gate electrodes  11   a ,  11   b  and gate insulating film  10 . A photoresist film  35  is coated onto the interlayer insulating film and photolithography is used to pattern the photoresist film  32 . The patterned photoresist film  35  is used as an etching mask while dry etching is performed to selectively remove the interlayer insulating films  12   a ,  12   b  and gate insulating films  10   a ,  10   b  such that these films remain on the embedded gate electrodes  11   a ,  11   b , as shown in  FIG. 16 , thereby opening source contact holes. Although not shown, gate contact holes are also opened in the interlayer insulating films  12   a ,  12   b  and gate insulating films  10   a ,  10   b  in a location differing from the source contact holes such that at least a portion of the gate surface electrode connected to the embedded gate electrodes  11   a ,  11   b  is exposed. Thereafter, the photoresist film  35  is removed through wet processing or the like. 
     Next, a metal layer such as a Ni film is deposited via sputtering or evaporation, and photolithography and RIE etc. are used to pattern the metal layer and form the first bottom barrier metal layers  13   a ,  13   b ,  13   c . Next, a metal layer such as a TiN film is deposited via sputtering etc., and photolithography and RIE etc. are used to pattern the metal layer and form the second bottom barrier metals  14   a ,  14   b . Next, metal layers such as Ti/TiN/Ti/Al films are continuously deposited via sputtering or the like. Photolithography and RIE etc. are used to pattern the metal layers such as the Ti/TiN/Ti/Al films, thus forming source electrode  16  and gate surface electrode (not shown) patterns that include the top barrier metal layer  15  on the bottom. This results in the patterns of the source electrode  16  and gate surface electrode separating. Next, sputtering or evaporation etc. is used to form the drain electrode  17  made of Au or the like on the entire bottom surface of the drain region  1 , as shown in  FIG. 1A . The insulated gate semiconductor device in the embodiment is completed in this manner. 
     In the manufacturing method of the insulated gate semiconductor device according to the embodiment, when the first embedded regions  5   a ,  5   b ,  5   c  are to be formed, the ion beam is tilted toward the off-angle direction relative to a line normal to the top surface of the current spreading layer  3 , in order to select the channeling direction and then perform multiple-stage ion implantation. This makes it possible to cause the channeling phenomenon to occur so as to make the projected range of the ions deep, and allows the positions of the bottom surfaces of the first embedded regions  5   a ,  5   b ,  5   c  to be formed in positions deeper than the positions of the bottom surfaces of the gate bottom protection regions  4   a ,  4   b . This makes it possible to provide an insulated gate semiconductor device that can protect the gate insulating films  10   a ,  10   b  on the bottom of the trenches  21   a ,  21   b  from high voltages during reverse bias. 
       FIG. 18  shows a profile that is the result of a Monte Carlo simulation when the tilt angle is changed in the off-angle direction and Al ions are implanted in four stages during ion implantation to form the first embedded regions  5   a ,  5   b ,  5   c  shown in  FIG. 1A . The first stage was 350 keV, 7.0×10 13  atoms/cm 2 , the second stage was 300 keV, 5.0×10 13  atoms/cm 2 , the third stage was 200 keV, 5.5×10 13  atoms/cm 2 , and the fourth stage was 120 keV, 2.0×10 13  atoms/cm 2 .  FIG. 19  shows tilt angle dependency for depths of 0.6 μm, 0.8 μm, and 1 μm shown in  FIG. 18 . From  FIG. 18  and  FIG. 19 , it can be understood that channeling is most likely to occur when the tilt angle is 4° (when the ion beam is parallel to the c-axis), with large channeling effects when the tilt angle is within 1° to 7° (when the tilt of the ion beam in the off-angle direction relative to the c-axis is ±3°), and with observable channeling effects even when the tilt angle is −1° to 9° (when the tilt of the ion beam in the off-angle direction relative to the c-axis is ±5°). 
       FIG. 20  shows simulation results when the tilt angle is changed relative to directions other than that of the tilt angle during ion implantation for forming the first embedded regions  5   a ,  5   b ,  5   c  shown in  FIG. 1A . From  FIG. 20 , it can be seen that if the tilt angle θ 2  is changed relative to directions other than that of the off-angle, the channeling phenomenon was not observed. 
       FIG. 21  shows simulation results from changing the temperature during ion implantation in a range of −100° C. to 900° C. during ion implantation for forming the first embedded regions  5   a ,  5   b ,  5   c  shown in  FIG. 1A . From  FIG. 21 , it can be understood that, the lower the temperature is during ion implantation, the easier it is for the channeling phenomenon to occur. 
     FIRST MODIFICATION EXAMPLE 
     A double trench structure will be described as an insulated gate semiconductor device according to the first modification example of the embodiment of the present invention. The double trench structure has a trench structure at the contact portion of the base bottom embedded region ( 5   b ,  6   b ) in addition to the trench structure at the gate portion. As shown in  FIG. 22 , the insulated gate semiconductor device according to the first modification example differs from the insulated gate semiconductor device shown in  FIG. 1A  in that, in a position separated from the trenches  21   a ,  21   b , a contact trench  22  is provided so as to go through the base region  7   b  sandwiched by the trenches  21   a ,  21   b , and a base contact plug  23  is embedded inside the contact trench  22 .  FIG. 22  illustratively shows only the trenches  21   a ,  21   b  and contact trench  22 , but in practice a plurality of structures in which the contact trench  22  is sandwiched by the trenches  21   a ,  21   b  as shown in  FIG. 22  may be provided. 
     The base contact plug  23  functions as a drawing electrode when breakdown occurs and avalanche current flows from the drain electrode  17  to the source electrode  16 . The material of the base contact plug  23  can be a metal such as Al, a polysilicon layer to which impurities have been added (doped polysilicon layer), a high concentration epitaxial layer, or the like, for example. The base contact plug  23  may be formed of the same material as the embedded gate electrodes  11   a ,  11   b , or may be formed of a differing material. 
     The p +  base bottom embedded region ( 5   b ,  6   b ) is provided on the bottom of the contact trench  22  with its bottom position deeper than the gate bottom protection regions  4   a ,  4   b . The base bottom embedded region ( 5   b ,  6   b ) includes the first embedded region  5   b  covering the bottom of the base contact plug  23  and the second embedded region  6   b  contacting the base regions  7   b ,  7   b  and the side surfaces of the base contact plug  23 . 
     The cross-sectional shape of the first embedded region  5   b  is an approximate trapezoid parallelogram, of which the upper base and the lower base are parallel, and a line connecting the midpoint of the upper base and the midpoint of the lower base has a tilt angle tilted relative to a line normal to the top surface of the drift layer  2  toward the off-angle direction. The tilt angle of the side surfaces of the first embedded region  5   b  is an angle within ±5° measured in the off-angle direction relative to the c-axis of the crystal structure forming the drift layer  2 , for example. The side surfaces of the second embedded regions  6   b  opposite to the base contact plug  23  are illustratively shown as being perpendicular to the top surface of the drift layer  2 , but may be parallel to the side surfaces of the first embedded region  5   b . The other configurations are the same as the insulated gate semiconductor device in the embodiment shown in  FIG. 1A , and thus redundant descriptions will be omitted. 
     In the insulated gate semiconductor device according to the first modification example, the bottom surfaces of the base bottom embedded region ( 5   b ,  6   b ) at the bottom of the contact trench  22  are provided in positions deeper than the bottom surfaces of the gate bottom protection regions  4   a ,  4   b  at the bottoms of the trenches  21   a ,  21   b , which makes avalanche current more likely to be drawn to the base bottom embedded region  5  when a high voltage is applied during reverse bias, thus making it possible to protect the gate insulating films  10   a ,  10   b  at the bottoms of the trenches  21   a ,  21   b  from high voltages during the reverse bias. 
     As one example of the method of manufacturing the insulated gate semiconductor device in the first modification example, the n +  current spreading layer  3  is formed as shown in  FIG. 6 , and then in order to form the first embedded region  5   b , the ion beam is tilted relative to a line normal to the top surface of the current spreading layer  3  toward the off-angle direction as shown in  FIG. 7 , in order to select the channeling direction and perform multiple-stage ion implantation. Furthermore, in order to form the gate bottom protection regions  4   a ,  4   b , multiple-stage ion implantation is performed from a direction perpendicular to the top surface of the current spreading layer  3  as shown in  FIG. 8 . Then, in order to form the second embedded region  6   b , multiple-stage ion implantation is performed from a direction perpendicular to the top surface of the current spreading layer  3 . In the ion implantation for forming the second embedded region  6   b , the ion beam may be tilted relative to a line normal to the top surface of the current spreading layer  3  toward the off-angle direction, in order to select the channeling direction and perform multiple-stage ion implantation. Thereafter, a heat treatment is performed in order to form the gate bottom protection regions  4   a ,  4   b , as shown in  FIG. 23 . At the same time, the base bottom embedded region ( 5   b ,  6   b ) made of the first embedded region  5   b  deeper than the gate bottom protection regions  4   a ,  4   b  and the second embedded regions  6   b  above the first embedded region  5   b  are formed. 
     Next, RIE etc. is used to form the trenches  21   a ,  21   b  shown in  FIG. 22 , and at the same time, the contact trench  22  is formed so as to go through the second embedded region  6   b  of the base bottom embedded region ( 5   b ,  6   b ) and so as to reach the first embedded region  5   b . Thereafter, a metal film is deposited so as to fill in the contact trench  22 , and the base contact plug  23  is formed by selectively removing the metal film. The other steps are the same as the method of manufacturing the insulated gate semiconductor device in the embodiment, and thus the description thereof will be omitted. 
     SECOND MODIFICATION EXAMPLE 
     The insulated gate semiconductor device in the second modification example of the embodiment of the present invention differs from the insulated gate semiconductor device of the first modification example shown in  FIG. 22  in that the base contact plug  23  is provided diagonally, as shown in  FIG. 24 . The base bottom embedded region ( 5   b ,  6   b ) is provided so as to cover the bottom of the base contact plug  23 . The base bottom embedded region ( 5   b ,  6   b ) has the first embedded region  5   b  for contacting the bottom surface of the base contact plug  23  and the second embedded regions  6   b  disposed on the top surface of the first embedded region  5   b.    
     The side surfaces of the base contact plug  23  are provided continuing in parallel to the oblique sides of the first embedded region  5   b  of the base bottom embedded region ( 5   b ,  6   b ). The side surfaces of the base contact plug  23  and the side surfaces of the first embedded regions  5   b  are tilted by selecting the channeling direction via tilting the ion beam relative to a line normal to the top surface of the current spreading layer  3  and drift layer  2  toward the off-angle direction. The side surfaces of the second embedded regions  6   b  on the sides opposite to the base contact plug  23  are illustratively shown as being parallel to the side surfaces of the base contact plug  23  and the side surfaces of the first embedded region  5   b , but the side surfaces of the second embedded regions  6   b  on the sides opposite to the base contact plug  23  are not limited to this. For example, the side surfaces of the second embedded regions  6   b  on the sides opposite to the base contact plug  23  may be perpendicular to the top surface of the current spreading layer  3  and drift layer  2 . The other configurations are the same as the insulated gate semiconductor device of the first modification example shown in  FIG. 22 , and thus redundant descriptions will be omitted. 
     In the insulated gate semiconductor device of the second modification example, the base bottom embedded region ( 5   b ,  6   b ) at the bottom of the contact trench  22  is provided with its bottom deeper than the gate bottom protection regions  4   a ,  4   b  at the bottom of the trenches  21   a ,  21   b , which makes avalanche current more likely to be drawn to the base bottom embedded region ( 5   b ,  6   b ) when a high voltage is applied during reverse bias, thus making it possible to protect the gate insulating films  10   a ,  10   b  at the bottoms of the trenches  21   a ,  21   b  from high voltages during reverse bias. 
     As one example of the method of manufacturing the insulated gate semiconductor device in the second modification example, p-type impurity ions are implanted via multiple-stage ion implantation in a direction perpendicular to the top surface of the current spreading layer  3  in order to form the gate bottom protection regions  4   a ,  4   b . Furthermore, in order to form the second embedded regions  6   b , p-type impurity ions are implanted via multiple-stage ion implantation in a channeling direction tilted relative to a line normal to the top surface of the current spreading layer  3  toward the off-angle direction. Thereafter, a heat treatment is performed in order to form the p +  gate bottom protection regions  4   a ,  4   b  and second embedded regions  6   b  as shown in  FIG. 25 . 
     Then, as shown in  FIG. 26 , the contact trench  22  is formed so as to have side walls along the channeling direction during ion implantation when forming the first embedded region  5   b . Next, multiple-stage ion implantation of p-type impurity ions is performed in the bottom surface of the contact trench  22  with the ion beam in the direction parallel to the side walls of the contact trench  22 , or in other words, in a channeling direction in which the ion beam is tilted relative to a line normal to the top surface of the current spreading layer  3  toward the off-angle direction. Thereafter, a heat treatment is performed in order to form the first embedded region  5   b  deeper than the gate bottom protection regions  4   a ,  4   b . Next, evaporation or sputtering etc. is used to deposit a metal film or the like, and etch-back is performed on the metal film in order to embed the base contact plug  23  in the contact trench  22 . The other steps are the same as the method of manufacturing the insulated gate semiconductor device in the embodiment, and thus the description thereof will be omitted. 
     When forming the first embedded region  5   b  and gate bottom protection regions  4   a ,  4   b , the p-type impurities may be implanted all at once via multiple-stage ion implantation instead of being individually implanted via multiple-stage ion implantation. In other words, after forming of the trenches  21   a ,  21   b  having parallel side walls in the depth direction and the contact trench  22  having side walls tilted relative to the depth direction, the p-type impurity ions may be implanted at the same time via multiple-stage ion implantation in the bottom of the trenches  21   a ,  21   b  and in the bottom of the contact trench  22  in the direction parallel to the side walls of the contact trench  22 . Thereafter, a heat treatment is performed in order to form the gate bottom protection regions  4   a ,  4   b , and it is possible to form the base bottom embedded region  5   b  deeper than the gate bottom protection regions  4   a ,  4   b.    
     Other Embodiments 
     The present invention was described using the embodiment and modification examples above, but the descriptions and drawings forming a part of the disclosure should not be construed as limiting the present invention. Various substitute embodiments, examples, and applied techniques should be clear to a person skilled in the art based on this disclosure. 
     In the embodiment, as shown in  FIG. 1A , an example was shown in which the p +  base bottom embedded regions ( 5   a ,  6   a ), ( 5   b ,  6   b ), ( 5   c ,  6   c ) have a two-layer structure of the first embedded regions  5   a ,  5   b ,  5   c  and second embedded regions  6   a ,  6   b ,  6   c , but as shown in  FIG. 27 , a single layer of the p +  base bottom embedded regions  5   a ,  5   b ,  5   c  may be provided. In ion implantation when forming the base bottom embedded regions  5   a ,  5   b ,  5   c , the acceleration voltage, dose amount, and implantation rounds etc. may be adjusted to make the base bottom embedded regions  5   a ,  5   b ,  5   c  thick in the depth direction. 
     In the embodiment, a MISFET having a trench structure was shown as an example, but the present invention is not limited to this and can be applied to an insulated gate semiconductor device having various types of trench structures such as an IGBT with a trench structure. As a trench gate IGBT, the n +  source regions  8   a ,  8   b ,  8   c ,  8   d  of the MISFET shown in  FIG. 1A  may be emitter regions, and a p +  collector region may be provided on the bottom surface side of the drift layer  2  instead of the n +  drain region  1 . 
     In the embodiment of the present invention, an example was shown in which the insulated gate semiconductor device uses SiC, but the present invention can be applied to insulated gate semiconductor devices using other wide bandgap semiconductors, such as gallium nitride (GaN) or diamond. Furthermore, the present invention is not limited to wide bandgap semiconductors and can also be applied to insulated gate semiconductor devices using silicon (Si). 
     Moreover, SiC has many polytypes (polyforms), such as cubic 3C-SiC, hexagonal 4H-SiC, 6H-SiC, etc., but the channeling direction is merely selected for each so as to cause channeling to occur. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents. In particular, it is explicitly contemplated that any part or whole of any two or more of the embodiments and their modifications described above can be combined and regarded within the scope of the present invention.