Patent Publication Number: US-2022231160-A1

Title: Semiconductor device, power conversion device and method of manufacturing semiconductor device

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor device, a power conversion device, and a method of manufacturing the semiconductor device, and particularly to a semiconductor device having a super junction structure. 
     BACKGROUND ART 
     A switching element such as a metal-oxide-semiconductor field-effect transistor (MOSFET) or an insulated-gate bipolar transistor (IGBT) and a rectifying element such as a Schottky barrier diode (SBD) are used as a semiconductor device for driving a load of an electrical motor such as a motor in a power electronics field. In the switching element, an ON state (conductive state) of low resistance and an OFF state (interrupting state) of high resistance are switched using a control signal inputted to a control terminal of the switching element. In the rectifying element, an ON state and an OFF state are switched in accordance with a state of a switching element connected to the rectifying element, for example. 
     High voltage is input in a usage of power electronics, thus it is important that the semiconductor device such as the switching element and the rectifying element has high withstand voltage in the OFF state. The switching element and the rectifying element in the OFF state generally extend a depletion layer in a drift layer, thereby maintaining voltage. Thus, these elements have higher withstand voltage as a thickness of the drift layer increases, and have higher withstand voltage as an impurity concentration of the drift layer decreases by reason that the depletion layer extends easily. 
     In the meanwhile, the semiconductor device needs to have low resistance in the ON state (ON resistance) to reduce a conduction loss. The resistance of the drift layer is one of components of the ON resistance, and is preferably reduced as much as possible. The resistance of the drift layer can be reduced by reducing the thickness of the drift layer or increasing the impurity concentration of the drift layer. However, when the thickness of the drift layer is reduced or the impurity concentration of the drift layer is increased as described above, the withstand voltage is reduced. As described above, the withstand voltage and the ON resistance of the semiconductor device have a trade-off relationship. 
     A super junction structure is known as a structure of the semiconductor device capable of improving the trade-off (for example, Patent Document 1 described below). That is to say, the semiconductor device having the super junction structure can reduce the ON resistance while keeping the withstand voltage or increase the withstand voltage while keeping the ON resistance compared with a semiconductor device which does not have the super junction structure. 
     In the super junction structure, a p-type pillar layer and an n-type pillar layer are alternately disposed on a surface vertical to a direction in which current flows in the semiconductor device, and charge balance is achieved to equalize an amount of effective impurity in the p-type pillar layer and an amount of effective impurity in the n-type pillar layer. Herein, the amount of effective impurity indicates an amount of impurity effectively acting as an acceptor in a p-type semiconductor and an amount of impurity effectively acting as a donor in an n-type semiconductor. A layer made up of the p-type pillar layer and the n-type pillar layer alternately disposed in a semiconductor layer in which the super junction structure is formed is referred to as “the super junction layer” hereinafter. 
     A shape of the p-type pillar layer and the n-type pillar layer includes a reed shape and a columnar shape, for example. For example, when each of the p-type pillar layer and the n-type pillar layer has the reed shape, the p-type pillar layer and the n-type pillar layer are disposed in a stripe form in a plan view. When the p-type pillar layer or the n-type pillar layer has the columnar shape, one pillar layer is disposed in a dotted form in the other pillar layer in a plan view. Particularly, the super junction layer having the stripe shape is compatible with a trench gate type semiconductor device, and is appropriate for reducing the resistance. There is an advantage that the super junction layer having the stripe shape has a simple structure compared with the super junction layer having the dotted shape, and a design and a process are relatively easy. 
     A method of forming the super junction structure mainly includes two method of a multi-epitaxial method and a trench-filling method. The multi-epitaxial method is a method of repeating an epitaxial growth of a semiconductor layer of a first conductivity type and an ion implantation of a second conductivity type impurity, and the number of repeating the process is determined by a necessary thickness of a super junction layer and an implantable depth of the ion implantation. The thickness of the super junction layer is generally set to several p.m, but is set to equal or larger than several tens of p.m in a device having high withstand voltage in some cases. The number of repeating the epitaxial growth and the ion implantation increases to form such a thick super junction layer by the multi-epitaxial method. 
     In the meanwhile, the trench-filling method is a method of epitaxially growing a semiconductor layer of a first conductivity type to have a thickness necessary for a super junction layer, forming a trench in the semiconductor layer by anisotropic etching, and then epitaxially growing a semiconductor layer of a second conductivity type to fill the trench. The trench-filling method has a small number of processes, and is excellent in mass productivity compared with the multi-epitaxial method. 
     For example, a step-flow growth epitaxially growing silicon carbide (SiC) on a specific crystal plane is general as an epitaxial growth of silicon carbide. An Off angle is provided in a general silicon carbide substrate to achieve the step-flow growth. In the epitaxial growth of silicon carbide, it is hard to perform the epitaxial growth on a crystal plane other than the specific crystal plane described above. Thus, when the super junction structure is formed on the semiconductor substrate made of silicon carbide by the trench-filling method, it is required that a longitudinal direction of the pillar layer of the second conductivity type coincides with a direction of the step-flow growth (the step-flow direction). Thus, a structure of alternately arranging the p-type pillar layer and the n-type pillar layer extending in the step-flow direction is general as the structure of the super junction layer having the stripe shape. 
     As described above, the super junction layer having the stripe shape has an advantage in reduction of the resistance and easiness of design and manufacture of the semiconductor device. Particularly, in a silicon carbide semiconductor device, a super junction layer having a stripe shape is mostly adopted for a reason of processes. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: International Publication No. 2017/183375 
     SUMMARY 
     Problem to be Solved by the Invention 
     Patent Document 1 discloses a termination structure including a plurality of frame-like withstand voltage holding structures surrounding an active region and having a conductivity opposite to that of a drift layer as a termination structure of a semiconductor device having a super junction structure. A plurality of p-type pillar layers are disposed in a stripe form in the active region, and each of the plurality of withstand voltage holding structure has a side extending in parallel to the p-type pillar layer and a side perpendicular to the p-type pillar layer in a plan view. 
     When the termination structure including the plurality of frame-like withstand voltage holding structures is used as with Patent Document 1, a distribution of potential in a revolving direction of the active region is smaller than a case of a typical termination structure including only one frame-like withstand voltage holding structure (for example, a junction termination extension (JTE) or a reduced surface field (RESURF)), and an electrical field concentration is reduced, thus the withstand voltage is increased. 
     However, it is considered that there is a large potential difference, which is several tens of percent of voltage applied to the semiconductor device, in the revolving direction of the active region around a corner part of the frame-like withstand voltage holding structure. The frame-like withstand voltage holding structure is electrically conductive over a whole outer periphery of the active region, thus cannot hold the potential difference in the revolving direction of the active region. Accordingly, the potential difference is held between a certain withstand voltage holding structure  56  and another withstand voltage holding structure  56  adjacent to an inner side or an outer side of the certain withstand voltage holding structure  56 , and an electrical field concentration occurs in that part. 
     The present invention is therefore has been made to solve problems as described above, and it is an object to reduce an electrical field concentration in a termination region in a semiconductor device having a super junction structure. 
     Means to Solve the Problem 
     A semiconductor device according to the present invention includes: a semiconductor substrate; a semiconductor layer formed on the semiconductor substrate and including a super junction layer in which a first pillar layer of a first conductivity type and a second pillar layer of a second conductivity type are alternately disposed; and a plurality of withstand voltage holding structures of a second conductivity type formed on an upper layer part of the semiconductor layer to surround an active region, wherein at least one of the withstand voltage holding structures overlaps with the super junction layer in a plan view, and at least one of the withstand voltage holding structures overlapping with the super junction layer in a plan view has at least one gap which is an intermittent part of at least one of the withstand voltage holding structures. 
     Effects of the Invention 
     According to the present invention, the withstand voltage structure has the gap, thus the withstand voltage holding structure can hold a potential difference in a revolving direction of the active region. Accordingly, an electrical field concentration in a termination region is reduced, and the withstand voltage of the semiconductor device can be increased. 
     These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  A schematic plan view of a semiconductor device including a super junction structure as a premise technique.
           FIG. 2  A schematic cross-sectional view illustrating a cross section vertical to a longitudinal direction of a p-type pillar layer of the semiconductor device as the premise technique.       

         FIG. 3  A schematic cross-sectional view illustrating a cross section parallel to the longitudinal direction of the p-type pillar layer of the semiconductor device as the premise technique. 
         FIG. 4  A graph illustrating a simulation result of a potential distribution in a surface of a semiconductor substrate of the semiconductor device as the premise technique. 
         FIG. 5  A schematic plan view around a corner part of a termination structure in a semiconductor device according to an embodiment 1. 
         FIG. 6  A schematic cross-sectional view illustrating a cross section vertical to a longitudinal direction of a p-type pillar layer of the semiconductor device according to the embodiment 1. 
         FIG. 7  A schematic cross-sectional view illustrating a cross section parallel to the longitudinal direction of the p-type pillar layer of the semiconductor device according to the embodiment 1. 
         FIG. 8  A drawing illustrating a modification example of the semiconductor device according to the embodiment 1. 
         FIG. 9  A drawing for explaining a method of manufacturing the semiconductor device according to the embodiment 1. 
         FIG. 10  A drawing for explaining a method of manufacturing the semiconductor device according to the embodiment 1. 
         FIG. 11  A drawing for explaining a method of manufacturing the semiconductor device according to the embodiment 1. 
         FIG. 12  A drawing for explaining a method of manufacturing the semiconductor device according to the embodiment 1. 
         FIG. 13  A drawing for explaining a method of manufacturing the semiconductor device according to the embodiment 1. 
         FIG. 14  A drawing for explaining a method of manufacturing the semiconductor device according to the embodiment 1. 
         FIG. 15  A drawing for explaining a method of manufacturing the semiconductor device according to the embodiment 1. 
         FIG. 16  A drawing for explaining a method of manufacturing the semiconductor device according to the embodiment 1. 
         FIG. 17  A drawing for explaining a method of manufacturing the semiconductor device according to the embodiment 1. 
         FIG. 18  A drawing for explaining a method of manufacturing the semiconductor device according to the embodiment 1. 
         FIG. 19  A drawing for explaining gap boundary end portions A and B. 
         FIG. 20  A schematic plan view around a corner part of a termination structure in a semiconductor device according to an embodiment 2. 
         FIG. 21  A schematic plan view around a corner part of a termination structure in a semiconductor device according to an embodiment 3. 
         FIG. 22  A schematic plan view around a corner part of a termination structure in a semiconductor device according to an embodiment 4. 
         FIG. 23  A schematic plan view around a corner part of a termination structure in a semiconductor device according to an embodiment 5. 
         FIG. 24  A schematic plan view around a corner part of a termination structure in a semiconductor device according to an embodiment 6. 
         FIG. 25  A schematic plan view around a corner part of a termination structure in a semiconductor device according to an embodiment 7. 
         FIG. 26  A schematic plan view around a corner part of a termination structure in a semiconductor device according to an embodiment 8. 
         FIG. 27  A block diagram of a power conversion device according to an embodiment 9. 
     
    
    
     DESCRIPTION OF EMBODIMENT(S) 
     In the present specification, a silicon carbide SBD is described as an example of a semiconductor device. In the description hereinafter, a first conductivity type is an n type and a second conductivity type is a p type. The drawings are schematically described hereinafter, thus a scale of each constituent element is not necessarily constant. Thus, dimensions and positional relationships of the constituent elements illustrated in the drawings are different from reality in some cases. The description of constituent elements which are unnecessary for explanation is omitted in all the drawings for convenience in drawing figures. 
     &lt;Premise Technique&gt; 
     A semiconductor device including a super junction structure as a premise technique is described before describing embodiments of the present invention.  FIG. 1  is a schematic plan view of a semiconductor device as a premise technique. This semiconductor device corresponds to that disclosed in Patent Document 1.  FIG. 2  is a cross-sectional view along an A 1 -A 2  line in  FIG. 1 , and  FIG. 3  is a cross-sectional view along a C 1 -C 2  line in  FIG. 1 . 
     The semiconductor device includes an n + -type semiconductor substrate  11  and an epitaxial crystal layer  12  which is an n-type semiconductor layer formed on a first main surface (a surface on an upper side in paper sheets of  FIG. 2  and  FIG. 3 ) of the semiconductor substrate  11 . The first main surface of the semiconductor substrate  11  has an OFF angle with respect to a specific crystal plane. The n +  type indicates that it has a higher impurity concentration than an n type. 
     Formed on an upper layer part of the epitaxial crystal layer  12  is a super junction layer  15  with a stripe shape in which an n-type pillar layer  13  (first pillar layer) and a p-type pillar layer  14  (second pillar layer) each having a reed shape with a longitudinal direction as a step-flow direction are alternately disposed in a plan view. Herein, a region on an outer side of the super junction layer  15  is defined as “an n-type pillar surrounding layer  16 ”. 
     A Schottky contact electrode  87  is formed on the super junction layer  15  except for an outer peripheral part of the super junction layer  15 , and an anode electrode  88  is formed thereon. In  FIG. 1 , illustrations of the Schottky contact electrode  87  and the anode electrode  88  are omitted. 
     A plurality of withstand voltage holding structures  56 , each of which is a p-type semiconductor region, are concentrically formed on an upper layer part of the super junction layer  15  to surround the Schottky contact electrode  87  in a plan view. A region surrounded by the withstand voltage holding structure  56  on an innermost side is an active region  1 , and a region on an outer side of an inner end of the withstand voltage holding structure  56  on the innermost side is a termination region  2 . 
     Each of the plurality of the withstand voltage holding structures  56  includes a straight part parallel to a longitudinal direction of the p-type pillar layer  14  and a straight part perpendicular to the longitudinal direction of the p-type pillar layer  14  in a plan view. Provided on a corner part of each of the plurality of withstand voltage holding structures  56  is a curved part smoothly connecting the straight part extending in parallel to the longitudinal direction of the p-type pillar layer  14  and the straight part perpendicular to the longitudinal direction of the p-type pillar layer  14 . 
     A cathode electrode  93  is formed on a second main surface (a surface on a lower side in paper sheets of  FIG. 2  and  FIG. 3 ) of the semiconductor substrate  11  via a back surface ohmic electrode  91 . 
       FIG. 4  is a graph illustrating a potential distribution on the semiconductor device in  FIG. 1  calculated using a technology computer-aided design (TCAD), and is a simulation result of a potential profile along the surface of the semiconductor substrate  11  in a case where a reverse bias of voltage VR is applied to the semiconductor device. In  FIG. 4 , the graph of a broken line, a dotted line, and a solid line indicates a potential profile along an A 1 -A 2  line, a B 1 -B 2  line, and a C 1 -C 2  line, respectively, in  FIG. 1 . In  FIG. 4 , a horizontal axis expresses a position in a direction along the A 1 -A 2  line, the B 1 -B 2  line, or the C 1 -C 2  line, and a vertical axis expresses potential. 
     In a simulation calculating the potential profile in  FIG. 4 , a parameter of a width of the n-type pillar layer  13 , a width of the p-type pillar layer  14 , the number of repeating the n-type pillar layer  13  and the p-type pillar layer  14 , the number of withstand voltage holding structures, and a width of each withstand voltage holding structure, for example, does not strictly coincide with that in  FIG. 1 . The potential profile in  FIG. 4  is not a result calculated on an assumption of a three-dimensional structure of the semiconductor device but a result calculated on an assumption that each cross section along the A 1 -A 2  line, the B 1 -B 2  line, and the C 1 -C 2  line in  FIG. 1  is independent of each other. That is to say, continuity of potential and electrical field in the revolving direction of the active region  1  is not considered. 
     When points having the same coordinate in the horizontal axis (corresponding to a distance from an end portion of the active region  1 ) in  FIG. 4  are compared, it is recognized that there is a large potential difference which is several tens of percent of the voltage VR applied to the semiconductor device. As described above, the potential profile illustrated in  FIG. 4  is a result calculated on the assumption that each cross section along the A 1 -A 2  line, the B 1 -B 2  line, and the C 1 -C 2  line in  FIG. 1  is independent of each other, however, actually, the withstand voltage holding structure  56  is electrically conductive over the whole outer periphery of the active region  1 , thus cannot hold the potential difference described above. Accordingly, the potential difference is held between a certain withstand voltage holding structure  56  and another withstand voltage holding structure  56  adjacent to an inner side or an outer side of the certain withstand voltage holding structure  56 , and an electrical field concentration occurs in that part. 
     Embodiment 1 
       FIG. 5  is a drawing illustrating a structure of the semiconductor device according to the embodiment 1 of the present invention, and is a schematic plan view around the corner part of the termination structure in the semiconductor device.  FIG. 6  is a cross-sectional view along a D 1 -D 2  line in  FIG. 5 , and  FIG. 7  is a cross-sectional view along a E 1 -E 2  line in  FIG. 5 . In these drawings, the same reference signs are assigned to elements each having functions similar to those in  FIG. 1  to  FIG. 3 . 
     A region illustrated in  FIG. 5  corresponds to an upper right part of the structure illustrated in  FIG. 1 . Although the illustration is omitted, the semiconductor device according to the embodiment 1 includes the structure illustrated in  FIG. 5  in each corner part. A structure except for the corner part may be basically similar to that of the semiconductor device of the premise technique. 
     The semiconductor device according to the embodiment 1 includes the n + -type semiconductor substrate  11  having low resistance and the epitaxial crystal layer  12  which is the n-type semiconductor layer formed on the first main surface (the surface on an upper side in paper sheets of  FIG. 6  and  FIG. 7 ) of the semiconductor substrate  11 . In the present embodiment, a silicon carbide substrate is used as the semiconductor substrate  11 . Silicon carbide is used as a semiconductor material, thus reduction in loss and increase in an operable temperature of the semiconductor device can be achieved. Used herein as the semiconductor substrate  11  is a silicon carbide substrate with a polytype of 4H having a first main surface with an OFF angle inclined at an angle of 4 degrees in a direction of [11-20] with respect to a (0001) plane. Nitrogen (N), for example, is used as an n-type impurity. 
     An impurity concentration of the epitaxial crystal layer  12  is equal to or larger than 1×10 13 cm −3  and equal to or smaller than 1×10 18 cm −3 , for example, but needs not necessarily be spatially constant, thus may have a concentration distribution in a vertical direction. A thickness of the epitaxial crystal layer  12  is equal to or larger than 0.1 μm and equal to or smaller than 100 μm. 
     Formed on the upper layer part of the epitaxial crystal layer  12  is the super junction layer  15  in which the n-type pillar layer  13  and the p-type pillar layer  14  each having a reed shape are alternately disposed in a plan view. An impurity concentration of the n-type pillar layer  13  and an impurity concentration of the p-type pillar layer  14  are equal to or larger than 1×10 13 cm −3  and equal to or smaller than 1×10 18 cm −3 , for example, but need not necessarily be spatially constant, thus may have a concentration distribution in each region. A width of the n-type pillar layer  13  and a width of the p-type pillar layer are equal to or larger than 1 μm and equal to or smaller than 50 μm, for example. A boundary line between the n-type pillar layer  13  and the p-type pillar layer  14  needs not necessarily be vertical to the first main surface of the semiconductor substrate  11 . Aluminum (Al), for example, is used as the p-type impurity. 
     An amount of n-type effective impurity included in one n-type pillar layer  13  and an amount of p-type effective impurity included in one p-type pillar layer  14  are set to be equal, thus charge balance is achieved. The thickness of the super junction layer  15  is equal to or larger than 1 μm and equal to or smaller than 150 μm, for example. The n-type pillar layer  13  and the p-type pillar layer  14  are disposed in a stripe form and have the longitudinal direction as the step-flow direction in a plan view. 
     A region on an outer side of the super junction layer  15  is the n-type pillar surrounding layer  16 . An impurity concentration of the n-type pillar surrounding layer  16  is equal to or larger than 1×10 13 cm −3  and equal to or smaller than 1×10 18 cm −3 , for example, and a thickness of the n-type pillar surrounding layer  16  is equal to or larger than 1 μm and equal to or smaller than 150 μm, for example. 
     As described hereinafter, in the present embodiment, the super junction layer  15  is formed by a trench-filling method of forming a trench in an n-type epitaxial crystal layer (first semiconductor layer) formed to have a constant thickness, and embedding a p-type epitaxial crystal layer (second semiconductor layer) in the trench, thereby forming the n-type pillar layer  13  and the p-type pillar layer  14 . That is to say, the n-type pillar layer  13  and the n-type pillar surrounding layer  16  are parts of the n-type epitaxial crystal layer where the p-type pillar layer  14  is not formed and the n-type epitaxial crystal layer remains, and particularly, a part sandwiched between the p-type pillar layers  14  is the n-type pillar layer  13 , and a part on an outer side of a region where the p-type pillar layer  14  is formed is the n-type pillar surrounding layer  16 . 
     The Schottky contact electrode  87  is formed on the super junction layer  15  except for the outer peripheral part of the super junction layer  15 , and the anode electrode  88  is formed thereon (in  FIG. 5 , illustrations of the Schottky contact electrode  87  and the anode electrode  88  are omitted). For example, titanium (Ti), molybdenum (Mo), tungsten (W), Al, the other metal, alloy, or a lamination body made of these materials can be used as materials of the Schottky contact electrode  87  and the anode electrode  88 . 
     A plurality of withstand voltage holding structures  56 , each of which is made up of a p-type semiconductor, are concentrically formed on the upper layer part of the super junction layer  15  and the n-type pillar surrounding layer  16  to surround the Schottky contact electrode  87  in a plan view. An impurity concentration of the withstand voltage holding structure  56  is higher than that of the n-type pillar layer  13  and the n-type pillar surrounding layer  16 , and is lower than 1×10 18 cm −3 , for example. A region surrounded by the withstand voltage holding structure  56  on the innermost side is the active region  1 , and a region on the outer side of the inner end of the withstand voltage holding structure  56  on the innermost side is a termination region  2 . 
     As illustrated in  FIG. 5 , each of the withstand voltage holding structures  56  includes the straight part parallel to the longitudinal direction of the p-type pillar layer  14  and the straight part perpendicular to the longitudinal direction of the p-type pillar layer  14  in a plan view. A shape of a chip of the semiconductor device of the present embodiment is a rectangular shape having a side parallel to the step-flow direction and a side vertical to the step-flow direction. Thus, each withstand voltage holding structure  56  extends in parallel to the p-type pillar layer  14  near the side of the semiconductor device parallel to the step-flow direction, and each withstand voltage holding structure  56  extends in the direction perpendicular to the p-type pillar layer  14  near the side thereof vertical to the step-flow direction. Provided on the corner part of each of the plurality of withstand voltage holding structures  56  is the curved part smoothly connecting the straight part extending in parallel to the longitudinal direction of the p-type pillar layer  14  and the straight part perpendicular to the longitudinal direction of the p-type pillar layer  14 . 
     In the present embodiment, at least one of the plurality of withstand voltage holding structures  56  is formed to overlap with a part of the Schottky contact electrode  87  in a plan view. More specifically, as illustrated in  FIG. 6  and  FIG. 7 , a part of the withstand voltage holding structure  56  on the innermost side is formed to overlap with the end portion of the Schottky contact electrode  87 . The withstand voltage holding structure  56  is formed to extend inside the super junction layer  15  to the n-type pillar surrounding layer  16  on the outer side of the super junction layer  15 . 
     The cathode electrode  93  is formed on the second main surface (the surface on the lower side in the paper sheets of  FIG. 2  and  FIG. 3 ) of the semiconductor substrate  11  via the back surface ohmic electrode  91 . Nickel (Ni), gold (Au), the other metal, alloy, or a lamination body made of these materials can be used as materials of the back surface ohmic electrode  91  and the cathode electrode  93 . 
     As illustrated in  FIG. 8 , a field insulating film  32  may be formed on the super junction layer  15  and the n-type pillar surrounding layer  16  in a part of the termination region  2 . In this case, the Schottky contact electrode  87  and the anode electrode  88  are formed to be partially located on an upper part of the field insulating film  32 . 
     Herein, in the present embodiment, as illustrated in  FIG. 5 , at least one of the withstand voltage holding structures  56  overlapping with the super junction layer  15  in a plan view includes a gap  57 . That is to say, the withstand voltage holding structure  56  including the gap  57  does not have a completely continuous frame-like shape, but is intermittently formed, and an intermittent part constitutes the gap  57 . In other words, the gap  57  passes across the withstand voltage holding structure  56  to which the gap  57  belongs, and connects an inner region and an outer region of the withstand voltage holding structure  56 . The part corresponding to the gap  57  may be an n-type semiconductor region or a p-type semiconductor region having a lower impurity concentration than the withstand voltage holding structure  56 . In the example in  FIG. 5 , a part of the gap  57  overlapping with the n-type pillar layer  13  is an n-type semiconductor region, and a part of the gap  57  overlapping with the p-type pillar layer  14  is a p-type semiconductor region. 
     When a part of the gap  57  is an n-type semiconductor region, an impurity concentration thereof may be the same as or different from that of the n-type pillar layer  13 . When a part of the gap  57  is a p-type semiconductor region, it is sufficient that it has a lower impurity concentration than the withstand voltage holding structure  56  and has an impurity concentration so as to be depleted at a time of application of a reverse bias. Alternatively, a part of the gap  57  may be formed of any material including an intrinsic semiconductor as long as it is not electrically conductive with the withstand voltage holding structure  56  at the time of application of the reverse bias. 
     In the embodiment 1, as illustrated in  FIG. 5 , the gaps  57  are provided in the curved parts of all of the withstand voltage holding structures  56  overlapping with the super junction layer  15  in a plan view. In the meanwhile, the gap  57  is not provided in the withstand voltage holding structure  56  (the withstand voltage holding structure  56  on the outermost side) which does not overlap with the super junction layer  15  in a plan view. The gap  57  is not provided in the straight part of the withstand voltage holding structure  56 . Moreover, the gaps  57  of the adjacent withstand voltage holding structures  56  are disposed to be displaced from each other so as not to be adjacent to each other in a radial direction of the withstand voltage holding structure  56  (that is to say, a direction directed from an inner side to an outer side of a frame of the withstand voltage holding structure  56 ) in a plan view. 
     A method of manufacturing the semiconductor device according to the embodiment 1 is described next.  FIG. 9  to  FIG. 18  are process drawings for explaining the manufacturing method. These process drawings correspond to a cross section illustrated in  FIG. 6 , that is to say, a cross section along a D 1 -D 2  line in  FIG. 5 . 
     As described above, the method of forming the super junction structure mainly includes two method of the multi-epitaxial method and the trench-filling method. The multi-epitaxial method is the method of repeating the epitaxial growth of the n-type semiconductor layer and the ion implantation of the p-type impurity. In the super junction structure, it is effective to increase the depth of the p-type pillar layer  14  to improve the withstand voltage. In the multi-epitaxial method, the number of repeating the process is determined by the necessary thickness of the super junction layer  15  and the implantable depth of the ion implantation. For example, when the p-type impurity can be implanted to the depth of 1 μm by the ion implantation, the repeat of the epitaxial growth and the ion implantation needs to be performed at least ten times to form the super junction layer  15  having the thickness of 10 μm. 
     In the meanwhile, the trench-filling method is the method of epitaxially growing the n-type first semiconductor layer to have the thickness necessary for the super junction layer  15  firstly, forming the trench in the semiconductor layer by anisotropic etching, and then epitaxially growing the p-type second semiconductor layer to fill the trench. Assuming that the super junction layer  15  having a practical thickness is formed, the trench-filling method has the small number of processes, and is excellent in mass productivity compared with the multi-epitaxial method. Thus, the trench-filling method is used in the present embodiment. 
     Firstly, the n + -type semiconductor substrate  11  is prepared as illustrated in  FIG. 9 . Next, as illustrated in  FIG. 10 , an epitaxial crystal layer  41  (first semiconductor layer) made of an n-type silicon carbide is epitaxially grown on the semiconductor substrate  11  by a chemical vapor deposition (CVD) method. The epitaxial crystal layer  41  becomes the epitaxial crystal layer  12 , the n-type pillar layer  13 , and the n-type pillar surrounding layer  16  in a subsequent process. A thickness of the epitaxial crystal layer  41  may be appropriately set in accordance with the thickness of the super junction layer  15  to be formed. 
     Next, a silicon oxide film  42  is deposited on a surface of the epitaxial crystal layer  41 , and the silicon oxide film  42  is patterned by selective etching using a photolithography technique, thus a mask pattern made up of the silicon oxide film  42  is formed as illustrated in  FIG. 11 . This mask pattern is used as a mask at a time of etching for forming the trench in which the p-type pillar layer  14  is embedded. In the present embodiment, the p-type pillar layer  14  is disposed in the stripe form, thus an opening having a stripe shape is provided in the mask pattern. A thickness of the silicon oxide film  42  may be appropriately set in accordance with a depth of the trench (a thickness of the p-type pillar layer  14 ) to be formed. 
     Subsequently, a trench  43  for embedding the p-type pillar layer  14  (“pillar formation trench  43 ” hereinafter) is formed in the epitaxial crystal layer  41  as illustrated in  FIG. 12  by etching using the mask pattern made up of the silicon oxide film  42  as a mask. The silicon oxide film  42  as the mask pattern is formed at intervals on the surface of the epitaxial crystal layer  41 , thus the plurality of pillar formation trenches  43  are formed at intervals. A shape of the p-type pillar layer  14  is regulated by a shape of the pillar formation trench  43 , thus this etching process is preferably performed by dry etching in which a shape of the trench can be easily controlled. 
     Moreover, an epitaxial crystal layer  44  (second semiconductor layer) made up of a p-type silicon carbide is grown by the epitaxial growth to embed the pillar formation trench  43  as illustrated in  FIG. 13 . The epitaxial crystal layer  44  becomes the p-type pillar layer  14  in a subsequent process. Thus, at the time of forming the epitaxial crystal layer  44 , an impurity concentration of the p-type epitaxial crystal layer  44  is set to be equal to the amount of the effective impurity of the n-type pillar layer  13  to achieve charge balance. 
     Performed next is a flattening process of removing unnecessary parts of the n-type epitaxial crystal layer  41  and the p-type epitaxial crystal layer  44  by chemical mechanical polishing (CMP) to expose the n-type epitaxial crystal layer  41  on an upper surface side of the semiconductor substrate  11  as illustrated in  FIG. 14 . The p-type epitaxial crystal layer  44  remaining after the flattening process becomes the p-type pillar layer  14 . 
     After the flattening process, the n-type epitaxial crystal layer  41  is divided into three regions of the epitaxial crystal layer  12 , the n-type pillar layer  13 , and the n-type pillar surrounding layer  16  illustrated in  FIG. 6 . Firstly, a region sandwiched between the p-type pillar layers  14  in the n-type epitaxial crystal layer  41  becomes the n-type pillar layer  13 . A region which is located in the same height as the n-type pillar layer  13  in a cross-sectional view but is not sandwiched between the p-type pillar layers  14  (a region on an outer side of a formation region of the p-type pillar layer  14 ) becomes the n-type pillar surrounding layer  16  in the n-type epitaxial crystal layer  41 . Moreover, a region between a bottom of the super junction layer  15  made up of the n-type pillar layer  13  and the p-type pillar layer  14  and the semiconductor substrate  11  becomes the epitaxial crystal layer  12  in the n-type epitaxial crystal layer  41 . 
     Next, an implantation mask in which a formation region of the withstand voltage holding structure  56  except for a portion of the gap  57  is opened is formed on the super junction layer  15  and the n-type pillar surrounding layer  16  using a photoresist, for example. That is to say, the formation region of the gap  57  is covered by the implantation mask. Then, p-type impurity such as Al ion, for example, is ion-implanted in the upper layer part of the semiconductor layer including the super junction layer  15  and the n-type pillar surrounding layer  16  from the implantation mask, thus the plurality of withstand voltage holding structures  56  are formed as illustrated in  FIG. 15 . At this time, the gap  57  is provided in at least one of the withstand voltage holding structures  56  overlapping with the super junction layer  15  in a plan view (the gap  57  is located in a depth direction in a paper sheet of  FIG. 15 , thus is not shown therein). After the withstand voltage holding structure  56  is formed, the implantation mask is removed. 
     The region surrounded by the withstand voltage holding structure  56  on the innermost side is the active region  1 , and the region on the outer side of the inner end of the withstand voltage holding structure  56  on the innermost side is the termination region  2 . 
     Next, anneal processing is performed at a temperature of 1500 to 2100° C. for thirty seconds to one hour, for example, in an inactive gas atmosphere such as argon (Ar) gas or vacuum. The implanted ions are electrical activated by the anneal processing. 
     Herein, when the termination region  2  partially includes the field insulating film  32  as illustrated in  FIG. 8 , a silicon oxide film is formed on a whole surface of the semiconductor layer including the super junction layer  15  and the n-type pillar surrounding layer  16  by a CVD method, for example. Then, the silicon oxide film is patterned by selective etching using a photolithography technique to form the field insulating film  32 . 
     Next, a film formation technique such as a sputtering method or a vacuum deposition method and a patterning technique such as a photolithography method are combined to form the Schottky contact electrode  87  on the super junction layer  15  as illustrated in  FIG. 16 . The Schottky contact electrode  87  is formed in a range including the whole active region  1  in a plan view. 
     Moreover, a film formation technique such as a sputtering method or a vacuum deposition method and a patterning technique such as a photolithography method are combined to form the anode electrode  88  on the Schottky contact electrode  87  as illustrated in  FIG. 17 . The anode electrode  88  is formed in a range including at least a part of the Schottky contact electrode  87  in a plan view. 
     Subsequently, the back surface ohmic electrode  91  and the cathode electrode  93  are formed on the second main surface of the semiconductor substrate  11  as illustrated in  FIG. 18  by a sputtering method or a vacuum deposition method, for example. Accordingly, the semiconductor device having the structure illustrated in  FIG. 5  to  FIG. 7  (or  FIG. 8 ) is completed. 
     Next, an operation of the semiconductor device according to the embodiment 1 is described in an ON state and an OFF state. The ON state is a state where positive voltage equal to or larger than a predetermined threshold value based on potential of the cathode electrode  93  is applied to the anode electrode  88 , and current flows from the anode electrode  88  to the cathode electrode  93 . The OFF state is a state where negative voltage is applied to the anode electrode  88  based on potential of the cathode electrode  93 , current does not flow, and an insulation breakdown does not occur. Particularly herein, a state where negative high voltage is applied to the anode electrode  88  and a depletion layer extends to the whole super junction layer  15  is referred to as the OFF state. 
     In the OFF state, a line of electric force is connected between the n-type pillar layer  13  and the p-type pillar layer  14  in a horizontal direction, thus the super junction layer  15  is depleted, and voltage in the semiconductor device in a vertical direction is held by the depleted super junction layer  15 . 
     Herein, in a semiconductor device which does not have the super junction structure, when an impurity concentration of an n-type conductive region is increased, the depletion layer hardly extends and withstand voltage decreases, thus the withstand voltage and the ON resistance has a trade-off relationship. In contrast, in the semiconductor device having the super junction structure, hardness of extension of the depletion layer occurring when the impurity concentration of the n-type conductive region is increased can be compensated by reducing a pitch of repeating the p-type pillar layer  14  and the n-type pillar layer  13 , thus a trade-off between the withstand voltage and the ON resistance can be improved. 
     In the actual semiconductor device, the end portion of the chip surface in the OFF state has the same potential as the cathode electrode  93 , thus also in the semiconductor device of the embodiment 1, the potential difference between the anode electrode  88  and the end portion of the chip surface is increased. Thus, an electrical field concentration in the horizontal direction of the semiconductor device needs to be reduced by using the withstand voltage holding structure  56 . 
     The example of the potential distribution in the horizontal direction of the chip in the semiconductor device as the premise technique in which the withstand voltage holding structure  56  does not have the gap  57  is as illustrated in  FIG. 4 . When the withstand voltage holding structure  56  does not have the gap  57 , each withstand voltage holding structure  56  has the same potential over the whole outer periphery of the active region  1 . This indicates that an equipotential line cannot pass across the withstand voltage holding structure  56 . Thus, in the semiconductor device as the premise technique, the potential difference which originally occurs is held in the region between a certain withstand voltage holding structure  56  and another withstand voltage holding structure  56  adjacent to the inner side or the outer side of the certain withstand voltage holding structure  56 , and an electrical field concentration occurs in that part. 
     In the semiconductor device of the embodiment 1, the gap  57  is provided in the withstand voltage holding structure  56  overlapping with the super junction layer  15  in a plan view to solve this problem. When the withstand voltage holding structure  56  has the gap  57 , an equipotential line can pass across the withstand voltage holding structure  56  through the gap  57 . Accordingly, there is no limitation that the withstand voltage holding structure  56  has the same potential over the whole outer periphery of the active region  1 , thus a degree of freedom in the potential distribution is increased, and the electrical field concentration is reduced. 
     Particularly when the gap  57  is disposed so that meandering of the equipotential line is suppressed as much as possible, a concentration of the equipotential line (that is to say, a concentration of an electrical field) can be further suppressed, thus such a configuration is effective. 
     An arrangement of the gap  57  suppressing the meandering of the equipotential line is described. Each gap  57  has two boundary lines with the withstand voltage holding structure  56  to which the gap  57  belongs, and herein, as illustrated in  FIG. 19 , an end portion, on a side farther away from a center of the active region  1 , of the boundary line closer to a central line of the active region  1  parallel to the longitudinal direction of the p-type pillar layer  14  (simply referred to as “the central line of the active region  1 ” hereinafter) in the two boundary lines is defined as “a gap boundary end portion A”. An end portion, on a side closer to the center of the active region  1 , of the boundary line on a side farther away from the central line of the active region  1  in the two boundary lines is defined as “a gap boundary end portion B”. 
     In this case, when the gap boundary end portion A is located on a side closer to the central line of the active region  1  than the gap boundary end portion B in each gap  57 , the meandering of the equipotential line is suppressed. That is to say, it is preferable that in each gap  57 , the gap boundary end portion A is located on a side closer to the central line of the active region  1  than a straight line parallel to the longitudinal direction of the p-type pillar layer  14  passing through the gap boundary end portion B. In  FIG. 19 , the gap boundary end portion A is preferably located closer to a left side than the gap boundary end portion B ( FIG. 19  is a drawing for explaining the gap boundary end portion A and the gap boundary end portion B, thus does not illustrate a preferable positional relationship between the gap boundary end portion A and the gap boundary end portion B). 
     The potential distribution illustrated in  FIG. 4  is calculated on the assumption of the super junction layer  15  having the stripe shape as illustrated in  FIG. 1 . In  FIG. 4 , the potential profiles along the A 1 -A 2  line, the B 1 -B 2  line, and the C 1 -C 2  line in  FIG. 1  are different from each other by reason that the p-type pillar layer  14  is not rotationally symmetric with respect to the center of the active region  1  in a plan view, that is to say, the super junction layer  15  is not rotationally symmetric with respect to the center of the active region  1  in a plan view. 
     Thus, the problem of the electrical field concentration in the premise technique described above (the problem of the electrical field concentration occurring when the potential difference is held in the region between the certain withstand voltage holding structure  56  and another withstand voltage holding structure  56  adjacent to the inner side or the outer side of the certain withstand voltage holding structure  56 ) is not a problem occurring only in the case where the super junction layer  15  has the stripe shape, but is a problem widely occurring in the case where the super junction layer  15  is not rotationally symmetric with respect to the center of the active region  1 . Accordingly, the present embodiment widely has an effect not only in the case where the super junction layer  15  has the stripe shape but also in the case where the super junction layer  15  is not rotationally symmetric with respect to the center of the active region  1  in a plan view. 
     In the semiconductor device according to the embodiment 1, the gap  57  is provided in the curved part of the withstand voltage holding structure  56 . The graph illustrated in  FIG. 4  indicates that the potential distribution significantly changes around the B 1 -B 2  line from a position of the A 1 -A 2  line toward the position of the C 1 -C 2  line via the position of the B 1 -B 2  line in  FIG. 1 , that is to say, at the curved part of the withstand voltage holding structure  56 , and the electrical field concentration occurs. The gap  57  is provided in the curved part of the withstand voltage holding structure  56 , thus the effect of reducing the electrical field can be increased, and the withstand voltage of the semiconductor device can be increased. 
     In the semiconductor device according to the embodiment 1, the gap  57  is not provided in the straight part of the withstand voltage holding structure  56 . The reason is that the potential distribution is small in the straight part of the withstand voltage holding structure  56 , the electrical field concentration which should be reduced is small, and if the gap  57  is provided in the straight part of the withstand voltage holding structure  56 , there is a possibility that the depletion of the n-type semiconductor region around the gap  57  is suppressed and the electrical field is hardly held. In other words, the gap  57  is not provided in the straight part of the withstand voltage holding structure  56 , thus there is a constant effect in increasing the withstand voltage of the semiconductor device. 
     In the semiconductor device according to the embodiment 1, the gaps  57  of the adjacent withstand voltage holding structures  56  are disposed to be displaced from each other so as not to be adjacent to each other in the radial direction of the withstand voltage holding structure  56  (that is to say, the direction directed from the inner side to the outer side of the frame of the withstand voltage holding structure  56 ) in a plan view. There is a possibility that the depletion of the n-type semiconductor region around the gap  57  is suppressed and the electrical field is hardly held, thus the gaps  57  are displaced from each other to prevent the region hardly holding the electrical field from being continuously located. In other words, the gaps  57  between the adjacent withstand voltage holding structures  56  are disposed not to be adjacent to each other in the radial direction of the withstand voltage holding structure  56 , thus a constant effect can be achieved in increasing the withstand voltage of the semiconductor device. 
     In the semiconductor device according to the embodiment 1, the semiconductor substrate  11  made of silicon carbide is used, and the super junction layer  15  and the epitaxial crystal layer in which the withstand voltage holding structure  56  is formed are also made of silicon carbide. Generally, ions implanted in the semiconductor layer are diffused at a time of thermal processing, thus it is hard to form an ion implantation region having a minute pattern. However, the diffusion hardly occurs in silicon carbide, thus when silicon carbide is used, the shape can be easily controlled in forming a minute pattern such as the gap  57 , and the effect of increasing the withstand voltage can be achieved more easily than the case of using silicon. 
     Moreover, in the semiconductor device according to the embodiment 1, the p-type impurity concentration of the withstand voltage holding structure  56  per unit area is set equal to or larger than 1×10 13 cm −2 . This indicates that when the withstand voltage holding structure  56  is formed by ion implantation, a dose amount is equal to or larger than 1×10 13 cm −2 . When the p-type impurity concentration of the withstand voltage holding structure  56  is smaller than a constant value, the depletion of the n-type semiconductor region around the withstand voltage holding structure  56  is not sufficiently performed, and causes the reduction in the withstand voltage. The p-type impurity concentration of the withstand voltage holding structure  56  per unit area is set equal to or larger than 1×10 13 cm −2 , thus the n-type semiconductor region around the withstand voltage holding structure  56  can be reliably depleted. In other words, the p-type impurity concentration of the withstand voltage holding structure  56  per unit area is set equal to or larger than 1×10 13 cm −2 , thus a constant effect can be achieved in increasing the withstand voltage of the semiconductor device. 
     Embodiment 2 
       FIG. 20  is a plan view illustrating a configuration of the semiconductor device according to the embodiment 2 of the present invention. The description of constituent elements which are unnecessary for explanation (the Schottky contact electrode  87  and the anode electrode  88 , for example) is omitted for convenience in drawing figures. Constituent elements of the semiconductor device according to the embodiment 2 are mainly similar to those in the embodiment 1, thus the description of the constituent elements similar to those in the embodiment 1 is omitted, and a configuration specific to the embodiment 2 is described. 
     In the semiconductor device according to the embodiment 1, the gap  57  is not provided in the withstand voltage holding structure  56  which does not overlap with the super junction layer  15  in a plan view, however, in the semiconductor device according to the embodiment 2, as illustrated in  FIG. 20 , the gap  57  is provided also in the withstand voltage holding structure  56  (the withstand voltage holding structure  56  on the outermost side) which does not overlap with the super junction layer  15  in a plan view. That is to say, the gap  57  is provided also in the withstand voltage holding structure  56  located in the n-type pillar surrounding layer  16 . 
     When the super junction layer  15  is not rotationally symmetric with respect to the center of the active region  1 , the potential distribution of the super junction layer  15  is rotationally asymmetric, and the potential distribution of the n-type pillar surrounding layer  16  is also rotationally asymmetric. Thus, the configuration of locating the gap  57  also in the withstand voltage holding structure  56  located in the position overlapping with the n-type pillar surrounding layer  16  has a constant effect in reducing the electrical field concentration. 
     The semiconductor device according to the present embodiment can be manufactured by the manufacturing method similar to that of the semiconductor device according to the embodiment 1 by appropriately changing the mask pattern used in the process of forming the withstand voltage holding structure  56  and the gap  57  ( FIG. 15 ). 
     Embodiment 3 
       FIG. 21  is a plan view illustrating a configuration of a semiconductor device according to an embodiment 3 of the present invention. The description of constituent elements which are unnecessary for explanation (the Schottky contact electrode  87  and the anode electrode  88 , for example) is omitted for convenience in drawing figures. Constituent elements of the semiconductor device according to the embodiment 3 are mainly similar to those in the embodiment 2, thus the description of the constituent elements similar to those in the embodiment 2 is omitted, and a configuration specific to the embodiment 3 is described. 
     In the semiconductor device ( FIG. 20 ) according to the embodiment 2, the withstand voltage holding structure  56  on the outermost side in the plurality of withstand voltage holding structures  56  does not overlap with the n-type pillar surrounding layer  16 , however, in the semiconductor device according to the embodiment 3, all of the withstand voltage holding structures  56  are disposed to overlap with the super junction layer  15  as illustrated in  FIG. 21 . According to this configuration, the electrical field concentration is further reduced in the region near the outer periphery of the chip, thus the withstand voltage of the semiconductor device can be increased. 
     The semiconductor device according to the present embodiment can be manufactured by the manufacturing method similar to that of the semiconductor device according to the embodiment 1 by appropriately changing the mask pattern used in the process of forming the pillar formation trench  43  in which the p-type pillar layer  14  is embedded ( FIG. 11  and  FIG. 12 ) and the mask pattern used in the process of forming the withstand voltage holding structure  56  and the gap  57  ( FIG. 15 ). 
     Embodiment 4 
       FIG. 22  is a plan view illustrating a configuration of a semiconductor device according to an embodiment 4 of the present invention. The description of constituent elements which are unnecessary for explanation (the Schottky contact electrode  87  and the anode electrode  88 , for example) is omitted for convenience in drawing figures. Constituent elements of the semiconductor device according to the embodiment 4 are mainly similar to those in the embodiment 3, thus the description of the constituent elements similar to those in the embodiment 3 is omitted, and a configuration specific to the embodiment 4 is described. 
     In the semiconductor device according to the embodiment 3 ( FIG. 21 ), the position of the outer periphery of the super junction layer  15  and position of the outer periphery of the withstand voltage holding structure  56  on the outermost side coincide with each other, however, in the semiconductor device according to the embodiment 4, as illustrated in  FIG. 22 , the outer periphery of the super junction layer  15  is located on an outer side of the outer periphery of the withstand voltage holding structure  56  on the outermost side in the corner part (curved part) of the withstand voltage holding structure  56 . That is to say, the n-type pillar layer  13  and the p-type pillar layer  14  extend to an outer side of the outer periphery of the withstand voltage holding structure  56  on the outermost side in the corner part of the withstand voltage holding structure  56 . According to this configuration, the electrical field concentration is further reduced in the curved part of the withstand voltage holding structure  56 , thus the withstand voltage of the semiconductor device can be increased. 
     The semiconductor device according to the present embodiment can be manufactured by the manufacturing method similar to that of the semiconductor device according to the embodiment 1 by appropriately changing the mask pattern used in the process of forming the pillar formation trench  43  in which the p-type pillar layer  14  is embedded ( FIG. 11  and  FIG. 12 ) and the mask pattern used in the process of forming the withstand voltage holding structure  56  and the gap  57  ( FIG. 15 ). 
     Embodiment 5 
       FIG. 23  is a plan view illustrating a configuration of a semiconductor device according to an embodiment 5 of the present invention. The description of constituent elements which are unnecessary for explanation (the Schottky contact electrode  87  and the anode electrode  88 , for example) is omitted for convenience in drawing figures. Constituent elements of the semiconductor device according to the embodiment 5 are mainly similar to those in the embodiment 4, thus the description of the constituent elements similar to those in the embodiment 4 is omitted, and a configuration specific to the embodiment 5 is described. 
     In the semiconductor device according to the embodiment 4 ( FIG. 22 ), the outer periphery of the super junction layer  15  is located on the outer side of the outer periphery of the withstand voltage holding structure  56  on the outermost side only in the corner part (curved part) of the withstand voltage holding structure  56 , however, in the semiconductor device according to the embodiment 5, as illustrated in  FIG. 23 , the outer periphery of the super junction layer  15  is located on the outer side of the outer periphery of the withstand voltage holding structure  56  on the outermost side in the whole part (curved part and straight part) of the withstand voltage holding structure  56 . That is to say, all of the plurality of withstand voltage holding structures  56  are included by the super junction layer  15 . According to this configuration, the electrical field concentration is further reduced on the outer side of the withstand voltage holding structure  56 , thus the withstand voltage of the semiconductor device can be increased. 
     The semiconductor device according to the present embodiment can be manufactured by the manufacturing method similar to that of the semiconductor device according to the embodiment 1 by appropriately changing the mask pattern used in the process of forming the pillar formation trench  43  in which the p-type pillar layer  14  is embedded ( FIG. 11  and  FIG. 12 ) and the mask pattern used in the process of forming the withstand voltage holding structure  56  and the gap  57  ( FIG. 15 ). 
     Embodiment 6 
       FIG. 24  is a plan view illustrating a configuration of a semiconductor device according to an embodiment 6 of the present invention. The description of constituent elements which are unnecessary for explanation (the Schottky contact electrode  87  and the anode electrode  88 , for example) is omitted for convenience in drawing figures. Constituent elements of the semiconductor device according to the embodiment 6 are mainly similar to those in the embodiment 5, thus the description of the constituent elements similar to those in the embodiment 5 is omitted, and a configuration specific to the embodiment 6 is described. 
     As illustrated in  FIG. 24 , in the semiconductor device according to the embodiment 6, the gap  57  of the withstand voltage holding structure  56  is formed to extend over the n-type pillar layer  13  and the p-type pillar layer  14 . In a plan view, the boundary between the gap  57  and the withstand voltage holding structure  56  to which the gap  57  belongs does not have contact (does not have an intersection point and a contact point) with the boundary between the n-type pillar layer  13  and the p-type pillar layer  14 . 
     Moreover, the boundary line on the side closer to the central line of the active region  1  parallel to the longitudinal direction of the p-type pillar layer  14  (simply referred to as “the central line of the active region  1 ” hereinafter) in the two boundary lines between the gap  57  and the withstand voltage holding structure  56  to which the gap  57  belongs is included by the p-type pillar layer  14  in a plan view. The boundary line on the side farther away from the central line of the active region  1  in the two boundary lines is included in the n-type pillar layer  13  in a plan view. 
     Shown from the graph in  FIG. 4  is that when the p-type pillar layer  14  is formed into the stripe shape, a distance from the end portion of the active region  1  to a position where the potential reaches a specific value (for example, VR/2 or VR) is larger in a case of a direction vertical to the longitudinal direction of the p-type pillar layer  14  (a direction of the A 1 -A 2  line in  FIG. 1 ) than in a case of a direction parallel to the longitudinal direction of the p-type pillar layer  14  (a direction of the C 1 -C 2  line in  FIG. 1 ). This indicates that when an equipotential line is illustrated in a plan view, it is illustrated as a concentric figure whose longitudinal direction is the longitudinal direction of the p-type pillar layer  14  (for example, a figure similar to an oval shape or a rectangular shape whose corners are rounded). 
     Thus, the equipotential line crossing from an outer side to an inner side of the withstand voltage holding structure  56  through the gap  57  enters an inner side of a portion of the withstand voltage holding structure  56  including the boundary line on a side farther away from the central line of the active region  1  through an outer side of a portion of the withstand voltage holding structure  56  including the boundary line on a side closer to the central line of the active region  1  in the two boundary lines between the gap  57  and the withstand voltage holding structure  56  to which the gap  57  belongs. Accordingly, the electrical field concentration in the gap  57  occurs in the end portion, on the side farther away from the center of the active region  1 , of the boundary line on the side closer to the central line of the active region  1  (the gap boundary end portion A in  FIG. 19 ) in the two boundary lines between the gap  57  and the withstand voltage holding structure  56  to which the gap  57  belongs and the end portion, on the side closer to the center of the active region  1 , of the boundary line on the side farther away from the central line of the active region  1  (the gap boundary end portion B in  FIG. 19 ) in the two boundary lines. 
     In the meanwhile, it is experientially known that when both the n-type semiconductor region and the p-type semiconductor region are located in non-one-dimensional form with respect to the direction of the electrical field, the equipotential line is distributed to bulge toward a low potential side in the n-type semiconductor region, and is distributed to bulge toward a high potential side in the p-type semiconductor region. 
     When the gap  57  is disposed as illustrated in  FIG. 24 , the gap boundary end portion A located in the outer peripheral part of the withstand voltage holding structure  56  is disposed on the p-type pillar layer  14 , and the gap boundary end portion B located in the inner peripheral part of the withstand voltage holding structure  56  is disposed on the n-type pillar layer  13 , thus the electrical field concentration in both the gap boundary end portions A and B are reduced, and the withstand voltage of the semiconductor device can be increased. That is to say, the gap boundary end portion A located to a side where potential is low is located on the p-type pillar layer  14 , thus the equipotential line bulges from the gap boundary end portion A toward the high potential side, and the electrical field of the gap boundary end portion A is reduced. The gap boundary end portion B located to a side where potential is high is located on the n-type pillar layer  13 , thus the equipotential line bulges from the gap boundary end portion B toward the low potential side, and the electrical field of the gap boundary end portion B is reduced. 
     The semiconductor device according to the present embodiment can be manufactured by the manufacturing method similar to that of the semiconductor device according to the embodiment 1 by appropriately changing the mask pattern used in the process of forming the pillar formation trench  43  in which the p-type pillar layer  14  is embedded ( FIG. 11  and  FIG. 12 ) and the mask pattern used in the process of forming the withstand voltage holding structure  56  and the gap  57  ( FIG. 15 ). 
     Embodiment 7 
       FIG. 25  is a plan view illustrating a configuration of a semiconductor device according to an embodiment 7 of the present invention. The description of constituent elements which are unnecessary for explanation (the Schottky contact electrode  87  and the anode electrode  88 , for example) is omitted for convenience in drawing figures. Constituent elements of the semiconductor device according to the embodiment 7 are mainly similar to those in the embodiment 6, thus the description of the constituent elements similar to those in the embodiment 6 is omitted, and a configuration specific to the embodiment 7 is described. 
     In the semiconductor device according to the embodiment 7, as illustrated in  FIG. 25 , the p-type pillar layer  14  including the boundary line on the side closer to the central line of the active region  1  parallel to the longitudinal direction of the p-type pillar layer  14  in the two boundary lines between the gap  57  and the withstand voltage holding structure  56  to which the gap  57  belongs and the n-type pillar layer  13  including the boundary line on the side farther away from the central line of the active region  1  are adjacent to each other, thus the semiconductor device according to the embodiment 7 is different from that according to the embodiment 6 ( FIG. 24 ) in that point. That is to say, in the present embodiment, the p-type pillar layer  14  where the gap boundary end portion A is located and the n-type pillar layer  13  where the gap boundary end portion B is located are adjacent to each other in a plan view. 
     According to this configuration, the region which does not include the withstand voltage holding structure  56  is reduced, thus it is suppressed that the depletion layer hardly extends. That is to say, the depletion layer is kept to extend easily, thus reduction in the withstand voltage of the semiconductor device can be prevented. 
     The semiconductor device according to the present embodiment can be manufactured by the manufacturing method similar to that of the semiconductor device according to the embodiment 1 by appropriately changing the mask pattern used in the process of forming the pillar formation trench  43  in which the p-type pillar layer  14  is embedded ( FIG. 11  and  FIG. 12 ) and the mask pattern used in the process of forming the withstand voltage holding structure  56  and the gap  57  ( FIG. 15 ). 
     Embodiment 8 
       FIG. 26  is a plan view illustrating a configuration of a semiconductor device according to an embodiment 8 of the present invention. The description of constituent elements which are unnecessary for explanation (the Schottky contact electrode  87  and the anode electrode  88 , for example) is omitted for convenience in drawing figures. Constituent elements of the semiconductor device according to the embodiment 8 are mainly similar to those in the embodiment 7, thus the description of the constituent elements similar to those in the embodiment 7 is omitted, and a configuration specific to the embodiment 8 is described. 
     In the semiconductor device according to the embodiment 7 ( FIG. 25 ), the boundary line between the gap  57  and the withstand voltage holding structure  56  to which the gap  57  belongs has a straight shape, however, in the semiconductor device according to the embodiment 8, the boundary line between the gap  57  and the withstand voltage holding structure  56  to which the gap  57  belongs has a curved shape as illustrated in  FIG. 26 . Thus, each of parts of the withstand voltage holding structure  56  divided by the two gaps  57  has an oval shape or band-like shape whose corners are rounded. 
     According to this configuration, the electrical field concentration is further reduced in the end portion of the boundary line between the gap  57  and the withstand voltage holding structure  56  to which the gap  57  belongs, and the withstand voltage of the semiconductor device can be increased.  FIG. 26  illustrates an example that the boundary line between the gap  57  and the withstand voltage holding structure  56  to which the gap  57  belongs is curved in the configuration in the embodiment 7. However, the embodiment 8 is applicable not only to the embodiment 7 but also to any of the embodiments 1 to 6. That is to say, the boundary line between the gap  57  and the withstand voltage holding structure  56  to which the gap  57  belongs is curved also in each configuration in the embodiments 1 to 6, thus the effect of increasing the withstand voltage can be obtained. 
     The semiconductor device according to the present embodiment can be manufactured by the manufacturing method similar to that of the semiconductor device according to the embodiment 1 by appropriately changing the mask pattern used in the process of forming the pillar formation trench  43  in which the p-type pillar layer  14  is embedded ( FIG. 11  and  FIG. 12 ) and the mask pattern used in the process of forming the withstand voltage holding structure  56  and the gap  57  ( FIG. 15 ). 
     Embodiment 9 
     In an embodiment 9, the semiconductor device according to the embodiments 1 to 8 is applied to a power conversion device. Described particularly herein is a case of applying a switching element (for example, a MOSFET) including the super junction layer  15  and the withstand voltage holding structure  56  corresponding to the embodiments 1 to 8 and a rectifying element (for example, an SBD) to a three-phase inverter. 
       FIG. 27  is a block diagram illustrating a configuration of a power conversion system to which a power conversion device according to the embodiment 9 is applied. The power conversion system illustrated in  FIG. 27  is made up of a power conversion device  301 , a power source  321 , and a load  331 . 
     The power source  321  is a power source converting a commercial AC power source into a DC power by an AC/DC converter, and supplies the DC power to the power conversion device  301 . 
     The power conversion device  301  is a three-phase inverter connected between the power source  321  and the load  331 , converts the DC power supplied from the power source  321  into an AC power, and supplies the AC power to the load  331 . As illustrated in  FIG. 27 , the power conversion device  301  includes a main conversion circuit  311  converting the DC power into the AC power and outputs the AC power, a drive circuit  312  outputting a drive signal driving a switching element constituting the main conversion circuit  311 , and a control circuit  313  outputting a control signal controlling the drive circuit  312  to the drive circuit  312 . 
     The load  331  is a three-phase electrical motor driven by the AC power supplied from the power conversion device  301 . 
     The main conversion circuit  311  includes a switching element and a rectifying element, and when the switching element is switched, the main conversion circuit  311  converts the DC power supplied from the power source  321  into the AC power, and supplies the AC power to the load  331 . Various specific circuit configurations may be applied to the main conversion circuit  311 , however, in the present embodiment, the main conversion circuit  311  is a three-phase full-bridge circuit with two levels. The three-phase full-bridge circuit can be made up of six switching elements and six rectifying elements antiparallelly connected to each switching element. The six switching elements are connected two by two in series to constitute upper and lower arms, and each pair of the upper and lower arms constitutes a U phase, a V phase, and a W phase of a full-bridge circuit. Output terminals of the pair of the upper and lower arms, that is to say, three output terminals of the main conversion circuit  311  are connected to the load  331 . 
     Each switching element and each rectifying element constituting the main conversion circuit  311  are a semiconductor device  314  according to any of the embodiments 1 to 8. 
     The drive circuit  312  generates the drive signal driving the switching element of the main conversion circuit  311 , and outputs the generated drive signal to a control electrode of the switching element of the main conversion circuit  311 . Specifically, the drive circuit  312  outputs a drive signal for making the switching element enter an ON state and a drive signal for making the switching element enter an OFF state to a control electrode of each switching element in accordance with a control signal outputted from the control circuit  313 . 
     The control circuit  313  controls the switching element of the main conversion circuit  311  so that a desired electrical power is supplied to the load  331 . For example, when the main conversion circuit  311  is operated by a pulse width modulation (PWM) control, a switching chart of the switching element is calculated based on an electrical power to be supplied to the load  331 , and the control signal for achieving this switching chart is outputted to the drive circuit  312 . The drive circuit  312  outputs the ON signal or the OFF signal as the drive signal to the control electrode of each switching element in accordance with the control signal. 
     The power conversion device according to the present embodiment includes the semiconductor device according to any one of the embodiments 1 to 8 as the semiconductor device  314  constituting the main conversion circuit  311 , thus the power conversion device having the high withstand voltage can be achieved. 
     &lt;Modification Example&gt; 
     In the embodiments 1 to 8, the SBD is described as the semiconductor device, however, the semiconductor device is not limited to the SBD, but a junction barrier diode (JBS), a pn junction diode, a MOSFET, a junction field-effect transistor (JFET), or an IGBT, for example, may also be applied. 
     The material of the semiconductor substrate  11  is not limited to silicon carbide, but may also be the other wide gap semiconductor such as silicon, GaN, diamond, a compound semiconductor, and an oxide semiconductor, for example. When the semiconductor substrate  11  has the OFF angle, and the surface on which the uniform epitaxial growth can be performed is limited to the specific crystal plane, the super junction layer  15  is required to have the stripe shape in a plan view regardless of the semiconductor material. Thus, when the semiconductor substrate  11  has the OFF angle, the embodiments 1 to 8 can be applied regardless of the semiconductor material. 
     In the embodiments 1 to 8, the first main surface of the semiconductor substrate  11  is inclined at the angle of 4 degrees in the direction of [11-20] with respect to the (0001) plane, however, the other crystal plane such as (000-1) plane may be used, and the inclination angle may be the other angle within a range of 0 degree to 8 degrees. A polytype of silicon carbide is not limited to 4H, however, the other polytype such as 3C and 6H may also be applied. 
     In the embodiments 1 to 8, the first conductivity type is the n type and the second conductivity type is the p type, however, it is also applicable that the first conductivity type is the p type and the second conductivity type is the n type. 
     In the embodiments 1 to 8, Al is used as the p-type impurity, however, the other III group element such as boron (B) or gallium (Ga) may also be used, for example. In the similar manner, in the embodiments 1 to 8, N is used as the n-type impurity, however, the other V group element such as phosphorus (P) or arsenic (As) may also be used, for example. 
     In the embodiments 1 to 5, the boundary line between the withstand voltage holding structure  56  and the gap  57  is the straight line extending in the radial direction of the withstand voltage holding structure  56 , however, the direction of the boundary line is not limited thereto. The shape of the boundary line is not limited to the straight line, however, an optional shape may also be applied. 
     In the embodiments 6 to 7, the boundary line between the withstand voltage holding structure  56  and the gap  57  is the straight line extending in the radial direction of the withstand voltage holding structure  14 , however, the direction of the boundary line is not limited thereto. The shape of the boundary line is not limited to the straight line, however, an optional shape may also be applied. 
     An optimal number, width, and arrangement of the gap  57  provided in the withstand voltage holding structure  56  are different depending on a whole design of the withstand voltage holding structure  56 , for example, thus are not regulated in detail in the embodiments 1 to 8. However, the optimal number, width, and arrangement of the gap  57  can be obtained using a TCAD when a design of the termination region is determined. Thus, optimal number, width, and arrangement of the gap  57  can be optimized without departing from the scope of the embodiments 1 to 8. 
     The effect obtained by the semiconductor device including the structure described in the embodiments 1 to 8 does not depend on the method of manufacturing the semiconductor device. That is to say, even when the semiconductor device including the structure described in the embodiments 1 to 8 is manufactured using the manufacturing method other than that described above, the effect similar to that described in the embodiments 1 to 8 can be obtained. 
     The gap is not provided in the withstand voltage holding structure  56  in the innermost periphery in  FIGS. 5, 19, 20, 21, 22, 23, 24, 25, and 26  described in the embodiments 1 to 8. In this case, the potential of the end portion of the Schottky contact electrode  87  having contact with the withstand voltage holding structure  56  in the innermost periphery can be constant, thus breakage of the semiconductor device caused by the local concentration of the current can be prevented. However, the gap may be provided also in the withstand voltage holding structure  56  in the innermost periphery, and when the gap is provided, the effect of reducing the electrical field in the curved part of the withstand voltage holding structure  56  can be obtained. 
     In the embodiment 9, the power source  321  is the power source in which the commercial AC power source is converted into the DC power by the AC/DC converter, however, the other type of power source is also applicable. The power source  321  may be a rectifying circuit connected to a commercial DC power source, a solar battery, a storage battery, or an AC power source, an output of an AC/DC converter, or an output of a DC/DC converter, for example. 
     The embodiment 9 exemplifies the three-phase inverter with two levels as the power conversion device, however, a range of application of the embodiments 1 to 8 is not limited to a specific power conversion device. The power conversion device may be an inverter with three level or multiple levels, for example, or may also be a single phase inverter. The embodiments 1 to 8 can also be applied to a DC/DC converter and an AC/DC converter. 
     In the embodiment 9, the load  331  is the three-phase electrical motor, however, the type of the load  331  is not limited thereto. Applicable as the load  331  is an electrical discharge machine, a laser beam machine, an induction heat cooking machine, a power source device of non-contact power supply system, or a power conditioner used in a solar power generation system or an electrical power storage system, for example. 
     According to the present invention, each embodiment can be arbitrarily combined, or each embodiment can be appropriately varied or omitted within the scope of the invention. 
     Although the present invention is described in detail, the foregoing description is in all aspects illustrative and does not restrict the invention. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention. 
     EXPLANATION OF REFERENCE SIGNS 
       1  active region,  2  termination region,  11  semiconductor substrate,  12  epitaxial crystal layer,  13  n-type pillar layer,  14  p-type pillar layer,  15  super junction layer,  16  n-type pillar surrounding layer,  32  field insulating film,  41  epitaxial crystal layer,  42  silicon oxide film,  43  pillar formation trench,  44  epitaxial crystal layer,  56  withstand voltage holding structure,  57  gap,  87  Schottky contact electrode,  88  anode electrode,  91  back surface ohmic electrode,  93  cathode electrode,  301  power conversion device,  311  main conversion circuit,  312  drive circuit,  313  control circuit.  314  semiconductor device,  321  power source,  331  load.