Patent Publication Number: US-9412831-B2

Title: Manufacturing method for silicon carbide semiconductor device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This present disclosure is a U.S. national stage application of PCT/JP2013/003123 filed on May 16, 2013 and is based on Japanese Patent Application No. 2012-114739 filed on May 18, 2012, the disclosures of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a method for manufacturing a silicon carbide (SiC) semiconductor device having a junction field effect transistor (JFET) of a trench structure. 
     BACKGROUND ART 
     PTL 1 discloses an SiC semiconductor device having a JFET with a trench structure. The JFET is formed as follows. 
     After an n −  type drift layer, a p +  type first gate region, and an n +  type source region have been formed on an n +  type SiC substrate in the stated order, a trench that penetrates through those regions is formed. Then, an n −  type channel layer and a p +  type second gate region are allowed to epitaxially grow within the trench, and embedded within the trench. Thereafter, a substrate surface is planarized, and unnecessary portions of the n −  type channel layer and the p +  type second gate region are removed to expose the n +  type source region. Subsequently, the substrate surface is etched with the use of a mask for exposing an outer peripheral region that surrounds a cell region in which the JFET is configured. The n +  type source region is removed in the outer peripheral region, and a first concave portion is formed in the outer peripheral region to form a first mesa portion. 
     Further, the substrate surface is etched with the use of a mask for exposing an outer edge of the first mesa portion in the outer peripheral region, the p +  type first gate region is further removed, and a second concave portion is formed within the first concave portion to form a second mesa portion. Thereafter, after a p type surface electric field relaxation (resurf) layer has been formed at a boundary position between a side surface and a bottom surface of the second concave portion, or ion implantation for forming a p type guard ring layer is conducted on the bottom surface of the second concave portion, the bottom surface of the second concave portion is activated through a heat treatment. In addition, the JFET disclosed in PTL 1 is formed through a process of forming an interlayer insulating film on a substrate front surface side, a process of forming a gate electrode and a source electrode, and a process of forming a drain electrode on a substrate rear surface side. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-A-2010-34381 (corresponding to US 2010/0025693 A1) 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, when the substrate surface is planarized after the n −  type channel layer and the p +  type second gate region have epitaxially grown within the trench, the removal amount of the n +  type source region in the planarizing process varies in the existing technique. Because a thickness of the residual n +  type source region is small, it is difficult to grasp an actual removal amount by optical evaluation using Fourier transform infrared spectroscopy (FT-IR). For that reason, the removal amount is grasped according to a difference between the substrate thicknesses before and after planarization in the present circumstances, but precision higher than ±0.5 μm level cannot be obtained. 
     For that reason, the removal of the n +  type source region becomes larger than a desired set value, and the thickness of the p +  type first gate region located in a lower layer of the first mesa portion may be thinned or eliminated when forming the first concave portion for forming the first mesa portion. This is one of causes of reduction in an element breakdown voltage. 
     In view of the above, an object of the present disclosure is to provide a method of manufacturing an SIC semiconductor device, which can inhibit a thickness of a first gate region from being unnecessarily thinned, or eliminated. 
     Solution to Problem 
     According to an aspect of the present disclosure, there is provided a method of manufacturing a silicon carbide semiconductor device for forming a JFET in a cell region of a semiconductor substrate, and forming a first concave portion that configures a first mesa portion in an outer periphery of the cell region, and a second concave portion that configures a second mesa portion in an outer peripheral position of the cell region than a stepped portion of the first mesa portion within the first concave portion. 
     In the method of manufacturing the silicon carbide semiconductor device, the semiconductor substrate is prepared. The semiconductor substrate includes a first conductivity type substrate that is made of silicon carbide, a drift layer of a first conductivity type that is formed on the first conductivity type substrate by epitaxial growth, a first gate region of a second conductivity type that is formed on the drift layer by the epitaxial growth, and a source region of the first conductivity type that is formed on the first gate region by the epitaxial growth or ion implantation. 
     A strip-like trench that penetrates through the source region and the first gate region, and reaches the drift layer with one direction as a longitudinal direction is formed, a channel layer of the first conductivity type is formed on an inner wall of the trench by epitaxial growth, a second gate region of the second conductivity type is formed on the channel layer by epitaxial growth, and the channel layer and the second gate region are planarized to expose the source region. 
     After the planarizing, the second concave portion having a depth deeper than the source region, and as deep as a boundary portion between the source region and the first gate region is exposed is formed in an outer peripheral region surrounding a cell region, with a region in which the trench is formed as the cell region in which the JFET is configured, by conducting selective etching. 
     After forming the second concave portion, a thickness of the source region is detected by observing a pn junction between the source region and the first gate region exposed by the second concave portion. Selective etching is conducted on the basis of the detection result, the first concave portion deeper than the thickness of the source region is formed inside of the outer peripheral region in an outer periphery of the cell region, and the second concave portion is made deeper than the second gate region. 
     After forming an interlayer insulating film on surfaces of the second gate region, the channel region, and the source region, contact holes are formed in the interlayer insulating film, and a gate electrode connected to at least one of the first gate region and the second gate region, and a source electrode connected to the source region are formed through the contact holes. A drain electrode is formed on a rear surface of the first conductivity type substrate. 
     In the method of manufacturing the silicon carbide semiconductor device, the first concave portion configuring the first mesa portion is formed after the second concave portion configuring the second mesa portion has been formed. As a result, the thickness of the source region can be detected by the SEM observation on the basis of the pn junction between the source region and the first gate region due to the stepped portion of the second concave portion before the selective etching for forming the first concave portion. Therefore, the etching can be precisely conducted by the degree of thickness of the source region when forming the first concave portion, and the etching depth can be precisely controlled. For that reason, the first gate region can be prevented from being unnecessarily thinned, or eliminated. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a top layout view of an SiC semiconductor device according to a first embodiment of the present disclosure; 
         FIG. 2  is a cross-sectional view of the SiC semiconductor device taken along a line II-II in  FIG. 1 ; 
         FIG. 3A  is a cross-sectional view partially illustrating a process of manufacturing the SiC semiconductor device; 
         FIG. 3B  is a cross-sectional view partially illustrating the process of manufacturing the SiC semiconductor device; 
         FIG. 3C  is a cross-sectional view partially illustrating the process of manufacturing the SiC semiconductor device; 
         FIG. 4A  is a cross-sectional view partially illustrating the process of manufacturing the SiC semiconductor device; 
         FIG. 4B  is a cross-sectional view partially illustrating the process of manufacturing the SiC semiconductor device; and 
         FIG. 4C  is a cross-sectional view partially illustrating the process of manufacturing the SiC semiconductor device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following respective embodiments, parts identical with or equivalent to each other are denoted by the same symbols for description. 
     (First Embodiment) 
     A first embodiment of the present disclosure will be described. As illustrated in  FIG. 1 , an SiC semiconductor device is structured to include a cell region R 1 , an electric field relaxation region R 2 , and an outer peripheral region R 3 . A JFET is formed in the cell region R 1 . The cell region R 1  has a square shape whose corners are rounded in a top shape. The electric field relaxation region R 2  functions to relax electric field concentration in an outer peripheral region of the cell region R 1 . The electric field relaxation region R 2  is arranged between the cell region R 1  and the outer peripheral region R 3 , and has a square frame shape whose corners are rounded so as to surround the periphery of the cell region R 1 . The outer peripheral region R 3  extensively spreads and terminates an electric field extending from the cell region R 1  on an outer peripheral side of the SiC semiconductor device, to thereby provide a high breakdown voltage. The outer peripheral region R 3  has a square frame shape whose corners are rounded so as to surround the periphery of the electric field relaxation region R 2  in a top shape. 
     Specifically, as illustrated in  FIG. 2 , the SiC semiconductor device includes an n +  type substrate (substrate)  1 , an n −  type drift layer (first semiconductor layer)  2 , a p +  type layer (second semiconductor layer)  3 , and an n +  type layer (third semiconductor layer)  4 . The n +  type substrate  1  has an impurity concentration equal to or higher than, for example, 1×10 19  cm −3 . The n −  type drift layer  2  has an impurity concentration lower than the n +  type substrate  1 , for example, 1×10 15  to 5×10 16  cm −3 . The p +  type layer  3  has an impurity concentration of, for example, 1×10 18  to 5×10 19  cm −3 . The n +  type layer  4  has an impurity concentration higher than the n −  type drift layer  2 , for example, 1×10 18  to 5×10 20  cm −3 . The n +  type substrate  1 , the n −  type drift layer  2 , the p +  type layer  3 , and the n +  type layer  4  are each made of SiC, and configure a semiconductor substrate  5 . As illustrated in  FIG. 1 , a center portion of the semiconductor substrate  5  forms the cell region R 1 , and the electric field relaxation region R 2  and the outer peripheral region R 3  are arranged with the cell region R 1  as a center in the stated order. 
     Also, as illustrated in  FIG. 2 , a trench  7   a  that penetrates through the n +  type layer  4  and the p +  type layer  3 , and reaches the n type drift layer  2  is formed in a main surface side of the semiconductor substrate  5  in the cell region R 1 . The trench  7   a  extends in a strip with one direction (a direction perpendicular to a paper plane in the present embodiment) on a substrate plane as a longitudinal direction. An n −  type layer (first conductivity type layer)  8 , and a p +  type layer (second conductivity type layer)  9  are formed in the stated order so as to be embedded in the trench  7   a . The n −  type layer  8  has a thickness of, for example, 0.1 to 0.5 μm, and an impurity concentration of 1.0×10 16  to 1.0×10 18  cm −3 . The p +  type layer  9  has an impurity concentration of 1×10 18  to 5×10 20  cm −3 . A first gate region  3   a  is formed by the p +  type layer  3 , a second gate region  9   a  is formed by the p +  type layer  9 , an n +  type source region  4   a  is formed by the n +  type layer  4 , and an n −  type channel layer  8   a  is formed by the n −  type layer  8 . 
     The impurity concentrations of the an n −  type channel layer  8   a  and the first and second gate regions  3   a  and  9   a , and the thickness of the n −  type channel layer  8   a  are set according to an operating mode of the JFET, and in the present embodiment, the JFET is set to operate in a normally off manner. 
     Also, a gate electrode  11  and a source electrode  12  are formed over the surfaces of the n +  type layer  4 , the n −  type layer  8 , and the p +  type layer  9  through an interlayer insulating film  10 . The gate electrode  11  is electrically connected to the second gate region  9   a  through a contact hole  10   a  formed in the interlayer insulating film  10 , and also electrically connected to the first gate region  3   a  in another cross-section different from that of  FIG. 2 . The source electrode  12  is electrically connected to the n +  type source region  4   a  through a contact hole  10   b  formed in the interlayer insulating film  10 . The gate electrode  11  is made of, for example, Al which is a material that enables an ohmic contact with the p +  type layer, and Ni laminated on Al. The source electrode  12  is made of, for example, Ni. 
     A drain electrode  13  electrically connected to an overall rear surface of the n +  type substrate  1  is formed on a rear surface of the semiconductor substrate  5 . The JFET is formed with the above structure, and the cell region R 1  in which plural JFETs are collected in a cell is formed. 
     Also, a first concave portion  18  from which the n +  type layer  4  of the semiconductor substrate  5  is removed by etching is formed in the electric field relaxation region R 2 . For that reason, a boundary portion of the electric field relaxation region R 2  with the cell region R 1  forms a first mesa portion in which a stepped portion is formed, and the p +  type layer  3  is exposed. 
     A trench  7   b  that reaches the n −  type drift layer  2  so as to divide between the cell region R 1  and the outer peripheral region R 3  (surround the periphery of the cell region R 1  in the present embodiment) is formed on the cell region R 1  side of the electric field relaxation region R 2 . The n −  type layer  8  and the p +  type layer  9  are arranged to be embedded in the trench  7   b . The n type layer  8  and the p +  type layer  9  in the electric field relaxation region R 2  function as an n type region  8   b  and a p type region  9   b  configuring a pn separation part.  FIG. 2  illustrates a structure in which only one trench  7   b  is formed to provide one pn separation part. Alternatively, plural trenches  7   b  may be concentrically arranged to surround the cell region R 1  to provide plural pn separation parts. 
     Also, a p −  type resurf layer  14  extends from the stepped portion forming the boundary portion of the electric field relaxation region R 2  with the outer peripheral region R 3  into the outer peripheral region R 3  which will be described later. The p −  type resurf layer  14  is 1.0×10 17  to 5.0×10 17  cm −3  in the p type impurity concentration. In the present embodiment, the stepped portion forming the boundary portion between the electric field relaxation region R 2  and the outer peripheral region R 3  has an inclined mesa shape, and the p −  type resurf layer  14  extends over the overall surface of the stepped portion, resulting in a structure where the p +  type layer  3  and the p −  type resurf layer  14  are connected to each other. A surge drawing electrode  15  is disposed in an outer periphery (an outer peripheral side of the outermost peripheral trench if the plural trenches  7   b  are provided) of the trench  7   b  so as to come in contact with a surface of the p +  type layer  3  through a contact hole  10   c  formed in the interlayer insulating film  10 . 
     Also, a second concave portion  19  from which the p +  type layer  3  and the n +  type layer  4  of the semiconductor substrate  5  are removed by etching is formed in the outer peripheral region R 3 . For that reason, in the outer peripheral region R 3 , the n −  type drift layer  2  is exposed, and the boundary portion between the electric field relaxation region R 2  and the outer peripheral region R 3  forms the stepped portion as a second mesa portion. In a surface layer portion of the n −  type drift layer  2 , the above-described p −  type resurf layer  14  extends toward the outer peripheral side of the cell region R 1 . Further, an n +  type layer  16  is formed to surround an outer periphery of the p −  type resurf layer  14 , and an equipotential ring (EQR) electrode  17  is disposed to be electrically connected to the n +  type layer  16  through a contact hole  10   d  formed in the interlayer insulating film  10 . 
     The SiC semiconductor device according to the present embodiment is configured with the above structure. Subsequently, a description will be given of the operation of the JFET provided in the cell region R 1  of the SiC semiconductor device configured as described above. 
     In the present embodiment, the JFET operates in a normally off manner. First, in a state before a gate voltage is applied to the first gate region  3   a  and the second gate region  9   a , the n −  type channel layer  8   a  pinches off by a depletion layer extending from both of the first gate region  3   a  and the second gate region  9   a  to the n −  type channel layer  8   a . For that reason, no channel region is set, and no current flows between the source and the drain. On the other hand, when the gate voltage is applied to the first gate region  3   a  and the second gate region  9   a , the amount of extension of the depletion layer extending from both of the first and second gate regions  3   a  and  9   a  toward the n −  type channel layer  8   a  side is controlled to reduce the amount of extension of depletion layer extending to the n″ type channel layer  8   a . For that reason, the channel region is set, and the current flows between the source and the drain. When the application of the gate voltage to the first gate region  3   a  and the second gate region  9   a  stops, the JFET turns off. 
     Also, when surge is generated, an avalanche breakdown is generated in the p −  type resurf layer  14 , a surge current flows along a current path illustrated in  FIG. 1 , and the surge current can be drawn from the surge drawing electrode  15  side. 
     In the above SiC semiconductor device, the element separation between the cell region R 1  and the outer peripheral region R 3  is conducted by the pn separation portion having the p type region  9   b  and the n type region  8   b  provided in the electric field relaxation region R 2 . For that reason, compared with a case where an oxide film is arranged within the trench to conduct the element separation, because there is no breakdown of the oxide film for element separation, a breakdown voltage between the cell region R 1  in which the JFET is formed and the outer peripheral region R 3  can be improved. 
     Also, because the p −  type resurf layer  14  extends to the stepped portion forming the boundary portion between the electric field relaxation region R 2  and the outer peripheral region R 3 , an electric field applied to the interlayer insulating film  10  formed on the surface of the p −  type resurf layer  14  can be reduced. For that reason, the breakdown caused by the electric field concentration of the interlayer insulating film  10  can be also suppressed. 
     Subsequently, a description will be given of a process of manufacturing the SiC semiconductor device illustrated in  FIG. 1  with reference to manufacturing process diagrams of  FIGS. 3A to 4C . 
     First, in a process illustrated in  FIG. 3A , the semiconductor substrate  5  is prepared in which the n −  type drift layer  2  having the impurity concentration of, for example, 1×10 15  to 5×10 16  cm −3 , the p +  type layer  3  having the impurity concentration of, for example, 1×10 18  to 5×10 19  cm −3 , and the n +  type layer  4  having the impurity concentration of, for example, 1×10 18  to 5×10 20  cm −3  are allowed to epitaxially grow on the n +  type substrate  1  having the impurity concentration of, for example, 1×10 16  cm −3  or higher. 
     In a process illustrated in  FIG. 3B , after a mask not shown has been arranged on the surface of the semiconductor substrate  5 , regions in which the trench  7   a  of the cell region R 1 , and the trench  7   b  of the electric field relaxation region R 2  are to be formed are opened. In addition, etching is conducted with the use of the mask to form the trenches  7   a  and  7   b  that penetrate through the n +  type layer  4  and the p +  type layer  3 , and reach the n −  type drift layer  2  at the same time. With the above process, the p +  type layer  3  and the n +  type layer  4  are divided into plural pieces by the trench  7   a , and the first gate region  3   a  and the n +  type source region  4   a  are formed by the p +  type layer  3  and the n +  type layer  4  located on the side surface of the trench  7 . Thereafter, the mask is removed. 
     In a process illustrated in  FIG. 3C , the n −  type layer  8  and the p +  type layer  9  are allowed to epitaxially grow, and laminated on the surface of the semiconductor substrate  5  in the stated order so as to be embedded within the trenches  7   a  and  7   b . In addition, the n −  type layer  8  and the p +  type layer  9  are left only within the trenches  7   a  and  7   b  to expose the surface of the n +  type layer  4  through the planarizing process using grinding or chemical mechanical polishing (CMP). With the above process, then n −  type channel layer  8   a  and the second gate region  9   a  can be formed within the trench  7   a  of the cell region R 1 , and the pn junction of the n type region  8   b  and the p type region  9   b  can be formed within the trench  7   b  of the electric field relaxation region R 2 . 
     In a process illustrated in  FIG. 4A , a mask that covers the cell region R 1  and the electric field relaxation region R 2  of the semiconductor substrate  5  is arranged, and the n +  type layer  4  and the p +  type layer  3  are removed in the outer peripheral region R 3  by etching. For example, anisotropic etching is conducted in a mixed gas atmosphere of SF 6 , O 2 , and Ar. With the above process, the second concave portion  19  is formed in the outer peripheral region R 3  to expose the p +  type layer  3 , and the boundary portion between the electric field relaxation region R 2  and the outer peripheral region R 3  forms the stepped portion as the second mesa portion. In this situation, it is preferable that the mesa portion of the boundary portion between the electric field relaxation region R 2  and the outer peripheral region R 3  is tapered. For example, the tapered mesa portion can be formed by adjusting a condition of the anisotropic etching, or conducting the anisotropic etching using a plane orientation dependence, or isotropic etching. 
     Thereafter, sacrificial oxidation or chemical dry etching is implemented for removal of the etching damage layer as needed. The etching damage layer is formed in a state where the surface is rough by etching for forming the second concave portion  19 . The surface roughness can be reduced by removal of the etching damage layer. Specifically, the etching damage layer can be easily removed by the sacrificial oxidation, and the etching damage layer can be removed in a shorter time by the chemical dry etching. 
     In a process illustrated in  FIG. 4B , a mask not shown that covers the cell region R 1  of the semiconductor substrate  5  is arranged, and etching is conducted in the electric field relaxation region R 2  and the outer peripheral region R 3  as deep as the thickness of the n +  type layer  4  can be removed. For example, anisotropic etching is conducted in a mixed gas atmosphere of SF 6 , O 2 , and Ar. With the above process, the first concave portion  18  is formed in the electric field relaxation region R 2 , the surface of the p +  type layer  3  is exposed, the second concave portion  19  is made deeper in the outer peripheral region R 3  whereby the step of the second mesa portion is displaced to a deeper position, and the n −  type drift layer  2  is exposed. In addition, the boundary portion between the cell region R 1  and the electric field relaxation region R 2  forms the stepped portion as the first mesa portion. 
     In the conventional art, because the process illustrated in  FIG. 4A  is conducted after the process illustrated in  FIG. 4B , there is no reference for grasping an etching depth at the time of etching for forming the first concave portion  18 . However, as in the present embodiment, the process illustrated in  FIG. 4B  is implemented after the process illustrated in  FIG. 4A  so that the second concave portion  19  has already been formed at the time of etching for forming the first concave portion  18 . For that reason, when the process of  FIG. 4A  is conducted, the pn junction between the p +  type layer  3  and the n +  type layer  4  is present in the stepped portion of the boundary portion between the electric field relaxation region R 2  and the outer peripheral region R 3 . The thickness of the n +  type layer  4  can be detected by observing that portion through SEM. Therefore, in the process illustrated in  FIG. 4B , because the etching depth can be accurately controlled, the p +  type layer  3  can be prevented from being too thinned, or eliminated. 
     In particular, if the etching damage layer is removed to reduce the surface roughness after the process illustrated in  FIG. 4A  described above, since the surface state becomes excellent, the pn junction can be easily evaluated by the SEM observation. 
     In a process illustrated in  FIG. 4C , a mask that opens in a region of the surface of the semiconductor substrate  5  in which the p −  type resurf layer  14  is to be formed is arranged, and p type impurities are ion-implanted. If the stepped portion of the boundary portion between the electric field relaxation region R 2  and the outer peripheral region R 3  is tapered as described above, the p type impurities can be also implanted into the stepped portion by only ion implantation from a direction perpendicular to the substrate. Even if the stepped portion of the boundary portion between the electric field relaxation region R 2  and the outer peripheral region R 3  is perpendicular to the surface of the semiconductor substrate  5 , if the p type impurities are obliquely ion-implanted, the p type impurities can be implanted even in the stepped portion. 
     Subsequently, after the mask used previously has been removed, a mask that opens in a region where the n +  type layer  16  is to be formed is arranged, and n type impurities are ion-implanted. In addition, a heat treatment is conducted to activate the implanted ions to form the p type resurf layer  14  and the n +  type layer  16 . 
     Thereafter, although not shown, after the interlayer insulating film  10  has been formed, the contact holes  10   a  to  10   d  are formed by patterning. Also, after metal films made of Al that enables an ohmic contact with the p type SiC, and Ni that enables an ohmic contact with the n type SiC have been formed, the metal films are patterned to form the gate electrode  11 , the source electrode  12 , the surge drawing electrode  15 , and the equipotential ring electrode  17 . In addition, the SiC semiconductor device according to the present embodiment is completed through a process of forming the drain electrode  13 . 
     As described above, in the present embodiment, the first concave portion  18  configuring the first mesa portion is formed after the second concave portion  19  configuring the second mesa portion has been formed. As a result, the thickness of the n +  type layer  4  can be detected by the SEM observation on the basis of the pn junction between the p +  type layer  3  and the n +  type layer  4  in the stepped portion of the boundary portion between the electric field relaxation region R 2  and the outer peripheral region R 3  due to the second concave portion  19  before the selective etching for forming the first concave portion  18 . Therefore, the etching can be precisely conducted by the degree of thickness of the n +  type layer  4  when forming the first concave portion  18 , and the etching depth can be precisely controlled. For that reason, the p +  type layer  3  for forming the first gate region  3   a  can be prevented from being unnecessarily thinned, or eliminated. 
     Also, after the second mesa portion has been formed by the formation of the second concave portion  19 , the first mesa portion is formed with the formation of the first concave portion  18 . For that reason, corners on the stepped portion of the second concave portion  19  are rounded by etching for forming the first concave portion  18 . For that reason, the electric field concentration on the stepped portion can be reduced, and the interlayer insulating film  10  formed on the stepped portion can be effectively inhibited from being broken down by the electric field concentration. 
     (Second Embodiment) 
     A second embodiment of the present disclosure will be described. In the present embodiment, the configuration of the leading ends of the trench  7   a  in the first embodiment is changed, and other configurations are identical with those in the first embodiment, and therefore only parts different from those in the first embodiment will be described. 
     In the present embodiment, in the SiC semiconductor device according to the first embodiment, the first concave portion  18  is formed to remove the n +  type source region  4   a  in the vicinity of both the leading ends of the trench  7   a  for forming the JFET. That is, the trench  7   a  illustrated in  FIG. 2  is laid out in a strip with a direction perpendicular to the paper plane as the longitudinal direction, and the first concave portion  18  extends to an inside of the trench  7   a  on positions of both the leading ends. With the above structure, the JFET structure is not formed on the leading ends of the trench  7   a.    
     The trench  7   a  is laid out in the strip as described above, but the thickness of the n −  type layer  8  becomes thicker than the side wall surface configuring long sides of the trench in both of the leading ends, due to migration at the time of epitaxial growth. For that reason, thresholds of other portions and the JFET are varied on both of the leading ends of the trench  7   a , the leakage of a drain voltage to the surface occurs when the gate voltage comes closer to the threshold at the time of driving the JFET, resulting in such a problem that an excessive drain current flows to reduce the element breakdown voltage. For that reason, as in the present embodiment, the n +  type source region  4   a  is removed on both of the leading ends of the trench  7   a  so that the JFET structure is not formed on those portions. With the above structure, because the threshold variation can be prevented, the leakage of the drain voltage to the surface can be prevented, and a reduction in the element breakdown voltage caused when the excessive drain current flows can be prevented from being generated. 
     Also, in this structure, as in the first embodiment, the second concave portion  19  is formed before the formation of the first concave portion  18  for removing the n +  type source region  4   a  on both of the leading ends of the trench  7   a , as a result of which the thickness of the n +  type source region  4   a  can be grasped by the SEM observation. For that reason, the etching can be precisely conducted by the degree of thickness of the n +  type source region  4   a  when forming the first concave portion  18 , and the etching depth can be precisely controlled. Hence, the same advantages as those in the first embodiment can be obtained. 
     (Other Embodiments) 
     In the above respective embodiments, a case in which the first gate region  3   a  and the second gate region  9   a  have the same potential has been described. Alternatively, those regions may have potentials different from each other such that the first gate region  3   a  has a first potential, and the second gate region  9   a  has a second potential. In this case, the first potential for controlling the first gate region  3   a , and the second potential for controlling the second gate region  9   a  may change to potentials independent from each other, or only any one potential can be controlled, and the other potential may be set to GND (source potential). For example, only the first potential for controlling the first gate region  3   a  may change whereas the second potential to be applied to the second gate region  9   a  may be fixed to GND. 
     In the above respective embodiments, a structure in which the equipotential ring electrode  19  is arranged in the outer peripheral region R 3  has been described as an example. Alternatively, a p type guard ring may be provided. That is, any of various structures known as an outer peripheral high breakdown voltage structure may be formed in the outer peripheral region R 3 . 
     Further, in the above embodiments, the epitaxially grown n +  type source region  4  has been described. Alternatively, the n +  type source region  4  may be formed by ion implantation of n type impurities into the first gate region  3 . Also, in this case, when the ion implantation is conducted so that the n +  type source region  4  extends to both of the leading ends of the trench  6 , the first concave portion  18  extends to both of the leading ends of the trench  7   a , thereby being capable of obtaining the same advantages as those in the second embodiment. 
     In the above respective embodiments, the JFET of the n-channel type in which the channel region is set in the n −  type channel layer  8   a  has been described as an example. Alternatively, the present disclosure can be applied to the JFET of a p-channel type in which the conductivity type of the respective components is reversed.