Abstract:
The trench has, in a cross-sectional view, a first corner portion which is an intersection between a first sidewall surface and a bottom portion and a second corner portion which is an intersection between a second sidewall surface and the bottom portion. A first layer has a second-conductivity-type region. In a cross-sectional view, the second-conductivity-type region is arranged to intersect with a line which passes through any of the first corner portion and the second corner portion and is in parallel to a &lt;0001&gt; direction of a silicon carbide crystal forming the silicon carbide layer. A ratio calculated by dividing SP by ST is not lower than 20% and not higher than 130%, where ST represents a total area of the trenches in a boundary surface between the first layer and a second layer and SP represents a total area of the second-conductivity-type regions in a plan view.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method for manufacturing a silicon carbide semiconductor device, and in particular to a method for manufacturing a silicon carbide semiconductor device including the step of performing screening of chips. 
       BACKGROUND ART 
       [0002]    In recent years, in order to achieve high breakdown voltage, low loss, and utilization of semiconductor devices under a high temperature environment, silicon carbide has begun to be adopted as a material for a semiconductor device. Silicon carbide is a wide band gap semiconductor having a band gap larger than that of silicon, which has been conventionally widely used as a material for semiconductor devices. Hence, by adopting silicon carbide as a material for a semiconductor device, the semiconductor device can have a high breakdown voltage, reduced on-resistance, and the like. Further, the semiconductor device thus adopting silicon carbide as its material has characteristics less deteriorated even under a high temperature environment than those of a semiconductor device adopting silicon as its material, advantageously. 
         [0003]    Among defects that may occur in silicon carbide substrates, micropipes are problematic in particular. For example, “Reliability consideration for recent Infineon SiC diode releases” by M. Holz and three others, Microelectronics Reliability, No. 47, Aug. 21, 2007, pp. 1741 to 1745 (NPD 1) describes a method for detecting micropipes. According to the document, micropipes are detected by applying an avalanche breakdown voltage to a device and checking a change in leakage current before and after the application of the voltage. 
       CITATION LIST 
     Non Patent Document   
       [0004]    NPD 1: “Reliability consideration for recent Infineon SiC diode releases” by M. Holz and three others, Microelectronics Reliability, No. 47, Aug. 21, 2007, pp. 1741 to 1745 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    When an epitaxial layer is formed on a silicon carbide substrate having micropipes, the micropipes are blocked by the epitaxial layer. In the present specification, a micropipe blocked by an epitaxial layer is called a blocked micropipe. In the stage of an ordinary inspection before shipment, a device having a blocked micropipe exhibits characteristics not inferior to those of a device having no blocked micropipe. However, the device having a blocked micropipe may have an increased leakage current after being used for two or three months, and thus it is desirable to screen out such a device by the inspection before shipment. 
         [0006]    In the method described in the above document, however, when a micropipe is present in a guard ring termination portion, no current flows through the guard ring termination portion even if an avalanche breakdown voltage is applied to a device, and thus there is no increase in leakage current based on the comparison between before and after the application of the voltage. Accordingly, when a micropipe is present in a guard ring portion, the micropipe cannot be detected by the method described in the above document, and thus the accuracy of detecting micropipes is not sufficient. As a result, there may be cases where chips including micropipes are shipped without being screened out. 
         [0007]    The present invention has been made to solve such a problem, and an object of the present invention is to provide a method for manufacturing a silicon carbide semiconductor device by which chips including micropipes can be screened out with high accuracy. 
       Solution to Problem 
       [0008]    A method for manufacturing a silicon carbide semiconductor device in accordance with the present invention includes the steps of: preparing a silicon carbide substrate having a first main surface and a second main surface opposite to the first main surface; causing etch pits including micropipes to appear in the first main surface by etching the first main surface; obtaining two-dimensional positional information on the micropipes in the first main surface; cutting the silicon carbide substrate into a plurality of chips; and performing screening of the chips based on the two-dimensional positional information, wherein the first main surface is a silicon plane or a plane having an off angle of less than or equal to 10° relative to the silicon plane. 
       Advantageous Effects of Invention 
       [0009]    As is clear from the above description, the present invention can provide a method for manufacturing a silicon carbide semiconductor device by which chips including micropipes can he screened out with high accuracy. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is a schematic cross sectional view schematically showing a structure of a silicon carbide semiconductor device in a first embodiment of the present invention. 
           [0011]      FIG. 2  is a schematic perspective view schematically showing a shape of a silicon carbide substrate of the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0012]      FIG. 3  is a flowchart schematically showing a method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0013]      FIG. 4  is a flowchart schematically showing a method for manufacturing a silicon carbide semiconductor device in a second embodiment of the present invention. 
           [0014]      FIG. 5  is a schematic cross sectional view schematically showing a first step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0015]      FIG. 6  is a schematic cross sectional view schematically showing a second step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0016]      FIG. 7  is a schematic cross sectional view schematically showing a fourth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0017]      FIG. 8  is a schematic cross sectional view schematically showing a fifth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0018]      FIG. 9  is a schematic cross sectional view schematically showing a sixth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0019]      FIG. 10  is a schematic cross sectional view schematically showing a seventh step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0020]      FIG. 11  is a schematic cross sectional view schematically showing an eighth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0021]      FIG. 12  is a schematic cross sectional view schematically showing a ninth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0022]      FIG. 13  is a schematic cross sectional view schematically showing a tenth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0023]      FIG. 14  is a schematic cross sectional view schematically showing an eleventh step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0024]      FIG. 15  is a schematic cross sectional view schematically showing a twelfth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0025]      FIG. 16  is a schematic cross sectional view schematically showing a thirteenth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0026]      FIG. 17  is a schematic cross sectional view schematically showing a fourteenth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0027]      FIG. 18  is a schematic cross sectional view schematically showing a fifteenth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0028]      FIG. 19  is a schematic cross sectional view schematically showing a sixteenth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0029]      FIG. 20  is a schematic plan view schematically showing a third step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0030]      FIG. 21  is a schematic plan view schematically showing a seventeenth step of the method for manufacturing the silicon carbide semiconductor device in the first embodiment of the present invention. 
           [0031]      FIG. 22  is a schematic plan view schematically showing the step of forming a cutting position pattern in the method for manufacturing the silicon carbide semiconductor device in the second embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0032]    Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that, in the below-mentioned drawings, the same or corresponding portions are given the same reference characters and are not described repeatedly. Further, in the crystallographic description in the present specification, an individual orientation is represented by [ ], a group orientation is represented &lt; &gt;, by an individual plane is represented by ( ), and a group plane is represented by { }. In addition, a negative crystallographic index is normally expressed by putting “−” (bar) above a numeral, but is expressed by putting the negative sign before the numeral in the present specification. 
         [0033]    First, an overview of the embodiments of the present invention will be described in (1) to (8) below. 
         [0034]    (1) A method for manufacturing a silicon carbide semiconductor device  1  in accordance with the present embodiment includes the steps of preparing a silicon carbide substrate  80  having a first main surface  80   b  and a second main surface  80   a  opposite to first main surface  80   b;  causing etch pits  3   a  including micropipes to appear in first main surface  80   b  by etching first main surface  80   b;  obtaining two-dimensional positional information on the micropipes in first main surface  80   b;  cutting the silicon carbide substrate into a plurality of chips C 12  to C 65 ; and performing screening of chips C 12  to C 65  based on the two-dimensional positional information, wherein first main surface  80   b  is a silicon plane or a plane having an off angle of less than or equal to 10° relative to the silicon plane. 
         [0035]    According to silicon carbide semiconductor device  1  in accordance with the present embodiment, the two-dimensional positional information on the micropipes in first main surface  80   b  is obtained, and screening of the chips is performed based on the two-dimensional positional information. Accordingly, no matter where a micropipe is present in first main surface  80   b  of silicon carbide substrate  80 , a chip including the micropipe can be detected. As a result, chips including micropipes can be screened out with high accuracy. 
         [0036]    (2) Preferably, in the method for manufacturing silicon carbide semiconductor device  1  in accordance with the present embodiment, a silicon carbide epitaxial layer  81  is formed in contact with second main surface  80   a.  Thereby, even if a micropipe is covered with silicon carbide epitaxial layer  81  and becomes a blocked micropipe, a chip including the micropipe can be screened out with high accuracy. 
         [0037]    (3) Preferably, in the method for manufacturing silicon carbide semiconductor device  1  in accordance with the present embodiment, a pattern  2  indicating a cutting position for chips C 12  to C 65  is formed on a front surface  10   a  of silicon carbide epitaxial layer  81 . In the step of cutting silicon carbide substrate  80  into the plurality of chips C 12  to C 65 , silicon carbide substrate  80  is cut along pattern  2 . By forming the pattern indicating the cutting position, a chip in which a micropipe is present can be specified by a simple method. 
         [0038]    (4) Preferably, in the method for manufacturing silicon carbide semiconductor device  1  in accordance with the present embodiment, the step of performing screening of chips C 12  to C 65  is performed by comparing the two-dimensional positional information on the micropipes with a position of pattern  2 . In a case where silicon carbide substrate  80  has small warpage, a chip in which a micropipe is present can be specified by a simple method and with high accuracy, by comparing the two-dimensional positional information on the micropipes in first main surface  80   b  with the position of pattern  2  formed on front surface  10   a , which is on a side closer to second main surface  80   a.    
         [0039]    (5) Preferably, in the method for manufacturing silicon carbide semiconductor device  1  in accordance with the present embodiment, the two-dimensional positional information is associated with identification numbers of chips C 12  to C 65 . Thereby, a chip in which a micropipe is present can be specified. 
         [0040]    (6) Preferably, in the method for manufacturing silicon carbide semiconductor device  1  in accordance with the present embodiment, first main surface  80   b  is polished after the step of causing etch pits  3  including the micropipes to appear. Thereby, warpage of silicon carbide substrate  80  generated in the step of causing the etch pits to appear can be reduced. Further, when first main surface  80   b  is polished after the step of forming silicon carbide epitaxial layer  81  in contact with second main surface  80   a , both warpage of silicon carbide substrate  80  generated by the step of forming silicon carbide epitaxial layer  81  and warpage of silicon carbide substrate  80  generated in the step of causing the etch pits to appear can be reduced. 
         [0041]    (7) Preferably, in the method for manufacturing silicon carbide semiconductor device  1  in accordance with the present embodiment, first main surface  80   b  is ground to remove at least portions of etch pits  3 . By removing small etch pits  3   b  other than etch pits  3   a  of micropipes, defects in a conforming chip can be removed. Further, when etch pits  3   a  of micropipes are also removed, first main surface  80   b  has small unevenness. Thus, planarization of an electrode  98  formed in contact with first main surface  80   b  can be improved. 
         [0042]    (8) Preferably, in the method for manufacturing silicon carbide semiconductor device  1  in accordance with the present embodiment, electrode  98  is formed in contact with first main surface  80   b  after the step of grinding first main surface  80   b.  Thereby, adhesion of electrode  98  to first main surface  80   b  can be improved. 
         [0043]    Next, the embodiments of the present invention will be described in more detail. 
       First Embodiment 
       [0044]    Referring to  FIGS. 1 and 2 , a structure of an MOSFET  1  as a silicon carbide semiconductor device in accordance with the present embodiment will be described. 
         [0045]    MOSFET  1  of the present embodiment mainly has a silicon carbide substrate  10 , a gate insulating film  91 , a gate electrode  92 , an interlayer insulating film  93 , a source electrode  94 , a source interconnection layer  95 , and drain electrode  98  (back surface electrode). Silicon carbide substrate  10  has, for example, silicon carbide substrate  80 , an n type drift region  81  (epitaxial layer), a p type base region  82 , an n type region  83 , and a p type contact region  84 . 
         [0046]    Silicon carbide substrate  80  is made of, for example, hexagonal silicon carbide, and has a polytype of 4H. Silicon carbide substrate  80  has, for example, n type (a first conductivity type). N type drift region  81  is an epitaxial layer formed on silicon carbide substrate  80 . N type drift region  81  has n type. Preferably, the impurity concentration in a type drift region  81  is lower than the impurity concentration in silicon carbide substrate  80 . The donor concentration in n type drift region  81  is preferably more than or equal to 1×10 15  cm −3  and less than or equal to 5×10 16  cm −3 , and is 8×10 15  cm −3 , for example. 
         [0047]    P type base region  82  has p type (a second conductivity type). P type base region  82  is provided on n type drift region  81 , The impurity concentration in p type base region  82  is 1×10 18  cm −3 , for example. N type region  83  has n type. N type region  83  is provided on p type base region  82  to be separated from n type drift region  81  by p type base region  82 , P type contact region  84  has p type. P type contact region  84  is connected to source electrode  94  and p type base region  82 . 
         [0048]    A trench TR is provided in front surface  10   a  of silicon carbide substrate  10 . 
         [0049]    Trench TR has a wall surface SW and a bottom portion BT. Wall surface SW penetrates n type region  83  and p type base region  82 , and reaches n type drift region  81 . On p type base region  82 , wall surface SW includes a channel surface of MOSFET  1 . 
         [0050]    Wall surface SW is inclined with respect to front surface  10   a  of silicon carbide substrate  10 , and trench TR spreads in a tapered manner toward an opening. Preferably, the plane orientation of wall surface SW is inclined with respect to a (000-1) plane by more than or equal to 50° and less than or equal to 65°. Bottom portion BT is located on n type drift region  81 , In the present embodiment, bottom portion BT is a surface which is substantially parallel to front surface  10   a  of silicon carbide substrate  10 . 
         [0051]    Gate insulating film  91  covers each of wall surface SW and bottom portion BT of trench TR. Gate electrode  92  is provided on gate insulating film  91 . Source electrode  94  is in contact with each of n type region  83  and p type contact region  84 . Source interconnection layer  95  is in contact with source electrode  94 . Source interconnection layer  95  is an aluminum layer, for example. Interlayer insulating film  93  insulates gate electrode  92  from source interconnection layer  95 . Drain electrode  98  (back surface electrode) is arranged in contact with silicon carbide substrate  80 . 
         [0052]    Next, a method for manufacturing MOSFET  1  in accordance with the first embodiment will be described with reference to  FIG. 3 . 
         [0053]    First, the step of preparing a silicon carbide substrate (S 10 :  FIG. 3 ) is performed. Silicon carbide substrate  80  ( FIG. 5 ) having n type conductivity type (the first conductivity type), and having first main surface  80   b  and second main surface  80   a  on an opposite side is prepared by slicing an ingot made of for example, hexagonal silicon carbide having a polytype of 4H formed by a sublimation method. It should be noted that first main surface  80   b  is a silicon plane or a plane having an off angle of less than or equal to 10° relative to the silicon plane, and second main surface  80   a  is a carbon plane or a plane having an off angle of less than or equal to 10° relative to the carbon plane. 
         [0054]    Next, the step of causing etch pits to appear (S 20 :  FIG. 3 ) is performed. In the step of causing etch pits to appear, etch pits  3  including etch pits  3   a  of micropipes appear in first main surface  80   b  of silicon carbide substrate  80 , by etching first main surface  80   b.  Referring to  FIG. 6 , etch pits  3  are formed as a result of spreading, by etching, of crystal defects such as screw dislocations (threading screw dislocations), edge dislocations (threading edge dislocations), composite dislocations, basal plane dislocations, and micropipes. That is, etch pits  3  include etch pits  3   a  of micropipes and etch pits  3   b  due to defects other than micropipes. Etch pits  3   b  due to defects other than micropipes include, for example, etch pits of screw dislocations (threading screw dislocations), etch pits of edge dislocations (threading edge dislocations), etch pits of composite dislocations (threading composite dislocations), etch pits of basal plane dislocations, and the like. 
         [0055]    The etching for causing the above etch pits to appear may be dry etching or wet etching. As the dry etching, for example, gas etching may be used. For the gas etching, nitrogen gas, chlorine gas, and oxygen gas may be used. Specifically, for example, silicon carbide substrate  80  having first main surface  80   b  is placed within a chamber, nitrogen is introduced into the chamber, and the pressure is set to 50 Pa and the temperature is set to 1050° C. within the chamber. Next, for example, chlorine gas is caused to flow through the chamber at a flow velocity of 0.2 slm for 45 minutes. Next, for example, a mixed gas of nitrogen and oxygen (nitrogen: 90%, oxygen: 10%) is caused to flow through the chamber at a flow velocity of 3 slm for five minutes. It should be noted that, after the chlorine gas is introduced into the chamber, the pressure within the chamber increases to 90000 Pa. When the mixed gas of nitrogen and oxygen is introduced, the pressure within the chamber is 50000 Pa, and thereafter increases to 90000 Pa. In addition, as the wet etching, KOH (potassium hydroxide) etching may be performed. Specifically, silicon carbide substrate  10  having first main surface  80   b  is immersed, for example, in molten KOH at 515° C., for eight minutes. Next, silicon carbide substrate  10  having first main surface  80   b  is washed with pure water. Hence, etch pits  3   a  of micropipes and etch pits  3   b  other than micropipes appear in first main surface  80   b.    
         [0056]    Next, the step of obtaining positional information on the micropipes (S 30 :  FIG. 3 ) is performed. In the step of obtaining positional information on the micropipes, two-dimensional positional information on etch pits  3   a  of micropipes in first main surface  80   b  of silicon carbide substrate  80  is obtained. Specifically, referring to  FIG. 20 , etch pits  3   a  of micropipes which have appeared in first main surface  80   b  of silicon carbide substrate  80  are optically observed using, for example, an optical microscope or the like. Observation of etch pits  3   a  of micropipes may be performed by placing the optical microscope on the first main surface  80   b  side of silicon carbide substrate  80  and observing first main surface  80   b  from the first main surface  80   b  side, or may be performed by placing the optical microscope on the second main surface  80   a  side of silicon carbide substrate  80  and observing first main surface  80   b  from the second main surface  80   a  side through silicon carbide substrate  10 . 
         [0057]    By analyzing an image observed by the optical microscope, two-dimensional positions of etch pits  3   a  of micropipes in first main surface  80   b  of silicon carbide substrate  80  are specified. Preferably, an image of entire first main surface  80   b  is obtained by the optical microscope, and two-dimensional positions of all etch pits  3   a  of micropipes present in first main surface  80   b  are specified. For example, a line which is parallel to an orientation flat  80   c  of silicon carbide substrate  80  and is located in first main surface  80   b  is defined as the x axis, and a line which is perpendicular to the x axis and is located in first main surface  80   b  is defined as the y axis. For example, in a case where the x axis is arranged at a position where first main surface  80   b  has a maximum width in the x axis direction and the y axis is arranged at a position where first main surface  80   b  has a maximum width in the y axis direction, the position where the x axis and the y axis intersect with each other is defined as the origin of coordinates (that is, x=0, y=0). Two-dimensional positional information on a micropipe in first main surface  80   b  is, for example, coordinates of a position of the center of etch pit  3   a  of the micropipe when first main surface  80   b  is considered as an xy coordinate system (for example, x=x 1 , y= 1 ). The two-dimensional positional information on the micropipes may be temporarily saved in a memory. In addition, whether an etch pit is etch pit  3   a  of a micropipe or etch pit  3   b  other than etch pit  3   a  of a micropipe may be determined, for example, as described below. For example, an etch pit of a size larger than a certain size may be determined as etch pit  3   a  of a micropipe. Alternatively, an etch pit larger than other etch pits  3   b  when compared with other etch pits  3   b  may be determined as etch pit  3   a  of a micropipe. 
         [0058]    Next, the step of polishing the second main surface (S 40 :  FIG. 3 ) is performed. Specifically, for example by chemical mechanical polishing using an abrasive material such as colloidal silica, second main surface  80   a  of silicon carbide substrate  80  is polished, and second main surface  80   a  is planarized to the extent that allows epitaxial growth. 
         [0059]    Next, the step of polishing the first main surface (S 45 :  FIG. 3 ) is performed. Specifically, for example by chemical mechanical polishing using an abrasive material such as colloidal silica, first main surface  80   b  of silicon carbide substrate  80  is polished, and first main surface  80   b  is planarized. It should be noted that the step of polishing the first main surface may be performed after the step of forming an epitaxial layer (S 50 :  FIG. 3 ) described later. Further, first main surface  80   b  and second main surface  80   a  may be polished simultaneously. 
         [0060]    Next, the step of forming an epitaxial layer (S 50 :  FIG. 3 ) is performed. Specifically, referring to  FIG. 7 , n type drift region  81  made of silicon carbide is formed on second main surface  80   a  of silicon carbide substrate  80  made of silicon carbide. Formation of n type drift region  81  can be performed, for example, by a CVD (Chemical Vapor Deposition) method. For example, nitrogen (N) or phosphorus (P) is introduced as an impurity into n type drift region  81 . P type base region  82  and n type region  83  are to be formed on n type drift region  81 . 
         [0061]    Next, the step of implanting ions (S 60 :  FIG. 3 ) is performed. Referring to  FIG. 8 , the step of implanting ions can be performed, for example, by performing ion implantation on the entire surface of n type drift region  81 . For forming p type base region  82 , ions of an impurity for imparting p type, for example such as aluminum (Al), are implanted into front surface  10   a  of n type drift region  81 . Further, in the ion implantation for forming n type region  83 , ions of an impurity for imparting n type, for example such as phosphorus (P), are implanted, It should be noted that, instead of ion implantation, epitaxial growth accompanied by addition of impurities may be used. Referring to  FIG. 9 , a resist film  60  is formed on n type region  83  of silicon carbide substrate  10 , Next, exposure and development are performed on resist film  60 . Thereby, a mask layer  61  ( FIG. 10 ) having an opening corresponding to a position where p type contact region  84  is to be formed is formed. Next, p type contact region  84  is formed by ion implantation using mask layer  61 . Next, mask layer  61  is removed ( FIG. 11 ). Thus, p type contact region  84  which connects front surface  10   a  of silicon carbide substrate  10  and p type base region  82  is formed by a photolithography method. 
         [0062]    Next, heat treatment is performed to activate the impurities. This heat treatment is preferably performed at a temperature of more than or equal to 1500° C. and less than or equal to 1900° C., and is performed at approximately 1700° C., for example. The heat treatment is performed for approximately 30 minutes, for example. The atmosphere for the heat treatment is preferably an inert gas atmosphere, and is an Ar atmosphere, for example. 
         [0063]    Referring to  FIG. 12 , a mask layer  40  having an opening is formed on a surface made of a type region  83  and p type contact region  84 , by the photolithography method. As mask layer  40 , for example, a silicon oxide film or the like can be used. The opening is formed corresponding to a position where trench TR ( FIG. 1 ) is to be formed. 
         [0064]    Next, the step of forming a recessed portion is performed. Specifically, referring to  FIG. 13 , a recessed portion TQ is formed in front surface  10   a  of silicon carbide substrate  10 , by performing plasma etching on silicon carbide substrate  10  having mask layer  40  formed thereon. Recessed portion TQ is formed by removing n type region  83 , p type base region  82 , and a portion of n type drill region  81  of silicon carbide substrate  10  by etching, through the opening in mask layer  40 . As a method for the etching, for example, dry etching, and more specifically, inductively-coupled-plasma reactive ion etching (ICP-RIE) can be used. By performing ICP-RIE on front surface  10   a  of silicon carbide substrate  10  using SF 6  or a mixed gas of SF 6  and O 2  as a reactive gas, for example, recessed portion TQ having a wall surface A substantially along a thickness direction of silicon carbide substrate  10  (a vertical direction in the drawing) as well as a bottom portion B is formed in a region where trench TR ( FIG. 1 ) is to be formed. 
         [0065]    Next, a thermal etching step is performed. Specifically, thermal etching is performed on recessed portion TQ formed in silicon carbide substrate  10 . In the thermal etching step, wall surface A of recessed portion TQ in silicon carbide substrate  10  is thermally etched within a furnace, while supplying a gas containing chlorine into the furnace. Silicon carbide substrate  10  is heated within the furnace, for example at more than or equal to 1000° C. and less than or equal to 1800° C. for approximately 20 minutes, and thereby wall surface A of recessed portion TQ in silicon carbide substrate  10  is etched. The temperature for the thermal etching of silicon carbide substrate  10  is preferably more than or equal to 800° C., more preferably more than or equal to 1300° C., and further preferably more than or equal to 1500° C. It should be noted that mask layer  40  made from silicon dioxide is not substantially etched during thermal etching of silicon carbide, because it has an extremely high selectivity with respect to silicon carbide. 
         [0066]    By performing the thermal etching step described above, wall surface A and bottom portion B of recessed portion TQ are etched by approximately more than or equal to 2 nm and 0.1 μm, for example, and thereby trench TR formed of wall surface SW and bottom portion BT is formed on silicon carbide substrate  10 , as shown in  FIG. 14 . Next, mask layer  40  is removed by any method such as etching. Trench TR is formed of wall surface SW serving as a side surface, and bottom portion BT connected with wall surface SW. Bottom portion BT may be a surface, or may be a line. When bottom portion BT is a line, trench TR has a V shape when viewed in a cross section. 
         [0067]    Next, the step of forming a gate insulating film is performed. Specifically, referring to  FIG. 15 , after trench TR is formed by thermally etching wall surface A of recessed portion TO described above, gate insulating film  91  is formed in contact with wail surface SW of trench TR. Gate insulating film  91 , which further covers each of wall surface SW and bottom portion BT of trench TR and is in contact with n type drift region  81 , p type base region  82 , n type region  83 , and p type contact region  84 , is formed. Gate insulating film  91  is made of silicon dioxide, and can be formed, for example, by thermal oxidation. 
         [0068]    After gate insulating film  91  is formed, NO annealing using nitric oxide (NO) as an atmospheric gas may be performed. Specifically, for example, silicon carbide substrate  10  having gate insulating film  91  formed thereon is held in a nitric oxide atmosphere, at a temperature of more than or equal to 1100° C. and less than or equal to 1300° C., for approximately one hour. Thereby, nitrogen atoms are introduced into an interface region between gate insulating film  91  and p type base region  82 . As a result, formation of an interface state in the interface region is suppressed, and thus channel mobility can be improved. It should be noted that a gas other than NO gas may be used as an atmospheric gas, if the gas allows such introduction of nitrogen atoms. 
         [0069]    After the NO annealing, Ar annealing using argon (Ar) as an atmospheric gas may be further performed. Preferably, the heating temperature for the Ar annealing is higher than the heating temperature for the NO annealing, and is lower than the melting point of gate insulating film  91 . This heating temperature is held for approximately one hour, for example. Thereby, formation of the interface state in the interface region between gate insulating film  91  and p type base region  82  is further suppressed. It should be noted that, as an atmospheric gas, other inert gas such as nitrogen gas may be used instead of Ar gas. 
         [0070]    Next, the step of forming a front surface electrode (S 70 :  FIG. 3 ) is performed. In the step of forming a front surface electrode, gate electrode  92  and source electrode  94  are formed. Specifically, referring to  FIG. 16 , gate electrode  92  is formed on gate insulating film  91 . Specifically, gate electrode  92  is formed on gate insulating film  91  to fill a region inside trench TR, with gate insulating film  91  interposed therebetween. Gate electrode  92  can be formed, for example, by formation of a film of a conductor or doped polysilicon and CMP. 
         [0071]    Next, referring to  FIG. 17 , interlayer insulating film  93  is formed on gate electrode  92  and gate insulating film  91  to cover an exposed surface of gate electrode  92 . Etching is performed such that an opening is formed in interlayer insulating film  93  and gate insulating film  91 . Through this opening, each of n type region  83  and p type contact region  84  is exposed on front surface  10   a.  Next, source electrode  94  which is in contact with each of n type region  83  and p type contact region  84  on front surface  10   a  is formed. Specifically, a metal film containing Ti, Al, and Si, for example, is formed in contact with each of n type region  83  and p type contact region  84 , by sputtering. Next, by heating silicon carbide substrate  10  having the metal film formed thereon, the metal film is alloyed, and thus source electrode  94  which is in ohmic contact with silicon carbide substrate  10  is formed. 
         [0072]    Next, the step of grinding the first main surface (S 80 :  FIG. 3 ) is performed. Specifically, by grinding first main surface  80   b  of silicon carbide substrate  80 , silicon carbide substrate  80  is removed by approximately 200 μm, for example. In the step of grinding first main surface  80   b,  preferably at least portions of etch pits  3  are removed. Referring to  FIG. 18 , first main surface  80   b  may be ground such that portions of etch pits  3   a  of micropipes are left and etch pits  3   b  other than micropipes are all removed. A region having etch pit  3   b  of a micropipe is a chip region which will be determined as defective by the step of performing screening of chips described later, and will be discarded, Accordingly, a portion of etch pit  3   a  of a micropipe may be left. On the other hand, etch pit  3   b  other than a micropipe is a chip region which will be determined as conforming by the step of performing screening of chips described later. Accordingly, by removing etch pits  3   b  other than micropipes, defect density in first main surface  80   b  can be reduced. 
         [0073]    Both etch pits  3   a  of micropipes and etch pits  3   b  other than micropipes may be removed, as shown in  FIG. 19 . If etch pits  3   a  of micropipes are left even partially, when back surface electrode  98  is formed on first main surface  80   b  in the step of forming a back surface electrode (S 90 :  FIG. 3 ) described later, back surface electrode  98  is formed such that it also enters the inside of etch pits  3   a  of micropipes. Accordingly, a front surface of back surface electrode  98  has large unevenness. When the front surface of back surface electrode  98  in the chip region which will be determined as defective in the step of performing screening of chips (S 110 :  FIG. 3 ) described later has large unevenness, the front surface of back surface electrode  98  in the chip region which will be determined as conforming also has large unevenness. By removing etch pits  3   a  of micropipes as shown in  FIG. 19 , planarization of first main surface  80   b  can be improved. Accordingly, planarization of back surface electrode  98  formed on first main surface  80   b  is improved. 
         [0074]    Next, the step of forming a back surface electrode (S 90 :  FIG. 3 ) is performed. Specifically, back surface electrode  98  is formed on first main surface  80   b  of silicon carbide substrate  80 . The material used for back surface electrode  98  may be the same as the material for the source electrode described above. 
         [0075]    Next, the step of cutting the silicon carbide substrate (S 100 :  FIG. 3 ) is performed. Referring to  FIG. 21 , silicon carbide substrate  80  is cut, for example, with a dicing saw, to form the plurality of chips C 12  to C 65 . That is, silicon carbide substrate  10  is cut into the plurality of chips C 12  to C 65 , The plurality of chips C 12  to C 65  include chips C 25  and C 43  each having etch pit  3   a  of a micropipe, and chips C 23 , C 32 , and C 55  each having etch pit  3   b  other than a micropipe. 
         [0076]    Next, the step of performing screening of the chips (S 110 :  FIG. 3 ) is performed. In the step of performing screening of the chips, screening of the chips is performed based on the two-dimensional positional information on the micropipes obtained in the step of obtaining positional information on the micropipes (S 30 :  FIG. 3 ). The step of performing screening of the chips has the step of associating the two-dimensional positional information on etch pits  3   a  of micropipes with identification numbers of the chips. For example, it is determined in which cut chip having which identification number the position of the center of etch pit  3   a  of a micropipe is located. Referring to  FIG. 20 , for example, it is determined in which region of first main surface  80   b  the position of the center of etch pit  3   a  of a micropipe is located. It is determined with which chip having which identification number a region of silicon carbide substrate  10  located from x 3  to x 4  in the x direction and from y 3  to y 4  in the y direction is associated, after silicon carbide substrate  80  is cut. When the position of the center of etch pit  3   a  of a micropipe is (x 1 , y 1 ), the chip including that etch pit  3   a  is determined as chip C 25 . Similarly, when the position of the center of etch pit  3   a  of a micropipe is (x 2 , y 2 ), the chip including that etch pit  3   a  is determined as chip C 43 . in the step of performing screening of the chips, a chip including etch pit  3   a  of a micropipe is determined as a defective product, and a chip not including etch pit  3   a  of a micropipe is determined as a conforming product. 
         [0077]    Next, the function and effect of the method for manufacturing MOSFET  1  in accordance with the first embodiment will be described. 
         [0078]    According to MOSFET  1  in accordance with the first embodiment, the two-dimensional positional information on etch pits  3   a  of micropipes in First main surface  80   b  is obtained, and screening of the chips is performed based on the two-dimensional positional information. Accordingly, no matter where a micropipe is present in first main surface  80   b  of silicon carbide substrate  80 , a chip including the micropipe can be detected. As a result, chips including micropipes can be screened out with high accuracy. 
         [0079]    Further, according to the method for manufacturing MOSFET  1  in accordance with the first embodiment, silicon carbide epitaxial layer  81  is formed in contact with second main surface  80   a.  Thereby, even if a micropipe is covered with silicon carbide epitaxial layer  81  and becomes a blocked micropipe, a chip including the micropipe can be screened out with high accuracy. 
         [0080]    Furthermore, according to the method for manufacturing MOSFET  1  in accordance with the first embodiment, the two-dimensional positional information is associated with identification numbers of chips C 12  to C 65 . Thereby, a chip in which a micropipe is present can be specified. 
         [0081]    Furthermore, according to the method for manufacturing MOSFET  1  in accordance with the first embodiment, first main surface  80   b  is polished after the step of causing etch pits  3  including the micropipes to appear. Thereby, warpage of silicon carbide substrate  80  generated in the step of causing the etch pits to appear can be reduced. Further, when first main surface  80   b  is polished after the step of forming silicon carbide epitaxial layer  81  in contact with second main surface  80   a,  both warpage of silicon carbide substrate  80  generated by the step of forming silicon carbide epitaxial layer  81  and warpage of silicon carbide substrate  80  generated in the step of causing the etch pits to appear can be reduced. 
         [0082]    Furthermore, according to the method for manufacturing MOSFET  1  in accordance with the first embodiment, first main surface  80   b  is ground to remove at least portions of etch pits  3 . By removing small etch pits  3   b  other than etch pits  3   a  of micropipes, defects in a conforming chip can be removed. Further, when etch pits  3   a  of micropipes are also removed, first main surface  80   b  has small unevenness. Thus, planarization of electrode  98  formed in contact with first main surface  80   b  can be improved. As a result, planarization of electrode  98  in a conforming chip can be improved. 
         [0083]    Furthermore, according to the method for manufacturing MOSFET  1  in accordance with the first embodiment, back surface electrode  98  is formed in contact with first main surface  80   b  after the step of grinding first main surface  80   b.  Thereby, adhesion of back surface electrode  98  to first main surface  80   b  can be improved. 
       Second Embodiment 
       [0084]    Next, a method for manufacturing MOSFET  1  in accordance with a second embodiment will be described with reference to  FIG. 4 . It should be noted that MOSFET  1  manufactured by the manufacturing method in accordance with the second embodiment has the same structure as that in the first embodiment. 
         [0085]    First, the step of preparing a silicon carbide substrate (S 10 :  FIG. 4 ) is performed, by the same method as that described in the first embodiment. Thereby, the silicon carbide substrate having first main surface  80   b  and second main surface  80   a  opposite to first main surface  80   b  is prepared. Next, the step of causing etch pits to appear (S 20 :  FIG. 4 ) is performed, by the same method as that described in the first embodiment. Thereby, etch pits  3  including etch pits  3   a  of micropipes appear in first main surface  80   b.  Next, the step of obtaining positional information on the micropipes (S 30 :  FIG. 4 ) is performed, by the same method as that described in the first embodiment. Thereby, two-dimensional positional information on etch pits  3   a  of micropipes in first main surface  80   b  of silicon carbide substrate  80  is obtained. Next, the step of polishing the second main surface (S 40 :  FIG. 4 ) is performed, by the same method as that described in the first embodiment. Thereby, second main surface  80   a  of silicon carbide substrate  80  is polished. Next, the step of forming an epitaxial layer (S 50 :  FIG. 4 ) is performed, by the same method as that described in the first embodiment. Thereby, silicon carbide epitaxial layer  81  is formed in contact with second main surface  80   a  of silicon carbide substrate  80 . 
         [0086]    Next, the step of forming a cutting position pattern (S 51 :  FIG. 4 ) is performed. Specifically, referring to  FIG. 22 , pattern  2  indicating a cutting position where the silicon carbide substrate is to be cut in the step of cutting the silicon carbide substrate (S 100 :  FIG. 4 ) described later is formed on front surface  10   a  of silicon carbide epitaxial layer  81  formed on silicon carbide substrate  80 . The pattern indicating the cutting position is formed by providing grooves having a linear pattern, for example, in front surface  10   a  of silicon carbide epitaxial layer  81 , for example. Pattern  2  is provided corresponding to the shape of cut chips, and may be the one in which, for example, a plurality of rectangular or square frames are arranged along the x axis direction and the y axis direction, when viewed from the direction of a normal to front surface  10   a  of silicon carbide epitaxial layer  81 . Thereby, outer edges of the cut chips can be clarified. 
         [0087]    Next, the step of observing the micropipes (S 52 :  FIG. 4 ) is performed. For example, an. optical microscope is placed at a position facing second main surface  80   a  of silicon carbide substrate  80 . Next, for example, the optical microscope is focused on first main surface  80   b , and an image of entire first main surface  80   b  is obtained by the optical microscope. Thereby, two-dimensional information on first main surface  80   b  is obtained. The two-dimensional information on first main surface  80   b  includes the two-dimensional positional information on etch pits  3   a  of micropipes in first main surface  80   b.  Two-dimensional positions, in first main surface  80   b,  of etch pits  3   a  of micropipes formed in first main surface  80   b  are specified, for example, based on the image of first main surface  80   b  obtained by the optical microscope. Thereafter, for example, the optical microscope is focused on second main surface  80   a,  and an image of entire second main surface  80   a  is obtained. The two-dimensional positional information on etch pits  3   a  of micropipes in the first main surface is compared with pattern  2  indicating the cutting position formed on second main surface  80   a.  It is specified where etch pits  3   a  of micropipes are present in pattern  2  indicating the cutting position. Information on identification numbers (addresses) of chips each including etch pit  3   a  of a micropipe is recorded in an external memory or the like. It should be noted that the micropipes may be specified by the same method as that described in the step of obtaining positional information on the micropipes (S 30 :  FIG. 3 ). 
         [0088]    Next, the step of implanting ions (S 60 :  FIG. 4 ), the step of forming a front surface electrode (S 70 :  FIG. 4 ), the step of grinding the first main surface (S 80 :  FIG. 4 ), and the step of forming a back surface electrode (S 90 :  FIG. 4 ) are performed by the same methods as those described in the first embodiment. 
         [0089]    Next, the step of cutting the silicon carbide substrate (S 100 :  FIG. 4 ) is performed. Specifically, silicon carbide substrate  80  is cut along pattern  2  indicating the cutting position formed by the step of forming the cutting position pattern (S 51 :  FIG. 4 ) on front surface  10   a  of silicon carbide epitaxial layer  81  formed on silicon carbide substrate  80 . Thereby, silicon carbide substrate  10  is cut into the plurality of chips C 12  to C 65 , as shown in  FIG. 21 . The plurality of chips C 12  to C 65  include chips C 25  and C 43  each having etch pit  3   a  of a micropipe, and chips C 23 , C 32 , and C 55  each having etch pit  3   b  other than a micropipe. 
         [0090]    Next, the step of performing screening of the chips (S 110 :  FIG. 4 ) is performed. Based on the information on the identification numbers (addresses) of the chips each including etch pit  3   a  of a micropipe recorded in the step of observing the micropipes (S 52 :  FIG. 4 ), a chip including etch pit  3   a  of a micropipe is determined as a defective product, and a chip not including etch pit  3   a  of a micropipe is determined as a conforming product. It should be noted that the identification numbers of the chips each including etch pit  3   a  of a micropipe specified as described above is information obtained in the step of observing the micropipes based on the two-dimensional positional information on etch pits  3   a  of micropipes in the first main surface. It should be noted that screening of the chips may be performed based on the two-dimensional positional information on the micropipes obtained in the step of obtaining positional information on the micropipes (S 30 :  FIG. 4 ), or based on information determined from both the information obtained in the step of observing the micropipes (S 52 :  FIG. 4 ) and the information obtained in the step of obtaining positional information on the micropipes (S 30 :  FIG. 4 ). 
         [0091]    Next, the function and effect of the method for manufacturing MOSFET  1  in accordance with the second embodiment will be described. 
         [0092]    According to the method for manufacturing MOSFET  1  in accordance with the second embodiment, pattern  2  indicating the cutting position for chips C 12  to C 65  is formed on front surface  10   a  of silicon carbide epitaxial layer  81 . In the step of cutting silicon carbide substrate  80  into the plurality of chips C 12  to C 65 , silicon carbide substrate  80  is cut along pattern  2 . By forming the pattern indicating the cutting position, a chip in which a micropipe is present can be specified by a simple method. 
         [0093]    Further, according to the method for manufacturing MOSFET  1  in accordance with the second embodiment, the step of performing screening of chips C 12  to C 65  is performed by comparing the two-dimensional positional information on the micropipes with the position of pattern  2 . In a case where silicon carbide substrate  80  has small warpage, a chip in which a micropipe is present can be specified by a simple method and with high accuracy, by comparing the two-dimensional positional information on the micropipes in first main surface  80   b  with the position of pattern  2  formed on front surface  10   a,  which is on a side closer to second main surface  80   a.    
         [0094]    It should be noted that, although the first conductivity type has been described as n type and the second conductivity type has been described as p type in each of the embodiments described above, the first conductivity type may be p type and the second conductivity type may be n type. Further, although an MOSFET has been described as an example of the silicon carbide semiconductor device in each of the embodiments described above, the silicon carbide semiconductor device may be an IGBT (Insulated Gate Bipolar Transistor), an SBD (Schottky Barrier Diode), or the like. 
         [0095]    It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the scope of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the scope of the claims. 
       REFERENCE SIGNS LIST 
       [0096]      1 : silicon carbide semiconductor device (MOSFET);  2 : pattern;  3 ,  3   a,    3   b : etch pit;  10 ,  80 : silicon carbide substrate;  10   a : front surface;  40 ,  61 : mask layer;  60 : resist film;  80   a : second main surface;  80   b : first main surface;  80   c : orientation flat;  81 : silicon carbide epitaxial layer (n type drift region);  82 : p type base region;  83 : n type region;  84 : p type contact region;  91 : gate insulating film;  92 : gate electrode;  93 : interlayer insulating film;  94 : source electrode;  95 : source interconnection layer;  98 : back surface electrode (drain electrode); A, SW: wall surface; B, BT: bottom portion; C 12  to C 65 , C 23 , C 25 , C 43 : chip; TQ: recessed portion; TR: trench.