Patent Publication Number: US-2023144542-A1

Title: Manufacturing method of semiconductor device and semiconductor device

Description:
The contents of the following Japanese patent application(s) are incorporated herein by reference: 
     NO. 2020-190961 filed in JP on Nov. 17, 2020 
     NO. PCT/JP2021/021995 filed in WO on Jun. 9, 2021 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a manufacturing method of a semiconductor device and a semiconductor device. 
     2. Related Art 
     Conventionally, as a field stopper layer of a semiconductor device, a configuration including a plurality of impurity concentration peaks is known (see, for example, Patent Document 1).
     Patent Document 1: WO 2013/89256   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    illustrates a top view showing an example of a semiconductor device  100 . 
         FIG.  2    illustrates an enlarged view of a region D in  FIG.  1   . 
         FIG.  3    illustrates a view showing an example of a cross section e-e in  FIG.  2   . 
         FIG.  4 A  illustrates a view showing an example of a doping concentration distribution, a hydrogen chemical concentration distribution, a helium chemical concentration distribution, and a recombination center concentration distribution taken along line F-F in  FIG.  3   . 
         FIG.  4 B  illustrates a view showing a relationship between an implantation depth (Rp) of ions and acceleration energy required for implantation. 
         FIG.  4 C  illustrates a view showing a relationship between the implantation depth (Rp) of ions and a straggling (ΔRp, standard deviation) in an implantation direction. 
         FIG.  5 A  illustrates a view showing an example of a doping concentration distribution, a hydrogen chemical concentration distribution, a helium chemical concentration distribution, and a recombination center concentration distribution in the buffer region  20 . 
         FIG.  5 B  illustrates a view showing an example of the doping concentration distribution, the hydrogen chemical concentration distribution, the helium chemical concentration distribution, and the recombination center concentration distribution in the buffer region  20 . 
         FIG.  6    illustrates a view showing another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . 
         FIG.  7    illustrates a view showing still another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . 
         FIG.  8    illustrates a view showing still another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . 
         FIG.  9    illustrates a view showing still another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . 
         FIG.  10 A  illustrates a view showing still another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . 
         FIG.  10   (B illustrates a view showing still another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . 
         FIG.  10 C  illustrates a view showing still another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . 
         FIG.  11    illustrates a full width at half maximum Wk of a helium chemical concentration peak  221 . 
         FIG.  12 A  illustrates a view showing an example of the doping concentration distribution and the hydrogen chemical concentration distribution in the buffer region  20 . 
         FIG.  12 B  illustrates a view showing some processes in a manufacturing method of the semiconductor device  100 . 
         FIG.  12 C  illustrates a view showing another example of the doping concentration distribution and the hydrogen chemical concentration distribution in the buffer region  20 . 
         FIG.  12 D  illustrates a view showing still another example of the doping concentration distribution and the hydrogen chemical concentration distribution in the buffer region  20 . 
         FIG.  12 E  illustrates a view showing still another example of the doping concentration distribution and the hydrogen chemical concentration distribution in the buffer region  20 . 
         FIG.  12 F  illustrates a view showing another example of the processes in the manufacturing method of the semiconductor device  100 . 
         FIG.  12 G  illustrates a view showing still another example of the processes in the manufacturing method of the semiconductor device  100 . 
         FIG.  13    shows an example of a carrier concentration distribution and a helium chemical concentration distribution in the buffer region  20  of a comparative example. 
         FIG.  14    illustrates a view showing another example of the cross section e-e. 
         FIG.  15    illustrates a view showing an example of a doping concentration distribution and a hydrogen chemical concentration distribution taken along line F-F in  FIG.  14   . 
         FIG.  16    illustrates a view showing an example of a method of forming the buffer region  20 . 
         FIG.  17    illustrates a view showing a cross-sectional shape of a collector region  22  according to the comparative example. 
         FIG.  18    illustrates a view showing a result of a breakdown voltage test of a semiconductor device. 
         FIG.  19    illustrates a view showing a result of a breakdown voltage test of the semiconductor device. 
         FIG.  20    illustrates a view showing another example of the semiconductor device  100 . 
         FIG.  21    illustrates a view showing another example of the manufacturing process of the semiconductor device  100 . 
         FIG.  22    illustrates a view showing an example of a doping concentration distribution and a hydrogen chemical concentration distribution of the semiconductor device  100  shown in  FIG.  21   . 
         FIG.  23    illustrates a view showing still another example of the cross section e-e. 
         FIG.  24    illustrates a view showing an example of a method of forming the buffer region  20  shown in  FIG.  23   . 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, the invention will be described through embodiments of the invention, but the following embodiments do not limit the invention according to claims. In addition, not all of the combinations of features described in the embodiments are essential to the solving means of the invention. 
     As used herein, one side in a direction parallel to a depth direction of a semiconductor substrate is referred to as “upper” and the other side is referred to as “lower”. One surface of two principal surfaces of a substrate, a layer or other member is referred to as an upper surface, and the other surface is referred to as a lower surface. “Upper” and “lower” directions are not limited to a direction of gravity, or a direction in which a semiconductor device is mounted. 
     In the present specification, technical matters may be described using orthogonal coordinate axes of an X axis, a Y axis, and a Z axis. The orthogonal coordinate axes merely specify relative positions of components, and do not limit a specific direction. For example, the Z axis is not limited to indicate the height direction with respect to the ground. Note that a +Z axis direction and a −Z axis direction are directions opposite to each other. When the Z axis direction is described without describing the signs, it means that the direction is parallel to the +Z axis and the −Z axis. 
     In the present specification, orthogonal axes parallel to the upper surface and the lower surface of the semiconductor substrate are referred to as the X axis and the Y axis. Further, an axis perpendicular to the upper surface and the lower surface of the semiconductor substrate is referred to as the Z axis. In the present specification, the direction of the Z axis may be referred to as the depth direction. Further, in the present specification, a direction parallel to the upper surface and the lower surface of the semiconductor substrate may be referred to as a horizontal direction, including an X axis direction and a Y axis direction. 
     Further, a region from the center of the semiconductor substrate in the depth direction to the upper surface of the semiconductor substrate may be referred to as an upper surface side. Similarly, a region from the center of the semiconductor substrate in the depth direction to the lower surface of the semiconductor substrate may be referred to as a lower surface side. 
     In the present specification, a case where a term such as “same” or “equal” is mentioned may include a case where an error due to a variation in manufacturing or the like is included. The error is, for example, within 10%. 
     In the present specification, a conductivity type of doping region where doping has been carried out with an impurity is described as a P type or an N type. In the present specification, the impurity may particularly mean either a donor of the N type or an acceptor of the P type, and may be described as a dopant. In the present specification, doping means introducing the donor or the acceptor into the semiconductor substrate and turning it into a semiconductor presenting a conductivity type of the N type or a semiconductor presenting a conductivity type of the P type. 
     In the present specification, a doping concentration means a concentration of the donor or a concentration of the acceptor in a thermal equilibrium state. In the present specification, a net doping concentration means a net concentration obtained by adding the donor concentration set as a positive ion concentration to the acceptor concentration set as a negative ion concentration, taking into account of polarities of charges. As an example, when the donor concentration is N D  and the acceptor concentration is N A , the net doping concentration at any position is given as N D −N A . In the present specification, the net doping concentration may be simply referred to as the doping concentration. 
     The donor has a function of supplying electrons to a semiconductor. The acceptor has a function of receiving electrons from the semiconductor. The donor and the acceptor are not limited to the impurities themselves. For example, a VOH defect which is a combination of a vacancy (V), oxygen (O), and hydrogen (H) existing in the semiconductor functions as the donor that supplies electrons. In the present specification, the VOH defect may be referred to as a hydrogen donor. 
     In the semiconductor substrate of the present specification, bulk donors of the N type are distributed throughout. The bulk donor is a dopant donor substantially uniformly contained in an ingot during the manufacture of the ingot from which the semiconductor substrate is made. The bulk donor of this example is an element other than hydrogen. The bulk donor dopant is, for example, phosphorous, antimony, arsenic, selenium, or sulfur, but the invention is not limited to these. The bulk donor of this example is phosphorous. The bulk donor is also contained in a P type region. The semiconductor substrate may be a wafer cut out from a semiconductor ingot, or may be a chip obtained by singulating the wafer. The semiconductor ingot may be manufactured by either a Czochralski method (CZ method), a magnetic field applied Czochralski method (MCZ method), or a float zone method (FZ method). The ingot in this example is manufactured by the MCZ method. An oxygen concentration contained in the substrate manufactured by the MCZ method is 1×10 17  to 7×10 17 /cm 3 . The oxygen concentration contained in the substrate manufactured by the FZ method is 1×10 15  to 5×10 16 /cm 3 . When the oxygen concentration is high, hydrogen donors tend to be easily generated. The bulk donor concentration may use a chemical concentration of bulk donors distributed throughout the semiconductor substrate, or may be a value between 90% and 100% of the chemical concentration. Further, as the semiconductor substrate, a non-doped substrate not containing a dopant such as phosphorous may be used. In that case, the bulk donor concentration (D0) of the non-doped substrate is, for example, from 1×10 10 /cm 3  or more and to 5×10 12 /cm 3  or less. The bulk donor concentration (D0) of the non-doped substrate is preferably 1×10 11 /cm 3  or more. The bulk donor concentration (D0) of the non-doped substrate is preferably 5×10 12 /cm 3  or less. Each concentration in the present invention may be a value at room temperature. As an example, a value at 300K (Kelvin) (about 26.9 degrees C.) may be used as the value at room temperature. 
     In the present specification, a description of a P+ type or an N+ type means a higher doping concentration than that of the P type or the N type, and a description of a P− type or an N− type means a lower doping concentration than that of the P type or the N type. Further, in the specification, a description of a P++ type or an N++ type means a higher doping concentration than that of the P+ type or the N+ type. In the specification, a unit system is the SI base unit system unless otherwise particularly noted. Although a unit of length is represented using cm, it may be converted to meters (m) before calculations. 
     A chemical concentration in the present specification indicates an atomic density of an impurity measured regardless of an electrical activation state. The chemical concentration can be measured by, for example, secondary ion mass spectrometry (SIMS). The net doping concentration described above can be measured by voltage-capacitance profiling (CV profiling). Further, a carrier concentration measured by spreading resistance profiling (SRP method) may be set as the net doping concentration. The carrier concentration measured by the CV profiling or the SRP method may be a value in a thermal equilibrium state. Further, in a region of the N type, the donor concentration is sufficiently higher than the acceptor concentration, and thus the carrier concentration of the region may be set as the donor concentration. Similarly, in a region of the P type, the carrier concentration of the region may be set as the acceptor concentration. In the present specification, the doping concentration of the N type region may be referred to as the donor concentration, and the doping concentration of the P type region may be referred to as the acceptor concentration. 
     Further, when a concentration distribution of the donor, acceptor, or net doping has a peak in a region, a value of the peak may be set as the concentration of the donor, acceptor, or net doping in the region. In a case where the concentration of the donor, acceptor or net doping is substantially uniform in a region, or the like, an average value of the concentration of the donor, acceptor or net doping in the region may be set as the concentration of the donor, acceptor or net doping. In the present specification, atoms/cm 3  or/cm 3  is used to indicate a concentration per unit volume. This unit is used for a concentration of a donor or an acceptor in a semiconductor substrate, or a chemical concentration. A notation of atoms may be omitted. 
     The carrier concentration measured by the SRP method may be lower than the concentration of the donor or the acceptor. In a range where a current flows when a spreading resistance is measured, carrier mobility of the semiconductor substrate may be lower than a value in a crystalline state. The decrease in carrier mobility occurs when carriers are scattered due to disorder (disorder) of a crystal structure due to a lattice defect or the like. 
     The concentration of the donor or the acceptor calculated from the carrier concentration measured by the CV profiling or the SRP method may be lower than a chemical concentration of an element indicating the donor or the acceptor. As an example, in a silicon semiconductor, a donor concentration of phosphorous or arsenic serving as a donor, or an acceptor concentration of boron (boron) serving as an acceptor is approximately 99% of chemical concentrations of these. On the other hand, in the silicon semiconductor, a donor concentration of hydrogen serving as a donor is approximately 0.1% to 10% of a chemical concentration of hydrogen. 
       FIG.  1    illustrates a top view showing an example of a semiconductor device  100 .  FIG.  1    shows a position at which each member is projected on an upper surface of a semiconductor substrate  10 .  FIG.  1    shows merely some members of the semiconductor device  100 , and omits illustrations of some members. 
     The semiconductor device  100  includes the semiconductor substrate  10 . The semiconductor substrate  10  is a substrate that is formed of a semiconductor material. As an example, the semiconductor substrate  10  is a silicon substrate. The semiconductor substrate  10  has an end side  162  in the top view. When merely referred to as the top view in the present specification, it means that the semiconductor substrate  10  is viewed from an upper surface side. The semiconductor substrate  10  of this example has two sets of end sides  162  opposite to each other in the top view. In  FIG.  1   , the X axis and the Y axis are parallel to any of the end sides  162 . In addition, the Z axis is perpendicular to the upper surface of the semiconductor substrate  10 . 
     The semiconductor substrate  10  is provided with an active portion  160 . The active portion  160  is a region where a main current flows in the depth direction between the upper surface and a lower surface of the semiconductor substrate  10  when the semiconductor device  100  operates. An emitter electrode is provided above the active portion  160 , but is omitted in  FIG.  1   . 
     The active portion  160  is provided with at least one of a transistor portion  70  including a transistor element such as an IGBT, and a diode portion  80  including a diode element such as a freewheeling diode (FWD). In the example of  FIG.  1   , the transistor portion  70  and the diode portion  80  are alternately arranged along a predetermined array direction (the X axis direction in this example) on the upper surface of the semiconductor substrate  10 , and the semiconductor device  100  is a reverse conduction type IGBT (RC-IGBT). The active portion  160  in another example may be provided with only one of the transistor portion  70  and the diode portion  80 . 
     In  FIG.  1   , a region where each of the transistor portions  70  is arranged is indicated by a symbol “I”, and a region where each of the diode portions  80  is arranged is indicated by a symbol “F”. In the present specification, a direction perpendicular to the array direction in the top view may be referred to as an extending direction (the Y axis direction in  FIG.  1   ). Each of the transistor portions  70  and the diode portions  80  may have a longitudinal length in the extending direction. In other words, the length of each of the transistor portions  70  in the Y axis direction is larger than the width in the X axis direction. Similarly, the length of each of the diode portions  80  in the Y axis direction is larger than the width in the X axis direction. The extending direction of the transistor portion  70  and the diode portion  80 , and the longitudinal direction of each trench portion described below may be the same. 
     Each of the diode portions  80  includes a cathode region of N+ type in a region in contact with the lower surface of the semiconductor substrate  10 . In the present specification, a region where the cathode region is provided is referred to as the diode portion  80 . In other words, the diode portion  80  is a region that overlaps with the cathode region in the top view. On the lower surface of the semiconductor substrate  10 , a collector region of P+ type may be provided in a region other than the cathode region. In the specification, the diode portion  80  may also include an extension region  81  where the diode portion  80  extends to a gate runner described below in the Y axis direction. The collector region is provided on a lower surface of the extension region  81 . 
     The transistor portion  70  has the collector region of the P+ type in a region in contact with the lower surface of the semiconductor substrate  10 . Further, in the transistor portion  70 , an emitter region of the N type, a base region of the P type, and a gate structure having a gate conductive portion and a gate dielectric film are periodically arranged on the upper surface side of the semiconductor substrate  10 . 
     The semiconductor device  100  may have one or more pads above the semiconductor substrate  10 . The semiconductor device  100  of this example has a gate pad  164 . The semiconductor device  100  may have a pad such as an anode pad, a cathode pad, and a current detection pad. Each pad is arranged in a region close to the end side  162 . The region close to the end side  162  refers to a region between the end side  162  and the emitter electrode in the top view. When the semiconductor device  100  is mounted, each pad may be connected to an external circuit via a wiring such as a wire. 
     A gate potential is applied to the gate pad  164 . The gate pad  164  is electrically connected to a conductive portion of a gate trench portion of the active portion  160 . The semiconductor device  100  includes a gate runner that connects the gate pad  164  and the gate trench portion. In  FIG.  1   , the gate runner is hatched with diagonal lines. 
     The gate runner of this example has an outer circumferential gate runner  130  and an active-side gate runner  131 . The outer circumferential gate runner  130  is arranged between the active portion  160  and the end side  162  of the semiconductor substrate  10  in the top view. The outer circumferential gate runner  130  of this example encloses the active portion  160  in the top view. A region enclosed by the outer circumferential gate runner  130  in the top view may be the active portion  160 . Further, the outer circumferential gate runner  130  is connected to the gate pad  164 . The outer circumferential gate runner  130  is arranged above the semiconductor substrate  10 . The outer circumferential gate runner  130  may be a metal wiring including aluminum. 
     The active-side gate runner  131  is provided in the active portion  160 . Providing the active-side gate runner  131  in the active portion  160  can reduce a variation in wiring length from the gate pad  164  for each region of the semiconductor substrate  10 . 
     The active-side gate runner  131  is connected to the gate trench portion of the active portion  160 . The active-side gate runner  131  is arranged above the semiconductor substrate  10 . The active-side gate runner  131  may be a wiring formed of a semiconductor such as polysilicon doped with an impurity. 
     The active-side gate runner  131  may be connected to the outer circumferential gate runner  130 . The active-side gate runner  131  of this example is provided extending in the X axis direction so as to cross the active portion  160  from one outer circumferential gate runner  130  to the other outer circumferential gate runner  130  substantially at the center of the Y axis direction, the outer circumferential gate runner  130  enclosing the active portion  160 . When the active portion  160  is divided by the active-side gate runner  131 , the transistor portion  70  and the diode portion  80  may be alternately arranged in the X axis direction in each divided region. 
     Further, the semiconductor device  100  may include a temperature sensing portion (not shown) that is a PN junction diode formed of polysilicon or the like, and a current detection portion (not shown) that simulates an operation of the transistor portion provided in the active portion  160 . 
     The semiconductor device  100  of this example includes an edge termination structure portion  90  between the active portion  160  and the end side  162  in the top view. The edge termination structure portion  90  of this example is arranged between the outer circumferential gate runner  130  and the end side  162 . The edge termination structure portion  90  reduces an electric field strength on the upper surface side of the semiconductor substrate  10 . The edge termination structure portion  90  may include at least one of a guard ring, a field plate, and a RESURF which are annularly provided to enclose the active portion  160 . 
       FIG.  2    illustrates an enlarged view of a region D in  FIG.  1   . The region D is a region including the transistor portion  70 , the diode portion  80 , and the active-side gate runner  131 . The semiconductor device  100  of this example includes a gate trench portion  40 , a dummy trench portion  30 , a well region  11 , an emitter region  12 , a base region  14 , and a contact region  15  which are provided inside the upper surface side of the semiconductor substrate  10 . The gate trench portion  40  and the dummy trench portion  30  each are an example of the trench portion. Further, the semiconductor device  100  of this example includes an emitter electrode  52  and the active-side gate runner  131  that are provided above the upper surface of the semiconductor substrate  10 . The emitter electrode  52  and the active-side gate runner  131  are provided in isolation each other. 
     An interlayer dielectric film is provided between the emitter electrode  52  and the active-side gate runner  131 , and the upper surface of the semiconductor substrate  10 , but the interlayer dielectric film is omitted in  FIG.  2   . In the interlayer dielectric film of this example, a contact hole  54  is provided passing through the interlayer dielectric film. In  FIG.  2   , each contact hole  54  is hatched with the diagonal lines. 
     The emitter electrode  52  is provided on the upper side of the gate trench portion  40 , the dummy trench portion  30 , the well region  11 , the emitter region  12 , the base region  14 , and the contact region  15 . The emitter electrode  52  is in contact with the emitter region  12 , the contact region  15 , and the base region  14  on the upper surface of the semiconductor substrate  10 , through the contact hole  54 . Further, the emitter electrode  52  is connected to a dummy conductive portion in the dummy trench portion  30  through the contact hole provided in the interlayer dielectric film. The emitter electrode  52  may be connected to the dummy conductive portion of the dummy trench portion  30  at an edge of the dummy trench portion  30  in the Y axis direction. 
     The active-side gate runner  131  is connected to the gate trench portion  40  through the contact hole provided in the interlayer dielectric film. The active-side gate runner  131  may be connected to a gate conductive portion of the gate trench portion  40  at an edge portion  41  of the gate trench portion  40  in the Y axis direction. The active-side gate runner  131  is not connected to the dummy conductive portion in the dummy trench portion  30 . 
     The emitter electrode  52  is formed of a material including a metal.  FIG.  2    shows a range where the emitter electrode  52  is provided. For example, at least a part of a region of the emitter electrode  52  is formed of aluminum or an aluminum-silicon alloy, for example, a metal alloy such as AlSi, AlSiCu. The emitter electrode  52  may have a barrier metal formed of titanium, a titanium compound, or the like below a region formed of aluminum or the like. Further, a plug, which is formed by embedding tungsten or the like so as to be in contact with the barrier metal and aluminum or the like, may be included in the contact hole. 
     The well region  11  is provided overlapping the active-side gate runner  131 . The well region  11  is provided so as to extend with a predetermined width even in a range not overlapping the active-side gate runner  131 . The well region  11  of this example is provided away from an end of the contact hole  54  in the Y axis direction toward the active-side gate runner  131  side. The well region  11  is a second conductivity type region in which the doping concentration is higher than the base region  14 . The base region  14  of this example is a P− type, and the well region  11  is a P+ type. 
     Each of the transistor portion  70  and the diode portion  80  includes a plurality of trench portions arranged in the array direction. In the transistor portion  70  of this example, one or more gate trench portions  40  and one or more dummy trench portions  30  are alternately provided along the array direction. In the diode portion  80  of this example, the plurality of dummy trench portions  30  are provided along the array direction. In the diode portion  80  of this example, the gate trench portion  40  is not provided. 
     The gate trench portion  40  of this example may have two linear portions  39  extending along the extending direction perpendicular to the array direction (portions of a trench that are linear along the extending direction), and the edge portion  41  connecting the two linear portions  39 . The extending direction in  FIG.  2    is the Y axis direction. 
     At least a part of the edge portion  41  is desirably provided in a curved shape in a top view. By connecting between end portions of the two linear portions  39  in the Y axis direction by the edge portion  41 , it is possible to reduce the electric field strength at the end portions of the linear portions  39 . 
     In the transistor portion  70 , the dummy trench portions  30  are provided between the respective linear portions  39  of the gate trench portions  40 . Between the respective linear portions  39 , one dummy trench portion  30  may be provided, or a plurality of dummy trench portions  30  may be provided. The dummy trench portion  30  may have a linear shape extending in the extending direction, or may have linear portions  29  and an edge portion  31  similar to the gate trench portion  40 . The semiconductor device  100  shown in  FIG.  2    includes both of the linear dummy trench portion  30  having no edge portion  31 , and the dummy trench portion  30  having the edge portion  31 . 
     A diffusion depth of the well region  11  may be deeper than the depth of the gate trench portion  40  and the dummy trench portion  30 . The end portions in the Y axis direction of the gate trench portion  40  and the dummy trench portion  30  are provided in the well region  11  in a top view. In other words, the bottom in the depth direction of each trench portion is covered with the well region  11  at the end portion in the Y axis direction of each trench portion. With this configuration, the electric field strength on the bottom portion of each trench portion can be reduced. 
     A mesa portion is provided between the respective trench portions in the array direction. The mesa portion refers to a region sandwiched between the trench portions inside the semiconductor substrate  10 . As an example, an upper end of the mesa portion is the upper surface of the semiconductor substrate  10 . The depth position of the lower end of the mesa portion is the same as the depth position of the lower end of the trench portion. The mesa portion of this example is provided extending in the extending direction (the Y axis direction) along the trench, on the upper surface of the semiconductor substrate  10 . In this example, a mesa portion  60  is provided in the transistor portion  70 , and a mesa portion  61  is provided in the diode portion  80 . In the case of simply mentioning “mesa portion” in the present specification, the portion refers to each of the mesa portion  60  and the mesa portion  61 . 
     Each mesa portion is provided with the base region  14 . In the mesa portion, a region arranged closest to the active-side gate runner  131 , in the base region  14  exposed on the upper surface of the semiconductor substrate  10 , is to be a base region  14 - e . While  FIG.  2    shows the base region  14 - e  arranged at one end portion of each mesa portion in the extending direction, the base region  14 - e  is also arranged at the other end portion of each mesa portion. Each mesa portion may be provided with at least one of a first conductivity type of emitter region  12 , and a second conductivity type of contact region  15  in a region sandwiched between the base regions  14 - e  in the top view. The emitter region  12  of this example is an N+ type, and the contact region  15  is a P+ type. The emitter region  12  and the contact region  15  may be provided between the base region  14  and the upper surface of the semiconductor substrate  10  in the depth direction. 
     The mesa portion  60  of the transistor portion  70  has the emitter region  12  exposed on the upper surface of the semiconductor substrate  10 . The emitter region  12  is provided in contact with the gate trench portion  40 . The mesa portion  60  in contact with the gate trench portion  40  may be provided with the contact region  15  exposed on the upper surface of the semiconductor substrate  10 . 
     Each of the contact region  15  and the emitter region  12  in the mesa portion  60  is provided from one trench portion to the other trench portion in the X axis direction. As an example, the contact region  15  and the emitter region  12  in the mesa portion  60  are alternately arranged along the extending direction of the trench portion (the Y axis direction). 
     In another example, the contact region  15  and the emitter region  12  in the mesa portion  60  may be provided in a stripe shape along the extending direction of the trench portion (the Y axis direction). For example, the emitter region  12  is provided in a region in contact with the trench portion, and the contact region  15  is provided in a region sandwiched between the emitter regions  12 . 
     The mesa portion  61  of the diode portion  80  is not provided with the emitter region  12 . The base region  14  and the contact region  15  may be provided on an upper surface of the mesa portion  61 . In the region sandwiched between the base regions  14 - e  on the upper surface of the mesa portion  61 , the contact region  15  may be provided in contact with each base region  14 - e . The base region  14  may be provided in a region sandwiched between the contact regions  15  on the upper surface of the mesa portion  61 . The base region  14  may be arranged in the entire region sandwiched between the contact regions  15 . 
     The contact hole  54  is provided above each mesa portion. The contact hole  54  is arranged in the region sandwiched between the base regions  14 - e . The contact hole  54  of this example is provided above respective regions of the contact region  15 , the base region  14 , and the emitter region  12 . The contact hole  54  is not provided in regions corresponding to the base region  14 - e  and the well region  11 . The contact hole  54  may be arranged at the center of the mesa portion  60  in the array direction (the X axis direction). 
     In the diode portion  80 , a cathode region  82  of the N+ type is provided in a region in direct contact with the lower surface of the semiconductor substrate  10 . On the lower surface of the semiconductor substrate  10 , a collector region of the P+ type  22  may be provided in a region where the cathode region  82  is not provided. The cathode region  82  and the collector region  22  are provided between a lower surface  23  of the semiconductor substrate  10  and a buffer region  20 . In  FIG.  2   , a boundary between the cathode region  82  and the collector region  22  is indicated by a dotted line. 
     The cathode region  82  is arranged away from the well region  11  in the Y axis direction. With this configuration, the distance between the P type region (the well region  11 ) having a relatively high doping concentration and formed up to the deep position, and the cathode region  82  is ensured, so that the breakdown voltage can be improved. The end portion in the Y axis direction of the cathode region  82  of this example is arranged farther away from the well region  11  than the end portion in the Y axis direction of the contact hole  54 . In another example, the end portion in the Y axis direction of the cathode region  82  may be arranged between the well region  11  and the contact hole  54 . 
     First Example 
       FIG.  3    illustrates a view showing an example of a cross section e-e in  FIG.  2   . The cross section e-e is an XZ plane passing through the emitter region  12  and the cathode region  82 . The semiconductor device  100  of this example includes the semiconductor substrate  10 , the interlayer dielectric film  38 , the emitter electrode  52 , and the collector electrode  24  in the cross section. 
     The interlayer dielectric film  38  is provided on the upper surface of the semiconductor substrate  10 . The interlayer dielectric film  38  is a film including at least one layer of a dielectric film such as silicate glass to which an impurity such as boron or phosphorous is added, a thermal oxide film, and other dielectric films. The interlayer dielectric film  38  is provided with the contact hole  54  described in  FIG.  2   . 
     The emitter electrode  52  is provided on the upper side of the interlayer dielectric film  38 . The emitter electrode  52  is in contact with an upper surface  21  of the semiconductor substrate  10  through the contact hole  54  of the interlayer dielectric film  38 . The collector electrode  24  is provided on a lower surface  23  of the semiconductor substrate  10 . The emitter electrode  52  and the collector electrode  24  are formed of a metal material such as aluminum. In the specification, the direction in which the emitter electrode  52  is connected to the collector electrode  24  (the Z axis direction) is referred to as a depth direction. 
     The semiconductor substrate  10  includes an N type or N+ type of drift region  18 . The drift region  18  is provided in each of the transistor portion  70  and the diode portion  80 . 
     In the mesa portion  60  of the transistor portion  70 , an N+ type of emitter region  12  and a P− type of base region  14  are provided in order from an upper surface  21  side of the semiconductor substrate  10 . The drift region  18  is provided below the base region  14 . The mesa portion  60  may be provided with an N+ type of accumulation region  16 . The accumulation region  16  is arranged between the base region  14  and the drift region  18 . 
     The emitter region  12  is exposed on the upper surface  21  of the semiconductor substrate  10  and is provided in contact with gate trench portion  40 . The emitter region  12  may be in contact with the trench portions on both sides of the mesa portion  60 . The emitter region  12  has a higher doping concentration than the drift region  18 . 
     The base region  14  is provided below the emitter region  12 . The base region  14  of this example is provided in contact with the emitter region  12 . The base region  14  may be in contact with the trench portions on both sides of the mesa portion  60 . 
     The accumulation region  16  is provided below the base region  14 . The accumulation region  16  is an N+ type region with a higher doping concentration than the drift region  18 . That is, the accumulation region  16  has a higher donor concentration than the drift region  18 . By providing the accumulation region  16  having the high concentration between the drift region  18  and the base region  14 , it is possible to improve a carrier injection enhancement effect (IE effect) and reduce an on-voltage. The accumulation region  16  may be provided to cover a whole lower surface of the base region  14  in each mesa portion  60 . 
     The mesa portion  61  of the diode portion  80  is provided with the P− type of base region  14  in contact with the upper surface  21  of the semiconductor substrate  10 . The drift region  18  is provided below the base region  14 . In the mesa portion  61 , the accumulation region  16  may be provided below the base region  14 . 
     In each of the transistor portion  70  and the diode portion  80 , an N+ type buffer region  20  may be provided below the drift region  18 . The doping concentration of the buffer region  20  is higher than the doping concentration of the drift region  18 . The buffer region  20  may have a concentration peak having a higher doping concentration than the doping concentration of the drift region  18 . The doping concentration of the concentration peak indicates a doping concentration at the local maximum of the concentration peak. Further, as the doping concentration of the drift region  18 , an average value of doping concentrations in the region where the doping concentration distribution is substantially flat may be used. 
     The buffer region  20  may have two or more concentration peaks in the depth direction (Z axis direction) of the semiconductor substrate  10 . The concentration peak of the buffer region  20  may be provided at the same depth position as, for example, a chemical concentration peak of hydrogen (proton) or phosphorous. The buffer region  20  may function as a field stopper layer which prevents a depletion layer expanding from the lower end of the base region  14  from reaching the collector region of the P+ type  22  and the cathode region  82  of the N+ type. In the present specification, a depth position of an upper end of the buffer region  20  is set as Zf. The depth position Zf may be a position at which the doping concentration is higher than the doping concentration of the drift region  18 . 
     In the transistor portion  70 , the collector region of the P+ type  22  is provided below the buffer region  20 . An acceptor concentration of the collector region  22  is higher than an acceptor concentration of the base region  14 . The collector region  22  may include an acceptor which is the same as or different from an acceptor of the base region  14 . The acceptor of the collector region  22  is, for example, boron. 
     Below the buffer region  20  in the diode portion  80 , the cathode region  82  of the N+ type is provided. A donor concentration of the cathode region  82  is higher than a donor concentration of the drift region  18 . A donor of the cathode region  82  is, for example, hydrogen or phosphorous. Note that an element serving as a donor and an acceptor in each region is not limited to the above described example. The collector region  22  and the cathode region  82  are exposed on the lower surface  23  of the semiconductor substrate  10  and are connected to the collector electrode  24 . The collector electrode  24  may be in contact with the entire lower surface  23  of the semiconductor substrate  10 . The emitter electrode  52  and the collector electrode  24  are formed of a metal material such as aluminum. 
     One or more gate trench portions  40  and one or more dummy trench portions  30  are provided on the upper surface  21  side of the semiconductor substrate  10 . Each trench portion passes through the base region  14  from the upper surface  21  of the semiconductor substrate  10 , and reaches the drift region  18 . In a region where at least any one of the emitter region  12 , the contact region  15 , and the accumulation region  16  is provided, each trench portion also passes through the doping regions of these to reach the drift region  18 . The configuration of the trench portion penetrating the doping region is not limited to the one manufactured in the order of forming the doping region and then forming the trench portion. The configuration of the trench portion penetrating the doping region includes a configuration of the doping region being formed between the trench portions after forming the trench portion. 
     As described above, the transistor portion  70  is provided with the gate trench portion  40  and the dummy trench portion  30 . In the diode portion  80 , the dummy trench portion  30  is provided, and the gate trench portion  40  is not provided. The boundary in the X axis direction between the diode portion  80  and the transistor portion  70  in this example is the boundary between the cathode region  82  and the collector region  22 . 
     The gate trench portion  40  includes a gate trench provided in the upper surface  21  of the semiconductor substrate  10 , a gate dielectric film  42 , and a gate conductive portion  44 . The gate dielectric film  42  is provided to cover the inner wall of the gate trench. The gate dielectric film  42  may be formed by oxidizing or nitriding a semiconductor on the inner wall of the gate trench. The gate conductive portion  44  is provided inside from the gate dielectric film  42  in the gate trench. That is, the gate dielectric film  42  insulates the gate conductive portion  44  from the semiconductor substrate  10 . The gate conductive portion  44  is formed of a conductive material such as polysilicon. 
     The gate conductive portion  44  may be provided longer than the base region  14  in the depth direction. The gate trench portion  40  in the cross section is covered by the interlayer dielectric film  38  on the upper surface  21  of the semiconductor substrate  10 . The gate conductive portion  44  is electrically connected to the gate runner. When a predetermined gate voltage is applied to the gate conductive portion  44 , a channel is formed by an electron inversion layer in a surface layer of the base region  14  at a boundary in contact with the gate trench portion  40 . 
     The dummy trench portions  30  may have the same structure as the gate trench portions  40  in the cross section. The dummy trench portion  30  includes a dummy trench provided in the upper surface  21  of the semiconductor substrate  10 , a dummy dielectric film  32 , and a dummy conductive portion  34 . The dummy conductive portion  34  is electrically connected to the emitter electrode  52 . The dummy dielectric film  32  is provided covering an inner wall of the dummy trench. The dummy conductive portion  34  is provided in the dummy trench, and is provided inside the dummy dielectric film  32 . The dummy dielectric film  32  insulates the dummy conductive portion  34  from the semiconductor substrate  10 . The dummy conductive portion  34  may be formed of the same material as the gate conductive portion  44 . For example, the dummy conductive portion  34  is formed of a conductive material such as polysilicon or the like. The dummy conductive portion  34  may have the same length as the gate conductive portion  44  in the depth direction. 
     The gate trench portion  40  and the dummy trench portion  30  of this example are covered with the interlayer dielectric film  38  on the upper surface  21  of the semiconductor substrate  10 . It is noted that the bottoms of the dummy trench portion  30  and the gate trench portion  40  may be formed in a curved-surface shape (a curved-line shape in the cross section) convexly downward. In the present specification, a depth position of a lower end of the gate trench portion  40  is set as Zt. 
     An upper-surface-side lifetime killer  210  may be provided on the upper surface  21  side of the semiconductor substrate  10 . The upper-surface-side lifetime killer  210  is a recombination center of a lattice defect or the like locally formed in the depth direction. In each drawing, the peak position of the density distribution of the lifetime killer in the depth direction is schematically indicated by an X mark. In the present specification, the peak position will be described as the position of the lifetime killer. The X marks are discretely arranged in the X axis direction, but the lifetime killer is uniformly provided in the X axis direction unless otherwise described. 
     The upper-surface-side lifetime killer  210  can be formed by implanting particles such as helium into a predetermined depth position from the upper surface  21  of the semiconductor substrate  10 . A concentration peak of particles such as helium may be arranged at the same depth position as that of the upper-surface-side lifetime killer  210 . The upper-surface-side lifetime killer  210  may be arranged below each trench portion. Further, the upper-surface-side lifetime killer  210  is preferably provided at a position not overlapping the gate trench portion  40  in the top view. With this configuration, the upper-surface-side lifetime killer  210  can be formed by implanting particles such as helium without damaging the gate dielectric film  42 . The upper-surface-side lifetime killer  210  of this example is provided over the entire diode portion  80  in the top view. The upper-surface-side lifetime killer  210  in  FIG.  3    is not provided in the transistor portion  70 , but in another example, the upper-surface-side lifetime killer  210  may be provided in a part of a region of the transistor portion  70 . 
     A lower-surface-side lifetime killer  220  is provided on the lower surface  23  side of the semiconductor substrate  10 . The lower-surface-side lifetime killer  220  may be formed by implanting particles such as helium from the lower surface  23  side of the semiconductor substrate  10 . A plurality of the lower-surface-side lifetime killers  220  may be arranged at different positions in the depth direction. In the example of  FIG.  3   , a first lower-surface-side lifetime killer  220 - 1  and a second lower-surface-side lifetime killer  220 - 2  are arranged at different depth positions. However, the lower-surface-side lifetime killers  220  may be provided at three or more depth positions. A peak of a helium chemical concentration may be provided at the same depth position as that of each of the lower-surface-side lifetime killers  220 . 
     In the buffer region  20 , two or more lower-surface-side lifetime killers  220  may be provided. This makes it easy to control the distribution of the lifetime killer in the buffer region  20 . Therefore, the carrier lifetime can be controlled precisely. 
     The lower-surface-side lifetime killer  220  may be provided over the entire diode portion  80  in the top view. Further, the lower-surface-side lifetime killer  220  may be provided over the entire transistor portion  70  in the top view. The lower-surface-side lifetime killer  220  may be provided over the entire active portion  160  in the top view, or may be provided over the entire semiconductor substrate  10  in the top view. The first lower-surface-side lifetime killer  220 - 1  and the second lower-surface-side lifetime killer  220 - 2  may be provided in the same range in the top view. 
       FIG.  4 A  illustrates a view showing an example of a doping concentration distribution, a hydrogen chemical concentration distribution, a helium chemical concentration distribution, and a recombination center concentration distribution taken along line F-F in  FIG.  3   . In  FIG.  4 A , the center position in the depth direction of the semiconductor substrate  10  is set as Zc. That is, the region on the upper surface  21  side of the semiconductor substrate  10  is a region between the upper surface  21  and the center position Zc, and the region on the lower surface  23  side is a region between the lower surface  23  and the center position Zc. 
     The emitter region  12  contains an N type dopant such as phosphorous. The base region  14  contains a P type dopant such as boron. The accumulation region  16  contains an N type dopant such as phosphorous or hydrogen. The doping concentration distribution may have respective concentration peaks in the emitter region  12 , the base region  14 , and the accumulation region  16 . 
     The drift region  18  is a region having a substantially flat doping concentration. A doping concentration Dd of the drift region  18  may be the same as the bulk donor concentration of the semiconductor substrate  10 , or may be higher than the bulk donor concentration. 
     The buffer region  20  of this example has a plurality of doping concentration peaks  25 - 1 ,  25 - 2 ,  25 - 3 , and  25 - 4  in the doping concentration distribution. Each doping concentration peak  25  may be formed by locally implanting hydrogen ions. In another example, each doping concentration peak  25  may be formed by implanting an N type dopant such as phosphorous. The collector region  22  contains a P type dopant such as boron. Further, the cathode region  82  shown in  FIG.  3    contains an N type dopant such as phosphorous. 
     The hydrogen chemical concentration distribution of this example has a plurality of local hydrogen chemical concentration peaks  103  in the buffer region  20 . By implanting hydrogen ions into the buffer region  20 , a VOH defect in which hydrogen, lattice defects, and oxygen are combined is formed and functions as a donor. The hydrogen chemical concentration peak  103  of this example is provided at the same depth position as that of the doping concentration peak  25 . Providing two peaks at the same depth position means that the local maximum of one peak is arranged within a range of the full width at half maximum of the other peak. When the concentration of the hydrogen chemical concentration peak  103  is not sufficiently high, a clear doping concentration peak  25  may not be observed at the same depth position as that of the hydrogen chemical concentration peak  103 . The hydrogen chemical concentration of this example steeply decreases immediately after entering the drift region  18  from the buffer region  20 . Thus, VOH defects are hardly formed in the drift region  18 . In another example, hydrogen may diffuse into the drift region  18  to form VOH defects. In this case, the doping concentration of the drift region  18  is higher than the bulk donor concentration. 
     The buffer region  20  has two or more helium chemical concentration peaks  221  arranged at different positions in the depth direction of the semiconductor substrate  10 . In this example, a first helium chemical concentration peak  221 - 1  and a second helium chemical concentration peak  221 - 2  are provided in the buffer region  20 . The second helium chemical concentration peak  221 - 2  is arranged farther away from the lower surface  23  than the first helium chemical concentration peak  221 - 1 . 
     As described above, the lower-surface-side lifetime killer  220  is formed in the vicinity of each helium chemical concentration peak  221 . The lower-surface-side lifetime killer  220  may be a recombination center that promotes carrier recombination. The recombination center may be a lattice defect. The lattice defects may be mainly composed of vacancies such as monatomic vacancies (V) and diatomic vacancies or divacancies (VV), may be dislocations, may be interstitial atoms, or may be transition metals or the like. For example, atoms adjacent to the vacancies have dangling bonds. In a broad sense, the lattice defects may also include donors and acceptors. However, in the present specification, the lattice defects mainly composed of vacancies may be referred to as vacancy-type lattice defects, vacancy-type defects, or simply lattice defects. In the present specification, the lattice defect may be simply referred to as a recombination center or a lifetime killer as a recombination center contributing to the carrier recombination. The lifetime killer may be formed by implanting helium ions into the semiconductor substrate  10 . Since the lifetime killer formed by implanting helium may be terminated by hydrogen existing in the buffer region  20 , the depth position of the density peak of the lifetime killer may not be identical to the depth position of the helium chemical concentration peak  221 . 
     By implanting helium into two or more depth positions of the buffer region  20 , the density distribution of the lower-surface-side lifetime killer  220  in the buffer region  20  can be easily controlled.  3 He or  4 He may be implanted into each depth position.  3 He is a helium isotope including two protons and one neutron.  4 He is a helium isotope including two protons and two neutrons. 
     When  3 He or  4 He is implanted, without passing through a buffer material (aluminum or the like), with the smallest acceleration energy at which the implantation depth is uniquely determined, a half-value width in the depth direction of the concentration peak of the helium chemical concentration can be reduced. 
       FIG.  4 B  illustrates a view showing a relationship between an implantation depth (Rp) of ions and acceleration energy required for implantation. In this example, helium ions are directly implanted into the silicon semiconductor substrate  10  without passing through the buffer material. In  FIG.  4 B , a horizontal axis represents a range Rp(μm), and a vertical axis represents the acceleration energy E(eV) required for implantation. In  FIG.  4 B , an example of  3 He is indicated by a solid line, and an example of  4 He is indicated by a broken line. 
     log 10 (Rp) is set as x, and log 10 (E) is set as y. 
     In  3 He, a relationship between the range Rp and an acceleration energy E may be given by Equation (1). 
         y= 4.52505 E− 03 x   6 −4.71471 E− 02 x   5 +1.67185 E− 01 x   4 −1.72038 E− 01 x   3 −2.92723 E− 01 x   2 +1.39782 E+ 00×+5.33858 E+ 00  Equation (1)
 
     Note that E−A is 10 −A , and E+A is 10 A . 
     The acceleration energy calculated by substituting an actual range Rp′ at the time of manufacturing the semiconductor device  100  into Equation (1) is set as E. When an actual acceleration energy E′ at the time of manufacture is within ±20% of the acceleration energy E calculated from Equation (1), it may be considered that  3 He is used. 
     In  4 He, the relationship between the range Rp and the acceleration energy E may be given by Equation (2). 
         y= 2.90157 E− 03 x   6 −3.66593 E− 02 x   5 +1.59363 E− 01 x   4 −2.31938 E− 01 x   3 −2.00999 E− 01 x   2 +1.45891 E+ 00×+5.27160 E+ 00  Equation (2)
 
     When the actual acceleration energy E′ at the time of manufacture is within ±20% of the acceleration energy E calculated from Equation (2) using the actual range Rp′, it may be considered that  4 He is used. 
     As illustrated in  FIG.  4 B , when the range Rp is equal to or larger than a boundary value with a value of a region where the range Rp is 8 μm to 10 μm set as the boundary value, the acceleration energy of  4 He is higher by approximately 10% than the acceleration energy of  3 He. When the range Rp is equal to or less than the boundary value, the acceleration energy of  3 He is higher by approximately 10% than the acceleration energy of  4 He. This is presumed to be due to changes in a balance between an electronic stopping power and a nuclear stopping power depending on the number of neutrons of the isotope. As an example, when the range Rp is 10 μm or less,  4 He may be used. Accordingly, helium ions can be implanted with approximately 10% lower acceleration energy. When the range Rp is larger than 10 μm,  3 He may be used. 
       FIG.  4 C  illustrates a view showing a relationship between the implantation depth (Rp) of ions and a straggling (ΔRp, standard deviation) in an implantation direction. The implantation direction in this example is the depth direction of the semiconductor substrate  10 . Also in this example, helium ions are directly implanted into the silicon semiconductor substrate  10  without passing through the buffer material. In  FIG.  4 C , a horizontal axis represents the range Rp (μm), and a vertical axis represents the straggling ΔRp (μm). In  FIG.  4 C , an example of  3 He is indicated by a solid line, and an example of  4 He is indicated by a broken line. 
     The straggling ΔRp may be calculated assuming that the helium concentration distribution is a Gaussian distribution. For example, the straggling ΔRp may be a distance (distribution width) between two points having a concentration of 0.60653 times the concentration peak value, or may be a distance between two points having a concentration of 0.6 times the concentration peak value. When a local minimum value or the like between adjacent concentration peaks is larger than 0.6 times the concentration peak value, a distance between inflection points such as the local minimum value of the concentration distribution may be used as the straggling ΔRp. 
     log 10 (Rp) is set as x, and log 10 (ΔRp) is set as y. 
     In  3 He, the relationship between the range Rp and the straggling ΔRp may be given by Equation (3). 
         y= 5.00395 E− 04 x   6 +9.91651 E− 03 x   5 −9.76015 E− 02 x   4 +2.12587 E− 01 x   3 +1.30994 E− 01 x   2 +2.25458 E− 01×−8.59463 E− 01  Equation (3)
 
     The straggling calculated by substituting the actual range Rp′ at the time of manufacturing the semiconductor device  100  into Equation (3) is set as ΔRp. When the actual straggling ΔRp′ at the time of manufacture is within ±20% of the straggling ΔRp calculated from Equation (3), it may be considered that  3 He is used. The actual straggling ΔRp′ preferably does not include helium diffusion due to thermal annealing. The actual straggling ΔRp′ may be a value measured after helium implantation and before thermal annealing, or may be a value obtained by subtracting the helium diffusion from the value measured after the thermal annealing. 
     In  4 He, the relationship between the range Rp and the straggling ΔRp may be given by Equation (4). 
         y= 3.10234 E− 03 x   6 −9.20762 E− 03 x   5 −6.13612 E− 02 x   4 +2.34304 E− 01 x   3 +3.88591 E− 02 x   2 +2.22955 E− 01×−8.01967 E− 01  Equation (4)
 
     When the actual straggling ΔRp′ at the time of manufacture is within ±20% of the straggling ΔRp calculated from Equation (4) using the actual range Rp′, it may be considered that  4 He is used. The actual straggling ΔRp′ preferably does not include helium diffusion due to thermal annealing. 
     As illustrated in  FIG.  4 C , when the range Rp is equal to or less than a boundary value with a value of a region where the range Rp is 10 to 20 μm set as the boundary value, the straggling ΔRp of  3 He is smaller by approximately 10% than the straggling ΔRp of  4 He. When the range Rp is equal to or larger than the boundary value, the stragglings ΔRp are substantially equal between  3 He and  4 He. This is presumed to be due to changes in a balance between an electronic stopping power and a nuclear stopping power depending on the number of neutrons of the isotope. 
     As an example, when the range Rp is 20 μm or less,  3 He may be used. Accordingly, the straggling ΔRp can be made approximately 10% smaller. Alternatively, in a case where a difference which is given to the helium chemical concentration distribution or electrical characteristics by the difference of approximately 10% in the straggling ΔRp is sufficiently small, even when the range Rp is 20 μm or less, it may be considered that the stragglings ΔRp are substantially equal between  3 He and  4 He. In this case, helium atoms implanted into the semiconductor substrate  10  may be  3 He or may be  4 He. 
     As an example, the full width at half maximum of the helium chemical concentration peak  221  when  4 He is implanted is 1 μm or less. The full width at half maximum of the helium chemical concentration peak  221  may be 0.5 μm or less. By arranging a plurality of helium chemical concentration peaks  221  having a small half-value width in the buffer region  20 , the distribution shape of the lower-surface-side lifetime killer  220  can be easily controlled. Further, it is possible to suppress VOH defects formed by helium implantation from being distributed in a wide range. Thus, the doping concentration distribution of the buffer region  20  can be suppressed from varying in a wide range. 
     Further, by providing the plurality of helium chemical concentration peaks  221 , the total concentration of the lower-surface-side lifetime killer  220  can be maintained high. Thus, the lifetime of the carrier can be shortened at the time of turning off the semiconductor device  100  or the like, and a tail current can be suppressed. 
     When the acceleration energy E of  3 He is He approximately 20 MeV or more (the range Rp is 270 μm or more), the straggling ΔRp is 10 μm or more. When the acceleration energy E of  4 He is approximately 21 MeV or more (the range Rp is 250 μm or more), the straggling ΔRp is 10 μm or more. In this case, the full width at half maximum of the helium chemical concentration peak  221  cannot be made sufficiently smaller than the width of the buffer region  20  in the depth direction. Thus, VOH defects are formed in a wide range of the buffer region  20 , and the doping concentration distribution is varied. Thus, an electric field may be locally concentrated in the buffer region  20 , so that a short-circuit current tolerance decreases. In contrast, by reducing the half-value width of the helium chemical concentration peak  221 , the short-circuit current tolerance can be easily maintained. Therefore, in the case of implanting either  3 He or  4 He, the acceleration energy E may be 20 MeV or less, or may be 10 MeV or less. Alternatively, the acceleration energy E of at least one or more or two or more helium chemical concentration peaks  221  of the plurality of helium chemical concentration peaks  221  may be equal to or less than 10 MeV, or may be equal to or less than 5 MeV. 
       FIG.  5 A  illustrates a view showing an example of the doping concentration distribution, the hydrogen chemical concentration distribution, the helium chemical concentration distribution, and the recombination center concentration distribution in the buffer region  20 . The concentration distributions may be similar to respective concentration distributions described in  FIG.  4 A . 
     The doping concentration distribution of this example has the doping concentration peaks  25 - 1 ,  25 - 2 ,  25 - 3 , and  25 - 4  in order from the lower surface  23  side of the semiconductor substrate  10 . The doping concentration peak  25 - 4  is an example of the deepest doping concentration peak that is arranged farthest away from the lower surface  23 . The depth positions of the respective doping concentration peaks  25  are set as Zd 1 , Zd 2 , Zd 3 , and Zd 4  in order from the lower surface  23  side. Each depth position Zd indicates a distance from the lower surface  23 . Note that any doping concentration peak  25  may not be a clear peak. For example, an inflection point (kink) of the slope of the doping concentration distribution may be set as the doping concentration peak  25 . The doping concentration peak  25 - 1  may be the doping concentration peak  25  having the largest concentration value. The doping concentration peak  25 - 2  may be the doping concentration peak  25  having the second largest concentration value. The doping concentration peak  25 - 3  may be the doping concentration peak  25  having the smallest concentration value. The doping concentration peak  25 - 4  may be the doping concentration peak  25  having a higher concentration than the doping concentration peak  25 - 3 . 
     The hydrogen chemical concentration distribution of this example has hydrogen chemical concentration peaks  103 - 1 ,  103 - 2 ,  103 - 3 , and  103 - 4  in order from the lower surface  23  side of the semiconductor substrate  10 . The depth positions of the respective hydrogen chemical concentration peaks  103  are set as Zh 1 , Zh 2 , Zh 3 , and Zh 4  in order from the lower surface  23  side. Each depth position Zh indicates a distance from the lower surface  23 . A depth position Zdk may be the same position as a depth position Zhk. k is an integer of 1 to 4. The hydrogen chemical concentration peak  103 - 1  may be the hydrogen chemical concentration peak  103  having the largest concentration value. The hydrogen chemical concentration peak  103 - 2  may be hydrogen chemical concentration peak  103  having the second largest concentration value. The hydrogen chemical concentration peak  103 - 3  may be the hydrogen chemical concentration peak  103  having the smallest concentration value. The hydrogen chemical concentration peak  103 - 4  may be the hydrogen chemical concentration peak  103  having a higher concentration than the hydrogen chemical concentration peak  103 - 3 . 
     The helium chemical concentration distribution of this example has the first helium chemical concentration peak  221 - 1  and the second helium chemical concentration peak  221 - 2  in order from the lower surface  23  side of the semiconductor substrate  10 . The depth positions of the respective helium chemical concentration peaks  221  are set as Zk 1  and Zk 2  in order from the lower surface  23  side. Each depth position Zk indicates a distance from the lower surface  23 . Further, the concentration values of the respective helium chemical concentration peaks  221  are set as Pk 1  and Pk 2  in order from the lower surface  23  side. 
     Two or more helium chemical concentration peaks  221  are arranged between the doping concentration peak  25 - 4 , which is the deepest doping concentration peak, and the lower surface  23  of the semiconductor substrate  10 . At least one helium chemical concentration peak  221  may be arranged between the depth positions Zd 1  and Zd 2 . In this example, all helium chemical concentration peaks  221  are arranged between the depth positions Zd 1  and Zd 2 . The full width at half maximum of the helium chemical concentration peak  221 - 2  may be larger than the full width at half maximum of the helium chemical concentration peak  221 - 1 . The full width at half maximum of the helium chemical concentration peak  221 - 1  may be different from the full width at half maximum of the helium chemical concentration peak  221 - 2  depending on a difference in acceleration energy. In this example, a plurality of lower-surface-side lifetime killers  220  can be arranged in the vicinity of the collector region  22 . 
       FIG.  5 B  illustrates a view showing an example of the doping concentration distribution, the hydrogen chemical concentration distribution, the helium chemical concentration distribution, and the recombination center concentration distribution in the buffer region  20 . In this example, the helium chemical concentration distribution and the recombination center concentration distribution are different from those in the example of  FIG.  5 A . Other distributions may be similar to those of the example of  FIG.  5 A . 
     The buffer region  20  of this example has one helium chemical concentration peak  221 - 0  and one lower-surface-side lifetime killer  220 - 0 . The position of the helium chemical concentration peak  221 - 0  in the depth direction is set as Zk 0 , and the concentration is set as Pk 0 . 
     The depth position Zk 0  of the helium chemical concentration peak  221 - 0  is arranged between the depth positions Zk 1  and Zk 2 . A recombination center concentration peak (lower-surface-side lifetime killer  220 - 0 ) is arranged in the vicinity of the depth position Zk 0 . Further, the concentration Pk 0  of the helium chemical concentration peak  221 - 0  may be higher than any of Pk 1  and Pk 2 . The lower-surface-side lifetime killer  220 - 0  may also be higher in concentration than any of the lower-surface-side lifetime killers  220 - 1  and  220 - 2 . 
     In the example of  FIGS.  5 A and  5 B , when the depletion layer expanding from the lower end of the base region  14  reaches the lower-surface-side lifetime killer  220  at the time of turn-off or the like, the recombination center functions as the generation center of the carrier. With this configuration, a leakage current may increase, the heat generation of the semiconductor device may be promoted, the temperature of the semiconductor device may increase, and the tolerance of turn-off or the like may decrease. As in the example of  FIG.  5 A , the peak concentration of the helium chemical concentration (recombination center concentration) can be decreased by arranging the plurality of lower-surface-side lifetime killers  220 . With this configuration, the concentration of the generation center of the carrier can also be decreased, the leakage current can be reduced, the temperature rise of the semiconductor device can also be suppressed, and the tolerance of turn-off or the like can be increased. Further, the implantation of hole carriers from the collector region  22  into the drift region  18  can be suppressed. 
     Further, in the example of  FIG.  5 A , a distance (Zk 2 −Zk 1 ) between the first helium chemical concentration peak  221 - 1  closest to the depth position Zd 1  and the second helium chemical concentration peak  221 - 2  closest to the depth position Zd 2  may be half or more of the distance (Zd 2 −Zd 1 ). With this configuration, the plurality of lower-surface-side lifetime killers  220  can be arranged over a certain range. Further, an interval (Zk 2 −Zk 1  in this example) between the adjacent helium chemical concentration peaks  221  in the depth direction may be 2 μm or more, may be 3 μm or more, may be 4 μm or more, or may be 5 μm or more. 
     The concentration values Pk of the helium chemical concentration peaks  221  may be the same. In another example, any of the concentration values Pk may be different from the other concentration values Pk. The implantation dose amount of helium ions corresponding to each helium chemical concentration peak  221  may be 1×10 11  (/cm 2 ) or more, may be 3×10 11  (/cm 2 ) or more, or may be 1×10 12  (/cm 2 ) or more. The implantation dose amount of helium ions corresponding to each helium chemical concentration peak  221  may be 1×10 13  (/cm 2 ) or less, may be 3×10 12  (/cm 2 ) or less, or may be 1×10 12  (/cm 2 ) or less. 
     Each helium chemical concentration peak  221  may be arranged at a depth position different from that of any hydrogen chemical concentration peak  103 . That is, the depth position Zk of the local maximum of each helium chemical concentration peak  221  does not fall within a range of the full width at half maximum of any hydrogen chemical concentration peak  103 . With this configuration, the lifetime killer formed by helium implantation is suppressed from being terminated by hydrogen, and the concentration of the lower-surface-side lifetime killer  220  is easily maintained. 
     In each helium chemical concentration peak  221 , the concentration value Pk may increase as a distance from the depth position Zh of the hydrogen chemical concentration peak  103  increases. With this configuration, it is possible to suppress the lifetime killer formed by helium implantation from forming VOH defects, and it is possible to suppress the variation in the shape of the doping concentration distribution in the buffer region  20 . 
     When the carrier concentration distribution measured by the SRP method is a doping concentration distribution, the doping concentration distribution may have a valley portion  35  at the same depth position as that of any of the helium chemical concentration peaks  221 . The valley portion  35  is a region where the doping concentration shows a local minimum value. In this example, since the lower-surface-side lifetime killer  220  is provided at the same depth position as that of the helium chemical concentration peak  221 , the carrier density at this position decreases. 
       FIG.  6    illustrates a view showing another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . The doping concentration distribution and the hydrogen chemical concentration distribution in  FIG.  6    are the same as those in the example of  FIG.  5 A . The helium chemical concentration distribution in this example has the first helium chemical concentration peak  221 - 1 , the second helium chemical concentration peak  221 - 2 , and the third helium chemical concentration peak  221 - 3  in order from the lower surface  23  side of the semiconductor substrate  10 . The depth positions of the respective helium chemical concentration peaks  221  are set as Zk 1 , Zk 2 , and Zk 3  in order from the lower surface  23  side. Further, the concentration values of the respective helium chemical concentration peaks  221  are set as Pk 1 , Pk 2 , and Pk 3  in order from the lower surface  23  side. The recombination center concentration also has a distribution similar to that of the helium chemical concentration. 
     Also in this example, all helium chemical concentration peaks  221  are arranged between the depth positions Zd 1  and Zd 2 . In another example, any of the helium chemical concentration peaks  221  may be arranged in another region of the buffer region  20 . 
     The first helium chemical concentration peak  221 - 1  may have a higher concentration value Pk than at least one of the second helium chemical concentration peak  221 - 2  and the third helium chemical concentration peak  221 - 3 . The first helium chemical concentration peak  221 - 1  may be the helium chemical concentration peak  221  with the largest concentration value Pk. Further, the concentration value Pk of the helium chemical concentration peak  221  may decrease as a distance from the lower surface  23  of the semiconductor substrate  10  increases. Further, the straggling ΔRp or the full width at half maximum of the helium chemical concentration peak  221  may increase as the distance from the lower surface  23  of the semiconductor substrate  10  increases. 
     The relative magnitude relationship of the concentrations of the respective lower-surface-side lifetime killers  220  may be the same as the relative magnitude relationship of the concentrations of the corresponding helium chemical concentration peaks  221 . That is, the concentration of the lower-surface-side lifetime killer  220  may increase as the concentration of the corresponding helium chemical concentration peak  221  increases. 
     According to this example, the high-concentration lower-surface-side lifetime killer  220  is arranged in the vicinity of the lower surface  23 . Thus, the implantation of hole carriers from the collector region  22  into the drift region  18  can be suppressed. Further, it is possible to suppress an increase in leakage current and to improve tolerance at the time of turn-off or the like. 
       FIG.  7    illustrates a view showing another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . The doping concentration distribution and the hydrogen chemical concentration distribution in  FIG.  7    are the same as those in the example of  FIG.  5 A . The helium chemical concentration distribution of this example is different from that of the example of  FIG.  6    in the relative magnitude relationship of the concentrations of the respective helium chemical concentration peaks  221 . The other structures are the same as those of the example in  FIG.  6   . The recombination center concentration also has a distribution similar to that of the helium chemical concentration. 
     The first helium chemical concentration peak  221 - 1  may have a lower concentration value Pk than at least one of the second helium chemical concentration peak  221 - 2  and the third helium chemical concentration peak  221 - 3 . The first helium chemical concentration peak  221 - 1  may be the helium chemical concentration peak  221  with the smallest concentration value Pk. Further, the concentration value Pk of the helium chemical concentration peak  221  may increase as the distance from the lower surface  23  of the semiconductor substrate  10  increases. Further, the straggling ΔRp or the full width at half maximum of the helium chemical concentration peak  221  may increase as the distance from the lower surface  23  of the semiconductor substrate  10  increases. 
     According to this example, the high-concentration lower-surface-side lifetime killer  220  is arranged in the vicinity of the drift region  18 . Thus, the lifetime of the carrier flowing from the drift region  18  to the lower surface  23  side can be shortened at the time of turning off the semiconductor device  100  or the like. Thus, a period during which the tail current flows can be shortened. Further, it is possible to suppress an increase in leakage current and to improve tolerance at the time of turn-off or the like. 
       FIG.  8    illustrates a view showing another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . The doping concentration distribution and the hydrogen chemical concentration distribution in  FIG.  8    are the same as those in the example of  FIG.  5 A . In this example, a peak interval between a helium chemical concentration peak  221 - k  and a helium chemical concentration peak  221 -( k +1) in the depth direction is set as Lk (in  FIG.  8   , L 1 , L 2 ). The other structures are the same as any of the examples described in  FIGS.  5 A to  7   . The peak interval (in  FIG.  8   , L 1 , L 2 ) between two adjacent helium chemical concentration peaks  221  in the depth direction may be uniform in the buffer region  20 . The recombination center concentration also has a distribution similar to that of the helium chemical concentration. 
       FIG.  9    illustrates a view showing another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . The doping concentration distribution and the hydrogen chemical concentration distribution in  FIG.  9    are the same as those in the example of  FIG.  5 A . In this example, each peak interval Lk is different from that of the example of  FIG.  8   . The other structures are the same as those of the example in  FIG.  8   . 
     In this example, the first peak interval L 1  is smaller than the second peak interval L 2  at a position farther away from the lower surface  23  than the first peak interval L 1  (L 1 &lt;L 2 ). That is, in the buffer region  20 , the helium chemical concentration peak  221  is arranged at a higher density toward the lower surface  23 . The recombination center concentration also has a distribution similar to that of the helium chemical concentration. 
     According to this example, many lower-surface-side lifetime killers  220  can be formed in the vicinity of the collector region  22 . Thus, the implantation of hole carriers from the collector region  22  into the drift region  18  can be suppressed. 
       FIG.  10 A  illustrates a view showing another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . The doping concentration distribution and the hydrogen chemical concentration distribution in  FIG.  10 A  are the same as those in the example of  FIG.  5 A . In this example, each peak interval Lk is different from that of the example of  FIG.  8   . The other structures are the same as those of the example in  FIG.  8   . 
     In this example, the first peak interval L 1  is larger than the second peak interval L 2  (L 1 &gt;L 2 ). That is, in the buffer region  20 , the helium chemical concentration peak  221  is arranged at a higher density toward the drift region  18 . The recombination center concentration also has a distribution similar to that of the helium chemical concentration. 
     According to this example, many lower-surface-side lifetime killers  220  can be formed in the vicinity of the drift region  18 . Thus, the lifetime of the carrier flowing from the drift region  18  to the lower surface  23  side can be shortened at the time of turning off the semiconductor device  100  or the like. Thus, a period during which the tail current flows can be shortened. 
       FIG.  10 B  illustrates a view showing another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . The doping concentration distribution and the hydrogen chemical concentration distribution in  FIG.  10 B  are the same as those in the example of  FIG.  5 A . 
     A region between two adjacent doping concentration peaks  25  in the depth direction is set as an inter-peak region  105 . A region between two adjacent hydrogen chemical concentration peaks  103  in the depth direction may be set as the inter-peak region  105 . In this example, a region between the depth positions Zd 1  and Zd 2  (or Zh 1  and Zh 2 ) is set as an inter-peak region  105 - 1 , a region between the depth positions Zd 2  and Zd 3  (or Zh 2  and Zh 3 ) is set as an inter-peak region  105 - 2 , and a region between the depth positions Zd 3  and Zd 4  (or Zh 3  and Zh 4 ) is set as an inter-peak region  105 - 3 . 
     In this example, the helium chemical concentration peaks  221  are arranged in two or more inter-peak regions  105 . The helium chemical concentration peak  221  may be arranged in two inter-peak regions  105  adjacent to each other. One or more helium chemical concentration peaks  221  may be arranged in each inter-peak region  105 . More helium chemical concentration peaks  221  may be arranged closer to the lower surface  23  in the inter-peak region  105 . In the example of  FIG.  10 B , two helium chemical concentration peaks  221  are arranged in the inter-peak region  105 - 1 , and one helium chemical concentration peak  221  is arranged in the inter-peak region  105 - 2 . 
     The magnitude relationship of the concentrations of the respective helium chemical concentration peaks  221  may be similar to that of any of the examples described in  FIGS.  5 A to  10 A . In the example of  FIG.  10 B , the concentration of the helium chemical concentration peak  221  decreases as the distance from the lower surface  23  increases. The interval of the helium chemical concentration peaks  221  may be similar to that of any of the examples described in  FIGS.  5 A to  10 A . The recombination center concentration may also have a distribution similar to that of the helium chemical concentration. 
       FIG.  10 C  illustrates a view showing another example of the helium chemical concentration distribution and the recombination center concentration distribution in the buffer region  20 . The doping concentration distribution and the hydrogen chemical concentration distribution in  FIG.  10 C  are the same as those in the example of  FIG.  5 A . 
     In this example, the helium chemical concentration peak  221  is not arranged in the inter-peak region  105  between two inter-peak regions  105  where the helium chemical concentration peaks  221  are arranged. In the example of  FIG.  10 C , two helium chemical concentration peaks  221  are arranged in the inter-peak region  105 - 1 , no helium chemical concentration peak  221  is arranged in the inter-peak region  105 - 2 , and one helium chemical concentration peak  221  is arranged in the inter-peak region  105 - 3 . The concentration of each helium chemical concentration peak  221  may be similar to that of the example of  FIG.  10 B . The recombination center concentration may also have a distribution similar to that of the helium chemical concentration. 
       FIG.  11    illustrates the full width at half maximum Wk of the helium chemical concentration peak  221 . In this example, the full width at half maximum of the hydrogen chemical concentration peak  103  is set as Wh. In  FIG.  11   , only one helium chemical concentration peak  221  and one hydrogen chemical concentration peak  103  are shown, and the other peaks are omitted. 
     The full width at half maximum Wk of each helium chemical concentration peak  221  is smaller than the full width at half maximum Wh of any hydrogen chemical concentration peak  103  arranged farther away from the lower surface  23  of the semiconductor substrate than each helium chemical concentration peak  221 . For example, the full width at half maximum of each of the helium chemical concentration peaks  221 - 1 ,  221 - 2 , and  221 - 3  shown in  FIG.  10 A  is smaller than the full width at half maximum of any of the hydrogen chemical concentration peaks  103 - 2 ,  103 - 3 , and  103 - 4 . Each full width at half maximum Wk may be equal to or less than half of the full width at half maximum Wh of the hydrogen chemical concentration peak  103  farther away from lower surface  23 . By reducing the full width at half maximum Wk of the helium chemical concentration peak  221 , it is possible to suppress a change in the shape of the doping concentration distribution of the buffer region  20  over a wide range. 
       FIG.  12 A  illustrates a view showing an example of the doping concentration distribution and the hydrogen chemical concentration distribution in the buffer region  20 . The doping concentration distribution and the hydrogen chemical concentration distribution may be similar to those in the examples described in  FIGS.  5 A to  11   . Further, the helium chemical concentration distribution is the same as that of any of the examples described in  FIGS.  5 A to  11   . 
     In this example, the two doping concentration peaks  25 - 3  and  25 - 4  farthest away from the lower surface  23  of the semiconductor substrate  10  are not observed as clear concentration peaks. A ratio of the minimum value of the doping concentration in the region between the doping concentration peak  25 - 3  and the doping concentration peak  25 - 4  to the larger one of the concentration values of the doping concentration peak  25 - 3  and the doping concentration peak  25 - 4  is set as n. The ratio n may be 50% or less, may be 20% or less, or may be 10% or less. 
     Further, a ratio of the minimum value of the hydrogen chemical concentration in the region between the hydrogen chemical concentration peak  103 - 3  and the hydrogen chemical concentration peak  103 - 4  to the larger one of the concentration values of two hydrogen chemical concentration peaks  103 - 3  and  103 - 4  farthest away from lower surface  23  of semiconductor substrate  10  is set as m. The ratio m may be larger than the ratio n. That is, in a range from the depth position Zd 3  to Zd 4 , the amplitude of a fluctuation in the hydrogen chemical concentration distribution may be larger than the amplitude of a fluctuation in the doping concentration distribution. 
     Further, a region from the depth position Zd 1  to the depth position Zd 2  is set as a region X, and a region from the depth position Zd 2  to the depth position Zd 4  is set as a region Y. In the region X, a ratio of the minimum value of the hydrogen chemical concentration to the minimum value of the doping concentration is set as α. Similarly, in the region Y, a ratio of the minimum value of the hydrogen chemical concentration to the minimum value of the doping concentration is set as β. The ratio α may be larger than the ratio β. Further, in the depth direction, the region Y may be longer than the region X. The length of the region Y may be 1.5 times or more the length of the region X, or may be 2 times or more the length of the region X. 
       FIG.  12 B  illustrates a view showing some processes in a manufacturing method of the semiconductor device  100 . In this example, in an upper-surface-side structure forming step S 1200 , the structure on the upper surface  21  side of the semiconductor substrate  10  is formed. The structure on the upper surface  21  side may include at least one of doped regions, such as the emitter region  12 , the base region  14 , and the accumulation region  16 , on the upper surface  21  side of the semiconductor substrate  10 . The structure on the upper surface  21  side may include each trench portion. The structure on the upper surface  21  side may include a structure such as the emitter electrode  52  above the upper surface  21  of the semiconductor substrate  10 . The structure on the upper surface  21  side may include the edge termination structure portion  90 . 
     Next, in a substrate grinding step S 1202 , the lower surface  23  of the semiconductor substrate  10  is ground to thin the semiconductor substrate  10 . In S 1202 , the semiconductor substrate  10  may be thinned to a thickness corresponding to the breakdown voltage to be possessed by the semiconductor device  100 . 
     Next, in a lower-surface-side region forming step S 1204 , the lower surface doped region of the semiconductor substrate  10  is formed. The lower surface doped region is a doped region in contact with an electrode, such as the collector electrode  24  formed in a later process, formed on the lower surface  23 . The lower surface doped region may include at least one of the cathode region  82  and the collector region  22 . 
     Next, in a first ion implantation step S 1206 , ions for forming the buffer region  20  are implanted into the semiconductor substrate  10 . In S 1206 , ions may be implanted from the lower surface  23  of the semiconductor substrate  10  into a region where the buffer region  20  is to be formed. In S 1206 , a donor ion such as a hydrogen ion (for example, a proton) or a phosphorous ion may be implanted. 
     Next, in a first annealing step S 1208 , the semiconductor substrate  10  is thermally annealed. In S 1208 , the semiconductor substrate  10  may be put into an electric furnace to anneal the entire semiconductor substrate  10  (or a wafer). The annealing temperature in S 1208  may be 320 degrees C. or higher and 420 degrees C. or lower. In S 1208 , annealing may be performed in an atmosphere containing hydrogen and nitrogen. 
     Next, in a second ion implantation step S 1210 , ions for forming the lower-surface-side lifetime killer  220  are implanted into the semiconductor substrate  10 . In S 1210 , ions may be implanted from the lower surface  23  of the semiconductor substrate  10 . In S 1210 , hydrogen ions such as protons or helium ions may be implanted. In this example, helium ions are implanted. 
     In S 1210 , the lower-surface-side lifetime killer  220  described in  FIGS.  5 A to  10 C  is formed. The lower-surface-side lifetime killers  220  can be formed at a plurality of positions in the depth direction by sequentially changing the acceleration energy of helium ions or the like. In S 1210 , helium ions or the like may be implanted in order from a position close to the lower surface  23  among the plurality of positions in the depth direction, or helium ions or the like may be implanted in order from a position far from the lower surface  23 . In this example, helium ions are implanted in order from a position far from the lower surface  23 . Further, in S 1210 , ions may be implanted in order from the lower-surface-side lifetime killer  220  having a large dose amount, or ions may be implanted in order from the lower-surface-side lifetime killer  220  having a small dose amount. 
     Next, in the second annealing step S 1212 , the semiconductor substrate  10  is thermally annealed. In S 1212 , the semiconductor substrate  10  may be put into an electric furnace to anneal the entire semiconductor substrate  10  (or a wafer). The annealing temperature in S 1212  may be lower than the annealing temperature in S 1208 . The annealing temperature in S 1212  may be 300 degrees C. or higher and 400 degrees C. or lower. In S 1212 , annealing may be performed in a nitrogen atmosphere or an atmosphere containing hydrogen and nitrogen. 
     S 1212  may be performed each time helium ions or the like are implanted into one depth position in S 1210 , or may be performed each time helium ions or the like are implanted into a plurality of depth positions. A set of processes of S 1210  and S 1212  may be repeated a plurality of times (S 1213 ). 
     Next, in a lower surface electrode forming step S 1214 , an electrode in contact with the lower surface  23  is formed. In S 1214 , the collector electrode  24  may be formed. Through such a process, the semiconductor device  100  can be formed. 
       FIG.  12 C  illustrates a view showing another example of the doping concentration distribution and the hydrogen chemical concentration distribution in the buffer region  20 . Except for the matters particularly described or illustrated in  FIG.  12 C , the doping concentration distribution and the hydrogen chemical concentration distribution are similar to those in the example of  FIG.  12 A . The doping concentration distribution in the buffer region  20  of this example has a flat portion  250  between any two doping concentration peaks  25 . The flat portion  250  is a region where the variation of the doping concentration in a predetermined depth range is within a predetermined variation range. The depth range may be 0.5 μm or more, or may be 1 μm or more. The variation range may be ±30% or less of the average value of the concentrations at both ends of the depth range, may be ±20% or less, or may be ±10% or less. The variation range of the concentration distribution is a difference between the maximum value and the minimum value of the doping concentration in the region. 
     Further, in the flat portion  250 , a variation ratio R 1  of the doping concentration is less than a variation ratio R 2  of the hydrogen chemical concentration. The variation ratio of the concentration distribution is a ratio of the maximum value to the minimum value of the concentration in the region. That is, the variation ratio is a value obtained by dividing the maximum value of the concentration by the minimum value. The variation ratio R 1  may be half or less of the variation ratio R 2 , may be ¼ or less, or may be 1/10 or less. 
     Further, the peak width of the doping concentration peak  25  in the flat portion  250  may be larger than the peak width of the corresponding hydrogen chemical concentration peak  103 . The peak width of the doping concentration peak  25  in the flat portion  250  may be a distance between the minimum portion on the upper surface  21  side and the minimum portion on the lower surface  23  side in the doping concentration peak  25 . In the flat portion  250 , the maximum value of the doping concentration may be 50% or less of the minimum value. In this case, the minimum value of the doping concentration is 50% or more of the maximum value, and a full width at half maximum FWHM of the doping concentration peak  25  cannot be defined. When the full width at half maximum FWHM of the doping concentration peak  25  can be measured, the full width at half maximum FWHM may be used as the peak width of the doping concentration peak  25 . The full width at half maximum FWHM may be used as the peak width of the hydrogen chemical concentration peak  103 . 
     In the example of  FIG.  12 C , the flat portion  250  is arranged between the doping concentration peak  25 - 3  and the doping concentration peak  25 - 4 . The doping concentration in the flat portion  250  is larger than the doping concentration Dd of the drift region  18 . The doping concentration in the flat portion  250  may be 2.5 times or more the doping concentration Dd of the drift region  18 . 
     The buffer region  20  may have a plurality of doping concentration peaks  25  having no flat portion between peaks on the upper surface  21  side of the flat portion  250 . The definition of the flat portion is similar to that of the flat portion  250 . The buffer region  20  in the example of  FIG.  12 C  has the doping concentration peaks  25 - 4 ,  25 - 5 ,  25 - 6 , and  25 - 7  on the upper surface  21  side of the flat portion  250 . The value of the doping concentration peak  25  on the upper surface  21  side of the flat portion  250  may be substantially the same, or may decrease as the distance from the lower surface  23  increases. The term “substantially the same” may refer to 30% or less of the variation of the adjacent doping concentration peaks  25 , may refer to 20% or less, or may refer to 10% or less. 
     In the plurality of doping concentration peaks  25  having no flat portion between the peaks, the valley portions  251  may be provided between the peaks. In each valley portion  251 , the gradient (differential value) of the doping concentration distribution may continuously change from a negative value to a positive value in a direction from the lower surface  23  toward the upper surface  21 . On the other hand, in the flat portion  250 , the gradient of the doping concentration distribution may have a continuous value of substantially zero in the direction from the lower surface  23  toward the upper surface  21 . Note that the gradient of the doping concentration distribution may be the average value of a plurality of measurement points in a predetermined measurement range, the measurement points being based on the CV profiling or the SRP method, or the average value may be a value calculated by well-known fitting. 
     Further, the positions in the depth direction of the plurality of doping concentration peaks  25  having no flat portion between peaks correspond to the positions in the depth direction of the hydrogen chemical concentration peaks  103 . Each doping concentration peak  25  arranged closer to the upper surface  21  side than the flat portion  250  and the corresponding hydrogen chemical concentration peak  103  may have the following relationship. 
     
       
      
       C 
       Hv 
       /C 
       Hp 
       &lt;N 
       v 
       /N 
       p  
      
     
     Note that C Hp  is the concentration of the hydrogen chemical concentration peak  103 , C Hv  is the concentration of the valley portion  252  adjacent to the hydrogen chemical concentration peak  103  on the upper surface  21  side, N p  is the concentration of the doping concentration peak  25 , and N v  is the concentration of the valley portion  251  adjacent to the doping concentration peak  25  on the upper surface  21  side. C Hv /C Hp  may be 0.8 times or less of N v /N p , may be 0.5 times or less, may be 0.2 times or less, may be 0.1 times or less, or may be 0.01 times or less. C Hv /C Hp  may be 0.001 times or more of N v /N p , may be 0.01 times or more, or may be 0.1 times or more. 
     Since the semiconductor device  100  has the plurality of doping concentration peaks  25  having no flat portion between peaks on the upper surface  21  side of the flat portion  250 , the distribution of the doping concentration can be made gentle, and a change in an electric field intensity when the depletion layer reaches the buffer region  20  can be made gentle. With this configuration, a sudden change in a voltage waveform can be suppressed. 
       FIG.  12 D  illustrates a view showing still another example of the doping concentration distribution and the hydrogen chemical concentration distribution in the buffer region  20 . Except for the matters particularly described or illustrated in  FIG.  12 D , the doping concentration distribution and the hydrogen chemical concentration distribution are similar to those in the example of  FIG.  12 C . In the buffer region  20  of this example, the concentration of the doping concentration peak  25  on the upper surface  21  side of the flat portion  250  decreases toward the upper surface  21 . Further, the concentration of the hydrogen chemical concentration peak  103  on the upper surface  21  side of the flat portion  250  also decreases toward the upper surface  21 . With such a structure, the variation of the doping concentration of the buffer region  20  in the vicinity of the drift region  18  can be made gentle. 
     The concentration of the hydrogen chemical concentration peak  103 - k  on the upper surface  21  side of the flat portion  250  may be half or less of the concentration of the hydrogen chemical concentration peak  103 -( k −1) adjacent on the lower surface  23  side, or may be ¼ or less. The concentration of the hydrogen chemical concentration peak  103 - k  may be 1/10 or more of the concentration of the hydrogen chemical concentration peak  103 -( k −1). The concentration of the doping concentration peak  25 - k  on the upper surface  21  side of the flat portion  250  may be half or less of the concentration of the doping concentration peak  25 -( k −1) adjacent on the lower surface  23  side, or may be ¼ or less. The concentration of the doping concentration peak  25 - k  may be 1/10 or more of the concentration of the doping concentration peak  25 -( k −1). Also in this example, the fluctuation (a difference between N v  and N p ) in the doping concentration on the upper surface  21  side of the flat portion  250  is smaller than the fluctuation (a difference between C HV  and C Hp ) in the hydrogen chemical concentration. Further, the half-value width of the doping concentration peak  25  is larger than the half-value width of the hydrogen chemical concentration peak  103 . 
     An envelope connecting the hydrogen chemical concentration peaks  103 - k  is set as a hydrogen peak envelope  231 . An envelope connecting the valley portions  104 - k  of the hydrogen chemical concentration is set as a hydrogen valley portion envelope  232 . Further, an envelope connecting the doping concentration peak  25 - k  is set as a doping peak envelope  233 . An envelope connecting the valley portions  26 - k  of the doping concentration is set as a doping valley portion envelope  234 . At any position X between the position Zd 4  and the position Zf, a first ratio of the hydrogen peak envelope  231  to the hydrogen valley portion envelope  232  may be larger than a second ratio of the doping peak envelope  233  to the doping valley portion envelope  234 . The first ratio may be larger than 2 times the second ratio, or may be larger than 3 times. Since the semiconductor device  100  has the plurality of doping concentration peaks  25  on the upper surface  21  side of the flat portion  250  and has the configuration in which the plurality of doping concentration peaks  25  decreases, the distribution of the doping concentration can be made gentle, and the change in the electric field intensity when the depletion layer reaches the buffer region  20  can be made gentle. With this configuration, a sudden change in a voltage waveform can be suppressed. 
       FIG.  12 E  illustrates a view showing another example of the doping concentration distribution and the hydrogen chemical concentration distribution in the buffer region  20 . Except for the matters particularly described or illustrated in  FIG.  12 E , the doping concentration distribution and the hydrogen chemical concentration distribution are similar to those in the example of  FIG.  12 D . In the buffer region  20  of this example, the doping concentration distribution gently varies in an adjacent region  240  in contact with the drift region  18 . The adjacent region  240  includes a plurality of hydrogen chemical concentration peaks  103  and is a region where the concentration of the hydrogen chemical concentration peak  103  decreases as the distance from the lower surface  23  increases. The adjacent region  240  of this example is a region from the depth position Zd 4  to Zf. A region between the drift region  18  and the flat portion  250  arranged closest to the upper surface  21  side in the buffer region  20  may be set as the adjacent region  240 . 
     In this example, a range (that is, a width) of the adjacent region  240  in the depth direction is larger than that in the example of  FIG.  12 D . The range of the adjacent region  240  can be adjusted by the interval between the hydrogen chemical concentration peaks  103  arranged on the upper surface  21  side of the flat portion  250 . The adjacent region  240  may occupy 30% or more of the buffer region  20  (Zd 1  to Zf) in the depth direction, or may occupy 50% or more. The width (Zf−Zd 4 ) of the adjacent region  240  in the depth direction may be larger than the width (Zd 4 −Zd 3 ) of the flat portion  250  in the depth direction. The width (Zf−Zd 4 ) may be 2 times or more the width (Zd 4 −Zd 3 ), may be 3 times or more, or may be 5 times or more. 
     The doping concentration distribution in the adjacent region  240  is approximated by a straight line  230 . The straight line  230  can be calculated by a least-squares method or the like. A slope α of the straight line  230  in the adjacent region  240  may be expressed using a semi-logarithmic slope. The position of one end of the adjacent region  240  is x1 [cm], and the position of the other end is x2 [cm]. In the example of  FIG.  12 E , x1 corresponds to the depth position Zd 4 , and x2 corresponds to the depth position Zf. The doping concentration in x1 is set as N1 [/cm 3 ], and the doping concentration in x2 is set as N2 [/cm 3 ]. The slope a of the straight line  230  is given by the following equation. 
       α=(|log 10 ( N 2)−log 10 ( N 1)|)/(| x 2− x 1|)
 
     The slope α of the straight line  230  of this example may be 20 (/cm) or more and 200 (/cm) or less. The slope α may be 40 (/cm) or more, or may be 60 (/cm) or more. The slope α may be 180 (/cm) or less, or may be 160 (/cm) or less. By making the slope α of the straight line  230  gentle, the expanding of the depletion layer (space charge region) reaching the adjacent region  240  at the time of switching of the semiconductor device  100  can be made gentle. 
       FIG.  12 F  illustrates a view showing another example of the processes in the manufacturing method of the semiconductor device  100 . The manufacturing method of this example is different from the example of  FIG.  12 B  in that the lower-surface-side region forming step S 1204  is performed after the first annealing step S 1208  and before the second ion implantation step S 1210 . Other processes are similar to those of the example of  FIG.  12 B . 
     The first ion implantation step S 1206  may include processes S 1601  to S 1604  described below. In this case, the doping concentration peak  25  closest to the lower surface  23  in the buffer region  20  can be formed without loss. Thus, even when the collector region  22  is formed in the lower-surface-side region forming step S 1204  after the first ion implantation step S 1206 , there is no problem that the depletion layer reaches the collector region  22  as described below. 
       FIG.  12 G  illustrates a view showing another example of the processes in the manufacturing method of the semiconductor device  100 . The manufacturing method of this example is different from the example of  FIG.  12 B  in that the lower-surface-side region forming step S 1204  is performed after the second annealing step S 1212  and before the lower surface electrode forming step S 1214 . Other processes are similar to those of the example of  FIG.  12 B . 
     Also in this example, the first ion implantation step S 1206  may include processes S 1601  to S 1604  described below. In this case, the doping concentration peak  25  closest to the lower surface  23  in the buffer region  20  can be formed without loss. Thus, even when the collector region  22  is formed in the lower-surface-side region forming step S 1204  after the first ion implantation step S 1206 , there is no problem that the depletion layer reaches the collector region  22  as described below. 
       FIG.  13    shows an example of the carrier concentration distribution and the helium chemical concentration distribution in the buffer region  20  of a comparative example. The buffer region  20  of this example has only one peak of helium chemical concentration formed by implanting  3 He. Further, in  FIG.  13   , the carrier concentration distribution when helium is not implanted is indicated by a solid line, and the carrier concentration distribution when helium is implanted is indicated by a broken line. The carrier concentration distribution when helium is not implanted is similar to the doping concentration distribution in  FIG.  5 A  or the like. 
     In this example, a single helium chemical concentration peak is provided in the buffer region  20 . Thus, it is difficult to control the distribution of the lifetime killer. Further, when the half-value width of the helium chemical concentration peak is large, the carrier concentration distribution varies in a wide range as compared with a case where helium is not implanted. In contrast, in the example of  FIGS.  1  to  12 B , since the plurality of helium chemical concentration peaks are arranged in the buffer region  20 , the distribution of the lifetime killer can be adjusted precisely. Further, by reducing the half-value width of the helium chemical concentration peak, it is possible to suppress the variation of the carrier concentration distribution in a wide range. 
     Second Example 
       FIG.  14    illustrates a view showing another example of the cross section e-e. In the semiconductor device  100  of the this example, the method of forming the buffer region  20  is different from that of the first example described in  FIGS.  1  to  13   . The method of forming the buffer region  20  will be described below. The other portions are similar to those in the first example Note that in the semiconductor device  100  of this example, the lower-surface-side lifetime killer  220  may be provided in the buffer region  20 , or may not be provided. That is, the helium chemical concentration peak  221  may be provided in the buffer region  20 , or may not be provided. 
       FIG.  15    illustrates a view showing an example of the doping concentration distribution and the hydrogen chemical concentration distribution taken along line F-F in  FIG.  14   . The doping concentration distribution and the hydrogen chemical concentration distribution may be similar to those of the example of  FIG.  5 A . Note that although  FIG.  15    shows an example in which each doping concentration peak in the doping concentration distribution can be clearly observed, any doping concentration peak may not be clearly observed as in the example of  FIG.  5 A . 
       FIG.  16    illustrates a view showing an example of the method of forming the buffer region  20 .  FIG.  16    shows an implantation process of implanting a dopant into the buffer region  20 . First, a first dopant of the N type is implanted into a first implantation position from the implantation surface of the semiconductor substrate  10  (S 1601 ). In this example, the implantation surface is the lower surface  23 , and the first implantation position is the depth position Zd 1  (or Zh 1 ) described in  FIG.  5 A  and the like. Further, the first dopant is, for example, a hydrogen ion or a phosphorous ion. 
     After the first dopant is implanted, a second dopant of the N type is implanted from the implantation surface (the lower surface  23  in this example) of the semiconductor substrate  10  into a second implantation position having a larger distance from the implantation surface than the first implantation position (S 1602 ). In this example, the second implantation position is the depth position Zd 2  (or Zh 2 ) described in  FIG.  5 A  and the like. Further, the second dopant is, for example, a hydrogen ion or a phosphorous ion. The second dopant may be the same element as the first dopant. For example, both the first dopant and the second dopant are hydrogen ions. In another example, one of the first dopant and the second dopant may be a phosphorous ion, and the other may be a hydrogen ion. 
     After the second dopant is implanted, a third dopant of the N type is implanted from the implantation surface (the lower surface  23  in this example) of the semiconductor substrate  10  into a third implantation position having a larger distance from the implantation surface than the second implantation position (S 1603 ). In this example, the third implantation position is the depth position Zd 3  (or Zh 3 ) described in  FIG.  5 A  and the like. Further, the third dopant is, for example, a hydrogen ion or a phosphorous ion. The third dopant may be the same element as the first dopant or the second dopant. For example, the first dopant, the second dopant, and the third dopant are all hydrogen ions. In another example, a part of the first dopant, the second dopant, and the third dopant may be hydrogen ions, and a part thereof may be phosphorous ions. 
     After the third dopant is implanted, a fourth dopant of the N type is implanted from the implantation surface (the lower surface  23  in this example) of the semiconductor substrate  10  into a fourth implantation position having a larger distance from the implantation surface than the third implantation position (S 1604 ). In this example, the fourth implantation position is the depth position Zd 4  (or Zh 4 ) described in  FIG.  5 A  and the like. Further, the fourth dopant is, for example, a hydrogen ion or a phosphorous ion. The fourth dopant may be the same element as the first dopant, the second dopant, or the third dopant. For example, the first dopant, the second dopant, the third dopant, and the fourth dopant are all hydrogen ions. In another example, a part of the first dopant, the second dopant, the third dopant, and the fourth dopant may be hydrogen ions, and a part thereof may be phosphorous ions. 
     In the implantation process, three or more N-type dopants including the first dopant and the second dopant may be implanted from the implantation surface of the semiconductor substrate  10  into the implantation positions having different depths from each other. In the example of  FIG.  16   , the dopant is implanted into four depth positions, but it is sufficient if the dopant is implanted into two or more depth positions. 
     When the dopant is implanted into the semiconductor substrate  10 , foreign matter such as particles may adhere to the implantation surface. When the dopant is further implanted from the implantation surface in a state where the foreign matter adheres to the implantation surface, the dopant is shielded by the foreign matter, and the dopant may not be able to be implanted precisely. In particular, when a distance between the depth position into which the dopant is implanted and the implantation surface is short, the acceleration energy of the dopant is small, and thus the dopant is easily shielded by the foreign matter. 
     According to this example, after the first dopant is implanted, the second dopant is implanted at a deeper position. Thus, even when the foreign matter adheres to the implantation surface in the process of implanting the second dopant (S 1602 ), the implantation of the first dopant is not affected. Thus, the implantation of the first dopant having a relatively small acceleration energy can be performed precisely. 
     In the implantation process, it is preferable to initially implant a dopant to be implanted into the implantation position closest to the lower surface  23  of the semiconductor substrate  10  among the plurality of dopants to be implanted into the buffer region  20 . In this example, the first dopant to be implanted into the implantation position closest to the lower surface  23  is implanted initially. Accordingly, the implantation of the first dopant having the smallest acceleration energy can be performed precisely. In another example, the buffer region  20  may include a dopant that is implanted after the first dopant and that is implanted closer to the lower surface  23  than the first dopant. 
     In the implantation process, among the plurality of dopants to be implanted into the buffer region  20 , a dopant to be implanted into the implantation position farthest from the lower surface  23  of the semiconductor substrate  10  may be implanted finally. In this example, the fourth dopant to be implanted into the implantation position farthest from the lower surface  23  is implanted finally. Accordingly, the implantation of each dopant having acceleration energy smaller than that of the fourth dopant can be performed precisely. 
     Further, as illustrated in  FIG.  16   , in the implantation process, dopants may be implanted in order from an implantation position close to the lower surface  23  of the semiconductor substrate  10 . Accordingly, the dopants can be implanted in order from the dopant having smaller acceleration energy, and thus the implantation of each dopant can be performed precisely. 
     A distance between the depth position Zd 4 , which is farthest from the lower surface  23  of the semiconductor substrate  10  among the implantation positions of the plurality of dopants to be implanted into the buffer region  20 , and the lower surface  23  of the semiconductor substrate  10  may be half or less of the thickness of the semiconductor substrate  10 . That is, the depth position Zd 4  is arranged between the center position Zc (see  FIG.  4 A ) of the semiconductor substrate  10  and the lower surface  23 . In the manufacturing process of the semiconductor device  100 , dopants of the same conductivity type to be implanted from the same implantation surface (the lower surface  23  in this example) into a region on the implantation surface side (the lower surface  23  side in this example) of the semiconductor substrate  10  may be implanted in order from a region closer to the implantation surface. 
     Further, in the top view, a range where the first dopant is implanted and a range where the second dopant is implanted may be the same. The implantation ranges of all the dopants of the first conductivity type to be implanted into the buffer region  20  in the implantation process may be the same. 
       FIG.  17    illustrates a view showing a cross-sectional shape of the collector region  22  according to the comparative example. In this example, dopants are implanted into the buffer region  20  in order from a position far from the lower surface  23 . In this case, for example, a dopant, such as the first dopant, having a shallow implantation position and small acceleration energy may be shielded by particles on the implantation surface. When the first dopant is locally shielded, the doping concentration peak  25 - 1  is locally missing in an XY plane. 
     When the doping concentration peak  25 - 1  is locally missing, the donor concentration in the region becomes low, so that the collector region  22  easily enters the region. As a result, as illustrated in  FIG.  17   , a portion protruding upward is generated in a part of the collector region  22 . Thus, when the semiconductor device  100  is turned off, the depletion layer expanding from the lower end of the base region  14  easily reaches the collector region  22 . In the transistor portion  70 , the collector region  22  of the p type is formed on the lower surface  23  of the semiconductor substrate  10 . Further, also in the edge termination structure portion  90  and a part of a region of the diode portion  80 , the collector region  22  may be formed on the lower surface  23 . In the region where the collector region  22  of the p type is formed on the lower surface  23  in this manner, when the doping concentration peak  25 - 1  is locally missing, the breakdown voltage decreases. 
       FIG.  18    illustrates a view showing a result of a breakdown voltage test of the semiconductor device. In  FIG.  18   , a horizontal axis represents the voltage applied between the emitter and the collector of the semiconductor device in an off state, and a vertical axis represents the current flowing between the emitter and the collector of the semiconductor device. In the semiconductor device of the comparative example described in  FIG.  17   , when an emitter-collector voltage Vce is 1400 V or less, a large emitter-collector current Ices flows. In contrast, in the semiconductor device  100  according to the example, even when the emitter-collector voltage Vce was approximately 1600 V, a large emitter-collector current Ices did not flow. That is, in the semiconductor device  100  according to the example, a breakdown voltage is improved compared with that of the comparative example. 
       FIG.  19    illustrates a view showing a result of a breakdown voltage test of the semiconductor device.  FIG.  19    shows the number of semiconductor devices determined to be defective by the breakdown voltage test. In the breakdown voltage test, a semiconductor device having a predetermined breakdown voltage or less is determined to be defective.  FIG.  19    shows test results of a semiconductor device of a reference example in which an implantation surface is cleaned and each dopant is implanted, in addition to the semiconductor device  100  according to the comparative example and the example shown in  FIG.  17   . In the reference example, the dopant was implanted into the buffer region  20  in the same implantation order as in the comparative example, and the implantation surface was cleaned with water each time the dopant was implanted. 
     As illustrated in  FIG.  19   , according to the example, the number of defects could be significantly reduced without changing the design of each concentration distribution in the buffer region  20  as compared with the comparative example. Further, the number of defects can be reduced in the example as compared with the reference example in which the implantation surface is cleaned. As described above, in the semiconductor device  100  in which the collector region  22  of the p type is formed on the lower surface  23 , the number of defects of the breakdown voltage can be significantly reduced. 
       FIG.  20    illustrates a view showing another example of the semiconductor device  100 . In the example described in  FIGS.  14  to  16   , an example in which the buffer region  20  has the plurality of doping concentration peaks  25  has been described. In the semiconductor device  100  of this example, the accumulation region  16  has a plurality of doping concentration peaks  25 . In  FIG.  20   , a process of implanting a dopant into the accumulation region  16  will be described. The buffer region  20  may or may not have a plurality of doping concentration peaks  25  formed in processes similar to those of the example of  FIGS.  14  to  16   . 
     In the process of implanting the dopant into the accumulation region  16 , each dopant may be implanted in an order similar to that of the process of implanting the dopant into the buffer region  20  described in  FIGS.  14  to  16   . Note that this example is different from the example of  FIGS.  14  to  16    in that the implantation surface is the upper surface  21  and the reference position of the implantation position of each dopant is the upper surface  21 . Other content may be the same as that of the example of  FIGS.  14  to  16   . For example, in the description of the implantation process in  FIG.  16   , the “buffer region  20 ” may be replaced with the “accumulation region  16 ”, and the “lower surface  23 ” may be replaced with the “upper surface  21 ”. 
     In the example of  FIG.  20   , first, the first dopant of the N type is implanted into the first implantation position from the implantation surface of the semiconductor substrate  10  (S 2001 ). In this example, the implantation surface is the upper surface  21 . Further, the first implantation position is a position away from the upper surface  21  by the distance Zd 1  or Zh 1 . Further, the first dopant is, for example, a hydrogen ion or a phosphorous ion. 
     After the first dopant is implanted, the second dopant of the N type is implanted from the implantation surface (the upper surface  21  in this example) of the semiconductor substrate  10  into the second implantation position having a larger distance from the implantation surface than the first implantation position (S 2002 ). In this example, the second implantation position is a position away from the upper surface  21  by the distance Zd 2  or Zh 2 . In this example, a first depth position (first implantation position) into which the first dopant is implanted and a second depth position (second implantation position) into which the second dopant is implanted are arranged in the accumulation region  16 . Further, the second dopant is, for example, a hydrogen ion or a phosphorous ion. The second dopant may be the same element as the first dopant. For example, both the first dopant and the second dopant are hydrogen ions. In another example, one of the first dopant and the second dopant may be a phosphorous ion, and the other may be a hydrogen ion. 
     In the example of  FIG.  20   , the accumulation region  16  has two doping concentration peaks  25 , but it is sufficient if the number of the doping concentration peaks  25  may be two or more. According to this example, after the first dopant is implanted, the second dopant is implanted at a deeper position. Thus, even when the foreign matter adheres to the implantation surface in the process of implanting the second dopant (S 2002 ), the implantation of the first dopant is not affected. Thus, the implantation of the first dopant having a relatively small acceleration energy can be performed precisely. 
       FIG.  21    illustrates a view showing another example of the manufacturing process of the semiconductor device  100 . In this example, the passed-through region forming process S 2101  is executed before the implantation process described with reference to  FIG.  16   . Further, any dopant implanted into the buffer region  20  is hydrogen ions. At least one of the first dopant and the second dopant having a relatively high doping concentration may be hydrogen ions. Further, the other dopant may be hydrogen ions. 
     In the passed-through region forming process S 2101 , charged particles are implanted from the lower surface  23 . The charged particles are hydrogen ions, helium ions, electron beams, or the like. The range of the charged particles is half or more of the thickness of the semiconductor substrate  10 . The range of the charged particles may be larger than the thickness of the semiconductor substrate  10 . The region of the semiconductor substrate  10  through which the charged particles have passed is referred to as a passed-through region. The passed-through region may include half or more of the drift region  18  in the depth direction, or may include the whole. 
     In the passed-through region through which the charged particles have passed in the semiconductor substrate  10 , the lattice defects mainly composed of vacancies such as monatomic vacancies (V) and diatomic vacancies or divacancies (VV) are formed by the charged particles passing therethrough. Atoms adjacent to the vacancies have dangling bonds. The lattice defects also include interstitial atoms, dislocations, and the like, and may include donors and acceptors in a broad sense. However, in the present specification, the lattice defects mainly composed of vacancies may be referred to as vacancy-type lattice defects, vacancy-type defects, or simply lattice defects. In the present specification, the concentration of lattice defects mainly composed of vacancies may be referred to as a vacancy concentration. Further, since many lattice defects are formed due to the implantation of the charged particles into the semiconductor substrate  10 , the crystallinity of the semiconductor substrate  10  may be strongly disturbed. In the present specification, this disturbance of crystallinity may be referred to as disorder. 
     After the passed-through region forming process S 2101 , an implantation process S 2103  is performed. An annealing process S 2102  of annealing the semiconductor substrate  10  may be performed between the passed-through region forming process S 2101  and the implantation process S 2103 . 
     The implantation process S 2103  includes processes S 1601  to S 1604  described in  FIG.  16   . As described above, in the implantation process S 2103 , hydrogen ions are implanted into at least one depth position of the buffer region  20 . Thus, the buffer region  20  contains hydrogen. 
     After the implantation process S 2103 , a hydrogen diffusion process S 2104  is performed. In the hydrogen diffusion process S 2104 , hydrogen in the buffer region  20  is diffused into the passed-through region by annealing the semiconductor substrate  10 . The annealing temperature in hydrogen diffusion process S 2104  may be equal to or lower than the annealing temperature in the annealing process S 2102 . 
     Oxygen is contained In the entire semiconductor substrate  10 . The oxygen is introduced intentionally or unintentionally during manufacturing a semiconductor ingot. In the semiconductor substrate  10 , hydrogen (H), vacancies (V), and oxygen (O) are combined to form a VOH defect. Further, by diffusing hydrogen after forming the passed-through region, lattice defects in the passed-through region are combined to hydrogen, and formation of the VOH defect is promoted. The VOH defect functions as a donor that supplies electrons. In the present specification, the VOH defect may be referred to simply as a hydrogen donor. 
     In the semiconductor substrate  10  of this example, the hydrogen donor is formed in a hydrogen ion passed-through region. The hydrogen donor in the passed-through region is formed when hydrogen terminates the dangling bond of vacancy-type lattice defects formed in the passed-through region and is further combined to oxygen. Therefore, the doping concentration distribution of the hydrogen donor in the passed-through region may follow a vacancy concentration distribution. The hydrogen chemical concentration in the passed-through region may be 10 times or more the vacancy concentration formed in the passed-through region, or may be 100 times or more. The hydrogen in the passed-through region may be hydrogen remaining after the passage of hydrogen ions, or may be hydrogen diffused from a hydrogen supply source described below. The doping concentration of the hydrogen donor is lower than the chemical concentration of hydrogen. When the ratio of the doping concentration of the hydrogen donor to the chemical concentration of hydrogen is defined as an activation ratio, the activation ratio may be a value of 0.1% to 30%. In this example, the activation ratio is 1% to 5%. 
     By forming a hydrogen donor in the passed-through region of the semiconductor substrate  10 , the donor concentration in the passed-through region can be made higher than the bulk donor concentration. Normally, it is necessary to prepare the semiconductor substrate  10  having a predetermined bulk donor concentration in correspondence to characteristics of an element to be formed on the semiconductor substrate  10 , particularly a rated voltage or a breakdown voltage. In this case, as described in  FIG.  4 A , the doping concentration of the drift region  18  is substantially equal to the bulk donor concentration. In contrast, according to the semiconductor device  100  illustrated in  FIG.  21   , the donor concentration of the semiconductor substrate  10  can be adjusted by controlling the dose amount of the charged particles or the hydrogen ions. Thus, the semiconductor device  100  having the drift region  18  with a predetermined doping concentration can be manufactured using a semiconductor substrate with a bulk donor concentration that does not correspond to the characteristics and the like of the element. The variation in the bulk donor concentration at the time of manufacturing the semiconductor substrate  10  is relatively large, but the dose amount of the hydrogen ions can be controlled with relatively high precision. Thus, the concentration of lattice defects generated by implanting hydrogen ions can also be controlled with high precision, and the donor concentration of the passed-through region can be controlled with high precision. 
     In the example of  FIG.  21   , the implantation process S 2103  is performed after the passed-through region forming process S 2101 . In another example, the passed-through region forming process S 2101  may be performed between the implantation process S 2103  and the hydrogen diffusion process S 2104 . 
       FIG.  22    illustrates a view showing an example of the doping concentration distribution and the hydrogen chemical concentration distribution of the semiconductor device  100  shown in  FIG.  21   .  FIG.  22    shows a concentration distribution at a position corresponding to the line F-F shown in  FIG.  3   . In this example, in the passed-through region forming process S 2101 , charged particles are implanted into the semiconductor substrate  10  in a range larger than the thickness of the semiconductor substrate  10 . That is, most of the charged particles passes through the semiconductor substrate  10 . 
     As described above, lattice defects are formed in a region through which the charged particles pass in the semiconductor substrate  10 . In this example, the entire semiconductor substrate  10  is a passed-through region. Then, the hydrogen diffused from the buffer region  20  in the hydrogen diffusion process S 2104  is combined to lattice defects to form VOH defects. Thus, the doping concentration in the passed-through region is higher than a bulk donor concentration D0. 
     Further, the hydrogen chemical concentration may monotonically decrease from the buffer region  20  toward the upper surface  21 , may be flat, or may monotonically increase. For example, when hydrogen ions are implanted as the charged particles in the passed-through region forming process S 2101 , the hydrogen chemical concentration may monotonically increase from the buffer region  20  toward the upper surface  21 . The doping concentration may monotonically decrease from the buffer region  20  toward the upper surface  21 , may be flat, or may monotonically increase. 
     Third Example 
       FIG.  23    illustrates a view showing another example of the cross section e-e. The semiconductor device  100  of this example is different from each example described in  FIGS.  1  to  22    in that the buffer region  20  has a plurality of doping concentration peaks  25  and a plurality of lower-surface-side lifetime killers  220 . The structure and the forming method of the plurality of doping concentration peaks  25  are the same as those in the second example described in  FIGS.  14    to  22 . Further, the structure and the forming method of the plurality of lower-surface-side lifetime killers  220  are similar to those of the first example described in  FIGS.  1  to  13   . The buffer region  20  has a plurality of helium chemical concentration peaks  221  corresponding to the plurality of lower-surface-side lifetime killers  220 , as in the first example described in  FIGS.  1  to  13   . The structure other than the buffer region  20  is the same as any of the examples described in  FIGS.  1  to  22   . 
       FIG.  24    illustrates a view showing an example of a method of forming the buffer region  20  shown in  FIG.  23   . In this example, first, in the implantation process S 2401 , dopants such as hydrogen ions are implanted into a plurality of depth positions of the buffer region  20 . The implantation process S 2401  includes processes S 1601  to S 1604  described in  FIG.  16   . 
     Next, in a first annealing process S 2402 , the semiconductor substrate  10  is annealed. Accordingly, the plurality of doping concentration peaks  25  can be formed in the buffer region  20 . 
     Next, in a helium implantation process S 2403 , helium ions are implanted into different depth positions of the buffer region  20  from the lower surface  23 . In the helium implantation process S 2403 , helium ions may be implanted in order from a depth position close to the lower surface  23 . In another example, helium ions may be implanted in a different order. In the helium implantation process S 2403 , helium ions also may be implanted in order from a depth position having a long distance from the lower surface  23 . Even when the helium chemical concentration peak  221  is locally missing, the protrusion of the collector region  22  as shown in  FIG.  17    is not formed. Further, by performing the implantation process S 2401  before the helium implantation process S 2403 , it is possible to prevent the dopant in the implantation process S 2401  from being shielded by the foreign matter adhering to the implantation surface in the helium implantation process S 2403 . 
     A second annealing process S 2404  of annealing the semiconductor substrate  10  may be performed after the helium implantation process S 2403 . Accordingly, excessive lattice defects or the like generated in the helium implantation process S 2403  can be terminated with hydrogen. The annealing temperature in the second annealing process S 2404  may be lower than the annealing temperature in the first annealing process S 2402 . 
     In this example, the helium implantation process S 2403  is performed after the implantation process S 2401 . In another example, the implantation process S 2401  may be performed after the helium implantation process S 2403 . An annealing process is preferably performed after each implantation process. 
     While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention. 
     The operations, procedures, steps, and stages of each process performed by an apparatus, system, program, and method shown in the claims, embodiments, or diagrams can be performed in any order as long as the order is not indicated by “prior to,” “before,” or the like and as long as the output from a previous process is not used in a later process. Even if the process flow is described using phrases such as “first” or “next” in the claims, embodiments, or diagrams, it does not necessarily mean that the process must be performed in this order.