Abstract:
A semiconductor device includes a semiconductor material, the semiconductor material including a base region and a field stop zone including a first side adjacent the base region and a second side opposite the first side. The field stop zone includes a first dopant implant and a second dopant implant. The first dopant implant has a first dopant concentration maximum and the second dopant implant has a second dopant concentration maximum with the first dopant concentration maximum being less than the second dopant concentration maximum, and being located closer to the second side than the second dopant concentration maximum.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to semiconductor components such as IGBTs (insulated gate bipolar transistors), diodes and thyristors having a field stop zone. 
     A field stop zone typically is located adjacent an n-base layer. The field stop zone thus may define two sides, with a side adjacent the n-base layer, and a side away from the n-base layer. In an IGBT, a p-type collector layer typically borders the side away from the base layer. In a modified design, the field stop zone can also be surrounded by the n-base layer. In this case, the side adjacent the n-base layer is defined as the side of the n-base layer which is farther away from the p-type collector. While the field stop zone typically is a doped area of an n material, in some semiconductor components it may be a doped area of a p material. 
     It is known to provide more than one dopant implant in the field stop zone, with the highest concentration implant nearest the side away from the base material. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       The present invention will be further elucidated with reference to the embodiments of the drawings, in which: 
         FIG. 1  shows schematically an embodiment of the present invention with an IGBT including a field stop zone; 
         FIGS. 2   a ,  2   b  and  2   c  show schematically possible field stop zone dopant profiles according to a first embodiment of the present invention; and 
         FIG. 3  shows schematically a further possible field stop zone dopant profile according to another embodiment of the present invention; and 
         FIG. 4  shows schematically an embodiment of the present invention with a diode having a field stop zone. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows schematically an embodiment of the present invention with an IGBT  10  having an emitter or cathode electrical contact  12  and a collector or anode electrical contact  14 , both made for example of copper or aluminium. A gate electrode  16 , made for example of polycrystalline silicon, is located in a dielectric material layer  18 , made for example of silicon dioxide. 
     A base material  20  made of a semiconductor material such as silicon has a p base region  22  introduced on an n-base region  24 . A pn junction thus is formed for the device  10 . An n+ emitter region  36  is located on the p base region  22 . 
     The field stop zone has a side  30  facing away from the base region  24 , and a side  32  adjacent the base region  24  and which can be defined as the area where an implanted dopant concentration approaches or reaches the dopant concentration level of the n− base region  24 . 
     A dopant concentration profile  34  thus can be defined with increasing depth from side  30  to side  32 . 
     A p emitter layer  28  is adjacent the field stop side  30  and is in contact with anode contact  14 . 
     When a voltage applied to the gate electrode  16  exceeds a threshold voltage of the device, the IGBT is turned on and the resistance in the base region  24  is reduced between the cathode contact  12  and anode contact  14 . 
     When the voltage at the gate electrode  16  is less than the threshold voltage, current flow between contacts  12 ,  14  will be blocked by the pn junction. 
     As shown in  FIG. 2   a , the present invention provides in a first embodiment that the field stop zone profile  34  is formed by a plurality of dopant implants  42 ,  44 ,  46 ,  48 . 
     The semiconductor material which forms n-base region  24  is also used as the basis for a field stop zone  26 , and the dopant implants  42 ,  44 ,  46 ,  48  are formed by creating defects in the silicon structure of the semiconductor material. For example, the dopant implants may be formed via proton implantations. After the proton implantations, the semiconductor material is subjected to heat treatment during an annealing step. Vacancy/hydrogen-related complexes are created which form donors in the field stop zone  26 , and the number of donors per volume defines the dopant concentration. 
     A first dopant implant  42  has a dopant concentration maximum  52  and a second dopant implant  44  having a dopant concentration maximum  54 . First dopant implant  52  may be formed for example by a proton implantation at at least 500 keV, for example at 550 keV. For this energy, the dopant concentration maximum  52  is located at a depth of about 6.5 micrometers. It may preferably have a dopant concentration of (1-5)E14/cubic centimeter for example. The second dopant implant  44  may be produced using a proton implantation at a higher energy than used during the first implantation, for example at an energy of 800 keV, and in this first embodiment advantageously provides a second dopant concentration maximum  54  with a concentration greater than the dopant concentration maximum  52 , for example at (1-5)E15/cubic centimeter or ten times the amount of the first dopant concentration maximum  52 . The second dopant concentration maximum  54  preferably is at least twice the amount of the first dopant concentration maximum  52 . The second dopant concentration maximum  54  also is located further away from side  30 , for example at about 11 micrometers. 
     The doping profile  34  of the present invention with the two maxima  52 ,  54  can optimize the turn-off response of the IGBT so that good leakage current characteristics and good short circuit ruggedness may be provided. 
     It is also desirable that turn-off response be soft. For such an effect, two further implants  46 ,  48  for example may be provided. Implant  46  may occur for example via a proton implantation at 1200 keV and provide a dopant concentration maximum  56  located at about 20 micrometer depth and with a dopant concentration of for example 5E13-2E14/cubic centimeter. Implant  48  may occur for example using a proton implantation at 1500 keV with a dopant concentration maximum  58  of for example (1-5)E13/cubic centimeter located at a depth of about 30 micrometers. 
     The solid line in  FIG. 2   a  thus represents the actual dopant profile through field stop zone  26 , with the dashed lines showing the dopant concentration of each implant  42 ,  44 ,  46 ,  48 . The dotted line shows the envelope of the actual dopant profile. 
     It is noted that the concentration scale is logarithmic and thus minor dopant concentrations of the second implant  44 , for example, at a depth of about 6.5 micrometers, have little effect on the maximum of the profile  34  caused by first maximum  52  of first implant  52 . 
       FIGS. 2   b  and  2   c  show other further possible profiles  34   a  and  34   b , respectively, according to the first embodiment. The field stop zone of  FIG. 2   b  has a fourth implant  48   a  having a fourth peak  58   a  having a maximum dopant concentration approximately equal to the maximum dopant concentration of peak  54 . The field stop zone  58   b  of  FIG. 2   c  has a fourth implant  48   b  having a maximum dopant concentration greater than the maximum dopant concentration of all other peaks  52 ,  54 ,  56   
     The implants may be produced by proton implantation from the direction of side  30 , in other words the back of a wafer containing the n-base material. After the implantations, the wafer may then be annealed at for example temperatures of 300° C.-500° C. for 30 min. to 4 hrs in order to activate the hydrogen-related donors and to reduce the concentration of recombination centers such as, for example, di-vacancies or oxygen-vacancy complexes. This annealing step can result in a broadening of the donor peaks. 
     Alternate methods to create the profile of the present invention such as implantations through the front of the wafer however may also be possible. 
     As opposed to proton implantation, another option is a multiple helium implantation in combination with a controlled formation of thermal donors. Furthermore, other dopants such as phosphorus or arsenic are also possible. 
     The depths of the maxima may be set via an appropriate selection of the acceleration energies for the individual implants and are freely selectable in principle. 
     Prior art field stop zone dopant profiles often had good short circuit ruggedness but poor leakage current yields for switching, typically where the maximum was located too close to the side away from the base material. The increase in leakage current can be explained by doping inhomogeneities in the implanted region. These doping inhomogeneities are induced by particles adhering on the wafer surface during implantation and, thus, locally reducing the penetration depth of the implanted ions. Other known dopant profiles may have had good leakage current yields, but had poor short circuit ruggedness, typically where the doping maximum was located further away from the side away from the base material. 
     The present profile advantageously provides for excellent short-circuit ruggedness of the IGBT. Holes are delivered from the p emitter  28 , and the holes partly compensate for a negative space charge on the anode side, so that the electric field gradient, and thus the maximum electric field intensity, does not assume excessively high values near the anode contact  14  when the electric field is switched over during short-circuit operation. Increased hole injection may be achieved since dopant maximum  52  has a lower dopant concentration than dopant maximum  54 . This relationship also reduces the current-dependence of the emitter efficiency of the anode-side pn junction in the IGBT. For a soft decay of the current in the end phase of the switching response during turn off, the present profile also advantageously raises the charge carrier concentration between the side  30  and the second maximum  54 . A sufficient amount of charge carriers in this area is available even during the end phase of the reverse recovery phase, so that the dI/dt (change in current with respect to change in time) gradient of current decay is sufficiently low and excessive overvoltages across parasitic inductivities are avoided. 
     Another advantage of the profile  34  is that the leakage current yield can be substantially improved. Using higher implantation energies, the radiation passes through even larger particles located on the semiconductor surface during the implantation process. A higher dopant concentration in the second implant compared to the first implant therefore delivers a better leakage current yield than in the reverse case. 
     P emitter  28  may be a shallow profile emitter. However, emitter variants may include, for example, thermally diffused p-emitters or p-emitters manufactured by laser annealing or emitters manufactured by a combination of these methods. In particular, annealed emitters have the advantage of a greater lateral homogeneity. Emitters manufactured in this way are characterized by higher emitting efficiency which may be further adjusted via a field stop profile according to the present invention, so that leakage currents and switching loss at nominal current and lower currents are reduced. In contrast, in the event of a short circuit, the profile has almost no effect on the emitter efficiency due to the high charge carrier concentration, as is desired. 
       FIG. 3  shows schematically a further possible field stop zone dopant profile  134  according to another embodiment of the present invention. Three implants  62 ,  64 ,  66  with dopant concentration maxima  72 ,  74 ,  76  may be provided. Implant  62  may be created for example via a proton implantation at 800 keV, implant  64  via a proton implantation at 1200 keV and implant  66  via a proton implantation at 1500 keV. The dopant concentration maxima depths are about 11, 20 and 30 micrometers for these energies and the concentration maxima may be 2E15, 2E14 and 5E14/cubic centimeter, respectively, for example. The dip in the second concentration maximum  74  may be advantageous for turning off properties. Thus, according to another embodiment of the present invention the second dopant concentration maximum, for example dopant concentration maximum  74 , has a concentration at least 10 percent smaller than either of a first or third dopant concentration maximum between which the second dopant concentration maximum is located, for example dopant concentration maximum  72  and dopant concentration maximum  76 . 
       FIG. 4  shows schematically a diode  110  having for example an anode contact  114  and a cathode contact  112 , as well as a p emitter  122 , an n+ emitter  127  and n− base  124  forming a pn junction with p-emitter  122 . A field stop zone  126  for example having the profile  134  may also be provided. Diode  110  may be formed using thin film technology as with the IGBT, and the formation of the field stop zone  126  may be similar to that for the IGBT. 
     While the field stop zone profile has been described with n channel devices, it may also be used with p channel type devices.