Patent Publication Number: US-2013248925-A1

Title: Power semiconductor device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2012-068629, filed on Mar. 26, 2012, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate to a power semiconductor device. 
     BACKGROUND 
     Recently, an insulated gate bipolar transistor (IGBT) has been widely used as a power semiconductor device having a high breakdown voltage and being able to control large current. The IGBT is used as a switching device and therefore requires a breakdown voltage according to uses. In order to obtain a predetermined breakdown voltage, when a termination portion has a guard ring structure, there is a problem in that a termination length may increase when termination efficiency increases. Further, in order to obtain higher breakdown voltage, there has been known a semiconductor device by forming trench gates, which extend into the semiconductor device so as to face each other, between electrodes that are disposed on upper and lower surfaces of the termination portion, to prevent the trench gates from extending to a counter electrode of a depletion layer. However, there is a problem in that the semiconductor device in which the trench gate structure is disposed in the termination portion has a complicated structure and is not easily manufactured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an IGBT according to a first embodiment; 
         FIG. 2  is a partial cross-sectional view taken along dashed dotted line A-A′ of  FIG. 1 ; 
         FIGS. 3A and 3B  are partial cross-sectional views illustrating a simulation structure at a termination portion of an IGBT device; 
         FIG. 4  is a voltage-current characteristic diagram illustrating current ICE (A) of the IGBT device illustrated in  FIGS. 3A and 3B ; 
         FIGS. 5A ,  5 B, and  5 C are equipotential maps inside the IGBT device illustrated in  FIGS. 3A and 3B ; 
         FIGS. 6A and 6B  are current distribution diagrams inside the IGBT device illustrated in  FIGS. 3A and 3B ; 
         FIGS. 7A and 7B  are graphs illustrating a change of a breakdown voltage of the IGBT device when an interval L between a bottom surface of a trench and a bottom surface of a guard ring is changed; 
         FIG. 8  is a partial cross-sectional view illustrating a schematic configuration of an IGBT device according to a second embodiment; and 
         FIG. 9  is a partial cross-sectional view illustrating a schematic configuration of an IGBT device according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to an embodiment, a power semiconductor device includes a semiconductor substrate, a base layer, a device portion, a guard ring, and an insulator. The semiconductor substrate includes a drift layer with a first conductive type. The base layer has a second conductive type and is selectively formed in a surface of the drift layer. The device portion is formed on the surfaces of the base layer and the drift layer. The guard ring has a second conductive type and is disposed in plural and is selectively formed in the surface of the drift layer around the device portion. The insulator is buried in at least one of the guard rings. 
     Hereinafter, a plurality of further embodiments will be described with reference to the drawings. In the drawings, the same reference numerals designate the same or similar portions. 
     A power semiconductor device according to a first embodiment will be described with reference to the accompanying drawings.  FIG. 1  is a plan view schematically illustrating a substrate surface pattern from which an emitter electrode and other electrodes of an insulated gate bipolar transistor (IGBT) that is a power switching device are removed. 
     As illustrated in  FIG. 1 , an IGBT  11  includes a device portion  12  with a rectangular shape at a central portion of a semiconductor substrate with a substantially rectangular shape. A termination portion  13  is disposed around the device portion  12 . A plurality of elongated trench gates  14  is disposed in parallel inside the device portion  12 . The termination portion  13  includes a plurality of guard rings  15  around the device portion  12 . An equivalent potential ring (EQPR) layer  16  with a ring shape is disposed at an outermost circumferential portion of the termination portion  13 . 
       FIG. 2  is a partial cross-sectional view taken along dashed dotted line A-A′ of  FIG. 1 . As illustrated in  FIG. 2 , a right portion of dashed dotted line B-B′ of  FIG. 2 . corresponds to the device portion  12 . The semiconductor substrate includes a p+ type collector layer  21 , an n+ type buffer layer  22 , and an n− type drift layer  23 . The p+ type collector layer  21 , the n+ type buffer layer  22 , and the n− type drift layer  23  are sequentially formed and stacked. The semiconductor substrate has the n− type drift layer  23  at a surface side. A p type base layer  24  is selectively formed in a surface of the n− type drift layer  23 . A plurality of trench gates  25  penetrating through the p type base layer  24  from the surface of the p type base layer  24  in a depth direction of the substrate and arriving inside the n− type drift layer  23  is disposed in the p type base layer  24 . The device portions  12  are selectively formed on the surfaces of the p type base layer  24  and the n− type drift layer  23 . The p+ type collector layer  21  has a higher impurity concentration than that of the p type base layer  24 . The n+ type buffer layer  22  has a higher impurity concentration than that of the n− type drift layer  23 . 
     The trench gate  25  includes a gate electrode  25 - 3  made of a polysilicon film, and the like. The gate electrode  25 - 3  is formed in a trench  25 - 1  formed in the p type base layer  24  via a gate insulating film  25 - 2  with a thin film. An n type emitter layer  26  is selectively formed in the surface of the p type base layer  24  at both sides of the trench gate  25 . An emitter electrode  27  connected with the n type emitter layer  26  is disposed at the surface of the p type base layer  24 . An insulating film  28  is respectively provided at a top portion of the trench gates  25  so as to insulate the trench gates  25  and the emitter electrode  27 . The emitter electrode  27  is connected to an emitter electrode terminal E, the p+ type collector layer  21  that is a lowermost layer of the substrate is connected to a collector electrode terminal C. The gate electrodes  25 - 3  of each trench gate  25  are connected to each other and are connected to a gate electrode terminal G (not illustrated). 
     A left portion of dashed dotted line B-B′ of  FIG. 2  corresponds to the termination portion  13   a.  The termination portion  13  includes a guard ring  31 , a guard ring  32 , and a guard ring  33 . The guard ring  31  is connected to the p type base layer  24  of the device portion  12  and is a p type innermost circumferential guard ring formed around the device portion  12  with a ring shape. The guard ring  32  is a p type outer circumferential guard ring formed at the outside of the guard ring  31  with a ring shape. The guard ring  33  is a p type outermost circumferential guard ring formed at the outermost circumferential side of the guard ring  32  in a ring shape. The guard ring  31 , the guard ring  32 , and the guard ring  33  each include at least one of trench  31 - 1 , trench  31 - 2 , trench  32 - 1 , and trench  33 - 1  formed by using reactive ion etching (RIE), for example. The trenches  31 - 1 ,  31 - 2  formed in the guard ring  31  are disposed in the vicinity of both ends of the guard ring  31 . The trench  32 - 1  formed in the guard ring  32  is disposed in the vicinity of the inner circumferential end of the guard ring  32 . The trench  33 - 1  formed in the guard ring  33  is disposed in the vicinity of the inner circumferential end of the guard ring  33 . 
     An insulator  50  made of a silicon oxide film formed by thermal oxidation and chemical vapor deposition (CVD) is buried in the trench  31 - 1 , the trench  31 - 2 , the trench  32 - 1 , and the trench  33 - 1 . Here, the silicon oxide film is used as the insulator  50 , but an undoped polycrystalline silicon film, an undoped amorphous silicon film, an insulating organic film (polyimide film, for example), and the like, may be used instead. In this case, a silicon thermally-oxidized film formed by thermally oxidizing the silicon substrate may be formed on a side and a bottom portion of the trench. The surface of the n− type base layer  23  on which the guard ring  31 , the guard ring  32 , and the guard ring  33  are formed is covered with an insulating film  34 . The insulating film  34  divides between the emitter electrode  27  and the guard ring  31  in the vicinity of the guard ring  31 . The insulating film  34  is partially removed in upper portions of the guard ring  32 , the guard ring  33 , and the EQPR layer  16 , and thus upper portions of the guard ring  32 , the guard ring  33 , and the EQPR layer  16  is exposed. E ach field plate electrode  35  is disposed on the exposed portions. The field plate electrode  35  is a floating electrode, for example, in which potential is not set. The guard ring  31  is connected to the emitter electrode  27  via the p type base layer  24  in a portion in which the insulating film  34  is not formed. 
     As described above, the guard ring  31 , the guard ring  32 , and the guard ring  33  each include the trench  31 - 1 , the trench  31 - 2 , the trench  32 - 1 , and the trench  33 - 1 , the insulator  50  is respectively buried in the trench  31 - 1 , the trench  31 - 2 , the trench  32 - 1 , and the trench  33 - 1 . For this reason, in the IGBT  11  of the embodiment, when a reverse bias is applied between the collector and the emitter, it is possible to increase an electric field around a bottom portion of the trench, and to disperse a field concentration point. Therefore, it is possible to improve a breakdown voltage of a device. 
     In the embodiment, the improvement of the breakdown voltage of the device is calculated by simulation, and thus the effect is confirmed. Hereinafter, the details thereof will be described with reference to  FIGS. 3 to 6 . 
       FIG. 3  is a partial cross-sectional view illustrating a simulation structure at a termination portion of an IGBT device.  FIG. 3A  illustrates a conventional structure and  FIG. 3B  illustrates a structure of the embodiment. In  FIGS. 3A and 3B , portions corresponding to the structure of the device termination portion illustrated in  FIG. 2  are denoted by the corresponding reference numerals and the detained description of the portions will be omitted. 
     As illustrated in  FIGS. 3A and 3B , in the simulation structure of the circumferential end of the IGBT device, the p+ type collector layer  21 , the n+ type buffer layer  22 , and the n− type drift layer  23  are sequentially formed and stacked. In the surface of the n− type drift layer  23 , the guard ring  31 , a guard ring  32   a,  a guard ring  32   b,  and the guard ring  33  is disposed around the device portion  12 . The guard ring  31  is the p type innermost circumferential guard ring with a ring shape. The guard ring  32   a  and the guard ring  32   b  are a p type outer circumferential guard ring with a ring shape at the outside of the guard ring  31 . The guard ring  33  is the outermost circumferential guard ring with a ring shape at the outermost circumferential sides of the guard ring  32   a  and the guard ring  32   b.  However, both the simulation structures of the circumferential ends of the IGBT devices illustrated in  FIGS. 3A and 3B  have the reversed left portion and right portion with respect to the structure of the IGBT device illustrated in  FIG. 2 . In  FIGS. 3A and 3B , the right portion is the termination portion and the left portion is the device portion (not illustrated). 
     In the simulation structure at the circumferential end of the IGBT device illustrated in  FIG. 3A , none of the guard ring  31 , the guard ring  32   a,  the guard ring  32   b,  and the guard ring  33  include the trench  31 - 1 , the trench  31 - 2 , the trench  32 - 1  and the trench  33 - 1  illustrated in  FIG. 2 . 
     In the simulation structure at the circumferential end of the IGBT device illustrated in  FIG. 3B , the trench  31 - 1  and the trench  31 - 2  buried the insulator in the guard ring  31  are formed. The guard ring  32   a,  the guard ring  32   b,  and the guard ring  33  do not include the trench. 
       FIG. 4  is a voltage-current characteristic diagram illustrating current ICE (A) of the IGBT device. In detail,  FIG. 4  is a characteristic diagram (voltage VCE (V)-current ICE (A) characteristic) illustrating a change of the current ICE (A) when reverse polarity voltage VCE (V) is applied between the emitter electrode  27  and the p+ type collector layer  21  of the IGBT device (illustrated in  FIGS. 3A and 3B ). A dotted line A represents the VCE-ICE characteristic diagram of the IGBT device with the termination portion illustrated in  FIG. 3A . A solid line B represents the VCE-ICE characteristic diagram of the IGBT device with the termination portion illustrated in  FIG. 3B . In the voltage VCE(V)-current ICE(A) characteristic illustrated by the dotted line A and the solid line B, the ICE (A) slowly increases with the increase in VCE(V), but the ICE(A) suddenly increases when the VCE (V) exceeds the breakdown voltage of the device. 
     As can be appreciated from characteristics illustrated in  FIG. 4 , the IGBT device with the termination portion of the embodiment, which is illustrated in  FIG. 3B , has the more improved breakdown voltage than that of the IGBT device with the conventional termination portion illustrated in  FIG. 3A . In detail, in the case of the embodiment, the breakdown voltage has 758 V, while in the conventional technology, the breakdown voltage has 740 V. 
       FIG. 5  is an equipotential map inside the IGBT device illustrated in  FIG. 3 .  FIG. 5A  is an equi-electric field map in which an electric field distribution inside the device is obtained based on a simulation and the obtained electric field distribution is diagrammed, in the state in which the same reverse polarity VCE voltage (voltage between the emitter electrode  27  and the p+ type collector layer  21 ) as the breakdown voltage is applied to the IGBT device illustrated in  FIG. 3A , that is, just before an avalanche breakdown is occurred. Similarly,  FIG. 5B  is an equi-electric field map in which the electric field distribution inside the device is obtained based on a simulation just before the avalanche breakdown is occurred, and the obtained electric field distribution is diagrammed. In  FIGS. 5A and 5B , only the guard ring  31 , the guard ring  32   a,  the guard ring  32   b,  and the guard ring  33  of the IGBT device are schematically illustrated and the description of the other structures will be omitted. In  FIGS. 5A and 5B , an equi-electric field intensity curve is changed from the upper portion of low potential to the lower portion of high potential. 
     Comparing  FIGS. 5A and 5B , the equi-electric field intensity curve under the bottom surface of the guard ring  31  of  FIG. 5A  is concentrated at both ends of the guard ring  31 , and a peak of electric field is formed at the portions, but the electric field is formed so as to be substantially parallel with the bottom surface of the guard ring  31  at the central portion. On the other hand, the equi-electric field intensity curve under the bottom surface of the guard ring  31  of  FIG. 5B  is concentrated just under the trenches  31 - 1 ,  31 - 2 , in addition to both ends of the guard ring  31  and the peak of the electric field is formed at the portions. 
       FIG. 5C  is a graph illustrating the electric field distributions corresponding to  FIGS. 5A and 5B . A vertical axis of  FIG. 5C  represents an electric field intensity (V/cm 2 ) and a horizontal axis represents a distance (μm) taken along a cross-section of the IGBT device. A dotted line A of  FIG. 5C  represents the electric field distribution of the IGBT device of a conventional technology illustrated in  FIG. 3A . A sold line B of  FIG. 5C  represents an electric field distribution of the IGBT device according to the embodiment illustrated in  FIG. 3B . 
     As illustrated in the dotted line A and the solid line B, the electric field of the IGBT device (illustrated in  FIG. 3B ) according to the embodiment is reduced at both ends of the bottom surface of the guard ring  31 , as compared with the IGBT (illustrated in  FIG. 3A ) of a conventional technology. In addition, the peak of electric field occurs just under the two trenches  31 - 1 ,  31 - 2 . 
     Therefore, the IGBT device (illustrated in  FIG. 3B ) according to the embodiment has the trenches  31 - 1 ,  31 - 2  formed in the guard ring  31 , such that the peak of electric field under the guard ring  31  is dispersed at the central portion of the guard ring as well as both ends of the bottom surface of the guard ring. As a result, as illustrated in  FIG. 4 , it is possible to totally improve the breakdown voltage of the device. In the electric field distribution graph illustrated in  FIG. 5C , an area surrounded by a curve representing the electric distribution, a horizontal axis of the graph, and a vertical axis of the graph represents the breakdown voltage. Here, comparing an area surrounded by the dotted line A, a horizontal axis of the graph, and a vertical axis of the graph with an area surrounded by the dotted line B, a horizontal axis of the graph, and a vertical axis of the graph, the area in the case of the solid line B is larger than that of the solid line A. 
       FIG. 6  is a current distribution diagram inside the IGBT device (illustrated in  FIG. 3A ).  FIG. 6A  is a current distribution diagram in which a current distribution inside the device is obtained based on a simulation, and the obtained current distribution is diagrammed, in the state in which the same reverse polarity VCE voltage (voltage between the emitter electrode  27  and the p+ type collector layer  21 ) as the breakdown voltage to the IGBT device (illustrated in  FIG. 3A ) is applied, that is, just before avalanche breakdown is generated. Similarly,  FIG. 6B  is a current distribution diagram in which the current distribution inside the device (illustrated in  FIG. 3B ) is obtained at the time of applying the reverse polarity VCE voltage based on a simulation and the obtained current distribution is diagrammed. Similarly to  FIGS. 5A and 5B ,  FIGS. 6A and 6B  schematically illustrate only the guard ring  31 , the guard ring  32   a,  the guard ring  32   b,  and the guard ring  33  of the IGBT device and the description of the other structures will be omitted. 
     Comparing  FIGS. 6A and 6B , in the conventional technology (illustrated in  FIG. 6A ), the current ICE between the collector and the emitter is concentrated on the outer circumferential end of the guard ring  31 . On the other hand, in the embodiment (illustrated in  FIG. 6B ), the current ICE is dispersed from the inner circumferential end of the guard ring  31  to the central portion of the guard ring  31 . Therefore, in the embodiment, current is not concentrated on one point but is dispersed, and thus the breakage of the device is suppressed. It is possible to improve the breakdown voltage of the device. 
     From the foregoing simulation results, in the embodiment, by providing the trench buried the insulator  50  in the guard ring, it is possible to increase the potential around the lower portion of the trench, and to disperse the electric field concentration point. Therefore, it is possible to improve the breakdown voltage. 
       FIG. 7  is a graph illustrating the change of the breakdown voltage of the IGBT device.  FIG. 7A  is a graph illustrating a result of simulating the change of the breakdown voltage of the IGBT device when an interval L between a bottom surface of a trench  71  and a bottom surface of a guard ring  72  that are illustrated in  FIG. 7B  is changed. In  FIG. 7A , a solid line C represents an interval L of 0 μm, a solid line D represents an interval L of 1 μm, a solid line E represents the guard ring structure without the trench, and a solid line F represents an interval L of 2 μm. 
     As illustrated in  FIG. 7A , when the interval L is 2 μm or more, the breakdown voltage of the IGBT device is increased in comparison with the guard ring structure (solid line E) without the trench, but when the interval L is 2 μm or less, the breakdown voltage of the IGBT device is reduced. 
     A power semiconductor device according to a second embodiment will be described with reference to the accompanying drawings.  FIG. 8  is a partial cross-sectional view illustrating a schematic configuration of an IGBT device. Meanwhile, in  FIG. 8 , components corresponding to the components of the IGBT device according to the first embodiment illustrated in  FIG. 2  are denoted by the same reference numerals and the detailed description will be omitted. 
     As illustrated in  FIG. 8 , in the IGBT device of the embodiment, the guard ring  31  includes the trench  31 - 1  and the trench  31 - 2 , but the guard ring  32  and the guard ring  33  do not include the trench. The other components are the same as those of the first embodiment. 
     A power semiconductor device according to a third embodiment will be described with reference to the accompanying drawings.  FIG. 9  is a partial cross-sectional view illustrating a schematic configuration of an IGBT device. Meanwhile, in  FIG. 9 , components corresponding to the components of the IGBT device according to the first embodiment illustrated in  FIG. 2  are denoted by the same reference numerals and the detailed description will be omitted. 
     As illustrated in  FIG. 9 , in the IGBT device according to the embodiment, the guard ring  31  does not include the trench, but the guard ring  32  includes the trench  32 - 1 , the guard ring  33  includes the trench  33 - 1 . The other components are the same as those of the first embodiment. 
     In the termination of the semiconductor device having the guard ring structure according to the foregoing embodiments, the electric field concentration point is dispersed and the breakdown voltage increases, by forming the trench buried the insulator in the guard ring. For the purpose, it is evident that other combinations can be implemented in addition to the foregoing embodiments. 
     The foregoing embodiments describe the IGBT device as the power semiconductor device, but the invention is not limited thereto but can also be applied to a power MOSFET as the power semiconductor device or a MOS type semiconductor device including the general power semiconductor device. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intend to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of the other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.