Patent Publication Number: US-2009230405-A1

Title: Diode having Schottky junction and PN junction and method for manufacturing the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2008-68260 filed on Mar. 17, 2008, the disclosure of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a diode having a Schottky junction and a PN junction and a method for manufacturing the same. 
     BACKGROUND OF THE INVENTION 
     A conventional Schottky diode includes a metal electrode for a Schottky junction connecting to a surface of a N conductive type semiconductor region. Further, a P conductive type semiconductor region is dispersively arranged on the surface of the N conductive type semiconductor region. This type of diode is defined as a JBS diode (i.e., junction barrier Schottky diode). The JBS diode is disclosed in, for example, JP-A-H10-321879. 
       FIG. 9  shows a JBS diode  100  according to a prior art. The diode  100  includes a semiconductor substrate  103 . The substrate  103  has a N +  conductive type cathode region  110  having a N type impurity with high impurity concentration, a N conductive type region  112  and a P conductive type region  114 . The P conductive type semiconductor region  114  is divided into multiple parts, which are dispersively arranged on the surface of the N conductive type region  112 . A cathode electrode  104  is formed on the backside of the substrate  103 . The cathode electrode  104  contacts a cathode region  110  with ohmic contact. An anode electrode  102  is formed on the foreside  103   a  of the substrate  103 . The anode electrode  102  contacts the surface of the N conductive type semiconductor region  112  and the surface of the P conductive type semiconductor region  114  with Schottky junction Js. 
     When a voltage higher than the cathode electrode  104  is applied to the anode electrode  102 , i.e., when a forward voltage is applied to the diode  100 , the current flows from the anode electrode  102  to the cathode electrode  104  via the Schottky junction Js, the N conductive type semiconductor region  112  and the cathode region  110 . 
     When the voltage higher than the anode electrode  102  is applied to the cathode electrode  104 , i.e., when an inverse voltage is applied to the diode  100 , a depletion layer expands from a P-N junction between the P conductive type semiconductor region  114  and the N conductive type semiconductor region  112 . When multiple P conductive type semiconductor regions  114  are dispersively arranged on the surface of the N conductive type semiconductor region  112 , the depletion layer widely expands, so that the diode  100  has high breakdown voltage. Thus, the breakdown voltage of the JBS diode  100  is superior to a conventional Schottky diode having no P conductive type semiconductor region  114 . 
     In the diode  100 , another Schottky junction Js is formed between the anode electrode  102  and the P conductive type semiconductor region  114 . Although the diode  100  includes the P conductive type semiconductor region  114 , the P-N junction diode between the P conductive type semiconductor region  114  and the N conductive-type semiconductor region  112  is not sufficiently utilized. If the P-N junction diode between the P conductive type semiconductor region  114  and the N conductive type semiconductor region  112  is sufficiently utilized, a resistance in the forward direction of the diode  100  is much reduced. However, the diode  100  does not provide this advantage. 
     It is preferred that the P conductive type semiconductor region  114  contacts the anode electrode  102  with ohmic contact so that the P-N junction diode between the P conductive type semiconductor region  114  and the N conductive type semiconductor region  112  functions as a PN junction diode. If the anode electrode  102  contacts the P conductive type semiconductor region  114  with ohmic contact, and further, the anode electrode  102  contacts the N conductive type semiconductor region  112  with a Schottky junction Js, the diode  100  may function as both of the JBS diode and the PN junction diode. However, there is no material for providing such anode electrode  102 . Accordingly, when the anode electrode  102  is made of one metallic material, only one of the ohmic contact and the Schottky junction Js is provided. 
     Accordingly, by forming the anode electrode  102  from two types of metallic electrodes, the diode provides both of the PN junction diode and the JBS diode. For example, one metallic electrode is formed on the surface  103   a  of the substrate  103  to contact the surface of the P conductive type semiconductor region  114  with ohmic contact so that the one metallic electrode provides the anode electrode of the PN junction diode, and another metallic electrode is formed on the surface  103   a  of the substrate  103  to contact the surface of the N conductive type semiconductor region  112  with Schottky contact so that the other one metallic electrode provides the anode electrode of the JBS diode. Thus, the diode provides both of the JBS diode and the PON junction diode. 
     In the conventional diode  100 , the P conductive type semiconductor region  114  and the N conductive type semiconductor region  112  are arranged on the surface  103   a  of the semiconductor substrate  103 . Thus, the method for forming two metallic electrodes separately is complicated. Specifically, the P conductive type semiconductor region  114  is selectively formed on a part of the surface  103   a  of the substrate  103 , and the metallic electrode contacting the P conductive type semiconductor region  114  with ohmic contact is selectively formed on the P conductive type semiconductor region  114 . For example, ions are implanted on the selected part of the surface  103   a  of the substrate  103  so that the P conductive type semiconductor region  114  is formed. Then, the metallic electrode for the ohmic contact is selectively formed on the selected part of the surface  103   a  of the substrate  103 , on which the P conductive type semiconductor region  114  is formed. Here, a step for limiting the ion implantation area and a step for limiting the metallic electrode forming area are different from each other. Thus, these steps are difficult and complicated to perform. 
     Thus, it is required to easily and simply manufacture the diode providing both of the JBS diode and the PN junction diode in such a manner that the one metallic electrode is formed on the N conductive type semiconductor region and the other metallic electrode is formed on the P conductive type semiconductor region. 
     SUMMARY OF THE INVENTION 
     In view of the above-described problem, it is an object of the present disclosure to provide a method for manufacturing a diode having a Schottky junction and a PN junction. It is another object of the present disclosure to provide a diode having a Schottky junction and a PN junction. 
     According to a first aspect of the present disclosure, a method for manufacturing a diode includes: forming a P conductive type semiconductor film on a N conductive type semiconductor layer with a crystal growth method; forming a first metallic film on the P conductive type semiconductor film so that the first metallic film contacts the P conductive type semiconductor film with an ohmic contact; forming a mask having an opening on the first metallic film; etching a part of the first metallic film and a part of the P conductive type semiconductor film via the opening of the mask so that a part of the N conductive type semiconductor layer is exposed; and forming a second metallic film on the part of the N conductive type semiconductor layer so that the second metallic film contacts the N conductive type semiconductor layer with a Schottky contact. 
     In the above method, a step for dispersively forming the P conductive type semiconductor film on the N conductive type semiconductor layer and a step for selectively forming the first metallic film on the P conductive type semiconductor film are performed at the same time. Thus, the diode having the Schottky diode structure and the PN junction diode structure is easily manufactured. 
     According to a second aspect of the present disclosure, a diode includes: a cathode layer; a N conductive type layer arranged on the cathode layer; a plurality of P conductive type regions arranged on the N conductive type layer, wherein the plurality of P conductive type regions is separated from each other by a predetermined distance; a plurality of ohmic electrodes, each of which is arranged on a corresponding P conductive type region; and a Schottky electrode covering a part of the N conductive type layer, which is exposed from the plurality of P conductive type regions. The Schottky electrode further covers the plurality of P conductive type regions and the plurality of ohmic electrodes, and the cathode layer has a N conductive type. 
     The above diode having the Schottky diode structure and the PN junction diode structure is easily manufactured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a diagram showing a cross section of a diode according to an example embodiment; 
         FIG. 2-7  are diagrams showing a method for manufacturing the diode shown in  FIG. 1 ; 
         FIG. 8  is a diagram showing a cross section of a diode according to another example embodiment; and 
         FIG. 9  is a diagram showing a cross section of a diode according to a prior art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  shows a hybrid diode  1  having a JBS diode and a PN junction diode according to an example embodiment. 
     The diode  1  includes a SiC semiconductor substrate  3 . A N +  type cathode region  10  and a N conductive type semiconductor region  20  are formed on the substrate  3  in this order. The diode  1  further includes multiple P conductive type semiconductor regions  30 , which are formed on the surface of the N conductive type semiconductor region  20 . The P conductive type semiconductor regions  30  are arranged on the N conductive type semiconductor region  20  at predetermined intervals so that a concavity  4  is formed between two adjacent P conductive type semiconductor regions  30 . The diode  1  further includes an ohmic electrode  40  as a first metallic film, which is formed on a surface  31  of each P conductive type semiconductor region  30 . The ohmic electrode  40  contacts the P conductive type semiconductor region  30  so that an ohmic junction Jr is formed therebetween. The ohmic electrode  40  is made of titanium, aluminum, nickel and their combination. Here, when the ohmic electrode  40  is made of two metallic materials, the ohmic electrode  40  is formed by stacking two metallic films. 
     The diode  1  includes a part of a surface of the N conductive type semiconductor region  20 , on which no P conductive type semiconductor region  30  is formed so that the part of the surface of the N conductive type semiconductor region  20  is exposed from P conductive type semiconductor regions  30 . Further, the diode  1  includes a sidewall  33  of the P conductive type semiconductor region  30 , a sidewall of the ohmic electrode  40 , and a Schottky electrode  50  for covering a surface  41  of the ohmic electrode  40 . The Schottky electrode  50  is made of molybdenum, titanium or nickel. The Schottky electrode  50  contacts the part of the surface of the N conductive type semiconductor region  20 , on which no P conductive type semiconductor region  30  is formed, so that a Schottky junction Js is formed therebetween. The Schottky electrode  50  and the ohmic electrode  40  provide an anode electrode  60 . The anode electrode  60  is electrically coupled with both of the P conductive type semiconductor region  30  and the N conductive type semiconductor region  20 . The diode  1  includes a surface wiring  62  for covering the surface of the anode electrode  60 . The surface wiring  62  fills the concavity  4 . The surface  62   a  of the surface wiring  62  is planarized. The surface wiring  62  is made of aluminum. The diode  1  further includes a cathode electrode  70 , which contacts on the backside of the cathode region  10  with ohmic contact. 
     The diode  1  provides a structure of the PN junction diode, i.e., a PN junction diode region J 1  and a structure of the Schottky diode, i.e., a Schottky diode region J 2 . In the PN junction diode region J 1 , the cathode electrode  70 , the cathode region  10 , the N conductive type semiconductor region  20 , P conductive type semiconductor region  30 , and the ohmic electrode  40  are stacked in this order. In the Schottky diode region J 2 , the cathode electrode  70 , the cathode region  10 , the N conductive type semiconductor region  20 , and the Schottky electrode  50  are stacked in this order. 
     In general, the Schottky diode has a forward voltage drop, which is lower than the PN junction diode so that the Schottky diode flows the current between the anode and the cathode with the low forward voltage. The PN junction diode has a forward resistance, which is lower than the Schottky diode so that the current density between the anode and the cathode is high in a range of the high forward voltage. The diode  1  includes the PN junction diode region J 1  and the Schottky diode region J 2 . Accordingly, the diode  1  can flows the current between the anode and the cathode with a low forward voltage since the Schottky diode region J 2  functions. Further, the diode  1  flows the current with high density between the anode and the cathode when the forward voltage is high since the PN junction diode region J 1  functions. The diode  1  has a low forward resistance between the anode and the cathode when the forward voltage is high. Thus, the diode  1  provides both of the characteristics of the Schottky diode and the characteristics of the PN junction diode. 
     When an inverse voltage is applied between the anode and the cathode, a depletion layer expands from the PN junction  30   a  between the P conductive type semiconductor region  30  and the N conductive type semiconductor region  20 . The depletion layer covers a junction surface of the Schottky junction Js. Accordingly, the breakdown voltage of the diode  1  is high in case of an inverse bias. Thus, the diode  1  provides both of the JBS diode function and the PN junction diode function. 
     A method for manufacturing the diode  1  will be explained with reference to  FIGS. 2-7 . 
     As shown in  FIG. 2 , the cathode region  10  having the N +  conductive type is prepared. The impurity concentration of the cathode region  10  is 1×10 18 /cm 3 . The thickness of the cathode region  10  is 350 μm. The N conductive type semiconductor region  20  is formed on the surface of the cathode region  10  by a crystal growth method. The crystal growth is performed under reduced pressure at 1600° C. in a mixed gas atmosphere of a silane gas, a propane gas, a hydrogen gas and a nitrogen gas. The nitrogen gas introduces an impurity. The impurity concentration of the N conductive type semiconductor region  20  is 5×10 15 /cm 3 . The thickness of the N conductive type semiconductor region  20  is 15 μm. In this embodiment, the cathode region  10  and the N conductive type semiconductor region  20  provide the semiconductor substrate  3 . 
     As shown in  FIG. 3 , the P conductive type semiconductor region  30  is formed on the surface of the N conductive type semiconductor region  20  by a crystal growth method. The crystal growth is performed under reduced pressure at 1600° C. in a mixed gas atmosphere of a silane gas, a propane gas, a hydrogen gas and a trimethyl-aluminum gas. The trimethyl-aluminum gas introduces an impurity. The impurity concentration of the P conductive type semiconductor region  30  is 1×10 20 /cm 3 . The thickness of the P conductive type semiconductor region  30  is 1 μm. 
     As shown in  FIG. 4 , the ohmic electrode  40  is formed on the surface of the P conductive type semiconductor region  30  so that the ohmic electrode  40  and the P conductive type semiconductor region  30  provide the ohmic junction Jr. The ohmic electrode  40  is formed by an electron beam evaporation method. The thickness of the ohmic electrode  40  is 0.5 μm. The ohmic electrode  40  is made of titanium, aluminum, nickel or their combination. Here, when the ohmic electrode  40  is made of two metallic materials, the ohmic electrode  40  is formed by stacking two metallic films. 
     As shown in  FIG. 5 , a mask M having an opening  5  is formed on the surface of the ohmic electrode  40 . The mask M is made of photo resist, and patterned by a photo lithography method. 
     As shown in  FIG. 6 , a part of the ohmic electrode  40  and a part of the P conductive type semiconductor region  30  are etched with using the opening  5  of the mask M. The part of the ohmic electrode  40  is dry-etched with using a chlorine based gas. The part of the P conductive type semiconductor region  30  is dry-etched with using a carbon tetrafluoride based gas. Then, the mask M is removed with using a sulphuric based remover. 
     As shown in  FIG. 7 , a metallic film made of molybdenum, titanium or nickel is formed on a whole surface of the substrate  3  by an electron beam evaporation method. The thickness of the metallic film is 0.5 μm. Thus, the metallic film provides the Schottky electrode  50 . The Schottky electrode  50  contacts a part of the surface of the n conductive type semiconductor region  20  with the Schottky junction Js. The Schottky electrode  50  is electrically connected to the ohmic electrode  40 . 
     Then, as shown in  FIG. 1 , the surface wiring  62  is formed on the Schottky electrode  50  so that the surface wiring  62  fills the concavity  4 . The surface wiring  62  is formed from aluminum by an evaporation method. The surface of the surface wiring  62  is planarized. Further, a nickel film is formed on the backside  3   b  of the substrate  3 , i.e., the nickel film is formed on the cathode region  10 , so that the cathode electrode  70  is formed. 
     When the part of the ohmic electrode  40  and the part of the P conductive type semiconductor region  30  are removed via the opening  5  of the mask M so that a part of the N conductive type semiconductor region  20  is exposed, as shown in  FIG. 6 , multiple P conductive type semiconductor regions  30  dispersively arranged on the surface  3   a  of the substrate is obtained, and further, the ohmic electrode  40  is selectively formed on the p conductive type semiconductor regions  30  at the same time. Thus, the step for forming the P conductive type semiconductor regions  30  dispersively on the N conductive type semiconductor region  20  and the step for forming the ohmic electrode  40  selectively on the surface  30  of the P conductive type semiconductor regions  30  are performed at the same time. The anode electrode  60  contacts the P conductive type semiconductor regions  30  with the ohmic junction Jr, and further, contacts the N conductive type semiconductor region  20  with the Schottky junction Js. The diode  1  having the Schottky diode region and the PN junction diode region is easily manufactured. 
     Further, as shown in  FIG. 7 , the Schottky electrode  50  is formed on the shole surface of the substrate  3 . Thus, the Schottky electrode  50  is electrically connected to the ohmic electrode  40  at the same time when the Schottky electrode is formed on the N conductive type semiconductor region  20 . 
     The P conductive type semiconductor region  30  is formed on the N conductive type semiconductor region  20  with using the crystal growth method. Accordingly, the P conductive type semiconductor region  30  is formed without implanting a P conductive type impurity. Further, it is not necessary to perform thermal treatment in order to activate the implanted P conductive type impurity. Thus, the surface of the N conductive type semiconductor region  20  is not substantially roughened. Therefore, a leak current in case of applying the inverse voltage is reduced, and the characteristics of the diode  1  are improved. 
     In the embodiment, the surface wiring  65  covers the whole surface of the Schottky electrode  50 . Alternatively, as shown in  FIG. 8 , a Schottky electrode  53  instead of the Schottky electrode  50  and the surface wiring  62  may be formed in a diode  2 . The surface of the Schottky electrode  50  is planarized. In this case, it is not necessary to form the surface wiring  62 . 
     The above embodiments have the following features. An ohmic electrode is formed on a whole surface of a P conductive type semiconductor region. There is no step between the side of the P conductive type semiconductor region and the side of the ohmic electrode. The Schottky electrode covers the part of the N conductive type semiconductor region, on which the P conductive type semiconductor region is not arranged, the side of the P conductive type semiconductor region, the side of the ohmic electrode, and the surface of the ohmic electrode. Further, the surface wiring covers the surface of the Schottky electrode. The surface of the surface wiring is planarized. As shown in  FIG. 8 , the Schottky electrode covers the part of the N conductive type semiconductor region, on which the P conductive type semiconductor region is not arranged, the side of the P conductive type semiconductor region, the side of the ohmic electrode, and the surface of the ohmic electrode. 
     According to a first aspect of the present disclosure, a method for manufacturing a diode includes: forming a P conductive type semiconductor film on a N conductive type semiconductor layer with a crystal growth method; forming a first metallic film on the P conductive type semiconductor film so that the first metallic film contacts the P conductive type semiconductor film with an ohmic contact; forming a mask having an opening on the first metallic film; etching a part of the first metallic film and a part of the P conductive type semiconductor film via the opening of the mask so that a part of the N conductive type semiconductor layer is exposed; and forming a second metallic film on the part of the N conductive type semiconductor layer so that the second metallic film contacts the N conductive type semiconductor layer with a Schottky contact. 
     Here, in general, the Schottky contact is a junction having a Schottky barrier between semiconductor and metal. At the Schottky junction, the barrier height of the semiconductor is different from the barrier height of the metal. On the other hand, ohmic contact is a junction having no Schottky type barrier substantially. At the ohmic junction, there is no big difference between the barrier height of the semiconductor and the barrier height of the metal. Thus, when a forward voltage is applied to the ohmic junction, the current in proportion to the voltage according to the Ohm&#39;s law flows through the ohmic junction. 
     When the etching a part of the first metallic film and a part of the P conductive type semiconductor film is performed, the P conductive type semiconductor film is divided into multiple columns dispersively arranged on the N conductive type semiconductor layer, and the first metallic film is selectively arranged on each column of the P conductive type semiconductor film. Thus, a step for dispersively forming the P conductive type semiconductor film on the N conductive type semiconductor layer and a step for selectively forming the first metallic film on the P conductive type semiconductor film are performed at the same time. Thus, the diode having the Schottky diode structure and the PN junction diode structure is easily manufactured. 
     Alternatively, the method for manufacturing the diode may further include: removing the mask between the etching and the forming the second metallic film. In the forming the second metallic film, the second metallic film is formed on a side of the P conductive type semiconductor film, a side of the first metallic film and a surface of the first metallic film. In this case, the forming the second metallic film provides to electrically couple the first metallic film and the second metallic film. 
     Alternatively, the N conductive type semiconductor layer and the P conductive type semiconductor film may be made of SiC. In the prior art, as shown in  FIG. 9 , the P conductive type semiconductor region  114  is formed such that a P type impurity is implanted on a part of the surface of the N conductive type semiconductor region  112 , and then, the P type impurity is activated in a thermal treatment. When the P conductive type semiconductor region  114  is formed with using an ion implantation method, a defect may be formed in the P conductive type semiconductor region  114 . Further, it is necessary to perform the heat treatment at a temperature equal to or higher than 1600° C. in order to activate the P type impurity when the P conductive type semiconductor region  114  is made of SiC. When the heat treatment is performed at a high temperature, the surface of the P conductive type semiconductor region  114  may be roughened since sublimation is occurred on the surface of the P conductive type semiconductor region  114 . Accordingly, a leak current in a case where an inverse voltage is applied to the diode increases, so that performance of the diode is reduced. In the present embodiment, the P conductive type semiconductor film is formed on the N conductive type semiconductor layer by the crystal growth method. Accordingly, it is not necessary to perform an ion implantation step and a heat treatment. Thus, the leak current in a case where an inverse voltage is applied to the diode is reduced, so that performance of the diode is improved. 
     Alternatively, the first metallic film may be made of at least one of titanium, aluminum and nickel, and the second metallic film may be made of one of molybdenum, titanium and nickel. In this case, the first metallic film sufficiently contacts the P conductive type semiconductor film with ohmic contact. Further, the second metallic film sufficiently contacts the N conductive type semiconductor layer with Schottky contact. 
     Further, the method for manufacturing the diode may further include: forming a surface wiring on a whole surface of the second metallic film. The surface wiring is made of aluminum, and the P conductive type semiconductor film and the N conductive type semiconductor layer are made of SiC. Furthermore, the N conductive type semiconductor layer may have an impurity concentration around 5×10 15 /cm 3 . The P conductive type semiconductor film may have an impurity concentration around 1×10 20 /cm 3 . The first metallic film has a thickness around 0.5 μm, and the second metallic film has a thickness around 0.5 μm. 
     According to a second aspect of the present disclosure, a diode includes: a cathode layer; a N conductive type layer arranged on the cathode layer; a plurality of P conductive type regions arranged on the N conductive type layer, wherein the plurality of P conductive type regions is separated from each other by a predetermined distance; a plurality of ohmic electrodes, each of which is arranged on a corresponding P conductive type region; and a Schottky electrode covering a part of the N conductive type layer, which is exposed from the plurality of P conductive type regions. The Schottky electrode further covers the plurality of P conductive type regions and the plurality of ohmic electrodes, and the cathode layer has a N conductive type. 
     The above diode having the Schottky diode structure and the PN junction diode structure is easily manufactured. 
     Alternatively, the ohmic electrode may be made of at least one of titanium, aluminum and nickel, and the Schottky electrode may be made of at least one of molybdenum, titanium and nickel. Each P conductive type region and the N conductive type layer may be made of SiC. Further, the diode may further include: a surface wiring arranged on a whole surface of the Schottky electrode. The surface wiring is made of aluminum. Furthermore, the cathode layer may have an impurity concentration around 1×10 18 /cm 3 , and the N conductive type layer may have an impurity concentration around 5×10 15 /cm 3 . Each P conductive type region may have an impurity concentration around 1×10 20 /cm 3 . The ohmic electrode has a thickness around 0.5 μm, and the Schottky electrode has a thickness around 0.5 μm. 
     While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments and constructions. The invention is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, which are preferred, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.