Patent Publication Number: US-9406743-B2

Title: Semiconductor device with counter doped layer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of U.S. patent application Ser. No. 14/220,447, filed Mar. 20, 2014 and claims priority from Japanese Patent Application No. 2013-057949 filed on Mar. 21, 2013, the contents of which are hereby incorporated by reference to this application. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a manufacturing method of a semiconductor device and a semiconductor device, and more particularly relates to a technique effectively applied to a semiconductor device having a junction field effect transistor (JFET) formed on a silicon carbide (SiC) substrate. 
     BACKGROUND 
     A junction field effect transistor (hereinafter, referred to as junction FET) which controls a channel with using a pn junction as a gate has been known as one of power semiconductor elements. In particular, a junction FET using SiC as a substrate material is excellent in withstand voltage characteristic because SiC has a dielectric breakdown field larger than that of Si, and since the pn junction has a high diffusion potential, a so-called normally-off FET, which can completely deplete a channel even without applying a negative voltage to a gate, can be achieved. 
     Japanese Patent Application Laid-Open Publications No. 2007-128965 (Patent Document 1) and No. 2011-171421 (Patent Document 2) disclose trench-type junction FETs. In the junction FETs disclosed in these Patent Documents, a trench is formed in an n − -type drift layer epitaxially grown on a SiC substrate and the sidewalls and the bottom surface of the trench are doped with p-type impurities such as Al (aluminum) by using an oblique ion implantation method and a vertical ion implantation method in combination, thereby forming a p-type gate region. 
     An on-resistance which is one of important characteristics representing the performance of a junction FET can be reduced by increasing an interval between adjacent gate regions. If doing so, however, the source and drain withstand voltages at the time of a reverse bias are decreased. More specifically, the on-resistance and the source and drain withstand voltages have a tradeoff relation with the interval between gate regions as a parameter. Therefore, the control of this parameter is very important for the improvement of the performance of the junction FET. 
     Mater. Sci, Forum 600-603. 1059 (2009) (Non-Patent Document 1) reports that it is possible to improve the above-described tradeoff relation between the on-resistance and the source and drain withstand voltages by making the impurity concentration profile of the p-type gate region steep. Although the Non-Patent Document 1 does not describe how to make the impurity concentration profile steep, for example, a method in which an oblique ion implantation method is used to dope the sidewalls of the trench with n-type impurities (for example, nitrogen), thereby compensating for the impurity concentration at an end of the p-type gate region may be adopted (see FIG. 3 of Non-Patent Document 1). 
     Japanese Patent Application Laid-Open Publication No. 10-294471 (Patent Document 3) relates to a planar-type junction FET. The Patent Document 3 describes that the performance of the junction FET can be further improved by making a retrograde profile in which the width of the p-type gate region on a drain side is wider than that on a source side. Here, the width of the p-type gate region is adjusted by ion implantation energy and the dose amount of impurities. 
     On the other hand, Japanese Patent Application Laid-Open Publication No. 2004-134547 (Patent Document 4) which relates to a trench-type junction FET discloses a method in which the width of the p-type gate region on the drain side is made wider than that on the source side by making the acceleration voltage at the time of ion implantation of impurities to the bottom surface of the trench lower than the acceleration voltage at the time of ion implantation of impurities to the sidewalls of the trench (see FIG. 5 of the Patent Document 4). 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2007-128965 
         Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2011-171421 
         Patent Document 3: Japanese Patent Application Laid-Open Publication No. 10-294471 
         Patent Document 4: Japanese Patent Application Laid-Open Publication No. 2004-134547 
       
    
     Non-Patent Documents 
     
         
         Non-Patent Document 1: Mater. Sci, Forum 600-603. 1059 (2009) 
       
    
     SUMMARY 
     For the accurate control of the interval between adjacent p-type gate regions and the impurity concentration profile of the p-type gate region in the above-described conventional trench-type junction FET, there are many parameters be controlled such as the taper angle of the trench, the film thickness and shape of an oxide film used as an etching mask for the trench, and angle accuracy of an ion implantation device in addition to the process dimensions of the trench. For this reason, in consideration of mass productivity, it is difficult to ensure a process margin to obtain stable high yields. 
     Also, for the improvement of the performance of the junction FET, in particular, for the reduction of the on-resistance, it is required to increase a ratio of the source area occupying the active region, and for its achievement, the reduction of the width of the gate region is necessary. In a conventional art, however, since the gate region is formed by doping the sidewalls of the trench with impurities by an oblique ion implantation method, if the width of the trench is narrowed, a ratio of the depth with respect to the width of the trench (aspect ratio) is increased, so that it becomes difficult to dope the sidewalls of the region with impurities. More specifically, it is difficult to reduce the width of the gate region in the conventional method in which the gate region is formed by doping the sidewalls of the trench with impurities by the oblique ion implantation method. 
     Other problems and novel features will become apparent from the description of the specification and the attached drawings. 
     One embodiment of this application is a manufacturing method of a semiconductor device having a junction field effect transistor formed on a main surface of a semiconductor substrate of a first conductivity type, and the method includes: 
     (a) a step of forming a source layer of the first conductivity type on a surface of a drift layer of the first conductivity type formed on the semiconductor substrate; 
     (b) after the step (a), a step of forming a plurality of trenches disposed at predetermined intervals by etching the surface of the drift layer with a first insulating film formed on the drift layer used as a mask; 
     (c) after the step (b), a step of forming a counter dope layer of the first conductivity type by doping the drift layer below each of the plurality of trenches with impurities by using a vertical ion implantation method; 
     (d) after the step (c), a step of forming a sidewall spacer on each sidewall of the first insulating film and the trenches; and 
     (e) after the step (d), a step of forming a gate layer of a second conductivity type by doping the drift layer below each of the plurality of trenches with third impurities by using the vertical ion implantation method. 
     According to the embodiment mentioned above, high-performance junction FETs can be manufactured with high yields. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a part of an active region of a SiC substrate having a vertical junction FET of a first embodiment formed thereon; 
         FIG. 2A  is a sectional view of a principal part of the SiC substrate showing a manufacturing method of the vertical junction FET of the first embodiment; 
         FIG. 2B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 2A ; 
         FIG. 2C  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 2B ; 
         FIG. 3  is a plan view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET of the first embodiment; 
         FIG. 4A  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 2C ; 
         FIG. 4B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 4A ; 
         FIG. 5  is a plan view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 2C ; 
         FIG. 6A  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 4B ; 
         FIG. 6B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 6A ; 
         FIG. 6C  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 6B ; 
         FIG. 7A  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 6C ; 
         FIG. 7B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 7A ; 
         FIG. 7C  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 7B ; 
         FIG. 8A  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 7C ; 
         FIG. 8B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 8A ; 
         FIG. 8C  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 8B ; 
         FIG. 9A  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 8C ; 
         FIG. 9B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 9A ; 
         FIG. 10A  is a sectional view of a principal part of a SiC substrate showing a manufacturing method of a vertical junction FET of a modification example of the first embodiment; 
         FIG. 10B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 10A ; 
         FIG. 10C  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 10B ; 
         FIG. 11A  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 10C ; 
         FIG. 11B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 11A ; 
         FIG. 12A  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 11B ; 
         FIG. 12B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 12A ; 
         FIG. 13  is a sectional view of a part of an active region of the SiC substrate having the vertical junction FET of the modification example of the first embodiment formed thereon; 
         FIG. 14A  is a sectional view of a principal part of a SiC substrate showing a manufacturing method of a vertical junction FET of a second embodiment; 
         FIG. 14B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 14A ; 
         FIG. 15A  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 14B ; 
         FIG. 15B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 15A ; 
         FIG. 16  is a sectional view of a part of an active region of the SiC substrate having the vertical junction FET of the second embodiment formed thereon; 
         FIG. 17  is a sectional view of a part of an active region of a SiC substrate having a vertical junction FET of a third embodiment formed thereon; 
         FIG. 18A  is a sectional view of a principal part of the SiC substrate showing a manufacturing method of the vertical junction FET of the third embodiment; 
         FIG. 18B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 18A ; 
         FIG. 19A  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 18B ; 
         FIG. 19B  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 19A ; 
         FIG. 20  is a sectional view of a principal part of the SiC substrate showing the manufacturing method of the vertical junction FET continued from  FIG. 19B ; and 
         FIG. 21  is a sectional view of a part of an active region of a SiC substrate having a vertical junction FET of another embodiment formed thereon. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments, and the repetitive description thereof will be omitted. In addition, in the embodiments below, the description of the same or similar portions is not repeated in principle unless particularly required. Also, in some drawings used in the following embodiments, hatching is used even in a plan view and hatching is omitted even in a sectional view so as to make the structure easily understood. 
     First Embodiment 
       FIG. 1  is a sectional view of a part of an active region of a SiC substrate having a vertical junction FET of a first embodiment formed thereon. 
     On a main surface of an n + -type SiC substrate  1  serving as a drain region of the vertical junction FET, an n − -type drift layer  2  having an impurity concentration lower than that of the n + -type SiC substrate  1  is formed, and on a surface of the n − -type drift layer  2 , a plurality of n + -type source layers  3  having an impurity concentration higher than that of the n + -type SiC substrate  1  are formed at predetermined intervals. These n + -type source layers  3  extend in a stripe shape along a first direction (vertical direction with respect to the paper sheet) of the main surface of the n + -type SiC substrate  1 . 
     On the surface of each of the n + -type source layers  3 , a source contact layer  11  made of a Ni (nickel) silicide film is formed. The source contact layer  11  is electrically connected to a source electrode  16  through a contact hole  15  formed in an interlayer insulating film  28  which covers the n + -type source layer  3 . The interlayer insulating film  28  is made of a silicon oxide film, and the source electrode  16  is made of a metal film containing Al (aluminum) as a main component. 
     In the surface of the n − -type drift layer  2 , shallow trenches  4  are formed between adjacent n + -type source layers  3  along an extending direction (first direction) of the n + -type source layers  3 , and on the n − -type drift layer  2  below each shallow trench  4 , a p-type gate layer  7  is formed. Also, a sidewall spacer  14  made of a silicon oxide film is formed on each sidewall of the shallow trenches  4 . Furthermore, in the n − -type drift layer  2  below the sidewall spacer  14 , an n-type counter dope layer  5  is formed so as to be adjacent to the p-type gate layer  7 . 
     One of the features of the vertical junction FET of the first embodiment is that, as shown in  FIG. 1 , the bottom surface of the shallow trench  4  is positioned lower than the n + -type source layer  3  and the sidewall spacer  14  made of an insulating material (silicon oxide) is formed on the sidewall of the shallow trench  4 , so that the n + -type source layer  3  and the p-type gate layer  7  are not in contact with each other. 
     Another feature of the vertical junction FET of the first embodiment is that, as will be described further below, the n − -type drift layer  2  below the shallow trench  4  is doped with impurities by using the vertical ion implantation method, thereby forming the n-type counter dope layer  5  and the p-type gate layer  7 . 
     On the surface of the p-type gate layer  7  formed on the n − -type drift layer  2  below the shallow trench  4 , a gate contact layer  12  made of a Ni silicide film is formed. The gate contact layer  12  is electrically connected to a gate electrode (gate electrode  17  described further below) made of a metal film of the same layer as the source electrode  16  via a contact hole formed in the interlayer insulating film  28  at an end of the active region (not shown). 
     On the uppermost portion of the main surface of the n + -type SiC substrate  1 , a surface protection film  19  made of a polyimide resin film is formed. As shown in  FIG. 1 , an opening  29  is formed in the surface protection film  19 , and the source electrode  16  exposed on the bottom of the opening  29  forms a source pad. Although not shown, another opening is formed in the surface protection film  19 , and the gate electrode  17  exposed on the bottom of this opening forms a gate pad. 
     Although  FIG. 1  shows only a part of the active region of the n + -type SiC substrate  1 , a termination layer (p − -type termination layer  8  described further below) for mitigating the electric field of the active region is formed on the circumference of the active region not shown in the drawing. The p − -type termination layer  8  is a p-type semiconductor region formed by ion-implanting impurities to the n − -type drift layer  2  on the circumference of the active region. Also, a guard ring (n + -type guard ring layer  3 G and guard ring wiring  18  described further below) is formed on a further outer side of the termination layer, that is, on the outer circumference of the n + -type SiC substrate  1 . The n + -type guard ring layer  3 G is an n-type semiconductor region formed by ion-implanting impurities to the n − -type drift layer  2  on the outer circumference of the n + -type SiC substrate  1 , and the guard ring wiring  18  is made of a metal film of the same layer as the source gate  16  and the gate electrode  17 . 
     On the other hand, a drain electrode  30  is formed on the back surface of the n + -type SiC substrate  1 . The drain electrode  30  is made of a conductive film containing Ni (nickel) silicide as a main component. As described above, the vertical junction FET of the first embodiment has a three-terminal structure having the source pad and the gate pad provided on the main surface side of the n + -type SiC substrate  1  and the drain electrode  30  provided on the back surface side of the n + -type SiC substrate  1 . 
     The operation of the vertical junction FET of the first embodiment is basically the same as the operation of a conventional vertical junction FET, and on/off of the current flowing between the source and the drain is switched by controlling the width of a depletion layer extending from the p-type gate layer  7  to the channel (n − -type drift layer  2  below n + -type source layer  3 ). More specifically, in an OFF state, a negate voltage is applied to the gate (p-type gate layer  7 ) to expand the depletion layer from the gate to the channel, thereby preventing carriers (electrons) from flowing between the source and the drain. On the other hand, in an ON state, a positive voltage is applied to the gate and the drain to reduce the depletion layer, thereby causing carries (electrons) to flow from the source to the drain. 
     Next, a manufacturing method of the vertical junction FET of the first embodiment is described with reference to the drawings in the order of the process. Here, a vertical junction FET having a withstand voltage equal to or higher than 600 V is assumed. 
     First, as shown in  FIG. 2A , on the main surface of the n + -type SiC substrate  1  doped with n-type impurities (nitrogen) at a high concentration, the n − -type drift layer  2  is formed by using an epitaxial growth method. The n − -type drift layer  2  has an impurity (nitrogen) concentration of about 2×10 16  atom/cm 3  and a thickness of about 6 μm. 
     Next, as shown in  FIG. 2B , a silicon oxide film (first insulating film)  20  is deposited on the main surface of the n + -type SiC substrate  1  by using CVD, and then the silicon oxide film  20  is patterned by dry etching with a photoresist film (not shown) used as a mask. Subsequently, with this silicon oxide film  20  used as a mask, n-type impurities (nitrogen) are ion-implanted to the n − -type drift layer  2 , thereby forming the n + -type source layer  3 . At this time, the n-type impurities are ion-implanted also to the n − -type drift layer  2  on the outer circumference of the n + -type SiC substrate  1 , thereby forming the n + -type guard ring layer  3 G surrounding the active region. The n + -type source layer  3  and the n + -type guard ring layer  3 G have an impurity concentration of about 1×10 20  atom/cm 3 . 
     Next, after the silicon oxide film  20  is removed, as shown in  FIG. 2C , a silicon oxide film  21  is deposited on the main surface of the n + -type SiC substrate  1  by using CVD, and the silicon oxide film  21  is patterned by dry etching with a photoresist film used as a mask. Subsequently, with this silicon oxide film  21  used as a mask, the n + -type source layer  3  and the n − -type drift layer  2  therebelow are subjected to dry etching, thereby forming a plurality of shallow trenches  4 . At this time, the n − -type drift layer  2  at an end of the active region is also subject to dry etching, thereby forming a shallow trench  4 C having a width wider than that of the shallow trenches  4 . 
     As shown in  FIG. 2C , the shallow trenches  4  and  4 C are formed so that their bottom surfaces are positioned lower than the n + -type source layers  3 . A depth from the surface of the n − -type drift layer  2  to the bottom surface of the shallow trenches  4  and  4 C is about 0.5 μm. Also, by forming the plurality of shallow trenches  4  in the n − -type drift layer  2  of the active region, the n + -type source layers  3  are separated via the shallow trenches  4 . The width (S) of each of the n + -type source layers  3 , in other words, an interval between adjacent shallow trenches  4  is about 1.0 μm. Also, each shallow trench  4  has a width (W) of about 1.0 μm. 
     As shown in  FIG. 3 , the shallow trenches  4  formed in the n − -type drift layer  2  of the active region extend in a stripe shape along one direction of the main surface of the n + -type SiC substrate  1 . 
     An object of forming the shallow trenches  4  and  4 C in the n − -type drift layer  2  is to dope a deep region of the n − -type drift layer  2  with impurities (impurities for forming the n-type counter dope layer  5  and impurities for forming the p-type gate layer  7 ) in the next ion implantation process. Therefore, when an energy ion implantation device with high acceleration voltage is used to dope the region with impurities, the depth of the shallow trenches  4  and  4 C may be shallower than the depth of the n + -type source layer  3 . 
     Next, as shown in  FIG. 4A , with the silicon oxide film used as a mask, n-type impurities (nitrogen) are ion-implanted to the n − -type drift layer  2  below the shallow trenches  4  and  4 C, thereby forming the n-type counter dope layers  5 . The ion implantation of the n-type impurities is performed by a vertical ion implantation method, and the n-type counter dope layer  5  has the impurity (nitrogen) concentration of about 1×10 17  atom/cm 3 . Also, the ion implantation of the n-type impurities is performed by multi-step implantation with varied acceleration voltages, and the depth of the n-type counter dope layer  5  is about 0.8 μm to 1 μm from the surface of the n − -type drift layer  2 . 
     The n-type counter dope layer  5  is formed so as to compensate for diffusion of impurities (aluminum) of the p-type gate layer  7  to be formed in the next process in a lateral direction (channel direction) and make the impurity concentration profile of the p-type gate layer  7  steep. 
     Next, as shown in  FIG. 4B  and  FIG. 5 , the sidewall spacer  6  is formed on each of the sidewalls of the silicon oxide film  21  and the shallow trenches  4  and  4 C. The sidewall spacer  6  is formed by depositing a silicon oxide film on the main surface of the n + -type SiC substrate  1  by using CVD and then performing anisotropic etching to this silicon oxide film. 
     Next, as shown in  FIG. 6A , with the silicon oxide film  21  and the sidewall spacers  6  used as masks, p-type impurities (aluminum or boron) are ion-implanted to the n − -type drift layer  2  below the shallow trenches  4 , thereby forming the p-type gate layers  7  in a self-alignment manner with respect to the sidewall spacers  6 . At this time, p-type impurities are ion-implanted also to the n − -type drift layer  2  at the end of the active region, thereby forming a p-type gate layer  7 C having a width wider than that of the p-type gate layer  7 . 
     Ion implantation of the p-type impurities is performed by a vertical ion implantation method, and the impurity concentration of the p-type gate layers  7  and  7 C is about 1×10 18  atom/cm 3 . Also, ion implantation of the p-type impurities is performed by multi-step implantation with varied acceleration voltages, and the depth of the p-type gate layers  7  and  7 C is approximately equal to the depth of the n-type counter dope layer  5  (about 0.8 μm to 1 μm from the surface of the n − -type drift layer  2 ). 
     As described above, in the first embodiment, after the sidewall spacers  6  are formed on the sidewalls of the shallow trenches  4 , the n − -type drift layer  2  below the shallow trenches  4  is doped with p-type impurities by using the vertical ion implantation method, thereby forming the p-type gate layers  7  in a self-alignment manner with respect to the sidewall spacers  6 . 
     In this manner, the width (G) of the p-type gate layer  7  can be made narrower than the width (W) of the shallow trench  4 . More specifically, when the width (W) of the shallow trench  4  is made narrow to a processing limit, the width (G) of the p-type gate layer  7  can be further made narrower than this processing limit. Also, since the width of the sidewall spacers  6  to be formed on the sidewalls of the shallow trenches  4  can be accurately controlled by defining the film thickness of the silicon oxide film which is a material of the sidewall spacers  6 , the width (G) of the p-type gate layer  7  can also be accurately controlled. Furthermore, since the p-type gate layers  7  are formed in the state where the sidewall spacers  6  are formed on the sidewalls of the shallow trenches  4 , the n + -type source layers  3  and the p-type gate layers  7  are prevented from being in contact with each other. In particular, in the first embodiment, since the shallow trenches  4  are formed deeper than the n + -type source layers  3 , the n + -type source layers  3  and the p-type gate layers  7  are further reliably prevented from being in contact with each other. 
     Next, after the silicon oxide film  21  and the sidewall spacers  6  are removed, as shown in  FIG. 6B , a silicon oxide film  22  is deposited on the main surface of the n + -type SiC substrate  1  by using CVD, and the silicon oxide film  22  is patterned by dry etching with a photoresist film used as a mask. Subsequently, with this silicon oxide film  22  used as a mask, p-type impurities (aluminum or boron) are ion-implanted to the n − -type drift layer  2  on the circumference of the active region, thereby forming the p − -type termination layer  8 . The p − -type termination layer  8  is a semiconductor region for the purpose of mitigating the electric field of the active region, and is formed in a region deeper than the p-type gate layer  7 C. Also, the impurity concentration of the p − -type termination layer  8  is about 1×10 17  atom/cm 3 . 
     Next, after the silicon oxide film  21  and the sidewall spacers  6  are removed, the n + -type SiC substrate  1  is annealed, thereby activating the n-type impurities (nitrogen) and the p-type impurities (aluminum or boron) with which the n − -type drift layer  2  has been doped in the above-described process. Here, the anneal temperature of the n + -type SiC substrate  1  is about 1700° C. to 1800° C. When the n + -type SiC substrate  1  is annealed, the main surface side and the back surface side of the n + -type SiC substrate  1  are coated with a carbon layer  9  as shown in  FIG. 6C  in order to prevent vaporization of Si constituting the n + -type SiC substrate  1 . 
     Next, after the carbon layer  9  is removed, as shown in  FIG. 7A , a silicon oxide film  23  is deposited on the main surface of the n + -type SiC substrate  1  by using CVD. Subsequently, a barrier metal film  24  made of a TiN (titanium nitride) film is deposited on the silicon oxide film  23  by using sputtering. 
     Next, as shown in  FIG. 7B , the barrier metal film  24  and the silicon oxide film  23  of the active region are subjected to anisotropic etching, thereby forming sidewall spacers  10  made of a laminated film of the barrier metal film  24  and the silicon oxide film  23  on the sidewalls of the shallow trenches  4 . At this time, the outside of the active region is covered with a photoresist film  26  having an opening  25  provided above the n + -type guard ring layer  3 G, and the barrier metal film  24  and the silicon oxide film  23  on the bottom of the opening  25  are etched, thereby exposing the surface of the n + -type guard ring layer  3 G. 
     Next, after the photoresist film  26  is removed, as shown in  FIG. 7C , a Ni film  27  is deposited on the main surface of the n + -type SiC substrate  1  by sputtering. Subsequently, the n + -type SiC substrate  1  is annealed, thereby causing each of the n + -type source layers  3  and the p-type gate layers  7  and  7 C to react with the Ni film  27  (silicidation reaction). 
     Next, the unreacted Ni film.  27  and the barrier metal film  24  are removed. In this manner, as shown in  FIG. 8A , the source contact layer  11  made of a Ni silicide film is formed on each surface of the n + -type source layer  3 , and the gate contact layer  12  made of a Ni silicide film is formed on each surface of the p-type gate layers  7  and  7 C. Also, a guard ring contact layer  13  made of a Ni silicide film is formed on the surface of the n + -type guard ring layer  3 G. 
     The source contact layer  11  is a conducive layer for the ohmic connection between the n + -type source layer  3  and the source electrode  16  to be formed in a later process, and the gate contact layer  12  is a conductive layer for the ohmic connection between the p-type gate layers  7  and  7 C and the gate electrode to be formed in a later process. Also, the guard ring contact layer  13  is a conductive layer for the ohmic connection between the n + -type guard ring layer  3 G and the guard ring wiring  18  to be formed in a later process. 
     Also, by removing the barrier metal films  24  constituting parts of the sidewall spacers  10 , the sidewall spacers  14  made of the silicon oxide film  23  are formed on the sidewalls of the shallow trenches  4  and  4 C. 
     Next, as shown in  FIG. 8B , after the interlayer insulating film  28  made of a silicon oxide film is deposited on the main surface of the n + -type SiC substrate  1  by using CVD, the interlayer insulating film  28  is patterned by dry etching with a photoresist film used as a mask, thereby forming a contact hole  15  on each of the n + -type source layers  3 , the p-type gate layer  7 C, and the n + -type guard ring layer  3 G. Also, in a region not shown in the drawings, a contact hole is formed also in the silicon oxide film  28  on the p-type gate layer  7 . 
     Next, as shown in  FIG. 8C , after a metal film containing Al as a main component is deposited on the main surface of the n + -type SiC substrate  1  by sputtering, this metal film is patterned by dry etching with a photoresist film used as a mask. In this manner, the source electrode  16  electrically connected to the source contact layer  11  on the surface of the n + -type source layer  3 , the gate electrode  17  electrically connected to the gate contact layer  12  of the p-type gate layer  7 C, and the guard ring wiring  18  electrically connected to the guard ring contact layer  13  on the surface of the n + -type guard ring layer  3 G are formed. In a region not shown in the drawings, the gate electrode  17  is electrically connected also to the gate contact layer  12  on the surface of the p-type gate layer  7 . 
     Next, as shown in  FIG. 9A , after the surface protection film  19  made of a polyimide resin film is formed on the uppermost part of the main surface of the n + -type SiC substrate  1 , the opening  29  is formed in the surface protection film  19  on the source electrode  16 . The source electrode  16  exposed to the bottom of this opening  29  forms a source pad. Also, in a region not shown in the drawings, an opening is formed in the surface protection film  19  on the gate electrode  17 , thereby forming a gate pad. 
     Next, as shown in  FIG. 9B , the drain electrode  30  is formed on the entire back surface of the n + -type SiC substrate  1 , thereby completing the vertical junction FET of the first embodiment shown in  FIG. 1 . The drain electrode  30  is made of a Ni silicide film and has a surface plated with Au (gold). 
     In the vertical junction FET of the first embodiment fabricated through the process described above, the following effects can be obtained. 
     Since the n-type counter dope layer  5  and the p-type gate layer  7  are formed in the n − -type drift layer  2  by using the vertical ion implantation method, the impurity concentration profile of the p-type gate layer  7  can be accurately controlled in comparison with a conventional manufacturing method of a trench-type junction FET, in which the sidewalls and bottom surface of a trench are doped with p-type impurities by using an oblique ion implantation method and a vertical ion implantation method in combination, thereby forming a p-type gate region. 
     Also, since it becomes unnecessary to consider diffusion variations of the impurities in a lateral direction caused due to the oblique ion implantation method, the dimensional accuracy of the width (G) of the p-type gate layer  7  can be improved. This means that a process margin for obtaining stable high yields can be easily ensured and a more severe on-resistance design can be made while ensuring a withstand voltage margin. 
     Furthermore, since the p-type gate layer  7  is formed in a self-alignment manner with respect to the sidewall spacers  6  formed on the sidewalls of the shallow trenches  4 , the width (G) of the p-type gate layer  7  can be made narrower than the processing limit. Therefore, since a ratio of an area of the n + -type source layer  3  serving as a current path occupying the active region can be increased, the density of the current flowing between the source and the drain can be improved and the on-resistance of the whole chip can be reduced. In other words, the chip size can be reduced without reducing the current density. 
     In the structure in which the n + -type source layer  3  and the p-type gate layer  7  are in contact with each other, the occurrence of a leakage current at a junction part therebetween is a matter of concern (see Patent Document 2). However, since the n + -type source layer  3  and the p-type gate layer  7  are not in contact with each other, the leakage current can be suppressed. 
     Modification Example of First Embodiment 
     In the first embodiment, after the shallow trenches  4  are formed in the n − -type drift layer  2 , impurities are ion-implanted to the n − -type drift layer  2  below the shallow trenches  4 , thereby forming the n-type counter dope layers  5  and the p-type gate layer  7 C. Alternatively, the n-type counter dope layers  5  and the p-type gate layer  7 C can be formed while omitting the shallow trenches  4  by the following method. 
     First, after the n − -type drift layer  2  is formed on the main surface of the n + -type SiC substrate  1  by a method similar to that of the first embodiment as shown in  FIG. 10A , a silicon oxide film  31  is deposited on the main surface of the n + -type SiC substrate  1  by using CVD, and the silicon oxide film  31  is patterned by dry etching with a photoresist film used as a mask as shown in  FIG. 10B . Subsequently, with this silicon oxide film  31  used as a mask, n-type impurities (nitrogen) are ion-implanted to the n − -type drift layer  2 , thereby forming the n + -type source layers  3 . 
     Next, as shown in  FIG. 10C , after a silicon nitride film  32  is deposited on the main surface of the n + -type SiC substrate  1  by using CVD, the silicon nitride film  32  is polished by using chemical-mechanical polishing so as to recede until the surface of the silicon nitride film  32  is exposed. 
     Next, as shown in  FIG. 11A , the silicon oxide film  31  is selectively etched by using a difference in etching rate between the silicon oxide film  31  and the silicon nitride film  32 , thereby leaving the silicon nitride film  32  on each of the n + -type source layers  3 . 
     Next, as shown in  FIG. 11B , n-type impurities (nitrogen) are ion-implanted to the n − -type drift layer  2  with the silicon nitride film  32  used as a mask, thereby forming the n-type counter dope layers  5 . As with the first embodiment, the n-type counter dope layers  5  are formed by using a vertical ion implantation method performed by multi-step implantation of n-type impurities with varied acceleration voltages. 
     Next, as shown in  FIG. 12A , after a silicon nitride film is deposited on the main surface of the n + -type SiC substrate  1  by using CVD, this silicon nitride film is subjected to anisotropic etching, thereby forming sidewall spacers  33  on sidewalls of the silicon nitride films  32 . 
     Next, as shown in  FIG. 12B , with the silicon nitride films  32  and the sidewall spacers  33  used as masks, p-type impurities (aluminum or boron) are ion-implanted to the n − -type drift layer  2 , thereby forming the p-type gate layers  7  in a self-alignment manner with respect to the sidewall spacers  33 . The p-type gate layers  7  are formed by using a vertical ion implantation method performed by multi-step implantation of p-type impurities with varied acceleration voltages. 
     The processes thereafter are approximately similar to those of the first embodiment, and therefore are not described here.  FIG. 13  is a sectional view of apart of an active region of the SiC substrate having the vertical junction FET of the modification example of the first embodiment formed thereon. 
     Also in this modification example, since the n-type counter dope layers  5  and the p-type gate layers  7  are formed by doping of impurities by using the vertical ion implantation method, the dimensional accuracy of the width (G) of the p-type gate layer  7  can be improved in comparison with the conventional manufacturing method of a trench-type junction FET. 
     Also, since the ratio of an area of the n + -type source layer  3  occupying the active region can be increased, the density of the current flowing between the source and the drain can be improved and the on-resistance of the whole chip can be decreased. 
     Furthermore, since the n + -type source layer  3  and the p-type gate layer  7  are not in contact with each other via the n-type counter dope layer  5 , a leakage current can also be suppressed. 
     Second Embodiment 
     A manufacturing method of a vertical junction FET of a second embodiment is described with reference to the drawings in the order of the process. 
     First, as shown in  FIG. 14A , after the n − -type drift layer  2  is formed on the main surface of the n + -type SiC substrate  1 , n-type impurities (nitrogen) are ion-implanted to the n − -type drift layer  2  of the active region, thereby forming the n + -type source layer  3 . Subsequently, as shown in  FIG. 14B , after the silicon oxide film  21  deposited on the main surface of the n + -type SiC substrate  1  is patterned, the n + -type source layer  3  and the n − -type drift layer  2  therebelow are subjected to dry etching with this silicon oxide film  21  used as a mask, thereby forming the plurality of shallow trenches  4 . The processes so far are similar to those shown in  FIG. 2A  to  FIG. 2C  of the first embodiment. 
     Next, as shown in  FIG. 15A , with the silicon oxide film  21  used as a mask, p-type impurities (aluminum or boron) are ion-implanted to the n − -type drift layer  2  below the shallow trenches  4 , thereby forming p-type gate layers (first gate layer)  35 . At this time, as with the p-type gate layers  7  of the first embodiment, ion implantation of p-type impurities is performed by a vertical ion implantation method, but the ion implantation is performed with a high acceleration voltage of about 200 KeV to 600 KeV in this case, thereby forming the p-type gate layers  35  in a deep region of the n − -type drift layer  2 . 
     Next, as shown in  FIG. 15B , the sidewall spacers  6  made of a silicon oxide film are formed on the sidewalls of the silicon oxide film  21  and the shallow trenches  4  by a method similar to that of the first embodiment. Then, with the silicon oxide film  21  and the sidewall spacers  6  used as masks, p-type impurities (aluminum or boron) are ion-implanted to the n − -type drift layer  2  below the shallow trenches  4 , thereby forming p-type gate layers  36  (second gate layer). At this time, as with the p-type gate layers  7  of the first embodiment, ion implantation of p-type impurities is performed by a vertical ion implantation method, but the ion implantation is performed with a low acceleration voltage lower than 200 KeV in this case, thereby forming the p-type gate layers  36  in a shallow region of the n − -type drift layer  2 , that is, on the p-type gate layers  35 . Also, the impurity concentration of the p-type gate layer  36  is approximately the same as the impurity concentration of the p-type gate layer  35 . 
     In this manner, the p-type gate layer has a retrograde structure composed of the p-type gate layer  35  formed in the deep region of the n − -type drift layer  2  and having a wide width and the p-type gate layer  36  formed in the shallow region of the n − -type drift layer  2  and having a narrow width. 
     The processes thereafter are approximately similar to those of the first embodiment, and therefore are not described here.  FIG. 16  is a sectional view of apart of an active region of the SiC substrate having the vertical junction FET of the second embodiment formed thereon. 
     According to the second embodiment, since the p-type gate layers  35  and  36  are formed by doping of impurities by using the vertical ion implantation method, the dimensional accuracy of the widths of the p-type gate layers  35  and  36  can be improved. 
     Also, since the p-type gate layer has the retrograde structure as described above, the performance of the vertical junction FET can be enhanced. 
     Furthermore, since the n + -type source layer  3  and the p-type gate layers  35  and  36  are not in contact with each other, a leakage current can be suppressed. 
     Third Embodiment 
     A vertical junction FET of a third embodiment is obtained by combining the vertical junction FET of the first embodiment and the vertical junction FET of the second embodiment together. More specifically, as shown in  FIG. 17 , in the vertical junction FET of the third embodiment, the p-type gate layer having the retrograde structure as that of the second embodiment is adopted in the vertical junction FET of the first embodiment. 
     A manufacturing method of the vertical junction FET of the third embodiment is described with reference to the drawings in the order of the process. 
     First, as shown in  FIG. 18A , the n − -type drift layer  2  is formed on the main surface of the n + -type SiC substrate  1 . Subsequently, the n + -type source layer  3  is formed on the n − -type drift layer  2 , and then the plurality of shallow trenches  4  are formed in the n − -type drift layer  2  by dry etching with the silicon oxide film  21  used as a mask. 
     Next, as shown in  FIG. 18B , with the silicon oxide film  21  used as a mask, n-type impurities (nitrogen) are ion-implanted to the n − -type drift layer  2  below the shallow trenches  4 , thereby forming the n-type counter dope layers  5 . The processes so far are the same as those shown in  FIG. 2A  to  FIG. 4A  of the first embodiment. 
     Next, as shown in  FIG. 19A , with the silicon oxide film  21  used as a mask, p-type impurities (aluminum) are ion-implanted to the n − -type drift layer  2  below the shallow trenches  4 , thereby forming the p-type gate layers  35 . This ion implantation of the p-type impurities is performed by a vertical ion implantation method, and as with the second embodiment, the ion implantation is performed with a high acceleration voltage of about 200 KeV to 600 KeV, thereby forming the p-type gate layers  35  in a deep region of the n − -type drift layer  2 . 
     Next, as shown in  FIG. 19B , the sidewall spacers  6  made of a silicon oxide film are formed on the sidewalls of the silicon oxide film  21  and the shallow trenches  4  by a method similar to that of the first embodiment. Then, as shown in  FIG. 20 , with the silicon oxide film  21  and the sidewall spacers  6  used as masks, p-type impurities (aluminum) are ion-implanted to the n − -type drift layer  2  below the shallow trenches  4 , thereby forming the p-type gate layers  35 . This ion implantation of the p-type impurities is performed by a vertical ion implantation method, and as with the second embodiment, the ion implantation is performed with a low acceleration voltage lower than 200 KeV, thereby forming the p-type gate layers  36  in a shallow region of the n − -type drift layer  2 . 
     In this manner, the p-type gate layer has a retrograde structure composed of the p-type gate layer  35  formed in the deep region of the n − -type drift layer  2  and having a wide width and the p-type gate layer  36  formed in the shallow region of the n − -type drift layer  2  and having a narrow width. Also, the n-type counter dope layer  5  is disposed so as to be adjacent to the p-type gate layer  36  formed in the shallow region. 
     According to the third embodiment, the effects of the first embodiment and the effects of the second embodiment described above can be obtained. 
     In the foregoing, the invention made by the inventors of the present invention has been concretely described based on the embodiments. However, it is needless to say that the present invention is not limited to the foregoing embodiments and various modifications and alterations can be made within the scope of the present invention. 
     For example, although the vertical junction FET of the first embodiment and the vertical junction FET of the second embodiment are combined together in the third embodiment, it is also possible to combine the vertical junction FET of the modification example of the first embodiment and the vertical junction FET of the second embodiment together as shown in  FIG. 21 . 
     In addition, some of contents described in the embodiments are described below. 
     (1) A manufacturing method of a semiconductor device having a junction field effect transistor formed on a main surface of a semiconductor substrate of a first conductivity type includes: 
     (a) a step of forming a drift layer of the first conductivity type on the semiconductor substrate of the first conductivity type; 
     (b) a step of forming a plurality of source layers of the first conductivity type disposed at predetermined intervals on a surface of the drift layer by doping the drift layer with first impurities with a first insulating film formed on the drift layer used as a mask; 
     (c) after the step (b), a step of removing the first insulating film and forming a second insulating film on each of the plurality of source layers; 
     (d) a step of forming a counter dope layer of the first conductivity type in the drift layer by doping the drift layer with second impurities by a vertical ion implantation method with the second insulating film used as a mask; 
     (e) after the step (d), a step of forming sidewall spacers made of a third insulating film on sidewalls of the second insulating film; and 
     (f) a step of forming a gate layer of a second conductivity type in the drift layer by doping the drift layer with third impurities by the vertical ion implantation method with the second insulating film and the sidewall spacers used as masks. 
     (2) In the manufacturing method of a semiconductor device described in (1), the counter dope layer has an impurity concentration lower than an impurity concentration of the source layer. 
     (3) In the manufacturing method of a semiconductor device described in (1), the counter dope layer is formed so as to be in contact with a side surface of the gate layer. 
     (4) In the manufacturing method of a semiconductor device described in (1), the semiconductor substrate is made of silicon carbide, the first and second impurities are nitrogen, and the third impurities are aluminum or boron.