Abstract:
A method of manufacturing a semiconductor device provided with a MOS field effect transistor having a channel region of a first conduction type formed in a surface layer portion of a semiconductor substrate, a source region of a second conduction type formed on a rim portion of a trench made to penetrate through the channel region, and a base region of the first conduction type formed in the surface layer portion of the semiconductor substrate adjacently to the source region. The method includes: a step of forming a mask layer having a base-region forming opening corresponding to the base region and a trench forming opening corresponding to the trench on the semiconductor substrate in which the channel region is formed; a base-region forming step of introducing impurities through the base-region forming opening; a trench forming step of forming the trench through the trench forming opening; and a step of forming a gate insulation film on an inner wall surface of the trench.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a manufacturing method of a semiconductor device in which is formed a MOS FET (Metal-Oxide-Semiconductor Field Effect Transistor) having a trench structure, and a semiconductor device suitably manufactured through the manufacturing method.  
           [0003]    2. Description of Related Art  
           [0004]    A semiconductor device includes a type provided with a MOS FET (MOS Field Effect Transistor) having a trench structure. In a semiconductor device of this type, a source region and a channel region are placed along the depth direction of the trench, which makes it possible to achieve miniaturization of elements and a reduction of power consumption.  
           [0005]    [0005]FIG. 3 is a schematic cross section showing a structure of a semiconductor device provided with a MOS FET having the trench structure, obtained through a conventional manufacturing method.  
           [0006]    An N −  epitaxial layer  52  is formed on the surface of a silicon substrate  51 , and a diffusion region  65  is formed on the N −  epitaxial layer  52 . Trenches  54 , each of which penetrates through the diffusion region  65  and halfway through the N −  epitaxial layer  52  in the thickness direction, are formed at regular intervals. Inside each trench  54  is provided a gate electrode  55  made of polysilicon, and a gate oxide film  56  is provided to surround the gate electrode  55 .  
           [0007]    N +  source regions  57  and P +  base regions  58  are formed in the surface layer portion of the diffusion region  65 , and the rest of the diffusion region  65  forms a P −  channel region  53 . The N +  source regions  57  are formed on the periphery (rim portion) of each trench  54 . The P +  base region  58  is formed between every two adjacent N +  source regions  57 , and is connected to the P −  channel region  53 .  
           [0008]    Insulation films  59  made of silicon oxide is formed to cover above each trench  54 . The insulation films  59  are also present on the periphery of each trench  54  (on the N +  source regions  57 ) when viewed in a plane. A space between every two adjacent insulation films  59  forms a contact hole  60 . An electrode film  61  made of metal, such as aluminum, is formed on the diffusion region  65  and the insulation films  59 . The electrode film  61  is placed to fill in the contact holes  60 .  
           [0009]    While the semiconductor device described above is operating, a current flows from the N +  source regions  57  toward the silicon substrate  51  through the P −  channel region  53  along the gate oxide films  56 .  
           [0010]    [0010]FIG. 4( a ), FIG. 4( b ), and FIG. 4( c ) are schematic cross sections used to explain a manufacturing method of the semiconductor device of FIG. 3.  
           [0011]    Initially, the N −  epitaxial layer  52  is formed on the silicon substrate  51 . Then, impurities used to control a conduction type to be a p-type are introduced into the surface layer portion of the N −  epitaxial layer  52 , whereby the P −  channel region  53  is formed. Subsequently, the P +  base regions  58  and the trenches  54  are formed. Although it does not matter which of the P +  base regions  58  and the trenches  54  are formed first, the following description will describe a case where the P +  regions  58  are formed first.  
           [0012]    A mask layer  71  having openings (hereinafter, referred to as base-region forming openings)  70  in portions corresponding to the P +  base regions  58  is formed on the P −  channel region  53 . Then, impurities are implanted and diffused into the P −  channel region  53  through the base-region forming openings  70 , whereby the P +  base regions  58  are formed (FIG. 4( a )). The mask layer  71  is then removed. Subsequently, the N +  source regions  57  are formed through the same method using another mask layer having openings.  
           [0013]    Then, a first resist film  73  having openings (hereinafter, referred to as trench forming openings)  72  in portions corresponding to the trenches  54  is formed on the P −  channel region  53 . Then, the N +  source regions  57 , the P −  channel region  53 , and the upper portion of the N −  epitaxial layer  52  are etched away through the trench forming openings  72 , whereby the trenches  54  are formed (FIG. 4( b )). The first resist film  73  is then removed, and the inner wall surface of each trench  54  is subjected to thermal oxidation, whereby the gate oxide film  56  is formed.  
           [0014]    Then, a polysilicon film is formed to fill in the trenches  54 . Impurities are introduced into the polysilicon film to make the polysilicon film electrically conductive, whereby the gate electrodes  55  are formed. The top surfaces of the respective gate electrodes  55  are flush with the surfaces of the P +  base regions  58  and the N +  source regions  57 .  
           [0015]    Subsequently, a silicon oxide film  76  is formed across the entire surface of the silicon substrate  51  having undergone the foregoing processes. A second resist film  75  having openings  74  in portions corresponding to the contact holes  60  is then formed on the silicon oxide film  76  (FIG. 4( c )). The silicon oxide film  76  is etched away through the openings  74  of the second resist film  75 , whereby the contact holes  60  are formed. Residual portions of the silicon oxide film  76  form the insulation films  59 . After the second resist film  75  is removed, the electrode film  61  is formed on the silicon substrate  51  having undergone the foregoing processes. The semiconductor device shown in FIG. 3 is thus obtained.  
           [0016]    The base-region forming openings  70  and the trench forming openings  72  are formed through the lithographic technique using a stepper (exposure apparatus). For this reason, the trench forming openings  72  are aligned and formed so that the trenches  54  will be formed at predetermined positions with respect to the P +  base regions  58 .  
           [0017]    Also, the openings  74  used to form the contact holes  60  are aligned and formed so as to avoid portions above the trenches  54  (gate electrodes  55 ).  
           [0018]    Referring to FIG. 3, because the P +  base regions  58  need to be spaced apart from the gate oxide films  56 , the base-region forming openings  70  are aligned with accuracy within a diffusion margin Md, which is equal to intervals between the P +  base regions  58  at the predetermined positions and the gate oxide films  56 . Also, because the insulation films  59  need to be present between the respective gate electrodes  55  and the electrode film  61 , the contact holes  60  are aligned with accuracy within a contact margin Mc, which is equal to intervals between the contact holes  60  at adequate positions and the gate electrodes  55 .  
           [0019]    Incidentally, in order to meet the demand to reduce power consumption of the power MOS FET, miniaturization of cell pitches has been advancing recently, and the diffusion margin Md and the contact margin Mc are also becoming smaller. On the other hand, according to the manufacturing method as described above, for example, a shift in alignment of approximately 0.3 μm is inevitably caused during exposure by the exposure apparatus. For these reasons, it has been becoming difficult to form a microscopic MOS FET having a trench structure through the method described above.  
         SUMMARY OF THE INVENTION  
         [0020]    It is therefore an object of the invention to provide a manufacturing method of a semiconductor device, capable of manufacturing a semiconductor device provided with a microscopic MOS FET having a trench structure.  
           [0021]    Another object of the invention is to provide a semiconductor device provided with a MOS FET having a trench structure that can be miniaturized.  
           [0022]    A manufacturing method of a semiconductor device of the invention is a method of manufacturing a semiconductor device provided with a MOS field effect transistor having a channel region of a first conduction type formed in a surface layer portion of a semiconductor substrate, a source region of a second conduction type formed on a rim portion of a trench made to penetrate through the channel region, and a base region of the first conduction type formed in the surface layer portion of the semiconductor substrate adjacently to the source region. The method includes: a step of introducing impurities used to control a conduction type to be the first conduction type into the surface layer portion of the semiconductor substrate in order to form the channel region; a step of forming a mask layer having a base-region forming opening corresponding to the base region and a trench forming opening corresponding to the trench on the semiconductor substrate in which the channel region is formed; a step of introducing the impurities used to control a conduction type to be the first conduction type into a surface layer portion of the channel region through the base-region forming opening in the mask layer in order to form the base region; a step of forming the trench that penetrates through the channel region by etching away the surface layer portion of the semiconductor substrate through the trench forming opening in the mask layer; and a step of forming a gate insulation film on an inner wall surface of the trench.  
           [0023]    According to the invention, the positions of the base region and the trench in the surface layer portion of the semiconductor substrate are determined by the base-region forming opening and the trench forming opening made in the mask layer. Hence, for example, in a case where the base region is formed first and then the trench is formed, the trench is formed while being aligned exactly with respect to the base region. Likewise, in a case where the trench is formed first, and then the base region is formed, the base region is formed while being aligned exactly with respect to the trench. The trench forming opening and the trench together form a single concave having a continuous inner sidewall surface.  
           [0024]    When the base region is formed, for example, the impurities may be introduced through the base-region forming opening by temporarily filling the trench forming opening with resist or the like. Likewise, when the trench is formed, for example, the surface layer portion of the semiconductor substrate may be etched away by temporarily filling the base-region forming opening with resist or the like. The resist is removed after the base region or the trench is formed.  
           [0025]    As has been described above, according to the manufacturing method of the semiconductor device, the base region and the trench are aligned automatically (self-aligned), and a process for performing exact alignment is no longer needed. It is thus possible to manufacture a semiconductor device provided with a microscopic MOS FET having a trench structure.  
           [0026]    It is preferable that the method further includes: a step of forming a polysilicon film in a region from inside the trench to a lower portion inside the trench forming opening and at a lower portion inside the base-region forming opening; a step of making the polysilicon film electrically conductive by introducing impurities into the polysilicon film; a polysilicon film oxidizing step of forming a silicon oxide film by oxidizing, of the polysilicon film, an upper portion of the polysilicon film inside the trench, the polysilicon film inside the trench forming opening, and the polysilicon film inside the base-region forming opening; a step of forming resist on the silicon oxide film inside the trench forming opening and inside the base-region forming opening after the polysilicon film oxidizing step; a step of forming a source-region forming opening corresponding to the source region between the base region and the trench by etching away the mask layer using the resist as a mask; and a step of introducing impurities used to control a conduction type to be the second conduction type into the surface layer portion of the channel region through the source-region forming opening in order to form the source region.  
           [0027]    For example, the polysilicon film may be formed on the semiconductor substrate entirely, and removed through etching, so that the polysilicon film is only left and formed inside the trench, at the lower portion inside the trench forming opening, and at the lower portion inside the base-region forming opening.  
           [0028]    In the step of oxidizing the polysilicon film, a silicon oxide film is formed in a portion from the upper portion of the trench to the lower portion of the trench forming opening. By forming an electrode film in the step later so as to cover above the silicon oxide film, the silicon oxide film then lies between the gate electrode and the electrode film. Hence, the silicon oxide film thus obtained can be used as an insulation film. The electrode film can be formed in such a manner so as to be connected to the source region through the use of a space between two adjacent insulation films as a contact hole.  
           [0029]    Of the polysilicon film that is made electrically conductive through introduction of impurities, part of the polysilicon film inside the trench is not oxidized and left intact as polysilicon. The polysilicon thus left forms a gate electrode.  
           [0030]    The gate electrode and the insulation film are both obtained from the polysilicon film that is formed inside the concave formed by the trench forming opening and the trench. Hence, the insulation film is formed directly above the gate electrode, and the side surface of the insulation film extends from the inside to the outside of the trench along the inner sidewall surface of the trench.  
           [0031]    As has been described, the insulation film is formed while being aligned automatically with respect to the trench. Hence, the contact hole is formed while being aligned automatically with respect to the trench, etc.  
           [0032]    Further, because the source-region forming opening is formed in such a manner that the opening portion (the base-region forming opening and the trench forming opening) and the non-opening portion of the mask layer are inverted, the position of the source region is also determined by the mask layer. Hence, the source region is formed while being aligned automatically with respect to the base region and the trench.  
           [0033]    As has been described, according to the manufacturing method of the semiconductor device, the base region, the trench, the source region, and the insulation film (contact hole) are aligned automatically, and a step of performing exact alignment is no longer needed. It is thus possible to manufacture a semiconductor device provided with a microscopic MOS FET having a trench structure.  
           [0034]    The mask layer may be a layer having resistance to an etching medium used in the trench forming step, and for example, it may be a layer made of silicon oxide. In this case, for example, the trench can be formed through dry etching.  
           [0035]    A semiconductor device of the invention includes: a channel region of a first conduction type formed in a surface layer portion of a semiconductor substrate; a source region of a second conduction type formed on a rim portion of a trench made to penetrate through the channel region; a base region of the first conduction type formed in the surface layer portion of the semiconductor substrate adjacently to the source region; a gate insulation film formed on an inner sidewall surface of the trench; a gate electrode placed inside the trench to oppose the channel region with the gate insulation film in between; and an insulation film provided from an inside to an outside of the trench above the gate electrode and having a side surface extending along an inner sidewall surface of the trench from the inside to the outside of the trench.  
           [0036]    The above and other objects, features, and advantages of the invention will become more apparent from the following description of embodiments with reference to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    [0037]FIG. 1 is a schematic cross section showing a structure of a semiconductor device according to one embodiment of the invention;  
         [0038]    [0038]FIG. 2( a ) through FIG. 2( i ) are schematic cross sections used to explain a group of processes in a manufacturing method of the semiconductor device of FIG. 1;  
         [0039]    [0039]FIG. 3 is a schematic cross section showing a structure of a semiconductor device provided with a MOS FET having a trench structure obtained through a conventional manufacturing method; and  
         [0040]    [0040]FIG. 4( a ), FIG. 4( b ), and FIG. 4( c ) are schematic cross sections used to explain a manufacturing method of the semiconductor device of FIG. 3. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    [0041]FIG. 1 is a schematic cross section showing a structure of a semiconductor device provided with a MOS FET having a trench structure according to one embodiment of the invention.  
         [0042]    An N −  epitaxial layer  2  is formed on the surface of a silicon substrate  1 , and a diffusion region  30  is formed on the N −  epitaxial layer  2 . Trenches  17 , each of which penetrates through the diffusion region  30  and halfway through the N −  epitaxial layer  2  in the thickness direction, are formed at regular intervals. The respective trenches  17  extend in parallel with one another in a direction perpendicular to the sheet plane of FIG. 1.  
         [0043]    Inside each trench  17  is placed a gate electrode  26  made of polysilicon that has been made electrically conductive through introduction of impurities. A gate oxide film  18  is provided to surround each gate electrode  26 .  
         [0044]    N +  source regions  25  and P +  base regions  14  are formed in the surface layer portion of the diffusion region  30 , and the rest of the diffusion region  30  forms a P −  channel region  4 . The N +  source regions  25  are formed on the periphery (rim portion) of each trench  17 , and the P +  base regions  14  are formed so that the N +  source region  25  comes adjacent to either side. The P +  base regions  14  are connected to the P −  channel region  4 .  
         [0045]    The P +  base regions  14  have a greater thickness than the N +  source regions  25 . In other words, the P −  channel region  4  is thinner in a portion adjacent to the P +  base regions  14  than in a portion adjacent to the N +  source regions  25 . Also, the P +  base regions  14  and the N +  source regions  25  are formed to have a high concentration of impurities and thereby have low resistance in comparison with the P −  channel region  4 . According to the arrangement as described above, in a case where the MOS FET is used as a switch, a surge current generated when the switch is turned OFF flows through a portion including the P +  base regions  14  having low resistance. This makes it possible to avoid an unwanted event that the semiconductor element heats to break. In short, the MOS FET has a large L load capacity.  
         [0046]    An insulation film  28  made of silicon oxide is formed above each gate electrode  26 . The insulation films  28  are formed in a region from the inside to the outside of the respective trenches  17 . The side surface  28   a  of each insulation film  28  has no step or the like and extends from the inside to the outside of the trench  17  along the inner sidewall surface of the trench  17 . A space between every two adjacent insulation films  28  forms a contact hole  31 . A metal electrode film  27  is formed on the diffusion region  30  and the insulation films  28 . The metal electrode film  27  is placed to fill in the contact holes  31 , and thereby comes in contact with the diffusion region  30  exposed inside the contact holes  31 .  
         [0047]    While the semiconductor device described above is operating, a current flows between the N +  source regions  25  and the silicon substrate  1  through the P −  channel region  4  along the gate oxide films  18 .  
         [0048]    [0048]FIG. 2( a ) through FIG. 2( i ) are schematic cross sections used to explain a manufacturing method of the semiconductor device of FIG. 1.  
         [0049]    Initially, the N −  epitaxial layer  2  is formed on the silicon substrate  1 . Then, the silicon substrate  1  on which is formed the N −  epitaxial layer  2  is heated, whereby a thermal oxidation film  3  is formed in the surface layer portion of the N −  epitaxial layer  2 . A thickness of the thermal oxidation film  3  is, for example, 100 to 1000 Å approximately.  
         [0050]    Then, boron ions are implanted into the surface layer portion of the N −  epitaxial layer  2  through the thermal oxidation film  3 , whereby the P −  channel region  4  is formed. This state is illustrated in FIG. 2( a ). When the boron ions are implanted, energy for accelerating boron ions is, for example, 100 keV approximately, and a concentration of boron ions is, for example, 1×10 13  to 10×10 13  atoms/cm 2 .  
         [0051]    Then, a silicon oxide film  5  is formed on the thermal oxidation film  3 , for example, through the CVD (Chemical Vapor Deposition) method. A thickness of the silicon oxide film  5  is, for example, 1000 to 10000 Å. The thermal oxidation film  3  and the silicon oxide film  5  together form a silicon oxide film  6 . Further, a first resist film  7  is formed on the silicon oxide film  6 , in which openings  8  and  9  are made at predetermined positions through lithography. The openings  8  and the openings  9  extend in a direction perpendicular to the sheet plane of FIG. 2( b ).  
         [0052]    The silicon oxide film  6  is then etched away through the openings  8  and  9  in the first resist film  7 . Consequently, in the silicon oxide film  6 , the base-region forming openings  10  are formed in portions corresponding to the openings  8  and the trench forming openings  11  are formed in portions corresponding to the openings  9 . The base-region forming openings  10  and the trench forming openings  11  are placed alternately.  
         [0053]    The P −  channel region  4  is exposed at the bottoms of the base-region forming openings  10  and the trench forming openings  11 . This state is illustrated in FIG. 2( b ). Widths of the base-region forming openings  10  and the trench forming openings  11  are, for example, 0.4 to 0.6 μm approximately. The first resist film  7  is then removed.  
         [0054]    Then, a second resist film  13  is formed, in which openings  12  are made at predetermined positions through lithography. Consequently, the base-region forming openings  10  are positioned inside the openings  12  and the trench forming openings  11  are filled with the second resist film  13 .  
         [0055]    Subsequently, boron ions are implanted into the surface layer portion of the P −  channel region  4  through the base-region forming openings  10  inside the openings  12 , whereby the P +  base regions  14  are formed (FIG. 2( c )). In this instance, the silicon oxide film  6  functions as a mask that prevents boron ions from being implanted into regions of the P −  channel region  4  other than those corresponding to the base-region forming openings  10 . A density of boron ions to be implanted is, for example, 1×10 15  to 10×10 15  atoms/cm 2 . The second resist film  13  is then removed.  
         [0056]    Then, a third resist film  16  is formed, in which openings  15  are made at predetermined positions through lithography. Consequently, the trench forming openings  11  are positioned inside the openings  15  and the base-region forming openings  10  are filled with the third resist film  16 .  
         [0057]    Subsequently, the trenches  17 , each of which penetrates through the P −  channel region  4  and halfway through the N −  epitaxial layer  2  in the thickness direction (to the top portion of the N −  epitaxial layer  2 ), are formed by means of etching through the trench forming openings  11  inside the openings  15  (FIG. 2( d )). A depth of the trenches  17  is determined by a thickness of the P −  channel region  4  or the like, and for example, it is determined to be 0.5 to 3.0 μm. Etching is performed, for example, through dry etching. In this case, the silicon oxide film  6  has resistance to an etching medium, and thereby functions as a hard mask that protects portions other than those corresponding to the trench forming openings  11  from the etching medium.  
         [0058]    Each trench forming opening  11  and the corresponding trench  17  form a single concave having a continuous inner sidewall surface along substantially the same plane. The third resist film  16  is then removed.  
         [0059]    Then, the silicon substrate  1  having undergone the foregoing processes is heated, whereby the gate oxide film  18  is formed in the vicinity of the inner surface of each trench  17  through thermal oxidation. In this instance, the vicinities of the surfaces of the P +  base regions  14  exposed in the base-region forming openings  10  are subjected to thermal oxidation concurrently.  
         [0060]    Then, a polysilicon film  19  is formed on the silicon substrate  1  having undergone the foregoing processes, for example, through the CVD method. The polysilicon film  19  is formed so as to fill in the trenches  17 , the trench forming openings  11 , and the base-region forming openings  10 . Subsequently, the polysilicon film  19  is etched away to leave the polysilicon film  19  at the lower portions inside the base-region forming openings  10 , inside the trenches  17 , and at the lower portions inside the trench forming openings  11 . This state is illustrated in FIG. 2( e ). Impurities are then implanted into the polysilicon film  19  and the polysilicon  19  is thereby made electrically conductive.  
         [0061]    Then, the silicon substrate  1  having undergone the foregoing processes is subjected to oxidation, which gives rise to oxidation of the entire polysilicon film  19  inside the base-region forming openings  10  and inside the trench forming openings  11 , and the upper portions of the polysilicon film  19  inside the trenches  17  (FIG. 2( f )).  
         [0062]    Consequently, the thermal oxidation films in the vicinity of the surfaces of the P +  base regions  14 , the oxidized polysilicon film  19 , and the silicon oxide film  6  together form a silicon oxide film  20 . The silicon oxide film  20  is provided with concaves  21  and  22  in portions corresponding to the base-region forming openings  10  and the trench forming openings  11 , respectively. The polysilicon film  19  let inside each trench  17  without being oxidized forms the gate electrode  26 .  
         [0063]    Then, a fourth resist film  23  is formed to fully cover the surface of the silicon oxide film  20 . Subsequently, the fourth resist film  23  is etched back, so that the fourth resist film  23  is present inside the concaves  21  and  22  alone (FIG. 2( g )).  
         [0064]    Then, the silicon oxide film  20  is etched away using the fourth resist mask  23  inside the concaves  21  and  22  as a mask. Etching is performed, for example, through dry etching (for instance, reactive ion etching (RIE)) Consequently, source-region forming openings  24  are formed in the silicon oxide film  20 . In other words, the source-region forming openings  24  are formed while being aligned exactly between the base regions  14  and the trenches  17  in such a manner that the opening portions (the base-region forming openings  10  and the trench forming openings  11 ) and the non-opening portions of the silicon oxide film  6  (see FIG. 2( b )) are inverted.  
         [0065]    In this state, the P −  channel region  4  between the trenches  17  and the P +  base regions  14  is exposed inside the respective source-region forming openings  24 . Also, the silicon oxide film  20  is present above each gate electrode  26  and above each P +  base region  14 .  
         [0066]    Subsequently, impurities used to control the conduction type to be an n-type are implanted into the surface layer portion of the P −  channel region  4  through the source-region forming openings  24 , and the silicon substrate  1  having undergone the foregoing processes is annealed, whereby the N +  source regions  25  are formed. The fourth resist film  23  is then removed. This state is illustrated in FIG. 2( h ).  
         [0067]    Then, a fifth resist film  29  is formed, in which openings  32  are made at predetermined positions through lithography. Consequently, the silicon oxide film  20  above each gate electrode  26  is covered with the fifth resist film  29 , and the silicon oxide film  20  above each P +  base region  14  is exposed inside the corresponding opening  32  (FIG. 2( i )).  
         [0068]    The exposed silicon oxide film  20  above each P +  base region  14  is then removed, for example, through wet etching. The fifth resist film  29   is  then removed. Further, a metal electrode film  27  is formed on the silicon substrate  1  having undergone the foregoing processes. The silicon oxide film  20  above each gate electrode  26  forms the insulation film  28  that lies between the gate electrode  26  and the metal electrode film  27 . In this manner, the semiconductor device shown in FIG. 1 is obtained.  
         [0069]    As described above, the insulation films  28  are obtained by oxidizing part of the polysilicon film  20  formed inside the trenches  17  and the trench forming openings  11 . For this reason, the side surface  28   a  of each insulation film  28  extends in a direction along which the inner sidewall surface of the trench  17  extends (a direction perpendicular or nearly perpendicular to the silicon substrate  1 ), and therefore has no step or the like.  
         [0070]    In the manufacturing method as described above, the positions of the P +  base regions  14  and the trenches  17  are determined respectively by the base-region forming openings  10  and the trench forming openings  11  made in the silicon oxide film  6  (see FIG. 2( c ) and FIG. 2( d )). The positions of the base-region forming openings  10  and the trench forming openings  11  are determined respectively by the openings  8  and  9  in the first resist film  7  (see FIG. 2( b )).  
         [0071]    Also, as can be understood from comparison between FIG. 2( g ) and FIG. 2( h ), the N +  source regions  25  are formed in portions corresponding to the silicon oxide film  6  (silicon oxide film  20 ) present between the bases-region forming openings  10  (concaves  21 ) and the trench forming openings  11  (concaves  22 ). Hence, the positions of the N +  source regions  25  are also determined by the positions at which the base-region forming openings  10  and the trench forming openings  11  are formed in the silicon oxide film  6 .  
         [0072]    Further, the positions of the insulation films  28  (the positions of the contact holes  31 ) are determined by the trench forming openings  11  in the silicon oxide film  6 .  
         [0073]    Hence, the relative positional relations among the P +  base regions  14 , the trenches  17 , the N +  source regions  25 , and the insulation films  28  (contact holes  31 ) are all determined by a single silicon oxide film  6 . This eliminates the need of separate alignment when each is formed. In other words, the P +  base regions  14 , the trenches  17 , the N +  source regions  25 , and the insulation films  28  (contact holes  31 ) can be aligned automatically (self-aligned).  
         [0074]    The openings  12  in the second resist film  13  need to be formed while being aligned with respect to the base-region forming openings  10  and the trench forming openings  11  (FIG. 2( c )). However, the openings  12  only have to be formed in such a manner that the end portion of each opening  12  is positioned on the silicon oxide film  6  present between the base-region forming opening  10  and the trench forming opening  11 . Thus, an alignment margin of the openings  12  is large in comparison with the diffusion margin Md and the contact margin Mc (see FIG. 3) in the conventional manufacturing method. As has been described, highly precise alignment is not needed when the openings  12  are formed.  
         [0075]    Likewise, large alignment margins are allowed for the openings  15  in the third resist film  16  (FIG. 2( d )) and for the openings  32  in the fifth resist film  29  (FIG. 2( i )).  
         [0076]    Also, the fourth resist film  23  can be formed to be present inside the concaves  21  and  22  alone by only controlling its etching thickness, which makes the alignment in the horizontal direction unnecessary.  
         [0077]    As has been described, according to the manufacturing method of the semiconductor device, because a process of performing exact alignment is no longer needed, it is possible to manufacture a semiconductor device provided with a microscopic MOS FET having a trench structure. Consequently, for example, even in a case where the elements are formed according the 0.4 μm rule using the conventional stepper (exposure apparatus), a cell density (the number of cells per unit area) can be improved markedly, that is, three to five times larger than the density conventionally achieved.  
         [0078]    For example, when it is designed that the width of the trenches  17  and the width of the P +  base regions  14  are both 0.4 μm, according to the manufacturing method of the semiconductor device of the invention, the cell pitch width can be set to as narrow as 1.5 to 2.0 μm, for example. When the cells are miniaturized, the number and the width of the P −  channel region  4  per unit area can be increased, which in turn makes it possible to enlarge the channel area. Consequently, the channel resistance can be reduced, and hence, the ON resistance of the semiconductor device can be reduced.  
         [0079]    While the above description described the embodiment of the invention, the invention can be implemented in another embodiment. For example, the embodiment above described a case of a semiconductor device provided with an n-type channel MOS FET. However, the semiconductor device may be provided with a p-type channel MOS FET.  
         [0080]    Also, in the embodiment above, the P +  base regions  14  are formed first (FIG. 2( c )) and then the trenches  17  are formed (FIG. 2( d )). However, the trenches  17  may be formed first followed by the P +  base regions  14 .  
         [0081]    While the above description described embodiments of the invention in detail, it should be appreciated that these embodiments represent examples to provide clear understanding of the technical contents of the invention, and the invention is not limited to these examples. The sprit and the scope of the invention, therefore, are limited solely by the scope of the appended claims.  
         [0082]    This application is based on Application No. 2002-137517 filed with the Japanese Patent Office on May 13, 2002, the entire content of which is incorporated hereinto by reference.