Abstract:
A semiconductor substrate includes a first conductivity type semiconductor layer and a second conductivity type semiconductor layer thereon. A first conductivity type semiconductor region is formed in a surface portion of the second conductivity type semiconductor layer and is divided into first and second regions. A trench is formed in the semiconductor substrate so as to penetrate the second conductivity type semiconductor layer and to reach the first conductivity type semiconductor layer. The first region is disposed around the trench so that the side surface of the first region is exposed to the trench. The second region is disposed to be distant from the trench and to be adjacent to the first region. A bottom face of the second region is located to a position deeper than that of said first region. As a result, when a high voltage is applied between a source and a drain, it is possible to cause a punch-through phenomenon to occur at the second region earlier than at the first region.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims priority from Japanese Patent Application No. H. 9-328991 filed Nov. 28, 1997, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a silicon carbide semiconductor device and in particular relates to a vertical type insulated gate field effect transistor for large electric power use (hereinafter, referred to as “vertical type power MOSFET). 
     2. Related Art and Discussion 
     FIG. 16 shows a sectional constitution of a vertical type power MOSFET described in JP-A-7-326755. The conventional vertical type power MOSFET is explained with reference to FIG.  16 . 
     In FIG. 16, an SiC substrate  104  is formed by successively depositing an n − -type epitaxial layer  102  and a p-type epitaxial layer  103  on an n + -type monocrystalline SiC semiconductor substrate  101 . 
     An n +  source region  105  constituting a semiconductor region is formed in the p-type epitaxial layer  103  by ion implantation or the like. Also, a trench  106  passing through the n +  source region  105  and the p-type epitaxial layer  103  and reaching the n − -type epitaxial layer  102  is formed by etching. Inside the trench  106 , a gate thermal oxide film (insulation film)  107  is formed, and a gate electrode layer  108  is formed thereon. Also, a source electrode layer  110  constituting a first electrode layer is formed on an interlayer insulation film  109 , the surface of the n + -type source region  105  and the surface of the p-type epitaxial layer  103 . A drain electrode layer  111  constituting a second electrode layer is formed on the back surface of the semiconductor substrate  104 . 
     In the construction described above, the surface of the p-type epitaxial layer  103  on the side surface of the trench  106  is a channel region. When a positive voltage is impressed on the gate electrode  108  and a channel is formed in the side surface of the p-type epitaxial layer  103 , current flows between the source and the drain. 
     However, when a high voltage is impressed between the source and the drain while the vertical type power MOSFET is in an off state (i.e., no voltage is applied to the gate electrode), the working life of the gate oxide film  107  is shortened because it may suffer some damage or a blocking voltage thereof becomes small comparing to a design value. 
     To solve the above-mentioned problems, the inventors built a prototype of the conventional vertical type power MOSFET and studied it. 
     When a voltage is applied between the source and the drain during the off state of the vertical type power MOSFET, a depletion layer is produced at a PN junction portion between n − -type epitaxial layer  102  and the p-type epitaxial layer  103 , whereby an electric field is generated. The distribution of the electric field depends on impurity concentrations of the n − -type epitaxial layer  102  and the p-type epitaxial layer  103  and the magnitude of voltage applied between the source and the drain. The blocking voltage of the power MOSFET is determined by the condition at which a punch-through phenomenon occurs, that is, the depletion layer extending on a side of the p-type epitaxial layer  103  reaches the n + -type source region  105 . 
     It was confirmed that a measured blocking voltage lowers rather than a design blocking voltage in the prototype of the conventional power MOSFET. As the cause thereof, it is considered that the side surface of the trench  106  is formed not to be perpendicular to the surface of the SiC substrate  104  but to be inclined to some extent with respect thereto. FIG. 17 shows a schematic view of the vertical type power MOSFET in which a high voltage is impressed between the source and the drain during the off state thereof. The reason why the measured blocking voltage lowers is described with reference to FIG.  17 . 
     The depletion layer is produced at the PN junction portion between the n − -type epitaxial layer  102  and the p-type epitaxial layer  103 . The end portion of the depletion layer which makes contact to the surface of the trench  106  (hereinafter, referred to as “depletion layer end portion) is terminated in a state that it is substantially perpendicular to the surface of the trench  106 . For this reason, if the side surface of the trench  106  is perpendicular to the surface of the SiC substrate  104 , the depletion layer end portion will be terminated in a state that it is substantially parallel to the surface of the SiC substrate  104 . However, when the trench  106  is formed by etching, in practice, the side surface of the trench  106  is formed to be inclined to some extent with respect to the surface of the SiC substrate  104 . Therefore, as shown in FIG. 17, the depletion layer end portion is terminated in a state that it is curved in the vicinity of the trench  106 . 
     As a result, the depletion layer end portion reaches the boundary between the p-type epitaxial layer  103  and the n + -type source region  105  earlier than the other portion of the depletion layer. For this reason, it is considered that a punch-through phenomenon occurs at an SiO 2 /SiC interface which is an interface with the gate thermal oxide film  107  comprising an SiO 2  film earlier than the other portion, whereby the actual blocking voltage lowers rather than the design value. 
     To confirm this consideration, a source-drain voltage causing the punch-through phenomenon was measured while a gate voltage is changed. As a result, it was confirmed that the source-drain voltage causing the punch-through phenomenon has strong dependence upon the gate voltage. This result means that the punch-through phenomenon mainly occurs at the SiO 2 /SiC interface and is in agreement with the above-mentioned consideration. 
     In view of the above, the inventors concluded that the cause of the gate oxide film damage and the shortened working life of the gate oxide film is in that current generated by the punch-through phenomenon is greatly accelerated along the SiO 2 /SiC interface and functions as a hot carrier, thereby deteriorating the SiO 2 /SiC interface and the gate oxide film. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a silicon carbide semiconductor device which can prevent the working life of the gate oxide film from being shortened. 
     To achieve this object, in the silicon carbide semiconductor device according to the present invention, a semiconductor substrate made of single crystal silicon carbide is formed by successively depositing a first conductivity type second semiconductor layer and a second conductivity type third semiconductor layer on a first conductivity type first semiconductor layer. A first conductivity type semiconductor region is formed in a surface portion of the third semiconductor layer and is divided into first and second regions. A trench is formed in the semiconductor substrate so as to penetrate the third semiconductor layer and reach the second semiconductor layer. The first region is disposed around the trench so that the side surface of the first region is exposed to the trench. The second region is disposed to be distant from the trench and to be adjacent to the first region. The thickness of the third semiconductor layer between the second region and the second semiconductor layer is made thinner than that of the third semiconductor layer between the first region and the second semiconductor layer. 
     When the thickness of the third semiconductor layer is set as described above, the punch-through phenomenon occurs on a second region side. As a result, it is possible to prevent the punch-through phenomenon from occurring at an interface between a gate insulation film formed in the trench and the third semiconductor layer, i.e., at an SiO 2 /SiC interface. Therefore, it is possible to prevent the working life of the gate insulation film from being shortened. 
     Alternatively, the second region can be formed by metal silicide or metal carbide. 
     Further, a stepped portion may be formed on the surface of the third semiconductor layer so that the surface of the third semiconductor layer in the region for the first region to be formed is made higher than that in the region for the second region to be formed. After that, by performing, for example, ion implantation toward the surface of the third semiconductor layer, the bottom face of the second region can be located deeper than that of the first region. 
     Furthermore, a groove may be formed in the surface of the third semiconductor layer so that the bottom face of the groove is located to a position deeper than the bottom face of the first region. From the bottom face of the groove, metal is thermally diffused in the third semiconductor layer to form metal silicide and metal carbide therein, or on the bottom face of the groove, a source electrode itself is deposited. Due to this arrangement, since the depletion layer extending on a third semiconductor layer side reaches the second region earlier than the first region, the punch-through phenomenon occurs on the second region side. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, features and characteristics of the present invention will be appreciated from a study of the following detailed description, the appended claims, and drawings, all of which form a part of this application. In the drawings: 
     FIG. 1 is a sectional view illustrating a vertical type power MOSFET according to a first preferred embodiment of the present invention; 
     FIGS. 2 through 8 are views illustrating a manufacturing process of the power MOSFET shown in FIG. 1 in a stepwise manner; 
     FIG. 9 is a sectional view illustrating a vertical type power MOSFET according to a second preferred embodiment; 
     FIG. 10 is a view for explaining a manufacturing process of a vertical type power MOSFET according to a third preferred embodiment; 
     FIG. 11 is a sectional view illustrating the vertical type power MOSFET according to the third preferred embodiment; 
     FIG. 12 is a sectional view illustrating a vertical type power MOSFET according to a fourth preferred embodiment; 
     FIG. 13 is a sectional view illustrating a vertical type power MOSFET according to a fifth preferred embodiment; 
     FIG. 14 is a sectional view illustrating a vertical type power MOSFET according to a sixth preferred embodiment; 
     FIG. 15 is a sectional view illustrating a modification of the vertical type power MOSFET as described above; 
     FIG. 16 is a sectional view illustrating a conventional vertical type power MOSFET; and 
     FIG. 17 is a schematic view for explaining a depletion layer produced in the power MOSFET shown in FIG.  16 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (First Embodiment) 
     A vertical type n-channel power MOSFET according to a first preferred embodiment of the present invention will be described with reference to FIG.  1 . 
     An n + -type silicon carbide (SiC) semiconductor substrate  1  serving as a low resistance semiconductor layer is made of hexagonal crystal system silicon carbide. An n − -type SiC semiconductor layer  2  serving as a high resistance semiconductor layer and a p-type SiC semiconductor layer  3  are successively layered on a main surface of the n + -type SiC semiconductor substrate  1 . As described above, a semiconductor substrate  4  made of single crystal silicon carbide is formed by the n + -type SiC semiconductor substrate  1 , the n − -type SiC semiconductor layer  2  and the p-type SiC semiconductor layer  3 . 
     A source region  5  made of an n +  semiconductor region is formed in a predetermined region at a surface portion of the p-type SiC semiconductor layer  3 . The source region  5  comprises a region (first semiconductor region)  5   a  adjacent to a side surface  6   a  of a trench  6  and a region (second semiconductor region)  5   b  distant from the trench side surface  6   a.  The junction depth of the region  5   b  to the p-type SiC semiconductor layer  3  is made deeper than that of the region  5   a,  that is, the bottom face of the region  5   b  is located to a position lower than that of the region  5   a.  As a result, the thickness of the p-type SiC semiconductor layer  3  between the region  5   b  and the n − -type SiC semiconductor layer  2  is made thinner than the thickness of the p-type SiC semiconductor layer  3  between the region  5   a  and the n − -type SiC semiconductor layer  2 . 
     The trench  6  passing through the source region  5  and the p-type SiC semiconductor layer  3  and reaching the n − -type SiC semiconductor layer  2  is formed in a predetermined region of the source region  5 . This trench  6  has the side surface  6   a  substantially perpendicular to the surface of the semiconductor substrate  4  and a bottom surface  6   b  parallel with the surface of the semiconductor substrate  4 . 
     A gate insulation film  7  is formed on the side surface  6   a  and bottom surface  6   b  of the trench  6 . A gate electrode layer  8  is filled inside the gate insulation film  7 . Further, an interlayer insulation layer  9  is formed to cover the gate insulation film  7  and the gate electrode layer  8 . 
     A source electrode (first electrode layer)  10  is then formed by aluminum or the like on the source region  5 , the low resistance p-type SiC semiconductor layer  3  and the interlayer insulation layer  9 . A drain electrode (second electrode layer)  11  is formed on the back surface of the n + -type SiC semiconductor substrate  1  (back surface of the semiconductor substrate  4 ). 
     In the vertical type power MOSFET structured as described above, the junction depth of the region  5   a  adjacent to the trench side surface  6   a  is made shallower than that of the region  5   b  distant from the trench side surface  6   a  (the region  5   b  making contact with the source electrode  10 ). For this reason, when a high voltage is impressed between the source and the drain, the depletion layer extending on a p-type SiC semiconductor layer  3  side reaches the region  5   b  earlier than the region  5   a  adjacent to the trench side surface  6   a.  As a result, a punch-through phenomenon occurs in the region  5   b  with the voltage lower than the voltage with which the punch-through phenomenon occurs at the SiO 2 /SiC interface (channel portion) present on the trench side surface  6   a.  Thereby, it is possible to prevent current generated by the punch-through phenomenon from flowing through the SiO 2 /SiC interface. 
     Consequently, not only deterioration of the SiO 2 /SiC interface and the gate oxide film  7  can be prevented, but also device breakdown due to current concentration can be prevented since the punch-through phenomenon occurs at the entire bottom face of the region  5   b.    
     Next, the manufacturing process of the vertical type n-channel power MOSFET will be described with reference to FIGS. 2 through 8. 
     First, as shown in FIG. 2, the n + -type SiC semiconductor substrate  1  serving as the low resistance semiconductor layer is prepared. The n − -type SiC semiconductor layer  2  is epitaxially grown on the main surface of the n + -type SiC semiconductor substrate  1 . Further, the p-type SiC semiconductor layer  3  is epitaxially grown on the n − -type SiC semiconductor layer  2 . In this way, the semiconductor substrate  4  is formed from the n + -type SiC semiconductor substrate  1 , the n − -type SiC semiconductor layer  2  and the p-type SiC semiconductor layer  3 . 
     After that, a power MOSFET as a semiconductor element is formed on the semiconductor substrate  4 . 
     First, as shown in FIG. 3, the region  5   a  is formed by implanting, for example, nitrogen ions into the p-type epitaxial layer  3  using a first mask  12 . After that, as shown in FIG. 4, the region  5   b  is formed by implanting, for example, nitrogen ions into the p-type epitaxial layer  3  using a second mask  13 . Acceleration voltages of ion implantations for forming the regions  5   a  and  5   b  are adjusted so that the junction depth of the region  5   b  is deeper than that of the region  5   a.  In this way, the source region  5  made up of the regions  5   a  and  5   b  is formed. 
     Next, as shown in FIG. 5, a trench  6  passing through the source region  5  and the p-type SiC semiconductor layer  3  and reaching the n − -type SiC semiconductor layer  2  is formed by dry etching using a mask  14 . This trench  6  has a side surface  6   a  not exactly perpendicular to but inclined to some extent with respect to the surface of the semiconductor substrate  4  and a bottom surface  6   b  parallel with the surface of the semiconductor substrate  4 . 
     After that, as shown in FIG. 6, a gate insulation film  7  is formed by carrying out thermal oxidation. As a result of this thermal oxidation, a thermal oxide film  7   a  having a small thickness is formed on the side surface  6   a  of the trench  6 , and thermal oxide films  7   b,    7   c  having a large thickness is formed on the bottom surface  6   b  of the trench  6  and on the surface of the semiconductor substrate  4 , respectively. 
     Then, as shown in FIG. 7, first and second polysilicon layers  8   a,    8   b  are successively filled into the trench  6  to form the gate electrode  8 . After that, an interlayer insulation layer  9  is formed by CVD on the gate oxide film  7  including the gate electrode  8 . Then, the gate insulation film  7  and the interlayer insulation layer  9  on the source region  5  and the p-type SiC semiconductor layer  3  where a source contact is to be located are removed by etching. A source electrode layer  10  is then formed on the source region  5 , the p-type SiC semiconductor layer  3  and the interlayer insulation layer  9 . Finally, a drain electrode layer  11  is formed on the back surface of the n + -type SiC semiconductor substrate  1 , whereby the trench gate type SiC power MOSFET shown in FIG. 1 is completed. 
     (Second Embodiment) 
     In the first embodiment, the region  5   a  and the region  5   b  are formed by separate steps so that the junction depth of the region  5   b  is deeper than that of the region  5   a.  In the second embodiment, the region  5   a  and the region  5   b  are formed at the same time by the same step. 
     In the second embodiment, as shown in FIG. 9, a groove  15  is formed on the surface of the p-type SiC semiconductor layer  3  by removing a predetermined region of the p-type SiC semiconductor layer  3  by a dry etching process before the regions  5   a  and  5   b  constituting the source region  5  are formed. After that, by implanting ions into the p-type SiC semiconductor layer  3 , the regions  5   a  and  5   b  can be formed at the same time. 
     In this case, because a stepped portion is formed on the surface of the p-type SiC semiconductor layer  3  by the groove  15 , when the thickness of the source region formed by ion implantation is constant, the region  5   b  is formed under the surface of the region (lower step portion) which has been subject to dry etching, and the region  5   b  is formed under the surface of the region (upper step portion) which has not been subject to dry etching. As a result, the region  5   b  formed in the lower step portion has a bottom face at a deeper position than the region  5   a  formed in the upper step portion. Therefore, the same effects as the first embodiment can be obtained by the second embodiment. In the second embodiment, the number of ion implantation steps necessary for forming the regions  5   a  and  5   b  can be reduced to one time. 
     (Third Embodiment) 
     In the first embodiment, the step for forming the region  5   b  is carried out following the step for forming the region  5   a.  In the third embodiment, the region  5   b  is formed after the interlayer insulation film  9  is formed. 
     That is, without performing the step shown in FIG. 4, the steps shown in FIGS. 5 through 8 are successively carried out. After that, as shown in FIG. 10, after the interlayer insulation film  9  and the gate insulation film  7  in a predetermined area on the surface of the semiconductor substrate  4  are removed by etching, ion implantation is carried out using the interlayer insulation film  9  as a mask to form the region  5   b  of the source region  5 . 
     After that, the interlayer insulation film  9  and the gate insulation film  7  in a predetermined area on the surface of the semiconductor substrate  4  are further removed by etching to allow the source electrode  10  to contact the p-type SiC semiconductor layer  3 . In this state, the source electrode layer  10  is formed on the semiconductor substrate  4 , to complete the vertical type SiC power MOSFET shown in FIG.  11 . 
     According to the third embodiment, a second mask formation step necessary for ion implantation process for forming the region  5   b  can be eliminated. 
     (Fourth Embodiment) 
     In the first through third embodiments, the region  5   b  of the source region  5  is formed by implanting, for example, nitrogen ions into the p-type SiC semiconductor substrate  3  and inverting the conductivity thereof to n +  type. In the fourth embodiment, a second region  20  is formed by metal silicide and metal carbide which have the same effects as the n + -type semiconductor layer, and play a role of the region  5   b  in the first, second, or third embodiment. 
     That is, without performing the step shown in FIG. 4, the steps shown in FIGS. 5 through 8 are successively carried out. After that, as shown in FIG. 12, after the interlayer insulation film  9  and the gate insulation film  7  in a predetermined area on the surface of the semiconductor substrate  4  are removed by etching, a nickel (Ni) layer  19  is formed in the region where the interlayer insulation film  9  and the gate insulation film  7  are removed. A heat treatment is then carried out with respect to the semiconductor device. By this heat treatment, the second region  20  which is composed by Ni silicide and Ni carbide having the same effects as the n + -type semiconductor layer is formed in the p-type SiC semiconductor layer  3  as shown in FIG.  12 . At this time, heat treatment time and temperature are adjusted so that the bottom face of the second region  20  is located to a position deeper than that of the source region  5  ( 5   a ). 
     Further, by the second region  20  electrically connecting the source electrode  10  to the source region  5 ( 5   a ), a contact resistance between the source electrode  10  made of aluminum or the like and the source region  5 ( 5   a ) can be reduced. Therefore, not only the same effects as the first embodiment but also, reduced contact resistance between the source region  5  and the source electrode  10  can be obtained by the fourth embodiment. 
     (Fifth Embodiment) 
     In the fourth embodiment, the Ni layer  19  is formed on the semiconductor substrate  4 , and the second region  20  composed of metal (Ni) silicide and metal carbide is formed by thermal diffusion. Further, as shown by the fifth embodiment, a groove may be formed in advance at a region where the second region  20  is to be formed. 
     Although, in the fourth embodiment, duration and temperature of the heat treatment are adjusted so that the bottom face of the second region  20  is located to a position is deeper than that of the source region  5 , there is a possibility that the bottom face of the second region  20  is not located to a position deeper than the bottom face of the source region, or it takes long time to realize the second region  20  thermally diffused deeper than the bottom face of the source region  5  in consideration of the amount of thermally diffused metal silicide and metal carbide. 
     For the reason described above, a groove  16  adjacent to the source region  5 ( 5   a ) is formed as shown in FIG.  13 . As a result, even when the amount of thermally diffused metal silicide and metal carbide is small, the bottom face of the second region  20  can be reliably reached to a position deeper than the bottom face of the source region  5  by the groove  16 . 
     In this way, even if the amount of thermally diffused metal silicide and metal carbide is limited, the same effects as the fourth embodiment can be obtained by forming the groove  16 . 
     It is to be noted that, because it is sufficient for the bottom face of the second region  20  to be located to a position deeper than the bottom face of the source region  5 , the depth of the groove  16  may be changed depending on the amount of thermally diffused metal silicide and metal carbide. For example, the depth of the groove  16  may be shallower than the junction depth of the source region  5 . 
     (Sixth Embodiment) 
     In the first through third embodiments, the source region  5  is divided into two regions  5   a  and  5   b  having different depths to which the bottom faces are located. In the sixth embodiment, the source region  5  is formed so that the bottom face thereof has a constant depth, and the bottom face of the source electrode  10  is extended to a position deeper than the bottom face of the source region  5 . In this way, the extended portion of the source electrode  10  can play a role of the region  5   b  in the first, second, or third embodiment. 
     FIG. 14 shows a schematic view of the vertical type power MOSFET according to the sixth embodiment. As shown in FIG. 14, a groove  17  passing through the source region  5  and reaching the p-type SiC semiconductor layer  3  is formed, and the source electrode  10  is extended to and filled in the groove  17 . 
     In this way, when the bottom face of the source electrode  10  is located to a position deeper than the bottom face of the source region  5 , the same effects as the above-described embodiments can be obtained. 
     In the sixth embodiment, since it is unnecessary to form the source region  5  by ion implantation as described in the first through third embodiments, the trench gate type power MOSFET can be produced by using a substrate in which a low resistance n-type SiC semiconductor layer is epitaxially grown on the p-type SiC semiconductor layer  3 . 
     (Modifications) 
     The source electrode layer  10  formed on the n + -type source region  5   b  and the p-type SiC semiconductor layer  3  can be made of a material different from that described before. Also, although the source electrode layer  10  makes contact with the p-type SiC semiconductor layer  3  to fix the potential thereof to a constant value, it may make contact at least with n + -type source region  5   b.    
     Further, as shown in FIG. 15, a low resistance p-type SiC semiconductor layer  12  may be provided in the predetermined area of the p-type SiC semiconductor layer  3 . 
     Furthermore, in the above-mentioned embodiments, the vertical type n-channel MOSFET is described as an example. However, the same effects as described above can be also obtained in a vertical type p-channel MOSFET. 
     The n + -type source region  5   a  can be formed after the formation of the trench  6 .