This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-093461, filed Mar. 30, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a semiconductor device and a method of manufacturing the same, particularly, to a semiconductor device having a high breakdown voltage and a method of manufacturing the particular semiconductor device.
FIG. 7 is a cross sectional view showing a conventional power (high breakdown voltage) vertical type MOSFET having a trench gate structure. A trench gate structure that permits improving the channel density is widely employed in a power vertical MOSFET in order to improve the on-voltage characteristics. As shown in FIG. 7, an N-type epitaxial semiconductor layer is formed on a first main surface (upper surface) of an N-type silicon semiconductor substrate 111. Formed in the N-type epitaxial semiconductor layer are an N-type drain region (Nxe2x88x92-type drain region) 106, a P-type base region 105, and an N-type source region (N+-type source region) 104. The P-type base region 105 is formed in a surface region of the N-type drain region 106. The N-type source region 104 is formed in a surface region of the P-type base region 105. Each of the P-type base region 105 and the N-type source region 104 is formed by an impurity diffusion. A gate trench is formed in a manner to extend from the surface of the N-type source region 104 through the N-type source region 104 and the P-type base region 105 so as to reach the N-type drain region 106. A gate insulating film 107 consisting of, for example, a silicon oxide film is formed on the side wall and the bottom of the gate trench. A gate electrode 108 made of, for example, a polycrystalline silicon (polysilicon) is buried in the gate trench. The gate insulating film 107 and the gate electrode 108 are covered with an interlayer insulating film 103 consisting of, for example, a silicon oxide film. A barrier metal layer 102 is formed on the entire substrate surface including the interlayer insulating film 103. A source electrode 101 consisting of a metal electrode such as aluminum is formed on the source region 104 and the base region 105 with the barrier metal layer 102 interposed therebetween, and the source region 104 and the base region 105 are electrically connected to each other. A drain electrode 112, which is a metal electrode, is formed on a second main surface (back surface) of the semiconductor substrate 111. FIG. 7 shows a unit cell portion of a semiconductor device, which corresponds to a unit cell portion of a semiconductor device in FIG. 9, as represented by region A in FIG. 9. Incidentally, the semiconductor device shown in FIG. 9 is for the exemplifying purpose. Therefore, the construction shown in FIG. 7 is not necessarily equal to the construction of the unit cell portion of the semiconductor device shown in FIG. 9. Similarly, FIG. 8 shows a unit cell portion of another conventional semiconductor device.
In the conventional power vertical MOSFET having a trench gate structure as shown in FIG. 7, the impurity concentration is increased in the epitaxial silicon semiconductor layer 106 formed on the silicon semiconductor substrate 111 in order to lower the on-voltage. A depletion layer is generated if a reversed bias is applied between the source electrode and the drain electrode. If the impurity concentration in the epitaxial silicon semiconductor layer 106 is increased, a depletion layer 113 is generated as shown in the drawing. As denoted by arrows in the drawing, the electric field is concentrated in the corner portion of the trench because of the presence of the depletion layer. It should be noted that the Vdss breakdown voltage is deteriorated by the electric field concentration occurring in the corner portion of the trench.
FIG. 8 is a cross sectional view showing a conventional power MOSFET of a planar structure. As shown in the drawing, an N-type epitaxial semiconductor layer (N-type drain region) 106 is formed on a first main surface (upper surface) of an N-type silicon semiconductor substrate 111. A P-type base layer 105 is formed in a surface region of the N-type drain region 106. Also, an N-type source region 104 is formed in a surface region of the P-type base region 105. A gate insulating film 107 such as a silicon oxide film is formed on the N-type source region 104, the P-type base region 105 and the N-type drain region 106 except the region where a source electrode is to be formed. Also, a gate electrode 108 made of, for example, polysilicon is formed on the gate insulating film 107. The gate insulating film 107 and the gate electrode 108 are covered with an interlayer insulating film 103 such as a silicon oxide film. A source electrode 101, which is a metal electrode made of, for example, aluminum, is formed on the N-type source region 104 and the P-type base region 105 so as to electrically connect the N-type source region 104 to the P-type base region 105. Further, a drain electrode 112 is formed on a second main surface (back surface) of the semiconductor substrate 111.
In the conventional power MOSFET of the planar structure shown in FIG. 8, the impurity concentration is also increased in the epitaxial silicon semiconductor layer 106 formed on the silicon semiconductor substrate 111 in order to lower the on-voltage, as in the power vertical MOSFET of the trench gate structure shown in FIG. 7. A depletion layer is generated when a reverse bias is applied between the source electrode and the drain electrode. If the impurity concentration is increased in the epitaxial silicon semiconductor layer 106, the electric field is concentrated in the extending portion of the P-type base region 105. It should be noted that the Vdss breakdown voltage is deteriorated by the electric field concentration taking place in the extending portion of the P-type base region 105.
An object of the present invention, which has been achieved in view of the situation described above, is to provide a semiconductor device in which the electric field concentration is relaxed in the corner portion of the trench gate or in the extending portion of the base region so as to improve the breakdown voltage and a method manufacturing the particular semiconductor device.
The present invention is featured in that formed in a part of the base region is an impurity diffusion region extending in a vertical direction of the semiconductor substrate and having an impurity concentration lower than that in the other portion of the base region. The impurity concentration of the impurity diffusion region is an average value of the concentration of the impurities diffused in the base region excluding the extending region. By forming the impurity diffusion region of a low impurity concentration, it is possible to extend the depletion layer toward the base region so as to improve the breakdown voltage. In the present invention, a trench is formed in a part of the base region, and an impurity of the conductivity type equal to that of the base region is introduced into the side wall and the bottom portion of the trench by ion implantation in a concentration lower than that in the base region, followed by diffusing the implanted impurity so as to form the impurity diffusion region of the low impurity concentration.
According to a first aspect of the present invention, there is provided a semiconductor device, comprising: a semiconductor substrate; a semiconductor layer of a first conductivity type formed on one main surface region of the semiconductor substrate; a drain region of the first conductivity type formed in the semiconductor layer of the first conductivity type; a base region of a second conductivity type formed in a surface region of the drain region; a source region of the first conductivity type formed in a surface region of the base region; a trench having a conductive layer and/or an insulating layer buried therein and extending from the surface of the base region through the base region to reach an inner region of the drain region; an impurity diffusion region of the second conductivity type formed in the periphery of the side wall of that portion of the trench which is positioned within the drain region and having an impurity concentration lower than that in the base region; a gate insulating film formed to cover a part of the surfaces of the drain region, the base region and the source region; and a gate electrode formed on the gate insulating film.
In the semiconductor device according to the first aspect of the present invention, the semiconductor substrate may be of the first conductivity type.
In the semiconductor device according to the first aspect of the present invention, the semiconductor substrate may be of the second conductivity type.
According to a second aspect of the present invention, there is provided a semiconductor device, comprising: a semiconductor substrate; a semiconductor layer of a first conductivity type formed on one main surface region of the semiconductor substrate; a drain region of the first conductivity type formed in the semiconductor layer of the first conductivity type; a base region of a second conductivity type formed in a surface region of the drain region; a source region of the first conductivity type formed in a surface region of the base region; a first trench having a conductive layer and/or an insulating layer buried therein and extending from the surface of the base region through the base region to reach an inner region of the drain region; an impurity diffusion region of the second conductivity type formed in the periphery of the side wall of that portion of the first trench which is positioned within the drain region and having an impurity concentration lower than that in the base region; a second trench extending from the surface of the source region or from the surface of that portion of the base region which is contiguous to the source region to reach an inner region of the drain region; a gate insulating film formed in the side wall and the bottom region of the second trench; and a gate electrode buried in the second trench in a manner to cover the gate insulating film.
In the semiconductor device according to the second aspect of the present invention, the semiconductor substrate may be of the first conductivity type.
In the semiconductor device according to the second aspect of the present invention, the semiconductor substrate may be of the second conductivity type.
In the semiconductor device according to the second aspect of the present invention, the depth of the first trench from the surface of the base region may be larger than that of the second trench from the surface of the source region or from the surface of that portion of the base region which is contiguous to the source region.
In the semiconductor device according to the second aspect of the present invention, in which the first trench is deeper than the second trench, the semiconductor substrate may be of the first conductivity type.
In the semiconductor device according to the second aspect of the present invention, in which the first trench is deeper than the second trench, the semiconductor substrate may be of the second conductivity type.
In the semiconductor device according to the second aspect of the present invention, in which the first trench is deeper than the second trench and the semiconductor substrate is of the second conductivity type, a buffer layer formed of a semiconductor layer of the first conductivity type having an impurity concentration higher than that in the semiconductor layer of the first conductivity type may be formed between the semiconductor substrate and the semiconductor layer of the first conductivity type.
According to a third aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of: forming a semiconductor layer of a first conductivity type on one main surface of a semiconductor substrate; forming a base region of a second conductivity type in a surface region of a drain region of the first conductivity type formed in the semiconductor layer of the first conductivity type; forming a source region of the first conductivity type in a surface region of the base region; forming a trench extending from the surface of the base region through the base region to reach an inner region of the drain region; forming an impurity diffusion region of the second conductivity type having an impurity concentration lower than that in the base region in the periphery of the side wall of that portion of the trench which is positioned within the drain region; burying a conductive layer and/or an insulating layer within the trench; forming a gate insulating film in a manner to cover a part of the surfaces of the drain region, the base region and the source region; and forming a gate electrode on the gate insulating film.
In the method of manufacturing a semiconductor device according to the third aspect of the present invention, the impurity diffusion region of the second conductivity type may be formed by implanting ions of the impurity of the second conductivity type.
According to a fourth aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of: forming a semiconductor layer of a first conductivity type on one main surface of a semiconductor substrate; forming a base region of a second conductivity type in a surface region of the drain region of the first conductivity type formed in the semiconductor layer of the first conductivity type; forming a source region of the first conductivity type in a surface of the base region; forming a first trench extending from the surface of the base region through the base region to reach an inner region of the drain region; forming an impurity diffusion region of the second conductivity type having an impurity concentration lower than that in the base region in the periphery of the side wall of that portion of the first trench which is positioned within the drain region; burying a conductive layer and/or an insulating layer within the first trench; forming a second trench extending from the surface of the source region or from the surface of that portion of the base region which is contiguous to the source region to reach an inner region of the drain region; forming a gate insulating film on the side wall and the bottom surface of the second trench; and forming a gate electrode within the second trench in a manner to cover the gate insulating film.
In the method of manufacturing a semiconductor device according to the fourth aspect of the present invention, the depth of the first trench from the surface of the base region may be larger than that of the second trench from the surface of the source region or from the surface of that portion of the base region which is contiguous to the source region.
Further, in the method of manufacturing a semiconductor device according to the fourth aspect of the present invention, the impurity diffusion region of the second conductivity type may be formed by ion implantation of an impurity of the second conductivity type.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
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