Abstract:
At a portion below a gate electrode of a vertical type MOS transistor having a gate electrode and source electrodes formed over the surface of a semiconductor substrate and a drain electrode formed over the back thereof, a P type impurity diffusion layer spaced away from P well diffusion layers which surround sources, is formed in the semiconductor substrate which serves as a drain region.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a structure of a power MOS transistor suitable for use in a motor control circuit or a power circuit or the like and a method of manufacturing the same. 
     2. Description of the Related Art 
     Various structures have heretofore been proposed as the structure of a power MOS transistor. However, the structure of a VDMOS (Vertical Diffusion MOS) transistor corresponding to a vertical type MOS transistor, featuring that it causes current to flow in the vertical direction of a semiconductor substrate, is generally basic to the structure of the power MOS transistor. This is because since the entire substrate is formed as a drain structure, the resistance thereof becomes low and a large current flow is provided and it is apt to withstand high voltages and reduce thermal resistance. 
     FIG. 1 is a cross-sectional view of a conventional VDMOS transistor and shows an N channel type. A drain region of the VDMOS transistor is formed of an N type semiconductor substrate  1 , whereas each source region thereof is composed of an N+ diffusion layer  6 , a P well layer  2  and a P+ diffusion layer  7  for applying a voltage. 
     The operation of the VDMOS transistor is as follows: When a desired voltage is applied to a gate electrode  5 , a current flows from the N type semiconductor substrate  1  used as the drain to the N+ diffusion layer  6  used as the source through the surface of a semiconductor disposed below the gate electrode  5 . The amount of the current at this time is controlled by gate and drain voltages in a manner similar to the normal type MOS transistor. 
     With the spread of application areas of a VDMOS transistor, there has recently been a demand for a VDMOS transistor made more powerful in performance. A reduction in on resistance, which is the utmost important characteristic of the power MOS transistor, is required to meet such a demand. The on resistance of the VDMOS transistor consists of a series of a channel resistance of a MOS transistor and a resistance (lying in the vertical direction) of an N type semiconductor substrate. However, the shorter the length of a gate of the transistor, the more the channel resistance can be reduced. It is thus quite natural that a reduction in on resistance should be achieved by shortening the gate length and reducing the resistance of the substrate. 
     As is however apparent from FIG. 1, the gate length of the VDMOS transistor is determined according to a junction depth of each P well diffusion layer  2  formed over the M type semiconductor substrate  1 . Since the P well diffusion layers  2  are respectively formed from gate ends on the source sides of the VDMOS transistor, a channel region is determined depending on the length of each P well diffusion layer  2 , which extend in the transverse direction thereof. It is thus necessary to form the P well diffusion layers  2  shallowly for the purpose of a reduction in gate length. 
     On the other hand, a high withstand voltage characteristic is known as one feature of the VDMOS transistor. While a withstand voltage of about 1000V is required according to purposes, a withstand voltage of about a few 100V is required even in the normal case. The P well diffusion layers  2  are formed deep for the purpose of high withstanding and the shape of each P well diffusion layer  2  by which the withstand voltage is determined, is set as smooth as possible, thereby reducing the curvature thereof and avoiding the concentration of an electric field on each P well diffusion layer  2 . It is necessary to select a substrate having a high resistance or resistivity of about 100 Ω·cm for the purpose of high withstanding from the viewpoint of a depletion layer even in the case of the substrate resistance. 
     SUMMARY OF THE INVENTION 
     As described above, it becomes necessary to deepen the P well diffusion layers and increase the resistance of the N type semiconductor substrate with a view toward maintaining a high withstand voltage characteristic. However, a problem arises in that this is contrary to the reduction in on resistance. 
     In order to solve the problem of a difficulty in reducing the on resistance, the present invention provides a vertical type insulated gate transistor wherein a P type impurity region for relaxing or reducing an electric field is formed between the N type semiconductor substrate and the corners of the P well diffusion layers both employed in the conventional VDMOS transistor. 
     According to the present invention, since an electric field produced between a drain and P well diffusion layers is reduced owing to a P type impurity diffusion layer (field relaxation layer) provided in the neighborhood of the corners of the P well diffusion layers, a transistor can withstand high voltages. Thus, since the P well diffusion layers can be shallowed in depth, the VDMOS transistor can be reduced in gate length. Further, since the depth of each P well diffusion layer and the expansion of a depletion layer therefrom, which are parameters for determining the thickness of the semiconductor substrate, are also reduced owing to a shallow junction between the P well diffusion layers and the semiconductor substrate, the thickness of the substrate can be formed thinner. It is therefore possible to reduce on resistance while a high withstand voltage characteristic is being held. 
     Typical ones of various inventions of the present application have been shown in brief. However, the various inventions of the present application and specific configurations of these inventions will be understood from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view of VDMOS transistor according to a prior art; 
     FIG. 2 is a cross-sectional view of a VDMOS transistor according to the present invention; 
     FIGS. 3 and 3A are alternative process views showing a method of manufacturing a VDMOS transistor according to the present invention; 
     FIG. 4 is a process view illustrating the method of manufacturing the VDMOS transistor according to the present invention; and 
     FIG. 5 is a process view depicting the method of manufacturing the VDMOS transistor according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the present invention will hereinafter be described with reference to the accompanying drawings. 
     FIG. 2 is a cross-sectional view for describing a structure (unit cell) of an N channel VDMOS transistor according to the present invention. A pair of P well diffusion layers  102  is formed over an N type semiconductor substrate  10  having about 10 Ω·cm to 100 Ω·cm. The density or concentration of the P well diffusion layer is 1E16cm-3 and the depth thereof is about 2 μm. This structure results in a unit cell of the VDMOS transistor. The N type semiconductor substrate  101  is used as a drain region. As shown in FIG. 2, a P type impurity layer  103  used as an electric field relaxation layer is formed in the vicinity of the corners of the P well diffusion layers, i.e., in the N type semiconductor substrate  101  interposed between the pair of P well diffusion layers  102 . 
     The P type impurity layer  103  is formed a few μm away from the P well diffusion layers  102  without making contact therewith. The concentration thereof is normally optimized to a range of 1E16cm-3 to 1E18cm-3 so as to reduce an electric field between the P well diffusion layers  102  and the N type semiconductor substrate (drain)  101 . Further, the depth thereof is optimally set in consideration of the depth of each P well diffusion layer  102 . 
     Reference numeral  104  indicates a field oxide film having a depth of about 1 μm, for separating between devices. The thickness of the field oxide film is set according to used voltages. Further, a gate oxide film  105  and a gate electrode  106  are formed as portions equivalent to expanding regions of the P well diffusion layers  102  and disposed over the surface of the semiconductor substrate. Furthermore, reference numerals  107  respectively indicate N+ diffusion layers which serve as source regions formed over the surfaces of the P well diffusion layers  102 . Reference numerals  108  indicate P+ diffusion layers for making electrodes of the P well diffusion layers respectively. `A method of manufacturing a VDMOS transistor according to the present invention will next be described with reference to FIGS. 3 through 5. Referring to FIG. 3, an N type semiconductor substrate  101  is first prepared. Alternatively, an N on N+ type semiconductor substrate may be used wherein a low-density impurity semiconductor is formed over a high-density impurity semiconductor. In this case, an N layer may be formed by an epitaxial method. A P type impurity layer  103 , which serves as an electric field relaxation layer, is formed in the N type semiconductor substrate  101 , as illustrated in FIG.  3 . After patterning has been effected using the normal photolithography and etching technique, boron is introduced by an ion implantation method to form the P type impurity layer  103  in semiconductor substrate  101 . The density thereof is 1E17cm-3 and the depth thereof is about 1 μm. Thereafter, an epitaxial layer  111  having resistance or resistivity which ranges from 10 Ω·cm to 100 Ω·cm and a thickness which ranges from 10 μm to 100 μm, is formed over the entire surface of the N type semiconductor substrate  101  by a CVD method, as also illustrated in FIG.  3 . The resistance and thickness of the epitaxial layer, etc. are optimized according to the depth of each P well diffusion layer and the required withstand voltage value or the like. In an alternative embodiment as illustrated in FIG. 3A, the P type impurity layer  103  is formed in a boundary between N type impurity diffusion layer region  115  and N type epitaxial layer  113 . 
     There is also known a method of directly introducing boron by a high-energy ion implantation method after the formation of an epitaxial layer. In this case, the process can be simplified because a diffusing process is omitted. This method does not necessarily require the epitaxial layer when it is used. The present structure can be also achieved by the normal N type silicon substrate alone. 
     Next, a field oxide film  104  used as a thick silicon oxide film for preventing a parasitic channel and lowering the capacity of each gate is formed in a field region by a thermal oxidation process as shown in FIG.  4 . Further, a gate oxide film  105  is formed in thicknesses ranging from about 100 Å to 1000 Å. Furthermore, a polysilicon film lowered in resistance, which serves as a gate electrode  106 , is formed in thicknesses ranging from about 2000 Å to 5000 Å by the CVD method. A polycide film obtained by laminating a polysilicon film on a high-melting point metal film (W, Ti or the like) may be used as a gate structure. 
     Next, P well diffusion layers  102  are formed over the N type semiconductor substrate  101  by the ion implantation method or the like of boron with a gate pattern as a mask as shown in FIG. 5. A dose of about 1E13cm-2 is used and a desired depth is diffused within the substrate by thermal diffusion. While the P well diffusion layers  102  are expanded even in the transverse direction with the ends of the gate as start points at this time, diffusion conditions may preferably be set so that the width of each expansion and the length of the gate substantially coincide with each other. Thereafter, each of source regions  107  is formed to a density of 1E20cm-3 and a depth of about 0.5 μm by a phosphorus or arsenic ion implantation method. 
     Further, P+ diffusion layers  108 , which serve as electrodes of the P well diffusion layers  102 , are formed within the P well diffusion layers  102  so as to be formed to a density of 1E20cm-3 and a depth of about 0.5 μm. 
     As described above, the VDMOS transistor according to the present invention has the following advantages. Since a P type impurity diffusion layer is placed between P well diffusion layers and an N type semiconductor substrate and serves as a field relaxation structure, a high electric field produced from a drain can be relaxed and an available voltage restriction can be expanded. 
     Since the depth of each P well diffusion layer can be made shallow owing to such a field relaxation effect, the shortening of the width of aMOS channel layer and a reduction in the thickness of the substrate can be achieved simultaneously. Thus, the on-resistance, which is of an important characteristic of the VDMOS transistor, can be reduced and the device can be rendered high in performance. 
     While the present invention has been described with reference to the illustrative embodiments,this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments,as well as other embodiments of the invention,will be apparent to those skilled in the art on reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.