Abstract:
According to the present invention, a lower electrode is formed on a semiconductor substrate and overgrows upward to form one electrode of a capacitor having a mushroom-shaped section. An insulation film is formed so as to at least cover the lower electrode. An upper electrode is formed so as to oppose the lower electrode and to cover at least the insulation film.

Description:
This application is a continuation of application Ser. No. 07/672,322, filed Mar. 20, 1991. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and a method for manufacturing the same and, more particularly, to a semiconductor device for use in a memory cell capacitor. 
     2. Description of the Related Art 
     A conventional cell capacitor is shown in FIG. 1. After a P-type silicon substrate 1 is thermally oxidized to form a field oxide film 2 having a thickness of 5000 Å, an oxide film 3 having a thickness of 100 Å is formed. Ions As +   are then implanted to form an N +   layer 4. A polysilicon 5 is deposited and patterned to serve as a plate electrode. 
     The cell capacitor having a plane structure as shown in FIG. 1 is simple to manufacture and has been widely applied to the extent of 1 M-bit dynamic random access memory (DRAM). Since the plane structure, however, reduces the area of a cell when the degree of integration of the cell is enhanced, the thickness of a gate oxide film (oxide film 3) needs to be thinned to retain stored charges to some extent, which degrades the reliability of the oxide film 3. The plane structure cannot be physically applied to a cell capacitor of a DRAM of 4 M-bit or more. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to provide a capacitor which has a high degree of integration and high reliability and which is simple to manufacture, and a method for manufacturing the same. 
     To attain the above object, there is provided a semiconductor device, comprising: a semiconductor substrate; a lower electrode formed on the semiconductor substrate and overgrowing upward to form one electrode of a capacitor having a mushroom-shaped section; an insulation film formed so as to cover at least the lower electrode; and an upper electrode opposing the lower electrode and formed so as to cover at least the insulation film. 
     Further, there is provided a method for manufacturing a semiconductor device, comprising the steps of: forming a lower electrode having a mushroom-shaped section on a semiconductor substrate so as to overgrow upward by a selective growth method, thereby forming one electrode of a capacitor; covering at least a surface of the lower electrode with an insulation film; and forming other electrode of the capacitor so as to cover at least the insulation film and oppose the lower electrode. 
     The semiconductor device according to the present invention is a modification to a stack capacitor cell (STC), in which one of electrodes of a capacitor is formed like a mushroom or a plurality of mushrooms stacked on one another, on a semiconductor substrate. The mushroom-shaped electrode is overgrown by a selective growth method or a selective epitaxial growth (SEG) method. If an insulation film and the other electrode of the capacitor are formed so as to cover the mushroom-shaped electrode, the degree of integration of the capacitor is increased. It is thus unnecessary to thin the insulation film between the electrodes. Since self-alignment can be attained in manufacturing the capacitor as described later, the capacitor can be improved in reliability. Using an N +  epitaxial growth method, N +   ions need not be implanted into the semiconductor substrate; accordingly, manufacturing steps can be simplified. 
     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 in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     FIG. 1 is a cross-sectional view showing a conventional cell capacitor; 
     FIGS. 2A and 2B are views showing steps of manufacturing a cell capacitor according to an embodiment of the present invention; and 
     FIG. 3 is a view showing a constitution and a manufacturing step of a cell capacitor according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described with reference to FIG. 2. As illustrated in FIG. 2A, an N +   diffusion layer 23 is formed on a P-type silicon substrate 21 and an oxide film 22 is deposited thereon by the CVD method. The oxide film 22 is patterned to overgrow an N +   epitaxial layer 24 on the N +   diffusion layer 23 by the SEG method. As illustrated in FIG. 2B, the oxide film 22 is removed by etching, and the surfaces of the N +   diffusion layer 23 and N +  epitaxial layer 24 are thermally oxidized to form an oxide film 25 of 100 Å in thickness. A polysilicon 26 is deposited on the surface of the oxide film 25 and then patterned to form a plate electrode 26. A high-integrated cell capacitor, since the N +   epitaxial layer 24 serving as a lower electrode has a mushroom-shaped section, the surface of the lower electrode is enlarged per unit of area; accordingly, the opposing surfaces of the N +   epitaxial layer 24 and the plate electrode 26 are increased in area and a cell capacitor having a large capacity can thus be formed within a small area. 
     To further enhance the degree of integration of the cell capacitor, as shown in FIG. 3, another CVD oxide film (not shown) is deposited before forming the plate electrode 26 shown in FIG. 2 or after forming a thermal oxide film 25 1  (corresponding to the oxide film 25 shown in FIG. 2) and then patterned and an N +   epitaxial layer 24 2  (corresponding to the N +   epitaxial layer 24 shown in FIG. 2) is overgrown on another N +   epitaxial layer 241 (corresponding to the N +  epitaxial layer 24 shown in FIG. 2). (When the CVD oxide film is patterned, the oxide film 251 on the N +   epitaxial layer 241 is also patterned.) A thermal oxide film 252 (corresponding to the oxide film 25 shown in FIG. 2) is then formed on the layer 24 2  and a polysilicon plate electrode 26 is formed. A two-layered N +   epitaxial layer (24 1 , 24 2 ) can thus be formed. Consequently, the degree of integration of the cell capacitor shown in FIG. 3 is higher than that of the cell capacitor shown in FIG. 2. 
     Using the above process, a plurality of N +   epitaxial layers can be formed on one another in the vertical direction so that a higher degree of integration can be achieved. 
     In forming the second N +   epitaxial layer in the cell capacitor shown in FIG. 3, if a plurality of hole patterns is formed in the CVD oxide film and then the second N+epitaxial layer is overgrown, the degree of integration of the cell capacitor can be made much higher. In the lithographic technique, the relationship in position between the adjacent overgrow epitaxial layers depends upon the shape of a mask. Since the epitaxial layers can be controlled so as not to contact each other by accident, self-alignment can be attained and the insulation film 25 between the electrodes need not be thinned, as described in the Background of the Invention. The cell capacitor can thus be improved in reliability. Since the N +   epitaxial layer 24 is used, no N +   ions need not be implanted and therefore the manufacturing steps can be considerably simplified. 
     The present invention is not limited to the above embodiments but can be applied to various devices. In the above embodiments, the electrode can be overgrown like a mushroom in shape, using a semiconductor, by the selective epitaxial growth method. For example, the same electrode can be overgrown, using metal such as tungsten (W), by the selective growth method. 
     As described above, the cell capacitor according to the present invention is increased in degree of integration, improved in reliability, and simple to manufacture. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative devices, and illustrated examples shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.