Abstract:
A capacitor is provided, which makes it easy to increase the opposing area size between the lower and upper electrode in spite of miniaturization, and which ensures a desired capacitance value large enough for stable operation of a semiconductor memory device in spite of miniaturization. The capacitor is comprised of a lower electrode formed over an interlayer dielectric layer of a substrate, an upper electrode, and a dielectric located between the lower and upper electrodes. The lower electrode has a first electrode part and a second electrode part connected to each other. The first electrode part includes a plate-shaped bottom subpart and a sidewall subpart extending upward from the periphery of the bottom subpart. The bottom subpart and the sidewall subpart form an inner space. At least part of the second electrode part is located in the inner space so that a first gap is formed between the bottom subpart and the second electrode part and a second gap is formed between the sidewall subpart and the second electrode part. The upper electrode is opposed to the bottom subpart of the first electrode part of the lower electrode and to the second electrode part thereof in the first gap, and is opposed to the sidewall subpart of the first electrode part of the lower electrode and to the second electrode part thereof in the second gap.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device and more particularly, to a capacitor structure of a semiconductor device applicable to a storage capacitor of a semiconductor memory device, and a method of fabricating the same. 
     2. Description of the Prior Art 
     Storage capacitors are one of the main components of the memory cell of a semiconductor memory device. 
     In general, the output voltage from the memory cell is proportional to the capacitance value of the storage capacitor of the cell and thereof, the capacitor need to have a satisfactorily large capacitance value to ensure stable operation of the cell or to improve the operation reliability of the cell. On the other hand, the capacitor needs to be further miniaturized with the recent progressing miniaturization and integration of the cell. Thus, in recent years, there has been the strong need to develop new capacitor structures that make it possible to realize a satisfactorily large capacitance value even if the cell is further miniaturized. To meet this need, various capacitor structures have been developed and disclosed, one of which is shown in FIG.  1 . 
     FIG. 1 shows a part of the memory cells of a semiconductor memory device, in which prior-art storage capacitors  130  are formed on the surface of a semiconductor substrate  101  along with Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs)  131 . One of the MOSFETs  131  and a corresponding one of the capacitors  130  constitute the cell. 
     An isolation dielectric  102  is selectively formed on the surface of the substrate  101 , defining active areas (not shown) thereon. In each of the active areas, a gate insulator  120  is selectively formed on the surface of the substrate  101 , a gate electrode  103  is formed on the gate insulator  120 , and a pair of source/drain regions  121   a  and  121   b  are formed in the substrate  101  at each side of the gate electrode  103 . The pair of source/drain regions  121   a  and  121   b,  the gate insulator  120 , and the gate electrode  103  constitute the MOSFET  131  in each of the active areas. 
     A first interlayer dielectric layer  104  is formed to cover the isolation dielectric  102 , the gate electrodes  103 , and the pairs of exposed source/drain regions  121   a  and  121   b.  A second interlayer dielectric layer  105  is formed on the first interlayer dielectric layer  104 . The layer  105  is thicker than the first interlayer dielectric layer  104 , because a wiring layer  106  is formed in the layer  105 . The wiring layer  106  is electrically connected to the respective source/drain regions  121   a.  The wiring layer  106  does not appear in the cross-section of the device shown on FIG.  1  and thus, it is illustrated by broken lines. 
     A silicon nitride (SiN x ) layer  107  is formed on the second interlayer dielectric layer  105 . The layer  107  serves as an etch stop layer in the process of etching the layers overlying the layer  107 . 
     Contact holes  122  are formed to penetrate the SiN x  layer  107  and the second and first interlayer dielectric layers  105  and  104 . The holes  122  reach the corresponding source/drain regions  121   b,  exposing the same. The holes  122  are filled with conductive contact plugs  117 . The bottoms of the plugs  117  are contacted with and electrically connected to the corresponding source/drain regions  121   b.    
     Lower electrodes  116 , which serve a charge storage electrodes of the respective memory cells, are formed on the SiN x  layer  107  to be overlapped with the respective active areas. These electrodes  116  are separated from each other by small gaps. As seen from FIG. 1, each of the electrodes  116  is formed by a circular-plate-shaped bottom.  116   a  and a cylindrical sidewall  116   b  connected to the periphery of the bottom  116   a.  The sidewall  116   b  extend upward from the periphery of the bottom  116   a.  The center of the bottom  116   a  is contacted with and electrically connected to the top of a corresponding one of the contact plugs  117 . 
     A capacitor dielectric  114  is formed to cover all the lower electrodes  116 . The dielectric  114  is contacted with not only the exposed areas of the electrodes  116  but also those of the silicon nitride layer  107  exposed from the gaps between the electrodes  116 . The dielectric  114  is commonly used for all the electrodes  116 . 
     An upper electrode  115  is formed on the capacitor dielectric  114  to be opposite to all the lower electrodes  116 . The electrode  115  is commonly used for all the electrodes  116 . The electrode  115  extends along the dielectric  114 . 
     The upper electrode  115 , the capacitor dielectric  114 , and one of the lower electrodes  116  constitute each of the storage capacitors  130 . Each of the MOSFETs  131  and a corresponding one of the capacitors  130  constitute the memory cell. 
     Next, a method of fabricating the prior-art semiconductor memory device shown in FIG. 1 is explained below with reference to FIGS. 2A to  2 H. 
     First, as shown in FIG. 2A, the isolation dielectric  102 , which is made of silicon dioxide (SiO 2 ), is selectively formed on the surface of the semiconductor substrate  101 , thereby defining the active areas. Next, a SiO 2  layer (not shown) is formed on the whole surface of the substrate  101  and an impurity-doped polysilicon layer (not shown) is deposited on the SiO 2  layer thus formed. The SiO 2  and polysilicon layers are patterned to have a specific shape, thereby forming the gate insulators  120  and the gate electrodes  103  on the surface of the substrate  101  in the respective active areas. 
     Using the isolation dielectric  102  and the gate electrodes  102  as a mask, an impurity is selectively implanted into the substrate  101 , thereby forming the pairs of the source/drain regions  121   a  and  121   b  in the respective active areas. Each pair of source/drain regions  121   a  and  121   b  is located in self-alignment with respect to a corresponding one of the gate electrodes  103 . 
     Thus, the MOSFETs  131  are fabricated on the substrate  101 , each which is formed by one of the pairs of source/drain regions  121   a  and  121   b,  a corresponding one of the gate insulators  120 , and a corresponding one of the gate electrodes  103 . 
     Subsequently, the first interlayer dielectric layer  104 , which is made of SiO 2 , is formed to cover the whole surface of the substrate  101 . At this time, the isolation dielectric  102  and the MOSFETs  131  are covered with the layer  104 , Then, the second interlayer dielectric layer  105 , which is made of BoroPhospoSilicate Glass (BPSG), is formed on the first interlayer dielectric layer  104 . The layer  105  contains in its inside the wiring layer  106  electrically connected to the respective source/drain regions  121   a.  The SiN x  layer  107  is formed on the second interlayer dielectric layer  105  thus formed by a Chemical Vapor Deposition (CVD) method. 
     On the SiN x  layer  107  thus formed, a patterned resist film  109  having openings  109   a  is formed. The openings  109   a  are used for forming the contact holes  122  and located at positions right over the respective source/drain regions  121   b.  The state at this stage is shown in FIG.  2 A. 
     Following this step, using the patterned resist film  109  as a mask, the SiN x  layer  107  and the second and first interlayer dielectric layers  105  and  104  are etched selectively and successively. Thus, as shown in FIG. 2B, the contact holes  122  are formed to penetrate the layers  107 ,  105 , and  104 , exposing the underlying source/drain regions  121   b.  Thereafter, the resist film  109  is removed. 
     A first conductive layer (not shown) having a thickness large enough for filling the contact holes  122  is formed on the SiN x  layer  107 . As the first conductive layer, for example, an impurity-doped (i.e., n- or p-type) polysilicon layer is used. The first conductive layer thus formed is then etched back until the underlying SiN x  layer  107  is exposed, thereby leaving selectively the first conductive layer in the holes  122 . Thus, as shown in FIG. 2C, the conductive contact plugs  117  are formed in the holes  122  by the remaining first conductive layer. 
     Subsequently, as shown in FIG. 2D, the first interlayer dielectric layer  108 , which is made of SiO 2 , is formed on the SiN x  layer  107 . The layer  108  is contacted with the tops of the plugs  117 . On the layer  108  thus formed, a patterned resist film  112  is then formed. The film  112  has openings  112   a  that expose selectively the areas where the lower electrodes  116  are to be formed. Using the film  112  as a mask, the first interlayer dielectric layer  108  is selectively etched to form a spacer layer  108   a  on the layer  107 , as shown in FIG.  2 E. The spacer layer  108   a  has openings  128  that expose the underlying SiN x  layer  107  and the tops of the contact plugs  117 , which is used for defining the lower electrodes  116 . 
     Next, as shown in FIG. 2F, a second conductive layer  113 , which is made of an impurity-doped (i.e., n- or p-type) polysilicon, is formed on the SiN x  layer  107  to cover the spacer layer  108   a  and the exposed contact plugs  117 . Then, a second dielectric layer  111 , which is made of SiO 2 , is deposited on the layer  113  by a CVD method. The layer  111  has a thickness large enough for burying entirely the openings  128  of the layer  108   a.    
     Thereafter, the second dielectric layer  111  and the second conductive layer  113  are successively etched back until the top of the spacer layer  108   a  is exposed. Thus, as shown in FIG. 2G, the parts of the layer  113  existing outside the openings  128  are selectively removed, leaving the layer  113  only in the opening  128 . As a result, the lower electrodes  116  are formed on the SiN x  layer  107  by the remaining parts of the layer  113 . 
     To completely remove the spacer layer  108   a  and the remaining second dielectric layer  111 , these layers  108   a  and  111  are further etched while the SiN x  layer  108  is used as an etch stop layer. The state at this stage is shown in FIG.  2 H. 
     Following this, as shown in FIG. 1, the capacitor dielectric  114 , which has a layered structure of SiO 2  and SiN x  layers, is formed to cover the lower electrode  116 . The dielectric  114  is contacted with the exposed areas of the SiN x  layer  107  through the gaps between the electrodes  116 . 
     Finally, as shown in FIG. 1, the upper electrode  115 , which is made of an impurity-doped polysilicon, is formed on the capacitor dielectric  114 . The electrode  115  extends along the dielectric  114 . Thus, the prior-art semiconductor memory device with the storage capacitors  130  is fabricated. 
     As explained above, with the prior-art storage capacitor  131  shown in FIG. 1, the opposing area between the lower and upper electrodes  116  and  115  can be increased due to existence of the cylindrical sidewall  116   b  of the lower electrode  116 , raising its capacitance value. However, a desired capacitance value that copes with further miniaturization and integration of the storage cell is unable to be accomplished. 
     Additionally, another prior-art structure of the storage capacitor is disclosed in the Japanese Non-Examined Patent Publication No. 9-275194 published in October 1997. In this structure, the lower electrode has a “double cylindrical structure” that the lower electrode has concentric inner and outer cylindrical parts and therefore, an obtainable capacitance value is larger than that of the prior-art capacitor structure shown in FIG.  1 . Even in this structure, however, a desired capacitance value coping with further miniaturization and integration of the storage cells is unable to be accomplished. 
     In summary, with the above-explained prior-art capacitor structures, any satisfactory amount of electric charge becomes unable to be stored in the storage capacitor due to their unsatisfactory capacitance value according to the progressing miniaturization. As a result, there is a problem that stable operation of the memory cells cannot be ensured and consequently, the operation reliability of the semiconductor memory device degrades. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a capacitor that makes it easy to increase the opposing area size between the lower and upper electrodes in spite of miniaturization, and a method of fabricating the capacitor. 
     Another object of the present invention is to provide a capacitor that ensures a desired capacitance value large enough for stable operation of a semiconductor memory device in spite of miniaturization, and a method of fabricating the capacitor. 
     The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description. 
     According to a first aspect of the present invention, a capacitor is provided, which is comprised of: 
     (a) a substrate having an interlayer dielectric layer; 
     (b) a lower electrode formed over the interlayer dielectric layer; 
     the lower electrode having a first electrode part and a second electrode part connected to each other; 
     the first electrode part including a bottom subpart and a sidewall subpart extending upward from a periphery of the bottom subpart; 
     the bottom subpart and the sidewall subpart forming an inner space; 
     at least part of the second electrode part being located in the inner space so that a first gap is formed between the bottom subpart and the second electrode part and a second gap is formed between the sidewall subpart and the second electrode part; 
     (c) a capacitor dielectric formed to extend along exposed areas of the bottom subpart and the sidewall subpart of the first electrode part and along an exposed area of the second electrode part in said first and second gaps; and 
     (d) an upper electrode formed on the capacitor dielectric; 
     the upper electrode being opposed to the bottom subpart of the first electrode part of the lower electrode and to the second electrode part thereof in the first gap; 
     the upper electrode being opposed to the sidewall subpart of the first electrode part of the lower electrode and to the second electrode part thereof in the second gap. 
     With the capacitor according to the first aspect of the present invention, the lower electrode has the first electrode part and the second electrode part connected to each other. The first electrode part includes the bottom subpart and the sidewall subpart extending upward from the periphery of the bottom subpart. At least part of the second electrode part is located in the inner space formed by the bottom subpart and the sidewall subpart of the first electrode part, so that the first gap is formed between the bottom subpart of the lower electrode and the second electrode part and at the same time, the second gap is formed between the sidewall subpart of the lower electrode and the second electrode part. 
     Also, the upper electrode is opposed to the bottom subpart of the first electrode part of the lower electrode and to the second electrode part thereof through the capacitor dielectric in the first gap. Furthermore, the upper electrode is opposed to the sidewall subpart of the first electrode part of the lower electrode and to the second electrode part thereof through the capacitor dielectric in the second gap. 
     Thus, the opposed area between the upper and lower electrodes can be expanded easily. In other words, the opposing area size between the lower and upper electrodes can be easily increased in spite of miniaturization. This ensures a desired capacitance value large enough for stable operation of a semiconductor memory device. 
     In a preferred embodiment of the capacitor according to the first aspect of the invention, the bottom subpart of the first electrode part has an opening and the interlayer dielectric layer has an opening. The second electrode part is contacted with and electrically connected to a wiring layer formed between the interlayer dielectric layer and the substrate or a conductive region formed in the substrate through the openings. 
     In this embodiment, it is preferred that the bottom subpart of the first electrode part is plate-shaped and that the second electrode part includes a plate-shaped main subpart and a contracting subpart extending downward from the main subpart. The main subpart is entirely located in the inner space of the lower electrode. The contacting subpart is contacted with and electrically connected to the wiring layer or the conductive region through the openings. 
     In another preferred embodiment of the capacitor according to the first aspect of the invention, the second electrode part of the lower electrode includes a main subpart and a contacting subpart extending downward from the main subpart toward the substrate. The contacting subpart penetrates the bottom subpart of the first electrode part to be contacted therewith. 
     In this embodiment, it is preferred that the contacting subpart of the second electrode part is contacted with and electrically connected to a wiring layer formed between the interlayer dielectric layer and the substrate or a conductive region formed in the substrate through the openings. 
     In still another preferred embodiment of the capacitor according to the first aspect of the invention, the bottom subpart of the first electrode part is circular-plate-shaped and the sidewall subpart thereof is cylindrical. The main subpart of the second electrode is circular-plate-shaped. 
     According to a second aspect of the present invention, a method of fabricating a capacitor is provided, which fabricates the capacitor according to the first aspect of the invention. This method is comprised of steps of (a) to (j). 
     In the step (a), a substrate having an interlayer dielectric layer is prepared. 
     In the step (b), a first spacer layer is formed over the interlayer dielectric layer. The first spacer layer has an opening penetrating the same. 
     In the step (c), a first conductive layer is formed on the interlayer dielectric layer to cover the first spacer layer. 
     In the step (d), a second layer is formed on the first conductive layer. 
     In the step (e), the second spacer layer, the first conductive layer, and the interlayer dielectric layer are selectively removed, thereby forming a contact hole penetrating the second spacer layer, the first conductive layer, and the interlayer dielectric layer. 
     In the step (f), a second conductive layer is formed on the second spacer layer. The second conductive layer is contacted with the first conductive layer. 
     In the step (g), the second and first conductive layers and the second spacer layer are selectively removed until the first spacer layer is exposed, thereby leaving selectively the second and first conductive layers and the second spacer layer in the opening of the first spacer layer. 
     The first conductive layer left in the opening of the first spacer layer serves as a first electrode part of a lower electrode, where the first electrode part includes a bottom subpart and a sidewall subpart extending upward from a periphery of the bottom subpart. The bottom subpart and the sidewall subpart form an inner space. 
     The second conductive layer left in the opening of the first spacer layer serves as a second electrode part of the lower electrode. At least part of the second electrode part is located in the inner space so that a first gap is formed between the bottom subpart and the second electrode part and a second gap is formed between the sidewall subpart and the second electrode part. 
     In the step (h), the second spacer layer left in the opening of the first spacer layer and the first spacer layer are entirely removed. 
     In the step (i), a capacitor dielectric is formed on exposed areas of the first electrode part of the lower electrode and the second electrode part thereof. 
     In the step (j), an upper electrode is formed on the capacitor dielectric so as to fill the first and second gaps between the first electrode part of the lower electrode and the second electrode part thereof. 
     With the method of fabricating a capacitor according to the second aspect of the present invention, the first spacer layer is formed over the interlayer dielectric layer in the step (b), where the first spacer layer has an opening selectively exposing the interlayer dielectric layer. Thee first conductive layer is formed on the interlayer dielectric layer to cover the first spacer layer in the step (c). The second spacer layer is formed on the first conductive layer in the step (d). Thereafter, the second spacer layer, the first conductive layer, and the interlayer dielectric layer are selectively removed, thereby forming the contact hole penetrating the second spacer layer, the first conductive layer, and the interlayer dielectric layer in the step (e). 
     Also, in the step (f), the second conductive layer is formed on the second spacer layer to be contacted with the first conductive layer. In the step (g), the second and first conductive layers and the second spacer layer are selectively removed until the first spacer layer is exposed, thereby leaving selectively the second and first conductive layers and the second spacer layer in the opening of the first spacer layer. 
     The first conductive layer left in the opening of the first spacer layer serves as the first electrode part of the lower electrode, where the first electrode part includes a bottom subpart and a sidewall subpart extending upward from a periphery of the bottom subpart. The bottom subpart and the sidewall subpart form an inner space. 
     The second conductive layer left in the opening of the first spacer layer serves as a second electrode part of the lower electrode. At least part of the second electrode part is located in the inner space so that a first gap is formed between the bottom subpart and the second electrode part and a second gap is formed between the sidewall subpart and the second electrode part. 
     Moreover, in the step (h), the second spacer layer left in the opening of the first spacer layer and the first spacer layer are entirely removed. 
     Accordingly, the capacitor according to the first aspect of the present invention can be fabricated. Since no special process is required, this method can be easily carried out, in other words, the capacitor can be fabricated easily. 
     In a preferred embodiment of the method according to the second aspect of the invention, the second conductive layer left in opening of the first spacer layer, which serves as the second electrode part of the lower electrode, is contacted with and electrically connected to a wiring layer formed between the interlayer dielectric layer and the substrate or a conductive region formed in the substrate through the contact hole. 
     In another preferred embodiment of the method according to the second aspect of the invention, the bottom subpart of the first electrode part is circular-plate-shaped and the sidewall subpart thereof is cylindrical. The second electrode part is circular-plate-shaped. 
     In still another preferred embodiment of the method according to second aspect of the invention, the step (g) is carried out by an etch back process. 
     In a further preferred embodiment of the method according to second aspect of the invention, a step of planarizing a surface of the upper electrode is additionally carried out after the step (j). 
     In a still further preferred embodiment of the method according to second aspect of the invention, the first and second spacer layers are simultaneously removed by an etching process in the step (h). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings. 
     FIG. 1 is a schematic, partial cross-sectional view showing the configuration of a prior-art storage capacitor of a semiconductor memory device. 
     FIGS. 2A to  2 H are schematic, partial cross-sectional views showing the process steps of a method of fabricating the prior-art storage capacitor of FIG.  1 . 
     FIG. 3 is a schematic, partial cross-sectional view showing the configuration of a storage capacitor of a semiconductor memory device according to an embodiment of the present invention. 
     FIGS. 4A to  4 H are schematic, partial cross-sectional views showing the process steps of a method of fabricating the storage capacitor of FIG.  3 . 
     FIG. 5 is a schematic plan view showing the layout of the lower and upper electrodes of the storage capacitor according to the embodiment of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail below while referring to the drawings attached. 
     FIG. 3 shows a part of the memory cells of a semiconductor memory device, in which storage capacitors  30  according to an embodiment of the invention are formed on the surface of a semiconductor substrate  1  along with MOSFETs  31 . The memory device includes a lot of memory cells arranged regularly on the substrate  1 . Here, each of the cells is constituted by one of the MOSFETs  31  and a corresponding one of the capacitors  30 . 
     An isolation dielectric  2  is selectively formed on the surface of the substrate  1 , defining active areas (not shown) thereon. In each of the active areas, a gate insulator  20  is selectively formed on the surface of the substrate  1 , a gate electrode  3  is formed on the gate insulator  20 , and a pair of source/drain regions  21   a  and  21   b  are formed in the substrate  1  at each side of the gate electrode  3 . The pair of source/drain regions  21   a  and  21   b,  the gate insulator  20 , and the gate electrode  3  constitute the MOSFETs  31  in each of the active areas. Thus, the MOSFETs  31  are located in the respective active areas. 
     A first interlayer dielectric layer  4  is formed to cover the isolation dielectric  2 , the gate electrodes  3 , and the pairs of source/drain regions  21   a  and  21   b.  A second interlayer dielectric layer  5  is formed on the first interlayer dielectric layer  4 . The layer  5  is thicker than the first interlayer dielectric layer  4 , because a wiring layer  6  is formed in the layer  5 . The wiring layer  6  is electrically connected to the source/drain regions  21   a.  The wiring layer  6  does not appear in the cross-section of the device shown in FIG.  3  and thus, it is illustrated by broken lines. 
     A SiN x  layer  7  is formed on the second interlayer dielectric layer  5 . The layer  7  serves as an etch stop layer in the process of etching the layers overlying the layer  7 , which is explained later. 
     Lower electrodes  16 , which serve as charge storage electrodes of the respective memory cells, are formed to be arranged regularly on the SiN x  layer  7 . Each of the electrodes  16  is formed by a first electrode part  10  and a second electrode part  13 . 
     The first electrode part  10  of the lower electrode  16  has a circular-plate-shaped bottom subpart  10   a  formed on the SiN x  layer  7 , and a cylindrical sidewall subpart  10   b  extending upward from the periphery of the bottom subpart  10   a.  These two subparts  10   a  and  10   b  form a cylindrical inner space thereon. 
     The second electrode part  13  has a circular-plate-shaped main support  13   a  having a smaller diameter than that of the sidewall subpart  10   b,  and a column-shaped contacting subpart  13   b  extending downward from the center of the subpart  13   a.  The whole bottom subpart  13   a  is located in the inner space defined by the subparts  10   a  and  10   b  of the first electrode part  10 . 
     The bottom face of the main subpart  13   a  is approximately parallel to and is opposed to the top face of the bottom subpart  10   a.  The outer side face of the main subpart  13   a  is approximately parallel to and is opposed to the inner side face of the sidewall subpart  10   b.  The main support  13   a  is apart from the bottom subpart  10   a  by a small gap. The main subpart  13   a  is apart from the sidewall subpart  10   b  by another small gap. 
     The contacting subpart  13   b  is located in a corresponding one of contact holes  22 . Each of the holes  22  penetrates the bottom subpart  10   a  of the first electrode part  10 , the SiN x  layer  7 , and the second and first interlayer dielectric layers  5  and  4 , reaching a corresponding one of the source/drain regions  21   b.    
     The inner end of the bottom subpart  10   a  is contacted with the outer face of the contacting subpart  13   b  in the hole  22 , thereby electrically connecting the second part  13  to the first part  10 . The bottom end of the subpart  13   b  is contacted with a corresponding one of the source/drain regions  21   b , thereby electrically connecting the lower electrode  16  to the corresponding source/drain regions  21   b.    
     A film-shaped capacitor dielectric  14  is formed to extend along the exposed areas of the first and second electrode parts  10  and  13  of the lower electrode  16 . The dielectric  14  is contacted not only with the exposed areas of the electrode  16  but also with the exposed areas of the SiN x  layer  7  through the gaps between the adjoining lower electrodes  16 . Thus, the exposed areas of the first and second electrode parts  10  and  13  are entirely contacted with one side of the dielectric  14 . The dielectric  14  is commonly used for all the lower electrodes  16 , i.e., for all the cells. 
     A common upper electrode  15  having a flat surface is formed on another side of the capacitor dielectric  14 . The electrode  15  extends to the gaps between the first and second electrode parts  10  and  13  of all the lower electrodes  16 . In other words, the gaps are filled with the dielectric  14  and the electrode  15 . The electrode  15  is commonly used for all the lower electrodes  16  or all the cells. 
     The lower electrode  16 , the capacitor dielectric  4 , and the upper electrode  15  constitute the storage capacitor  30  of each memory cell. 
     Next, a method of fabricating the semiconductor memory device with the storage capacitors  30  according to the embodiment of FIG. 3 is explained below with reference to FIGS. 4A to  4 H. 
     First, as shown in FIG. 4A, the isolation dielectric  2 , which is made of SiO 2 , is selectively formed on the surface of the silicon substrate  1 , thereby defining the active areas. Next, a SiO 2  layer (not shown) is formed on the whole surface of the substrate  1  and an impurity-doped (i.e., n- or p-type) polysilicon layer (not shown) is deposited on the SiO 2  layer thus formed. The SiO 2  and polysilicon layer are then patterned to have a specific shape, thereby forming the gate insulators  20  and the gate electrodes  3  on the surface of the substrate  1  in the respective active areas. 
     Using the isolation dielectric  2  and the gate electrodes  3  as a mask, n- or p-type impurity is selectively implanted into the substrate  1 , thereby forming the pairs of the source/drain regions  21   a  and  21   b  in the respective active areas. The pairs of source/drain regions  21   a  and  21   b  are formed in self-alignment with respect to the corresponding gate electrode  3 . 
     Thus, the MOSFETs  31  are formed on the substrate  1 , each of which is formed by the pair of source/drain regions  21   a  and  21   b,  the gate insulator  20 , and the gate electrode  3 . 
     Subsequently, the first interlayer dielectric layer  4 , which is made of SiO 2 , is formed to cover the whole substrate  1 . The isolation dielectric  2  and the MOSFETs  31  are covered with the layer  4 . The second interlayer dielectric layer  5 , which is made of BPSG, is formed on the first interlayer dielectric layer  4 . The layer  5  contains the wiring layer  6  electrically connected to the respective source/drain regions  21   a  in its inside. The wiring layer  6  may be made of tungsten silicide (WSi 2 ). 
     The SiN x  layer  7  is formed on the second interlayer dielectric layer  5  by a CVD method. A first dielectric layer  8 , which is made of SiO 2 , is formed on the SiN x  layer  7 . 
     A patterned resist film  9  having openings  9   a  is then formed on the layer  8 , exposing selectively the areas where the lower electrodes  16  are formed. The state at this stage is shown in FIG.  4 A. 
     Following this step, using the patterned resist film  9  as a mask, the first dielectric layer  8  made of SiO 2  is selectively etched, thereby forming a first spacer layer  8   a  on the SiN x  layer  7 , as shown in FIG.  4 B. The first spacer layer  8   a , which is made of SiO 2 , has circular openings  28  at the locations corresponding to the lower electrodes  16 . The resist film  9  is then removed. The state at this stage is shown in FIG.  4 B. 
     Thereafter, as shown in FIG. 4C, an impurity-doped polysilicon layer is deposited on the SiN x  layer  7  by a CVD method, thereby forming a first conductive layer  40 . The layer  40  is contacted with the first spacer layer  8   a  and the exposed areas of the SiN x  layer  7 . For example, the layer  40  has a thickness of 30 nm. 
     Subsequently, a SiO 2  layer is deposited on the first conductive layer  40  by a CVD method, thereby forming a second spacer layer  11 , as shown in FIG.  4 C. The second spacer layer  11  extends along the first conductive layer  40 . For example, the layer  11  has a thickness of 80 nm. The state at this stage is shown in FIG.  4 C. 
     As shown in FIG. 4D, a patterned resist film  12  is then formed on the second spacer layer  11 . The film  12  has openings  12   a  located at the corresponding positions to the respective source/drain regions  21   b.  The state at this stage is shown in FIG.  4 D. 
     Using the patterned resist film  12  as a mask, the second spacer layer  11 , the first conductive layer  40 , the SiN x  layer  7 , and the second and first interlayer dielectric layers  5  and  4  are etched selectively and successively. Thus, the contact holes  22  are formed to penetrate the layers  11 ,  40 ,  7 ,  5 , and  4 , exposing the underlying source/drain regions  21   b,  as shown in FIG.  4 E. The resist film  12  is then removed. The state at this stage is shown in FIG.  4 E. 
     Following this, a thick, impurity-doped polysilicon layer is deposited on the second spacer layer  11  by a CVD method, thereby forming a second conductive layer  43 , as shown in FIG.  4 F. The layer  43  has a thickness large enough for filling the contact hoes  22  and the inside of the openings  28   a.  For example, the layer  43  has a thickness of 500 nm. The state at this stage is shown in FIG.  4 F. 
     Next, the second conductive layer  43 , the second spacer layer  11 , and the first conductive layer  40  are successively etched back until the top of the first spacer layer  8   a  is exposed. Thus, the parts of the layers  40  and  43  existing outside the openings  28  are selectively removed, as shown in FIG.  4 G. Thus, the first and second conductive layers  40  and  43  are selectively left only in the openings  28 . The first conductive layer  40  left in the openings  28  constitute the first electrode parts  10  of the lower electrodes  16 , which are located on the SiN x  layer  7 . The second conductive layer  43  left in the openings  28  constitute the second electrode parts  13  of the lower electrodes  16 , which are slightly raised from the SiN x  layer  7 . The state at this stage is shown in FIG.  4 G. 
     Using the SiN x  layer  7  as an etch stop layer, the remaining first and second spacer layers  8   a  and  11  are completely removed. Thus, the gaps  24  are formed between two adjoining ones of the first electrode parts  10  and at the same time, the gaps  25  are respectively formed between the first and second electrode parts  10  and  13 , as shown in FIG.  4 H. The gaps  24  separate the first electrode parts  10  from each other. The gaps  25  expose the inner faces of the first and second electrode parts  10  and  13 . The state at this stage is shown in FIG.  4 H. 
     Furthermore, as shown in FIG. 3, the capacitor dielectric  14  is formed to cover the exposed areas of the first and second electrode subparts  10  and  13  of the lower electrodes  16  and those of the SiN x  layer  7 . As the layer  7 , for example, a layered structure of a SiO 2  layer and a SiN x  layer is used. 
     Finally, an impurity-doped polysilicon layer, which has a thickness of, for example, 200 nm, is deposited on the capacitor dielectric  14  by a CVD method. The top of the polysilicon layer thus deposited is planarized according to the necessity. Thus, the common upper electrode  15  is formed, as shown in FIG.  3 . 
     Through the above-explained process steps, the semiconductor memory device equipped with the storage capacitors  30  and the MOSFETs  31  in the memory cells are fabricated. 
     With the storage capacitors  30  according to the embodiment of the present invention, as shown in FIG. 3, the lower electrode  16  is formed by the first and second electrode parts  10  and  13 . The first electrode part  10  include the circular-plate-shaped bottom subpart  10   a  and the cylindrical sidewall subpart  10   b  extending upward from the periphery of the subpart  10   a.  The second electrode part  13  is located in the inner space formed by the first electrode part  10 . The bottom and sidewall subparts  10   a  and  10   b  of the first electrode part  10  are separated from the second electrode part  13  by the gap  25 . The capacitor dielectric  14  extends along the opposing surfaces of the first and second electrode parts  10  and  13  of the lower electrode  16  and the upper electrode  15 . The upper electrode  15  is formed to fill the gaps  24  and  25 . 
     As a result, the effective surface areas between the lower and upper electrodes  10  and  15  can be increased easily and therefore, a satisfactorily large capacitance value can be realized even if the capacitor  30  is miniaturized. Thus, stable operation of the semiconductor memory device can be ensured, which enhances the operation reliability of the device. 
     Additionally, with the above-explained method of fabricating the capacitors  30 , the gaps  24  and  25  are formed by the use of the first and second spacer layer  8   a  and  11 . Thus, the storage capacitors  30  can be easily fabricated. 
     As seen from the above explanation, the first electrode part  10  of the lower electrode  16  has an approximately cylindrical shape and the second electrode part  13  has an approximately circular plate shape in the above embodiment. However, it is needles to say that the invention is not limited to these shapes. Each of the first and second electrode part  10  and  13  may have any other shape than those above if it satisfies the limitation defined in the claims. 
     While the preferred form of the present invention has been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the present invention, therefore, is to be determined solely by the following claims.