Abstract:
A capacitor of a semiconductor memory of the present invention includes: a lower electrode which covers the surface of a storage node hole from the bottom to at least one of the sidewalls up to a level lower than the top surface of a second interlayer insulating film; a capacitive insulating film which covers the lower electrode; and an upper electrode which covers the capacitive insulating film.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2004-312959 filed in Japan on Oct. 27, 2004, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a semiconductor memory and a method for manufacturing the same. In particular, it relates to a semiconductor memory such as a DRAM and a method for manufacturing the same. 
     (b) Description of Related Art 
     So far, concave capacitive electrodes have been used for DRAMs of a capacitor-under-bitline (CUB) structure having charge storing capacitive elements under bit lines. With use of the concave capacitive electrode in a DRAM, the surface of an interlayer insulating film formed on the DRAM becomes flat. Therefore, the concave capacitive electrode has been paid attention for its usefulness for device miniaturization (e.g., see Japanese Unexamined Patent Publication No. 2002-141424). 
     Hereinafter, referring to  FIGS. 8A and 8B , an explanation is given of a configuration of a conventional embedded semiconductor DRAM including a concave capacitive electrode.  FIGS. 8A and 8B  are a sectional view and a plan view illustrating a configuration the conventional embedded semiconductor DRAM, respectively. As shown in  FIG. 8A , the conventional DRAM includes: a silicon wafer  120 ; isolation trenches (STIs)  101  formed to surround part of the silicon wafer  120  where a transistor will be formed; a gate insulating film  102   a  and a gate electrode  102   b  formed on the silicon wafer  120 ; a source/drain diffusion layer  103  formed in part of the silicon wafer  120  located at each side of the gate electrode; a first interlayer insulating film  104  formed on the silicon wafer  120 , a first bit line contact  105  and a capacitor contact  106  which penetrate the first interlayer insulating film  104  to reach the source/drain diffusion layer  103 , respectively; an insulating film  107  formed on the first interlayer insulating film  104 ; a second interlayer insulating film  108  formed on the insulating film  107 ; a lower electrode  110  covering the bottom and the sidewalls of a storage node hole  116  which penetrates the second interlayer insulating film  108  to reach the top surface of the capacitor contact  106 ; a capacitive insulating film  111  covering the lower electrode  110  and the second interlayer insulating film  108  outside the storage node hole  116 ; an upper electrode  112  covering the capacitive insulating film  111 ; a third interlayer insulating film  113  formed over the upper electrode  112  to bury the storage node hole  116 ; a second bit line contact  114  which penetrates the third interlayer insulating film  113  and the second interlayer insulating film  108  to reach the first bit line contact  105 ; and a first wiring layer  115  formed on the third interlayer insulating film  113  to be connected to the second bit line contact  114 . 
     In the step of manufacturing the semiconductor device shown in  FIG. 8A , the upper electrode  112  is subjected to etching using a mask (not shown) formed thereon, thereby forming an opening  117  for forming the second bit line contact  114 . The opening  117  is formed above the second interlayer insulating film  108  where the storage node hole  109  is not formed. Then, a contact hole (not shown) is formed to penetrate the third interlayer insulating film  113  and the second interlayer insulating film  108  below the opening  117  and then filled with a conductive film (not shown), thereby forming the second bit line contact  114 . 
     In a plan view of the conventional embedded semiconductor DRAM, as shown in  FIG. 8B , a margin c is provided between the storage node hole  116  having a short side length a and a long side length b (planar shape of a capacitor) and the opening  117 . The margin c is provided to absorb misalignment of the mask used to form the opening  117 . 
     However, the margin c is narrowed as the device is further miniaturized. Therefore, if the mask for forming the opening  117  is misaligned and etching is carried out with the misaligned mask, the capacitive insulating film  111  and the lower electrode  112  formed in the storage node hole  116  are likely to be etched away. As shown in  FIG. 8A , at the top end part H 3  of the storage node hole  116  (part of the sidewalls of the hole  116  closer to the top surface of the second interlayer insulating film  108 ), the upper electrode  112  is removed, whereby the capacitive insulating film  111  and the lower electrode  110  are exposed or partially etched away. As a result, the lower electrode  110  decreases in area, thereby reducing the amount of storable capacitance. Further, the capacitive insulating film which contributes to the capacitance is damaged by the etching to cause leakage current. 
     In the DRAM which employs the concave capacitive electrode, capacitance cannot be stored in any other parts than the capacitive insulating film formed in the storage node hole  116 . Therefore, if the electrode area decreases as a result of the miniaturization, the amount of storable capacitance also decreases. This brings about a problem in that the required amount of capacitance for memory operation cannot be maintained. 
     SUMMARY OF THE INVENTION 
     Under these circumstances, an object of the present invention is to provide an embedded semiconductor DRAM capable of realizing device miniaturization, suppressing leakage current and storing a large amount of capacitance, and a method for manufacturing the same. 
     A first semiconductor memory according to the present invention comprises a capacitor which covers the surface of a trench formed in part of an insulating film, wherein the capacitor includes: a lower electrode which covers the surface of the trench from the bottom to at least one of the sidewalls up to a level lower than the top surface of the insulating film; a capacitive insulating film which covers the lower electrode and part of said at least one of the sidewalls of the trench higher than part of said at least one of the sidewalls of the trench covered with the lower electrode; and an upper electrode which is formed on the capacitive insulating film and covers the part of said at least one of the sidewalls of the trench higher than the part covered with the lower electrode with the capacitive insulating film lying in between. 
     The lower electrode, capacitive insulating film and upper electrode may be formed at least one of the sidewalls of the trench. 
     In such a semiconductor memory, the lower electrode is positioned below the top surface of the insulating film. Accordingly, the upper electrode is formed along the trench surface at the top end part of the trench (part of the trench sidewalls closer to the top surface of the insulating film). As a result, part of the upper electrode at the top end part of the trench is given with a large thickness (thickness in the vertical direction). Therefore, even if an opening, which is supposed to be formed in a region other than the capacitor region, is misaligned to overlap the capacitor and the upper electrode is over-etched in the vertical direction, part of the capacitive insulating film which stores the capacitance of the capacitor will not be exposed. The part of the capacitive insulating film which stores the capacitance of the capacitor mentioned above is part of the capacitive insulating film sandwiched between the upper and lower electrodes. For example, the lower electrode is not formed at the top end part of the trench on which part of the capacitive insulating film is directly formed. Therefore, even if this part of the capacitive insulating film is exposed by over-etching for forming the opening, adverse effect such as the occurrence of leakage current is not caused. Thus, in the first semiconductor memory of the present invention, the capacitive insulating film which contributes to the capacitance of the capacitor will not be damaged even if the opening is formed to overlap the capacitor. This eliminates the need of providing an alignment margin between the opening and the trench, though it has been necessary in conventional techniques. As a result, the trench is enlarged to be in contact with the opening at the top end part thereof, while the memory cell size is not changed. Thus, a larger amount of capacitance is stored in the capacitor while device miniaturization is achieved. 
     Specifically, the above-mentioned opening is an opening for forming a bit line contact in a semiconductor memory of a CUB structure. In such a semiconductor memory, the upper electrode is formed over the capacitive insulating film in the trench and the insulating film outside the trench, an opening is formed in at least part of the region of the upper electrode formed on the top surface of the insulating film to reach the insulating film, and the semiconductor memory further comprises a bit line contact which penetrates part of the insulating film below the opening. 
     If the opening is misaligned from the desired position, the opening is formed above the top surface of the insulating film to overlap the capacitor and the bottom of part of the opening overlapping the capacitor is positioned higher than the upper end of the lower electrode. 
     If the semiconductor memory is a DRAM, it includes: a semiconductor substrate; a gate insulating film formed on the semiconductor substrate; a gate electrode formed on the gate insulating film; an impurity diffusion layer formed in part of the semiconductor substrate located to each side of the gate electrode; an interlayer insulating film formed on the semiconductor substrate; and a capacitor contact which penetrates the interlayer insulating film to reach the impurity diffusion layer, wherein the insulating film is formed above the interlayer insulating film and the bottom of the trench reaches the top of the capacitor contact. 
     The upper end of the lower electrode is positioned lower than the top surface of the insulating film by a distance which is 0.3 or more times larger than the thickness of the upper electrode. With this configuration, part of the capacitive insulating film which stores the capacitance is surely prevented from being exposed by over-etching for forming the opening. 
     A second semiconductor memory of the present invention comprises a capacitor which covers the surface of a trench formed in part of an insulating film, wherein the insulating film includes a first insulating film and a second insulating film which is formed on the first insulating film and protrudes toward the inside of the trench beyond the first insulating film on at least one of the sidewalls of the trench and the capacitor includes a lower electrode which covers part of the surface of the trench where the first insulating film is exposed, a capacitive insulating film which covers the lower electrode and part of the surface of the trench where the second insulating film is exposed and an upper electrode which covers the capacitive insulating film. 
     In such a semiconductor memory, part of the capacitive insulating film which covers the second insulating film forming the trench surface does not store the capacitance of the capacitor. Therefore, even if an opening, which is supposed to be formed in a region other than the capacitor region, is misaligned to overlap the capacitor and the upper electrode is over-etched in the vertical direction, part of the capacitive insulating film which stores the capacitance of the capacitor will not be exposed. Thus, in the second semiconductor memory of the present invention, the capacitive insulating film which contributes to the capacitance will not be damaged even if the opening is formed to overlap the capacitor. This eliminates the need of providing an alignment margin between the opening and the trench, though it has been necessary in conventional techniques. As a result, the trench is enlarged to be in contact with the opening at the top end part thereof, while the memory cell size is not changed. Thus, a larger amount of capacitance is stored in the capacitor while device miniaturization is achieved. 
     The upper end of the lower electrode is preferably thinner than the other part of the lower electrode. By so doing, the upper electrode is reduced in level difference which is generated at part thereof covering the boundary between the first and second insulating films forming the trench sidewalls. As a result, even if the opening is misaligned to overlap the capacitor, the capacitive insulating film formed on the first insulating film forming the trench sidewalls is less likely to be removed. 
     The second insulating film preferably protrudes toward the inside of the trench beyond the first insulating film by a length larger than the difference in thickness between the lower electrode and the capacitive insulating film. By so doing, part of the capacitive insulating film which stores the capacitance is surely prevented from being removed. 
     Specifically, the above-mentioned opening is an opening for forming a bit line contact in a semiconductor memory of a CUB structure. In such a semiconductor memory, the upper electrode is formed over the capacitive insulating film in the trench and the insulating film outside the trench, an opening is formed in at least part of the region of the upper electrode formed on the top surface of the insulating film to reach the insulating film, and the semiconductor memory further comprises a bit line contact which penetrates part of the insulating film below the opening. 
     If the semiconductor memory is a DRAM, it includes: a semiconductor substrate; a gate insulating film formed on the semiconductor substrate; a gate electrode formed on the gate insulating film; an impurity diffusion layer formed in part of the semiconductor substrate located to each side of the gate electrode; an interlayer insulating film formed on the semiconductor substrate; and a capacitor contact which penetrates the interlayer insulating film to reach the impurity diffusion layer, wherein the insulating film is formed above the interlayer insulating film and the bottom of the trench reaches the top of the capacitor contact. 
     The first insulating film may be made of phospho-silicate glass (PSG) and the second insulating film may be made of non-doped silicate glass (NSG). The NSG is lower in wet-etch rate than the PSG. Therefore, if the trench is formed by wet etching, the second insulating film remains protruding toward the inside of the trench beyond the first insulating film. 
     The second insulating film may be made of the same material as the first insulating film and doped with boron. The wet-etch rate decreases with an increase in boron concentration. Therefore, if the trench is formed by wet etching, the second insulating film remains protruding toward the inside of the trench beyond the first insulating film. 
     The present invention further provides a first method for manufacturing a semiconductor memory comprising a capacitor including a lower electrode, an upper electrode and a capacitive insulating film interposed between the lower electrode and the upper electrode. The method comprises the steps of: (a) forming a trench in part of an insulating film; (b) forming a conductive film which covers the bottom surface and the sidewalls of the trench, forming a mask which covers part of the conductive film located on a lower part of the trench and removing part of the conductive film uncovered with the mask by anisotropic etching to form the lower electrode which covers the surface of the trench from the bottom to the sidewalls up to a level lower than the top surface of the insulating film; (c) forming the capacitive insulating film on the lower electrode; and (d) forming the upper electrode on the capacitive insulating film. 
     According to the manufacturing method, the lower electrode is positioned below the top surface of the insulating film. Accordingly, the upper electrode is formed along the trench surface at the top end part of the trench. As a result, part of the upper electrode at the top end part of the trench is given with a large thickness (thickness in the vertical direction). Therefore, even if an opening, which is supposed to be formed in a region other than the capacitor region after the step (d), is misaligned to overlap the capacitor and the upper electrode is over-etched in the vertical direction, part of the capacitive insulating film which stores the capacitance of the capacitor will not be exposed. Thus, the capacitive insulating film which contributes to the capacitance of the capacitor will not be damaged even if the opening is formed to overlap the capacitor. This eliminates the need of providing an alignment margin between the opening and the trench, though it has been necessary in conventional techniques. As a result, the trench is enlarged to be in contact with the opening at the top end part thereof, while the memory cell size is not changed. Thus, a larger amount of capacitance is stored in the capacitor while device miniaturization is achieved. 
     Specifically, the above-mentioned opening is an opening for forming a bit line contact in a semiconductor memory of a CUB structure. According to this manufacturing method, in the step (d), the upper electrode is formed over the capacitive insulating film in the trench and the insulating film outside the trench. The method further comprises the steps of: (e) forming on the upper electrode a mask having an opening corresponding to part of the region of the upper electrode formed on the top surface of the insulating film and forming an opening in the upper electrode by etching; and (f) forming a bit line contact which penetrates part of the insulating film below the opening. 
     The present invention further provides a second method for manufacturing a semiconductor memory comprising a capacitor including a lower electrode, an upper electrode and a capacitive insulating film interposed between the lower electrode and the upper electrode. The method comprises the steps of: (a) forming a first insulating film and forming a second insulating film having a lower wet-etch rate than the first insulating film on the first insulating film; (b) wet-etching the first insulating film and the second insulating film to form a trench which penetrates the second insulating film and the first insulating film and in which the second insulating film protrudes toward the inside of the trench beyond the first insulating film; (c) forming the lower electrode which covers part of the surface of the trench where the first insulating film is exposed; (d) forming the capacitive insulating film on the lower electrode; and (e) forming the upper electrode on the capacitive insulating film. 
     According to the manufacturing method, part of the capacitive insulating film which covers the second insulating film forming the trench surface does not store the capacitance of the capacitor. Therefore, even if an opening, which is supposed to be formed in a region other than the capacitor region after the step (e), is misaligned to overlap the capacitor and the upper electrode is over-etched in the vertical direction, part of the capacitive insulating film which stores the capacitance of the capacitor will not be exposed. Thus, the capacitive insulating film which contributes to the capacitance will not be damaged even if the opening is formed to overlap the capacitor. This eliminates the need of providing an alignment margin between the opening and the trench, through it has been necessary in conventional techniques. As a result, the trench is enlarged to be in contact with the opening at the top end part thereof, while the memory cell size is not changed. Thus, a larger capacitance is stored in the capacitor while device miniaturization is achieved. 
     Specifically, the above-mentioned opening is an opening for forming a bit line contact in a semiconductor memory of a CUB structure. According to this manufacturing method, in the step (e), the upper electrode is formed over the capacitive insulating film in the trench and the second insulating film outside the trench. The method further comprises the steps of: (f) forming on the upper electrode a mask having an opening corresponding to part of the region of the upper electrode formed on the top surface of the second insulating film and forming an opening in the upper electrode by etching; and (g) forming a bit line contact which penetrates the second insulating film and the first insulating film through the opening. 
     In the step (c), a conductive film is formed over the entire surface of the trench, a mask is formed to fill part of the trench below the boundary between the first insulating film and the second insulating film and part of the conductive film uncovered with the mask is removed by etching to form the lower electrode. By so doing, the upper end of the lower electrode becomes thinner than the other part of the lower electrode. In this case, the upper electrode is reduced in level difference which is generated at a part thereof covering the boundary between the first and second insulating films forming the trench sidewalls. As a result, even if the opening is misaligned to overlap the capacitor, the capacitive insulating film formed on the first insulating film forming the trench sidewalls is less likely to be removed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A to 1C  are sectional views illustrating the steps of manufacturing a DRAM of a CUB structure according to Embodiment 1 of the present invention. 
         FIGS. 2A to 2C  are sectional views illustrating the steps of manufacturing the DRAM of a CUB structure according to Embodiment 1 of the present invention. 
         FIG. 3  is a sectional view illustrating the step of manufacturing the DRAM of a CUB structure according to Embodiment 1 of the present invention. 
         FIG. 4  is a plan view illustrating a configuration of a semiconductor memory according to Embodiment 1 of the present invention. 
         FIGS. 5A to 5C  are sectional views illustrating a DRAM of a CUB structure according to Embodiment 2 of the present invention. 
         FIGS. 6A to 6C  are sectional views illustrating the DRAM of a CUB structure according to Embodiment 2 of the present invention. 
         FIG. 7  is a sectional view illustrating the step of manufacturing the DRAM of a CUB structure according to Embodiment 2 of the present invention. 
         FIGS. 8A and 8B  are a sectional view and a plan view illustrating a configuration of a conventional embedded semiconductor DRAM, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiment 1 
     Hereinafter, an explanation is given of a method for manufacturing a semiconductor memory according to Embodiment 1 of the present invention with reference to the drawings.  FIGS. 1A to 1C ,  2 A to  2 C and  3  are sectional views illustrating the steps of manufacturing a DRAM of a CUB structure according to Embodiment 1 of the present invention. 
     According to the method of the present embodiment, in the step shown in  FIG. 1A , an STI  1  is formed first to surround part of a silicon substrate  20  where a transistor will be formed. Then, a gate insulating film  2   a  of 6 nm in thickness is formed on the silicon substrate  20 . Further, polysilicon (not shown) is deposited on the gate insulating film  2   a  and then dry-etched using a mask (not shown) formed on the polysilicon, thereby forming a gate electrode  2   b  of 150 nm in thickness which functions as a word line. Then, ion implantation is carried out using the gate electrode  2   b  as a mask to form an impurity diffusion layer  3  of 100 nm in thickness in part of the silicon substrate  20  located at each side of the gate electrode  2   b . Then, a first interlayer insulating film  4  made of BPSG (boron phospho-silicate glass) is deposited on the silicon substrate  20  to cover the gate electrode  2   b . The first interlayer insulating film  4  is then planarized by CMP so that it has a uniform thickness of about 500 nm. 
     Then, in the step shown in  FIG. 1B , a mask (not shown) is formed on the first interlayer insulating film  4  and etching is carried out to form a bit line contact hole (not shown) and a capacitor contact hole (not shown) which penetrate the first interlayer insulating film  4  to reach a drain region and a source region in the impurity diffusion layer  3 , respectively. Then, the surfaces of the bit line contact hole and the capacitor contact hole are covered with barrier metals  5   a  and  6   a  made of TiN of 10 nm in thickness, respectively, and the contact holes are filled with conductive films  5   b  and  6   b  made of tungsten, respectively. Thus, a bit line contact  5  and a capacitor contact  6  are formed. Then, an insulating film  7  such as a silicon nitride film of 50 nm in thickness is deposited on the first interlayer insulating film  4 . 
     Further, in the step shown in  FIG. 1C , a second interlayer insulating film  8  of 800 nm in thickness made of BPSG is formed on the insulating film  7 . Then, a resist mask (not shown) having an opening corresponding to a capacitor region is formed on the second interlayer insulating film  8  and dry etching is carried out to remove the second interlayer insulating film  8  and the insulating film  7  in the capacitor region. Thus, a storage node hole  9  is formed. When viewed in plan, the top end part H 1  of the storage node hole  9  (part of the sidewalls of the storage node hole  9  closer to the top surface of the interlayer insulating film  8 ) is in the form of a rectangle having a long side length of 0.5 μm and a short side length of 0.2 μm. The storage node hole  9  is formed by two-step dry etching. More specifically, first dry etching is carried out using the insulating film  7  as an etch-stop to remove the second interlayer insulating film  8  until the surface of the insulating film  7  is exposed, and then second dry etching is carried out to selectively remove the exposed insulating film  7 . 
     Then, in the step shown in  FIG. 2A , a TiN film of 30 nm in thickness (not shown) is formed over the bottom surface and the sidewalls of the storage node hole  9  and the surface of the second interlayer insulating film  8  outside the storage node hole  9 . Then, a resist (not shown) is applied to the substrate and light exposure is carried out over the entire surface of the substrate to leave the resist (not shown) only in the storage node hole  9 . Then, anisotropic etching is carried out to selectively remove the TiN film, thereby forming a lower electrode  10  made of TiN of 30 nm in thickness on the bottom surface and the sidewalls of the storage node hole  9 . At this time, the anisotropic etching is carried out such that the TiN film is removed by about 80 nm in thickness, thereby positioning the upper end of the lower electrode  10  lower than the top surface of the second interlayer insulating film  8  by about 50 nm. That is, the lower electrode  10  is formed on the sidewalls of the storage node hole  9  at a deeper position than the top surface of the second interlayer insulating film  8  by about 50 nm or more. 
     Then, in the step shown in  FIG. 2B , a capacitive insulating film  11  made of Ta 2 O 5  of 20 nm in thickness is deposited over the lower electrode  10  and the second interlayer insulating film  8 . Since the lower electrode  10  is positioned lower than the top surface of the second interlayer insulating film  8 , part of the sidewalls of the storage node hole  9  which is not in contact with the lower electrode  10 , i.e., the surface of the top end part H 1  of the storage node hole  9 , comes into direct contact with the capacitive insulating film  11 . Thereafter, a TiN film of about 50 nm in thickness (not shown) is formed on the capacitive insulating film  11  to form an upper electrode  12 . 
     Then, in the step shown in  FIG. 2C , a mask (not shown) having an opening in a bit line contact region is formed on the upper electrode  12  and etching is carried out to form an opening  17 . At this time, the upper electrode  12  is over-etched by about 30%, i.e., about 15 nm in terms of the TiN film, so as not to leave the upper electrode  12  and the capacitive insulating film  11  in a bit line contact region the memory cell region and the other regions than the memory cell region.  FIG. 2C  shows that the opening  17  is misaligned from the desired position and the left end of the opening  17  reaches the capacitor in the storage node hole  9 . The misalignment of the opening  17  is derived from the mask (not shown) for forming the opening  17  which is misaligned from the desired position. 
     Then, in the step shown in  FIG. 3 , a third interlayer insulating film  13  of 600 nm in thickness made of BPSG is deposited on the upper electrode  12  and planarized by CMP such that the thickness thereof on the upper electrode  12  outside the storage node hole  9  is reduced to 200 nm. Then, a bit line contact  14  is formed through the third interlayer insulating film  13  and the second interlayer insulating film  8  below the opening  17  to reach the bit line contact  5 . Then, a first wiring layer  15  is formed on the third interlayer insulating film  13  to be in contact with the bit line contact  14 . Through the above steps, a capacitor of a CUB structure is obtained. 
     Hereinafter, the effect of the manufacturing method of the present embodiment will be described in comparison with that of the conventional method. In the conventional method, a margin c shown in  FIG. 8B  is narrowed as the device is further miniaturized. Therefore, as shown in  FIG. 8A , an opening  117  is misaligned and an upper electrode  112  at the top end part H 3  of a storage node hole  116  is removed to expose a capacitive insulating film  111 . 
     On the other hand, in the present embodiment, the lower electrode  10  is positioned lower than the top surface of the second interlayer insulating film  8  as shown in  FIG. 2C . Accordingly, at the top end part H 1  of the storage node hole  9 , the upper electrode  12  is formed along the sidewalls of the storage node hole  9 . As a result, part of the upper electrode  12  at the top end part H 1  of the storage node hole  9  is given with a large thickness as compared with that obtained by the conventional technique. Therefore, in the step of forming the opening  17 , even if the opening  17  is misaligned to overlap the capacitor and the upper electrode  12  is over-etched in the vertical direction, part of the capacitive insulating film  11  which stores the capacitance of the capacitor will not be exposed. The part of the capacitive insulating film  11  which stores the capacitance of the capacitor mentioned herein is part of the capacitive insulating film  11  sandwiched between the upper and lower electrodes  12  and  10 . For example, the lower electrode  10  does not exist at part of the capacitive insulating film  11  sandwiched between the top end part H 1  of the storage node hole  9  and the upper electrode  12 . Therefore, even if the capacitive insulating film  12  is exposed as shown in  FIG. 2C , adverse effect such as the occurrence of leakage current is not caused. 
     As described above, if a capacitor is formed by the method of the present embodiment, the capacitance of the capacitor will not be affected even if the opening  17  is formed to overlap the capacitor. This eliminates the need of providing an alignment margin c between the opening  17  and the capacitor in the storage node hole  9 , though it has been necessary in the conventional method. That is, according to the present embodiment, the storage node hole  9  is enlarged to be in contact with the opening  17  at the top end part H 1  as shown in  FIG. 4 , while the memory cell size is not changed. Thus, a larger amount of capacitance is stored in the capacitor while device miniaturization is achieved.  FIG. 4  is a view illustrating a planar configuration of the semiconductor memory of Embodiment 1 of the present invention. 
     Next, a comparison as to the total area of the capacitor is made between the capacitor of the present embodiment and a conventional capacitor. The total area of the capacitor mentioned herein is the sum of the area of the capacitor at the bottom of the storage node hole  9  (bottom area) and the area of the capacitor at the sidewalls of the storage node hole  9  (lateral area). The conventional capacitor shown in  FIG. 8B  has a short side length a of 0.2 μm, a long side length b of 0.45 μm and an alignment margin c of 0.05 μm and the storage node hole  9  has a height of 0.85 μm (height of the capacitor region). Under these conditions, the total area of the conventional capacitor is calculated as follows. 
     Bottom area: 0.2×0.45=0.09 (μm 2 ) 
     Lateral area: 0.85×(0.45×2+0.2×2)=1.105 (μm 2 ) 
     Total area: 0.09+1.105=1.195 (μm 2 ) 
     On the other hand, the present embodiment eliminates the need of the alignment margin. Therefore, the long side length of the capacitor increases by 0.05 μm in a memory cell having the same area as the conventional memory cell. Further, at the top end part of the storage node hole  9 , the lower electrode  10  is formed at a deeper position than the top surface of the second interlayer insulating film  8  by 50 nm or more. Therefore, the height of the capacitor becomes smaller by 0.05 μm than the conventional capacitor. As a result, the capacitor of the present embodiment has a short side length a of 0.2 μm, a long side length b (including the margin c) of 0.5 μm and the height of 0.8 μm. Under these conditions, the total area is calculated as follows. 
     Bottom area: 0.2×0.5=0.1 (μm 2 ) 
     Lateral area: 0.8×(0.5×2+0.2×2)=1.12 (μm 2 ) 
     Total area: 0.1+1.12=1.22 (μm 2 ) 
     From the above results, it is understood that the capacitor of the present embodiment is given with a larger area than the conventional capacitor. 
     Embodiment 2 
     Hereinafter, with reference to the drawings, an explanation is given of a method for manufacturing a semiconductor memory according to Embodiment 2 of the present invention.  FIGS. 5A to 5C ,  6 A to  6 C and  7  are sectional views illustrating the steps of manufacturing a DRAM of a CUB structure according to Embodiment 2 of the present invention. 
     According to the method of the present embodiment, an STI  1  is formed in a silicon substrate  20 , and then a gate insulating film  2   a , a gate electrode  2   b , an impurity diffusion layer  3  and a first interlayer insulating film  4  are formed in the step shown in  FIG. 5A  in the same manner as Embodiment 1. Then, in the step shown in  FIG. 5B , a bit line contact  5 , a capacitor contact  6  and an insulating film  7  are formed. 
     Then, in the step shown in  FIG. 5C , a second interlayer insulating film  21  of 750 nm in thickness is formed on the insulating film  7  and an insulating film  22  of 50 nm in thickness is formed on the second interlayer insulating film  21 . The second interlayer insulating film  21  is made of a material which is wet-etched at a higher rate than the insulating film  22 . For example, the second interlayer insulating film  21  may be made of PSG and the insulating film  22  may be made of NSG. As described later, the wet etching mentioned herein is carried out to etch the second interlayer insulating film  21  and the insulating film  22  to form a storage node hole  9 . 
     Then, a resist (not shown) having an opening corresponding to the capacitor region is formed on the insulating film  22  and dry etching is carried out using the resist as a mask, thereby forming a storage node hole  9  which penetrates the insulating film  22 , the second interlayer insulating film  21  and the insulating film  7  to reach the top surface of the capacitor contact  6 . Then, wet etching is carried out using a hydrofluoric acid solution under such conditions that the second interlayer insulating film  21  is etched faster than the insulating film  22 . Thus, in the storage node hole  9 , the second interlayer insulating film  21  is removed more than the insulating film  22 . As a result, part of the storage node hole  9  where the second interlayer insulating film  21  forms the sidewalls thereof becomes about 40 nm larger in diameter than part of the storage node hole  9  where the insulating film  22  forms the sidewalls thereof. 
     Instead of forming the second interlayer insulating film  21  and the insulating film  22  from different materials, boron may be implanted into the top part of an interlayer insulating film (not shown) so that the top part of the interlayer insulating film is given with an etch rate different from that of the other part. For example, the interlayer insulating film is formed to have a thickness of 800 nm and then boron is implanted therein down to a depth of 50 nm from the top surface thereof. Since the wet-etch rate decreases with an increase in boron concentration, the top part of the interlayer insulating film becomes lower in etch rate than the other part. 
     Then, in the step shown in  FIG. 6A , a TiN film of 30 nm in thickness (not shown) is formed on the bottom surface and the sidewalls of the storage node hole  9 . Thereafter, a resist (not shown) is applied to the substrate and light exposure is carried out over the entire surface of the substrate, thereby leaving the resist only in the storage node hole  9 . Then, etching is carried out to remove only 130 nm of the TiN film to form a lower electrode  23  made of TiN. In this etching step, the upper end of the lower electrode  23  which is uncovered with the resist is thinned down to about 10 nm in thickness, while the other part of the lower electrode  23  remains unetched because it is covered with the resist. 
     Then, in the step shown in  FIG. 6B , a capacitive insulating film  24  made of Ta 2 O 5  of 20 nm in thickness is formed on the lower electrode  23  in the storage node hole  9  and the insulating film  22  outside the storage node hole  9 . Then, an upper electrode  25  made of TiN of 50 nm in thickness is formed on the capacitive insulating film  24 . 
     Then, in the step shown in  FIG. 6C , a mask  26  having an opening corresponding to a bit line contact region is formed on the upper electrode  25 , and then dry etching is carried out using the mask  26  to form an opening  27 .  FIG. 6C  shows the opening  27  which is misaligned from the desired position and the left end of the opening  27  reaches the capacitor in the storage node hole  9 . The misalignment of the opening  27  is derived from the mask  26  which is misaligned from the desired position. 
     Then, in the step shown in  FIG. 7 , a third interlayer insulating film  13 , a bit line contact  14  and a first wiring layer  15  are formed in the same manner as Embodiment 1. Through the above-described steps, a capacitor of a CUB structure is obtained. 
     In the capacitor of the present embodiment, as shown in  FIG. 5C , the side edge of the insulating film  22  at the top end part H 2  of the storage node hole  9  protrudes toward more inside of the storage node hole  9  than the side edge of the second interlayer insulating film  21 . Since the lower electrode  23  is not formed on the protruding side edge of the insulating film  22  at the top end part H 2  of the storage node hole  9 , the capacitive insulating film  24  formed on the insulating film  22  at the top end part H 2  of the storage node hole  9  does not store the capacitance. Therefore, no problems arise even if the opening  27  is misaligned to overlap the capacitor as shown in  FIG. 6C , thereby removing parts of the upper electrode  25  and the capacitive insulating film  24  formed at the top end part H 2  of the storage node hole  9 . On the other hand, the capacitance is stored in the lower electrode  23 , capacitive insulating film  24  and upper electrode  25  which are formed at a level lower than the top end part H 2  of the storage node hole  9 . As shown in  FIG. 7 , since the insulating film  22  at the top end part H 2  of the storage node hole  9  protrudes toward inside of the storage node hole  9  to generate a dent part below it, the upper electrode  25  is reduced in level difference which is generated at part thereof covering the boundary between the insulating film  22  and the dent part below it. Therefore, even if the opening  27  is misaligned to overlap the capacitor, the lower electrode  23 , capacitive insulating film  24  and upper electrode  25  formed in the storage node hole  9  at a level lower than the insulating film  22  are less likely to be removed. 
     As described above, in the capacitor manufactured by the method of the present embodiment, the capacitive insulating film  24  which contributes to the capacitance will not be damaged even if the opening  27  overlaps the capacitor. Therefore, unlike the conventional method, there is no need of providing an alignment margin between the opening  27  and the capacitor in the storage node hole  9 . More specifically, in the present embodiment, the capacitor in the storage node hole  9  is enlarged to be in contact with the opening  17 , while the memory cell size is not changed. As a result, a larger amount of capacitance is stored while device miniaturization is achieved. 
     In the present embodiment, the following conditions are preferably satisfied so as to prevent the capacitive insulating film  24  which contributes to the capacitance from damage even when the opening  27  overlaps the capacitor. As a first condition, the side edge of the insulating film  22  is formed to protrude toward more inside of the storage node hole  9  than the side edge of the second interlayer insulating film  21  by the length same as or larger than the thickness of the lower electrode  23 . If the protruding length of the side edge of the insulating film  22  is smaller than the thickness of the lower electrode  23 , as a second condition, the upper electrode  25  is formed to have a thickness larger than the difference between the thickness of the lower electrode  23  and the protruding length of the insulating film  22  (the thickness of the lower electrode  23  minus the protruding length). If one of these conditions is satisfied, the capacitive insulating film  24  which contributes to the capacitance will not be damaged by dry etching for forming the opening  27 . The opening  27  is allowed to overlap the capacitor as long as it does not exceed a boundary d shown in  FIG. 6C , i.e., as long as the capacitive insulating film  24  formed at the bottom of the storage node hole  9  is not exposed.