Abstract:
A method for fabricating a semiconductor memory device with a tree-type capacitor having increased area for reliable storage of electrical charges representative of data thereon. The tree-type capacitor includes a storage electrode having a trunk-like conductive layer coupled to at least one branch-like conductive layer, which can be structured in various shapes that allow the branch-Like conductive layer to have increased surface area. The branch-like conductive layers are formed by successively depositing at least one insulating layer and at least one conductive layer over the substrate such that the conductive layer makes a series of twists and turns, defining the shape of the branch-like conductive layer. The surface of the built-up wafer is removed until the conductive layer is divided into a number of segments. A contact hole is formed through the conductive layer to a drain/source region of a transistor in the device, and is filled with a conductive layer, forming the trunk-like layer. The insulating material is wet-etched away, leaving the conductive segment attached to the truck-like layer as a branch-like conductive layer. A dielectric layer is formed over exposed surfaces of the trunk-like conductive layer and the branch-like conductive layer, and a further conductive layer is formed overlaying the dielectric layer to serve as an opposing electrode of the tree-type capacitor.

Description:
This is a divisional of U.S. patent application Ser. No. 09/055,400, filed Apr. 6, 1998, U.S. Pat. No. 6,153,964 which is a divisional of U.S. patent application Ser. No. 08/681,352, filed Jul. 23, 1996, now U.S. Pat. No. 5,612,486. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention: 
     This invention relates in general to semiconductor memory devices, and more particularly to a structure of a dynamic random access memory (DRAM) cell substantially composed of a transfer transistor and a charge storage capacitor. 
     2. Description of the Related Art: 
     FIG. 1 is a circuit diagram of a memory cell for a DRAM device. As shown in the drawing, a DRAM cell is substantially composed of a transfer transistor T and a charge storage capacitor C. A source of the transfer transistor T is connected to a corresponding bit line BL, and a drain thereof is connected to a storage electrode  6  of the charge storage capacitor C. A gate of the transfer transistor T is connected to a corresponding word line WL. An opposing electrode  8  of the capacitor C is connected to a constant power source. A dielectric film  7  is provided between the storage electrode  6  and the opposing electrode  8 . 
     In the DRAM manufacturing process, a two-dimensional capacitor called a planar type capacitor is mainly used for a conventional DRAM having a storage capacity less than 1M (mega=million) bits. In the case of a DRAM having a memory cell using a planar type capacitor, electric charges are stored on the main surface of a semiconductor substrate, so that the main surface is required to have a large area. This type of a memory cell is therefore not suited to a DRAM having a high degree of integration. For a high integration DRAM such as a DRAM with more than 4M bits of memory, a three-dimensional capacitor, called a stacked-type or a trench-type capacitor, has been introduced. 
     With the stacked-type or trench-type capacitors, it has been made possible to obtain a larger memory in a similar volume. However, to realize a semiconductor device of an even higher degree of integration, such as a very-large-scale integration (VLSI) circuit having a capacity of 64M bits, a capacitor of such a simple three-dimensional structure as the conventional stacked-type or trench-type, turns out to be insufficient. 
     One solution for improving the capacitance of a capacitor is to use the so-called fin-type stacked capacitor, which is proposed in Ema et al., “3-Dimensional Stacked Capacitor Cell for 16M and 64M DRAMs”, International Electron Devices meeting, pp. 592-595, December 1988. The fin-type stacked capacitor includes electrodes and dielectric films which extend in a fin shape in a plurality of stacked layers. DRAMs having the fin-type stacked capacitor are also disclosed in U.S. Pat. No. 5,071,783 (Taguchi et al.); U.S. Pat. No. 5,126,810 (Gotou), U.S. Pat. No. 5,196,365 (Gotou); and U.S. Pat. No. 5,206,787 (Fujioka). 
     Another solution for improving the capacitance of a capacitor is to use the so-called cylindrical-type stacked capacitor, which is proposed in Wakamiya et al., “Novel Stacked Capacitor Cell for 64-Mb DRAM”, 1989 Symposium on VLSI Technology Digest of Technical Papers, pp. 69-70. The cylindrical-type stacked capacitor includes electrodes and dielectric films which extend in a cylindrical shape to increase the surface areas of the electrodes. A DRAM having the cylindrical-type stacked capacitor also is disclosed in the U.S. Pat. No. 5,077,688 (Kumanoya et al.). 
     With the trend toward increased integration density, the size of the DRAM cell in a plane (the area it occupies in a plane) must be further reduced. Generally, a reduction in the size of the cell leads to a reduction in charge storage capacity (capacitance). Additionally, as the capacitance is reduced, the likelihood of soft errors arising from the incidence of α-rays is increased. Therefore, there is still a need in this art to design a new structure of a storage capacitor which can achieve the same capacitance, while occupying a smaller area in a plane, and a suitable method of fabricating the structure. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a method for fabricating a semiconductor memory device which is structured with a tree-type capacitor that allows an increased area for charge storage. 
     In accordance with the foregoing and other objects of the invention, a new and improved method for fabricating a semiconductor memory device are provided. 
     The invention provides a method for fabricating a semiconductor memory device. The semiconductor memory device includes a substrate, a transfer transistor having source/drain regions, formed on the substrate, and a charge storage capacitor electrically coupled to one of the source/drain regions. According to the method, a first insulating, layer is first formed over the substrate, such that it covers the transfer transistor. An insulating pillar is then formed over the first insulating layer, the insulating pillar defining recess areas on either side thereof. A first film of insulating material and a second film of conductive material are next alternately formed over the first insulating layer in a recess area and over the insulating pillar. A selected part of the second film that lies above the insulating pillar is then removed and a first conductive layer is formed which penetrates at least through the second film, the first film, and the first insulating layer so as to be electrically coupled to one of the source/drain regions The first conductive layer and the second film in combination thus form a storage electrode of the charge storage capacitor. The insulating pillar and the first film are then removed. A dielectric layer is formed over exposed surfaces of the first conductive layer and the second film, and a second conductive layer is formed over the dielectric layer. The second conductive layer thus functions as an opposing electrode of the charge storage capacitor. 
     A semiconductor memory device according to the invention is therefore formed having a tree-type capacitor of increased area for reliable storage thereon of electrical charges representative of data. By varying the number of conducting layers formed, interleaved with insulating layers, during fabrication, the total surface area of the capacitor electrodes can be controlled. The size, shape, and placement of the insulating pillar and the size, shape, and construction of the second conductive layer may also be varied to change the shape of the tree-type capacitor in order to satisfy particular design needs. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The description is made with reference to the accompanying drawings in which. 
     FIG. 1 is a circuit diagram of a memory cell of a DRAM device; 
     FIGS. 2A through 2G are cross-sectional views for explaining a first embodiment of a semiconductor memory cell having a tree-type capacitor according to the invention, and a method for fabricating the same according to the invention; 
     FIGS. 3A through 3D are cross-sectional views for explaining a second embodiment of a semiconductor memory cell having a tree-type capacitor according to the invention and a method for fabricating the same according to the invention; 
     FIGS. 4A and 4B are cross-sectional views for explaining a third embodiment of a semiconductor memory cell having a tree-type capacitor according to the invention and a method for fabricating the same according to the invention; 
     FIGS. 5A through 5D are cross-sectional views for explaining a fourth embodiment of a semiconductor memory cell having a tree-type capacitor according to the invention and a method for fabricating the same according to the invention; 
     FIGS. 6A and 6B are cross-sectional views for explaining a fifth embodiment of a semiconductor memory cell having a tree-type capacitor according to the invention and a method for fabricating the same according to the invention; 
     FIGS. 7A and 7B are cross-sectional views for explaining a sixth embodiment of a semiconductor memory cell having a tree-type capacitor according to the invention and a method for fabricating the same according to the invention; 
     FIGS. 8A through 8F are cross-sectional views for explaining a seventh embodiment of a semiconductor memory cell having a tree-type capacitor according to the invention and a method for fabricating the same according to the invention; 
     FIGS. 9A through 9D are cross-sectional views for explaining an eighth embodiment of a semiconductor memory cell having a tree-type capacitor according to the invention and a method for fabricating the same according to the invention; and 
     FIG. 10A through 10D are cross-sectional views for explaining a ninth embodiment of a semiconductor memory cell having a tree-type capacitor according to the invention and a method for fabricating the same according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Preferred Embodiment 
     A description will be given of a first embodiment of a semiconductor memory device having a tree-type charge storage capacitor according to the invention, by referring to FIGS. 2A through 2G. This embodiment of the semiconductor memory device can be produced by a first preferred method for fabricating a semiconductor memory device according to the invention. 
     Referring to FIG. 2A, a surface of a silicon substrate  10  is subjected to thermal oxidation by the LOCOS (local oxidation of silicon) technique, and thereby a field oxidation film  12  having a thickness of approximately 3000 Å (angstroms), for example, is formed. Next, a gate oxidation film  14  having a thickness of approximately 150 Å, for example, is formed by subjecting the silicon substrate  10  to the thermal oxidation process. Then a polysilicon film having a thickness of approximately 2000 Å, for example, is deposited on the entire surface of the silicon substrate  10  by CVD (chemical vapor deposition) or LPCVD (low pressure CVD). In order to obtain a polysilicon film of low resistance, suitable impurities such as phosphorus ions, for example, are diffused into the polysilicon film. Preferably, a refractory metal layer is deposited over the polysilicon film, and then an annealing process is carried out to form polycide, so that the film&#39;s resistance is further decreased. The refractory metal may be tungsten (W), and its thickness is, for example, approximately 2000 Å. Thereafter, the polycide is subjected to a patterning process to form gate electrodes (or word lines) WL 1  through WL 4 , as shown in FIG.  2 A. Then, arsenic ions, for example, are diffused into the silicon substrate  10  at an energy of 70 KeV to provide an impurity concentration of approximately 1×10 15  atoms/cm 2 , for example. In this step, the word lines WL 1  through WL 4  are used as mask films. Thereby, drain regions  16   a  and  16   b  and source regions  18   a  and  18   b  are formed in the silicon substrate  10 . 
     Referring next to FIG. 2B, in the subsequent step the CVD method is used to deposit a planarization insulating layer  20  of, for example, borophosphosilicate glass (BPSG), to a thickness of approximately 7,000 Å, for example. Then the same method is used to form an etching protection layer  22 , which can be, for example, a silicon nitride layer, having a thickness of approximately 1,000 Å, for example. After that, a thick insulating layer of, for example silicon dioxide, is deposited over the wafer to a thickness of approximately 7,000 Å, for example. Conventional photolithographic and etching processes are then used to define an insulating pillar  24  bounded by recesses  23 . Although FIG. 2B shows the insulating pillar  24  in a number of separate locations, the insulating pillar  24  is actually an integrated body, which is apparent when viewed from above. 
     Referring next to FIG. 2C, in the subsequent step the CVD method is used successively to form a first insulating layer  26 , a polysilicon layer  28 , and a second insulating layer  30 . The first and second insulating layers  26 ,  30  are preferably formed of silicon oxide. The first insulating layer  26  and the polysilicon layer  28  are each deposited to a thickness of approximately 1,000 Å, for example, and the second insulating layer  30  is deposited to a thickness of approximately 7,000 Å, for example. Arsenic (As) ions can be diffused into the polysilicon layer  28  so as to increase its conductivity. 
     Referring next to FIG. 2D, in the subsequent step, chemical mechanical polishing (CMP) is performed on the surface of the wafer of FIG. 2C until an upper part of the polysilicon layer  28  is polished away. The remaining part of the polysilicon layer  28  includes a number of separate sections as designated by the numerals  28   a ,  28   b  shown in FIG.  2 D. 
     Referring next to FIG. 2E, conventional photolithographic and etching processes are then carried out to selectively etch, in sequence, the insulating layer  30 , polysilicon layer sections  28   a  and  28   b , insulating layer  26 , etching protection layer  22 , insulating layer  20 , and gate oxidation film  14 . As a result, storage electrode contact holes  32   a  and  32   b  are formed. The storage electrode contact holes  32   a  and  32   b  extend respectively from a top surface of the insulating layer  30  to a top surface of the drain regions  16   a  and  16   b . A polysilicon film is then deposited and etched back to refill the storage electrode contact holes  32   a  and  32   b  with polysilicon layers  34   a  and  34   b.    
     Referring next to FIG. 2F, in the subsequent step wet etching is performed on the wafer with the etching protection layer  22  as the etch end point, so as to remove the insulating layers  26 ,  30  and the insulating pillar  24 . The remaining tree trunk-like polysilicon layers  34   a ,  34   b  and the branch-like polysilicon layers  28   a ,  28   b , in combination form a tree-like storage electrode for the capacitor of the DRAM. The trunk-like polysilicon layers  34   a ,  34   b  are respectively electrically coupled to the drain regions  16   a  and  16   b  of the transfer transistors in the DRAM. The branch-like polysilicon layers  28   a ,  28   b  are each substantially L-shaped in cross section and have the substantially horizontal sections in electrical contact with the trunk-like polysilicon layers  34   a ,  34   b . With this particular shape, the storage electrodes are hereinafter in this specification referred to as “tree-like storage electrodes”, and the capacitors thus made are referred to as “tree-type capacitors”. 
     Referring next to FIG. 2G, in the subsequent step dielectric films  36   a ,  36   b  are respectively formed over the tree-like storage electrode ( 34   a ,  28   a ) and the tree-like storage electrode ( 34   b ,  28   b ). The dielectric films  36   a ,  36   b  can be formed of, for example, silicon dioxide, silicon nitride, NO (silicon nitride/silicon dioxide), ONO (silicon dioxide/silicon nitride/silicon dioxide), or the like. Next, an opposing electrode  38  of polysilicon, that opposes the storage electrodes ( 34   a ,  28   a ) and ( 34   b  and  28   b ), is formed over the dielectric films  36   a ,  36   b . The process for forming the opposing electrode  38  includes a first step of depositing a polysilicon layer by the CVD method to a thickness of, for example, approximately 1,000 Å, a second step of diffusing N-type impurities into the polysilicon layer so as to increase the conductivity thereof, and a final step of using conventional photolithographic and etching processes to etch away selected parts of the polysilicon layer. The fabrication of the tree-type capacitors in the DRAM is then complete. 
     To complete the fabrication of the DRAM chip, the subsequent steps include fabricating bit lines, bonding pads, interconnections, passivations, and packaging. These steps involve only conventional techniques and are not within the spirit and scope of the invention, so that a detailed description thereof will not be provided herein. 
     Second Preferred Embodiment 
     In the foregoing first embodiment, the disclosed tree-type capacitor has only a single branch electrode. However, the number of branches is not limited to one and can be two or more. In the following, a second embodiment of the tree-type capacitor, which includes two branches of electrodes, is described with reference to FIGS. 3A through 3D. The tree-type capacitor of the second embodiment is based on the wafer structure of FIG.  2 B. Elements in FIGS. 3A through 3D that are identical to those in FIG. 2B are labeled with the same numerals. 
     Referring to FIG. 3A together with FIG. 2B, the CVD method is used to successively form alternate layers of insulation and polysilicon, including a first insulating layer  40 , a first polysilicon layer  42 , a second insulating layer  44 , a second polysilicon layer  46 , and a third insulating layer  48 . The insulating layers  40 ,  44 ,  48  are formed preferably of, for example, silicon oxide. The insulating layers  40 ,  44  and the polysilicon layers  42 ,  46  are each deposited to a thickness of approximately 1,000 Å, for example, and the insulating layer  48  is deposited to a thickness of approximately 7,000 Å, for example. The polysilicon layers  42 ,  46  can be diffused with arsenic (As) ions so as to increase the conductivity thereof 
     Referring next to FIG. 3B, in the subsequent step the CMP technique is applied to the surface of the wafer shown in FIG. 3A, so as to polish away an upper part of the polysilicon layers  42 ,  46 . The remaining part of the polysilicon layers  42 ,  46  includes a number of separate sections designated by the numerals  42   a ,  46   a , and  42   b ,  46   b.    
     Referring next to FIG. 3C, in the subsequent step conventional photolithographic and etching processes are used to form storage electrode contact holes which extend from the top surface of the insulating layer  48  (see FIG. 3B) to the surface of the drain regions  16   a  and  16   b . The storage electrode contact holes are then refilled with polysilicon layers  50   a ,  50   b  by first using the CVD method to deposit a polysilicon layer, and then etching back part of the polysilicon layer. Subsequently, wet etching is performed on the wafer, with the etching protection layer  22  as the etch end point, so as to remove the insulating layers  40 ,  44 ,  48  and the insulating pillar  24 . The remaining trunk-like polysilicon layers  50   a ,  50   b  and the branch-like polysilicon layers  42   a ,  46   b  and  42   b ,  46   b  in combination form two tree-like storage electrodes. The trunk-like polysilicon layers  50   a ,  50   b  are respectively electrically coupled to the drain regions  16   a  and  16   b  of the transfer transistors in the DRAM. The branch-like polysilicon layers  42   a ,  46   a  and  42   b ,  46   b  are each substantially L-shaped in cross section, and have substantially horizontal sections in contact with the trunk-Like polysilicon layers  50   a ,  50   b.    
     Referring next to FIG. 3D, in the subsequent step dielectric films  52   a ,  52   b  are respectively formed on the tree-like storage electrodes ( 50   a ,  46   a ,  42   a ) and ( 50   b ,  46   b ,  42   b ). Next, an opposing polysilicon electrode  54  is formed over the dielectric films  52   a ,  52   b . The process for forming the opposing electrode  54  includes a first step of depositing a polysilicon layer by the CVD method, a second step of diffusing N-type impurities into the polysilicon layer so as to increase the conductivity thereof, and a final step of using conventional photolithographic and etching processes to etch away selected part of the polysilicon layer. After that, the fabrication for the tree-type capacitors in the DRAM is complete. 
     Third Preferred Embodiment 
     In the foregoing first and second embodiments, the bottom-most layer of the branch-like part of the tree-like storage electrode is separated from the etching protection layer  22 . However, the invention is not limited to such a structure. In the following, a third embodiment of the invention in which the bottom-most layer of the branch-like part of each tree-like storage electrode is in contact with the etching protection layer  22  is described, with reference to FIGS. 4A and 4B. 
     The tree-type capacitors of the third embodiment are also based on the structure of FIG.  2 B. Elements in FIGS. 4A through 4D that are identical to those in FIG. 2B are labeled with the same numerals. 
     Referring first to FIG. 4A together with FIG. 2B, the CVD method is used successively to form alternate layers of insulation and polysilicon including a first polysilicon layer  60 , a first insulating layer  62 , a second polysilicon layer  64 , and a second insulating layer  66 . 
     Referring next to FIG. 4B, in the subsequent step the CMP technique is applied to the surface of the wafer shown in FIG. 4A, so as to polish away an upper part of the polysilicon layers  60 ,  64 . The remaining parts of the polysilicon layers  60 ,  64  include a number of separate sections designated by the numerals  60   a ,  64   a , and  60   b ,  64   b . Next, conventional photolithographic and etching processes are used to form storage electrode contact holes. The storage electrode contact holes are then refilled with polysilicon layers  68   a ,  68   b . After that, wet etching is performed on the wafer, with the etching protection layer  22  as the etch end point, so as to remove the insulating layers  62 ,  66 . 
     The remaining trunk-like polysilicon layers  68   a ,  68   b  and the branch-like polysilicon layers  60   a ,  64   b  and  60   b ,  64   b  in combination form two tree-like storage electrodes. The trunk-like polysilicon layers  68   a ,  68   b  are respectively electrically coupled to the drain regions  16   a  and  16   b  of the transfer transistor in the DRAM. The branch-like polysilicon layers  60   a ,  64   a  and  60   b ,  64   b  are each substantially L-shaped in cross section, and have substantially horizontal sections in contact with the trunk-like polysilicon layers  68   a ,  68   b . In this embodiment, the branch-like polysilicon layers  60   a ,  60   b  of the tree-like storage electrodes are in contact with the etching protection layer  22 . A dielectric film and opposing polysilicon electrode may now be formed as described previously for the first, second and third embodiments. After that, the fabrication for the tree-type capacitors in the DRAM is complete. 
     Fourth Preferred Embodiment 
     In the foregoing three embodiments, the trunk-like part of the tree-like storage electrode of each tree-type capacitor is an integrally formed semiconductor element. However, the invention is not limited to such a structure. In the following, a fourth embodiment, in which the trunk-like part of each tree-like storage electrode is composed of a plurality of semiconductor elements, is described, with reference to FIGS. 5A and 5D. 
     The tree-type capacitor of the fourth embodiment is also based on the structure of FIG.  2 A. Elements in FIGS. 5A through 5D that are identical to those in FIG. 2A are labeled with the same numerals. 
     Referring first to FIG. 5A together with FIG. 2A, the CVD method is used to deposit a planarization insulating layer  70  over the wafer of, for example, BPSG. Then the same method is used to deposit an etching protection layer  72  of, for example, silicon nitride. After that, conventional photolithographic and etching processes are used to etch selected parts of the etching protection layer  72  and the planarization insulating layer  70 , so as to form storage electrode contact holes  76   a ,  76   b  which extend from the top surface of the etching protection layer  72  to the top surface of the drain regions  16   a  and  16   b . Next, the CVD method is used to deposit over the wafer a polysilicon layer which fills the storage electrode contact holes  76   a ,  76   b . The polysilicon layer can be diffused with impurities so as to increase the conductivity thereof Conventional photolithographic and etching processes then are used to define T-shaped elements  74   a ,  74   b , to form respective bottom parts of capacitor charge storage electrodes for memory cells in the DRAM. 
     Referring next to FIG. 5B, in the subsequent step a thick insulating layer of, for example, silicon dioxide, is deposited over the wafer. Then, conventional photolithographic and etching processes are used to etch away selected parts of the insulating layer, so as to form insulating pillars  78 . Next, the CVD method is used successively to form a first insulating layer  80 , a polysilicon layer  82 , and a second insulating layer  84 . 
     Referring next to FIG. 5C, in the subsequent step the CMP technique is applied to the surface of the wafer shown in FIG. 5B, so as to polish away an upper part of the polysilicon layer  82 . The remaining part of the polysilicon layer  82  includes a number of separate sections designated by the numerals  82   a ,  82   a.    
     Referring next to FIG. 5D, in the subsequent step conventional photolithographic and etching processes are used to successively etch away selected parts of the second insulating layer  84 , polysilicon layers  82   a ,  82   b , and the first insulating layer  80 , so as to form contact holes which extend from the top surface of the insulating layer  84  to the top surface of the T-shaped elements  74   a ,  74   b  of the tree-like storage electrodes. Then, the contact holes are refilled with polysilicon so as to form upper parts  86   a ,  86   b  of the tree-like storage electrodes. The process for refilling the polysilicon into the contact holes includes a first step of depositing a polysilicon layer by the CVD method, and a second step of etching back the same. After that, wet etching is performed on the wafer, with the etching protection layer  72  as the etch end point, so as to remove the insulating layers  84 ,  80  and the insulating pillar  78 . This completes the fabrication of the storage electrodes of the tree-type capacitors in the DRAM. The embodiment differs from that of FIG. 2F in that the storage electrodes each include in addition a substantially horizontal section extended from the T-shaped elements,  74   a ,  74   b  on the bottom. A dielectric film and opposing polysilicon electrode may now be formed as described previously for the first, second and third embodiments. After that, the fabrication for the tree-type capacitors in the DRAM is complete. 
     Fifth Preferred Embodiment 
     In the foregoing four embodiments, the trunk-like part of the tree-like storage electrode is a solid semiconductor element. However, the invention is not limited to such a structure. The following description discloses a fifth embodiment with reference to FIGS. 6A and 6B, in which the trunk-like part of each tree-like storage electrode is hollow. 
     The tree-type capacitor of the fifth embodiment is based on the structure of FIG.  2 D. Elements in FIGS. 6A and 6B that are identical to those in FIG. 2D are labeled with the same numerals. 
     Referring first to FIG. 6A together with FIG. 2D, after the fabrication has reached the stage shown in FIG. 2D, conventional photolithographic and etching processes are used to etch away selected parts of the insulating layer  30 , the branch-like polysilicon layer  28   a ,  28   b , the insulating layer  26 , the etching protection layer  22 , the planarization insulating layer  20 , and the gate oxidation film  14 , so as to form storage electrode contact holes  87   a ,  87   b  which extend from the top surface of the insulating layer  30  to the top surfaces of the drain regions  16   a  and  16   b . Next, the CVD method is used to deposit a polysilicon layer in such a manner that the polysilicon layer is formed only on the inner walls of the storage electrode contact holes  87   a ,  87   b , and do not fill up the holes. After that, conventional photolithographic and etching processes are used to define trunk-like polysilicon layers  88   a ,  88   b  for the respective storage electrodes of the memory cells in the DRAM. As illustrated in FIG. 6A, the trunk-like polysilicon layers  88   a ,  88   b  are each substantially U-shaped in cross section, which provides an increased area on which the storage electrodes can store large amounts of electric charge. 
     Referring next to FIG. 6B, in the subsequent step wet etching is performed on the wafer, with the etching protection layer  22  as the etch end point, so as to remove the insulating layers  30 ,  26  and the insulating pillar  24 . This completes the fabrication of the storage electrodes of the tree-type capacitors in the DRAM. The embodiment differs from that of FIG. 2F in that the trunk-like parts of the storage electrodes, namely the trunk-like polysilicon layers  88   a ,  88   b , are hollow and have U-shaped cross sections, which provide the storage electrodes with an increased surface area. A dielectric film and opposing polysilicon electrode may now be formed as described previously for the first, second and third embodiments. After that, the fabrication for the tree-type capacitors in the DRAM is complete. 
     Sixth Preferred Embodiment 
     A sixth embodiment of the invention is illustrated in FIGS. 7A and 7B. In this embodiment also, the trunk-like part of each tree-like storage electrode is hollow. The tree-type capacitors of the sixth embodiment are based on the structure of FIG.  5 C. Elements in FIGS. 7A and 7B that are identical to those in FIG. 5C are labeled with the same numerals. 
     Referring first to FIG. 7A together with FIG. 5C, after the fabrication has reached the stage shown in FIG. 5C, conventional photolithographic and etching processes are used to etch assay selected pars of the insulating layer  84 , the polysilicon layers  82   a ,  82   b , and the insulating layer  80 , so as to form contact holes  90   a ,  90   b  which extend downward from the top surface of the insulating layer  84  to the top surfaces of the T-shaped elements  74   a ,  74   b  of the storage electrodes. Next, the CVD method is used to deposit a polysilicon layer which is then etched back so as to form sidewall spacers  92   a ,  92   b  on the inner walls of the contact holes  90   a ,  90   b . The sidewall spacers  92   a ,  92   b  constitute upper trunk-like parts of the tree-like storage electrodes, and are hollow with U-shaped cross sections, which provides the storage electrode with increased surface area. 
     Referring next to FIG. 7B, in the subsequent step wet etching is performed on the wafer, with the etching protection layer  72  as the etch end point, so as to remove the insulating layers  84 ,  80  and the insulating pillar  78 . This completes the fabrication for the storage electrodes of the tree-type capacitors in the DRAM. The embodiment differs from that of FIG. 5D in that the upper part of each trunk-like electrode is hollow, and has a U-shaped cross section. A dielectric film and opposing polysilicon electrode may now be formed as described previously for the first, second and third embodiments. After that, the fabrication for the tree-type capacitors in the DRAM is complete. 
     Seventh Preferred Embodiment 
     In the foregoing six embodiments, the branch-like part of the tree-like storage electrode is L-shaped in cross section, so that it is crooked, with two straight segments. However, the invention is not limited to such a structure. The number of straight segments can be increased to three or more. The following description, with reference to FIGS. 8A and 8F, is of a seventh embodiment in which the branch-like part of each tree-like storage electrode is crooked, with four straight segments. 
     The tree-type capacitors of the seventh embodiment are based on the structure of FIG.  2 A. Elements in FIG. 8A through 8F that are identical to those in FIG. 2A are labeled with the same numerals. 
     Referring first to FIG. 8A together with FIG. 2A, after the fabrication has reached the stage shown FIG. 2A, the CVD method is used to deposit a planarization insulating layer  100  of, for example, BPSG. Then the same method is used to deposit an etching protection layer, which can be, for example, a silicon nitride layer  102 . A thick insulating layer of, for example, silicon dioxide, is then deposited over the wafer. After that, a conventional photolithographic process is used to form a photoresist layer  106  and then anisotropic etching is performed on the exposed silicon dioxide layer, so as to form protruding insulating layers  104  and an underlying insulated layer  103 . 
     Referring next to FIG. 8B, in the subsequent step a photoresist erosion technique is performed to erode away part of the photoresist layer  106 , so as to form a photoresist layer  106   a  that is reduced both in breadth and thickness (height). Part of the surface of the protruding insulating layers  104  formerly underlying the uneroded photoresist layer  106  is thereby exposed. 
     Referring next to FIG. 8C, in the subsequent step anisotropic etching is performed on the exposed surface of the protruding insulating layers  104  and the underlying insulating layer  103 , until the silicon nitride layer  102 , which serves as etching protection layer, is exposed. Protruding insulating layers  104   a  with stair-like sidewalls are thus formed. After that, the photoresist layer is removed. 
     Referring next to FIG. 8D, the subsequent steps are the same as those shown in FIGS. 2C and 2D in which the CVD method is used successively to form a first insulating layer  108 , a polysilicon layer, and a second insulating layer  112 , and then the CMP technique is applied to the surface of the wafer so as to polish away an upper part of the polysilicon layer. The remaining part of the polysilicon layer thus includes a number of separate sections designated by the numerals  110   a ,  110   b.    
     Referring next to FIG. 8E, in the subsequent step conventional photolithographic and etching processes are used to etch away successively selected parts of the insulating layer  112 , the polysilicon layers  110   a ,  110   b , the insulating layer  108 , the silicon nitride layer  102 , the planarization insulating layer  100 , and the gate oxidation film  14 , so as to form storage electrode contact holes  114   a ,  114   b  which extend from the top surface of the insulating layer  112  to the top surface of the drain regions  16   a  and  16   b . After that, the storage electrode contact holes  114   a ,  114   b  are refilled with polysilicon layers  116   a ,  116   b  by first using the CVD method to deposit a polysilicon layer, and then etching back part of the polysilicon layer. 
     Referring next to FIG. 8F, in the subsequent step, wet etching is performed on the wafer, with the silicon nitride layer  102  as the etch end point, so as to remove the insulating layers  112 ,  108  of silicon dioxide and the insulating pillar  104   a . This completes the fabrication of the storage electrodes of the tree-type capacitors in the DRAM. A dielectric film and opposing polysilicon electrode may now be formed as described previously for the first, second and third embodiments. After that, the fabrication for the tree-type capacitors in the DRAM is complete. 
     As illustrated in FIG. 8F, the storage electrodes of the tree-type capacitors include trunk-like polysilicon layers  116   a ,  116   b  and branch-like polysilicon layers  110   a ,  110   b  which are each crooked, with four straight segments. The trunk-like polysilicon layers  116   a ,  116   b  are electrically coupled to the drain regions  16   a  and  16   b  of the transfer transistor in the DRAM. The bottom-most, horizontal segments of the branch-like polysilicon layers  110   a ,  110   b  are in contact with the trunk-like polysilicon layers  116   a ,  116   b.    
     The insulating pillars or protruding insulating layers of this embodiment are modified in shape so as to form the branch-like polysilicon layers with increased area for charge storage. However, the particular shapes of the insulating pillars and protruding insulating layers are not limited to those disclosed. Thus, referring to FIG. 2B, for example, isotropic etching or wet etching can be used instead of anisotropic etching to etch away part of the thick insulating layer This permits the formation of near triangular-shaped insulating layers instead of the rectangular ones shown. In addition, also referring to FIG. 2B, after the insulating pillar  24  is formed, sidewall insulating layers can be formed on the sidewalls of the insulating pillar  24 , so as to form insulating pillars of different shape. Therefore, the branch-like polysilicon layers can be modified into various shapes. 
     If it is desired to fabricate the branch-like polysilicon layers with an increased number of straight segments, the wafer structure of FIGS. 8B and 8C can be used as the base and subsequently, the photoresist erosion technique and anisotropic etching can be used repeatedly to form the protruding insulating layers with an increased number of step-like segments. 
     Eighth Preferred Embodiment 
     In the foregoing seven embodiments, the CMP technique is used to divide a single layer of polysilicon into separate sections used respectively to form individual storage electrodes. However, the invention is not limited to the use of the CMP technique for that purpose. Instead, according to an eighth embodiment of the invention illustrated in FIGS. 9A through 9D, conventional photolithographic and etching processes can be substituted for the CMP method, for dividing the single layer of polysilicon into the separate sections. 
     The tree-type capacitor of the eighth embodiment is based on the structure of FIG.  3 A. Elements in FIGS. 9A through 9D that are identical to those in FIG. 3A are labeled with the same numerals. 
     Referring first to FIG. 9A together with FIG. 3A, after the fabrication has reached the stage shown in FIG. 3A, the topmost layer of silicon dioxide  48  is etched away or polished by the CMP technique, until the topmost polysilicon layer  46  is exposed. The resultant wafer structure is shown in FIG.  9 A. 
     Referring next to FIG. 9B, a conventional photolithographic process is used to form a photoresist layer  120 . After that, anisotropic etching is performed successively on the exposed parts of the polysilicon layer  46 , the silicon dioxide layer  44 , and the polysilicon layer  42 . By such etching, the polysilicon layers  42 ,  46  are divided into a number of separate sections designated by the numerals  42   c ,  42   d , and  46   c ,  46   d.    
     Referring next to FIG. 9C, conventional photolithographic and etching processes are then applied to form storage electrode contact holes  122   a ,  122   b  which extend from the top surface of the insulating layer  48  to the top surface of the drain regions  16   a  and  16   b . Next, the storage electrode contact holes  122   a ,  122   b  are refilled with polysilicon layers  124   a ,  124   b , by first using the CVD method to deposit a polysilicon layer and then etching back part of the polysilicon layer. 
     Referring next to FIG. 9D, in the subsequent step wet etching is performed on the wafer, with the etching protection layer  22  as the etch end point, so as to remove the insulating layers  40 ,  44 ,  48  of silicon dioxide and the insulating pillar  24 . This completes the fabrication of the storage electrodes of the tree-type capacitors. A dielectric film and opposing polysilicon electrode may now be formed as described previously for the first, second and third embodiments. After that, the fabrication for the tree-type capacitors in the DRAM is complete. 
     These electrodes are composed of trunk-like polysilicon layers  124   a ,  124   b  and branch-like polysilicon layers  42   c ,  46   c  and  42   d ,  46   d , each consisting of three straight segments. The trunk-like polysilicon layers  124   a ,  124   b  are electrically coupled respectively to the drain regions  16   a  and  16   b  of the transfer transistors in the DRAM. The branch-like polysilicon layers  42   c ,  46   c  and  42   d ,  46   d  have their respective bottom-most, horizontal segments in contact with the trunk-like polysilicon layers  50   a ,  50   b.    
     Ninth Preferred Embodiment 
     In the foregoing first through seventh embodiments, the branch-like polysilicon layers have their topmost segments aligned substantially in the same horizontal plane; and in the eighth embodiment, the branch-like polysilicon layers have their topmost segments aligned substantially in the same vertical plane. However, the invention is not limited to such structures. Instead, according to a ninth embodiment of the invention illustrated in FIGS. 10A through 10D, the topmost segments of the branch-like polysilicon layers are not aligned. 
     The tree-type capacitor of the ninth embodiment is based on the structure of FIG.  9 A. Elements in FIG. 10A through 10D that are identical to those in FIG. 9A are labeled with the same numerals. 
     Referring first to FIG. 10A together with FIG. 9A, after the fabrication has reached the stage shown in FIG. 9A, a conventional photolithographic process is used to form a photoresist layer  130  and anisotropic etching is performed on the exposed parts of the polysilicon layer  46  and the silicon dioxide layer  44 . Through this process, the polysilicon layer  46  is divided into a number of separate sections designated by the numerals  46   e ,  46   f.    
     Referring next to FIG. 10B, in the subsequent step the photoresist erosion technique is used to erode away part of the photoresist layer  130 , so as to form a photoresist layer  130   a  of reduced breadth and thickness. Part of the top surface of the polysilicon layers  46   e ,  46   f  is thus exposed. Then, anisotropic etching is performed on the exposed parts of the polysilicon layers  46   e ,  46   f , and  42 . Through this process, parts of the polysilicon layers  46   e ,  46   f  are further etched away, thereby forming polysilicon layers  46   g ,  46   h  of reduced size. After that, anisotropic etching is again performed on the exposed parts of the silicon dioxide layers  44 ,  40  until the topmost surfaces of the polysilicon layers  42   g ,  42   h  are exposed. The photoresist layer is then removed. 
     Referring next to FIG. 10C, in the subsequent step conventional photolithographic and etching processes are used to form storage electrode contact holes  132   a ,  132   b  which extend from the top surface of the insulating layer  48  to the top surfaces of the drain regions  16   a  and  16   b . Then, the storage electrode contact holes  132   a ,  132   b  are refilled with polysilicon layers  134   a ,  134   b , by first using the CVD method to deposit a polysilicon layer, and then etching back part of the polysilicon layer. 
     Referring finally to FIG. 10D, in the subsequent step, wet etching is performed on the wafer, with the etching protection layer  22  as the etch end point, so as to remove the insulating layers  40 ,  44 ,  48  of silicon dioxide and the insulating pillar  24 . This completes the fabrication of the storage electrodes of the tree-type capacitors in the DRAM. A dielectric film and opposing polysilicon electrode may now be formed as described previously for the first, second and third embodiments. After that, the fabrication for the tree-type capacitors in the DRAM is complete. 
     The storage electrodes include trunk-like polysilicon layers  134   a ,  134   b  and branch-like polysilicon layers  42   g ,  46   g  and  42   h ,  46   h  having L-shaped cross sections. The trunk-like polysilicon layers  134   a ,  134   b  are electrically coupled respectively to the drain region  16   a  and the drain region  16   b , of the transfer transistors in the DRAM. The branch-like polysilicon layers  42   g ,  46   g  and  42   h ,  46   h  have bottom-most, horizontal segments in respective contact with the trunk-like polysilicon layers  134   a ,  134   b , and the substantially vertical segments of the branch-like polysilicon layers  46   g ,  46   h  are more elevated than that of the branch-like polysilicon layers  42   g ,  42   h.    
     It will be apparent to those skilled in the art of semiconductor fabrication that the foregoing disclosed embodiments can be applied either alone or in combination so as to provide storage electrodes of various sizes and shapes on a single DRAM chip. These variations are all within the scope of the invention. 
     Although in the accompanying drawings the embodiments of the drains of the transfer transistors are based on diffusion areas in a silicon substrate, other variations, for example trench type drain regions, are possible. 
     Elements in the accompanying drawings are schematic diagrams for demonstrative purpose and not depicted in the actual scale. The dimensions of the elements of the invention as shown should by no means be considered limitations on the scope of the invention. 
     While the invention has been described by way of example and in terms of preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims, which define the invention, should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.