Abstract:
A semiconductor structure having a body capacitance plate, which is formed with a process that assures that the body capacitance plate is self-aligned to both the source line (SL) diffusion and the bitline diffusion is provided. Thus the amount of overlap between the SL and the bitline diffusions and the body capacitance plate is precisely controlled. More specifically, the present invention forms the structure of a 1T-capacitorless SOI body charge storage cell having sidewall capacitor plates using a process that assures that there is 1) minimal overlap between plate and source/drain diffusions, and 2) that the minimal overlap obtained in the present invention is precisely controlled and is not subject to alignment tolerances. The inventive cell results in larger signal margin, improved performance, smaller chip size, and reduced dynamic power dissipation relative to the prior art.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a semiconductor structure and a method of fabricating the same. More particularly, the present invention relates to a body capacitor for a silicon-on-insulator (SOI) memory device. The present invention also provides a method for fabricating such a semiconductor structure in which the processing steps assure that there is minimal overlap capacitance between the capacitor plate and the source/drain diffusions that is controlled and is not subjected to alignment tolerances. The present invention also relates to a semiconductor memory cell layout that includes the inventive SOI body capacitor used in a folded bitline design.  
       BACKGROUND OF THE INVENTION  
       [0002]     Several one transistor (1T)-capacitorless cells that store charge in the body of a silicon-on-insulator (SOI) metal oxide semiconductor field effect transistor (MOSFET) are known in the art. Both the Fazan cell (See, for example, IEEE Electron Device Letters, Vol. 23, No. 2, February 2002) and the Toshiba cell (See, for example, IEEE Journal of Solid-State Circuits, Vol. 37, No. 11, November 2002) are examples of such cells.  
         [0003]     The Toshiba cell improves upon the Fazan cell by enhancing the body capacitance with a conducting plate coupled to the sidewall of the SOI layer through a dielectric. The plate is connected to the substrate through the buried oxide-(BOX) of the SOI. Although the Toshiba cell results in enhanced body charge, and improves the distinction between a “0” (minimum quantity of body majority carriers) and a “1” (maximum accumulation of majority carriers), due to alignment tolerances the amount of overlap between the plate-bitline diffusion and plate-source diffusion varies randomly from cell to cell, and across the chip. This random variation in overlap adds a parasitic capacitance that increases the average bitline capacitance. This increased bitline capacitance results in a combination of slower performance, increased chip area (because larger drivers are required to compensate for the larger parasitic capacitance), and increased dynamic power dissipation.  
         [0004]     In view of the above drawbacks with prior art 1T-capacitorless cells, there is a need to provide such as cell that avoids excessive overlap of the body capacitor plate with the bitline diffusion.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention provides a semiconductor structure having a body capacitor plate, which is formed with a process that assures that the body capacitor plate is self-aligned to both the source line (SL) diffusion and the bitline (BL) diffusion. Thus, the amount of overlap between the SL and the BL diffusions and the body capacitor plate is precisely controlled. Unlike the Toshiba cell, the inventive 1-T dynamic access memory (DRAM) cell minimizes the body-capacitor overlap capacitances among source and drain junctions and provides coupling nearly exclusively to the body regions to effectively hold the body charges.  
         [0006]     More specifically, the present invention forms the structure of a 1T-capacitorless SOI body charge storage cell having sidewall capacitor plates using a process that assures that there is 1) minimal overlap between plate and source/drain diffusions, and 2) that the minimal overlap obtained in the present invention is precisely controlled and is not subject to alignment tolerances. The inventive cell results in larger signal margin, improved performance, smaller chip size, and reduced dynamic power dissipation relative to the prior art.  
         [0007]     In broad terms, the semiconductor structure of the present invention comprises  
         [0008]     a silicon-on-insulator substrate comprising an upper patterned Si-containing layer located atop a buried insulating layer, said patterned Si-containing layer having source/drain diffusions located therein;  
         [0009]     a plurality of transistors, each including a wordline gate conductor, located on a surface of said patterned Si-containing layer, wherein a bitline stud which extends to an overlaying bitline is in contact with one of said source/drain diffusions;  
         [0010]     a source line located atop said patterned Si-containing layer adjacent to selected transistors, said source line is in contact with another of said source/drain diffusions; and  
         [0011]     a capacitor plate beneath each wordline gate conductor and located within said patterned Si-containing layer and extending down through said buried insulating layer, wherein said source/drain diffusions, said wordline gate conductor and said capacitor plate have edges that are aligned to each other.  
         [0012]     In the present invention, the source line (SL) is present atop one of the source/drain diffusions of a transistor and thus that diffusion region can be referred to as the SL diffusion. Also, the other diffusion region of the transistor is in contact with the bitline (BL) through the bitline stud and thus it can be referred to herein as the BL diffusion.  
         [0013]     In addition to the semiconductor structure described above, the present invention also provides a method of fabricating the same. Specifically, and in broad terms, the method of the present invention comprises:  
         [0014]     providing a structure that includes a patterned material stack located atop a patterned Si-containing layer, said patterned Si-containing layer is located on a buried insulating layer of a silicon-on-insulator substrate;  
         [0015]     forming a dielectric on exposed sidewalls of at least said patterned Si-containing layer;  
         [0016]     forming isolation regions on exposed areas of said buried insulating layer that lie adjacent to said patterned Si-containing layer;  
         [0017]     providing a recess in a portion of said isolation regions through said buried insulating layer and forming a sidewall capacitor plate in said recess;  
         [0018]     forming a plurality of transistors on said patterned Si-containing layer, each transistor comprising a wordline gate conductor and underlying source/drain diffusions; and  
         [0019]     forming a source line atop one of said diffusions and forming a bitline stud and a bitline atop said other diffusion, wherein said bitline stud separates a pair of adjacent wordline gate conductors, and said source/drain diffusions, said wordline gate conductor and said capacitor plate have edges that are aligned to each other.  
         [0020]     The present invention also relates to a memory cell layout that comprises an array of memory cells, each memory cell comprising  
         [0021]     a plurality of transistors, each including a wordline gate conductor, located on a surface of a patterned Si-containing layer of a silicon-on-insulator substrate, wherein a bitline stud which extends to an overlaying bitline is in contact with one of said source/drain diffusions;  
         [0022]     a source line located atop said patterned Si-containing layer adjacent to each transistor, said source line is in contact with another of said source/drain diffusions; and  
         [0023]     a capacitor plate beneath each wordline gate conductor and located within said patterned Si-containing layer and extending down through an underlying buried insulating layer, wherein said source/drain diffusions, said wordline gate conductor and said capacitor plate have edges that are aligned to each other, and each wordline gate conductor gates a cell in alternate crossing bitlines. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIGS. 1-20  are pictorial representation (through various views) illustrating the various processing steps that are employed the present invention for fabricating a self-aligned SOI body capacitor structure.  
         [0025]      FIG. 21  is a first exemplary layout of a portion of an array cell employing the inventive self-aligned SOI body capacitor.  
         [0026]      FIG. 22  is a second exemplary layout of a portion of an array cell employing the inventive self-aligned SOI body capacitor. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     The present invention, which provides a self-aligned SOI body capacitor and a method of fabricating the same, will now be described in greater detail by referring to the drawings that accompany the present application. The drawings are provided herein for illustrative purposes and thus they are not drawn to scale.  
         [0028]     In  FIGS. 1-20 , the structure through various processing steps is illustrated in different views. Drawing A represents a top-down view where cross-sections A-A and B-B, and in some instances C-C, are shown. Drawing B is a cross sectional view along the line A-A, Drawing C is a cross sectional view along the line B-B, and Drawing D is a cross sectional view along the line C-C. “A-A” is a cross section in a vertical cutting a plane parallel to a wordline location, and through the center of the wordline location. “B-B” is a cross section in a vertical cutting plane parallel to a bitline location, and through the center of the bitline location. “C-C” is a cross section in a vertical cutting plane parallel to a bitline location, and through a capacitor plate.  
         [0029]     The process of fabricating a self-aligned SOI body capacitor begins by first providing the structure  10  shown in  FIG. 1A-1C . The structure  10  includes a Si-on-insulator (SOI) substrate  12 , a patterned material stack  20  located on an upper surface of the SOI substrate  12  and a patterned resist  30  located on an upper surface of the patterned material stack  20 .  
         [0030]     The SOI substrate  12  includes a semiconductor layer  14 , a buried insulating layer  16  and a Si-containing layer  18 . The semiconductor layer  14  of the SOI substrate  12  comprises any semiconductor material known in the art. Illustrative examples of semiconductor materials that can be employed as the semiconductor layer  14  include, but are not limited to: Si, SiC, SiGe, SiGeC, Ge alloys, GaAs, InAs, InP as well as other III/V or II/VI compound semiconductors. Typically, the semiconductor layer  14  is a Si-containing semiconductor such as, Si, SiC, SiGe, or SiGeC. The thickness of the semiconductor layer  14  is inconsequential to the present invention.  
         [0031]     The buried insulating layer  16  is typically comprised of an oxide, nitride, oxynitride or multilayers thereof. More typically, the buried insulating layer  16  is comprised of an oxide. The thickness of the buried insulating layer  16  may vary depending on the origin of the layer. Typically, however, the buried insulating layer  16  has a thickness from about 5 to about 500 nm, with a thickness from about 50 to about 200 nm being more highly preferred.  
         [0032]     The Si-containing layer  18  of the SOI substrate  12  is comprised of a silicon containing semiconductor including, for example, Si, SiC, SiGe, or SiGeC. The Si-containing layer  18  of the SOI substrate  12  is preferably monocrystalline. The thickness of the Si-containing layer  18  may vary depending on the technique that was used in forming the SOI substrate  12 . Typically, the Si-containing layer  18  of the SOI substrate  12  has a thickness from about 2 to about 300 nm, with a thickness from about 5 to about 150 nm being more highly preferred.  
         [0033]     The crystal orientation of the semiconductor layer  14  and the Si-containing layer  18  may be the same or different, with the same crystal orientation being typical for SOI substrates made by separation of silicon by ion implantation of oxygen (SIMOX). Illustratively, the crystal orientation of layers  14  and  18  are typically chosen from (100), (110) or (111). The Si-containing layer  18  of the SOI substrate  12  can be unstrained, strained or a combination thereof.  
         [0034]     The SOI substrate  12  shown in  FIGS. 1A-1C  can be fabricated using techniques that are well known in the art. For example, the SOI substrate  12  may be formed by an ion implantation process referred to as SIMOX in which ions such as oxygen ions are implanted into a starting wafer and thereafter the ion implanted wafer is subjected to an annealing process that causes the formation of the buried insulating layer  16  within the substrate. Alternatively, the SOI substrate  12  can be formed by a layer transfer process that includes wafer bonding.  
         [0035]     The patterned material stack  20  shown in  FIGS. 1A-1C  comprises at least three layers, with a fourth layer being optional. The bottom most layer of material stack  20  is a nitride layer  22 . An optional oxide marker layer  24  may be located on the nitride layer  22 . The purpose of using the optional oxide marker layer  24  will be discussed in relation to  FIGS. 10A-10C . A polysilicon layer  26  may be located on the oxide marker layer  24 , if present, or on the nitride layer  22 . The uppermost layer of the material stack  20  comprises a nitride layer  28 .  
         [0036]     The thickness of the patterned material stack  20  may vary depending on the number of layers within the stack. Typically, the overall thickness of the patterned material stack  20  is from about 20 to about 600 nm, with a thickness from about 35 to about 300 nm being more typically. This overall thickness for the patterned material stack  20  includes a thickness for the nitride layer  22  from about 5 to about 200, preferably from about 10 to about 100 nm, a thickness for the optional oxide marker layer  24  from about 3 to about 10, preferably, from about 4 to about 8 nm, a thickness for the polysilicon layer  26  from about 5 to about 200, preferably from about 10 to about 100 nm, and a thickness for the nitride layer  28  from about 5 to about 200, preferably from about 10 to about 100 nm.  
         [0037]     The patterned resist  30  is comprised of a conventional photoresist material and its thickness is well known to those skilled in the art.  
         [0038]     The patterned material stack  20  is formed by first forming the various material layers using one or more conventional blanket deposition techniques such as, for example, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), evaporation, chemical solution deposition, or atomic layer deposition. The polysilicon layer  26  can be formed by a CVD process. In addition to deposition processes, the various insulating layers of the material stack  20  can be formed by thermal means including oxidation and nitridation. A combination of the aforementioned techniques can also be used.  
         [0039]     After forming the various layers of the material stack, a resist is applied to the uppermost layer of the material stack  20  utilizing a conventional deposition process such as, for example, spin-on coating, and then the resist is subjected to a conventional lithographic process. The lithographic process includes exposing the resist material to a pattern of radiation and developing the resist utilizing a conventional developer. After the resist has been patterned, the pattern is transferred into the material stack, stopping on the upper surface of the SOI substrate  12 , i.e., on top of the Si-containing layer  18 , utilizing one or more, preferably one, etching process. The etching process used to pattern the material stack includes a dry etching process (including reactive ion etching, ion beam etching, plasma etching or laser ablation), wet etching, or a combination thereof. Preferably, the etching used to pattern the material stack comprises anisotropic reactive ion etching wherein the chemistry of the etchant is changed to selectively etch the exposed material layer. The patterned resist  30  can, in some embodiments, be removed from the structure following the formation of the patterned material  20  utilizing a conventional resist stripping process.  
         [0040]     Prior to forming the material stack on the SOI substrate, a pre-implant step is performed in which ions (n- or p-type) are implanted through the Si-containing layer  18  and the buried insulating layer  16  stopping within the upper surface of the semiconductor layer  14 . That is, a heavily doped (on the order of about 1×10 19  atoms/cm 3  or greater) region (not shown in the drawings) can be formed at or near the interface between the buried insulating layer  16  and the semiconductor layer  14 . The dopant polarity of the heavily doped region is selected to be the same as the dopant polarity of the polysilicon body capacitor sidewall plate, to be subsequently formed and contacted to the substrate. This heavily doped region in the semiconductor layer  14  beneath the buried insulating layer  16  will serve to distribute the voltage that is applied to the sidewall plates, by enhancing conduction near the semiconductor layer  14 /buried insulating layer  16  interface.  
         [0041]     Note that in  FIGS. 1A-1C  the patterned material stack  20  protects portions of the Si-containing layer  18 , while leaving other portions of the Si-containing layer  18  exposed. Also, the patterned material stack  20  defines (i.e., covers) the active region in which the transistor of the inventive structure will be subsequently formed. The optional oxide maker layer  24  is shown in  FIGS. 1A-1C  and is not shown again in the remaining drawings.  
         [0042]     Next, and as shown in  FIGS. 2A-2C , exposed portions of the Si-containing layer  18 , not protected by the patterned material stack  20 , are then etched using the upper patterned nitride layer  28  as an etch mask. This etching step removes all the exposed portions of the Si-containing layer  18 , stopping on the upper surface of the buried insulating layer  16 . The etching step is performed utilizing an etching process such as anisotropic reactive ion etching that selectively removes the Si-containing layer  18 . If the patterned resist  30  was not previously stripped from the structure, it can be removed following this etching process.  
         [0043]     Note that in  FIGS. 2A-2C  the Si-containing layer  18  is patterned such that the sidewalls thereof all substantially aligned with the sidewalls of the patterned material stack  20 . Moreover, in  FIGS. 2A-2C , portions of the buried insulating layer  16 , not directly beneath the patterned material stack  20 , are now exposed.  
         [0044]     A dielectric  32  is then formed on the exposed sidewalls of the etched Si-containing layer  18  as well as the exposed sidewalls of the polysilicon layer  26 . The dielectric  32  serves as the insulator between the sidewall plate (to be subsequently formed) of the body capacitor and the remaining Si-containing layer  18  (which will form the body of a MOSFET). The dielectric  32  may comprise SiO 2 , silicon oxynitride, or a high k material (k greater than 4.0, preferably greater than 7.0) such as, for example, Al 2 O 3  or Ta 2 O 5 .  
         [0045]     The dielectric  32 , which can be formed by a variety of techniques including deposition (such as, for example, CVD or PECVD) or thermal (such as oxidation or oxynitridation), has a thickness from about 2 to about 20 nm. More typically, the dielectric  32  has a thickness from about 3 to about 6 nm. The structure including the dielectric  32  that is formed on the exposed sidewalls of the remaining Si-containing layer  18  and the polysilicon layer  26  is shown, for example, in  FIGS. 3A-3C .  
         [0046]      FIGS. 4A-4C  show the structure that is formed after an insulating material  34 , such as an oxide, is formed over the surface of the exposed buried insulating layer  16  and atop the patterned material stack  20 . Any conventional deposition process such as CVD or PECVD can be used to form a layer of the insulating material  34 .  
         [0047]     After forming the insulating material  34 , the insulating material  34  is planarized so that an upper surface of the insulating material  34  is coplanar with the upper surface of the polysilicon layer  26  of the patterned material stack  20 . That is, a planarization process such as chemical mechanical polishing (CMO) and/or grinding is used to provide a structure as shown in  FIGS. 5A-5C  in which the upper surface of the insulating material  34  is substantially planar to the upper surface of the polysilicon layer  26 . It is noted that the planarization process used in this step of the present invention removes the upper nitride layer  28  of the patterned material stack  20 . As shown in  FIGS. 5A-5C , isolation regions  34  are formed adjacent to the remaining portions of layers  26 , optionally  24 ,  22  and layer  18 .  
         [0048]     A pad nitride layer  36  is then deposited by conventional techniques over the planarized structure shown in  FIGS. 5A-5C  providing the structure shown, for example, in  FIGS. 6A-6C . The pad nitride layer  36  is a relatively thick layer since it determines the height of the wordline wiring to be subsequently formed. Typically, the pad nitride layer  36  has a thickness from about 30 to about 150 nm.  
         [0049]     Stripes (i.e., openings or troughs)  38  are then formed through the pad nitride layer  36  stopping on the upper surface of the polysilicon layer  26  in the active region (see  FIG. 7C ) and on the surface of insulating material  34  in the region that lies to the periphery of the active region (see  FIG. 7B ). The stripes  38  are formed by lithography and etching. The etching step is performed by utilizing an anisotropic reactive ion etch for silicon nitride that is selective to silicon and silicon oxide. The stripes  38  define the location of the sidewall capacitor plates, which will be self-aligned to the body (i.e., the remaining Si-containing layer  18 ), and the location of the wordline gate conductors. Both the sidewall capacitor plates and the wordlines are self-aligned to each other since they will be defined by and registered to the same structural features (i.e., through the opening in the pad nitride layer  36 ). The structure including the stripes  38  is shown, for example, in  FIGS. 7A-7C .  
         [0050]     The exposed surfaces of the insulating material  34  outside of the active region shown along line A-A are then etched by a dry etching process such as anisotropic reactive ion etching through layer  34  and the underlying buried insulating layer  16  stopping on the upper surface of semiconductor layer  14 . The etching process used in this step of the present invention, which provides the structure shown in  FIG. 8A-8C , is selective to silicon and silicon nitride. In  FIG. 8B , reference numeral  40  is used to define the recessed region formed by this step of the present invention; note that no etching occurs in the active area during this step of the present invention, since it is protected by either polysilicon layer  26  or pad nitride  36 .  
         [0051]     The recessed region  40  and the stripes  38  are then filled with a conductor  42  and then the conductor  42  is planarized to the upper surface of pad nitride  36  providing the structure shown in  FIGS. 9A-9C . The filling process includes any conventional deposition process, while planarization is performed utilizing CMP and/or grinding. The conductor  42  comprises a metal, a metal alloy, a metal silicide, polysilicon or a combination thereof. Preferably, polysilicon is used as the conductor  42 . When polysilicon is used as the conductor  42 , it is typically formed using an in-situ doping deposition process. The polysilicon is doped with the same dopant polarity as the buried layer that was previously formed into the semiconductor layer  14  as described in connection with the structure shown in  FIGS. 1A-1C . Preferably, the dopant polarity of the polysilicon conductor  42  and of the deep implant region described above in  FIGS. 1A-1C  are P-type since a P-type workfunction for the capacitor plate will result in maximum body hole concentration, for the nMOSFETs in this exemplary embodiment.  
         [0052]     The exposed conductor  42  is then etched selective to nitride and oxide, recessing the top surface of the conductor  42  slightly below the top surface of the Si-containing layer  18 . See, for example, the structure shown in  FIG. 10B . In this step, all the exposed conductor  42  over the active region as well as portions of the polysilicon layer  28  are removed which exposes a top surface of nitride layer  22 . See, for example, the structure shown in  FIG. 10C . In embodiments in which the optional oxide marker  24  is present in the structure, and when that layer is reached during the course of etching the conductor  42  in the active region, the oxide signature is detected to indicate that the remaining amount of conductor  42  to be etched is slightly thicker than the lower nitride layer  22 . The use of the optional oxide marker layer  24  results in excellent control of the depth of the conductor recess below the top surface of the Si-containing layer  18 . The recessed conductor forms the sidewall plate conductors  42 ′.  
         [0053]     Next, and as shown in  FIGS. 11A-11B , an oxide layer  44  having a precisely controlled thickness is formed over the top surface of the recessed conductor  42 ′. This oxide layer serves as an insulating layer between the body capacitor plate and the subsequently formed overlying wordline conductor. The oxide layer  44  is formed utilizing an oxidation process to a thickness from about 5 to about 20 nm. The surface of the Si-containing layer  18  in the active region (See,  FIG. 11C ) is protected during this thermal oxidation step by the nitride layer  22 . Oxide layer  44  is however formed on the exposed sidewalls of polysilicon layer  26  in the active region.  
         [0054]     The exposed nitride layer  22  in the active region that overlays the Si-containing layer  18  is then removed, preferably with an anisotropic etch to minimize undercutting of the nitride layer  22  elsewhere on the Si-containing layer  18 . At this point in the process, channel doping (not shown) may be introduced into the exposed portions of the Si-containing layer  18  using conventional ion implantation techniques. A screen oxide (also not shown) may optionally be formed prior to the implantation process and thereafter removed. The resultant structure that is formed after the foregoing step of removing the exposed nitride layer  22  overlaying the Si-containing layer  18  is shown in  FIGS. 12A-12C , for example. Reference numeral  45  is used to denote the openings formed during this step of the present invention.  
         [0055]     A transfer gate dielectric  46  is then formed on the exposed surface of Si-containing layer  18  through opening  45 . The transfer gate dielectric  46  comprises an oxide, oxynitride or a high k (k greater than 4.0, preferably greater than 7.0) dielectric. The thickness of the transfer gate dielectric  46  is typically from about 1 to about 10 nm. Any conventional deposition process or thermal process can be used to form the transfer gate dielectric  46 .  FIGS. 13A-13C  show the structure after the transfer gate dielectric  46  has been formed.  
         [0056]      FIGS. 14A-14C  shows the structure after wordline gate conductor  48  and an insulating cap  50  are formed. Specifically, the wordline gate conductor  48  is formed through the opening  45  atop the transfer gate dielectric  46  by deposition. Following the deposition process, the wordline gate conductor  48  is planarized to the upper surface of the pad nitride layer  36 . The planarized wordline gate conductor  48  is then recessed utilizing a timed etching process such as reactive ion etching. The wordline gate conductor  48  is comprised of a conductive material including, for example, a metal, metal alloy, metal silicide, polysilicon or any combination thereof. The insulating cap  50 , typically an oxide, is then deposited and planarized utilizing conventional processes well known in the art.  
         [0057]     The exposed pad nitride layer  36  is then removed utilizing a well known etching method such as, for example, hot phosphoric acid or reactive ion etching, exposing the polysilicon layer  26  and insulating material  34 . The resultant structure is shown in  FIGS. 15A-15C .  
         [0058]     The exposed polysilicon layer  26  over the active region is then removed by etching, e.g., reactive ion etching, thus exposing the nitride layer  22  over the active region. The structure formed after this step of the present invention has been performed is shown, for example, in  FIGS. 16A-16C .  
         [0059]     At this point of the inventive process, an oxide spacer  52  is formed on each sidewall of the wordline conductor  48 . The purpose of the spacer  52  is to slightly widen the footprint of the wordline to avoid any possibility of etching into the seam between the wordline and the oxide layer  44  over the body capacitor plate  42 ′ when the nitride layer  22  is subsequently removed. If the spacer  52  was not present at this time, there would be a remote possibility that the etch to remove the lower nitride layer  22  may damage the protective oxide layer  44  between the wordline  48  and the body capacitor plate  42 ′. The oxide spacer  52  is formed using well known methods such as deposition of a conformal CVD oxide film, followed by an anisotropic etch.  FIGS. 17A-17C  show the structure including the oxide spacer  52 .  
         [0060]     The exposed lower nitride layer  22  over active Si-containing layer  18  is then removed, for example, by reactive ion etching, stopping on the Si-containing layer  18  surface outside of the wordlines  48 . The resultant structure in shown, for example, in  FIGS. 18A-18C . A silicon nitride etch chemistry is typically employed to avoid erosion of the insulating material  34  and the insulating cap  50 . Conventional processing follows, with formation of oxide spacers (if not previously formed), and source/drain implantation. Since the sidewall plates  42 ′ and the source/drain diffusions  54  to be subsequently formed in the next processing step (See  FIGS. 19A-19C ) are each separately self-aligned with the wordline troughs  38 , they are self-aligned with each other. The self-alignment of the sidewall plates  42 ′ and the source/drain diffusions  54  is not taught or suggested in any prior art SOI body capacitor known to the applicants of the present invention.  
         [0061]     Source-drain implants (including any extensions and halos) and anneals are now done forming at least the source-drain diffusions  54  shown in  FIGS. 19A-19C .  
         [0062]      FIGS. 20A-20D  exemplify a typical layout of a portion of the 1T-capacitorless memory array with self-aligned sidewall body plates  42 ′. Specifically, in  FIGS. 20A-20D , a structure including two transistors that share a common bitline diffusion  54 BL in the memory array is illustrated. The source line diffusion shown in these drawings are labeled as  54 SL.  
         [0063]     The structure shown in  FIGS. 20A-20D  includes source-line (SL) conductors  56  that are formed adjacent to selected wordlines using well known metal deposition and damascene processes. An interlayer dielectric (ILD, not shown) is then typically deposited. Bitline studs  58  are formed to selected diffusions, and bitline conductors  60  are formed using standard methods. Higher level insulating and wiring layers are then formed to complete the fabrication of the chip. Several specific layouts and technology options for a memory array employing the self-aligned body-capacitor will be shown next. It is noted that in the present invention, the wordlines lie perpendicular to the bitlines and arrays of transistors are arranged in rows and columns.  
         [0064]     Note that a new cross-section (C-C) has been added in  FIG. 20 . C-C is a cut parallel to B-B but passing through the capacitor plate  42 ′ of the self-aligned body capacitor. A phantom of the SOI region (i.e., Si-containing layer  18 ) and source-drain diffusions  54  has also been incorporated into cross-sectional view C-C to illustrate the location of the capacitor plate  42 ′ with respect to the diffusions  54  in the Si-containing layer  18 , as well as the wordline conductor  48 . It is emphasized that the edges of the capacitor plate  42 ′, the wordline conductor  48 , and the diffusions  54  are all self-aligned and are not subject to alignment tolerance.  
         [0065]     The above discussion, in reference to  FIGS. 1-20 , illustrates the various processing steps that are used in fabricating the inventive self-aligned SOI body capacitor structure shown in  FIG. 20 .  FIGS. 21 and 22  illustrate some cell layouts that can include the inventive structure shown in  FIG. 20 . In  FIGS. 21 and 22 , like reference numerals are used to describe the components that are present in the structure shown in  FIG. 20 .  
         [0066]     A first exemplary layout of a portion of an array of cells employing the inventive self-aligned SOI body capacitor illustrated in  FIG. 20  is shown in  FIG. 21 . Specifically,  FIG. 21  illustrates an open bitline architecture design where each wordline  48  gates a cell in each crossing bitline  60 . Locations of the sidewall capacitor plate  42 ′, self-aligned to the wordlines and diffusions, are delineated by the dashed line rectangular regions. This embodiment defines continuous stripes of SOI (running horizontally in  FIG. 21 ). Rows of SOI stripes are separated by the inventive capacitor plate and regions of STI. Bodies of adjacent MOSFETs sharing the same SOI stripe are isolated by source-drain diffusions, which extend all the way to the BOX (as previously described by the process flow). If the technology were designed to use shallow source-drain diffusions that extend only partly to the BOX body charge of adjacent MOSFETs would not be isolated. In that case STI would be needed to provide isolation between bodies of MOSFETs sharing a common bitline. The case of source-drain diffusions extending partly to the BOX is discussed in  FIG. 22 .  
         [0067]     Specifically,  FIG. 22  illustrates a second exemplary layout of a portion of an array of cells employing the inventive self-aligned SOI body capacitor. In the folded bitline architecture design shown in  FIG. 22 , each wordline  48  gates a cell in alternate crossing bitlines. Locations of the sidewall capacitor plate, self-aligned to the wordlines and diffusions  60 , are delineated by the-dashed line rectangular regions. Regions of SOI (i.e., the active Si-containing layer  18 ) are indicated by rectangles bordered by dotted lines. These regions are labeled with reference numeral  100  in  FIG. 22 . A plurality of isolation regions (i.e., STIs) isolates SOI regions that lie under each bitline. Each one of the plurality of isolated SOI regions contains a single MOSFET with a single bitline contact diffusion on a first end and a single source line (SL) contact diffusion on a second end of said isolated SOI region. Corresponding SOI regions in adjacent rows jog laterally by 1 minimum feature dimension (1F), thus resulting in the folded bitline layout. Since this second exemplary layout uses intervening STI to isolate bodies of MOSFETs sharing a common bitline, there is no restriction on the source-drain diffusion depth. No art has been found which illustrates a folded bitline layout for a body-charge storage type of memory device. All the known art for this type of cell employ continuous stripes of SOI, thus necessitating deep diffusions. It may be advantageous to employ shallow diffusions for improved scalability.  
         [0068]     While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the spirit and scope of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.