Abstract:
The present invention relates to a memory cell with a memory capacitor ( 110 ) on an active semiconductor region ( 104 ), the memory capacitor having a first capacitor-electrode layer, which, in a cross-sectional view of the memory cell, has first ( 218.1 ) and second ( 218.2 ) electrode-layer sections that extend on the active semiconductor region in parallel to the surface of the active semiconductor region at a vertical distance to each other and that are electrically connected by a third electrode-layer section extending vertically, that is, perpendicular to the surface of the active semiconductor region. A control transistor ( 112 ) is connected with a conductive second capacitor electrode layer that extends between the first and second electrode-layer sections and is electrically isolated from them by an isolation layer ( 116 ). Achieved advantages comprise a high manufacturing yield can, reduced fabrication cost and reduced risk of junction leakage by a small area required for the memory cell.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a memory cell and to a memory device. 
       BACKGROUND OF THE INVENTION 
       [0002]    W. Mueller et. al., in “Challenges for the DRAM Cell Scaling to 40 nm”, IEDM Tech. Dig., 2005, pp. 336-339, give a review on concepts for scaling DRAM memory cells down to 40 nm. Technology issues discussed for the DRAM capacitor concern trench capacitors on one hand and stack capacitors on the other hand. The scaling of a trench capacitor at constant cell capacitance is described as requiring an increase of the aspect ratio to a value of up to 120 and/or the use of alternative high-k dielectric materials. Similar requirements are reported to hold for stack capacitor structures. 
         [0003]    A disadvantage of employing trench capacitors with high aspect ratios in sub-100 nm technologies is a lower manufacturing yield. A further disadvantage is an increase of manufacturing costs. Furthermore, the use of novel high-k materials for dielectric layers is problematic with respect to the process temperatures used in a typical CMOS process. Furthermore, the scaling of trench and stack capacitors bears [bears down?] an increased risk of junction leakage, which may show a negative impact on the memory effect. 
         [0004]    A further technological issue of scaling DRAM memory cells relates to the cell transistor. When scaling the cell transistor gate length below 100 nm, the doping level required to meet the off-current criterion of I off &lt;1 fA gets so high that the electric field at the node junction may exceed 0.5 MV/cm, thus initiating trap-assisted tunneling leakage. 
         [0005]    It would be desirable to provide a memory cell structure and a memory device that allow a high manufacturing yield at low cost and that mitigate or completely remove the mentioned problems of a scaled memory cell suitable for DRAM. 
       SUMMARY OF THE INVENTION 
       [0006]    According to a first aspect of the present invention, a memory cell is provided, comprising:
       a semiconductor substrate with an active semiconductor region, which is laterally defined on a surface of the semiconductor substrate by isolation regions adjacent to the active semiconductor region,   a control transistor having semiconductor transistor-electrode regions within lateral bounds of the active semiconductor region and the isolation regions, and   a memory capacitor on the active semiconductor region, the memory capacitor having a first capacitor-electrode layer, which, in a cross-sectional view of the memory cell, has first and second electrode-layer sections that extend on the active semiconductor region in parallel to the surface of the active semiconductor region at a vertical distance to each other and that are electrically connected by a third electrode-layer section extending vertically, that is, perpendicular to the surface of the active semiconductor region,   wherein the control transistor is connected with a conductive second capacitor electrode layer that extends between the first and second electrode-layer sections and is electrically isolated from them by an isolation layer.       
 
         [0011]    The memory cell of the first aspect of the present invention overcomes the mentioned disadvantages of the prior-art memory cell structures. Since no fabrication of deep trenches is required for the memory capacitor, a high manufacturing yield can be achieved. 
         [0012]    The memory cell can further be fabricated without addition of a critical mask level in comparison to known processes. This keeps the manufacturing cost low. The risk of junction leakage is reduced by a small area of the junction. Furthermore, standard back-end interconnect processing is enabled in comparison with known memory cells comprising stack capacitors. The close contact between the structural elements of memory cell and the semiconductor substrate enables an efficient temperature dissipation under operation and in the manufacturing process, which reduces the relevance of the earlier mentioned temperature issues associated with the described prior-art technologies. 
         [0013]    The memory cell of the first aspect of the present invention therefore is particularly suitable for memory devices with highly scaled structural elements, such as embedded DRAM (eDRAM), as used for instance in CMOS technology nodes employing transistor-gate lengths below 100 nm, and even at 65 nm and below. 
         [0014]    The memory cell of the invention has the further advantage that trap-assisted tunneling leakage can be reduced. The junction leakage is linked to junction area. As the capacitor surrounds the drain junction, the leakage path is drastically reduced. 
         [0015]    In the following, embodiments of the memory cell of the invention will be described. Unless stated otherwise or obvious from the description, the described embodiments can be combined with each other to form further embodiments. 
         [0016]    The specific shape of the memory capacitor on the active semiconductor region is characterized by its “planar” (as opposed to trench or stacked architectures) three-dimensional memory capacitor with a particular shape of the first capacitor-electrode layer, as described above with reference to the first, second and third electrode-layer sections of this first capacitor-electrode layer. 
         [0017]    Within the framework of the above definition, several embodiments of the structure of the first electrode-layer are possible. In one embodiment, the first electrode-layer has a shape, which in the mentioned cross-sectional view resembles the upper-case letter U turned on its side. That means, the first and second electrode-layer section of the first capacitor-electrode layer form longitudinal bars of the letter U, which correspond to the vertical bars of the upright letter U. The third electrode-layer section forms the “connecting piece” between them, which in an upright letter U forms the horizontal bar. That is, in this embodiment the letter U rests on one of its longitudinal bars. The two longitudinal bars of the first capacitor-electrode layer extend in parallel to the surface of the active semiconductor region, along their longitudinal direction in the mentioned cross-sectional view. 
         [0018]    The longitudinal U-bars need not have the same length, or, in other words, longitudinal extension in the cross-sectional view under consideration. The longitudinal extension of the first and second electrode-layer sections can differ to the point where the shape of the first electrode layer resembles the upper-case letter J turned on its side. For the purpose of definition, an embodiment can be considered to have a J-shape, if the ratio of the longitudinal extensions of the longer to the shorter electrode-layer section of the first and second electrode-layer sections is larger than 2, but normally smaller than 100. Suitably, for the J-shape, the ratio is smaller than 10, preferably even smaller than 5. If the ratio is smaller than 2, the memory capacitor is considered to have a U-shape for the purpose of the present definition. A perfect U-shape is present if the value of the ratio equals 1. 
         [0019]    The J-shape is considered to be different from an L-shape, for which the mentioned ratio would be mathematically infinite. For the purpose of clarification, the memory-capacitor embodiment with the shape of the letter J turned on its side has first and second electrode-layer sections parallel to each other, one of them corresponding to the long vertical bar of the upright upper-case letter J, and the other one corresponding to the short bar of the upright upper-case letter J. Both are connected through the third electrode-layer section. In contrast, the upright upper-case letter L has no short second electrode-layer section parallel to a long first electrode-layer section, i.e., to the vertical L bar, but only a horizontal (third) electrode-layer section. The J-shape and the U-shape of the memory-capacitor in the present invention allows the previously mentioned reduction of the junction area. This is not the case for the L shape. 
         [0020]    With the memory capacitor according to the memory cell of the first aspect of the present invention, an optimization of the capacitance value of the memory capacitor in relation to its geometrical intension is achieved in comparison with an L-shaped memory capacitor. Of course, the present comparison with the shapes of upper-case letters serves primarily to support the intuitive understanding of the geometrical structure of the embodiments. No typographical details like the presence or absence of serifs of printed letters are taken into consideration for the purpose of the present discussion of the geometrical shape of the memory capacitor in the cross-sectional view. 
         [0021]    Given otherwise identical geometrical parameters, an embodiment with a U-shape will have a larger capacitance than one with a J-shape. The capacitance value is defined by the common surface between the first and second capacitor electrode layer. Since the first capacitor-electrode layer is segmented into three electrode-layer sections, it is the common surface shared by the second capacitor-electrode layer and the three electrode-layer sections that is to be considered. The larger longitudinal extension of the shorter electrode-layer section in comparison with the longer electrode-layer section in the J-shape, the higher is the capacitance of the memory capacitor, provided no other geometrical parameters of the capacitor structure are changed. 
         [0022]    However, aspects of technological feasibility may additionally influence the selection of a particular capacitor shape for a particular memory-cell structure. 
         [0023]    Suitably, to make fabrication easier, the electrode-layer section with the shorter longitudinal extension is that with a larger vertical distance to the surface of the active semiconductor region. Note, however, that in some embodiments, the electrode-layer section with a larger vertical distance from the surface of the active semiconductor region has, in the cross-sectional view, a longer longitudinal extension than the electrode-layer section that is closer to the surface of the active semiconductor region. 
         [0024]    In preferred embodiments, the memory cell has a semiconductor intermediate layer, which is arranged on the surface region in the active semiconductor region. 
         [0025]    Furthermore, a semiconductor electrode layer is arranged on the intermediate layer and comprises at least one of the transistor-electrode regions. Furthermore, the semiconductor electrode layer continues laterally into the second capacitor electrode layer. In one form of this embodiment, the semiconductor electrode layer and the second capacitor layer form adjacent regions of the same semiconductor layer. An advantage of the present embodiment is that it is particularly easy to implement into existing CMOS and BiCMOS technologies. 
         [0026]    In the structure of this embodiment, the semiconductor intermediate layer is preferably made of a semiconductor material, which is selectively removable with respect to the semiconductor substrate. A suitable choice of materials is for instance silicon for the substrate material and the second capacitor electrode layer, and silicon-germanium SiGe for the semiconductor intermediate layer. 
         [0027]    The control transistor is typically a MOSFET having source and drain regions arranged as transistor-electrode regions in the semiconductor electrode layer. In this embodiment, which is based on the structure of the previous embodiment using the intermediate layer on the surface region in the active semiconductor region, the drain region continues laterally into the second capacitor-electrode layer. This means, the semiconductor electrode layer contains the source and drain regions of the control transistor as well as the second capacitor-electrode layer in different sections. This embodiment is particularly compact. 
         [0028]    The control MOSFET of this memory-cell embodiment preferably has a gate-electrode layer, which is made of an electrically conductive material which may be the same as the material of the first capacitor-electrode layer. For this embodiment, the first capacitor-electrode layer and the gate-electrode layer can be deposited in the same deposition step. 
         [0029]    Later on, they can be separated by patterning, as will be described in the context of preferred embodiments of the method of the third aspect of the invention with respect to the Figures. 
         [0030]    However, in an alternative embodiment, the MOSFET has a gate-electrode layer, which is made of an electrically conductive material, which is different from that first capacitor-electrode layer. This embodiment requires an additional mask for the separate definition of the gate-electrode layer and the first capacitor-electrode layer. It has the advantage, that the materials compositions of theses layers can be tailored independently according to the needs of a particular application. A second aspect of the present invention is formed by a memory device comprising a plurality of memory cells according to the first aspect of the invention or one of its embodiments. 
         [0031]    The memory device of the second aspect of the invention shares the advantages of the memory cell of the first aspect of the invention. It can be produced with a high manufacturing yield, at low cost, with particularly favorable temperature-dissipation properties in a bulk substrate. 
         [0032]    In the following, embodiments of the memory device of the second aspect of the invention will be described. As before, embodiments of the memory device can be combined with each other, to form additional embodiments, unless stated otherwise or obvious from the description as forming alternatives. 
         [0033]    In typical embodiments, the memory cells are provided in a matrix arrangement, each memory cell being connected to a respective unique combination of word and bit lines via two of its transistor-electrode regions. For instance, the respective word line is connected with the gate electrode of the control transistor of the memory cell, and the respective bit line is connected with the source electrode of the same control transistor of the same memory cell. 
         [0034]    The advantage of the memory cell of the first aspect of the invention is reflected by several alternative memory architectures forming embodiments of the memory device of the second aspect of the invention. 
         [0035]    In one architecture, adjacent memory cells of the matrix arrangement are provided in a back-to-front arrangement in a direction parallel to the bit lines. For the purpose of definition, the third electrode-layer section is to be considered the back side and the control transistor is considered as forming the front side of a memory cell. 
         [0036]    In an alternative architecture, adjacent memory cells of the matrix arrangement are in a back-to-back arrangement in a direction parallel to the bit lines. Here, the same definitions of back and front applies. 
         [0037]    The memory cells in back-to-back architectures can either be immediately connected through respective first capacitor-electrode layers, or separated from each other by a lateral distance, which can be filled with an isolating material. In this embodiment, the regions facing each other back-to-back are suitably arranged on the isolation regions that define adjacent active semiconductor regions, i.e., the shallow trench isolations. 
         [0038]    According to a third aspect of the invention, a method for fabricating a memory cell is provided. The method comprises the steps
       providing a semiconductor substrate with an active semiconductor region, which is laterally defined on a flat surface region of the semiconductor substrate by isolation regions adjacent to the active semiconductor region,   fabricating a memory capacitor on the active semiconductor region with a first capacitor-electrode layer, which, in a cross-sectional view of the memory cell, has first and second electrode-layer sections that extend on the active semiconductor region in parallel to the surface of the active semiconductor region at a vertical distance to each other and that are electrically connected by a third electrode-layer section extending vertically, that is, perpendicular to the surface of the active semiconductor region, and with a second capacitor electrode layer that extends between the first and second electrode-layer sections and is electrically isolated from them by an isolation layer;   fabricating a control transistor having semiconductor transistor-electrode regions within lateral bounds of the active semiconductor region and the isolation regions,   wherein fabricating the control transistor comprises providing a connection of the control transistor with the second capacitor electrode layer.       
 
         [0043]    The method of the third aspect of the invention shares the advantages of the memory cell of the first aspect of the invention. 
         [0044]    The expression “within lateral bounds of the active semiconductor region and the isolation regions” refers to a lateral region that is limited by lateral side faces of the isolation regions facing away from the active semiconductor region. 
         [0045]    In the following, embodiments of the method can be combined with each other, to form additional embodiments, unless stated otherwise or obvious from the description as forming alternatives. 
         [0046]    In one embodiment, fabricating the memory capacitor comprises:
       depositing a layer stack of a first semiconductor layer on the surface of the active region and a second semiconductor layer on the first semiconductor layer, the material of the first semiconductor layer being chosen such that it is selectively removable without removing the second semiconductor layer and the material of the active semiconductor region;   selectively removing a section of the first semiconductor layer, the section corresponding to the extension of one of the first and second electrode-layer sections of the first electrode layer, thus forming a tunnel section between the surface of the active semiconductor region and the side of the second semiconductor layer facing the surface of the active semiconductor region;   depositing an isolation layer on the surface of the active semiconductor region and on the exposed surface of the second semiconductor layer;       
 
         [0050]    depositing a gate layer on the oxide layer, thereby also filling the tunnel section; 
         [0051]    patterning the gate layer on top of the second semiconductor layer to separate a transistor gate from on the other one of the first and second electrode-layer sections of the first electrode layer, wherein the patterning includes protecting the third electrode-layer section from removal. 
         [0052]    Preferred embodiments of the invention are also defined in the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0053]    The foregoing and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. In the following drawings 
           [0054]      FIG. 1  shows a schematic cross-sectional view of a memory cell according to a first embodiment of the invention. 
           [0055]      FIGS. 2 to 7  show schematic cross-sectional views of a memory cell according to a second embodiment at different stages of its fabrication, for illustrating an embodiment of a process flow according to an embodiment of the fabrication method of the invention. 
           [0056]      FIGS. 8 and 9  show schematic top and cross-sectional views of a memory cell according to a fourth embodiment of the invention. [It is always the same structure  FIG. 8  corresponds to a top view of the first architecture] 
           [0057]      FIG. 10  shows a memory device with a back-to-front architecture. 
           [0058]      FIGS. 11 to 13  show top and sectional views of a memory device with a first back-to-back architecture. 
           [0059]      FIGS. 14 to 16  show top and sectional views of a memory device with a second back-to-back architecture 
           [0060]      FIG. 17  is a top view of a memory device for illustrating the cell area required by a memory cell. [in case of the second back to back architecture] 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0061]      FIG. 1  shows a schematic cross-sectional view of a memory cell  100  according to a first embodiment of the invention. The memory cell  100  is provided on a silicon substrate  102 . On the silicon substrate active semiconductor regions  104  are laterally defined by shallow trench isolation (STI) regions  106 , as is well known from CMOS technology.  FIG. 1  only shows a section of the substrate  102 . The section comprises one active semiconductor region, which has a substantially flat surface  108 . The substrate  102  is only shown to a depth that does not exceed the depth of the shallow trench isolation. Note that the graphical representation of the memory cell in  FIG. 1  is purely schematic and not meant to reflect geometrical proportions adequately. 
         [0062]    On the active semiconductor region  104 , the active components of the memory cell  100  are provided. That is, a memory capacitor  110  and a control MOSFET  112  are arranged on the active semiconductor region. Note that the surface of the active semiconductor region  104  and of the shallow trench isolation regions  106  are flat, implying that the memory cell is built on the substrate  102  without a need to pattern the substrate surface other than by providing the STI regions  106 . 
         [0063]    The memory capacitor  110  extends on the active semiconductor region and on a part of the shallow-trench isolation region  106 . However, the memory capacitor  110  does not extend over the whole lateral extension of the active semiconductor region  104 . In the present embodiment slightly less than half of active semiconductor region, as seen in the cross-sectional of  FIG. 1 , is covered by a semiconductor intermediate layer  114 . The semiconductor intermediate layer  114  is made of silicon germanium SiGe in the present embodiment. The memory capacitor  110  is separated from the silicon substrate  102  by an isolation layer  116 . 
         [0064]    The shape of the memory capacitor  110 , as seen in the cross-sectional view of  FIG. 1 , resembles that of the letter U turned on its side. More specifically, the memory capacitor  110  has a first capacitor-electrode layer  118 , which, in the cross-sectional view of  FIG. 1 , has a first electrode-layer section  118 . 1  next to the surface  108  of the active semiconductor region  104 . A second electrode-layer section  118 . 2  is arranged in parallel to the first electrode-layer section  118 . 1  at a vertical distance. The first and second electrode-layer sections  118 . 1  and  118 . 2  correspond to the longitudinal bars of the letter U. Note that the first electrode-layer section  118 . 1  has a larger longitudinal extension than the second electrode-layer section. The first and second electrode-layer section  118 . 1  and  118 . 2  of the first capacitor-electrode layer  118  are connected by a vertical third electrode-layer section  118 . 3 , which corresponds to the bottom section of a upright capital letter U. 
         [0065]    The vertical distance between the first and second electrode-layer sections  118 . 1  and  118 . 2  is filled with a second capacitor-electrode layer  120  and an insulating layer that electrically isolates the first and second capacitor electrode layers from each other. That is, the second capacitor-electrode layer fills the space between the longitudinal bars of the letter U formed by the first capacitor-electrode layer  118 . Regarding materials of the first and second capacitor electrode layers  118  and  120 , the first capacitor-electrode layer  118  can be form of a metal or a suitably doped semiconductor material. The second capacitor-electrode layer  120  is formed of doped semiconductor material. 
         [0066]    As is visible from  FIG. 1 , the second capacitor electrode layer  120  forms an integral part of a semiconductor electrode layer  122 , which comprises source and drain regions  124  and  126  of the control transistor  112 . The source and drain regions  124  and  126  are separated by a channel region  128  of the semiconductor electrode layer  122 . The semiconductor electrode layer  122  is arranged on the semiconductor intermediate layer  114  and on the stack formed by the isolation layer  116  and the first electrode-layer section  118 . 1  of the first capacitor-electrode layer. Accordingly, the thickness of the semiconductor intermediate layer  114  and of this mentioned layer stack is equal. The semiconductor electrode layer  122  is in the present embodiment made of silicon, which is suitably doped in the source and drain regions  124  and  126  and in the second capacitor-electrode layer  120 . The conductivity type of the doped regions  124 ,  126  and  120  is identical. In the present embodiment, the conductivity type is n-conductivity. 
         [0067]    On top of the semiconductor electrode layer  122 , a gate stack  130  is provided. 
         [0068]    The gate stack comprises a gate insulation layer  132  and a gate electrode layer  134 . In the present embodiment, the gate electrode layer is made of the same material as the first capacitor-electrode layer  118 . However, the materials are different in other embodiments, which allow tailoring the gate electrode and the capacitor according to specific application requirements. This individual tailoring of the gate and capacitor-electrode materials, however, requires some additional processing involving an additional mask. Insulating lateral spacers  136  and  138  are arranged on the lateral sidewalls of the gate stack  130 . Similarly, lateral spacers  140  and  142  are arranged on the sidewalls of the first capacitor-electrode layer  118 . 
         [0069]    Further structural details for electrically connecting the memory cell  100  to other circuit elements of a memory device are omitted in  FIG. 1  for reasons of simplicity of the graphical representation. However, contacting of the memory cell  100  will be addressed in the context of  FIGS. 9 ,  10  and of other embodiments. 
         [0070]      FIGS. 2 to 7  show schematic cross-sectional views of a memory cell  200  according to a second embodiment at different stages of its fabrication. The structure of the memory cell  200  strongly resembles that of the memory cell  100  of  FIG. 1 . The fabrication method described in the following can therefore also be applied for fabricating the memory cell  100  and for the other memory-cell embodiments presented herein. Minor differences in the structure of the memory cell  200  in comparison with that of  FIG. 1  are limited to the memory capacitor and will be discussed later with reference to  FIG. 7 . 
         [0071]    Reference numerals used for the description of  FIGS. 2 to 7  resemble those of corresponding structural elements of the memory cell  100  of  FIG. 1 . The only difference lies in the first digit of the reference labels, which is “2” instead of “1”, for respective structural elements of the present embodiment corresponding to those of the memory cell  100  of  FIG. 1 . 
         [0072]    The fabrication of memory cell  200  starts with providing a semiconductor substrate  202  with shallow-trench isolation (STI) regions  206  to define active semiconductor regions  204 . The semiconductor substrate  202  is a silicon wafer. On the surface of the active semiconductor regions  204  of the silicon wafer  202  a layer stack of a semiconductor intermediate layer  214  and a semiconductor electrode layer  222  is deposited, typically using an epitaxial deposition technique such as metal organic vapor phase epitaxy (MOVPE). The semiconductor intermediate layer is made of SiGe and the semiconductor electrode layer  222  is made of silicon. 
         [0073]    Subsequently, a mask  223  is deposited and patterned. The mask covers the layer stacks  214 ,  222  and at a section of the shallow trench isolation region  206  to the left of the active semiconductor region  206 . The STI region, which in  FIG. 1  appears to right of the active semiconductor region  204 , is not covered by the mask  223 . This allows lateral access of an etching agent to the semiconductor intermediate layer  214  in a subsequent processing step. The etching agent used to remove the SiGe of the semiconductor intermediate layer  214  is selected so as to not attack the material of the semiconductor electrode layer  222  and of the active semiconductor region, i.e. it does not attack silicon. A suitable etching agent for this purpose is HCl. 
         [0074]    The etching process is continued for a predetermined duration so as to control the lateral extension of a tunnel section  225  that is created between the active semiconductor region  204  and the semiconductor electrode layer  222 . The tunnel section  225  extends over approximately half of the lateral extension of the active semiconductor region  204  between the shallow-trench isolation regions on its opposite lateral sides. The result of this processing is shown in  FIG. 3 . The source and drain region implants are realized after the gate stack patterning and capacitor electrode patterning. 
         [0075]    The capacitor electrode can extend in the lateral direction approximately up to a spacer  238  corresponding to the future gate stack. Indeed if the electrode extends under the future gate, a parasitic transistor would appear which could degrade electrical characteristics of architecture. 
         [0076]    The doping of the SiGe layer  214  doping is adapted for optimizing the selectivity during the tunnel etching with HCl. The channel doping is adapted to have an adjusted threshold voltage for the selection transistor. 
         [0077]    With reference to  FIG. 4 , after removal of the mask  223 , isolation layer  216  is fabricated, which covers all exposed surfaces of the substrate. In particular, the isolation layer  216  also covers the lower phase  222 . 1  of the semiconductor electrode layer  222  and the exposed surface  204 . 1  of the active semiconductor region  204 , as well as the exposed side faces of the semiconductor intermediate layer  214 . The isolation layer  216  can be fabricated as an oxide layer, for instance by thermal oxidation. However, it can also be deposited by other known methods. 
         [0078]    Subsequently, as can be seen in  FIG. 5 , the material of the first electrode layer and of the gate electrode of the control transistor is deposited as a coherent material layer  227 . The material can be doped polysilicon or a metal. A metal-layer stack can also be used. 
         [0079]    Subsequently, as shown in  FIG. 6 , a photolithography step and a subsequent gate-stack etching step is applied to pattern the material layer  227  to separate the first capacitor-electrode layer  218  from the gate stack  230 . Then, as shown in  FIG. 7 , lateral spacers  236  to  242  are formed on the gate stack  230  and on the first capacitor electrode layer  218 . 
         [0080]    As can be seen in the cross-sectional views of  FIGS. 6 and 7 , the first capacitor-electrode layer  218  differs slightly from the first capacitor-electrode layer  118  of the embodiment of  FIG. 1 . In the present embodiment, the shape of the first capacitor-electrode layer  218  resembles that of the letter J turned on its side, because the first electrode-layer section  218 . 1  has a longitudinal extension that is more than twice as large as that of the second electrode-layer section  218 . 2 . Note that in the comparison of the shape of the first capacitor-electrode layer  218  no distinction is made between a J-shape and a mirrored J-shape.  FIG. 7  shows a mirrored J-shape of the first capacitor-electrode layer  218 , but viewing the cross section from the opposite direction would result in a J-shape. Therefore, there is no actual difference between these two shapes. 
         [0081]    The process flow stays the same also for the matrix integration scheme according  FIG. 10  below, and regardless of the form, be it U- or J-shaped. 
         [0082]    Summarizing the processing according to specific embodiments of the fabrication method of the invention, a silicium germanium epitaxy is made on a silicon substrate. After this operation, the SiGe layer is selectively removed only one side (that requires the addition of an extra mask) or on both sides according the matrix structure of the memory plan (see matrix integration in  FIGS. 10 to 12 ). This step prepares the future implementation of the capacitor. Then a standard CMOS process flow continues with an oxide deposition and a polysilicon deposition (or other gate material). The gate stack of the selection transistor is in one embodiment realized at the same time of capacitor electrode ( 118 ). In that case no specific mask is needed for the capacitor definition. If, however, different dielectric and top electrode materials for the selection-transistor gate stack compared to the capacitor electrode ( 118 ) are desired, the process is the following: the electrode of selection transistor is built in first. After an oxide deposition is realized, a metal gate or other gate material is deposited. In this case the photolithography of the capacitor electrode that requires an additional mask. After the capacitor electrode is realized by etching the following stack. The rest of process flow stays the same that the standard CMOS process flow. 
         [0083]      FIGS. 8 and 9  show schematic top and cross-sectional views, respectively, of a memory cell according to a fourth embodiment of the invention. The views are aligned laterally so that lateral extensions along a direction x indicated on the right side of both figures are identical in the figures However, note that the lateral proportions shown in the figures are chosen for purposes of legibility of the figures and need not reflect actual lateral proportions used in real devices. The reference labels used in  FIGS. 8 and 9  for the structural elements of memory cell  300  correspond to those used for the previous embodiments of  FIG. 1  and  FIGS. 2 to 7  in their second and third digits, wherever structural elements of the present embodiment correspond to those of the earlier embodiments. The structure of the memory cell  300  is based on the memory cell  100  of  FIG. 1 . Note that the lateral extension of the semiconductor active region  304  is shown larger than for the memory cell  100 . However, this is only a matter of graphical representation in the figures. The lateral extensions are not drawn to scale and not shown with proportion that correspond to those of a real device. 
         [0084]    The memory cell  300  forms an integral part of a memory device, of which only a section is shown in  FIGS. 8 and 9 . The memory cell  300  is indicated by a dashed outline. The gate electrode  334  of the gate stack  330  forms an integral part of a word line  350 . The source electrode S of the control transistor  310  is connected to a bit line  352  through a vertical connect element  354 . This way, the memory cell can be addressed individually during operation of the memory device. 
         [0085]      FIG. 10  shows a memory device  400  with a back-to-front architecture. Two neighboring active semiconductor regions  404   a  and  404   b  are shown. The memory device  400  is based on memory cells of the type of memory cell  300  shown in  FIGS. 8 and 9 . It thus forms the matrix arrangement of the elementary structure presented  FIG. 8  and  FIG. 9 . 
         [0086]    A single memory cell is again indicated by shaded hatching and a dashed outline. Each active semiconductor region has one word line extending along the y-direction indicated on the right side of  FIG. 10 . The bit lines  452 A to  452 D extend along the x-direction. Memory capacitors are formed in the sections  418   a  and  418   b  in each semiconductor active region, respectively. 
         [0087]      FIGS. 11 to 13  show top and sectional views of a memory device with a first back-to-back architecture.  FIG. 12  is a cross-sectional view of the memory device  500  of  FIG. 11  along the dashed line XII-XII.  FIG. 13  is a cross-sectional view of the memory device  500  along the dashed line labeled XIII-XIII in  FIG. 11 . Note that the three representations of the memory device  500  shown in  FIGS. 11 to 13  do not correspond to each other in their lateral scaling. 
         [0088]    The memory device  500  of  FIGS. 11 to 13  differs from memory device  400  of  FIG. 10  in that the density of memory cells per active semiconductor region is increased. A single memory cell  501  is again indicated by a dashed outline. 
         [0089]    Referring to  FIG. 13 , it is seen that two memory cells are arranged within the lateral extension of the active semiconductor regions  504   a  and  504   b  in x-direction. To this end, two control transistors  510  and  510 ′ and two memory capacitors are arranged on opposite lateral sides of a connecting element  554 , which couples the memory cells to the corresponding bit line  452 D. The memory capacitors  512  and  512 ′ and the control transistors  510  and  510 ′ are arranged according to a mirror symmetry, in the present case in a back-to-back arrangement, on the opposite sides of the connecting element  554 . To this end, the semiconductor intermediate layer is processed from both lateral ends during the fabrication to form two tunnel sections for accommodating the respective first electrode-layer sections  518 . 1  and  518 . 1 ′ of respective memory cells. 
         [0090]      FIG. 12  shows that the lateral extension of the active semiconductor regions in the y-direction corresponds approximately to the extension of the memory capacitor  512 . 
         [0091]    As can be seen in  FIG. 13 , adjacent active semiconductor regions  504   a  and  504   b  are connected through a semiconductor layer  560 , which couples adjacent memory capacitors  512 ′ and  512 ″, which are arranged back-to-back, such that the third electrode-layer sections  518 . 3 ′ and  518 . 3 ″ face each other. In this back-to-back arrangement, in the direction parallel to the bit lines  552 A to  552 D, the respective word lines  550 ′ and  550 ″ are to be considered the front of the respective memory cell. 
         [0092]    The principle of this embodiment can be further exploited in the embodiment shown in  FIGS. 14 to 16 .  FIGS. 14 to 16  show top and sectional views of a memory device  600  with a second back-to-back architecture.  FIG. 15  is a cross-sectional view of the memory device  600  of  FIG. 14  along the dashed line XV-XV.  FIG. 16  is a cross-sectional view of the memory device  600  along the dashed line labeled XVI-XVI in  FIG. 14 . Again, the three representations of the memory device  600  shown in  FIGS. 14 to 16  do not correspond to each other in their lateral scaling. 
         [0093]    The embodiment resembles that of the previous  FIGS. 11 to 13  with the exception that the capacitor electrodes  612 ′ and  612 ″ in adjacent semiconductor active regions  604   a  and  604   b  are directly contacting each other over their whole extension in the z-direction. 
         [0094]    This embodiment allows a more integrated architecture. It is not necessary to have a shallow-trench isolation (STI) large enough to have two capacitor electrodes not in short. 
         [0095]    The memory capacitors can be addressed individually by means of the word line.  FIG. 17  is a top view of a memory device for illustrating the cell area required by a memory cell.  FIG. 17  is a schematic illustration for calculating the area of the memory cell. Assuming a basic minimal lateral extension F, corresponding to a technological node under consideration that can be fabricated by a given technology, the area of a memory cell corresponds to 4F×2F=8F 2 . For a F corresponding to 65 nm-CMOS technology node, we obtain a capacitance of 0.5 fF for the memory capacitor. In comparison, standard EDRAM requires a cell area of 25 F 2 . Therefore, if one uses the same area as a standard 65 nm EDRAM one obtains a capacitance equal to 3fF For a 65 nm node with a weak junction leakage a capacitance of 3 fF seems suitable. 
         [0096]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
         [0097]    Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
         [0098]    Note that the terms “horizontal” and “vertical” are only used with an internal reference to the surface of the active semiconductor region, which surface is assumed horizontal in the present context. However, this assumption is made only for the purpose of creating an intuitive mental reference picture, and without intending to restrict the claimed memory-cell structure to a certain orientation with respect to external reference directions, reference planes, or the like. 
         [0099]    In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. 
         [0100]    Any reference signs in the claims should not be construed as limiting the scope.