Abstract:
A storage capacitor includes at least one first electrode adjacent to at least one second electrode, whereby a lateral capacity is formed between these electrodes. The electrodes comprise stacks of metal parts and connecting contact elements. The second electrodes can be arranged around the first electrodes, and at least some of the second electrodes can be used jointly with adjacent ones of the first electrodes to form adjacent storage capacitors.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a storage capacitor, in particular one that can be used for 1T-, 2T- and 3T-memory cells in, for example, system-on-chip applications, which provides the highest possible capacity with low surface area usage.  
         BACKGROUND OF THE INVENTION  
         [0002]    In dynamic RAM memories, the information to be stored by a memory cell is generally held on a capacitor known as a storage capacitor. In system-on-chip applications in a pure logic technology, storage capacitors are often created by means of the gate capacity of an MOS transistor (e.g. a MoSyS-1T-SRAM, a 1T-cell of a static random access memory from MoSys Inc.) or by means of connection in parallel of gate and diffusion capacity (e.g. IFX-2T concept, a 2T-cell concept from Infineon).  
           [0003]    Because of the leakage currents, a storage capacitor slowly loses its charge which can lead to the loss of the information stored on the capacitor. In order to counter this, in microelectronic circuits the charge of all storage capacitors is refreshed again at certain intervals, so that the information is retained. This interval depends, amongst other things, on the size of the memory capacity.  
           [0004]    As a result of increasing integration the leakage currents in the abovementioned system-on-chip applications are becoming ever greater and new leakage current sources such as the gate leakage currents are arising due to the increasingly thin oxide coatings. The total memory capacity is also becoming smaller.  
           [0005]    An object of the invention is to provide a storage capacitor with the highest possible memory capacity and the lowest possible surface area usage, which in particular in a pure logic technology can be created without additional, and thus more expensive, process steps.  
         SUMMARY OF THE INVENTION  
         [0006]    Within the context of the present invention, the storage capacitor comprises coupling capacitances of metals. Here, according to the invention, an inner electrode and an outer electrode of the storage capacitor are constructed from stacks enclosing metal pieces and the contact elements connecting these metal pieces. In addition, according to the invention several outer electrodes are grouped around an inner electrode, in order to maximise the capacity for the storage capacitor and to guarantee screening from the adjacent storage capacitors or memory cells.  
           [0007]    In a memory cell arrangement, several storage capacitors according to the invention of uniform shape can be arranged next to one another, with joint use being made of the outer electrodes of the adjacent storage capacitors. For production considerations and because of the better screening from adjacent storage capacitors a hexagonal shape is preferred here. All outer electrodes of the memory cell arrangement can be connected together with a further metal part that is applied to a reference potential or a supply voltage. The present invention is preferably suited to use in microelectronic circuits in order, for example in system-on-chip applications, to create 1T-, 2T- or 3T-memory cells. The invention is obviously not restricted to this preferred area of application, however.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The present invention will now be explained in more detail with reference to the attached drawing and using preferred embodiments.  
         [0009]    [0009]FIG. 1 shows a side view of a storage capacitor according to a preferred embodiment of the invention.  
         [0010]    [0010]FIG. 2 shows a top view of a storage capacitor according to the invention.  
         [0011]    [0011]FIGS. 3A-3C show examples of even arrangements of outer (second) electrodes and inner (first) electrodes in storage capacitors according to the invention.  
         [0012]    [0012]FIGS. 4A and 4B show examples of connections between the outer electrodes of adjacent storage capacitors.  
         [0013]    [0013]FIGS. 5A-5C show examples of the control of a storage capacitor according to the means of 1T-, 2T- and 3T-memory cells.  
     
    
     DETAILED DESCRIPTION  
       [0014]    [0014]FIG. 1 shows a side view of a storage capacitor with an inner (first) electrode  1  in the centre, two adjacent outer (second) electrodes  2  and a metal part  3  connecting the outer electrodes.  
         [0015]    In a manner according to the invention, lateral capacities between adjacent conductors are used to construct the storage capacitor. For this purpose, in each case a stack of metal parts  5  and contact elements  6  connecting these metal parts  5  is constructed, in order to form the corresponding electrode  1  or  2  of the storage capacitor, as shown in FIG. 1. Between a stack and an adjacent stack the desired memory capacity is then created, with the two stacks in particular being arranged in parallel.  
         [0016]    When used in microelectronic circuits the metal parts  5  are so-called landing-pads and are each positioned in a metal layer  4 . The contact elements  6  connecting the metal parts  5  are so-called vias and are positioned between the metal layers  4 .  
         [0017]    Advantages of this solution compared with the normal design of a storage capacitor in microelectronic circuits include the reduction in leakage currents within the memory capacity itself, by avoiding MOS or diffusion capacitances, and simple implementation by means of standard metallization in a purely standard CMOS process.  
         [0018]    In order to maximise the memory capacity and to guarantee screening from adjacent storage capacitors, several outer electrodes  2  are arranged around an inner electrode  1 , as shown by way of example in FIG. 2.  
         [0019]    [0019]FIG. 2 shows a top view of a storage capacitor according to the invention without metal part with an inner electrode  1  in the centre and four adjacent outer electrodes  2  in the shape of a diamond, indicating the lateral memory capacities between the outer electrodes  2  and the inner electrode  1 .  
         [0020]    The outer electrodes  2  are connected via contact elements with a metal part  3 , which is applied to a reference potential or a supply voltage (see FIG. 1). The inner electrode is coupled with a selection circuit (e.g. a selection transistor) via a contact element.  
         [0021]    Several of the storage capacitors described above can be arranged alongside one another to form a memory cell arrangement, with joint use of the outer electrodes being made by the adjacent storage capacitors.  
         [0022]    It is, furthermore, possible to assign a storage capacitor not just one inner electrode, but several, which are then connected in parallel by means of separate connections, in order to thereby increase the capacity of the storage capacitor. Essentially, however, the principle according to the invention can be put into practice by just one inner (first) electrode and one outer (second) electrode, each with a stack-like construction as described.  
         [0023]    The outer electrodes of a storage capacitor can be arranged in various forms around the corresponding inner electrode. Since in microelectronic circuits as a rule the metal landing-pads are used as a basis for the construction of the stacks described above, a rectangular shape, a diamond shape and a hexagonal shape are the most advantageous shapes. FIGS. 3A-3C show these three most advantageous shapes for an even arrangement of the outer electrodes  2  around inner electrodes  1  in a memory cell arrangement according to the invention.  
         [0024]    The three shapes shown in FIGS. 3A-3C differ essentially by the space used. Taking the side length of a landing pad as a reference length A and also selecting the same distance between two adjacent landing-pads, then the necessary relative area for the rectangular shape (FIG. 3A) comes to 16A 2 , the relative area for the hexagonal shape (FIG. 3B) comes to 12A 2  and the relative area for the diamond shape (FIG. 3C) comes to 8A 2 . Because it is easier lithographically and can be created with a higher yield, the hexagonal shape, which in its three-dimensional form looks like a honeycomb, is preferred for production considerations over the other two shapes. In addition, the screening from the adjacent cells is better with the hexagonal shape than with the other shapes.  
         [0025]    Through further connections of the outer electrodes, as shown for example in FIGS. 4A and 4B, the capacity of a storage capacitor can be further increased. Here, the connection between adjacent outer electrodes  2  within a storage capacitor can be achieved both in just one of the metal layers  4  and also in several or all metal layers  4 . The same applies to a possible connection between storage capacitors arranged adjacently between outer electrodes  2 . It is likewise also in principle conceivable to connect in this way several inner electrodes  1  of a storage capacitor.  
         [0026]    In microelectronic circuits through the connection of two adjacent outer electrodes, in FIGS. 4A and 4B respectively vertical or horizontal, in each metal layer the storage capacity of the rectangular shape is increased without loss of surface area (see FIG. 4A). Likewise, by connecting two outer, each horizontal, electrodes in each metal layer the memory capacity in the hexagonal shape is increased without loss of surface area (see FIG. 4B). Furthermore, the hexagonal shape has lower surface area usage than the rectangle.  
         [0027]    [0027]FIGS. 5A-5C shows possible applications of a storage capacitor  7  according to the invention in 1T-, 2T- and 3T-semiconductor memories or corresponding memory cell arrangements.  
         [0028]    According to FIG. 5A, the storage capacitor  7  is controlled via its inner electrodes via a selection transistor  10 , which in turn is addressed via a bit line  8  and a word line  9 . Furthermore, the outer electrodes of the storage capacitor  7  are connected with a reference potential Vref (1T-memory cell concept).  
         [0029]    According to FIG. 5B, the storage capacitor is controlled via two selection transistors  10   a ,  10   b . Selection transistor  10   a  is addressed via a first bit line  8   a  and a first word line  9   a,  while selection transistor  10   b  is addressed via a second bit line  8   b  and a second word line  9   b  (2T-memory cell concept). The storage capacitor  7  is connected via its inner electrodes with the two selection transistors  10   a ,  10   b , while the outer electrodes are again applied to the reference potential Vref.  
         [0030]    According to FIG. 5C, three transistors  10   a ,  10   b  and  10   c  are used, which are connected to the storage capacitor  7  as shown. The first bit line  8   a  and first word line  9   a  assigned to selection transistor  10   a  serve to write or store information in the storage capacitor  7 , which is applied to a first reference potential Vref 1 . The second bit line  8   b  and second word line  9   b  assigned to selection transistor  10   c  serve to read out the information stored in the storage capacitor. The transistor  10   b  connecting the storage capacitor  7  with the selection transistor  10   c  is applied to a second reference potential Vref 2  (3T-memory cell concept).  
         [0031]    Of course, the connections of the inner and outer electrodes of the storage capacitor to the selection transistor(s) and the reference potential (or the supply potential) can also be swapped over.  
         [0032]    Although exemplary embodiments of the invention are described above in detail, this does not limit the scope of the invention, which can be practiced in a variety of embodiments.