Abstract:
A memory circuit arrangement includes a switching element per column that can be used to connect or disconnect two bit lines for memory cells of a column. The switching element leads to a reduction of the chip area and/or to an improvement in the electronic properties of the memory circuit arrangement.

Description:
PRIORITY AND CROSS REFERENCE TO RELATED APPLICATION 
   This application is a continuation of International Application No. PCT/EP2004/050356 filed Mar. 24, 2004, which claims priority to German application 103 23 244.3, filed May 22, 2003, both of which are incorporated in their entirety by reference herein. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a memory circuit arrangement, and in particularly to a memory arrangement having a switching element for connection of bit lines for memory cells. 
   2. Description of the Related Art 
   Memory cells of a memory circuit are arranged in rows and columns or in matrix-type fashion. Each memory cell has a transistor that may have a floating gate. Word lines and bit lines are transversely arranged where the word lines are electrically connected to control electrodes (also referred to as the gate) of the transistors of a row. The bit lines are connected to terminal electrodes of the transistors of a column. The terminal electrodes are formed, for example, by doped regions in a substrate, and may be referred to as a drain and a source for field effect transistors. 
   The memory cells may store content in such a way that the content is erased when an operating voltage is switched off. Alternatively, the memory cells may retain their content when the operating voltage is switched off. In this context, reference is made to memory cells that effect volatile storage and memory cells that effect nonvolatile storage, respectively. 
   Each column of memory cells is connected to two bit lines, namely to a drain line and a source line. The source line is utilized for memory cells of two adjacent columns. The floating gate transistor has a floating gate, or memory gate, that is an electrode insulated on all sides. Insofar as the term column is used hereinafter, it designates—unless differentiated more precisely—the memory cells of a column of the entire memory arrangement. The memory cells have high power consumption and may assume a large chip area. The memory arrangements may also have a complex architectures where two metallic bit lines, namely a metallic source line and a metallic bit line, are in each case required per column. 
   Accordingly, there is a need to provide a memory circuit of simple construction having improved electronic properties and which uses a small chip area and low power consumption. It is desirable for the memory circuit arrangement to make it possible to maintain previous potential conditions or to utilize other potential conditions for different operating modes. 
   SUMMARY OF THE INVENTION 
   The present invention relates to a memory circuit arrangement having multiple memory cells arranged in rows and columns in a matrix-type fashion where each memory cell has at least one transistor, such as a floating gate transistor. Word lines are electrically conductively connected to control electrodes, or the gate, of the transistors of memory cells of a row of the matrix and bit lines are connected to terminal electrodes of the transistors of memory cells of a column of the matrix. The memory cell may be configured to provide volatile storage or nonvolatile storage. 
   In one aspect, the memory circuit arrangement includes, per column of memory cells, at least one switching element that enables the connection and disconnection of an electrically conductive connection of two bit lines of the same column of the matrix. In a first operating mode of the circuit arrangement, the two bit lines carry the same potential and are electrically conductively connected with the aid of the switching element. In second operating mode of the circuit arrangement, different potentials may be present on the two bit lines, and the bit lines are disconnected from one another with the aid of the switching element. In the first operating mode, the potential of one bit line can thus also be applied to the other bit line by means of a simple circuitry measure. A potential that is present on the connected bit line in other operating modes is then turned off, for example for all the memory cells of the circuit arrangement or for a portion of the memory cells of the circuit arrangement. By way of example, the potential is turned off for all the memory cells of the memory or for only for the memory cells of a memory segment in which the affected column is situated. In both cases, a transistor can be used for disconnection. 
   The switching element provides degrees of freedom that permit new operating modes of the memory circuit arrangement. By way of example, it is possible to carry the same potential on the disconnected voltage feed in a plurality or even in all of the operating modes. The switching power required for changing the potential becomes lower. 
   In an embodiment, one bit line that is electrically conductively connected to the switching element is arranged in a doped region of a substrate, such as a silicon substrate. By contrast, the other bit line which is electrically conductively connected to the switching element is a bit line of metal layer or is a bit line having metal. The doped region is arranged, for example, between two isolation trenches for isolating the memory cells of mutually adjacent columns. As an alternative, the doped region can be arranged in such an isolation trench. The metal line has a smaller resistance in comparison with a doped region, so that leading the operating voltage as far as the switching element is associated with a comparatively small voltage drop and power loss. It is only at the switching element that the doped region is connected in. If the length of the doped region is adapted to the conductivity thereof, then voltage drops or losses with regard to the writing, reading or erasing times are acceptable in comparison with the exclusive connection of bit lines made of metal. If a bit line comprises metal, then the disadvantages associated with the use of doped regions with regard to the specific conductivity and a voltage drop associated therewith do not occur at least with regard to these bit lines. Moreover, what can be achieved by using a plurality of switching elements per column or by skillful arrangement of the switching element or of the switching elements, for example on half the length of the two bit lines, is that the disadvantages associated with the doped region do not substantially impair the operation of the circuit arrangement. 
   In a next embodiment, the bit lines are local bit lines that can be electrically conductively connected to global bit lines via further switching elements. The local bit lines define a memory segment by virtue of the fact that they are connected only to a portion of the cells of a column of the memory circuit arrangement. The circuit arrangement contains at least two memory segments. The use of memory segments affords the possibility of driving the memory circuit arrangement in a similar manner to a magnetic storage disk that likewise contains segments. Moreover, what is achieved by the use of local and global bit lines is that the global bit lines can be arranged in upper metallization layers and, by way of example, have a larger cross-section than the local bit lines or the distance with respect to one another is increased. The former reduces the bit line resistance (R) and the latter reduces the capacitive load (C) of the bit lines. As a result, the RC delay due to the global bit lines is shorter and the access times of the circuit arrangement are improved. The use of memory segments additionally affords the simple possibility of limiting the length of the local bit lines, in particular of the local bit lines routed in doped regions. 
   In another embodiment, the number of global bit lines in the circuit arrangement is half as large as the number of metallic local bit lines. If source lines in different columns of a memory segment have to carry potentials that are different from one another in one operating mode and if drain lines have to carry potentials that are different from one another in one operating mode, without the use of the switching elements, more global bit lines than in accordance with the development are necessary even in the case of double utilization of global lines for two local lines. 
   In another embodiment, the switching element is arranged at an end of a row of memory cells of a column of a memory segment. Alternatively or in addition, the switching element is arranged between the memory cells of a column of a memory segment, preferably in the center of the column, in order to feed a current into two equal length sections of the bit lines arranged in a doped region. The voltage drop across the bit line in the doped region can thus be reduced further in a simple way. 
   In a second aspect, the invention relates to a memory circuit arrangement having at least one further switching element per column that enables the production and the disconnection of an electrically conductive connection of a collective line running in the row direction to a respective bit line. The development is based on the consideration that a local production and disconnection of the connection to the collective line is associated with a low power demand in comparison with a segment-related or global production and disconnection of a connection to the collective line. Moreover, operating modes are possible with potential conditions which would require additional lines in the column direction without the invention according to the second aspect. 
   In particular, a combination of the two circuit arrangements according to the invention leads to a circuit arrangement of simple construction which has particularly good electrical properties, for example, with regard to the power consumption. Moreover, operating modes are again made possible with potential conditions which would require additional lines in the column direction without the combination. Furthermore, the required chip area is small on account of the combination. 
   In one embodiment, the circuit arrangement contains a control unit, which drives the switching elements for connecting two bit lines alternately to the switching elements for connecting the collective line to the bit line. The alternate driving ensures that no potential conflicts occur on a bit line. 
   In another embodiment, for a column, the switching element for connecting the two bit lines is adjacent to the switching element for connecting a bit line to the collective line. As an alternative, the two switching elements are arranged at different locations, in particular at different ends of the column. 
   In a next embodiment the circuit arrangement contains a drive circuit constructed in such a way that a memory cell transistor is programmed and/or erased with a uniform channel (UCP Uniform Channel Programming), so that the tunnel oxide is stressed uniformly. The advantages obtained by means of the switching elements are particularly great in particular with this type of programming. 
   In another embodiment, the bit lines of a column of memory cells are arranged such that they overlap one another in the direction normal to a substrate main area. In this case, a substrate main area is an area having an area content with respect to other areas, e.g. with respect to edge areas, that is greater than the area content of these areas. That is, the bit lines are arranged such that the two bit lines lie one above the other if the substrate main area lies in the horizontal. 

   
     DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are explained below with reference to the accompanying drawings. 
       FIG. 1  illustrates voltage conditions at memory cells during programming, erasing and reading. 
       FIG. 2  illustrates a circuit diagram of a memory cell array. 
       FIG. 3  illustrates a circuit diagram of a column of the memory cell array. 
       FIG. 4  illustrates the layout of a memory circuit. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Identical, functionally identical, or similar elements and signals are referred to with the same reference symbols in the figures unless stated otherwise. 
     FIG. 1  shows an exemplary embodiment of voltage conditions at memory cells during programming, during erasing and during reading of memory cells of a circuit arrangement explained in greater detail below with reference to  FIGS. 2 to 4 . The voltage values cited in connection with  FIG. 1  are only examples and may be chosen deviating within wide limits from the voltage values cited, e.g. deviating by plus or minus 50% or 30% of the respective voltage value. Operating modes with fundamentally different voltage values can also be realized. In the explanation of  FIG. 1 , reference is made to memory transistors T 11  to T 22 , the arrangement of which in a memory cell array is explained in more detail below with reference to  FIG. 2 . The memory transistor T 11  is chosen by way of example for explaining the operating modes. It goes without saying that further transistors can be written to, erased or read in the same way. 
   During programming, as illustrated in  FIG. 1 , a voltage of +14 Volts is applied to a gate electrode  12  of the memory transistor T 11 . During programming a voltage of −3 Volts in each case is present at a source region  14 , a drain region  18  and at a substrate region  16  of the memory transistor T 11 . On account of these voltage conditions, a tunneling current flows from an inversion channel in the substrate  16  over the whole area through a tunnel oxide into the all-around insulated electrode  20  of the transistor T 11 . 
   During programming, the following voltages are present at transistors in the same row as the memory transistor T 11  but in a different column, e.g. at the transistor T 21 :
         +14 Volts at a gate electrode  22 ,   +3 Volts at a source region  24  and at a substrate region  26 , and   +3 Volts at a drain region  28  of the transistor T 21 .       

   The following potentials are present at transistors in the same column as the transistor T 11  but in a different row of the memory cell array, e.g. at the transistor T 12 :
         0 Volts or −3 Volts at a gate electrode  32 ,   3 Volts at a source region  34  and at a substrate region  36 , and   3 Volts at a source region  38  of the transistor T 12 .       

   The following voltages are present at transistors in different rows and in different columns than the transistor T 11 , e.g. at the transistor T 22 :
         0 Volts at a gate electrode  42 ,   +3 Volts at a source region  44  and at a substrate region  46 , and   +3 Volts at a drain region  48 .       

   During erasure, all the memory transistors of a memory segment are erased simultaneously so that the same potential conditions exist at all the memory transistors T 11  to T 22 .  FIG. 1  illustrates the erasure operation for the memory transistor T 11  as representative of all the memory transistors T 11  to T 22 . During erasure the following voltages are present at the transistor T 11 :
         −14 Volts at the gate electrode  12 ,   +3 Volts at the source region  14  and at the substrate region  16 , and   +3 Volts at the drain region  18 .       

   The following potentials are present when reading the memory state of the memory transistor T 11 :
         +2.5 Volts at the gate electrode  12 ,   0 Volts at the source region  14  and at the substrate region  16 , and   1 Volt at the drain region  18 .       

     FIG. 2  shows a circuit diagram of a memory circuit  50 , which contains, inter alia, a cell array subdivided into a plurality of memory segments  60 ,  62 . The memory segments  60 ,  62  are constructed substantially identically, so that only the construction of the memory segment  60  is explained below. The memory segment  60  may include multiple memory transistors T 11  to Tmn, where m designates the number of columns and n designates the number of rows in a memory segment. Each memory cell of the memory cell array comprises a memory transistor, e.g. the transistor T 11 . The memory cells of the memory segment are arranged in matrix-type fashion. The gate electrodes of the memory transistors T 11 , T 21  of a row are connected to a word line WL 1 . The gate electrodes of the memory transistors T 12 , T 22  to T 2   m  are connected to a word line WL 2 . Likewise, for example a further 14 word lines of the memory segment  60  are in each case connected to the gate electrodes of the memory transistors of a row. 
   The memory transistors T 11  to Tmn of a memory segment  60  are all constructed substantially identically. Therefore, reference is made to the explanations concerning  FIG. 1  for the memory transistor T 11 . 
   Drain regions of memory transistors of a column of the memory segment  60  are connected to a local bit line, e.g. the drain regions (D) of the memory transistors T 11  and T 12  are connected to a local bit line BL 1 . A local bit line BL 2  is connected to the drain regions (D) of the transistors of the second column, in particular to the drain regions (D) of the memory transistors T 21  and T 22 . Further memory cells  70  of the memory segment  60  are indicated by dots. By way of example, the memory segment  60  contains 1024 columns. 
   The source regions (S) and also the substrate regions of the memory transistors of a column are in each case connected via a doped well W 1 , W 2  to Wm. Isolation trenches in each case lie between the wells W 1 , W 2 , etc. A well W 1 , W 2  is formed for example by a p-doped layer and an underlying n-doped layer. The contact-connection of the source regions (S) of the memory transistors T 11  to T 22  to a well W 1 , W 2  is produced for example by means of a siliciding and a contact region doped into the well, see e.g. U.S. Pat. No. 6,438,030 B1, which is incorporated by reference in its entirety herein. 
   Moreover, global drain lines run via all the memory segments  60 ,  62  in the column direction, of which two global drain lines GDL 1  and GDL 2  are illustrated in  FIG. 2 . In an alternative exemplary embodiment, a global drain line is utilized for two local bit lines of a memory segment  60 ,  62  with the aid of selection transistors. Selection transistors for connecting the local bit lines BL 1  and BL 2  to the global drain lines GDL 1  and GDL 2  are not illustrated in  FIG. 2 . 
     FIG. 3  shows a circuit diagram of the first column of the memory segment  60 . The other columns of the memory segment  60  are constructed like the first column and are therefore not explained in any further detail. In addition to the elements already explained with reference to  FIG. 2 , the first column of the memory segment  60  contains two configuration transistors TS 1   a  and TS 1   b . The operating path of the configuration transistor TS 1   a  lies between the well W 1  and the local bit line BL 1 , that is to say source at the well W 1  and drain at the local bit line BL 1 . The control electrode of the configuration transistor TS 1   a  is connected to a control line SLa, which is also connected to the control electrodes of the configuration transistors TS 2   a , TS 3   a , etc. of the other columns of the memory segment  60 . 
   The configuration transistor TS 1   b  lies at the lower end of the first column. Its operating path lies between the well W 1  and a ground line M carrying a ground reference or a potential of 0 Volts. The control electrode of the configuration transistor TS 1   b  is connected to a control line SLb, to which the control electrodes of the other lower configuration transistors TS 2   b , TS 3   b  etc. of the memory segment  60  are also connected. 
   In another exemplary embodiment, the configuration transistor TS 1   a  lies between two circuit points  80 ,  82  at the lower end of the well W 1  and at the lower end of the local bit line BL 1 . In a further exemplary embodiment, the configuration transistor TS 1   a  lies between the eighth and ninth memory cells of the first column of the memory segment  60 . 
   In addition to the configuration transistors TS 1   a  and TS 1   b , for the first column of the memory segment  60  there is also a selection transistor (not illustrated) for connecting the local bit line BL 1  to the global drain line GDL 1 . However, this selection transistor is not illustrated in  FIG. 3 . 
   During the operation of the circuit arrangement  50 , the configuration transistors TS 1   a  and TS 1   b  are alternately driven. That is the configuration transistors TS 1   a  and TS 1   b  are driven so that in each case one configuration transistor turns off and the other configuration transistor turns on. If the configuration transistor TS 1   a  turns on, then the bit line BL 1  is connected to the well W 1 . The well W 1  and thus also the source line of the first column carry the potential of the bit line BL 1 . If the transistor TS 1   a  turns on, then the transistor TS 1   b  isolates the well W 1  and the source line from the ground line M. If, by contrast, the configuration transistor TS 1   b  turns on, then the ground potential is applied to the well W 1  and the source line. If the configuration transistor TS 1   b  turns on, the configuration transistor TS 1   a  is turned off, so that no potential conflict occurs between the potential of the well W 1  and the potential on the bit line BL 1 . 
   The configuration transistor TS 1   a  is in the on state in the case of writing to a memory cell of the first column of the memory segment  60  or to some other memory cell of the memory segment  60 . The transistor TS 1   a  is additionally in the on state if a memory cell in the memory segment  60  that is not arranged in the first column is being read. 
   The configuration transistor TS 1   b  is in the on state if a memory cell of the first column of the memory segment  60  is being read. The configuration transistor TS 1   a  is in the on state in the case of erasure. 
     FIG. 4  shows the layout of part of the circuit arrangement  50 . The meaning of the reference symbols has already been explained above with reference to  FIGS. 1 to 3 . Trenches G 0  to Gm were introduced into a substrate and filled with an insulating material, such as silicon dioxide. The insulating wells W 1 , W 2 , etc. were produced between the trenches G 0 , G 1 , G 2 , etc. or before the production of the trenches G 0 , G 1 , G 2 , etc. By way of example, an n-doped layer was introduced in a lightly p-doped substrate and a p-doped layer was introduced above that.  FIG. 4  does not illustrate the wells W 1 , W 2 , etc. since they are covered by the active areas of the transistors which are situated at the substrate surface. 
   The memory transistors T 11  to T 22  or Tmn of the memory cell array have a layout such as has also been used hitherto. Therefore, this layout is not illustrated completely in  FIG. 4 , but rather is only indicated by two break lines  100 . 
   In the first metallization layer, the local bit lines BL 1 , BL 2 , etc. run in the vertical direction over the cell array. These bit lines end at the drain contact of the transistor T 1   n , T 2   n , etc., that is to say at the drain contact of the last transistor of a column. Also situated in the first metallization plane is the ground line M, which runs in the horizontal direction in  FIG. 4 , that is to say at right angles to the local bit lines BL 1 , BL 2 , etc. 
   The local bit lines BL 1 , BL 2 , etc. are connected via contacts K 1  to the drain regions of the configuration transistors TS 1   a , TS 1   b , etc. Moreover, the bit lines BL 1 , BL 2  are connected to the drain regions of the memory transistors of the memory cell array. The ground line M is connected via contacts K 3  to the source region of the configuration transistor TS 1   b , TS 2   b , etc. 
   In the second metallization plane, the global drain or bit lines GDL 1 , GDL 2 , etc. run in the vertical direction. As already mentioned, the global drain lines GDL 1 , GDL 2  are connected to the respective local bit line BL 1 , BL 2 , etc. via selection transistors that are not illustrated. 
   Proceeding from the active source regions of the memory transistors T 11  to Tmn, there are in each case contact-connections to the wells W 0 , W 1 , etc.; see for example the contact-connection K 2  of the memory transistor T 11  and the contact-connection  80  of the memory transistor T 1   n  in the first column. The contact-connections K 2  and  80  are also depicted in  FIG. 3 . 
   The potentials when programming the memory transistor T 11  are specified below:
         −3 Volts on the bit line BL 1 ,   +3 Volts on the bit line BL 2 ,   +2.5 Volts on the control line SLa,   +14 Volts on the selected word line (here WL 1 ),   0 or −3 Volts on the nonselected word lines (not illustrated),   −3 Volts on the control line SLb, and thus   −3 Volts at the well W 0  and +3 Volts at the well W 1 .       

   Consequently, what is achieved by using the configuration transistors is that adjacent wells W 0  and W 1  are at different potentials. Additional vertical lines for the application of the potentials in addition to the local bit lines BL 1 , BL 2  are not necessary, with the result that only a small chip area is required for the circuit arrangement  50 . 
   The following potentials are applied when reading T 11 :
         +1 volt on the local bit line BL 1 ,   0 Volts on the local bit line BL 2 ,   0 Volts on the control line SLa,   +2.5 Volts on the word line WL 1 ,   0 Volts on the nonselected word lines (not illustrated),   +2.5 Volts on the control line SLb, and thus   0 Volts in each case at the wells W 0  and W 1 .       

   The use of the configuration transistors makes it possible, in the read operating mode, to connect the well W 1  and the bit line BL 1  to different potentials, and the well W 2  and the bit line BL 2  to identical potentials. This is possible even though the configuration transistors TS 1   a , TS 2   a , etc. are driven via a common control line SLa and the configuration transistors TS 1   b , TS 2   b , etc. are driven via a common control line SLb. 
   In another exemplary embodiment, a local bit line BL 1  is optionally contact-connected to the underlying well via at least two configuration transistors TS 1   a . In addition or as an alternative, there are multiple ground lines M for each sector, so that each well can be contact-connected with the aid of at least two configuration transistors TS 1   b . By virtue of this measure, a fast read access is possible and only comparatively small voltage drops arise along the wells and hence the source lines. 
   The circuit arrangement explained makes it possible to obtain the smallest possible offset between adjacent bit lines BL 1 , BL 2 . Only a single metallic bit line may be provided per column. 
   In an alternative exemplary embodiment, adjacent bit lines are not brought as close as the minimum possible offset, because the metallic bit lines are made wider or because the capacitances between adjacent bit lines are intended to be reduced on account of an increase in the distance between these bit lines. Improved electronic properties of the memory circuit thus result instead of an area saving. A compromise between reducing the chip area and improving the electrical properties is also obtained in another exemplary embodiment. By reducing the number of metal lines, moreover, the susceptibility to defects is reduced in production and so a higher yield is achieved in the production of the circuit arrangement  50 . 
   However, the invention is not restricted to the memory circuit arrangement explained. Thus, by way of example, the concept can be modified through the use of configuration transistors such that programming is still effected using the CHE principle (Channel Hot Electrons) and erasure is effected by means of a tunneling current. However, a tunneling current that is distributed uniformly over the tunnel oxide between floating gate and substrate is used for erasure, thus resulting in less damage to the tunnel oxide than is the case with the previously used tunneling current in only a partial region of the tunnel oxide. Further applications of the concept according to the invention relate in particular to reducing the power consumption of the memory circuit. 
   In particular, the invention can also be used when only bit lines in doped regions are used, or when bit lines are routed in isolation trenches.