Abstract:
The present invention relates to an integrated circuit including at least one matrix network of identical elements capable of being individually addressed at least in a first direction and including, at least for this first direction, at least one redundancy element, and a circuit that reversibly inhibits the operation of a defective element and maintains the circuit operation by using the redundancy element. The integrated circuit also may include a circuit that definitely inhibits the operation of a defective element and maintains the circuit operation by using the redundancy element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/154,434 filed May 23, 2002, now abandoned, which is a continuation of Ser. No. 10/024,458, filed Dec. 17, 2001, which is a continuation of Ser. No. 09/883,868, filed Jun. 18, 2001, now abandoned, which is a continuation of Ser. No. 09/390,478, filed Sep. 7, 1999, now abandoned, which is a continuation of Ser. No. 09/086,625, filed May 29, 1998, entitled MEMORY CIRCUIT WITH DYNAMIC REDUNDANCY, now issued U.S. Pat. No. 5,982,679. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to memories implemented in the form of a matrix network of memory cells in an integrated circuit. The present invention more specifically applies to memories provided with a redundancy element for functionally replacing a defective element of the memory (for example, a column or a row). 
   2. Discussion of the Related Art 
   The use of a redundancy element to replace a defective element must be performed transparently for the user. Memory circuits are thus generally associated with redundancy or repair circuits, meant to permanently modify the selection of the memory columns when one of these columns has failed a test. For this purpose, fuses operated during a test phase of the manufacturing process are generally used. Several architectures may be used to organize the routing of the decoded column addresses. 
   A first architecture provides a redundancy column next to a group of columns forming the matrix, and associates each column, that might be replaced, with a fuse to connect, in the place of a defective column, the redundancy column. 
   A second architecture uses a fuse matrix to store the address of the defective column and a comparator receiving the address of the column and the defective address stored by the fuse matrix. The comparator directs, upon each reading from or writing into the memory, the current datum to the redundancy column if the current address corresponds to the address stored by the fuse matrix. 
   More recently, redundancy circuits reducing the number of necessary fuses to implement replacement of a defective element of the memory have been provided. Such circuits consist of shifting, from one column and from column to column, the addressing of the memory towards the redundancy column, each column in the memory being capable of being used as a replacement column for the preceding column. The advantage of such circuits with respect to prior solutions is that they make the times of access to the memory cells uniform, even in case of a use of the redundancy column. Examples of redundancy circuits of this type are described in patents WO-A-9406082 and EP-A-0477809 which patents are incorporated herein by reference. 
     FIG. 1  shows a conventional example of a redundancy memory circuit of the type described in patent WO-A-9406082. This drawing shows a memory circuit including two groups or matrix networks  1 ,  1 ′ of memory cells  2 ,  2 ′ likely to each store one data bit. Each network  1 ,  1 ′ includes  m  rows and n+1 columns, the n+1-th column forming a redundancy column for replacing a column containing a defective cell among the  n  first columns. Cells  2 ,  2 ′ are addressed by means of an address bus  3  carrying, in the form of a binary address, the coordinates (column and row) of a memory word formed of several bits. 
   In the example shown, it is assumed that a memory word includes two bits and that each network  1 ,  1 ′ is associated with one bit of a memory word. The binary address of a memory word is carried by bus  3  over  k  bits. This address is decoded by row and column decoders  4  and  5 , for extracting from address A carried by bus  3  a row address Ar and a column address Ac, that is, the vertical and horizontal coordinates of the memory word in the memory. Generally, the column address is carried by least significant bits of address A, while the row address is carried by most significant bits.  m  outputs of decoder  4  form row conductors  6  enabling to select the row of the addressed memory word, a single one of the row conductors being active. Column decoder  5  is meant to activate a column conductor  7  of each group  1 ,  1 ′ corresponding to the column address of the memory word. 
   Each matrix network  1 ,  1 ′ is associated with a data input/output  8 ,  8 ′ on which the memory word is input or read. Each terminal  8 ,  8 ′ is associated with a read amplifier  9 ,  9 ′ and with a write amplifier  10 ,  10 ′. Each network  1 ,  1 ′ is further associated with a multiplexer  11 ,  11 ′ for routing the data bit to a column conductor  7 ,  7 ′ of network  1 ,  1 ′. The selection of conductor  7 ,  7 ′ is performed based on column address Ac. 
   A redundancy circuit  12  is interposed between  n  outputs of decoder  5  and n+1 control inputs of multiplexers  11 ,  11 ′. Circuit  12  is formed by a fuse circuit  13  and a routing circuit  15 . Circuits  13  and  15  are, in practice, overlapping and have the function of shifting the electric connection from an output conductor  14  of decoder  5  to a conductor  16  of next rank at the output of circuit  15  if the conductor  16  of same rank as conductor  14  is associated with a column  7  or  7 ′ including a defective cell  2  or  2 ′. Thus, circuit  13  includes as many fuses as decoder  5  includes outputs  14  and circuit  15  includes as many output  16  as each of networks  1  and  1 ′ include columns  7 ,  7 ′. If the connection of a conductor  14  is shifted to a conductor  16  of next rank, this shifting is repeated for all conductors  14  of higher rank so that the conductor  14  of highest rank is associated with the redundancy column. 
   A disadvantage of conventional redundancy architectures is that the shifting, as concerns the addressing, of a defective column to the next column to use the redundancy column is performed simultaneously for all networks  1 ,  1 ′ as soon as one of these networks includes a defective cell. Thus, the circuit must include as many redundancy columns as there are matrix networks  1 ,  1 ′ associated with a data bit even when only a single defective column in the entire memory can be corrected or repaired by the redundancy columns. 
   Another disadvantage of conventional circuits equipped with redundancy elements is that they use destructive fusible elements (for example, which can be fused by laser or electric current) or the state of which is modified irreversibly to modify, from the addressing viewpoint, the organization of the columns of the matrix networks. This leads, in practice, to implementing a redundancy element only during tests performed during the memory manufacturing. Indeed, it is generally not desirable to enable an end user to act in a definitive manner upon the internal structure of an integrated circuit. Now, the failure of the memory cell may occur during the operation of a system with which the memory is associated. In such a case, the memory circuit conventionally becomes unusable even when a redundancy element can remain available. 
   SUMMARY OF THE INVENTION 
   The present invention aims at overcoming the disadvantages of conventional redundancy memory circuits. 
   The present invention more generally relates to any integrated matrix structure of identical elements associated with at least one redundancy element to take over the operation of a defective element. These can be, for example, identical operators implemented in the form of a matrix network in an integrated circuit in one or two directions (for example, operators organized in systolic architecture), or even more complex elements (for example, processors) which are either used individually by an adapted selection addressing in a matrix network including several of these elements, or used in parallel by all receiving common information in a first direction and one or several individualized pieces of information in a second direction. Except for memories, the more the integrated element is complex, the more this element risks to have a temporary operation failure linked, for example, to a non-reproducible malfunction during a system initialization procedure. 
   According to a first aspect, the present invention aims at enabling a dynamic and reversible repair of a defect in a memory cell by using a redundancy element. 
   More generally, according to this first aspect, the present invention aims at enabling the use of a redundancy element in an integrated circuit including a matrix structure of identical elements by repairing, dynamically and reversibly, a failure of one of the elements by the use of the redundancy element. 
   The present invention also aims at combining a reversible repair with an irreversible repair. 
   According to a second aspect, the present invention aims at optimizing the use of redundancy lines (rows or columns) in a memory by inhibiting the operation of a line including a defective cell, which is different from one matrix network to the other in a circuit including several matrix networks all simultaneously addressed according to a first direction. 
   To achieve these and other objects, the present invention provides an integrated circuit including at least one matrix network of identical elements capable of being individually addressed at least in a first direction and including, at least for this first direction, at least one redundancy element, and means for reversibly inhibiting the operation of a defective element and for maintaining the circuit operation by using the redundancy element. 
   According to an embodiment of the present invention, the circuit further includes a means for definitively inhibiting the operation of a defective element. 
   According to an embodiment of the present invention, applied to a circuit including several matrix networks of identical elements to be simultaneously addressed in a second direction, this circuit includes, for each matrix network, a redundancy circuit for organizing the use of a redundancy element associated with this matrix network independently from the other matrix networks. 
   The present invention also relates to a memory provided with at least one matrix network of cells and at least one redundancy element associated with a first direction, and including at least one redundancy circuit for reversibly modifying the addressing in the first direction to use the redundancy element in the presence of a defective memory cell in the matrix network. 
   According to an embodiment of the present invention, applied to a memory including at least n+1 successive columns of cells, the redundancy circuit includes as many logic units as the matrix network includes columns,  n  first units including a selector for routing an address conductor associated with the corresponding column, either to the memory cells associated with this column, or to the memory cells associated with the next column. 
   According to an embodiment of the present invention, each of the n first units further includes a fuse for definitively inhibiting the operation of a defective column. 
   According to an embodiment of the present invention, the memory includes several matrix networks and, for each network, at least one redundancy element associated with a first direction and at least one redundancy circuit, each redundancy circuit being individually controllable. 
   The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 , previously described, is meant to show the state of the art and the problem to solve; 
       FIG. 2  shows a memory circuit architecture according to an embodiment of the present invention; 
       FIG. 3  shows an embodiment of a redundancy circuit according to a first embodiment of the present invention; and 
       FIG. 4  partially shows a redundancy circuit according to a second embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   The same elements have been referred to with the same references in the different drawings. For clarity, only those elements which are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. 
     FIG. 2  shows an example of an architecture of a redundancy memory circuit according to an embodiment of the present invention. As previously, the memory circuit is, for example, organized in matrix networks or groups  1 ,  1 ′,  1 ″ of memory cells  2 ,  2 ′,  2 ″, each group  1 ,  1 ′,  1 ″ being associated with one of the bits of a memory word containing several bits. Each network includes, for example,  m  rows and n+1 columns, the column of rank n+1 forming, for each network  1 ,  1 ′,  1 ″, a redundancy column or element. 
   The present invention will be described hereafter in relation with a memory circuit provided with redundancy columns. It should however be noted that the present invention also applies to the case where the redundancy elements are formed by memory cell rows as well as to the case where redundancy elements are provided in both directions. 
   Row address Ar is, as previously, determined by a conductor in the active state among  m  row conductors and is the same for all matrix networks  1 ,  1 ′,  1 ″. Column address Ac corresponds to a conductor among n conductors in an active state. Addresses Ac and Ar correspond, for example, to the address conductors issued by decoders  4  and  5  of the circuit shown in FIG.  1 . 
   According to the present invention, each network  1 ,  1 ′,  1 ″ of memory cells is associated with a redundancy circuit  20 ,  20 ′,  20 ″ and reorganizes, independently from the possible reorganization of the column addresses of the other networks, the addressing of the column conductors in the presence of a defective memory cell. 
   Each group  1 ,  1 ′,  1 ″ also includes a circuit  21 ,  21 ′,  21 ″ for routing (SEL_MUX) the datum input on a terminal  8 ,  8 ′,  8 ″ to the column conductor (not shown in  FIG. 2 ) corresponding to the addressed memory cell among the n+1 columns of the group. In the embodiment shown in  FIG. 2 , circuits  21 ,  21 ′,  21 ″ also integrate input/output amplifiers (I/O AMP), one input/output amplifier being associated with each column conductor of group  1 ,  1 ′,  1 ″ as close as possible to the memory cells of this group. Indeed, recent memories are most often provided to operate with signal levels, especially in the read mode, which are very low, which leads to placing the input/output amplifiers as close as possible to the memory cells. Such an architecture appears in particular in the case of a DRAM. 
   Each redundancy circuit receives the  n  column addresses Ac and includes n+1 column conductor outputs, the n+1-th output being used in case of a failure of a cell of a column of lower rank. Preferably, the reorganization of the column addresses is performed, as in the conventional case shown in  FIG. 1 , by a successive shifting of the column conductors from the conductor of the defective cell. Each circuit  20 ,  20 ′,  20 ″ also includes an input  22 ,  22 ′,  22 ″ of reception of a shift control signal (repair) and an output  23 ,  23 ′,  23 ″ indicative of the use of the redundancy column. 
   The signals of repair and indication of the use of the redundancy column are issued and used by a circuit of control and detection of defective cells. This device can be formed of a conventional test system or can be formed of a test circuit  24  (shown in dotted lines in  FIG. 2 ) for automatically detecting a defective cell and then controlling the shifting of the address of the column including this defective cell to the next column, and so on. 
   According to the present invention, the means used to organize the shifting of the column conductors to the next conductors are volatile means, that is, they are reset upon each powering-off of the memory circuit. 
     FIG. 3  shows a first embodiment of a redundancy circuit  20 ,  20 ′, or  20 ″ of FIG.  2 . This circuit is formed of  n  units  29  associated with the  n  first column conductors of the matrix network  1 ,  1 ′, or  1 ″ and of a unit  29 ′ associated with the conductor of the redundancy column. The  n  units  29  include a selector  30  for routing the address signal present on a conductor Ac 0 , Ac 1 , . . . , Ac(n−1), either to column C 0 , C 1 , . . . , C(n−1) of same rank, or to the next column C 1 , . . . , C(n−1), Cn. Each selector  30  here is formed of two switches  31 ,  32 , which enable switching between a current column and the next column. Each unit  29 ,  29 ′ includes an element  33 , for example, a NAND gate, of detection of the state of the preceding cell and an element  34 , for example a MOS transistor, having the function of a reversible fuse of the corresponding cell. Transistors  34  are controlled by a signal Rs forming an order of cell repair by the use of the next column and so on until the redundancy column is reached. Although signal Rs is applied on the gates of all transistors  34 , it will be seen hereafter that the control is only effective on the transistor of the addressed column, that is, the signal Ac of which is in the active state. 
   In the example shown in  FIG. 3 , the memory circuit is formed of several sets of network  1 ,  1 ′,  1 ″ and the selection of a set of networks or another is performed via a predecoding address As which enables identification of which of the network sets is selected. Such an architecture is optional. In the present example, the selection means is formed, for each unit  29 , of a NOR gate  35 , a first input of which receives selection address As, and a second input of which receives the output of switch  31  of the current unit. Column conductor C 0 , C 1 , C(n−1), Cn corresponds to the output terminal of gate  35  and is connected to the gate of a MOS transistor  36  connected in series between a first terminal of a transistor  34  of the corresponding cell and the ground. In the absence of a predecoding, the column conductor is connected, via an inverter, to the output of switch  31 . A second terminal of transistor  34  is connected to a non-inverting control terminal of switch  31 , to an inverting input of switch  32  and to a first input of gate  33  of the corresponding unit  29 . A second input of gate  33  of each unit  29 ,  29 ′, except for the first one, is connected to the second terminal of transistor  34  of the unit of lower rank. The output of gate  33  of each unit  29  is connected to the input of an inverter  37 , the output of which is connected to the first input of gate  33 . The output of gate  33  of a given unit  29  is also connected to the gate of a MOS transistor  38  of the next unit  29 ,  29 ′, this transistor  38  being connected in series with a MOS transistor  39  between a positive supply terminal Vdd and the output terminal of the switch  32  of the preceding cell. This output terminal of each switch  32  is also connected to the second input terminal of gate  35  of the next unit. The output of gate  33  of each unit  29  is further connected to an inverting control terminal of switch  31  and to a non-inverting control terminal of the switch  32  of this unit. 
   The second input of gate  33  of the first unit  29  is connected to voltage Vdd. It should be noted that gate  33  of the first unit  29  is optional. It is however used in practice because it simplifies the reproduction of the redundancy circuit by enabling a repetition of units having the same components. The output terminal of inverter  37  of the last unit  29  forms terminal  23  ( FIG. 2 ) indicative of the use of the (or of the last) redundancy column. 
   The testing of a memory circuit according to the present invention can be performed conventionally, during manufacturing, from a memory bitmap which is compared to a previously stored model. In case of divergences, the position of the defects enables to determine from which column conductor the shifting has to be performed. 
   An advantage of the present invention is that it corrects other defects than defects proper to networks  1 ,  1 ′,  1 ″ of memory cells. Indeed, these can be a defect in the input/output amplifier associated with the column conductor, in the routing multiplexer of the datum present on terminal  8 ,  8 ′,  8 ″ associated with the column in which the cell is identified as defective, and in any other possible component individually associated with each of the n+1 columns of each memory cell network. 
   The operation of a redundancy circuit such as shown in  FIG. 3  is the following. If the identification of the defective column can be performed conventionally, the shifting from this defective column is performed, according to the present invention, by a column scanning. 
   If no error is identified, the cell of rank n+1 of circuit  20  and the corresponding column of memory cells is not used. In this case, signal Rs is in the inactive state (for example, “0”) and, for each of units  29 , switch  31  is in the on state while switch  32 , operating in reverse fashion, is in the blocked or non-conducting state. The switch  31  of each selector  30  thus connects the address conductor of the unit corresponding to the first input of gate  35  associated with the corresponding column conductor. 
   Since the gate of transistor  38  of the first cell is connected to the ground, this first transistor  38  is always on. Transistor  39  of each unit  29 ,  29 ′ is off as long as its gate is in the high state (“1”), that is, transistor  39  of units  29 ,  29 ′ is off for all cells of lower rank than a possibly defective column. The second input of gate  33  of the first unit always is at state “1” by being connected to voltage Vdd. 
   If a defective column is detected, signal Rs is placed in its active state, which turns on transistor  34  as well as transistor  36  of the addressed cell. Indeed, since the gate of transistor  36  is connected to column conductor C 0 , C 1 , C(n−1) or Cn of the corresponding unit, this transistor is on only when the unit containing it is addressed. The turning-on of transistors  34  and  36  forces the output of gate  33  to state “1”, with switch  32  turning on and switch  31  turning off. As a result, address Ac associated with the column being addressed is transferred by switch  32  onto the second input of gate  35  of the next column. State “1” of gate  33  is transferred, by being inverted, to the first input of gate  33  associated with the next column. Thus, selectors  30  of the next columns which are controlled from the output of the corresponding gate  33  are all controlled to cause a shifting of the column conductor to the next column conductor, from the column where the error has appeared to the redundancy column. 
   Unit  29  associated with the defective column is, as for itself, disconnected from the general redundancy circuit. Indeed, once transistors  34  and  36  have been simultaneously turned on, they can only be turned back off by powering off the circuit. Signal Rs can thus return to state “0” for the addressing of the next column. 
   From a static point of view, the outputs of all gates  33  of the columns of lower rank than a defective column are at state “0” and the outputs of gates  33  of the defective column are at state “1”. When the system is powered off, this state disappears and the column shifting also disappears, which enables to perform a new test procedure upon each powering-on of the memory circuit. 
   An advantage of the present invention is that it enables a dynamic and reversible operation of the redundancy circuit. Thus, defects appearing during the operation of the memory circuit can be isolated, by a test procedure. 
   In other applications of the redundancy circuit to repetitive elements other than memories, this advantage is particularly substantial if temporary defects are likely to appear. The redundancy column is then used to correct a mobile defect, for example, upon each powering-on of the system. 
     FIG. 4  shows a second embodiment of a redundancy circuit according to the present invention. In  FIG. 4 , a single cell of a redundancy circuit according to this second embodiment has been shown. 
   A feature of this second embodiment is to associate a reversible disconnection means ( 34 ,  36 ) of each column with a fuse  40  of definitive disconnection of this column. 
   According to the present invention, fuse  40  is, preferably, fused during a memory circuit testing phase during manufacturing, to use the redundancy column in the presence of a manufacturing defect which is, by nature, permanent. 
   Thus, while enabling the definitive suppression of defects during the manufacturing of the memory circuit, the use of a free redundancy column, that is, of a column unused to correct a manufacturing defect, is authorized, dynamically and reversibly during the operation of the memory circuit. 
   The general structure of a unit  29  shown in  FIG. 4  is substantially similar to the structure discussed in relation with  FIG. 3  for the dynamic and reversible operation. Only the differences between the two embodiments linked to the addition of a fuse  40  will be discussed. To enable the programming of a redundancy circuit according to this second embodiment, the circuit includes an additional terminal Rd for causing the fusing of fuse  40  of the unit  29  being addressed during the testing to isolate the corresponding column. Inverter  37  has been shown in detail in FIG.  4  and is formed of two MOS transistors  41 ,  42 , connected in series between a first terminal of fuse  40  and the ground, a second terminal of fuse  40  being connected to voltage Vdd. The gates of MOS transistors  41  and  42  are connected to the output of gate  33  which forms a terminal  43 ′ connected to the gate of transistor  38  (not shown) of the next unit. The midpoint of the series association of transistors  41  and  42  forms the output of inverter  37  connected to the first input of gate  33  of the current unit. The first terminal of fuse  40 , which provides the biasing of inverter  37 , is connected to the ground via a MOS transistor  44 . 
   If fuse  40  has not fused, that is, if the corresponding column has not been considered as defective during the manufacturing test, the operation of unit  29  shown in  FIG. 4  is similar to that discussed in relation with FIG.  3 . Transistor  44  is used as a fusing means for fuse  40 . Transistor  44  is controlled by an assembly based on MOS transistors  45 ,  46 ,  47 ,  48 , controlled by a switching to the high state of signal Rd. Two transistors  47  and  48  are connected in parallel between a series association of two transistors  45  and  46  and the ground, transistor  45  receiving signal Rd. The gate of transistor  45  is connected to a terminal  43 , that is, to the output of gate  33  of the preceding unit. The gate of transistor  46  is connected to the output of inverter  37  of the current unit, and thus to a terminal  49 ′ corresponding to the second input of gate  33  of the next unit. Transistors  47  and  48  form a switch similar to switch  31  or  32  and illustrate an embodiment of these switches by analogy. 
   In the example shown in  FIG. 4 , it is assumed that a single redundancy column is used. Thus, the fusing of a fuse  40  of a given column occurs at the condition that the signal present on terminal  43  is at state “0”, that is, that no preceding column has been corrected yet. Transistor  45  to  48  are on and transistor  44  is off. It is assumed that signal Rd switches to state “1” to indicate a need for a definitive column repair. This switching to state “1” turns on transistor  44 , which fuses fuse  40 . Thus, transistor  44 , turns on, which causes the fusing of fuse  40 . As indicated hereabove, this fusing introduces a permanent “0” on the output of inverter  37 , which prevents any return to normal operation of the corresponding column. 
   When fuse  40  has fused, a permanent state “0” is forced on the first input of gate  33  by inverter  37 . Thus, selector  30  of the corresponding unit directs address Ac via conduction means  32  and via a link  50 ′ to the next unit. This state “0” at the output of inverter  37  also inhibits the operation of transistors  34  and  36  under the action of a possible signal Rs. Further, this permanently forces the turning-on of transistor  39  and, accordingly, a state “1” on the second input of gate  35 , and thus a state “0” on its output. Thus, transistor  36  is permanently off, which eliminates any power consumption in the defective memory column. 
   The output of gate  33  is forced to state “1”, which blocks (renders non-conducting) transistors  45  and  47  of the next unit, preventing the breakdown of another fuse. 
   If a single redundancy column is used per network of memory cells, a definitive repair forbids the use of a reversible repair. Conversely, if each network is associated with several redundancy columns, there can remain available redundancy columns to perform a reversible repair in addition to the definitive repair performed during the manufacturing of the memory circuit. The modifications to be brought to units  29  are within the abilities of those skilled in the art based on the desired functionality. 
   It should be noted that the embodiments described in relation with  FIGS. 3 and 4  may be modified according to the respective states of repair control signals Rs, Rd. Further, other switching means may be used provided that they respect the functional indication given hereabove. 
   An advantage of the present invention is that it optimizes the use of a memory circuit equipped with redundancy elements by enabling the use of the redundancy elements associated with each network  1 ,  1 ′,  1 ″ ( FIG. 2 ) of the memory independently from one another. 
   Another advantage of the present invention is that it associates a definitive repair of an element of the memory to a repair available to the user. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, although the present invention has been described in relation with a memory circuit, the present invention applies, more generally, to any integrated circuit provided with matrix networks of identical elements. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalent thereto.