Abstract:
A memory having built-in self repair with column shifting is provided. The total single columns are divided into smaller column groups and a bad column group is repaired with a redundant column group. Each column group is multiplexed into a pair of column group bitlines, which are fed into a shift circuit for the column group and a shift circuit for an adjacent column group. The shift circuit for the column group nearest the redundant column group receives the bitlines for that column group and the redundant column group bitlines. If a bad column group is detected, then starting with the column group furthest from the redundant column group, the shift circuit for each column group before the bad column group is deactivated. The shift circuit for the bad column group and the shift circuit for each column group after the bad column group are activated. Therefore, the bad column group is shifted out of the memory and the redundant column group fills the void.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to memory design and, in particular, to built-in self repair. Still more particularly, the present invention provides a method and apparatus for providing built-in self repair with column shifting.  
           [0003]    2. Description of the Related Art  
           [0004]    A memory circuit is like an electronic checkerboard, with each square holding one bit of data or instruction. Each square, also referred to as a “cell,” has a separate address and can be manipulated independently. Cells are addressed as columns of cells. When manufacturing a memory circuit, or chip, columns may be combined for input/output (I/O) operations. For example, eight columns may be multiplexed (MUXed) together into a single I/O. This is referred to as an 8:1 MUX. Thirty-two columns may be multiplexed together into an I/O, referred to as a 32:1 MUX.  
           [0005]    Due to difficulties in manufacturing a memory with a high number of elements, bad columns or groups of columns are likely to occur. After fabrication, a memory chip may be tested to determine whether bad columns or groups of columns exists. However, discarding a memory chip for one bad column is costly. Therefore, memory circuits are designed with built-in self repair (BISR).  
           [0006]    In order to achieve memory column built-in self repair, some redundant columns are required to replace the bad columns in the memory itself. One BISR schemes replaces a bad column by using a redundant I/O and its associated columns (32 columns for a 32:1 mux). This technique is referred to as “I/O repair” and is easy to implement. FIG. 1 is a diagram illustrating an example of an I/O repair scheme. The memory circuit includes a plurality (N+1) of groups (I/Os) of thirty-two columns. I/O  0   102 , I/O  1   104 , and I/O N  106  are shown. The memory circuit has a 32:1 MUX, multiplexing thirty-two single columns to a single data output or input.  
           [0007]    The memory circuit in FIG. 1 also includes redundant I/O  108  for repairing a bad I/O. In this case, the redundant memory consists of a total of thirty-two columns. Thus, an I/O is a group of column groups with each column group including eight columns. MUX and buffer  112  may be selectively controlled to provide access to I/O  0   102  or redundant I/O  108 . Similarly, MUX and buffer  114  may be selectively controlled to provide access to I/O  1   104  or redundant I/O  108  and MUX and buffer  116  may be selectively controlled to provide access to I/O N  106  or redundant I/O  108 .  
           [0008]    In the example shown in FIG. 1, I/O  1   104  has bad columns. Therefore, MUX and buffer  114  may be selected to provide access to the redundant I/O rather than I/O  1 . This may be done after fabrication, such as by burning a fuse. Alternatively, the columns may be tested and control signals may be set for the multiplexors at system start-up.  
           [0009]    The I/O repair scheme shown in FIG. 1 has several disadvantages. First, the area penalty for the redundant columns may become very large for high column multiplexing. For example, a 32:1 MUX multiplexes thirty-two columns into a single I/O bit. For a memory with ten I/O bits and a 32:1 MUX, there would be three hundred twenty columns and thirty-two redundant columns for a 10% area penalty. Furthermore, there is a significant performance penalty, because the redundant I/O has to be multiplexed with all regular I/O. The long wire  120  through the entire I/O width has a very large capacitance. Furthermore, an additional multiplex and buffer state must be added to the critical path.  
           [0010]    Next, with reference to FIG. 2, a single column repair scheme is illustrated. The memory circuit includes a plurality (N+1) of groups of thirty-two columns. I/O  0   202 , I/O  1   204 , and I/O N  206  are shown. Each I/O includes single redundant columns  212 ,  214 ,  216  that may be multiplexed into the I/O. This repair scheme is more difficult to implement because it requires a control circuit for each single column to decide whether or not to repair the single column. In addition, the layout of the circuit is difficult to fit into the narrow single column pitch. Therefore, a lot of extra area needs to be added to the column MUX area in addition to the redundant columns.  
           [0011]    Therefore, it would be advantageous to provide an improved method and apparatus for built-in self repair of columns in a memory circuit.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides a built-in self repair with column shifting for memory circuits. The columns in the memory are divided into smaller column groups and a bad column group is repaired with a redundant column group. Each column group is multiplexed into a pair of column group bitlines, which are fed into a shift circuit for the column group and a shift circuit for an adjacent column group. The shift circuit for the column group nearest the redundant column group receives the bitlines for that column group and the redundant column group bitlines. If a bad column group is detected, then starting with the column group furthest from the redundant column group, the shift circuit for each column group before the bad column group is deactivated. The shift circuit for the bad column group and the shift circuit for each column group after the bad column group are activated. Therefore, the bad column group is shifted out of the memory and the redundant column group fills the void.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0014]    [0014]FIG. 1 is a diagram illustrating an example of an I/O repair scheme;  
         [0015]    [0015]FIG. 2 illustrates a single column repair scheme;  
         [0016]    [0016]FIG. 3 is an example diagram illustrating a column group repair scheme with column shifting in accordance with a preferred embodiment of the present invention;  
         [0017]    [0017]FIG. 4 is a diagram illustrating an example column group repair by shifting out a bad column group in accordance with a preferred embodiment of the present invention;  
         [0018]    [0018]FIG. 5 is an example diagram illustrating a column shifting column group repair scheme with 8:1 MUX in accordance with a preferred embodiment of the present invention;  
         [0019]    [0019]FIG. 6 is a block diagram illustrating a memory with column shifting column group repair in accordance with a preferred embodiment of the present invention;  
         [0020]    [0020]FIG. 7 is a circuit diagram of an example shift circuit in accordance with a preferred embodiment of the present invention; and  
         [0021]    [0021]FIG. 8 is a flowchart of the operation of a column shifting repair scheme in accordance with a preferred operation of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0022]    The description of the preferred embodiment of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention the practical application to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.  
         [0023]    With reference now to the figures and in particular with reference to FIG. 3, an example diagram illustrating a column group repair scheme with column shifting is shown in accordance with a preferred embodiment of the present invention. The memory circuit includes a plurality (N+1) of groups of thirty-two columns. I/O  0   302  and I/O  1   304  are shown. For simplicity, only two I/Os are shown and the columns are divided into groups of eight columns. However, the memory may include more or fewer I/Os and may divide the columns into more or fewer columns depending upon the implementation.  
         [0024]    In this example, the first stage has eight single columns with eight pairs of bitlines multiplexed into one column group with one pair of column group bitlines. For example, I/O  0   302  has four groups of eight columns multiplexed into column group bitlines cgbl 0 , cgbl 1 , cgbl 2 , cgbl 3 . Similarly, I/O  1   304  has four groups of eight columns multiplexed into column group bitlines cgbl 4 , cgbl 5 , cgbl 6 , cgbl 7 . The memory also includes redundant column group  306 , which is one group of eight columns multiplexed into redundant column group bitlines rcgbl.  
         [0025]    In accordance with a preferred embodiment of the present invention, the memory includes shifting stage  330  including a shift circuit for each column group. Each pair of column group bitlines is fed into a shift circuit for the column group and a shift circuit for an adjacent column group. When the memory is tested and a bad column group is identified, the shift circuits for the bad column group and each column group after the bad column group are activated. Thus, the bad column group is shifted out and redundant column group  306  is shifted in at the end.  
         [0026]    In a third stage, each pair of shifted column group bitlines is fed into a sense amplifier and the four sense amplifier outputs for each I/O are then multiplexed into a I/O. In the example shown in FIG. 3, the shifted column group bitlines for I/O  0  are fed into sense amplifiers  322  and the outputs of sense amplifiers  322  are multiplexed into data output do( 0 ). Similarly, the shifted column group bitlines for I/O  1  are fed into sense amplifiers  324  and the outputs of sense amplifiers  324  are multiplexed into data output do( 1 ). The redundant column group bitlines from the shifting stage may also be fed into sense amplifier  326  and accessed as do(R).  
         [0027]    The column group repair scheme with column shifting of the present invention has a small area penalty, since only a small redundant column group of, for example, eight columns is provided rather than an entire I/O. The column shifting repair scheme has a minor time penalty due to shifting stage  330 ; however, the design is simplified and the area penalty is significantly reduced while still providing an effective repair scheme for a bad column group.  
         [0028]    Turning now to FIG. 4, a diagram is shown illustrating an example column group repair by shifting out a bad column group in accordance with a preferred embodiment of the present invention. I/O  0   402 , I/O  1   404 , and redundant column group  406  are shown. The memory also includes shifting stage  430  including a shift circuit for each column group. In the example shown in FIG. 4, the memory is tested and a bad column group is identified in I/O  1   404 , the bad column group having column group bitlines cgbl 5 . The shift circuits for cgbl 5  and the column group bitlines after the bad column group are activated. Thus, cgbl 0 , cgbl 1 , cgbl 2 , cgbl 3 , and cgbl 4  are allowed to pass through the shifting stage; however, cgbl 5  are shifted out and cgbl 6  are shifted into their place. Similarly, cgbl 7  are shifted into the place of cgbl 6  and the redundant column group bitlines, rcgbl, are shifted into the place of cgbl 7 .  
         [0029]    In a third stage, each pair of shifted column group bitlines is fed into a sense amplifier and the four sense amplifier outputs for each I/O are then multiplexed into a I/O. In the example shown in FIG. 4, the shifted column group bitlines for I/O  0  are cgbl 0 , cgbl 1 , cgbl 2 , and cgbl 3 . These column group bitlines are fed into sense amplifiers  422  and the outputs of sense amplifiers  422  are multiplexed into data output do( 0 ). However, the shifted column group bitlines for I/O  1  are cgbl 4 , cgbl 6 , cgbl 7 , and rcgbl. These column group bitlines are fed into sense amplifiers  424  and the outputs of sense amplifiers  424  are multiplexed into data output do( 1 ).  
         [0030]    With reference now to FIG. 5, an example diagram illustrating a column shifting column group repair scheme with 8:1 MUX is shown in accordance with a preferred embodiment of the present invention. The memory circuit includes a plurality (N+1) of groups of thirty-two columns. I/O  0   502  and I/O  1   504  are shown. For simplicity, only two I/Os are shown and the columns are divided into groups of eight columns. However, the memory may include more or fewer I/Os and may divide the columns into more or fewer columns depending upon the implementation.  
         [0031]    In this example, the first stage has eight single columns with eight pairs of bitlines multiplexed into one column group with one pair of column group bitlines. For example, I/O  0   502  has four groups of eight columns multiplexed into column group bitlines cgbl 0 , cgbl 1 , cgbl 2 , cgbl 3 . Similarly, I/O  1   504  has four groups of eight columns multiplexed into column group bitlines cgbl 4 , cgbl 5 , cgbl 6 , cgbl 7 . The memory also includes redundant column group  506 , which is one group of eight columns multiplexed into redundant column group bitlines rcgbl.  
         [0032]    In accordance with a preferred embodiment of the present invention, the memory includes shifting stage  530  including a shift circuit for each column group. Each pair of column group bitlines is fed into a shift circuit for the column group and a shift circuit for an adjacent column group. When the memory is tested and a bad column group is identified, the shift circuits for the bad column group and each column group after the bad column group are activated. Thus, the bad column group is shifted out and redundant column group  506  is shifted in at the end.  
         [0033]    In a third stage, each pair of shifted column group bitlines is fed into a sense amplifier and each sense amplifier output is then provided as an I/O. In the example shown in FIG. 5, the shifted column group bitlines for I/O  0  are fed into sense amplifiers  522  and the outputs of sense amplifiers  522  are provided as data outputs do( 0 ), do( 1 ), do( 2 ), and do( 3 ). Similarly, the shifted column group bitlines for I/O  1  are fed into sense amplifiers  524  and the outputs of sense amplifiers  524  are provided as data outputs do( 4 ), do( 5 ), do( 6 ), and do( 7 ). The redundant column group bitlines from the shifting stage may also be fed into sense amplifier  526  and accessed as do(R).  
         [0034]    The examples shown in FIGS.  3 - 5  are not meant to imply architectural limitations. For instance, the number of columns per group, the number of columns per I/O, the number of levels of multiplexing, and the ratio at which the columns are multiplexed may vary depending upon the implementation. Furthermore, the number of redundant columns may also vary. For example, the column group repair scheme may include two redundant column groups within the scope of the invention. The testing and repair may be performed after fabrication, such as by burning a fuse. Alternatively, the column groups may be tested and control signals may be set for the shift circuits at system start-up. In addition, the memory circuits are shown with arrows showing data outputs. However, memory access may take the form of input and output. Since the shift circuits are set and remain set as long as the memory is in use, data may be read from and written to the columns without risk of corruption.  
         [0035]    With reference to FIG. 6, a block diagram illustrating a memory with column shifting column group repair is depicted in accordance with a preferred embodiment of the present invention. The memory includes memory columns  602  and redundant columns  604 . Shift circuits  606  are selectively activated to shift out bad columns from memory columns  602  and redundant columns  604  are shifted in to fill the void. The memory also includes sense amplifiers and/or multiplexors  608  to provide access to the appropriate number of I/Os.  
         [0036]    In accordance with a preferred embodiment of the present invention, the memory includes test and repair logic  610 . Techniques for testing for bad columns are known in the art and are not the focus of the present invention. Test and repair logic  610  identifies a bad column group and sets the appropriate SHIFT control signals for shift circuits  606 .  
         [0037]    Next, with reference to FIG. 7, a circuit diagram of an example shift circuit is shown in accordance with a preferred embodiment of the present invention. Each shift circuit receives a shift control signal (SHIFT), a first pair of bitlines (BL 0 , BLN 0 ), and a second pair of bitlines (BL 1 , BLN 1 ). The shift circuit includes four pairs of transistors. Transistors  702 ,  706 ,  712 ,  716  are positive-channel metal oxide semiconductor (PMOS) transistors. Transistors  704 ,  708 ,  714 ,  718  are N-channel metal oxide semiconductor (NMOS) transistors. If SHIFT is deasserted (low), then transistors  702 ,  704 ,  712 ,  714  allow current to flow from bitlines BL 0 , BLN 0  to the shifted bitlines SBL, SBLN. Thus, BL 0 , BLN 0  may be set to the column group bitlines for the shift circuit.  
         [0038]    Alternatively, if SHIFT is asserted (high), then transistors  706 ,  708 ,  716 ,  718  allow current to flow from bitlines BL 1 , BLN 1  to shifted bitlines SBL, SBLN. Thus, BL 1 , BLN 1  may be set to the column group bitlines adjacent to the column group to which the shift circuit corresponds.  
         [0039]    The shift circuit shown in FIG. 7 is intended as an example and is not meant to imply architectural limitations. Other components and configurations may be used to achieve the same or similar function. A shift circuit, as shown in FIG. 7, may be included for each column group in the memory; however, other arrangements of shift circuits may be used at various levels of multiplexing or with a variety of column groupings.  
         [0040]    With reference now to FIG. 8, a flowchart of the operation of a column shifting repair scheme is illustrated in accordance with a preferred operation of the present invention. The process begins and tests the memory columns (step  802 ). Next, a determination is made as to whether the memory passes or fails the test (step  804 ). If the memory passes the test, the process passes the memory (step  806 ) and ends.  
         [0041]    If the memory fails the test in step  804 , a determination is made as to whether the memory is repairable (step  808 ). If the memory is repairable in step  808 , the process deactivates the shift circuits for each column group before the bad column group, starting with the column group farthest from the redundant column group (step  810 ). Then, the process activates the shift circuit for the bad column group and each column group after the bad column group ( 812 ). Thereafter, the process returns to step  802  to retest the columns. If the memory is not repairable in step  808 , the process fails the memory (step  814 ) and ends.  
         [0042]    Thus, the present invention solves the disadvantages of the prior art by providing a column group repair scheme that includes a shifting stage. A plurality of shift circuits are selectively activated to shift out a bad column group. A redundant column group is provided to fill the void. Thus, the column group repair scheme of the present invention is more simple to implement than the single column repair scheme and has a smaller area and performance penalty than the I/O repair scheme.