Abstract:
A memory array  2  has an address decoder  12  responsive to a repair signal to operate either in a normal mode or a repair mode. In the normal mode a data bit is stored within a single memory cell  6 . In the repair mode a data bit is stored within multiple memory cells  6  of a common column of memory cells sharing bit lines  8 . This provides increased defect resistance when operating in the repair mode at the cost of reduced memory capacity.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to the field of memory arrays within integrated circuits. More particularly, this invention relates to the repair of defective circuits within memory arrays. 
         [0003]    2. Description of the Prior Art 
         [0004]    It is know to provide memory arrays with repair circuits which seek compensate for defective circuit elements within those memory arrays. As an example, it is known to provide redundant column and redundant row circuitry whereby, if a column or a row is defective, then it can be functionally substituted with a redundant column or a redundant row. Whilst these techniques are effective in improving the yield of useable integrated circuits, they suffer from the disadvantage of requiring additional circuit area overhead to provide the redundant columns and rows. Furthermore, a finite number such redundant columns may be provided and, if the number of defective rows or columns exceeds the number of redundant rows or columns, then repair then cannot be made and the integrated circuit cannot be used. 
         [0005]    As process geometry is falling in size, the vulnerability of circuit elements within memory arrays to defects in their manufacture increases. As an example, the SRAM bits cell current on-to-off ratio has been decreasing rapidly as processed technology decreases in size. This reduction in the on-to-off ratio is primarily the result of sub-threshold leakage or “off” current of transistors used within such bit cells becoming a significant fraction of the “on” current due to what are commonly known as “short-channel” effects. Furthermore, the bit cell “on” current is not increasing in line with the historical scaling trends due to many reasons, including scaling limitations of gate oxide and threshold voltage. In addition, the increased significance of two dimensional electric field effects and other effects relating to the physics of operation of small scale circuit elements further increase the “off” current due to phenomenon such as gate tunneling and band-to-band tunneling which make the ratio of the on-to-off current even worse. As a consequence of the above factors, a bit cell being read might not be able to provide sufficient current for the bit line pair to develop a sufficient voltage differential during the time period required for sensing the stored data value. This would result in either an error in the state being read or the speed requirements for the memory not being met. If there is a large number of such bit cells within a single block of memory beyond the number that can be repaired by traditional redundancy techniques, then the integrated circuit will have to be discarded. It should be noted that the traditional redundancy schemes, and particularly redundant rows, have a significant area and timing overhead. In addition, they are ill suited to repairing for drifts in process parameters, weak or erratic bits or defects of a gross nature. 
         [0006]    One approach to the above would be to design the memory arrays with a significant margin to cover expected process variations. However, as the process scales to smaller geometries, acceptable levels of margin in the design still result in a significant number of integrated circuits being produced which have errors beyond those which can be repaired by traditional row and column redundancy techniques. This problem is likely to become more severe as the geometries scale to smaller sizes. 
       SUMMARY OF THE INVENTION 
       [0007]    Viewed from one aspect the present invention provides an integrated circuit comprising: 
         [0008]    at least one array of memory cells having a plurality of rows of memory cells and a plurality of columns of memory cells; 
         [0009]    a plurality of bit lines, each of said plurality of bit lines being coupled to a respective column of said memory cells; 
         [0010]    a plurality of word lines carrying respective word line signals, each of said plurality of word lines being coupled to a respective row of said memory cells to control coupling of said respective row of memory cells to said plurality of bit lines so as to provide access to data bits stored therein; 
         [0011]    an address decoder responsive to input memory addresses to generate said word line signals; and 
         [0012]    repair control circuitry responsive to a repair signal to control said address decoder during a memory access operation to an memory input address such that: 
         [0013]    in a normal mode, a single row of memory cells is selected for access by one word line signal on one word line such that for each column of memory cells one memory cell is coupled to a corresponding bit line of said plurality of bit lines to provide access to a data bit as stored within said one memory cell; and 
         [0014]    in a repair mode, a plurality of rows of memory cells are selected for access by a plurality of word line signals on a plurality of word lines such that for each column of memory cells a plurality of memory cells are coupled to said corresponding bit line of said plurality of bit lines to provide access to said data bit as stored within said plurality of memory cells. 
         [0015]    The present technique recognises that rather than discard an integrated circuit containing a memory array with defects that cannot be repaired in other ways, it is beneficial to instead salvage the integrated circuit by providing the capability to switch between a normal mode, in which a single bit cell which stores a data value, to a repair mode in which more than one memory cell (typically two) is used to store each data value. With multiple memory cells being used to store each data value there is an increased likelihood of the integrated circuit being able to be salvaged, although at the cost of reduced memory storage capability. The multiple memory cells used are normal functional memory cells. In contrast to the normal redundancy techniques in which defective memory cells are substituted by the memory cells in a redundant row with one data bit still being stored in one memory cell, the present technique overcomes the weakness of a given memory cell by combining it with another memory cell such that the two memory cells working together as a combined entity are able to properly function to store a data bit. The memory cells which are combined in this way share bit lines such that they can be read together through those shared bit lines and written together through those shared bit lines. It will be appreciated that whilst the memory array has a reduced storage capacity compared to the normal mode, in which a data bit is stored in a single memory cell, the repair mode provides a significantly reduced storage capacity due to the need to share memory cells to store a single data bit, but nevertheless provides a working integrated circuit which would otherwise have been scrap. It will be appreciated that this technique cannot repair all possible defects, but there are a significant number of types of defects which maybe overcome using this technique. 
         [0016]    Whilst the address decoder can take a variety of different forms, a form in which the address decoder is formed of a pre-decoder and a word line decoder is well suited for adaptation in accordance with the current technique whereby the pre-decoder can be modified such that in the repair mode a single input memory address to the pre-decoder generates pre-decoded signals passes to the word line decoder that match pre-decoded signals that would be passed in response to decoding of a plurality of separate addresses when operating the normal mode. This conveniently allows multiple rows of memory cells to be simultaneously selected for connection to the word lines. 
         [0017]    Whilst the repair control circuitry can take many forms, a simple and effective one is that it acts to set a plurality of the signals passed to the word line decoder to fixed values such that multiple word line selecting signals are generated by the word line decoder. It is convenient to set the signals corresponding to a most significant bit of an input address to be fixed values as this provides that two memory cells will be used to store each data bit and it is normal for memory addresses having a different most significant bit to physically correspond to widely space memory cells within the memory array in a manner in which reduces the likelihood that both memory cells will simultaneously be defective. 
         [0018]    Whilst the present technique can be usefully employed within an integrated circuits including only a single array of memory cells, the technique is also useful within an integrated circuits containing multiple arrays of memory cells. In practice these different arrays of memory cells can be individually placed in normal mode or repair mode such that it is likely that only some of arrays will need to operate in the repair mode at reduced storage capacity thereby salvaging what might otherwise have been a scrap integrated circuit and yet only losing storage capacity within some of the arrays of memory cells. 
         [0019]    Within the context of integrated circuits containing a plurality of arrays of memory cells, an array select signal can be used to control the repair control circuitry to select one of the plurality of arrays of memory cells to operated in the repair mode. A single array requiring repair by operating in the repair mode is statistically the most likely occurrence and accordingly simplifying the repair control circuitry to operating in this manner represents a good trade off between complexity and usefulness. 
         [0020]    It will be appreciated that when operating in the repair mode whilst sharing memory cells to drive values out onto the bit lines may increase read speed, the need to write into more than one memory cell during a write could reduce write speed and accordingly it is desirable to provide a write signal driver of increased strength and of at least sufficient strength to write to the number of memory cells which act together in the repair mode. 
         [0021]    The same issue can also be addressed by providing timing and control circuitry able to extend the write cycle time and potentially reduce the read cycle time. 
         [0022]    It will be appreciated that the repair signal may be generated in a number of different ways and may, for example, be a signal from an external pin of the integrated circuit or a signal generated by a fusible link within the integrated circuit or a combination. The array select signals may also be generated in a similar way, either by one or more external pins or one or more fusible links or a combination. 
         [0023]    The present technique is applicable to arrange of different types of memory array, such as arrays having a single bit line per memory cell, bit line pairs per memory cell, DRAM memory, SRAM memory and other types of memory. 
         [0024]    It will also be appreciated that whilst the present technique can be used upon integrated circuits comprising dedicated memory integrated circuits, the technique is also useful within integrated circuits carrying a variety of other on-chip elements, such as general purpose processors and the like as is increasingly commonly found within so called system-on-chip integrated circuits. 
         [0025]    Viewed from another aspect the present invention provides an integrated circuit comprising: 
         [0026]    at least one array of memory cell means having a plurality of rows of memory cell means and a plurality of columns of memory cell means; 
         [0027]    a plurality of bit line means, each of said plurality of bit line means being coupled to a respective column of said memory cell means; 
         [0028]    a plurality of word line means carrying respective word line signals, each of said plurality of word line means being coupled to a respective row of said memory cell means to control coupling of said respective row of memory cell means to said plurality of bit line means so as to provide access to data bits stored therein; 
         [0029]    address decoder means for generating said word line signals in response to input memory addresses; and 
         [0030]    repair control means for controlling said address decoder means during a memory access operation to an input memory address and a repair signal such that: 
         [0031]    in a normal mode, a single row of memory cell means is selected for access by one word line signal on one word line means such that for each column of memory cell means one memory cell means is coupled to a corresponding bit line means of said plurality of bit line means to provide access to a data bit as stored within said one memory cell means; and 
         [0032]    in a repair mode, a plurality of rows of memory cell means are selected for access by a plurality of word line signals on a plurality of word line means such that for each column of memory cell means a plurality of memory cell means are coupled to said corresponding bit line means of said plurality of bit line means to provide access to said data bit as stored within said plurality of memory cell means. 
         [0033]    Viewed from a further aspect the present invention provides a method of operating an integrated circuit having at least one array of memory cells with a plurality of rows of memory cells and a plurality of columns of memory cells, a plurality of bit lines, each of said plurality of bit lines being coupled to a respective column of said memory cells, a plurality of word lines carrying respective word line signals, each of said plurality of word lines being coupled to a respective row of said memory cells to control coupling of said respective row of memory cells to said plurality of bit lines so as to provide access to data bits stored therein and an address decoder responsive to input memory addresses to generate said word line signals; said method comprising 
         [0034]    in response to a repair signal, controlling said address decoder during a memory access operation to an input memory address such that: 
         [0035]    in a normal mode, a single row of memory cells is selected for access by one word line signal on one word line such that for each column of memory cells one memory cell is coupled to a corresponding bit line of said plurality of bit lines to provide access to a data bit as stored within said one memory cell; and 
         [0036]    in a repair mode, a plurality of rows of memory cells are selected for access by a plurality of word line signals on a plurality of word lines such that for each column of memory cells a plurality of memory cells are coupled to said corresponding bit line of said plurality of bit lines to provide access to said data bit as stored within said plurality of memory cells. 
         [0037]    The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1  schematically illustrates a memory array operating in a normal mode; 
           [0039]      FIG. 2  schematically illustrates a memory array operating in a repair mode; 
           [0040]      FIG. 3  schematically illustrates a pre-decoder of a known form; 
           [0041]      FIG. 4  schematically illustrates the pre-decoder of  FIG. 3  modified to operate in accordance with one example of the present techniques; 
           [0042]      FIG. 5  schematically illustrates a memory incorporating multiple memory arrays each using one example of the current repair technique; 
           [0043]      FIG. 6  illustrates a pre-decoder and word line decoder using one example of the present techniques; and 
           [0044]      FIG. 7  schematically illustrates a system-on-chip integrated circuit incorporating multiple memory arrays. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0045]      FIG. 1  schematically illustrates a memory array  2  comprising a plurality of rows of memory cells  4 . An individual memory cell  6  is shown in expanded form coupled to a bit line pair  8  and a word line  10 . It will be appreciated that the memory array  2  can be considered to be formed of a plurality of rows of memory cells, those memory cells also lining up to form a plurality of columns of memory cells  4 . The memory cells of a given column of memory cells are coupled to the same bit lines  8  which run along the column. The memory cells  6  within a row of memory cells  4  share a common word line  10 . When an address decoder  12  decodes an input address (or part of such an address), it operates in a normal mode of operation to generate a single word line select signal on a single word line  10  which connects the memory cells  6  within that row with the bit lines of their respective columns. When the memory cells  6  are connected to the bit lines  8 , data can be read out of the memory cell  6  or written into the memory cells  6  depending upon whether a read operation or a write operation is being performed. When a write operation is being performed, a write driver circuit  14  acts to drive the appropriate voltage levels onto the bit lines  10  for a write cycle period in order for the desired data bits to be captured and stored within the selected memory cell  6 . The operation of the memory array  2  illustrated in the normal mode in  FIG. 1  is conventional and will be familiar to workers in this technical field. It will be noted that the effective address range of the memory array  2  in the normal mode spans the full memory array  2 . 
         [0046]      FIG. 2  illustrates the memory array  2  of  FIG. 1  operating in a repair mode. In the repair mode a single address input to the address decoder  12  results in two word line signals being generated and supplied to two separate rows of memory cells  16 ,  18 . The memory cells  6  of both of these rows of memory cells  16 ,  18  are connected to the bit lines  8  of the column of memory cells in which they lie. As illustrated in expanded form in  FIG. 2 , the memory cells  6  from both rows  16 ,  18  serve to drive the bit lines  8  when the operation is a read and accordingly weakness in one of the memory cells  6  may be compensated for by the other of the memory cells  6 . When the operation is a write, the data value being generated by the write driver  14  is written into both of the memory cells  6 . In this way, it will be seen that a given data bit is stored within two memory cells  6  in this example embodiment and these two memory cells are simultaneously connected to the bit lines during read and write operations. Also illustrated in  FIG. 2  is the effective address range of the memory array  2  in the repair mode. Since each data bit requires two memory cells  6  for its storage, the storage capacity of the memory array  2  in the repair mode is halved and thus the effective address range is shown as halved with the remainder of the memory array  2  being used for the duplicated cells. 
         [0047]    It will be appreciated that whilst the example of  FIG. 2  shows a given data bit been stored in two memory cells, it would be possible for the data bit to be stored in more than two memory cells, but this would be at the cost of further reducing the storage capacity. 
         [0048]      FIG. 3  schematically illustrates a known type of pre-decoder that can form part of the address decoder  12 . This is a 4-to-1 decoder in which two address bits ai, aj are input in both true and inverted form to four NAND gates and serve to generate a select signal on one of the outputs. This type of pre-decoder will be familiar to those in this technical field. 
         [0049]      FIG. 4  illustrates a modification of the pre-decoder of  FIG. 3  in accordance with one example of the current techniques. A repair signal is input on signal line  20  and serves to render non-conductive transition gates  22  relating to one of the input address bits ai and tie off the corresponding inputs to the NAND gates to a fixed level using the transistors  24 . Thus, for any given address input to the pre-decoder of  FIG. 4 , two of the output signals will be active (i.e. corresponding to the input of two different addresses) and accordingly multiple word line signals will be generated when the word line decoder acts upon the output of the pre-decoder. 
         [0050]      FIG. 5  illustrates a memory  26  comprising a plurality of memory arrays  28 ,  30 ,  32 ,  34  each of which can separately be placed into the repair mode if required. An array selecting signal input online  36  is a two-bit signal indicating which of the memory arrays  28 ,  30 ,  32 ,  34  is to be placed in repair mode. In this example embodiment, only a single memory array  28 ,  30 ,  32 ,  34  can be placed into the repair mode at any one time, but it would be possible in other embodiments to have multiple arrays simultaneously in the repair mode if desired. However, it will be most common only to need to place one array into the repair mode at any given time. Sub-array decoders  38 ,  40 ,  42 ,  44  are associated with each array  28 ,  30 ,  32 ,  34  serve to decode the array select signal and generate an output if a match occurs. This output is ANDed with a global repair signal by AND gates  46 ,  48 ,  50 ,  52  to generate a repair signal for the particular sub-arrays. This repair signal is fed to the pre-decoder of the particular sub-array concerned and processed as illustrated in  FIG. 4 . It will be appreciated that the array select signal  36  and the global repair signal can be input via external pins of the integrated circuit or maybe generated internally by fusible links or a combination of these techniques. 
         [0051]      FIG. 6  illustrates an address decoder having a plurality of pre-decoders  54 ,  56 ,  58  and a word line decoder  60 . In this example, only the pre-decoder  58  is subject to having its action modified by the repair signal as this will be sufficient to ensure two word line signals will be generated in the repair mode thereby activating two rows of memory cells when required. It will be seen in  FIG. 6  that the word line decoder  60  comprises a set of 3-input NAND gates connected to respective combinations of output signals of the pre-decoders  54 ,  56 ,  58  to generate sixty four different respective word line signals. The word line decoder  60  is one suited for a memory array having 64 rows of memory cells. It will be appreciated that depending upon the form and size of the memory array concerned, the address decoder illustrated in  FIG. 6  could take a variety of different forms. The repair signal serving in these different forms when active will cause two word line signals to be asserted for each input address in a manner in which will cause the bit line signals to be shared by two memory cells  6 . 
         [0052]      FIG. 7  schematically illustrates an integrated circuit  62  in a form of a system-on-chip including multiple memory arrays comprising a main memory  64  and a tightly coupled memory  66 . Also provided upon the integrated circuit  62  are a general purpose microprocessor  68  and a digital signal processor  70  (which may contain additional memory and other circuit elements).  FIG. 7  is schematic and it will be appreciated that such an integrated circuit will typically contain many further circuit elements. The multiple memory arrays  64 ,  66  provided upon the integrated circuit  62  can be in the form SRAM, DRAM or other forms of memory, such as ROM. Each of these different memory arrays  64 ,  66  can separately use the repair mode technique described above or it maybe that only one of these memory  64 ,  66  needs to use this technique. 
         [0053]    Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.