Abstract:
A memory device includes circuitry for replacing row pairs of primary memory having odd-even or even-odd addresses with rows of redundant memory having a corresponding odd-even or even-odd address pairing. The replacement memory rows maintain the odd-even address pairing of the primary memory rows being replaced which facilitates memory testing. The memory is loaded with a uniform test pattern, such as a checkerboard. When adjacent row pairs found to be defective are replaced with a corresponding pair of redundant memory rows, the uniform checkerboard pattern is maintained. Consequently, it is not necessary to load the test pattern into the redundant memory rows after replacement, thereby reducing the number of process steps and time required.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to redundancy repair of integrated circuit memory devices, and more particularly to a memory device having row redundancy repair. 
     2. Brief Description of the Related Art 
     Electronic memory, including read only memory (ROM), flash memory, and random access memory (RAM) devices, such as DRAMs and SRAMs, typically utilize memory arrays having rows and columns of memory cells for storing data. Manufactured memory arrays are tested for integrity, and defects are repaired. Typical tests involve writing data to the array using an external pattern, and reading the pattern back to confirm that the data is stored accurately. 
     When testing reveals a defective primary row or column, a non-defective redundant row or column is used to replace the defective primary row or column. This is achieved with fuses or anti-fuses that set hardwired latches associated with the defective row or column and map the address of the defective primary row or column to a fully-operable redundant row or column. With this re-mapping, all attempts to access the defective row or column will be redirected to the redundant row or column known to be properly working. The remapping process and logical substitution of the redundant region is transparent to the end user. U.S. Pat. No. 6,314,030 to Keeth, the entire disclosure of which is incorporated herein by reference, further describes memory circuit device testing and repair using redundant memory. 
     Testing memory devices by writing data to the array is a relatively slow process, especially for flash memory. During such memory testing, one bit pattern which is used employs a repetitive bit pattern, for example, of alternating ones and zeroes arranged in a “checkerboard” or inverse “checkerboard” pattern throughout the memory array. The pattern is written in blocks to the flash memory array. 
     The checkerboard testing pattern is obtained by loading each row of the flash memory array with an alternating series of 1&#39;s and 0&#39;s. The checkerboard pattern alternates from row to row, thus, even rows (rows 0, 2, 4, etc.) are loaded with a repeating pattern of 0&#39;s and 1&#39;s (0-1-0-1 . . . ), while odd rows (1, 3, 5, etc.) are loaded with a repeating pattern of 1&#39;s and 0&#39;s (1-0-1-0 . . . ). 
     A short between adjacent cells in two adjacent memory rows is a typical fault in a flash memory array, which requires replacement of the shorted pair of rows with a corresponding pair of redundant rows. Each pair of bad rows will be either an even and odd row (e.g., rows 2 and 3, for example), or an odd and even row (e.g., rows 3 and 4). 
     A flash memory device typically is divided into several memory banks, each of which is further divided into a plurality of memory blocks. Each memory block is provided with a number of primary rows and a lesser number of redundant rows, the latter bit available for repairing faulty primary rows in the associated block. Pattern writing during testing includes writing the pattern to the primary rows of the memory block, as well as to the redundant rows of the block, as these too must be tested for integrity. If a defective primary row is replaced by a redundant row during testing, the testing pattern must be rewritten to the block with the redundant row receiving the same bit pattern as the replaced defective row, for further testing. This is a time consuming process. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides an easier way to test primary rows and redundant rows in memory blocks, and to substitute redundant rows for defective primary rows while permitting continued testing without requiring a rewrite of the testing pattern. In the invention, pairs of defective rows of memory are replaced with redundant rows having an odd-even row pairing which matches the odd-even row pairing of the defective row pair. Consequently, a loaded test pattern of a redundant row used for repair matches that of the defective row which requires repair. This avoids having to rewrite the test pattern when making a repair using redundant rows and saves time in testing the memory. 
     A memory device according to the present invention includes memory blocks, each block having rows of primary memory cells having either an even or an odd address. Each memory block also includes at least two rows of redundant memory cells. Like each primary memory row, each row of redundant memory has an even or an odd row address. In each adjacent row pair, whether a primary or redundant row, one row has an even address, and the other row has an odd address. 
     During testing, the memory block is pre-loaded with an external test pattern, such as a checkerboard pattern, the arrangement of which alternates a “1” and “0” pattern at each column and with each row, as shown in the example in the Table below: 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE  
               
               
                   
               
               
                 Memory cells loaded with alternating (checkerboard) pattern. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 EVENROW &lt;0&gt; 
                 0 
                 1 
                 0 
                 1 
                 0 
                 1 
                 0 
                 . . . 
               
               
                 ODDROW &lt;1&gt; 
                 1 
                 0 
                 1 
                 0 
                 1 
                 0 
                 1 
                 . . . 
               
               
                   
               
             
          
         
       
     
     Upon detection of a failed odd-even or even-odd pair of primary memory rows, circuitry in the memory device remaps using programmed coding the addresses of the defective pair of adjacent rows, to a corresponding odd-even or even-odd pair of redundant rows as replacements. Advantageously, the replacement redundant rows have addresses that correspond correctly (odd or even) with the odd or even addresses of the replaced primary rows. By using the correct odd-even or even-odd pairing of redundant rows as replacements, the pre-loaded test pattern (checkerboard) will not change and need not be rewritten during further testing, so the pattern that is read out from the replacement redundant rows does not change from that of the replaced primary rows of the block during the further testing. The test pattern used need not be a checkerboard. Any pattern can be utilized that distinguishes even and odd rows, and preferably allows detection of failed row pairs. 
     Although preferred embodiments of the present invention are described with respect to repairing rows of memory arrays, the invention is not so limited. Repairs of shorted columns, for example, also could take place in odd/even pairs utilizing the teachings of the present invention. 
    
    
     The above advantage and features of the invention will be better understood from the following detailed description which is provided in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustration of a decoding arrangement for a main block memory array and an associated redundant memory array according to the present invention. 
     FIG. 2 is a schematic diagram illustrating a row repair circuit according to the present invention; 
     FIG. 3 is a detailed diagram of a portion of one of the latch/XOR circuits shown in the schematic diagram of FIG.  2 . 
     FIG. 4 is a block diagram of a memory device including the row repair circuit according to the present invention. 
     FIG. 5 is a block diagram illustrating a processor system containing a memory device having a row repair circuit according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to FIG. 1, a main block of a flash memory according to the present invention is illustrated. The main block includes a primary memory array  2  having 256 rows of memory cells and redundant memory array  4  having four rows of memory cells. The main and redundant arrays also have, for example, 4096 columns of memory cells. A row decoder (Rows X-Decoder)  6  decodes row addresses for memory array  2 . A redundant row decoder (RedRow X-Decoder)  8  decodes row addresses for redundant array  4 . 
     In a typical adaptation of the present invention a flash memory device  50  has, for example, four memory banks  52 ,  54 ,  56 ,  58  as shown in FIG. 4, each having a plurality of memory blocks, each memory block being arranged as shown in FIG.  1 . 
     The four rows of memory cells in redundant memory array  4  are associated with main memory array  2  for repairing defective rows of memory in the main array. Redundant row decoder  8  stores addresses of defective rows requiring repair and compares them to incoming row addresses for selecting a redundant row for substitution for a primary row which is addressed. 
     Redundant row decoder  8  is illustrated in greater detail in FIG.  2 . The redundant row decoder  8  includes a latch/XOR circuit  24  for receiving the five higher order bits of a memory address of a defective primary row, and even latch/XOR circuit  16  for receiving the lower three bits of an even row of a defective primary memory row, and a latch/XOR circuit  18  for receiving the lower three bits of a defective primary row. Accordingly, once a pair of adjacent odd and even primary rows are identified as defective during testing the address of the even row of the pair is programmed into the latch/XOR circuits  24  and  16 , while the address of the odd row of the pair is programmed into latches  24  and  18 . 
     The manner in which the addresses are loaded into the latch/XOR circuits  24 ,  16 , and  18  is illustrated in FIG. 3 which shows the loading of a one bit of the five bit address into the latch/XOR circuit  24 . As shown in FIG. 3, a programmable circuit  51  illustrated as a fuse circuit is connected to a fuse latch  52  within the latch/XOR circuit  24  to set the latch to a value determined by the programmed state of the fuse. The output of the fuse latch  52  is coupled to one input each of the XOR gates  48  and  50 . XOR gate  48  has another input for receiving a memory read address, while XOR gate  50  has another input for receiving an incoming write address. When a particular bit location from an incoming read address matches that stored in the fuse latch  52 , XOR gate  48  provides an output indicating a read match for that bit. Similarly, when an incoming write address bit matches the bit stored in fuse latch  52 , the XOR gate  50  provides an output indicating a matched write address for that bit. 
     Referring back to FIG. 2, the outputs of the latch/XOR circuits  24 , 16  and  18  are supplied to the four logic gates illustrated as AND gates  28 ,  30 ,  32  and  34 . AND gate  28  supplies an output signal when an incoming read address is an even address and matches an even address of a defective row stored in latch/XOR circuits  24  and  16 . AND gate  30  supplies an output signal for an incoming write address which matches the address stored in latch/XOR circuits  24  and  16 . AND gate  32  supplies an output signal when there is a match in an incoming read address with the read address stored in latch/XOR circuits  24  and  18 . AND gate  34  provides an output signal when there is a match between an incoming write address and an address stored in latch/XOR circuits  24  and  18 . Thus, gates  28  and  30  provide selection signals for a read or write even address which has been matched which are provided to gating circuit  36 . Gating circuit  36  receives enable signals as READ, WRITE, RED EN READ, and RED EN WRITE. Accordingly, when a read command is recognized by logic circuits elsewhere in the memory device a signal is applied on the READ command line. When the redundant logic circuitry is operating a signal is also present on the redundant enable (RED EN READ) line which enables a read of a redundant row. When the READ and RED EN READ signals are present and a signal appears at the output of gate  28 , gating circuit  36  passes this two line  40  where it is used to select an even redundant row for substitution for the defective even primary row whose address was set in the latch/XOR circuits  24  and  16 . Likewise for a write operation if gate  30  supplies a write signal this will be gated to select line  40  to select an even redundant row for substitution for a defective primary row when the WRITE and RED EN WRITE signals are present. 
     Similarly, line  42  is used to select an odd redundant row for operation. Thus, when gate  32  indicates that an incoming address matches an odd row address stored in latch/XOR circuits  24  and  18  during a read operation, the presence of the READ and RED EN RED signals at gating circuit  38  will cause the output of gate  32  to be applied to line  42  and select an odd redundant row for substitution for the defective primary row whose address is stored in latch/XOR circuits  24  and  18 . Likewise, gate  34  supplies a signal to gating circuit  38  when an incoming write address matches the odd row address stored in latch/XOR circuits  24  and  18  and this is gated by gating circuit  38  to line  42  when the WRITE and RED EN WRITE signals are present. 
     The gates  28 ,  30 ,  32 ,  34  are also enabled by an enable latch  26  which can be programmed to enable or disable operation of the decoder  8  of FIG. 2 as desired. 
     Thus, whenever a read or write address is presented to the decoder  8 , it is compared to the even address stored in the latch/XOR circuits  24  and  16  and the odd address stored in latch/XOR circuits  24  and  18  to determine if an even or odd redundant row needs to be substituted for the primary row being addressed. 
     By using the same odd-even or even-odd redundant row relationship, the test pattern (checkerboard) bit pattern initially loaded in the memory at the outset of testing will remain consistent. Consequently, subsequent testing based on the pre-loaded checkerboard pattern can take place without re-writing the pattern to the memory. By replacing the rows using the proper odd-even/even-odd relationship, the test pattern read from memory does not change even when defective main rows have been replaced with a pair of redundant memory rows. 
     Referring to FIG. 5, a processor system  60  is shown which uses a flash memory employing memory repair according to the present invention. Processor system  60  generally comprises a central processing unit (CPU)  62 , such as a microprocessor, that communicates with one or more input/output (I/O) devices  64  over a bus  66 . The processor system  60  also includes random access memory (RAM) such as DRAM  68 . One or more flash memory devices  70  also communicate with CPU  62 . In the case of a computer, the system may include peripheral devices such as a floppy disk drive  72  and a compact disk (CD) ROM drive  74  which also communicate with CPU  62  over the bus  66 . At least flash memory device  70  is constructed as an integrated circuit which includes circuitry to allow for redundant row repair as previously described. The redundancy repair of the invention can also be used in DRAM  68 . It may also be desirable to integrate the CPU  62  and memory  68  and/or  70  on a single IC chip. 
     It should be noted that while the preferred embodiment of the invention is described as applied to a flash memory device having typical row addressable architecture, the invention is not so limited and may be applied to other memory devices, including but not limited to DRAM, SRAM, SDRAM, as well as other memory devices, having different architectures and various sizes. Additionally, while the invention has been described with reference to four banks of memory containing a plurality of main blocks, the invention is not so limited and any number of banks and blocks can be used. Likewise, the number of rows and columns of cells in a memory block, and the number (of redundant rows can be varied from the memory block structure described above without departing from the spirit and scope of the invention. It is also noted that the FIG. 2 circuit can be replicated, that is, a plurality of FIG. 2 circuits can be provided, to address additional even and odd redundant rows, beyond the two shown in FIG.  2 . 
     The present invention provides an apparatus and method for repairing pairs of rows in a memory array with matching odd-even or even-odd redundant rows utilizing fuses  51  for storing the addresses of failed primary rows of the memory array. The fuses are used for setting latches  52  to store address information of defective rows. It will be evident to those of skill in the art that other types of programmable memory devices, such as anti-fuses, or other programmable devices, could be substituted in place of the fuses to set latches  52 . 
     While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, deletions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as limited by the foregoing description but is only limited by the scope of the appended claims.