Repairable block redundancy scheme

[0001] The invention relates to a method and apparatus for achieving block redundancy in a non-volatile memory.

[0002] During the testing of a fabricated memory device, it is often discovered that certain portions of a memory array are faulty, unreliable or defective in some way. A typical way to correct this problem is to flag the region as defective by blowing fuses or anti-fuses which set hardwired latches associated with the defective region and which map the defective region to a non-defective fully-operable redundant region. With this re-mapping, all attempts to access the defective region will be redirected to the redundant region known to be properly working.

[0003] Many memory systems implement this re-routing on a column-by-column or row-by-row basis. However, non-volatile memory like flash memory lends itself well to a block redundancy approach where redundant blocks of memory cells are substituted for defective main memory blocks. Fuses or anti-fuses are associated with the redundant memory blocks and are programmed with the addresses of the defective blocks. The fuse or anti-fuse programming sets corresponding latches which are used to redirect accesses from a defective memory block to a redundant memory block.

[0004] Fuses or anti-fuses and their associated latches and program circuitry take up a considerable area of a memory die and, once programmed to specific addresses cannot later be reprogrammed. As a result, there is a limited number of defective main memory blocks which can be identified for mapping to redundant memory blocks. In order to add more defective address possibilities, additional fuse and/anti-fuse sets and associated latches are required which increases die area.

[0005] In one aspect the invention provides a method and apparatus for achieving reduced complexity of redundant memory block addressing while allowing more memory block addresses to be used than there are fuse sets for loading and latching those addresses. In the invention, latches are used to store addresses of defective memory blocks, but the latches can be reprogrammed from a look-up table which holds a plurality of addresses which can be used for redundant block substitution. Many more redundant addresses are available in the look-up table than is capable of being held by the latches. An address decoder is used to select available programmed addresses from the look-up table which can be loaded into the latches for redundant block substitution whenever certain address transitions are detected.

[0006] The look-up table can be programmed to substitute a redundant memory block for a defective main memory block, a redundant memory block for another redundant memory block, a main memory block for another main memory block, or a main memory block for a redundant block.

[0007] These and other aspects feature an advantage of the invention would be more clearly understood from the following detailed description of the invention which is provided in connection with the accompanying drawings.

[0018] The present invention can generally be used to substitute one memory block for another memory block and particularly may be used for substituting a non-defective memory block for a defective memory block within a memory system. It is particularly useful in non-volatile memory systems such as flash memory.

[0019] As shown in FIG. 1, it is known to divide a memory device such as flash memory into memory banks and to further divide the memory banks into memory blocks. FIG. 1 shows memory banks 0–3 each having 64 main memory blocks and 8 redundant memory blocks contained therein. Thus, bank 0 contains main memory blocks addressable as <63:0> and redundant memory blocks addressable as <7:0>, while memory banks 1–3 respectively contain memory blocks <127:64>, redundant blocks <15:8>, main blocks <191:128>, redundant blocks <23:16>, and main blocks <255:192>, and redundant blocks <31:24> respectively. In FIG. 1 all of the main memory blocks and redundant blocks are of the same size. It should be noted that the size of the memory array in terms of the number of main and redundant blocks is merely exemplary. Many different memory arrays having more or less numbers of main blocks and redundant blocks may be employed with more or fewer than the four memory banks illustrated.

[0020] When a defective main block is found in one of the memory banks during a memory test, its address is loaded into fuse sets 108 in the redundant logic circuit 104. These fuse sets are configured as programmable fuses (or anti-fuses) which are connected to latches 106. Thus, an address is hardwired by blowing the fuses or fuse sets 108 for setting the latch contents. The latch contents are used in an address comparator. Thus, when an incoming address 110 matches a programmed address set in the fuse sets 108 and held in the latches 106, the address comparator causes redundant logic 104 to select one of the redundant blocks in a given bank as a substitute for the defective main block in the same bank.

[0021] A limitation of the FIG. 1 arrangement is that once the programmable fuses (or anti-fuses) of a fuse set 108 are set there is a permanent relationship established between an address of a defective main block set in the fuses and a substituted redundant memory block within the same bank. In addition, the number of defective addresses which can be programmed into the fuses of the fuse sets 108 is limited by the number of available fuse sets 108 and associated latches 106.

[0022] FIG. 2 illustrates a first embodiment of the invention. FIG. 2 is identical to FIG. 1 except the redundant logic 104 of FIG. 1 is now replaced by redundant logic 204. Redundant logic 204 contains programmable latches 208 which interface with a lookup table to 212 which is programmed and stores a plurality of redundant block addresses which can be used by the redundant logic 204 to select a redundant memory block to substitute for a defective main memory block identified by an incoming address.

[0023] FIG. 3 shows the redundant logic 204 of FIG. 2 in greater detail. The latches 208 handle an 8-bit address and are connected to a 4:1 multiplexer 308 which supplies 1 of 4 portions of a 32 bit address taken from the column lines of a decoded row of lookup table 212. The lookup table 212 contains a plurality of rows of memory elements and each row contains memory elements storing 32 bits. For example, 64 rows may be provided in the lookup table 212 which are selected by a row decoder 13 in response to incoming block addresses <21:16>. The 4-1 multiplexer 308 is controlled to select one of the four sets of 8 bits contained in the 32 bits from the lookup table 212 in accordance with applied bank address signals <23:22>. By using the 4:1 multiplexer 308 along with the bank address signals <23:22>, addresses for 4 different memory banks can be held in each row of the lookup table 212 in four segments, each representing an address for a respective bank. Thus, the bank addresses <22:23> select which bank is being accessed and cause the 8 bits in a row corresponding to the bank address to be applied by way of multiplexer 308 to the 8 bit latch 208. As noted, the block addresses <21:16> select a row through row decoder 213 with the selected memory bits of the row being presented to the multiplexer 308. The programmable elements loaded into latch 208 are thus determined by an incoming bank address <23:22> and block address <21:16>.

[0024] FIG. 3 further illustrates the signals which emanate from the redundant logic 204. Included are enable and disable signals, described below, a read bank signal (redbnk<1:0>), and a read block address signal (redblkadr<3:0>). These signals are used to select one of the four banks and a block within one of the four selected banks for block substitution.

[0025] Additional details of the circuit depicted in FIG. 3 are shown in FIG. 4. The selected redundant memory bank and block are designated by the signals redblkadr<3:0> and redbnk<1:0> respectively, are provided by output logic 416. The row decoder 213 selects a row of the lookup table 212 in accordance with the applied block address signals A<21:16>. Each row of lookup table 212 contains 64 flash memory transistors 219 which can be pre-programmed row-by-row by programming decoder 215 with addresses of memory blocks which are to be substituted. Sixty-four transistors 219 are provided because the 32 bits stored in each row of the lookup table 212 are stored as the bits themselves and their complements; hence 64 transistors. In this embodiment 64 rows are used, so a total of 64 rows×4 memory blocks per row yields 256 possible block addresses which can be programmed into the lookup table 212. The row decoder 213 selects a new row of lookup table 212 in response to an incoming block address on the block address input <21:16>. The address transition detector 404 causes the programmable latches l_0-7 which form latch set 208 to load a new address from lookup table 212 into latches 208 in response to a signal from the address transition detector 404 on line 405. The detector 404 senses an incoming address transition and causes the loading of address information from the selected row line which has been enabled by row decoder 213. The multiplexer passes only a selected 8 bits of the 32 bits stored in a row under control of bank select signal A<23:22>. The F bias line acts to decouple the latches l_0-7 from the lookup table 212 when the transistors 219 of the lookup table are programmed.

[0026] The 8 bits of data stored for each block address contain 6 bits which are used for block selection, two of those bits being output in a bank select address (redbnk<1:0>), 4 being output as a block address (redblkadr<3:0>) and 2 bits designated as an enable bit and a disable bit. The output logic 416 sees the 8 bit address stored in latches l_0-7 and supplies the noted 8 bit output signals.

[0027] FIG. 5 shows an expanded view of the connections between the multiplexer 308 and the output logic 416. From FIG. 5 it can be seen that from the 8 bits generated by multiplexer 308, 2 bits eventually become the enable and disable bits, 4 bits are the redundant block address (redblkadr), and 2 bits are the redundant bank address (redbnkadr). The enable bit activates the output gates 508 through NOR gates 513 and NAND gate 515 when a block redundancy operation is selected by the signal blkredun. thereby making the latched addresses available to select a designated memory block of a selected memory bank. This bit can be programmed as desired so a selected bank and block address is used or not for block substitution. The disabled bit is used to disable the output from programmable latches and also may also be used to disable other operations of the memory device. It is possible a user would want to disable any redundant addressing when certain block addresses are selected. This could be useful if it were discovered that a replacement memory block was also defective.

[0028] Although each row of lookup table 212 is shown in FIG. 5 as having 8 selected bits by multiplexer 308, it is important to remember that each memory cell of a flash memory typically has 2 complementary bit lines which store the cell content. Accordingly, although FIG. 5 shows four 8-bit rows (32 bits total) loaded from the lookup table 212 to the 4:1 multiplexer 308, a 64 bit bus is necessary, as shown in FIG. 3.

[0029] Although the embodiment described above employs a lookup table which uses stored complementary bits which are output on respective complementary bit lines to load the latches, as an alternative embodiment each of the latches can also be individually loaded using a single stored bit loaded into the latch from an associated bit line of the look up table. This latter arrangement has the advantage of using fewer memory cells within, and out put bit lines from, the look up table to store and load the look up table information into the latches.

[0030] Referring back to FIG. 2, any of the redundant blocks <31:0> can be substituted for any of the main block <255:00> because the substituted addresses stored in the lookup table for any presented bank and block address include a new bank and block address for a redundant block of memory.

[0031] In a second embodiment of the invention shown in FIG. 6, any memory block of a memory can be substituted for any other memory block including substituting a redundant block for a main block, a main block for a redundant block, a redundant block for a redundant block or a main block for a main block. The second embodiment accomplishes this by re-addressing all blocks so that each block has its own unique address as best can be illustrated by comparing the block diagram of FIG. 6 with that of FIG. 2. In FIG. 2 it was understood that there are 8 redundant blocks associated with every 64 main blocks, with the address of the redundant blocks being held on the rdnbnk and rdnblk address lines (FIGS. 3, 4, and 5). In FIG. 6, there are also 8 redundant blocks for every 64 main blocks but the redundant blocks are no longer addressed differently than the main blocks. Instead, all block addressing is done using the same address lines coming from block remapping logic 604. In this embodiment, each memory block whether main or redundant has its own unique address. Thus, for example, if memory block 62 is discovered to be bad or defective, block 271 can be substituted therefor, or perhaps block 191.

[0032] As shown in FIG. 7, the second embodiment also differs from the first in that it has an 88 bit data path instead of 64 bits between the latches 208′ and lookup table 212′. This is because the second embodiment addresses more memory blocks than the first. Thus, 9 bits are required for memory block addresses with 2 bits being used as the enable and disable bits. Accordingly, each stored block address within a row of the memory table is formed of 11 bits. As in the first embodiment, each row of the lookup table contains 4 block addresses so each row stores a total of 88 bits, 44 programming bits and their complements. The second embodiment also uses the bank address <23:22> to do the 4:1 multiplexing, but in the second embodiment block re-mapping logic 604 can address any block within the entire memory array. Thus, the output of the fuse set latches 208′ are block addresses redblkadr<24:16>.

[0033] The second embodiment of the invention also employs 2 bits as a disable and enable bit respectively. FIGS. 8 and 9 show the remapping logic 604 of the second embodiment in greater detail. FIG. 8 is similar to FIG. 4 of the first embodiment, but illustrates the output logic 416′ as supplying the enable and disable bits as well as the block address blkadr <24:16> which is able to select any memory block of the memory array. Otherwise, FIG. 8 operates in the same manner of FIG. 4. FIG. 9 is similar to FIG. 5 and also shows output gates 508′ supplying the block address signal blkadr <24:16>.

[0034] As with the first embodiment, the embodiment described above employs a lookup table which uses stored complementary bits which are output on respective complementary bit lines to load the latches, as an alternative embodiment each of the latches can also be individually loaded using a single stored bit loaded into the latch from an associated bit line of the look up table. This latter arrangement has the advantage of using fewer memory cells within, and out put bit lines from, the look up table to store and load the look up table information into the latches.

[0035] FIG. 10 illustrates an exemplary processing system 1000 which may utilize the memory device 200 of the present invention. The processing system 1000 includes one or more processors 1001 coupled to a local bus 1004. A memory controller 1002 and a primary bus bridge 1003 are also coupled the local bus 1004. The processing system 1000 may include multiple memory controllers 1002 and/or multiple primary bus bridges 1003. The memory controller 1002 and the primary bus bridge 1003 may be integrated as a single device 1006.

[0036] The memory controller 1002 is also coupled to one or more memory buses 1007. Each memory bus accepts memory components 1008 which include at least one memory device 200 of the present invention. The memory components 1008 may be a memory card or a memory module. The memory components 1008 may include one or more additional devices 1009. For example, the additional device 1009 might be used to configuration memory. The memory controller 1002 may also be coupled to a cache memory 1005. The cache memory 1005 may be the only cache memory in the processing system. Alternatively, other devices, for example, processors 1001 may also include cache memories, which may form a cache hierarchy with cache memory 1005. If the processing system 1000 include peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller 1002 may implement a cache coherency protocol. If the memory controller 1002 is coupled to a plurality of memory buses 1007, each memory bus 1007 may be operated in parallel, or different address ranges may be mapped to different memory buses 1007.

[0037] The primary bus bridge 1003 is coupled to at least one peripheral bus 1010. Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus 1010. These devices may include a storage controller 1011, an miscellaneous I/O device 1014, a secondary bus bridge 1015, a multimedia processor 1018, and an legacy device interface 1020. The primary bus bridge 1003 may also coupled to one or more special purpose high speed ports 1022. In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port (AGP), used to couple a high performance video card to the processing system 1000.

[0038] The storage controller 1011 couples one or more storage devices 1013, via a storage bus 1012, to the peripheral bus 1010. For example, the storage controller 1011 may be a SCSI controller and storage devices 1013 may be SCSI discs. The I/O device 1014 may be any sort of peripheral. For example, the I/O device 1014 may be an local area network interface, such as an Ethernet card. The secondary bus bridge may be used to interface additional devices via another bus to the processing system. For example, the secondary bus bridge may be an universal serial port (USB) controller used to couple USB devices 1017 via to the processing system 1000. The multimedia processor 1018 may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to one additional devices such as speakers 1019. The legacy device interface 1020 is used to couple legacy devices, for example, older styled keyboards and mice, to the processing system 1000.

[0039] The processing system 1000 illustrated in FIG. 10 is only an exemplary processing system with which the invention may be used. While FIG. 10 illustrates a processing architecture especially suitable for a general purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications can be made to configure the processing system 1000 to become more suitable for use in a variety of applications. For example, many electronic devices which require processing may be implemented using a simpler architecture which relies on a CPU 1001 coupled to memory components 1008 and/or memory devices 200. These electronic devices may include, but are not limited to audio/video processors and recorders, gaming consoles, digital television sets, wired or wireless telephones, navigation devices (including system based on the global positioning system (GPS) and/or inertial navigation), and digital cameras and/or recorders. The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices.

[0040] While the invention has been described and illustrated with reference to specific exemplary embodiments, it should be understood that many modifications and substitutions can be made without departing from the spirit and scope of the 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.