Patent Publication Number: US-2012026816-A1

Title: Defective memory block identification in a memory device

Description:
RELATED APPLICATION 
     This a Continuation of U.S. application Ser. No. 12/568,912 titled “DEFECTIVE MEMORY BLOCK IDENTIFICATION IN A MEMORY DEVICE,” filed Sep. 29, 2009 (allowed), which is a Continuation of U.S. application Ser. No. 11/454,464, filed Jun. 16, 2006, U.S. Pat. No. 7,610,525 issued on Oct. 27, 2009, which is a Continuation of U.S. application Ser. No. 10/930,087, filed Aug. 31, 2004, U.S. Pat. No. 7,272,458 issued on Sep. 18, 2007 which are commonly assigned and incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to memory devices and in particular the present invention relates to a flash memory devices. 
     BACKGROUND 
     Memory devices are typically internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory. 
     Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems. 
     Unlike most semiconductor memory, flash memory devices can be sold with a limited quantity of defective memory blocks. Flash memory devices typically have a scheme to communicate the locations of the defective blocks to a controller or processor. 
     One such scheme employs markers at specific locations in the defective memory block. The markers are generated during the manufacturing and testing process before being sold to the user. When the user powers-up the memory device, the memory controller would go through these specific locations in memory and track the defective blocks. Since there can be over 2000 memory blocks in a typical memory device, this is considerable overhead for the controller to handle. 
     In another error detection scheme, the memory controller reads the memory cells in a memory block and if the data is anything but FFH it would consider that block as a bad block. The controller then moves on to the next block. When all of the block&#39;s memory locations are read as FFH, the controller considers that block as good and starts to extract the information that it expected to see in block  0 . 
     However, if the memory defect is such that the controller is incapable of writing to any location in that block, the FFH data could not be altered from the initial erased condition. In this case, the controller would assume that the block is good since it read an FFH. Upon trying to write desired data into this block, the system would not allow it due to the write defect. 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an improved method for handling defective memory locations with minimal controller overhead. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a flowchart of one embodiment of a method for indicating defective memory blocks in a flash memory device. 
         FIG. 2  shows a schematic diagram of one embodiment of a NAND flash memory array of the present invention. 
         FIG. 3  shows a block diagram of one embodiment of an electronic system of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof. 
       FIG. 1  illustrates a flowchart of one embodiment of a method for identifying defective memory blocks in a flash memory device. In one embodiment, this method would be performed during the manufacturing and testing processes at the memory device manufacturer. This reduces the overhead operations required of the memory controller once the user begins operating the device. 
     The method performs a memory check  101  to determine if any defective blocks are present in the memory array. This memory check is well known in the art and can comprise writing a predetermined pattern to the memory cells then reading the data out. If the pattern that was read out does not match what was written, the location is considered defective and the block should be marked as defective. The error detection may also check for bitlines that are shorted together, multilevel cells that have one or more defective bits, or other types of errors. The present invention is not limited to any one error checking routine. 
     If a defective memory cell or cells were discovered  103 , the memory is erased  105  and the block number or numbers containing the defective locations are stored  107 . The memory needs to be erased due to the patterns left in the memory from the memory check process. The user typically expects to receive the memory in the erased state (i.e., FFH). 
     The defective block numbers can be stored at the end of an error free block. One or more data bytes may be required to store the information, depending on the number of defective blocks. 
     In one embodiment, the error free block is block  0  of the memory array. In an alternate embodiment, block  0  may not be the error free block. In such an embodiment, the first block determined to be defect-free is used. In another embodiment, the defective locations may be stored at the end of each block that contains at least one defective location. In this case, the stored defect indication might be the columns that are defective or some other defect indication that can be read by a user. 
     If no defective memory cells were discovered  103 , the memory is erased  109 . In this case, the memory locations all read FFH, or some other erased state, and the memory is ready for use. 
     One type of error in a flash memory device is that some cells cannot be programmed. They may be stuck in the erased state of FFH. With such a defect, the end user may not be able to tell that the block is bad since the FFH simply indicates that the block is erased. In this embodiment, a predetermined pattern (e.g., AAH) can be stored at some predefined location in the block (e.g., the end of the block) after the memory device has been tested and it has been determined that the block can be programmed. The end user can then go to this defect indication location and check for the predefined pattern. If the pattern is present, the user knows that the block is good and can be programmed. If the pattern is missing, the block is not programmable and is flagged as defective by the memory controller. 
     Flash memory devices that incorporate the embodiments of the defective memory block indication method of the present invention may be manufactured in a NOR architecture, a NAND architecture, or some other type of flash memory. In a NOR configuration, the cells are arranged in a matrix. The gates of each floating gate memory cell of the array matrix are connected by rows to wordlines and their drains are connected to column bitlines. The source of each floating gate memory cell is typically connected to a common source line. 
       FIG. 2  illustrates a simplified diagram of a typical NAND flash memory array of the present invention. The memory array of  FIG. 2 , for purposes of clarity, does not show all of the elements typically required in a memory array. For example, only two bitlines are shown (BL 1  and BL 2 ) when the number of bitlines required actually depends upon the memory density. The bitlines are subsequently referred to as (BL 1 -BLN). 
     The array is comprised of an array of floating gate cells  201  arranged in series strings  204 ,  205 . Each of the floating gate cells  201  are coupled drain to source in each series chain  204 ,  205 . A word line (WL 0 -WL 31 ) that spans across multiple series strings  204 ,  205  is coupled to the control gates of every floating gate cell in a row in order to control their operation. The bitlines (BL 1 -BLN) are eventually coupled to sense amplifiers (not shown) that detect the state of each cell. 
     In operation, the wordlines (WL 0 -WL 31 ) select the individual floating gate memory cells in the series chain  204 ,  205  to be written to or read from and operate the remaining floating gate memory cells in each series string  204 ,  205  in a pass through mode. Each series string  204 ,  205  of floating gate memory cells is coupled to a source line  206  by a source select gate  216 ,  217  and to an individual bitline (BL 1 -BLN) by a drain select gate  212 ,  213 . The source select gates  216 ,  217  are controlled by a source select gate control line SG(S)  218  coupled to their control gates. The drain select gates  212 ,  213  are controlled by a drain select gate control line SG(D)  214 . 
     Each cell can be programmed as a single bit per cell (SBC) or multiple bits per cell (i.e., multilevel cell—MLC). Each cell&#39;s threshold voltage (V t ) determines the data that is stored in the cell. For example, in a single bit per cell, a V t  of 0.5V might indicate a programmed cell while a V t  of −0.5V might indicate an erased cell. The multilevel cell may have multiple V t  windows that each indicate a different state. Multilevel cells take advantage of the analog nature of a traditional flash cell by assigning a bit pattern to a specific voltage range stored on the cell. This technology permits the storage of two or more bits per cell, depending on the quantity of voltage ranges assigned to the cell. 
     During a typical prior art programming operation, the selected wordline for the flash memory cell to be programmed is biased with a programming pulse at a voltage that is greater than 16V. A verification operation with a wordline voltage of 0V is then performed to determine if the floating gate is at the proper voltage (e.g., 0.5V). The unselected wordlines for the remaining cells are typically biased at approximately 10V during the program operation. Each of the memory cells is programmed in a substantially similar fashion. 
       FIG. 3  illustrates a functional block diagram of a memory device  300  that is coupled to a processor  310 . The processor  310  may be a microprocessor or some other type of controlling circuitry. The memory device  300  and the processor  310  form part of an electronic system  320 . The processor  310  is capable of generating memory commands to the memory device  300 . The memory device  300  has been simplified to focus on features of the memory that are helpful in understanding the present invention. 
     The array of flash memory cells  330  may be comprised of the flash memory cells as described previously with reference to  FIG. 2 . In an alternate embodiment, the memory array  330  is a NOR memory array. 
     The memory array  330  is arranged in banks of rows and columns. The control gates of each row of memory cells is coupled with a wordline while the drain and source connections of the memory cells are coupled to bitlines. As is well known in the art, the connections of the cells to the bitlines determines whether the array is a NAND architecture or a NOR architecture. 
     An address buffer circuit  340  is provided to latch address signals provided on address input connections A 0 -Ax  342 . Address signals are received and decoded by a row decoder  344  and a column decoder  346  to access the memory array  330 . It will be appreciated by those skilled in the art, with the benefit of the present description, that the number of address input connections depends on the density and architecture of the memory array  330 . That is, the number of addresses increases with both increased memory cell counts and increased bank and block counts. 
     The memory device  300  reads data in the memory array  330  by sensing voltage or current changes in the memory array columns using sense/buffer circuitry  350 . The sense/buffer circuitry, in one embodiment, is coupled to read and latch a row of data from the memory array  330 . Data input and output buffer circuitry  360  is included for bi-directional data communication over a plurality of data connections  362  with the controller  310 . Write circuitry  355  is provided to write data to the memory array. 
     Control circuitry  370  decodes signals provided on control connections  372  from the processor  310 . These signals are used to control the operations on the memory array  330 , including data read, data write (program), and erase operations. The control circuitry  370  may be a state machine, a sequencer, or some other type of controller that can execute the embodiments of the defective memory indication method of the present invention. 
     The control circuitry  370  knows which memory address in one or more memory blocks, depending on the embodiment, is the defect indication location. The control circuitry  370  can then read this location when powered-up by the user to determine which blocks are not usable. 
     The flash memory device illustrated in  FIG. 3  has been simplified to facilitate a basic understanding of the features of the memory. A more detailed understanding of internal circuitry and functions of flash memories are known to those skilled in the art. 
     CONCLUSION 
     In summary, the embodiments of the memory block defect identification method of the present invention provide a flash memory device with a non-volatile indication of bad memory blocks. The memory defect indication is generated by the manufacturer during the initial testing process, thus relieving the controller from having to regenerate the defective memory block map when the memory powered up at a later date by the memory user. 
     In another embodiment, by writing a predetermined pattern in a predefined location in each block, the end user can check this location to determine if the block has been checked for errors. The pattern indicates the result of the error check while also indicating that the block is writable. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.