Abstract:
Method of operating a memory include programming a memory cell and reading the memory cell to determine a programmed threshold voltage of the memory cell. If the programmed threshold voltage is within a threshold voltage distribution of a plurality of threshold voltage distributions, the memory cell is reprogrammed, and if the programmed threshold voltage is not within a threshold voltage distribution of the plurality of threshold voltage distributions, the memory cell is allowed to remain at the programmed threshold voltage.

Description:
RELATED APPLICATION 
     This Application is a Continuation of U.S. application Ser. No. 13/548,342, titled “MULTIPLE STEP PROGRAMMING IN A MEMORY DEVICE,” filed Jul. 13, 2012, (Allowed) which is commonly assigned and incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present embodiments relate generally to memory and a particular embodiment relates to multiple step programming in a memory device. 
     BACKGROUND 
     Memory is typically provided as an integrated circuit(s) formed in and/or on semiconductor die(s), whether alone or in combination with another integrated circuit(s), and is commonly found 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 memories have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memories typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the cells, through programming of a charge storage structure, such as floating gates or trapping layers or other physical phenomena, determine the data state of each cell. Common uses for flash memory include personal computers, digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, cellular telephones, and removable memory modules. 
     Single level memory cells (SLC) can store a single bit of data. Multi-level memory cells (MLC) can store two or more bits of data. 
     One problem that can occur with programming MLC memory is floating gate-to-floating gate capacitive coupling. The coupling can result in one memory cell disturbing adjacent memory cells, thus causing erroneous data to be stored in the adjacent memory cells. 
     Multiple step programming algorithms have been used to reduce the floating gate-to-floating gate coupling while also improving threshold voltage distribution widths. One particular multiple step programming operation comprises a prior art touch-up programming operation. This type of programming comprises programming an even page of memory first, reading the even page of memory, programming an odd page of memory then “touching-up” the even page of memory with additional programming pulses.  FIGS. 1A-1C  illustrate plots of threshold voltage distributions that can result from using a typical prior art multiple step programming operation. 
       FIG. 1A  illustrates threshold voltage distributions after an even page of memory has been programmed. This figure shows an erased state (111) as well as seven programmed states (000-011). 
     The states of  FIG. 1A  are each represented by three bits that are the programmed “hard” data of a multiple bit programmed word. The hard data are the actual data, of the multiple bit programmed word, that are used. The programmed word can also comprise “soft” data that are used to indicate a more precise location of the programmed state. For example, the area between each distribution that is indicated by the arrows is the soft data portion of the programmed word that indicates a location of its associated state to the right of the arrow. The soft data might be four bits of the multi-bit programmed word. Thus, the soft data can be considered the least significant bits (LSB) of the programmed word while the hard data can be considered the most significant bits (MSB) of the programmed word. 
     An even page read operation is performed after the even page has been programmed. Since programming of the memory pages might not be sequential, data stored in a page buffer for programming might be overwritten, after programming, by subsequent data to be programmed to the memory. Thus, during the even page read operation, the even page is read back out into the page buffer so that it can be further programmed during a subsequent touch-up programming operation, as discussed subsequently. This even page read can introduce errors into the programming operation, as subsequently described. 
     After the even page read is performed, the odd page of the memory cells is programmed.  FIG. 1B  illustrates the threshold voltage distributions after the odd page of memory has been programmed. The distributions have widened out due to the disturb effects of both program disturb (e.g., multiple programming voltages on the same word line) as well as floating gate-to-floating gate coupling. 
     It can be seen in  FIG. 1B  that the overlapping states have the potential to cause errors during reading of the memory since it could be unclear whether the read data belonged in, for example, the 001 state or the adjacent 101 state. In order to tighten up the distributions, an even page touch-up programming operation is performed. 
     The typical prior art even page touch-up programming operation comprises performing an additional program operation comprising additional programming pulses in order to program in the even page data read during the previous even page read operation. The even page touch-up programming operation programs the memory cells at the lower ends of the distributions to a high threshold voltage such that the memory cells at the lower ends of the distributions are moved up, thus tightening the distributions.  FIG. 1C  illustrates the distributions after the touch-up programming operation. 
     A problem with the above-described typical prior art multiple step programming operation is that, since the even page read operation does not use error correction coding, read errors are passed through uncorrected. This uncorrected data is then used during the touch-up programming operation. If the uncorrected data contains errors, the data is re-programmed with the errors during the touch-up operation. This can result in misplacement errors as shown in  FIG. 1C  by the “tails”  101 - 107  that are part of each distribution. These tails overlap with an adjacent distribution and represent the hard errors (e.g., error bits that are assigned a low probability of error by an error correction code (ECC) engine) transformed from the original soft errors (e.g., error bits that are assigned a high probability of error by the ECC engine) that can occur when assigning data to the wrong distribution during a read subsequent to the touch-up operation. 
     For the reasons stated above and for other reasons that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art to reduce these programming errors caused by misplacement of data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  show plots of threshold voltage distributions resulting from typical prior art multiple step programming. 
         FIG. 2  shows a schematic diagram of one embodiment of a portion of a memory array. 
         FIG. 3  shows a flow chart of one embodiment of a method for programming memory using a modified touch-up operation. 
         FIGS. 4A-4C  show plots of threshold voltage distributions in accordance with the method for programming of  FIG. 3 . 
         FIG. 5  shows a shows a block diagram of one embodiment of a system that can incorporate the multiple step programming method using the modified touch-up operation. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof and in which is shown, by way of illustration, specific embodiments. In the drawings, like numerals describe substantially similar components throughout the several views. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense. 
       FIG. 2  illustrates a schematic diagram of one embodiment of a portion of a NAND architecture memory array  201  comprising series strings of non-volatile memory cells. The present embodiments of the memory array are not limited to the illustrated NAND architecture. Alternate embodiments can use NOR, AND, PCM, or other architectures. 
     The memory array  201  comprises an array of non-volatile memory cells (e.g., floating gate) arranged in columns such as series strings  204 ,  205 . Each of the cells is coupled drain to source in each series string  204 ,  205 . An access line (e.g., word line) WL 0 -WL 31  that spans across multiple series strings  204 ,  205  is coupled to the control gates of each memory cell in a row in order to bias the control gates of the memory cells in the row. Data lines, such as even/odd bit lines BL_E, BL_O, are coupled to the series strings and eventually coupled to sense circuitry that detects the state of each cell by sensing current or voltage on a selected bit line. 
     Each series string  204 ,  205  of memory cells is coupled to a source line  206  by a source select gate  216 ,  217  (e.g., transistor) and to an individual bit line BL_E, BL_O by a drain select gate  212 ,  213  (e.g., transistor). 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 . 
     In a typical prior art programming of the memory array, each memory cell is individually programmed as either a single level cell (SLC) or a multiple level cell (MLC). The prior art uses a cell&#39;s threshold voltage (V t ) as an indication of the data stored in the cell. For example, in an SLC, a V t  of 2.5V might indicate a programmed cell (e.g., logical “0” state) while a V t  of −0.5V might indicate an erased cell (e.g., logical “1” state). An MLC uses multiple V t  ranges that each indicates a different state. Multiple level cells can take advantage of the analog nature of a traditional flash cell by assigning a specific bit pattern (e.g., 000-110) to a specific V t  range. 
       FIG. 3  illustrates a flow chart of one embodiment of a method for programming memory using a modified touch-up operation. The even page of a group of memory cells is programmed  301  from data in a page buffer. For example, the group of memory cells might comprise a block memory of memory cells. 
     The programming can be accomplished by a series of programming pulses applied to a word line coupled to control gates of the memory cells being programmed. A program verify operation after each programming pulse determines whether the memory cell has been programmed to its desired threshold voltage as dictated by the respective data to be programmed. When the memory cell turns on in response to a read voltage on the respective word line and produces a current or voltage on a respective bit line, as detected by the sense circuitry, the memory cell has been programmed. 
       FIG. 4A  illustrates the threshold voltage distributions that can result from the even page programming. The x-axis of the plot is the threshold voltage V t  and the y-axis is the number of memory cells at each threshold voltage. The distributions are the result of the fact that memory cells program at different rates. Thus, one programming pulse might move a first memory cell to the middle of the “011” state while another memory cell might only move to the left side of the “011” state after the same programming pulse. 
     While a large number of the memory cells end up being programmed to within the distributions, some of the memory cells end up in uncertain areas  401 - 407 . When this uncertain data is read, ECC correction is not used when it is later re-programmed. For example, if uncertain data is read from the uncertain area  403  between the threshold voltage distributions for states “001” and “101”, they can be either one of the states, thus possibly resulting in the previously described misplacement errors if the data is read and later programmed as the wrong state. 
     Since the memory pages are not always programmed sequentially, the programmed even page or pages are read back out  303  to the page buffer. As subsequently described, this data is used later during a touch-up operation. During the reading of the page of data subsequent to the touch-up operation, an ECC engine checks the data for errors and attempts to perform corrections on the errors. 
     In order to reduce the hard errors caused by the touch-up operation passing through the read data “as-is” without ECC correction, the uncertain data is excluded, inhibited, or removed from the page buffer  305 . Thus, the uncertain data is left in the uncertain areas  401 - 407  between the distributions and are not further programmed during the subsequent touch-up operation. 
     The odd memory page or pages are then programmed  307 . This can be accomplished in a substantially similar manner to the even page or pages programming in that the data are programmed to their respective memory cells from the page buffer by increasing the threshold voltages of the respective memory cells to the respective threshold voltage of each desired state. 
       FIG. 4B  illustrates the threshold voltage distributions after the odd page or pages being programmed. It can be seen that the disturb caused by the additional programming and floating gate-to-floating gate coupling of the memory cells has widened the distributions such that they overlap. In order to tighten up the distributions, a touch-up programming operation is performed  309 . 
     The touch-up programming operation comprises programming the data from the page buffer, that was previously read from the even page or pages, back to the memory cells. In one embodiment, the data is programmed back a certain voltage (e.g., 400 mV) higher. This has the effect of moving the lower ends of the distributions to higher threshold voltages and tightening the distributions. 
     The uncertain data from the uncertain locations  401 - 407  of  FIG. 4A  were not moved thus resulting in “tails”  410 - 416  on the distributions representing the uncertain data. However, these uncertain data are now “soft” errors (e.g., the least significant bits) instead of the “hard” errors (e.g., most significant bits) that resulted from the prior art touch-up programming operation. 
     The previous description, for purposes of illustration, started with programming the even page of data prior to programming the odd page of data. An alternate embodiment can comprise programming the odd page first, reading the odd page, programming the even page, then reprogramming the odd page. 
       FIG. 5  illustrates a functional block diagram of a memory device  500  as part of a memory system  520 . The memory device  500  is coupled to a controller  510 . The controller  510  may be a microprocessor or some other type of controlling circuitry. The memory device  500  has been simplified to focus on features of the memory that are helpful in understanding the present invention. 
     The memory device  500  includes an array  530  of non-volatile memory cells, such as the one illustrated previously in  FIG. 2 . The memory array  530  is arranged in banks of word line rows and bit line columns. In one embodiment, the columns of the memory array  530  are comprised of series strings of memory cells as illustrated in  FIG. 2 . As is well known in the art, the connections of the cells to the bit lines determines whether the array is a NAND architecture, an AND architecture, a NOR architecture, or another architecture. 
     Address buffer circuitry  540  is provided to latch address signals received through I/O circuitry  560 . Address signals are received and decoded by a row decoder  544  and a column decoder  546  to access the memory array  530 . 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  530 . That is, the number of addresses increases with both increased memory cell counts and increased bank and block counts. The page buffer  573 , as previously described, is coupled to the memory array for storing data to be programmed or that has been read. 
     The memory device  500  reads data in the memory array  530  by sensing voltage or current changes in the memory array columns using sense circuitry  550 . The sense circuitry  550 , in one embodiment, is coupled to read and latch a row of data from the memory array  530 . The I/O circuitry  560  provides bidirectional data communication as well as address communication over a plurality of data connections  562  with the controller  510 . Write circuitry  555  is provided to write data to the memory array. 
     Memory control circuitry  570  decodes signals provided on control connections  572  from the controller  510 . These signals are used to control the operations on the memory array  530 , including data read, data write (program), and erase operations. The memory control circuitry  570  may be a state machine, a sequencer, or some other type of control circuitry to generate the memory control signals. In one embodiment, the memory control circuitry  570  is configured to execute the method for programming with the modified touch-up programming operation. 
     The flash memory device illustrated in  FIG. 5  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, one or more embodiments include an improved multiple step programming method that reduces the chances of “hard” errors caused by an ECC engine assigning uncertain data to the wrong state. This can be accomplished by excluding the uncertain data from reprogramming during the touch-up operation. 
     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.