Abstract:
Various aspects of a nonvolatile memory have an improved erase suspend procedure. A bias arrangement is applied to word lines of an erase sector undergoing an erase procedure interrupted by an erase suspend procedure. As a result, another operation performed during erase suspend, such as a read operation or program operation, has more accurate results due to decreased leakage current from any over-erased nonvolatile memory cells of the erase sector.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/875,003 filed 2 Sep. 2010 entitled Method and Apparatus for the Erase Suspend Operation. This application is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Description of Related Art 
         [0003]    In a memory integrated circuit, once an erase operation is initiated, several erase phases follow: a pre-programming phase, an erase phase, and a soft programming phase. Although a purpose of these multiple erase phases is to repair over-erased memory cells, the resulting multi-phase erase operation is time-consuming. 
         [0004]    For some applications, a memory accepts an erase suspend command even during an erase operation. An example of the erase suspend command implementation during an erase operation is discussed in U.S. Pat. No. 5,805,501, incorporated by reference. U.S. Pat. No. 5,805,501 discusses at column 5, line 66 to column 6, line 4 a program step accomplished by establishing a large positive voltage on the gate relative to source; at column 10, lines 29-31 high voltage operations in program and program verify periods; 
         [0005]    Once an erase suspend command is received, control is taken over by a new command in 20 μs. The new command is valid in sectors not undergoing the suspended erase. However, current leakage is a problem after the 20 us while performing the new command. 
         [0006]    In serial flash, a small sector of approximately 4 kB, is the erase unit. However, a well or bit line is shared by a quantity of memory cells are larger than approximately 4 kB, for example, approximately 64 kB or approximately 128 kB. Current leakage from over-erased memory cells in the 4 kB erase sector will affect the other approximately 64 kB or approximately 128 kB which share the same well or bit line. So, if the erase suspend operation is performed in the erase operation during the erase phase or the soft program phase, the following read or program command will suffer from current leakage of the over-erased memory cells in the 4 kB erase sector. 
       SUMMARY 
       [0007]    One aspect is an apparatus comprising a memory array of an integrated circuit, a plurality of word lines, and control logic of the integrated circuit. 
         [0008]    The memory array includes a plurality of erase sectors of nonvolatile memory cells. The plurality of word lines accesses the nonvolatile memory cells. 
         [0009]    The control logic is responsive to an erase command by performing a multi-phase erase procedure on an erase sector of the plurality of erase sectors of the memory array. 
         [0010]    The control logic is responsive to an erase suspend command by performing an erase suspend procedure suspending the multi-phase erase procedure and allowing a non-erase command to perform a non-erase procedure on the memory array. The erase suspend procedure applies a bias arrangement to at least one of the plurality of word lines accessing the erase sector during the non-erase procedure. The bias arrangement decreases leakage from over-erased nonvolatile memory cells of the erase sector. 
         [0011]    In one embodiment, the erase suspend procedure applies the bias arrangement, responsive to the erase suspend procedure suspending the multi-phase erase procedure when the multi-phase erase procedure is in an erase phase. 
         [0012]    In one embodiment, the erase suspend procedure applies the bias arrangement, responsive to the erase suspend procedure suspending the multi-phase erase procedure when the multi-phase erase procedure is in a soft program phase after an erase phase. 
         [0013]    In one embodiment, the erase suspend procedure does not apply the bias arrangement, responsive to the erase suspend procedure suspending the multi-phase erase procedure when the multi-phase erase procedure is in a pre-program phase before an erase phase. 
         [0014]    In one embodiment, the erase suspend procedure does not apply the bias arrangement, responsive to the erase suspend procedure suspending the multi-phase erase procedure when the multi-phase erase procedure is in a verify phase. 
         [0015]    In one embodiment, the non-erase command is a read command and the non-erase procedure is a read procedure. 
         [0016]    In one embodiment, the non-erase command is a program command and the non-erase procedure is a program procedure. 
         [0017]    In one embodiment, the non-erase command is a program command and the non-erase procedure is a program procedure including a program verify phase, and the erase suspend procedure applies the bias arrangement during the program verify phase. 
         [0018]    In one embodiment, the non-erase procedure is performed on a selected memory cell sharing a same well with at least one of the over-erased nonvolatile memory cells of the erase sector. 
         [0019]    In one embodiment, the non-erase procedure is performed on a selected memory cell sharing a same bit line with at least one of the over-erased nonvolatile memory cells of the erase sector. 
         [0020]    Another aspect is method, comprising:
       performing an erase suspend procedure on a memory array of the integrated circuit, by suspending a multi-phase erase procedure of an erase sector of the memory array;   performing a non-erase procedure on the memory array; and   applying a bias arrangement to at least one word line accessing the erase sector during the non-erase procedure, the bias arrangement decreasing leakage from over-erased nonvolatile memory cells of the erase sector.       
 
         [0024]    Various embodiments are described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a simplified flowchart diagram of an erase procedure with an improved erase suspend procedure, that applies negative voltage on the word lines of a sector undergoing erase, as part of switching from an erase procedure to an erase suspend procedure. 
           [0026]      FIG. 2  is a simplified flowchart diagram of an improved erase suspend procedure, that applies negative voltage on the word lines of a sector undergoing erase, while performing a non-erase procedure during the erase suspend procedure. 
           [0027]      FIG. 3  is a simplified block diagram of an exemplary nonvolatile memory array that implements the improved erase suspend procedure. 
           [0028]      FIG. 4  shows multiple graphs, of (i) current versus voltage for an over-erased nonvolatile memory cell that causes leakage current, and (ii) another nonvolatile memory cell whose apparent state is distorted by the leakage current of the over-erased nonvolatile memory cell. 
           [0029]      FIG. 5  shows multiple graphs, of (i) current versus voltage for an over-erased nonvolatile memory cell that no longer causes leakage current due to the negative voltage applied to word lines of a sector undergoing an erase procedure and then an erase suspend procedure, and (ii) another nonvolatile memory cell whose apparent state is no longer distorted by the leakage current of the over-erased nonvolatile memory cell. 
           [0030]      FIG. 6  is a block diagram of a memory integrated circuit with the improvements described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIG. 1  is a simplified flowchart diagram of an erase procedure with an improved erase suspend procedure, that applies negative voltage on the word lines of a sector undergoing erase, as part of switching from an erase procedure to an erase suspend procedure. 
         [0032]    In response to the integrated circuit receiving an erase command (such as an erase command operation code and a sector address), the erase procedure starts  12 . The erase procedure has multiple phases, including a pre-program phase  14 , an erase phase  18 , a soft program phase  22 , and a verify phase  26 . 
         [0033]    During the pre-program phase, memory cells in an erase sector undergoing erase, are programmed to approximately the same level. This avoids having some cells that were either un-programmed or that have lost charge during normal operations, to be over-erased during the erase procedure. Because all the memory cells in an erase sector are erased as a block, all memory cells receive the same erase pulse during the erase phase, regardless of their individual charge state. 
         [0034]    During the erase phase, all of the memory cells in the erase sector undergoing erase, are given an erase pulse. In one embodiment with floating gate nonvolatile memory, the program procedure adds electrons to the floating gate, and the erase pulse removes electrons from the floating gate. Other embodiments include charge trapping nonvolatile memory. 
         [0035]    During the soft program phase, memory cells are identified whose threshold voltages are below a pre-determined minimum value. Such memory cells receive a soft programming pulse. This repeats until all memory cells in the erase sector have their threshold voltages brought to above the pre-determined minimum value. 
         [0036]    During the verify phase, all of the memory cells undergo verification to confirm adequate threshold voltage. Such verification can occur at different phases of the procedure. In response to verify failure, the failing phase is repeated, or the erase procedure is repeated. 
         [0037]    Erase suspend is a useful operation that prevents a user from waiting for long periods for the complete erase procedure to finish. However, because the erase procedure is suspended prior to completion of the erase procedure, the memory cells undergoing erase may have been over-erased, with the accompanying problem of current leakage. Current leakage from over-erased memory cells can distort the result of a program operation or read operation that is performed during the erase suspend period. For example, current leakage from over-erase memory cells will increase the apparent current flow from a memory cell undergoing a program operation or read operation, in the event that the substrate or bit line is shared with the over-erased memory cell. 
         [0038]    After each of the phases, there is an option to perform erase suspend. During the pre-program phase  14 , if there is an erase suspend  16 , then erase suspend begins  30 . Otherwise, the erase procedure continues with the erase phase  18 . During the erase phase  18 , if there is an erase suspend  20 , then negative voltage is applied on the word lines of the erase sector undergoing erase  28 , and then erase suspend begins  30 . Otherwise, the erase procedure continues with the soft program phase  22 . During the soft program phase  22 , if there is an erase suspend  24 , then negative voltage is applied on the word lines of the erase sector undergoing erase  28 , and then erase suspend begins  30 . Otherwise, the erase procedure continues with the verify phase  26 . During the verify phase  26 , if there is an erase suspend  24 , then erase suspend begins  30 . Otherwise, the erase procedure completes. 
         [0039]    When accepting an erase suspend command in the erase phase or the soft-program phase of a multi-phase erase procedure, a negative voltage is applied on all word lines of the sector or small sector to eliminate the leakage of over-erased memory cells. In the erase phase, over-erase can occur, and in the soft-program phase, fixing an over-erased memory cell may be incomplete. The step of applying negative voltage to the world lines can be finished in 20 μs, which can meet the required timing for a suspend command to be performed and defer to a new command such as a program command or read command. 
         [0040]    For those over-erased memory cells whose threshold voltage is less then 0 V, a background leakage current is induced, making the read operation or program verify phase of a program operation fail. Adding negative voltage on word lines can turn off the channel and eliminate the background leakage. So, a read operation and program verify phase of a program operation can pass, even though the object memory cell of the read operation or program verify phase of a program operation shares the same bit line or well as the over-erased memory cells. 
         [0041]      FIG. 2  is a simplified flowchart diagram of an improved erase suspend procedure, that applies negative voltage on the word lines of a sector undergoing erase, while performing a non-erase procedure during the erase suspend procedure. 
         [0042]    In  42 , erase suspend begins. In  44 , a non-erase procedure begins, such as a read operation or a program operation, while negative voltage is applied to word lines of the erase sector. In  46 , if the non-erase procedures are not over, than another non-erase procedure is performed  44 . Otherwise, the erase suspend procedure continues. In  48 , if there is another erase suspend, then the erase suspend procedure loops back to  44 . Otherwise, the erase suspend ends and the erase operation continues  50 . 
         [0043]      FIG. 3  is a simplified block diagram of an exemplary nonvolatile memory array that implements the improved erase suspend procedure. 
         [0044]    The memory cells in a block  62  are divided into a plurality of sectors  63  or a plurality of small sectors (N small sectors in one sector). Accordingly, the duration of an erase procedure for the sector will be smaller than the duration of an erase procedure for the entire block. 
         [0045]    During an erase procedure, an erase sector selected to be erased, such as sector  1   63 , shares a well or bit line  66  with another erase sector that is not selected to be erased, such as sector x  64 . During the erase suspend, the word lines  65  of the erase sector receive a negative voltage to suppress current leakage from over-erased memory cells. 
         [0046]      FIG. 4  shows multiple graphs, of (i) current versus voltage for an over-erased nonvolatile memory cell that causes leakage current, and (ii) another nonvolatile memory cell whose apparent state is distorted by the leakage current of the over-erased nonvolatile memory cell. 
         [0047]    The over-erased memory cell has a threshold voltage less than 0V, and causes leakage current. 
         [0048]    Another nonvolatile memory cell, which shares a bit line or well with the over-erased memory cell, undergoes a read operation or a program operation (including a program verify phase) IV curve. The apparent IV curve is the sum of the background current leakage of the over-erased memory cell, and its own actual current. 
         [0049]      FIG. 5  shows multiple graphs, of (i) current versus voltage for an over-erased nonvolatile memory cell that no longer causes leakage current due to the negative voltage applied to word lines of a sector undergoing an erase procedure and then an erase suspend procedure, and (ii) another nonvolatile memory cell whose apparent state is no longer distorted by the leakage current of the over-erased nonvolatile memory cell. 
         [0050]    Because a negative voltage is applied on the over-erased memory cell, the apparent behavior of the threshold voltage is larger than 0V. 
         [0051]    Another nonvolatile memory cell, which shares a bit line or well with the over-erased memory cell, undergoes a read operation or a program operation (including a program verify phase) IV curve. Because the over-erased memory cell is not causing background current leakage, the measured memory cell current is its own actual current. 
         [0052]      FIG. 6  is a block diagram of a memory integrated circuit with the improvements described herein. 
         [0053]      FIG. 6  is a simplified block diagram of an integrated circuit  650  including a memory array  600 . A word line (or row) and block select decoder  601  is coupled to, and in electrical communication with, a plurality  602  of word lines and string select lines, and arranged along rows in the memory array  600 . A bit line (column) decoder and drivers  603  are coupled to and in electrical communication with a plurality of bit lines  604  arranged along columns in the memory array  600  for reading data from, and writing data to, the memory cells in the memory array  600 . Addresses are supplied on bus  605  to the word line decoder and drivers  601  and to the bit line decoder  603 . Sense amplifiers and data-in structures in block  606 , including current sources for the read, program and erase modes, are coupled to the bit line decoder  603  via the bus  607 . Data is supplied via the data-in line  611  from input/output ports on the integrated circuit  650 , to the data-in structures in block  606 . Data is supplied via the data-out line  615  from the sense amplifiers in block  606  to input/output ports on the integrated circuit  650 , or to other data destinations internal or external to the integrated circuit  650 . Program , erase, and read bias arrangement state machine circuitry  609  implements the improved erase suspend procedure that applies a negative voltage to word lines of an erase sector undergoing an erase procedure, during an erase suspend procedure, and controls biasing arrangement supply voltages  608 . 
         [0054]    While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.