Abstract:
Various discussed approaches improve the over erase issue and the coupling effect, and include (A) multilevel contacts between (i) the first outer selected word line of an erase group, and (ii) the first unselected word line outside the ease group neighboring the first outer selected word line; (B) a sufficient separation distance between (i) the first outer selected word line of an erase group, and (ii) the first unselected word line outside the ease group neighboring the first outer selected word line. These are examples of electrically isolating (i) the first outer selected word line of an erase group, from (ii) the first unselected word line outside the ease group neighboring the first outer selected word line.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The technology relates to memory integrated circuits, in particular the erase operation of a nonvolatile memory integrated circuit. 
         [0003]    2. Description of Related Art 
         [0004]    Improvement of the over-erase effect from the erase operation has become an increasingly serious problem. Because of higher and higher memory array density, the floating gate coupling effect becomes even more serious. 
         [0005]    However, present approaches of the erase operation of nonvolatile memory cells are inefficient. For example, in the erase operation of  FIGS. 1-7  over-erased memory cells can result. In another example, such as US Patent Application Publication 2008/0175069, erase verify errors result in indiscriminate subdivision of an erase sector into two groups, followed by further subdivision into further groups, which results in a relatively complicated erase operation. In another example, the excessive use of dummy word lines (WLs) and dummy memory cells in U.S. Pat. No. 7,417,895 is wasteful of potentially usable memory cells. 
       SUMMARY 
       [0006]    Embodiments of the technology address the over-erase issue associated with the floating gate coupling effect of edge WLs. The discussed approaches can even be used without requiring an increased array size. Improved endurance and erase time performance of memory cells result from addressing the over-erase issue in an erase operation. 
         [0007]    Various discussed approaches that improve the over erase issue and the coupling effect, include (A) multilevel contacts (which can be drain or source side contacts that are bit lines/data lines that combine the drain side or source side of cells) between (i) the first outer selected word line of an erase group, and (ii) the first unselected word line outside the ease group neighboring the first outer selected word line; (B) a sufficient separation distance between (i) the first outer selected word line of an erase group, and (ii) the first unselected word line outside the ease group neighboring the first outer selected word line. These are examples of electrically isolating (i) the first outer selected word line of an erase group, from (ii) the first unselected word line outside the ease group neighboring the first outer selected word line. 
         [0008]    Various names for an erase group are sector, block, bank, array, small sector, and small block. 
         [0009]    One aspect of the technology is a memory apparatus with a plurality of memory cells, a plurality of word lines accessing the plurality of memory cells, a first plurality of multilevel contacts, and control circuitry. 
         [0010]    The plurality of word lines is divided into a plurality of erase groups. Responsive to an erase command, an erase group is selected from the plurality of erase groups to receive an erase bias arrangement. The erase group includes at least a first outer selected word line bounding (i) the erase group of word lines selected to receive the erase bias arrangement, from (ii) unselected word lines outside the erase group. In some embodiments, the erase group also includes inner selected word lines, and the first outer selected word line of the erase group is positioned between the inner selected word lines of the erase group and the unselected word lines outside the erase group. The first outer selected word line neighbors a first unselected word line outside the erase group. 
         [0011]    The first plurality of multilevel contacts is between (i) the first outer selected word line and (ii) the first unselected word line neighboring the first outer selected word line. 
         [0012]    The control circuitry is responsive to the erase command by selecting the erase group from the plurality of erase groups to receive the erase bias arrangement, and applying the erase bias arrangement to the erase group. 
         [0013]    In one embodiment, wherein responsive to the erase command, the control circuit applies an erase inhibit voltage to at least the first unselected word line (e.g., may also include other unselected word lines), at least partly while applying the erase bias arrangement to the erase group. In another embodiment, responsive to the erase command, the control circuit allows the first unselected word line to float (and other unselected word lines grounded or receiving the erase inhibit voltage), at least partly while applying the erase bias arrangement to the erase group. 
         [0014]    In various embodiments, the first unselected word line is a dummy word line; or the first unselected word line is not a dummy word line, and during regular operation accesses memory cells of the plurality of memory cells. 
         [0015]    In one embodiment, the erase group includes a second outer selected word line bounding (i) the erase group of word lines selected to receive the erase bias arrangement, from (ii) unselected word lines outside the erase group. The second outer selected word line neighbors a second unselected word line outside the erase group. For example, the first and second outer selected word lines are on opposite sides of the erase group (e.g., first WL and last WL of an erase group). The apparatus includes a second plurality of multilevel contacts between (i) the second outer selected word line and (ii) the second unselected word line neighboring the second outer selected word line. 
         [0016]    One embodiment further comprises a plurality memory elements under the plurality of word lines, including a first memory element under the first outer selected word line, and a second memory element under the first unselected word line neighboring the first outer selected word line. The first plurality of multilevel contacts is between (i) the first memory element under the first outer selected word line and (ii) the second memory element under the first unselected word line neighboring the first outer selected word line. Examples of the memory element are floating gates and charge trapping material. Examples of the array are NAND arrays and NOR arrays and embedded memory. 
         [0017]    One aspect of the technology is a memory apparatus with a plurality of memory cells, a plurality of word lines accessing the plurality of memory cells, and control circuitry. 
         [0018]    The plurality of word lines is divided into a plurality of erase groups. Responsive to an erase command, an erase group is selected from the plurality of erase groups to receive an erase bias arrangement. The erase group includes at least a first outer selected word line and a first inner selected word line. The outer selected word line bounds (i) the erase group of word lines selected to receive the erase bias arrangement, from (ii) unselected word lines outside the erase group. The first inner selected word line is adjacent to the first outer selected word line. The first outer selected word line neighbors (i) the first inner selected word line and (ii) a first unselected word line outside the erase group. A first distance separates the first outer selected word line from the first unselected word line. A second distance separates the first inner selected word line from the first outer selected word line. The first distance is greater than the second distance. Examples of these distances are shown in  FIG. 15 . 
         [0019]    The control circuitry is responsive to the erase command by selecting the erase group from the plurality of erase groups to receive the erase bias arrangement, and applying the erase bias arrangement to the erase group. 
         [0020]    In one embodiment, wherein responsive to the erase command, the control circuit applies an erase inhibit voltage to the first unselected word line, at least partly while applying the erase bias arrangement to the erase group. In another embodiment, responsive to the erase command, the control circuit allows the first unselected word line to float, at least partly while applying the erase bias arrangement to the erase group. 
         [0021]    In various embodiments, the first unselected word line is a dummy word line; or the first unselected word line is not a dummy word line, and during regular operation accesses memory cells of the plurality of memory cells. 
         [0022]    An aspect of the technology is an operation method of a memory, including the following steps:
       performing on a memory integrated circuit having a plurality of word lines divided into a plurality of erase groups, an erase command selecting an erase group to receive an erase bias arrangement including:
           applying the erase bias arrangement to word lines in the erase group, wherein the erase group includes:
               at least a first outer selected word line bounding (i) the erase group of word lines selected to receive the erase bias arrangement, from (ii) unselected word lines outside the erase group,   wherein the first outer selected word line neighbors a first unselected word line outside the erase group; and   
               
           electrically isolating (i) a first memory element under the first outer selected word line from (ii) a second memory element under the first unselected word line neighboring the first outer selected word line.       
 
         [0028]    In one embodiment the step of electrically isolating, is performed by a plurality of multilevel contacts between (i) the first outer selected word line and (ii) the first unselected word line neighboring the first outer selected word line. 
         [0029]    In one embodiment the first outer selected word line neighbors (i) the first unselected word line outside the erase group and (ii) a first inner selected word line in the erase group. The step of electrically isolating is performed by a first distance separating the first outer selected word line from the first unselected word line. The first distance is greater than a second distance separating the inner selected word line from the first outer selected word line. An example is shown by  FIG. 14 , which shows that the distance D 1  is greater than the distance D 2 . 
         [0030]    One embodiment further comprises:
       responsive to the erase command, applying an erase inhibit voltage to the first unselected word line, at least partly while applying the erase bias arrangement to the erase group.       
 
         [0032]    Another embodiment further comprises:
       responsive to the erase command, allowing the first unselected word line to float, at least partly while applying the erase bias arrangement to the erase group.       
 
         [0034]    In various embodiments, the first unselected word line is a dummy word line; or the first unselected word line is not a dummy word line, and during regular operation accesses memory cells of the plurality of memory cells. 
         [0035]    Examples of the first and second memory elements are floating gates and charge trapping material. Examples of the array are NAND arrays and NOR arrays and embedded memory. 
         [0036]    Various other embodiments are discussed herein. 
         [0037]    An aspect of the technology is a method, including the following steps: 
         [0038]    Various other embodiments are discussed herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]      FIG. 1  is a block diagram of a memory array portion with multiple erase groups, showing the floating gate coupling effect between adjacent erase groups. 
           [0040]      FIGS. 2 and 3  illustrate the floating gate coupling effect, by contrasting different floating gate voltages that result from being coupled to an adjacent floating gate of a dummy word line, versus being coupled to an floating gate of a word line that is part of the same erase group, where  FIG. 2  shows the adjacent word lines receiving different voltages (such as a negative voltage applied to word lines of a selected erase group, and a positive voltage applied to other word lines outside the selected erase group) and  FIG. 3  shows the adjacent word lines receiving the same voltages. 
           [0041]      FIG. 4  shows equations of the floating gate voltages accounting for capacitive coupling, and applies the equations to show the floating gate coupling effect from the different arrangements of  FIGS. 2 and 3 . 
           [0042]      FIG. 5  shows a top view of an array structure of word lines with a short distance separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in the floating gate coupling effect. 
           [0043]      FIG. 6 , corresponding to  FIG. 5 , shows a side view of an array structure of word lines with a short distance separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in the floating gate coupling effect. 
           [0044]      FIG. 7  shows multiple steps of an erase operation with multiple sub-operations of erase and erase verify, to show that relying on an erase verify sub-operation on an outer word line of the erase group, to perform an erase sub-operation on all word lines of the erase group, results in over-erasing of center word lines in the erase group. 
           [0045]      FIG. 8  is a block diagram of a memory array portion with multiple erase groups, showing that electrical isolation between an outer word line of an erase group and a neighboring word line outside the erase group, helps to suppress the floating gate coupling effect between adjacent word lines of different erase groups. 
           [0046]      FIGS. 9 and 10  illustrate a suppressed floating gate coupling effect, by showing common floating gate voltages that result from being adjacent to a multilevel contact that electrically isolates floating gates of outer word lines of an erase group from floating gates of neighboring word lines outside the erase group. 
           [0047]      FIG. 11  shows equations of the floating gate voltages accounting for capacitive coupling, and applies the equations to show the absence of a floating gate coupling effect from the arrangements of  FIGS. 9 and 10 . 
           [0048]      FIG. 12  shows a top view of an array structure of word lines with a multilevel contact separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in suppressed floating gate coupling effect. 
           [0049]      FIG. 13 , corresponding to  FIG. 12 , shows a side view of an array structure of word lines with a multilevel contact separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in suppressed floating gate coupling effect. 
           [0050]      FIG. 14  shows a top view of an array structure of word lines with a large distance separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in suppressed floating gate coupling effect. 
           [0051]      FIG. 15 , corresponding to  FIG. 14 , shows a side view of an array structure of word lines with a large distance separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in suppressed floating gate coupling effect. 
           [0052]      FIG. 16  shows multiple steps of an erase operation with multiple sub-operations of erase and erase verify, to show that electrical isolation between an outer word line of an erase group and a neighboring word line outside the erase group, results in no over-erasing of center word lines in the erase group. 
           [0053]      FIG. 17  is a simplified block diagram of an integrated circuit including a memory array implemented using any of the erase operation improvements as described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0054]      FIG. 1  is a block diagram of a memory array portion with multiple erase groups, showing the floating gate coupling effect between adjacent erase groups. 
         [0055]    Common examples of an erase group are erase sectors and erase blocks that include word lines which access memory cells erased together, responsive to an erase command identifying a particular erase sector or erase block to be erased. 
         [0056]    In particular,  FIG. 1  shows that neighboring memory cells have a coupling effect when the neighboring memory cells are accessed by WLs having different voltage levels. Accordingly, the outer WLs of an erase group have a floating gate coupling effect during an erase operation. The coupling effect impacts Vt uniformity after an erase an operation due to the coupling effect between a floating of an outer WL of a selected erase group with a floating gate of a neighboring dummy WL, or between a floating of an outer WL a selected erase group and the floating gate of an outer WL of an adjacent unselected erase group. 
         [0057]      FIGS. 2 and 3  illustrate the floating gate coupling effect, by contrasting different floating gate voltages that result from being coupled to an adjacent floating gate of a dummy word line, versus being coupled to an floating gate of a word line that is part of the same erase group, where  FIG. 2  shows the adjacent word lines receiving different voltages (such as a negative voltage applied to word lines of a selected erase group, and a positive voltage applied to other word lines outside the selected erase group) and  FIG. 3  shows the adjacent word lines receiving the same voltages. 
         [0058]    In  FIGS. 2 and 3 , V FG  is the floating gate voltage of a cell, CFF is the cell to cell parasitic capacitance, CFB is the cell to body capacitance, CFD is the cell to drain capacitance, CFW is the cell to word line capacitance, and CFWD is the cell to dummy word line capacitance. 
         [0059]      FIG. 4  shows equations of the floating gate voltages accounting for capacitive coupling, and applies the equations to show the floating gate coupling effect from the different arrangements of  FIGS. 2 and 3 . 
         [0060]    The various α values represent coupling to the floating gate from a neighboring node. In particular, αFW is the coupling value for CFW (word line), αFF is the coupling value for CFF (neighboring floating gate), αFB is the coupling value for CFB (body) and αFD is the coupling value for CFD (drain). Q refers to the charge stored on the respective floating gates, and CT is the nominal capacitance of the floating gate. 
         [0061]    When the voltage of WL 0  is equal to the voltage of WL 1 , and the voltages of DWL and WL the same too, then C FG  (coupling capacitance value, Q T =C FG *V FG ) will be unchanged, and the voltage VFG 1  is largely equal to the voltage VFG 0 . But the voltage magnitude of WL 2  is greater than the voltage magnitude of DWL, so the voltage difference Δ|VFG 0 | (due to floating gate coupling with floating gate FG 0 ) is smaller than the voltage difference Δ|VFG 1 | (due to floating gate coupling with floating gate FG 1 ). Accordingly, due to floating gate coupling between neighboring floating gates FGD and FG 0 , and floating gate coupling between neighboring floating gates FG 1  and FG 2 , VFG 0 ≠VFG 1 . 
         [0062]      FIG. 5  shows a top view of an array structure of word lines with a short distance separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in the floating gate coupling effect. The short distance is because the cells of the outer word lines have a soruce side which connects with the dummy word line cells. 
         [0063]      FIG. 6 , corresponding to  FIG. 5 , shows a side view of an array structure of word lines with a short distance separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in the floating gate coupling effect. 
         [0064]      FIG. 7  shows multiple steps of an erase operation with multiple sub-operations of erase and erase verify, to show that relying on an erase verify sub-operation on an outer word line of the erase group, to perform an erase sub-operation on all word lines of the erase group, results in over-erasing of center word lines in the erase group. 
         [0065]    In  710 , the WLs of an erase group are shown as the center WLs of an erase group, between edge WLs of the erase group. In  720 , the threshold voltage distributions are shown for the center and edge WLs. In  730 , ‘n’ erase shots are performed on all WLs. In the threshold voltage distributions shown for the center and edge WLs  740 , the center WLs of an erase pass erase verify, but edge WLs fail erase verify, due to the floating gate coupling effect. In  750 , ‘m’ erase shots are performed on all WLs. In the threshold voltage distributions shown for the center and edge WLs  760 , although the edge WLs have passed erase verify, the center WL region has over-erased. Accordingly, the floating gate coupling effect caused the Vt distribution to over-erase in the center WL region. 
         [0066]      FIG. 8  is a block diagram of a memory array portion with multiple erase groups, showing that electrical isolation between an outer word line of an erase group and a neighboring word line outside the erase group, helps to suppress the floating gate coupling effect between adjacent word lines of different erase groups. 
         [0067]    The top erase group includes word lines WL 0  to WL 31 , with outer word lines WL 0  and WL 31 , and inner word lines WL 1  through WL 30 . The bottom erase group includes word lines WL 32  to WL 63 , with outer word lines WL 32  and WL 63 , and inner word lines WL 33  through WL 62 . 
         [0068]    In particular,  FIG. 8  shows that neighboring memory cells have no coupling effect or suppressed coupling effect, with electrical isolation, via drain side shielding, between floating gates of adjacent WLs, despite the adjacent WLs having different voltage levels. Accordingly, the outer WLs of an erase group have suppressed floating gate coupling effect during an erase operation. 
         [0069]    The erase voltage Vers is applied to world lines of the selected regions, to erase memory cells that are inside the erase group selected for erase. Also shown is the inhibit voltage Vinhibit applied to word lines of deselected regions, which inhibits erase in memory cells that are outside of the erase group selected for erase. 
         [0070]      FIGS. 9 and 10  illustrate a suppressed floating gate coupling effect, by showing common floating gate voltages that result from being adjacent to a multilevel contact (shown as drain side contacts) that electrically isolates floating gates of outer word lines of an erase group from floating gates of neighboring word lines outside the erase group. 
         [0071]    In  FIGS. 9 and 10 , V FG  is the floating gate voltage of a cell, CFF is the cell to cell parasitic capacitance, CFB is the cell to body capacitance, CFD is the cell to drain capacitance, CFW is the cell to word line capacitance, and CFWD is the cell to dummy word line capacitance. 
         [0072]      FIG. 11  shows equations of the floating gate voltages accounting for capacitive coupling, and applies the equations to show the absence of a floating gate coupling effect from the arrangements of  FIGS. 9 and 10 . 
         [0073]    The various α values represent coupling to the floating gate from a neighboring node. In particular, αFW is the coupling value for CFW (word line), αFF is the coupling value for CFF (neighboring floating gate), αFB is the coupling value for CFB (body) and αFD is the coupling value for CFD (drain). Q refers to the charge stored on the respective floating gates, and CT is the nominal capacitance of the floating gate. 
         [0074]    The multilevel bit line drain contact electrically isolates the neighboring floating gates FGD and FG 0  from each other. The multilevel bit line drain contact also electrically isolates the neighboring floating gates FG 1  and FG 2  from each other. Despite the voltage magnitude of WL 2  being greater than the voltage magnitude of DWL, such electrical isolation suppresses floating gate coupling between neighboring floating gates FGD and FG 0 , and suppresses floating gate coupling between neighboring floating gates FG 1  and FG 2 . Accordingly, VFG 0 =VFG 1 , despite the different floating gate voltages of the adjacent floating gates VFGD≠VFG 2 , which in the absence of the multilevel bit line drain contact, would cause VFG 0 ≠VFG 1 . 
         [0075]      FIG. 12  shows a top view of an array structure of word lines with a multilevel contact (shown as drain side contacts between dummy word line and WL 0 ) separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in suppressed floating gate coupling effect. 
         [0076]    The array structure is divided into multiple erase groups, of sectors  0  (SE 0 ) through sector n (SEn). Each sector includes outer word lines (1 st  WL and last WL) and inner word lines (2 nd  WL through next to last WL). 
         [0077]    The bottom dummy word line and the last word line of sector n (SEn: last WL) share a common drain multilevel contact, to eliminate the coupling effect. The top dummy word line and the first word line of sector  0  (SE 0 : 1st WL) share a common drain multilevel contact, to eliminate the coupling effect. 
         [0078]    The number of dummy word lines can be 1, 2, or more. 
         [0079]    The last word line of sector  0  (SE 0 : last WL) and the first word line of sector  1  (SE 1 : 1st WL) share a common drain multilevel contact, to eliminate the coupling effect. Generally between sector  0  to sector n, the last word line of a preceding sector and the first word line of a subsequent sector share a common drain multilevel contact, to eliminate the coupling effect. 
         [0080]    During erase of the selected erase group, the voltage of the dummy word line is coupled to a Vinhibit voltage or left floating. During erase of the selected erase group, the voltage of the other word lines outside the selected erase group, are also coupled to a Vinhibit voltage or left floating. 
         [0081]      FIG. 13 , corresponding to  FIG. 12 , shows a side view of an array structure of word lines with a multilevel contact separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in suppressed floating gate coupling effect. 
         [0082]    The dummy word line and the outer word line (e.g., first WL or last WL) of a sector share a common drain multilevel contact, to eliminate the coupling effect. Also, the neighboring outer word lines of neighboring sectors (e.g., last WL of sector x and first WL of sector (x+1) share a common drain multilevel contact, to eliminate the coupling effect. Generally between sector  0  to sector n, The last word line of a preceding sector and the first word line of a subsequent sector share a common drain multilevel contact, to eliminate the coupling effect. 
         [0083]    In some embodiments, the multilevel contacts are used to eliminate the coupling effect only for dummy word lines, or only between neighboring sectors, or only between a subset of neighboring sectors, or some combination. 
         [0084]      FIG. 14  shows a top view of an array structure of word lines with a large distance separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in suppressed floating gate coupling effect. 
         [0085]    The array structure is divided into multiple erase groups, of sectors  0  (SE 0 ) through sector n (SEn). Each sector includes outer word lines (1 st  WL and last WL) and inner word lines (2 nd  WL through next to last WL). 
         [0086]    The bottom dummy word line and the last word line of sector n (SEn: last WL) are separated by a large space, to eliminate the coupling effect. The top dummy word line and the first word line of sector  0  (SE 0 : 1st WL) are separated by a large space, to eliminate the coupling effect. 
         [0087]    The number of dummy word lines can be 1, 2, or more. 
         [0088]    The last word line of sector  0  (SE 0 : last WL) and the first word line of sector  1  (SE 1 : 1st WL) are separated by a large space, to eliminate the coupling effect. Generally between sector  0  to sector n, the last word line of a preceding sector and the first word line of a subsequent sector are separated by a large space, to eliminate the coupling effect. 
         [0089]    During erase of the selected erase group, the voltage of the dummy word line is coupled to a Vinhibit voltage or left floating. During erase of the selected erase group, the voltage of the other word lines outside the selected erase group, are also coupled to a Vinhibit voltage or left floating. 
         [0090]      FIG. 15 , corresponding to  FIG. 14 , shows a side view of an array structure of word lines with a large distance separating an outer word line of an erase group from a neighboring word line outside the erase group, resulting in suppressed floating gate coupling effect. 
         [0091]    The dummy word line and the outer word line (e.g., first WL or last WL) of a sector are separated by a large space, to eliminate the coupling effect. Also, the neighboring outer word lines of neighboring sectors (e.g., last WL of sector x and first WL of sector (x+1) are separated by a large space, to eliminate the coupling effect. Generally between sector  0  to sector n, The last word line of a preceding sector and the first word line of a subsequent sector are separated by a large space, to eliminate the coupling effect. 
         [0092]    An example of a large separation, is any word line separation distance larger than the word line separation distance used between other word lines without the coupling effect issue. 
         [0093]    In some embodiments, the large separation distances are used to eliminate the coupling effect only for dummy word lines, or only between neighboring sectors, or only between a subset of neighboring sectors, or some combination. 
         [0094]      FIG. 16  shows multiple steps of an erase operation with multiple sub-operations of erase and erase verify, to show that electrical isolation between an outer word line of an erase group and a neighboring word line outside the erase group, results in no over-erasing of center word lines in the erase group. 
         [0095]    In  1610 , the WLs of an erase group are shown as the inner WLs of an erase group, between outer WLs of the erase group. In  1620 , the threshold voltage distributions are shown for the inner and outer WLs. In  1630 , ‘n’ erase shots are performed on all WLs. In the threshold voltage distributions shown for the inner and outer WLs  1640 , all WLs have failed verify and have about the same Vt level due to no floating gate effect. In  1650 , ‘m’ erase shots are performed on all WLs. In the threshold voltage distributions shown for the inner and outer WLs  1660 , although the outer WLs have passed erase verify, and the inner region has not over-erased. Accordingly, the Vt distribution has not over-erased. 
         [0096]    No over-erase resulted in the Vt distribution, due to the electrical isolation between the floating gates of the outer WLs and the floating gates of neighboring WLs outside the erase group. Because the floating gate coupling effect is eliminated from the outer WLs, all WLs can be verified and erased, removing the over-erase effect. 
         [0097]      FIG. 17  is a simplified block diagram of an integrated circuit including a memory array implemented using any of the erase operation improvements as described herein. 
         [0098]      FIG. 17  is a simplified block diagram of an integrated circuit  1750  including a memory array  1700 . The memory array  1700  has suppressed coupling between floating gates of word lines in different erase groups. For example, a multilevel bit line contact is between the outer word line of an erase group and a neighboring word line outside the erase group, to provide electrical isolation between their respective floating gates. In another example, a distance separates the outer word line of an erase group and a neighboring word line outside the erase group, to provide electrical isolation between their respective floating gates. A word line (or row) and block select decoder  1701  is coupled to, and in electrical communication with, a plurality  1702  of word lines and string select lines, and arranged along rows in the memory array  1700 . A bit line (column) decoder and drivers  1703  are coupled to and in electrical communication with a plurality of bit lines  1704  arranged along columns in the memory array  1700  for reading data from, and writing data to, the memory cells in the memory array  1700 . Addresses are supplied on bus  1705  to the word line decoder and drivers  1701  and to the bit line decoder  1703 . Sense amplifiers and data-in structures in block  1706 , including current sources for the read, program and erase modes, are coupled to the bit line decoder  1703  via the bus  1707 . Data is supplied via the data-in line  1711  from input/output ports on the integrated circuit  1750 , to the data-in structures in block  1706 . Data is supplied via the data-out line  1715  from the sense amplifiers in block  1706  to input/output ports on the integrated circuit  1750 , or to other data destinations internal or external to the integrated circuit  1750 . State machine and improved clock circuitry are in circuitry  1709 , controlling biasing arrangement supply voltages  1708 . 
         [0099]    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.