Abstract:
The switch transistors in the NAND strings have combinations of threshold voltage levels that vary across the levels of a three dimensional memory array. A bias arrangement is applied to the select lines electrically coupled to the switch transistors. The NAND strings on a particular level of a three dimensional memory array are selected. The NAND strings on other levels are deselected.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    This technology relates to high density memory devices with decoding structures in the z-direction, or depth direction. Such decoding distinguishes between different levels, or layers, of memory cells arranged to provide a three-dimensional memory array. 
         [0003]    2. Description of Related Art 
         [0004]    In Patent Application Publication 2010/0226195, a three dimensional memory array has the decoding function for the z-direction, or depth direction, removed from the actual array. In one example, a word line-like structure electrically connects the gates of only transistors that are in a same level of the three dimensional memory array, and are electrically disconnected from the gates transistors that are in a different level of the three dimensional memory array. In another example, the ends of NAND strings on a same level of the three dimensional memory array are electrically connected together, and are electrically disconnected from the ends of NAND strings on that are in a different level of the three dimensional memory array. Neither of these examples performs decoding of the level at the three dimensional memory array. Instead, actual decoding is performed in remotely located circuitry that then determines which level of NAND strings is selected for a particular operation. Complexity results from the structures and interconnects which connect the decoded level signals to the different levels of the three dimensional memory array. 
       SUMMARY 
       [0005]    One aspect of the technology is an integrated circuit, which includes a three dimensional memory array, a plurality of select lines, and control circuitry. 
         [0006]    The three dimensional memory array includes a plurality of levels. The levels include respective two dimensional arrays of NAND strings. The NAND strings include memory cells and switch transistors. The switch transistors have combinations of threshold voltage levels that vary across the plurality of levels. In one example, there are two threshold voltage levels, each defined by an acceptable range of threshold voltages. Other examples have more than two threshold voltage levels. 
         [0007]    The plurality of select lines are electrically coupled to the switch transistors. For example, a select line is orthogonally arranged across the NAND strings to control the gate voltage of switch transistors in the NAND strings. 
         [0008]    The control circuitry applies a bias arrangement to the plurality of select lines. Due to the bias arrangement, the NAND strings on a particular level of the plurality of levels are selected by the switch transistors, and the NAND strings on other levels of the plurality of levels are deselected by the switch transistors. The NAND strings on a particular level are selected, when the switch transistors are turned on for the switch transistors in the NAND strings on the particular level. The NAND strings on other levels are deselected, when at least one of the switch transistors is turned off for the switch transistors in the NAND strings on the particular level. Because the NAND strings are series connected, a minimum of one series transistor being turned off is sufficient to deselect the NAND string. 
         [0009]    In some embodiments, threshold voltages of the switch transistors controlled by one of the plurality of select lines are from a quantity of switch threshold voltage levels, and the quantity of threshold voltage levels is equal to a number of levels in the plurality of levels of the three dimensional array. Some examples of the quantity of switch threshold voltage levels and the number of levels in the plurality of levels of the three dimensional array, are 4 levels, 8 levels, 16 levels, other multiples of 2, and other numbers of levels. 
         [0010]    In some embodiments, the plurality of select lines includes two select lines, and the switch transistors electrically coupled to the two select lines distinguish the NAND strings on the particular level from the NAND strings on the other levels of the plurality of levels. In this example, the two select lines control two switch transistors in a NAND string. The NAND strings on a particular level are deselected if one or both switch transistors are off. The NAND strings on a particular level are selected if both switch transistors are on. 
         [0011]    In some embodiments, threshold voltages of the switch transistors controlled by one of the plurality of select lines are from a quantity of switch threshold voltage levels, and the quantity of threshold voltage levels is equal to a square root of a number of levels in the plurality of levels of the three dimensional array. Some examples of the quantity of switch threshold voltage levels and the number of levels in the plurality of levels of the three dimensional array, are 4 switch threshold voltage levels and 16 levels in the three dimensional array, 8 switch threshold voltage levels and 64 levels in the three dimensional array, and other numbers of levels. 
         [0012]    In some embodiments, the plurality of select lines includes four select lines, and the switch transistors electrically coupled to the four select lines distinguish NAND strings on the particular level from the NAND strings on the other levels of the plurality of levels. In this example, the four select lines control four switch transistors in a NAND string. The NAND strings on a particular level are deselected if one, two, three, or four switch transistors are off. The NAND strings on a particular level are selected if the four switch transistors are on. 
         [0013]    In some embodiments, threshold voltages of the switch transistors coupled to different select lines of the plurality of select lines are from corresponding sets of switch threshold voltage levels, which may be different or the same. For example, a first set of switch transistors controlled by a first select line has threshold voltages from a first set of switch threshold voltage levels, and a second set of switch transistors controlled by a second select line has threshold voltages from a second set of switch threshold voltage levels. In one example, the first set of switch threshold voltage levels includes 0.5V&lt;Vth&lt;1.2V, 1.2V&lt;Vth&lt;2.2V, 2.2V&lt;Vth&lt;3V, and Vth&gt;3V; and the second set of switch threshold voltages includes Vth&gt;4V, 3.2V&lt;Vth&lt;4V, 2.5V&lt;Vth&lt;3.2V, and 1.5V&lt;Vth&lt;2.5V. The sets of threshold voltages are different not because of different ordering (increasing versus decreasing), but because the threshold voltage ranges are different, for example Vth&gt;3V versus Vth&gt;4V. In this example, the quantity of threshold voltage levels is the same for the first and second sets of switch threshold voltage levels. 
         [0014]    In some embodiments, the plurality of select lines includes a first select line and a second select line. The switch transistors include a first set of switch transistors electrically coupled to the first select line, and a second set of switch transistors electrically coupled to the second select line. The NAND strings include memory cells in series with a first switch transistor from the first set of switch transistors, and a second switch transistor from the second set of switch transistors. Threshold voltages of the first set of switch transistors are from a first set of switch threshold voltage levels having a quantity of switch threshold voltage levels, and threshold voltages of the second set of switch transistors are from a second set of switch threshold voltage levels having the same quantity of switch threshold voltage levels. The number of levels in the plurality of levels of the three dimensional array is equal to the same quantity of switch threshold voltage levels. The first and second sets of switch threshold voltage levels may be the same or different. 
         [0015]    In some embodiments, the plurality of select lines includes a first select line, a second select line, a third select line, and a fourth select line. The switch transistors include a first set of switch transistors electrically coupled to the first select line, a second set of switch transistors electrically coupled to the second select line, a third set of switch transistors electrically coupled to the third select line, and a fourth set of switch transistors electrically coupled to the fourth select line. The NAND strings include memory cells in series with a first switch transistor from the first set of switch transistors, a second switch transistor from the second set of switch transistors, a third switch transistor from the third set of switch transistors, and a fourth switch transistor from the fourth set of switch transistors. Threshold voltages of the first set of switch transistors are from a first set of switch threshold voltage levels having a first quantity of switch threshold voltage levels. Threshold voltages of the second set of switch transistors are from a second set of switch threshold voltage levels having the same first quantity of switch threshold voltage levels. Threshold voltages of the third set of switch transistors are from a third set of switch threshold voltage levels having a second quantity of switch threshold voltage levels. Threshold voltages of the fourth set of switch transistors are from a fourth set of switch threshold voltage levels having the same second quantity of switch threshold voltage levels. The number of levels in the plurality of levels of the three dimensional array is equal to a product of the first quantity of switch threshold voltage levels and the second quantity of threshold voltage levels. 
         [0016]    In some embodiments, the plurality of select lines includes a first select line and a second select line. The switch transistors include a first set of switch transistors electrically coupled to the first select line, and a second set of switch transistors electrically coupled to the second select line. The NAND strings include memory cells in series with a first switch transistor from the first set of switch transistors and a second switch transistor from the second set of switch transistors. Threshold voltages of the first set of switch transistors are paired with threshold voltages of the second set of switch transistors. A first threshold voltage of the first switch transistors on the particular level at an i-th threshold voltage level in a first set of N ordered threshold voltage levels, is paired with a second threshold voltage of the second switch transistors on the particular level at an i-th threshold voltage level in a second set of N decreasingly ordered threshold voltage levels. 
         [0017]    For example, if the set of N=4 ordered threshold voltages includes (1V, 2V, 3V, 4V) then an increasing set of threshold voltages is (1V, 2V, 3V, 4V), and a decreasing set of threshold voltages is (4V, 3V, 2V, 1V). Examples of paired threshold voltages of first and second switch transistors are 1V and 4V; 2V and 3V; 3V and 2V; and 4V and 1V. The voltage values are examples only, and can be other integer values and noninteger values. 
         [0018]    In another example, if the set of N=8 ordered threshold voltages includes (1V, 2V, 3V, 4V, 5V, 6V, 7V, 8V) then an increasing set of threshold voltages is (1V, 2V, 3V, 4V, 5V, 6V, 7V, 8V), and a decreasing set of threshold voltages is (8V, 7V, 6V, 5V, 4V, 3V, 2V, 1V). Examples of paired threshold voltages of first and second switch transistors are 1V and 8V; 2V and 7V; 3V and 6V; and 4V and 5V; 5V and 4V; 6V and 3V; 7V and 2V; and 8V and 1V. 
         [0019]    The voltage values are examples only, and can be other integer values and noninteger values. Also, threshold voltages do not have to be symmetrical between the increasing and decreasing sets of threshold voltages. The increasing set of threshold voltages can have one or more values absent from the decreasing set of threshold voltages. The decreasing set of threshold voltages can have one or more values absent from the increasing set of threshold voltages. 
         [0020]    In some embodiments, the plurality of select lines includes a first select line, a second select line, a third select line, and a fourth select line. The switch transistors includes a first set of switch transistors electrically coupled to the first select line, a second set of switch transistors electrically coupled to the second select line, a third set of switch transistors electrically coupled to the third select line, and a fourth set of switch transistors electrically coupled to the fourth select line. The NAND strings include memory cells in series with a first switch transistor from the first set of switch transistors, a second switch transistor from the second set of switch transistors, a third switch transistor from the third set of switch transistors, and a fourth switch transistor from the fourth set of switch transistors. Threshold voltages of the first set of switch transistors, the second set of switch transistors, the third set of switch transistors, and the fourth set of switch transistors are from corresponding sets of switch threshold voltage levels having N switch threshold voltage levels. Threshold voltages of the first set of switch transistors are paired with threshold voltages of the second set of switch transistors, such that the bias arrangement applied to the first select line and the second select line selects the NAND strings on a fraction (1/N) of the plurality of levels. Threshold voltages of the third set of switch transistors are paired with threshold voltages of the fourth set of switch transistors, such that the bias arrangement applied to the third select line and the fourth select line selects the NAND strings on the fraction (1/N) of the plurality of levels. The intersection of the NAND strings selected by the first select line and the second select line, and the NAND strings selected by the third select line and the fourth select line, selects the NAND strings on the particular level of the plurality of levels. 
         [0021]    The intersection of the fraction (1/N) of the plurality of levels selected by the first select line and the second select line, and the fraction (1/N) of the plurality of levels selected by the third select line and the fourth select line, selects the fraction (1/N 2 ) of the plurality of levels. For example, when N=4 threshold voltage levels, the fraction (¼) of the levels are selected by the first select line and the second select line, the fraction (¼) of the levels are selected by the third select line and the fourth select line, and the intersection selects ( 1/16) of the levels, such that from 16 levels of an array, one level is selected. 
         [0022]    Another aspect of the technology is a method comprising: 
         [0023]    selecting NAND strings including memory cells and switch transistors on a particular level of a plurality of levels of a three dimensional memory array, and deselecting NAND strings on other levels of the plurality of levels, by applying a bias arrangement to a plurality of select lines electrically coupled to the switch transistors having combinations of threshold voltages that vary across the plurality of levels. 
         [0024]    Various embodiments and aspect are disclosed herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a perspective illustration of a 3D NAND memory structure with multiple layers of NAND strings, and switch transistors in the NAND strings that select the NAND strings in a particular layer of the 4-layer 3D NAND memory structure. 
           [0026]      FIG. 2  is a table showing an example of threshold voltage combinations of the pairs of switch transistors in NAND strings on different layers of the 4-layer 3D NAND memory structure corresponding to the circuit of  FIG. 3 . 
           [0027]      FIG. 3  is a circuit schematic showing the threshold voltages of the pairs of switch transistors in the NAND strings on different layers of the 4-layer 3D NAND memory structure, corresponding to the table of  FIG. 2 . 
           [0028]      FIGS. 4-7  are circuit schematics showing how various bias arrangements applied to the two select lines select different layers of NAND strings in the 3D NAND memory structure, where the bias arrangements applied to the two select lines control the two switch transistors in the NAND strings, corresponding to the table of  FIG. 2  and the circuit schematic of  FIG. 3 . 
           [0029]      FIG. 8  is a table showing an example of threshold voltage combinations of the pairs of switch transistors in NAND strings on different layers of the 8-layer 3D NAND memory structure corresponding to the circuit of  FIG. 9 . 
           [0030]      FIG. 9  is a circuit schematic showing the threshold voltages of the pairs of switch transistors in the NAND strings on different layers of the 8-layer 3D NAND memory structure, corresponding to the table of  FIG. 8   
           [0031]      FIG. 10  is a table showing an example of threshold voltage combinations of the quartets of switch transistors in NAND strings on different layers of the 16-layer 3D NAND memory structure corresponding to the circuit of  FIG. 11 . 
           [0032]      FIG. 11  is a circuit schematic showing the threshold voltages of the quartets of switch transistors in the NAND strings on different layers of the 16-layer 3D NAND memory structure, corresponding to the table of  FIG. 10 . 
           [0033]      FIGS. 12-13  are tables showing examples of threshold voltage combinations of the pairs of switch transistors in NAND strings on different layers of the 4-layer 3D NAND memory structure, corresponding to the circuit of  FIG. 14 . 
           [0034]      FIG. 14  is a circuit schematic showing the threshold voltages of the pairs of switch transistors in the NAND strings on different layers of the 4-layer 3D NAND memory structure, corresponding to the tables of  FIGS. 12 ,  13 , and  15 . 
           [0035]      FIG. 15  is a table showing an example of threshold voltage combinations of the pairs of switch transistors in NAND strings on different layers of the 4-layer 3D NAND memory structure, corresponding to the circuit of  FIG. 14 . 
           [0036]      FIG. 16  is a simplified block diagram of an integrated circuit according to an embodiment of the present technology. 
       
    
    
     DETAILED DESCRIPTION 
       [0037]      FIG. 1  is a schematic diagram showing 2 levels of memory cells, each having memory cells arranged in a NAND configuration of several NAND strings. This configuration is representative of a cube which can include many levels and many word lines. The shown cube includes 3 stacks of memory cells, with each stack of memory cells including NAND strings stacked 2 levels high and 1 string wide. 
         [0038]    The 2 levels of memory cells are defined at the cross-points of conductive lines  160  and  161  acting as word line WLn−1 and word line WLn, with a first stack of conductive strips, a second stack of conductive strips and a third stack of conductive strips. The 2 levels of switch transistors are defined at the cross-points of conductive lines  162  and  163  acting as string select line SSL 1  and string select line SSL 2 , with a first stack of conductive strips, a second stack of conductive strips and a third stack of conductive strips. Other embodiments have a different number of string select lines, such as four string select lines. 
         [0039]    The first level corresponds with an upper level in the cube and includes memory cells  70 ,  71  and switch transistors  72 ,  85  in a NAND string on a conductive strip; memory cells  73 ,  74  and switch transistors  75 ,  88  in a NAND string on a conductive strip; and memory cells  76 ,  77  and switch transistors  78 ,  89  in a NAND string on a conductive strip. 
         [0040]    The second level corresponds with a bottom level in the cube in this example, and includes memory cells (e.g.  80 ,  82 ,  84 ) and switch transistors arranged in NAND strings in a similar manner those in the first level. 
         [0041]    As shown in the figure, the conductive line  161  acting as word line WLn includes vertical extensions in order to couple the conductive line  161  to the memory cells (cells  71 ,  74 ,  77  in the first level) in the interface regions in the trenches between the conductive strips in all of the levels. The other conductive lines  159 ,  160 ,  162 ,  163  include similar vertical extensions. 
         [0042]    String select transistor pairs  72 ,  85 ;  75 ,  88 ; and  78 ,  89  are connected between respective NAND strings and bit line BL N+1 , bit line BL N , bit line BL N−1  in this arrangement. Likewise, similar string select transistors on a bottom level in the cube are connected between respective NAND strings and bit line BL N + 1    106 , bit line BL N    107 , bit line BL N−1    108  in this arrangement. Bit line BL N+1    106 , bit line BL N    107 , bit line BL N−1    108  are connected respectively to ends of the NAND strings in the stack of conductive strips to the left, center, and right in the figure. 
         [0043]    Block select transistors  90 - 95  are arranged at the opposite ends of the NAND strings and are used to couple the NAND strings in a selected cube to a reference source such as ground. The ground select line GSL  159  in this example is coupled to the gates of the block select transistors  90 - 95 , and can be implemented in the same manner as the conductive lines  160 ,  161  and  162 . The string select transistors and block select transistors can use the same dielectric stack as a gate oxide as the memory cells in some embodiments. In other embodiments, a typical gate oxide is used instead. Also, the channel lengths and widths can be adjusted as suits the designer to provide the switching function for the transistors. 
         [0044]    Examples of setting the voltages of the switch transistors are programming the switch transistors, and manufacturing the switch transistors such as with appropriate implant doses. 
         [0045]      FIG. 2  is a table showing an example of threshold voltage combinations of the pairs of switch transistors in NAND strings on different layers of the 4-layer 3D NAND memory structure. 
         [0046]      FIG. 3  is a circuit schematic showing the threshold voltages of the pairs of switch transistors in the NAND strings on different layers of the 4-layer 3D NAND memory structure, corresponding to the table of  FIG. 2 . 
         [0047]    The NAND strings on the various layers of the 4-layer 3D NAND memory structure have 2 series-coupled switch transistors. 
         [0048]    The specific threshold voltages are examples. The pair of switch transistors—controlled by 2 different string select lines—have threshold voltages that are paired, such that a bias arrangement applied to the 2 different string select lines selects the layer of NAND strings having the appropriate pair of threshold voltages. More generally, switch transistor threshold voltages are from the set of threshold voltages (Vth 1 , Vth 2 , Vth 3 , Vth 4 ) such that Vth 1 &lt;Vth 2 &lt;Vth 3 &lt;Vth 4 . 
         [0049]    The different layers of the NAND memory structure have pairs of switch transistors with a corresponding pair of threshold voltages (Vth of switch transistor controlled by SSL 1 , Vth of switch transistor controlled by SSL 2 ) as follows. In a first layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 1 , Vth 4 ). In a second layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 2 , Vth 3 ). In a third layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 3 , Vth 2 ). In a fourth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 4 , Vth 1 ). 
         [0050]    The physical order of the just described layers within the 3D NAND memory structure may be consecutive, from the top layer to the bottom layer, or from the bottom layer to the top layer. The order of the just described layers within the 3D NAND memory structure also may be nonconsecutive. 
         [0051]      FIGS. 4-7  are circuit schematics showing how various bias arrangements applied to the two select lines select different layers of NAND strings in the 3D NAND memory structure, where the bias arrangements applied to the two select lines control the two switch transistors in the NAND strings, corresponding to the table of  FIG. 2  and the circuit schematic of  FIG. 3 . 
         [0052]    In  FIG. 4 , the bias arrangement applied to the strings select lines (voltage applied to SSL 1 , voltage applied to SSL 2 ) is (1V, 4V). The 1V bias applied to SSL 1  turns on the switch transistors of layer  1 . The 4V bias applied to SSL 2  turns on the switch transistors of all layers. The intersection of these layers is only layer  1 , thereby decoding layer  1 . 
         [0053]    In  FIG. 5 , the bias arrangement applied to the strings select lines (voltage applied to SSL 1 , voltage applied to SSL 2 ) is (2V, 3V). The 2V bias applied to SSL 1  turns on the switch transistors of layers  1  and  2 . The 3V bias applied to SSL 2  turns on the switch transistors of layers  2 ,  3 , and  4 . The intersection of these layers is only layer  2 , thereby decoding layer  2 . 
         [0054]    In  FIG. 6 , the bias arrangement applied to the strings select lines (voltage applied to SSL 1 , voltage applied to SSL 2 ) is (3V, 2V). The 3V bias applied to SSL 1  turns on the switch transistors of layers  1 ,  2 , and  3 . The 2V bias applied to SSL 2  turns on the switch transistors of layers  3  and  4 . The intersection of these layers is only layer  3 , thereby decoding layer  3 . 
         [0055]    In  FIG. 7 , the bias arrangement applied to the strings select lines (voltage applied to SSL 1 , voltage applied to SSL 2 ) is (4V, 1V). The 4V bias applied to SSL 1  turns on the switch transistors of all layers. The 1V bias applied to SSL 2  turns on the switch transistors of layer  4 . The intersection of these layers is only layer  4 , thereby decoding layer  4 . 
         [0056]      FIG. 8  is a table showing an example of threshold voltage combinations of the pairs of switch transistors in NAND strings on different layers of the 8-layer 3D NAND memory structure. 
         [0057]      FIG. 9  is a circuit schematic showing the threshold voltages of the pairs of switch transistors in the NAND strings on different layers of the 8-layer 3D NAND memory structure, corresponding to the table of  FIG. 8   
         [0058]    The NAND strings on the various layers of the 8-layer 3D NAND memory structure have 2 series-coupled switch transistors. 
         [0059]    The specific threshold voltages are examples. The pair of switch transistors—controlled by 2 different string select lines—have threshold voltages that are paired, such that a bias arrangement applied to the 2 different string select lines selects the layer of NAND strings having the appropriate pair of threshold voltages. More generally, switch transistor threshold voltages are from the set of threshold voltages (Vth 1 , Vth 2 , Vth 3 , Vth 4 , Vth 5 , Vth 6 , Vth 7 , Vth 8 ) such that Vth 1 &lt;Vth 2 &lt;Vth 3 &lt;Vth 4 &lt;Vth 5 &lt;Vth 6 &lt;Vth 7 &lt;Vth 8 . 
         [0060]    The different layers of the NAND memory structure have pairs of switch transistors with a corresponding pair of threshold voltages (Vth of switch transistor controlled by SSL 1 , Vth of switch transistor controlled by SSL 2 ) as follows. In a first layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 1 , Vth 8 ). In a second layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 2 , Vth 7 ). In a third layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 3 , Vth 6 ). In a fourth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 4 , Vth 5 ). In a fifth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 5 , Vth 4 ). In a sixth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 6 , Vth 3 ). In a seventh layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 7 , Vth 2 ). In a eighth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 8 , Vth 1 ). 
         [0061]    The physical order of the just described layers within the 3D NAND memory structure may be consecutive, from the top layer to the bottom layer, or from the bottom layer to the top layer. The order of the just described layers within the 3D NAND memory structure also may be nonconsecutive. 
         [0062]      FIG. 10  is a table showing an example of threshold voltage combinations of the quartets of switch transistors in NAND strings on different layers of the 16-layer 3D NAND memory structure. 
         [0063]      FIG. 11  is a circuit schematic showing the threshold voltages of the quartets of switch transistors in the NAND strings on different layers of the 16-layer 3D NAND memory structure, corresponding to the table of  FIG. 10 . 
         [0064]    The NAND strings on the various layers of the 16-layer 3D NAND memory structure have 4 series-coupled switch transistors. 
         [0065]    The specific threshold voltages are examples. The quartet of switch transistors—controlled by 4 different string select lines—have threshold voltages that are quartets. 
         [0066]    More generally, switch transistor threshold voltages are from the set of threshold voltages (Vth 1 , Vth 2 , Vth 3 , Vth 4 ) such that Vth 1 &lt;Vth 2 &lt;Vth 3 &lt;Vth 4 . 
         [0067]    The quartet of switch transistors controlled by 4 different string select lines is divided into a first pair of switch transistors controlled by a first pair of string select lines, and a second pair of switch transistors controlled by a second pair of string select lines. The two pairs of switch transistors and string select transistors are discussed separately because of the different assignment of threshold voltages for each pair of switch transistors and string select transistors. 
         [0068]    The first pair of switch transistors is controlled by a first pair of string select lines SSL 1  and SSL 2 , and the second pair of switch transistors is controlled by a second pair of string select lines SSL 3  and SSL 4 . 
         [0069]    The different layers of the NAND memory structure have first pairs of switch transistors with a corresponding first pair of threshold voltages (Vth of switch transistor controlled by SSL 1 , Vth of switch transistor controlled by SSL 2 ) as follows. In a first, fifth, ninth, and thirteenth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 1 , Vth 4 ). In a second, sixth, tenth, and fourteenth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 2 , Vth 3 ). In a third, seventh, eleventh, and fifteenth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 3 , Vth 2 ). In a fourth, eighth, twelfth, and sixteenth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 4 , Vth 1 ). 
         [0070]    The physical assignment of the just described layers within the 3D NAND memory structure may be different according to different mathematical permutations, so long as a bias arrangement applied to the string select lines SSL 1  and SSL 2  selects (¼) of the layers. 
         [0071]    The different layers of the NAND memory structure have second pairs of switch transistors with a corresponding second pair of threshold voltages (Vth of switch transistor controlled by SSL 3 , Vth of switch transistor controlled by SSL 4 ) as follows. In a first, second, third, and fourth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 1 , Vth 4 ). In a fifth, sixth, seventh, and eighth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 2 , Vth 3 ). In a ninth, tenth, eleventh, and twelfth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 3 , Vth 2 ). In a thirteenth, fourteenth, fifteenth, and sixteenth layer of the NAND memory structure, the pair of switch transistors in the NAND strings have the threshold voltage pair (Vth 4 , Vth 1 ). 
         [0072]    The physical assignment of the just described layers within the 3D NAND memory structure may be different according to different mathematical permutations, so long as a bias arrangement applied to the string select lines SSL 3  and SSL 4  selects (¼) of the layers. 
         [0073]    An intersection of the NAND strings selected by the first select line SSL 1  and the second select line SSL 2  ((¼)*16 layers=4 layers), and the NAND strings selected by the third select line SSL 3  and the fourth select line SSL 4  ((¼)*16 layers=4 layers), selects the NAND strings on the particular level of the plurality of levels ((¼)*(¼)*16 layers=1 layer). 
         [0074]    In other embodiments, the first pair of select lines is not necessarily SSL 1  and SSL 2  but another permutation of string select lines (e.g., SSL 3  and SSL 4 , SSL 1  and SSL 3 , SSL 2  and SSL 1 ), and the second pair of select lines is not necessarily SSL 3  and SSL 4  but another permutation of string select lines (e.g., SSL 1  and SSL 2 , SSL 2  and SSL 4 , SSL 4  and SSL 3 ). 
         [0075]      FIGS. 12-13  and  15  are tables showing examples of threshold voltage combinations of the pairs of switch transistors in NAND strings on different layers of the 4-layer 3D NAND memory structure, corresponding to the circuit of  FIG. 14 .  FIGS. 12-13  and  15  are more general than  FIG. 2 .  FIGS. 12-13  and  15  do not require symmetry between the increasing threshold voltage for the switch transistors of one select line and the decreasing threshold voltages of another select line. 
         [0076]      FIG. 12  shows that the threshold voltages of the switch transistors increase with layer number for the switch transistors controlled by SSL 1 .  FIG. 12  also shows that the threshold voltages of the switch transistors decrease with layer number for the switch transistors controlled by SSL 2 . 
         [0077]      FIG. 13  shows that the threshold voltages of the switch transistors decrease with layer number for the switch transistors controlled by SSL 1 .  FIG. 13  also shows that the threshold voltages of the switch transistors increase with layer number for the switch transistors controlled by SSL 2 . 
         [0078]    For  FIGS. 12 and 13 , it is not necessary to have symmetrical threshold voltages Vt between the switch transistors controlled by SSL 1  and SSL 2 . For example, it is not required that Va=Vw, Vx=Vb, Vy=Vc, or Vz=Vd. 
         [0079]      FIG. 15  shows a specific example of threshold voltage levels that are not symmetrical between the increasing threshold voltage for the switch transistors of one select line and the decreasing threshold voltages of another select line. The first set of switch threshold voltage levels includes 0.5V&lt;Vth&lt;1.2V, 1.2V&lt;Vth&lt;2.2V, 2.2V&lt;Vth&lt;3V, and Vth&gt;3V; and the second set of switch threshold voltages includes Vth&gt;4V, 3.2V&lt;Vth&lt;4V, 2.5V&lt;Vth&lt;3.2V, and 1.5V&lt;Vth&lt;2.5V. 
         [0080]      FIG. 14  is a circuit schematic showing the threshold voltages of the pairs of switch transistors in the NAND strings on different layers of the 4-layer 3D NAND memory structure, corresponding to the tables of  FIGS. 12 ,  13 , and  15 .  FIG. 14  is more general than  FIG. 3 .  FIG. 14  does not require symmetry between the increasing threshold voltage for the switch transistors of one select line and the decreasing threshold voltages of another select line. 
         [0081]    In other embodiments, symmetry is not required between the increasing threshold voltage for the switch transistors of one select line and the decreasing threshold voltages of another select line, for the examples shown in  FIGS. 8-11 . 
         [0082]      FIG. 16  is a simplified block diagram of an integrated circuit according to an embodiment of the present technology. The integrated circuit line  975  includes a 3D NAND flash memory array  960 , implemented as described herein, on a semiconductor substrate. A row decoder  961  is coupled to a plurality of word lines  962 , and arranged along rows in the memory array  960 . A column decoder  963  is coupled to a plurality of bit lines  964  arranged along columns in the memory array  960  for reading and programming data from the memory cells in the array  960 . A level decoder  958  is coupled to a plurality of levels in the memory array  960  via SSL lines  959 . Level decoding is performed by applying bias arrangements to the SSL lines as described above. Addresses are supplied on bus  965  to column decoder  963 , row decoder  961  and plane decoder  958 . Sense amplifiers and data-in structures in block  966  are coupled to the column decoder  963  in this example via data bus  967 . Data is supplied via the data-in line  971  from input/output ports on the integrated circuit  975  or from other data sources internal or external to the integrated circuit  975 , to the data-in structures in block  966 . In the illustrated embodiment, other circuitry  974  is included on the integrated circuit, such as a general purpose processor or special purpose application circuitry, or a combination of modules providing system-on-a-chip functionality supported by the NAND flash memory cell array. Data is supplied via the data-out line  972  from the sense amplifiers in block  966  to input/output ports on the integrated circuit  975 , or to other data destinations internal or external to the integrated circuit  975 . 
         [0083]    A controller implemented in this example using bias arrangement state machine  969  controls the application of bias arrangement supply voltage generated or provided through the voltage supply or supplies in block  968 , such as read, erase, program, erase verify and program verify voltages. The controller can be implemented using special-purpose logic circuitry as known in the art. In alternative embodiments, the controller comprises a general-purpose processor, which may be implemented on the same integrated circuit, which executes a computer program to control the operations of the device. In yet other embodiments, a combination of special-purpose logic circuitry and a general-purpose processor may be utilized for implementation of the controller. 
         [0084]    Although the shown string select lines controlling switch transistors are shown adjacent to each other, the string select lines can be nonadjacent, such as having one or more select line one side of the word lines, and one or more other select lines on the other side of the word lines. 
         [0085]    In various embodiments, the memory cells accessed by word lines are nonvolatile memory cells such as charge trapping memory cells or flash memory cells, or volatile memory cells. In various embodiments the switch transistors accessed by select lines are nonvolatile memory cells such as charge trapping memory cells or flash memory cells, or volatile memory cells. 
         [0086]    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.