Abstract:
Methods for minimizing current consumption in a memory array during programming of non-volatile memory cells, such as NROM cells, in the array include: programming a cell without having a direct current flowing from a positive supply to ground through the array, programming a plurality of cells with programming pulses without discharging a global bit line carrying a programming voltage between programming pulses, and programming a cell with transient currents.

Description:
RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. application Ser. No. 10/864,500, which was filed in the U.S. Patent and Trademark Office on Jun. 10, 2004. The disclosures of all these applications, including all appendixes thereof, are incorporated herein by reference— 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to non-volatile cells generally and to methods of programming them in particular. 
       BACKGROUND OF THE INVENTION 
       [0003]    Non-volatile memory cells are known in the art, as are nitride read only memory (NROM) cells, which store two bits per cell. A memory chip typically includes an array of these cells, with each bit of each cell being individually accessible. 
         [0004]      FIGS. 1A and 1B , to which reference is now made, illustrate two exemplary, prior art memory chips, which includes the array, labeled  10 , of NROM cells  12 , an X decoder  14 , a Y decoder  16  and a Y multiplexer (Y-MUX)  18 . The array of  FIG. 1A  is a segmented virtual ground array and is described in U.S. Pat. No. 6,633,496, assigned to the common assignee of the present invention and the array of  FIG. 1B  is based on the segmented array structure as described in U.S. Pat. No. 6,614,692, assigned to the common assignee of the present invention. 
         [0005]    In both embodiments, array  10  comprises word lines WL(i) and local bit lines BL j  to which the NROM cells  12  are connected and through which NROM cells  12  are accessed. Each NROM cell, such as cells  12 A and  12 B ( FIG. 1A ), comprises a gate terminal G, and two diffusions F acting as the source or drain of the cell. Each word line WL(i) connects together a row of NROM cells  12  by connecting their gates G together. Moreover, the diffusions F of each two neighboring cells of a row are connected together. Each local bit line BL j  connects together two columns of NROM cells  12 , connecting together their diffusions F. Thus, bit line BL 8  connects the diffusions F of cells  12 A and  12 B to the diffusions F of cells connected to other WL&#39;s but sharing the same column (like cells  12 D and  12 E). Array  10  also comprises global bit lines GBL(n) and select cells  20 . Global bit lines GBL(n) are connected between Y-MUX  18  and select cells  20  and select cells  20  connect the global bit lines GBL(n) to local bit lines BL j . In the embodiment of  FIG. 1B , there are isolation areas  22  which electrically isolate one or more columns of cells  12  from its neighboring columns. 
         [0006]    X decoder  14  activates a row of NROM cells  12  by activating an individual word line WL(i). Y decoder  16  decodes the Y address of a bit to be accessed and instructs Y-MUX  18  to select the global bit lines GBL(n) connectable to the local bit lines BL j  of the cell housing the bit to be accessed. X decoder  14  also activates the SELECT lines associated with the select cells  20  needed to connect the selected global bit line GBL(n) to the desired local bit line BL j . 
         [0007]    For example, to program a bit, all the global bit lines GBL are initially discharged and floating. To program a bit in cell  12 C, X decoder  14  may first activate word line WL(i), to activate the gate of cell  12 C. As word line WL(i) rises to its programming voltage level, X-decoder  14  may select the bit line access path by activating select lines SEL- 5  and SEL- 4  to activate select cells  20 A and  20 B, respectively. Y-MUX  18  may drive GBL(N) and GBL(N+1) to GND and may leave all the other global bit lines floating. 
         [0008]    In order to apply the programming pulse that will program the cell&#39;s bit (the right side charge storage area as in  FIG. 1A  or the left side storage area as in  FIG. 1B ), Y-MUX  18  drives global bit line GBL(N) to a high voltage (HV), about 4-6V, global bit line GBL(N+1) to GND (0V) or another close to GND potential, and all other global bit lines typically remain floating. Select cell  20 A passes the high voltage of global bit line GBL(N) to local bit line BL 12  and thus, to the diffusion F acting as a source of cell  12 C. Select cell  20 B passes the ground voltage of global bit line GBL(N+1) to local bit line BL 13  and thus, to the diffusion F acting as the drain of cell  12 C. The dotted line indicates the flow of current from Y-MUX  18 , through cell  12 C and back to Y-MUX  18 . The programming pulse ends by driving global bit line GBL(N) low. Reference is now made to  FIG. 2 , which illustrates the activation timing for programming a multiplicity of cells. This programming method is discussed in US Publication 2003-0145176-A1, which application is co-owned by the common assignee of the present invention. Initially (graph (a)), the selected word line WL(i) is driven to a high voltage (e.g. 9V). At about the same time, or shortly thereafter, the relevant select lines SEL-x are also activated (graph (b)) to define the conducting path to the cell to be accessed. In graph (c), a GND or close to GND potential is driven to the drain and source terminals of the NROM cell to be accessed. Finally, as shown in graph (d), a high voltage pulse is applied to the global bit line (GBL) to be connected to the cell&#39;s drain terminal, to provide a programming pulse. The programming pulse (PGM&#39;ing Pulse) is followed by a program verify operation (PGM Verify) (which requires different voltage levels at the NROM cell terminals). 
         [0009]    When programming a large number of cells (e.g. in a 512 B page write operation), programming pulses may be applied to all the cells on the selected word line WL(i) to be programmed before applying the program verify test. This is shown in graphs (b) and (d) by the multiple transitions. Reference is now made to  FIG. 3 , which schematically illustrates the flow of direct current I DC  during a programming pulse. DC current I DC  flows from a power supply  30 , through a high voltage supply system  32  (which may include charge pumps, regulators, or boosters), along a first, long global bit line GBL( 1 ), through cell  12  to be programmed, along a second long global bit line GBL( 2 ), until it reaches a ground supply  34 . Along this current path there are usually additional conducting transistors (not shown in  FIG. 3 ) which serve as switches. The high voltage supply system  32  generates programming voltage levels (HV) which are significantly higher than the voltage level of power supply  30  (Vcc). The power efficiency (η) of the high voltage supply system is typically low (less than 50%) and therefore the current consumed from the power supply  30  (I CC,DC ) is significantly larger than I DC . Specifically, 
         [0000]      I CC,DC =HV*I DC /Vcc/η 
         [0000]    The DC current I DC  may be relatively large during programming, especially if many cells need to be programmed in parallel in order to achieve high programming rates. In addition, in the programming sequence shown in  FIG. 2 , each programming pulse charges and discharges the associated GBL. In high density memory devices, the parasitic capacitances C BL  associated with the global bit lines may be relatively large. The larger the capacitance C BL  is, the larger the dynamic current consumed from the high voltage supply system  32 . In addition, in the sequence shown in  FIG. 2 , the capacitances C BL  are charged and discharged many times during a programming operation. The dynamic current consumed from the high voltage supply system  32  is high, and the current consumed from the power supply  30  is even higher (due to the low efficiency of the high voltage supply system  32 ). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which: 
           [0011]      FIGS. 1A and 1B  are schematic illustrations of two exemplary, prior art memory chips having memory arrays; 
           [0012]      FIG. 2  is a timing diagram illustration of the activation timing for programming a multiplicity of cells in the arrays of  FIGS. 1A and 1B ; 
           [0013]      FIG. 3  is a schematic illustration of the flow of direct current I DC  during a programming pulse in the arrays of  FIGS. 1A and 1B ; 
           [0014]      FIGS. 4A and 4B  are schematic illustrations of the flow of current in one embodiment of the present invention; 
           [0015]      FIG. 5  is a graphical illustration of the voltages over time on three local bit lines, useful in understanding the operation of the embodiment of  FIGS. 4A and 4B ; 
           [0016]      FIG. 6  is a graphical illustration of the changes in a threshold voltage V t  over time in the presence of two different operating conditions, useful in understanding the operation of the embodiment of  FIGS. 4A and 4B ; 
           [0017]      FIG. 7A  is a timing diagram illustration of a second embodiment of the present invention; 
           [0018]      FIG. 7B  is a schematic illustration of the array of  FIG. 1A  when using the timing diagram of  FIG. 7A ; 
           [0019]      FIG. 8A  is a timing diagram illustration of a third embodiment of the present invention; and 
           [0020]      FIG. 8B  is a schematic illustration of the array of  FIG. 1A  when using the timing diagram of  FIG. 8A . 
       
    
    
       [0021]    It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention. 
         [0023]    The present invention may be novel programming methods which may reduce the amount of power required for programming NROM cells. One method may minimize the DC current components which flow from power supply  30  to GND  34 . Another method may utilize transient currents to perform the programming operation. A third method may avoid or minimize the switching of parasitic capacitances C BL . 
         [0024]    Reference is now made to  FIGS. 4A and 4B , which schematically illustrate the flow of current in one embodiment of the present invention. In this embodiment, when the programming pulse may be applied, ground  34  may be disconnected from the global bit line, such as global bit line GBL( 2 ), which may be connectable to the diffusion, labeled S, acting as the source terminal of cell  12  to be accessed. As a result, DC current I DC  may not flow between pump  32  and ground GND  34 . 
         [0025]    In  FIG. 4A , ground  34  may be disconnected by inactivating the select transistor  20  connecting the local bit line, here labeled BL 3 , to global bit line GBL( 2 ). Select transistor  20  is therefore shown with dashed lines. In  FIG. 4B , ground  34  may be disconnected by disconnecting a switch  40  connecting ground supply  34  to global bit line GBL( 2 ) 
         [0026]    Despite the disconnection, cell  12  may still be successfully programmed. The global bit line, such as global bit line GBL( 1 ), connectable to the diffusion, labeled D, acting as the drain terminal, may be driven to the high programming voltage (such as 4-6V). The remaining global bit lines (not shown) in the array may remain floating. 
         [0027]    Since word line WL(i) may have been driven high, as GBL( 1 ) may be driven high, current may start flowing through to drain terminal D of cell  12 . The bit on the drain side of the cell may start to program and current may begin to flow through cell  12 . However, since global bit line GBL( 2 ) may not be grounded, the voltage on the source side local bit line (marked BL 3 ) may start to rise. This is shown in  FIG. 5 , to which reference is now briefly made. 
         [0028]      FIG. 5  graphs the voltages over time on three local bit lines, the drain side local bit line BL 1 , the source side local bit line BL 2  for the neighboring cell, labeled  13 , and the source side local bit line BL 3  for cell  12 . As can be seen, the voltage V 1  on the drain side local bit BL 1  rises quickly and sharply. The voltage V 2  on the source side local bit line BL 2  for neighboring cell  13  rises quickly but not as sharply since bit line BL 2  was not initially grounded (it was floating). The voltage V 3  on source side local bit line BL 3  for cell  12  rises the slowest as it starts from a ground voltage. The result is that the drain to source voltage V ds  for cell  12  may be sufficiently large to program. It should not be sufficiently large to program cell  13 . However, as voltage V 3  may rise, the programming rate of cell  12  may decrease (due to the lowering V ds ) and its body effect may increase. The result is a reduced cell current. As long as the drain to source voltage V ds  is sufficiently high for long enough, as illustrated in  FIG. 6  to which reference is now briefly made, the bit may be programmed. 
         [0029]      FIG. 6  graphs a program pulse (graph (a)) and the changes in a threshold voltage V t  (which defines the programmed state of a bit) over time in the presence of two different operating conditions (graph (b)). The program pulse may last until a time T. 
         [0030]    Curve  50  graphs the changes for the following operating conditions (gate voltage V g =9V, drain voltage V d =5V, source voltage=0V) and curve  52  graphs the changes for the following operating conditions (gate and drain voltages as in curve  50 , drain voltage V d =4V). 
         [0031]    As can be seen, the threshold voltage V t  initially increases at a fast rate, during which the programming (direct) current I DC  may be relatively efficiently used, and then (at about time t 1  in  FIG. 6 ), it saturates with a very small rate of increase afterwards, during which the programming (direct) current I DC  may not be efficiently used. In the embodiment of the present invention, as long as V ds  remains large enough for a sufficient amount of time such that the threshold voltage V t  reaches the saturation region, then the bit will be programmed with a minimal amount of current applied. 
         [0032]    It will be appreciated that the time t 1  may vary due to the operating conditions (e.g. temperature) and the programming state of the cell (i.e. how many times the two bits of the cell have been programmed). 
         [0033]    It will further be appreciated that, in order to prevent the source voltage V s  from rising too fast (resulting in a non-effective programming pulse), the source side BL capacitance should not be too small. One way to ensure that is to leave the now floating global bit line GBL( 2 ) connected to the source side bit line BL 3 , as shown in  FIG. 4B . 
         [0034]    The programming pulse may be ended by discharging global bit lines GBL( 1 ) and GBL( 2 ) to GND, or by discharging the WL, or by de-selecting the accessed cell. 
         [0035]    Reference is now made to  FIGS. 7A and 7B , which together illustrate a second preferred embodiment of the present invention. In this embodiment, the current consumption may be minimized by reducing the number of charge and discharge events of a global bit line GBL driven to high programming voltage. Once a global bit line GBL may be driven to high voltage, insertion of this high voltage into a local bit line BL j  may be controlled by select transistors  20 .  FIG. 7A  is a timing diagram for this embodiment and  FIG. 7B  is the array of  FIG. 1A  operated in accordance with this embodiment. It will be appreciated that the present invention includes operating the array of  FIG. 1B  and other similar arrays in accordance with this embodiment. Initially, the selected word line WL (graph (a)) is driven high, after which, in accordance with a preferred embodiment of the present invention, the selected global bit line (graph (b)), such as GBL(N) of  FIG. 7B , may be driven high. The global bit line connectable to the source side bit line, such as global bit line GBL(N+1), may be left floating or may be driven to GND. 
         [0036]    Once both the word line WL(i) and the global bit line GBL(N) connectable to the drain bit line have been driven to their programming levels, the relevant select transistors which insert the power to the drain side bit lines may be activated (graph (c)). Thus, in  FIG. 7B , select transistor SEL 5  may be turned ON for the required programming pulse duration, thereby providing power to bit line BL 13  and programming the drain side bit of the circled cell until select transistor SEL 5  may be turned OFF. 
         [0037]    Global bit line GBL(N) may then remain at high voltage but may not drive any more current to a cell. The drain side bit line BL 13  will quickly drop to a voltage low enough to end the programming operation. Global bit line GBL(N+1), which may be connectable to the source side bit line BL 12 , may be driven to GND, causing the local bit lines adjacent to the circled cell to discharge to GND after select transistor SEL 5  may be turned OFF. 
         [0038]    In this embodiment, the next bits to be programmed may be the ones in the same row as the circled cell (i.e. having the same word line WL(i)) that uses global bit line GBL(N) as its high voltage global bit line. For example, the square cell of  FIG. 7B . 
         [0039]    To access the square cell and program its left side bit, select transistor SEL 8  may be turned ON to connect the source side, local bit line BL 16  to GBL(N+1), and select transistor SEL 7  may be turned ON to supply the high voltage to the drain bit of the square cell and to start its programming pulse. The programming pulse may substantially end when select transistor SEL 7  is turned OFF. Again, discharge of the local bit lines may be performed at the end of the programming pulse. 
         [0040]    This procedure may continue until all the bits that require programming and use the already charged global bit line GBL(N) as their high voltage global bit line have received a programming pulse. 
         [0041]    When more than a single bit is accessed in parallel, such as in the case of a full word access, they typically share the same WL and are located far away from each other. In this case, the programming may be defined by the data of the word and only some of the bits may need to be applied with a programming pulse. For the bits not to be programmed, the programming pulse may be prevented by either discharging the global bit line GBL(N) connected to the not-to-be-programmed bit or by using different-signals to control the SELECT transistors and prevent the programming voltage HV from being inserted into the drain local bit line BL of the cell that does not need to be programmed. 
         [0042]    Reference is now made to  FIGS. 8A and 8B , which together illustrate a third preferred embodiment of the present invention.  FIG. 8A  is a timing diagram for this embodiment and  FIG. 8B  is the array of  FIG. 1A  operated in accordance with this embodiment. It will be appreciated that the present invention includes operating the array of  FIG. 1B  and other similar arrays in accordance with this embodiment. 
         [0043]    In this embodiment, a column or portion of a column of bits are programmed one after the other, by selectively activating word lines WL and without charging or discharging the local bit line BL and the global bit line GBL connected to it. 
         [0044]    As an example, in order to program the cells marked with a circle and a square in  FIG. 8B , GBL(N) may first be driven high (graph (a) of  FIG. 8A ), after which it may be connected (graph (b)), through select transistors SEL- 5  to the drain side local bit line BL 13  of those cells. The source side local bit line BL 12  connected to GBL(N+1) may be driven to GND or left floating. With the programming voltage at the cell, word line WL(i) may be driven to its programming level. The circled cell may start programming. The square cell will not program since its word line WL(i+1) is low. The programming pulse may be ended by driving word line WL(i) low (typically to GND or to a close to GND level). 
         [0045]    The next programming pulse may be provided by driving word line WL(i+1) to its programming level. This may cause the square cell to start programming. Its programming may be ended by driving word line WL(i+1) low. Once all of the bits in the column have finished programming, select transistor SEL- 5  may be deactivated and global bit line GBL(N) may be driven low. 
         [0046]    When more than a single bit is accessed in parallel, such as in the case of a full word access, the programming may be defined by the data of the word and only some of the bits may need to be applied with a programming pulse. For the bits not to be programmed the programming pulse may be prevented by either discharging the global bit line GBL connected to the not-to-be-programmed bit, by using different signals to control the SELECT transistors and prevent the programming voltage HV from being inserted to the drain local bit line BL of the cell that does not need to be programmed, by segmenting the word line WL and using different control signals for the different word line WL segments. 
         [0047]    It will be appreciated that, in the embodiments hereinabove, the source side local bit line may be left floating after being discharged, and programming may be “unforced” as in the first embodiment. That is, there may be initial conditions setting up programming but there may be little or nothing to maintain the programming conditions. Programming may stop being efficient when the cell no longer has effective programming conditions therein. The select transistor may disconnect the charged GBL afterwards. 
         [0048]    While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.