Abstract:
A method of making a non-volatile memory (NVM) cell structure comprises the formation of a first NVM cell, a second NVM cell and an SRAM cell that includes first and second data nodes. A first pass gate structure is connected between the first NVM cell and the first data node of the SRAM cell, the first pass gate structure being responsive to first and second states of a first pass gate signal to respectively couple and decouple the first NVM cell and the SRAM cell. A first equalize structure is formed to connect the first pass gate structure and the first NVM cell and is responsive to a first equalize signal to connect the first NVM cell to ground. A second pass gate structure is connected between the second NVM cell and the second data node of the SRAM cell, the second pass gate structure being responsive to first and second states of a second pass gate signal to respectively couple and decouple the second NVM cell and the SRAM cell. A second equalize structure is connected between the second pass gate structure and the second NVM cell, the second equalize structure being responsive to a second equalize signal to connect the second NVM cell to ground. Appropriate biasing conditions are applied to the NVM cell structure to implement program/operations.

Description:
PRIORITY CLAIM 
       [0001]    This divisional patent application claims priority from U.S. patent application Ser. No. 11/656,609, filed Jan. 23, 2007, by Poplevine et al. and titled “Non-Volatile Memory Cell with Improved Programming Technique.” application Ser. No. 11/656,609, which is the subject of a Notice of Allowance issued by the U.S. Patent Office on Jul. 11, 2008, is hereby incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to integrated circuit memory devices and, in particular, to a method of making a non-volatile memory (NVM) 4-transistor single cell structure that includes a shared static random access memory (SRAM) cell and to program biasing techniques for the NVM cell. 
       BACKGROUND OF THE INVENTION 
       [0003]    Co-pending and commonly-assigned U.S. patent application Ser. No. 11/183,198, file on Jul. 15, 2005, by Poplevine et al. titled “Non-volatile Memory Cell with Improved Programming Technique” and which is the subject of a Notice of Allowance issued by the U.S. Patent Office on Nov. 2, 2006, discloses a 4-transistor PMOS non-volatile memory (NVM) cell that includes an embedded static random access memory (SRAM) cell. The NVM cell utilizes a reverse Fowler-Nordheim tunneling programming technique with a very low programming current that allows an entire NVM cell array to be programmed at a single cycle. application Ser. No. 11/183,198 is incorporated herein by reference to provide background information regarding the present invention. 
         [0004]    Co-pending and commonly-assigned U.S. patent application Ser. No. 11/182,115, filed on Jul. 15, 2005, by Poplevine et al., titled “Reverse Fowler-Nordheim Tunneling Programming for Non-volatile Memory Cell,” discloses a low current programming method for a non-volatile memory (NVM) cell utilizing reverse Fowler-Nordheim tunneling. Application Ser. No. 11/182,115 is incorporated herein by reference to provide background information regarding the present invention. 
         [0005]    Co-pending and commonly-assigned U.S. patent application Ser. No. 11/235,834, filed on Sep. 26, 2005, by Poplevine et al., titled “Method of Hot Electron Injection Programming of a Non-volatile Memory (NVM) Cell Array in a Single Cycle,” discloses a 4-transistor non-volatile memory (NVM) cell that includes a static random access memory (SRAM) cell. The cell utilizes a hot electron injection programming technique which, in combination with the SRAM cell and a sequence of cascaded pass gates, allows an entire NVM cell array, or a selected row or sector of the array, to be programmed at a single cycle. Application Ser. No. 11/235,834 is incorporated herein by reference to provide background information regarding the present invention. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provide a method of making a non-volatile memory (NVM) cell structure, The method comprises the formation of a first NVM cell, a second NVM cell and an SRASM cell that includes first and second data nodes. A first pass gate structure is connected between the first NVM cell and the first data node of the SRAM cell, the first pass gate structure being responsive to first and second states of a first pass gate signal to respectively couple and decouple the first NVM cell and the SRAM cell. A first equalize structure is formed to connect the first pass gate structure and the first NVM cell and is responsive to a first equalize signal to connect the first NVM cell to ground. A second pass gate structure is connected between the second NVM cell and the second data node of the SRAM cell, the second pass gate structure being responsive to first and second states of a second pass gate signal to respectively couple and decouple the second NVM cell and the SRAM cell. A second equalize structure is connected between the second pass gate structure and the second NVM cell, the second equalize structure being responsive to a second equalize signal to connect the second NVM cell to ground. Appropriate biasing conditions are applied to the NVM cell structure to implement program operations. 
         [0007]    The features and advantages of the present invention will be more fully understood and appreciated upon review and consideration of the following detailed description of the invention and the accompanying drawings that set forth illustrative embodiments of the invention. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic drawing illustrating a known dual non-volatile memory (NVM) cell with a 6 transistor static random access memory (SRAM) cell. 
           [0009]      FIG. 2A  is a schematic drawing illustrating a dual NVM cell with a shared SRAM, DC-NPG and CM-NMOS in accordance with the present invention. 
           [0010]      FIG. 2B  is a schematic drawing illustrating a dual NVM cell with a shared SRAM cell and full pass-gate in accordance with the present invention. 
           [0011]      FIG. 3  is a schematic drawing illustrating an array implementation of the NVM cell structure of  FIG. 2A  or  FIG. 2B  in accordance with the present invention. 
           [0012]      FIG. 4  is a schematic drawing illustrating a dual NVM cell with a 6 transistor SRAM cell. 
           [0013]      FIG. 5  is a schematic drawing illustrating a single NVM cell with a shared SRAM cell, DC-NPG and CM-NMOS in accordance with the present invention. 
           [0014]      FIG. 6  is a schematic drawing illustrating an array implementation of the NVM cell structure of  FIG. 5  in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 1  shows a schematic of a known dual 4T NVM cell  100  with an embedded SRAM cell  102 . In this dual NVM cell approach, one NVM cell  100  stores the data and the other NVM cell  100  serves as a reference. This approach increases the cell area by approximately 30% with the additional reference cell. To decrease the cell area, or in other words, to increase the capacity of the memory bit, the present invention provides a single NVM cell approach with a shared SRAM cell. 
         [0016]    As shown in  FIG. 2A , a NVM cell structure in accordance with the present invention is based upon the  FIG. 1  dual 4T NVM cell with a 6T static random access memory (SRAM) cell embedded. In addition, the cell includes two decoupling N-channel pass gates DC-NPG 1 , DC-NPG 2  and two equalize N-channel devices CM-NMOS 1 , CM-NMOS 2 . The dual NVM cells, i.e. cell  200   a  and cell  200   b , are treated as two separate memory bits, sharing a single SRAM cell  202 . Both NVM cells  200  have their own program control lines WLP 1 , WEQ 1 , WEQS 1  and WLP 2 , WEQ 2 , WEQS 2 , as shown in  FIG. 2A . 
         [0017]    With the inclusion of the pass gates and the equalize devices, both NVM cells  200  can be programmed independently. Since the control line for a program operation is globally connected, to program NVM cell  200   a , NVM cell  200   b  must be inhibited from the cell  200   a  program and vice versa. This is done, in the case of cell  200   a  program, for example, by setting the cell  200   b  signals as follows: WLP 2  to ground, WEQ 2  to Vdd and WEQS 2  to Vdd. Turning on the equalize device CM-NMOS 2  associated with cell  200   b  passes voltage at WEQ 2  to program device Vp 2  of cell  200   b . Since it is an NMOS, program device Vp 2  will be at Vdd-Vthn. This will inhibit cell  200   b  from being programmed. To program cell  200   a , signal WLP 1  and signal WEQS 1  are set to Vdd and signal WEQ 1  is set to ground. The pass gate DC-NPG 1  associated with cell  200   a  will pass the data from the SRAM cell  202  to Vp 1 . Vp 1  will be at ground level if node RT in the SRAM cell  202  is at ground. Vp 1  will be a Vdd-Vthn if node RT is at Vdd level. Ground level at Vp 1  or data zero at node RT of the SRAM will cause cell to be programmed. Vdd-Vthn level at Vp 1  or Vdd at RT of SRAM  202  will inhibit cell  200   a  from being programmed. 
         [0018]    Similarly, to program cell  200   b , cell  200   a  is inhibited from being programmed. This is done by setting cell  200   a  as follows: WLP 1  to ground, WEQ 1  and WEQS 1  to Vdd. The equalize device CM-CMOS 1  associated with cell  200   a  will pass voltage at WEQS 1  to Vp 1 . Since it is an NMOS device, Vp 1  will be at Vdd-Vthn. This will inhibit cell  200   a  from being programmed. To program cell  200   b , WLP 2  and WEQS 2  are brought to Vdd and WEQ 2  is brought to ground. Pass gate DC-NPG 2  will pass the data from the SRAM  202  to Vp 2 . Vp 2  will be at ground level if SRAM node RB is at ground. Vp 2  will be at Vdd-Vthn if node RB is at Vdd level. Ground level at Vp 2  or data zero at node RB of SRAM  202  will cause cell  200   b  to be programmed. Vdd-Vthn level at Vp 2  or Vdd at node RB of SRAM will inhibit cell  200   b  from being programmed. To further enhance the cell by passing full VDD (on Vp 1 /Vp 2 ) to better inhibit the cell from program disturb, an alternate embodiment cell structure is proposed as shown in  FIG. 2B . The operation of the  FIG. 2B  cell structure is identical to the operation of the  FIG. 2A  cell. 
         [0019]    To further enhance the cell by passing full VDD (on Vp 1 /Vp 2 ) to better inhibit the cell from program disturb, an alternate embodiment cell structure is proposed as shown in  FIG. 2B . The operation of the  FIG. 2B  cell structure is identical to the operation of the  FIG. 2A  cell. 
         [0020]    With the inclusion of the full CMOS pass gates and the full CMOS equalize devices, both NVM cells  200  can be programmed independently. Since the control line for a program operation is globally connected, to program NVM cell  200   a , NVM cell  200   b  must be inhibited from the cell  200   a  program and vice versa. This is done, in the case of cell  200   a  program, for example, by setting the cell  200   b  signals as follows: WLP 2  to ground, WEQ 2  to Vdd and WEQS 2  to Vdd. Turning on the equalize device CM-fullCMOS 2  associated with cell  200   b  passes voltage at WEQ 2  to program device Vp 2  of cell  200   b . Since these are a PMOS and NMOS (full CMOS pass gate) program device Vp 2  will be at full Vdd level. This will inhibit cell  200   b  from being programmed. To program cell  200   a , signal WLP 1  and signal WEQS 1  are set to Vdd and signal WEQ 1  is set to ground. The full pass gate DC-fullCMOS 1  associated with cell  200   a  will pass the data from the SRAM cell  202  to Vp 1 . Vp 1  will be at ground level if node RT in the SRAM cell  202  is at ground. Vp 1  will be a full Vdd if node RT is at Vdd level. Ground level at Vp 1  or data zero at node RT of the SRAM will cause cell to be programmed. Vdd level at Vp 1  or Vdd at RT of SRAM  202  will inhibit cell  200   a  from being programmed. 
         [0021]    Similarly, to program cell  200   b , cell  200   a  is inhibited from being programmed. This is done by setting cell  200   a  as follows: WLP 1  to ground, WEQ 1  and WEQS 1  to Vdd. The equalize device CM-fullCMOS 1  associated with cell  200   a  will pass voltage at WEQS 1  to Vp 1 . Since it is an NMOS device, Vp 1  will be at Vdd-Vthn. This will inhibit cell  200   a  from being programmed. To program cell  200   b , WLP 2  and WEQS 2  are brought to Vdd and WEQ 2  is brought to ground. The pass gate DC-fullCMOS 2  will pass the data from the SRAM  202  to Vp 2 . Vp 2  will be at ground level if SRAM node RB is at ground. Vp 2  will be at full Vdd level if node RB is at Vdd level. Ground level at Vp 2  or data zero at node RB of SRAM  202  will cause cell  200   b  to be programmed. Vdd level at Vp 2  or Vdd at node RB of SRAM  200   b  will inhibit cell  200   b  from being programmed. 
         [0022]    In the case of the  FIG. 2A  cell architecture, the two NVM cells  200   a ,  220   b  can be read either simultaneously by having two sense amplifiers or sequentially through a multiplexer. During a read operation, signals WEQ 1  and WEQ 2  are at Vdd. WLP 1 , WLP 2 , WEQS 1  and WEQS 2  are at ground. The SRAM  202  is decoupled from both NVM cells. Signals WEQ 1 , WEQS 1  and WEQ 2 , WEQS 2  set up a common ground at Wp 1  and Wp 2 , respectively. The data is sensed by comparing the current/voltage with a global reference current/voltage. The reference current/voltage is usually provided by a single or a group of the 4T cell replica. 
         [0023]    During program mode, Vr and Ve are connected to Vdd to prevent the read (Pr 1 , Pr 2 ) and erase (Pe 1 , Pe 2 ) transistors from being programmed. Vdd can be any voltage that is high enough to provide shielding against program but low enough not to cause any disturb. To program a cell, a data zero/ground has to be passed into the Vp 1 /Vp 2  from the SRAM  202 . To shield the 4T cell from program disturb, a data one is passed to Vp 1 /Vp 2  via the pass gate DC-NPG. Since the pass gate DC-NPG is a NMOS, the voltage level appears on Vp 1 /Vp 2  in Vdd-Vthn. The transmission is done by setting (WLP 1 , WLP 2 ) to logic high and (WEQ 1 , WEQ 2 ) to logic low. (WEQS 1 , WEQS 2 ) are always high during program mode. 
         [0024]    During Program 
         [0025]    (A) The control gate voltage Vc which is globally connected is swept from 0V to V cmax  in program time Tprog and stay at V cmax  for another Tprog period. Tprog is around 50 ms-100 ms, which will affect the amount of charge tunneling to the floating gates. V cmax  depends on the tunneling threshold and the amount of negative charge to put on the floating gate. This voltage varies from technology to technology. Voltage applied to Vc is coupled through control transistor (Pc 1 , Pc 2 ) to the floating gate (FG 1 , FG 2 ), increasing floating gate voltage. When the voltage at the floating gate reaches the tunneling threshold, electrons tunnel from the drain/source of the program (whichever Pp 1  or Pp 2  that is at ground) transistor to the floating gate, making the gate more negative. At the end of the program cycle, Vc is ramped down to 0v. The floating gates will be left at a net negative charge from the Reverse FN-tunneling program. 
         [0026]    During erase mode, RWL, (WEQ 1 , WEQ 2 ) are at logic high while (WLP 1 , WLP 2 , WEQS 1 , WEQS 2 ) are at logic low. The rest of signals are grounded. The erase voltage Ve is applied (˜10V for 70 Å, ˜16V for 120 Å). Erase will affect all cells. Ve varies from technology to technology. 
         [0027]    Except for program operation, signals WLP 1 , WLP 2 , WEQS 1  and WEQS 2  are always low and signals WEQ 1  and WEQ 2  are always high for erase and read operation. The shared SRAM  202  is always decoupled from NVM cells  200   a ,  200   b  except during program mode. 
         [0028]      FIG. 3  shows the implementation of an array using either the cell architecture shown in  FIG. 2A  or in  FIG. 2B . The array  300  has N rows and M columns. The SRAM word line (WL) selects the row to be written or read from the SRAM cell  202 . The NVM read word line (RWL) selects the row to be read from the dual 4T NVM cell. The erase voltage (Ve), the control voltage (Vc) and the read voltage (Vr) are applied to all cells directly without any high voltage switches or other supporting circuitry, thereby significantly simplifying the connection from external or internal power sources. As discussed above, WLP 1 , WLP 2 , WEQS 1 , WEQS 2  and WEQ 1 , WEQ 2  are used to decouple the SRAM  202  from NVM cells during read and to couple the SRAM to the selected NVM cell during program. 
         [0029]    The following is a description of the erase, program and read modes pertaining to the array  300  in  FIG. 3 . 
         [0030]    Program Mode 
         [0031]    The program mode consists of five steps.
           1) Erase mode   2) Write data into SRAM which will be programmed into cell 1     3) Enable program cycle   4) Write data into SRAM which will be programmed into cell 2     5) Enable program cycle           
 
         [0037]    1) Erase Mode: 
         [0038]    First the whole array  300  is erased in a single cycle. In the erase mode, RWL(N−1), WEQ are logic high, the erase voltage Ve is applied (˜10V for 70 Å gate oxide, ˜16V for 120 Å gate oxide), and the rest of signals, including WLP, are grounded. Erase affects all cells in the array  300 . Ve varies from technology to technology. 
         [0039]    2) SRAM Write Mode: 
         [0040]    In the SRAM write mode, signals RWL( 0 ) . . . RWL(N−1) are logic high. One of the SRAM word line (WL), e.g., WL( 0 ), should be logic high; the rest of the word lines WL, WL( 1 ) . . . WL(N−1) should be logic low. Signals WLP 1 , WLP 2 , WEQS 1 , WEQS 2  are set to logic low and signals WEQ 1 , WEQ 2  are set to logic high to decouple the shared SRAM cell  202  and to set up common ground at Vp 1  and Vp 2 . In order to program cell  200   a  later, a zero must be written to node RT of the SRAM  202 . The corresponding write bit line, e.g., BT( 0 ), should be logic low and bit line BB( 0 ) at logic high. To write a one (cell  200   a  remains erased) to the SRAM cell  202 , the corresponding write bit line, e.g., BB( 0 ), should be logic low and bit line BT( 0 ) at logic high. The number of write cycles depends upon the number of row (N) and the number of columns (M) in the array  300 . 
         [0041]    3) Dual 4T Cell Program Mode: 
         [0042]    Prior to program mode, the SRAM array is preloaded with data as described in (2) and the NVM array is preconditioned in the erase cycle. Signals WLP 1 , WEQS 1  are set to logic high and signal WEQ 1  is at logic low. WLP 2  is set to logic low and (WEQ 2 , WEQS 2 ) are set to logic high to inhibit cell  200   b  from being programmed. RWL( 0 ) . . . RWL(N−1) are logic high. WL( 0 ) . . . WL(N−1) are logic low. The written SRAM cell provides the logic to program cell  200   a . Vc is swept from 0v to V cmax . V cmax  should be larger than the tunneling condition and depends on technology. The Vc timing sequence is the same as described in (A). Only one cycle is needed. 
         [0043]    4) SRAM Write Mode: 
         [0044]    RWL( 0 ) . . . RWL(N−1) are logic high. One of the SRAM word lines (WL), e.g., WL( 0 ), should be logic high; the rest of word lines WL, WL( 1 ) . . . WL(N−1) should be logic low. Signals WLP, WLP 2 , WEQS 1 , WEQS 2  are set to logic low and signals WEQ 1 , WEQ 2  are set to logic high to decouple the SRAM  202  and to set up common ground at Vp 1  and Vp 2 . In order to program cell  200   b  later, a zero needs to be written to node RB of SRAM  202 . The corresponding write bit line, e.g., BB( 0 ), should be logic low and bit line BT( 0 ) at logic high. To write a one (cell  200   b  remains erased) to the SRAM cell  202 , the corresponding write bit line, e.g., BT( 0 ), should be logic low and bit line BB( 0 ) at logic high. The number of write cycles depend upon the number of rows (N) and the number of columns (M) in the array  300 . 
         [0045]    5) Dual 4T Cell Program Mode: 
         [0046]    Prior to program mode, the SRAM array is preloaded with data as descried in (4). (WLP 2 , WEQS 2 ) is set to logic high and WEQ 2  is at logic low. WLP 1  is set to logic low and (WEQ 1 , WEQS 1 ) is set to logic high to inhibit cell  200   a  from program disturb and second program sequence. RWL( 0 ) . . . RWL(N−1) are logic high. WL( 0 ) . . . WL(N−1) are logic low. The written SRAM cell provides the logic to program cell  200   b . Vc is swept from 0v to V cmax . V cmax  should be larger than the tunneling condition and depends on technology. The Vc timing sequence is the same as described in (A). Only one cycle is needed. 
         [0047]    NVM Read Mode 
         [0048]    One of the NVM read word lines (RWL), e.g., RWL( 0 ), should be at logic low; the rest of RWL, RWL( 1 ). . . RWL(N−1) should be logic high. Set (WLP 1 , WLP 2 , WEQS 1 , WEQS 2 ) to logic low and (WEQ 1 , WEQ 2 ) to logic high to decouple the SRAM  202  from the NVM cell and to set up a common ground at Vp 1  and Vp 2 . On all read bit lines (RBL( 0 ) . . . RBL 1 (M−1) and RBL 2 ( 0 ) . . . RBL 2 (M−1), high current or voltage will be seen if the cell was programmed; a low current or voltage will be seen if the cell was erased. The read voltage Vr is applied to all the cells. The RBL 1 ( 0 ) . . . RBL(M−1) and RBL 2 ( 0 ) . . . RBL 2 (M−1) will be sensed using two sense amplifier by comparing the current/voltage with a global reference current/voltage. The reference current/voltage is usually provided by a single or a group of the 4T cell replica. These sensed data are latched to output as QM( 0 ) . . . QM(M). 
         [0049]    SRAM Read 
         [0050]    SRAM read mode: RWL( 0 ) . . . RWL(N−1) are logic high. One of the SRAM word lines (WL), e.g., WL( 0 ), should be logic high; the rest of WL, WL( 1 ) . . . (L(N−1) should be logic low. Signals WLP 1 , WLP 2 , WEQS 1 , WEQS 2  are set to logic low and signals WEQ 1 , WEQ 2  are set to logic high. If the cell was written zero, bit line BT will be discharged to ground. If the cell was written one, bit line BB will be discharged to ground. These bit lines are sensed using the SRAM differential sense amplifiers and latched to the output as QN( 0 ) . . . QN(M). 
         [0051]    The advantage of the proposed enhancement is to increase the capacity of the memory. The bit cell is decreased by approximately 30% by sharing the SRAM between two NVM bit cell. 
         [0052]    The same proposed method can be applied to hot carrier programming method. An embodiment of a cell for this application is based upon a known dual 4 transistor NVM cell with shared embedded SRAM, shown in  FIG. 4 . 
         [0053]    The new cell, shown in  FIG. 5 , includes two 4T NVM cells  500   a ,  500   b  with a 6T SRAM cell  502  embedded. The dual NVM cells  500   a ,  500   b  are treated as two separate memory bits, sharing a single SRAM cell  502 . Both NVM cells  500   a ,  500   b  have their own program control lines, PWL 1  and PWL 2 , respectively, as shown in  FIG. 5 . The corresponding array implementation is shown in  FIG. 6 . 
         [0054]    Utilizing the  FIG. 5  cell architecture, both NVM cells  500   a ,  500   b  can now be programmed independently. Since the control line for program is globally connected, to program cell  500   a , cell  500   b  has to be inhibited from cell  500   a  program and vice versa. This is done by setting PWL 2  to ground. To program cell  500   a , set PWL 1  to Vdd. A Vdd at node Va will program cell  500   a . When a high voltage, e.g. 5V, is applied to Vp and PWL 1  is Vdd, the high lateral electric field between Vp and ground ( FIG. 2A ) creates hot electrons. Voltage applied to Vc is coupled through the control transistor to the floating gate (FG 1 ). The gate provides the high perpendicular electric field that attracts electrons to reach the FG. A ground at node Va will inhibit Cell  500   a  from program. 
         [0055]    To program cell  500   b , cell  500   a  is inhibited from being programmed. This is done by setting PWL 1  to ground. This will inhibit cell  500   a  from being programmed. To program cell  500   a , set PWL 1  to Vdd. A Vdd at node Va will program Cell  500   a . When a high voltage, e.g. 5V, is applied to Vp and PWL 2  is Vdd, the high lateral electric field between Vp and ground ( FIG. 5 ) creates hot electrons. Voltage applied to Vc is coupled through control transistor to the floating gate (FG 2 ). The gate provides the high perpendicular electric field that attracts electrons to reach the FG. A ground at node Va will inhibit cell  500   b  from program. 
         [0056]    Although only specific embodiments of the present invention are shown and described herein, the invention is not to be limited by these embodiments. Rather, the scope of the invention is to be determined by these descriptions taken together with the attached claims and their equivalents. 
         [0057]    What is claimed is: