Abstract:
A method and circuit for sensing multi states of a NAND memory cell by varying source bias, at a constant gate voltage, preferably zero volts, generating a memory cell current in response to the source bias, and sensing the memory cell state.

Description:
FIELD OF THE INVENTION 
     The present invention relates to NAND memory cells used in non-volatile flash memory architecture. More specifically, it relates to sensing multi-levels in a NAND memory cell by applying a source bias voltage. 
     BACKGROUND OF THE INVENTION 
     A conventional NAND EEPROM (Electrically erasable programmable read only memory) array block is formed by a series of floating gate transistors coupled in series between a select drain transistor and a select source transistor. The select drain transistor is coupled to a data transfer line called bit line (BL) and the select source transistor is coupled to a source line. Each floating gate transistor is a memory cell having a floating gate which is programmed and erased using techniques well known to one skilled in the art. The memory cell transistors are floating gate MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). 
     Conventionally, prior to programming the floating gate of a memory cell is biased to a negative voltage relative to the substrate by storing electrons into the floating gate. A floating gate of a memory cell is then programmed by turning the select source and drain transistors off to isolate the series of memory cells, biasing a control gate at the programming voltage, and grounding the body region. The substrate is biased, while the control gate is grounded, thereby driving the electrons from the floating gate back into the substrate. 
     Each NAND memory cell can be programmed into one of several states which can be designated, for example as follows: 
     (0,0) denotes an erased state; 
     (0,1) denotes a partially erased state; 
     (1,0) denotes a partially programmed state; and 
     (1,1) denotes a programmed state. 
     Currently, several reference voltages (Vref 1 , Vref 2  and Vref 3 ) are applied to NAND memory cells for sensing the state of a memory cell. The reference voltage and the state of the memory cell determine a cell current in a sensing circuit. For example a memory cell is conductive when erased, and hence pulls down the sense node. If the memory cell is programmed then it is not conductive and the sense node is pulled up. The state of the memory cell can be determined by analyzing the variation in the current in the sensing circuit caused by applying a reference voltage. “A Non-Volatile semiconductor memory device for storing multivalue data and readout/write-in method” is disclosed in U.S. Pat. No. 5,751,634. (Itoh). Similar to the method described above, in Itoh, reference voltages are applied at individual memory cells during data writing and data readout time, generating memory cell current in response to the reference voltages. The variation in memory cell currents provide the state of the memory cell. The disadvantage of Itoh is that when high reference voltages are applied to a memory cell, the reference voltage may cause disturbance in memory cells adjacent to the memory cell that is sensed at a given time. 
     Another method to sense the state of a NAND memory cell is by applying an external bias current at 0v and evaluating a cell current generated in response to the external bias current. The disadvantage of such a method is that only two states (0,0) and (1,1) can be sensed. 
     Therefore, what is desired is a circuit and a method that efficiently senses the levels of a multi state NAND memory cell without causing significant disturbance to memory cells adjacent to the memory cell sensed at any given time. 
     SUMMARY OF THE INVENTION 
     The present invention is a method and circuit for sensing the state of a multi-state NAND memory cell in a NAND string connected to a bit line by defining plural NAND memory cell states, applying a source bias voltage at the source terminal, at zero gate voltage and detecting a cell current in response to the applied source bias voltage. Finally, sensing the NAND memory cell state by measuring the cell current in response to the source bias voltage. The source bias generates reverse and forward voltage. 
     The present invention has the advantage of sensing plural states of NAND memory cells at zero gate voltage and hence minimizes any disturbance due to various reference voltages applied to NAND memory cells. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional diagram of a floating type memory cell. 
     FIG. 2 is a schematic diagram of a NAND array block. 
     FIG. 3 is a cross sectional diagram of the string of FIG. 2 as disposed on a substrate. 
     FIG. 4 a  is a diagram showing four states of a NAND memory cell. 
     FIGS. 4 b  and  4   c  are the I-V characteristic of a NAND memory cell at zero gate voltage. 
     FIG. 5 shows a NAND cell sensing circuit of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a cross-sectional diagram of a floating gate memory cell  100  of a NAND array block. Memory cell  100  is a floating gate transistor having a control gate  102  coupled to a voltage line  122  for applying a voltage of V g . on control gate  102 . Control gate  102  is separated from a floating gate  106  by an upper insulating layer  104 , the floating gate  106  being separated from a substrate  110  by a lower insulating layer  108 . 
     Substrate  110  includes an n+ source region  112  coupled to a voltage line  132  for applying a voltage of V s  on n+ source region  112 , a p-doped body region  114  coupled to a voltage line  134  for inducing a voltage on p-doped body region  114 , and an n+ drain region  116  coupled to a voltage line  136  for applying a voltage of V D  on n+ drain region  116 . 
     FIG. 2 is a schematic diagram of a conventional NAND array block  200 . Block  200  includes an external bit line BL coupled to a sense circuit  220  for detecting a voltage change on external bit line BL. Select drain transistor SG 1  couples bit line BL to string  210  while string  210  includes select drain transistor SG 1  and internal bit line IBL coupled in series between external bit line BL and a source line  240 . 
     Internal bit line IBL connects a NAND structure of  16  floating gate memory cells MC 1  to MC 16  connected in series between select gate transistors SG 1  and SG 2 .(for clarity reasons, only memory cells MC 1  to MC 3  and MC 15  to MC 16  are shown in FIG.  2 ). Other configurations of  4 ,  16  or  32  memory cells may also be used. Memory cell MC 3  can be implemented by memory cell  100  of FIG.  1 . One terminal of memory cell MC 16  in the string  210  is coupled to select source transistor SG 2  that is connected to source line  240 . 
     Each control gate of memory cells MC 1  to MC 16 , WL 1  to WL 16  is coupled to a pass transistor (one of transistors T 1  to T 16 ) while the control gate of select drain transistor SD is coupled by transistor TO to select drain line SDL. Each of transistors To to T 17  is coupled to charge pump  230  by pump line PL for applying voltages on lines SDL and WL 1  to WL 16  to the respective control gates of select drain transistor SG 1  and SG 2 , and memory cells MC 1  to MC 16 . 
     FIG. 3 is a cross sectional diagram of string  200  of FIG. 2 as disposed on a substrate  300 . Memory cells MC 1  to MC 16 , of which, for clarity reasons, only memory cells MC 1  to MC 4  and MC 14  to MC 16  are shown in FIG. 3, are fabricated on a substrate  300 . 
     Substrate  300  includes a p-well region  302 , which is a body region for memory cells MC 1  to MC 16 . P-well region  302  is coupled to voltage line  134  for asserting voltage V pw  on p-well region  302 . Substrate  300  also has n+ regions  304  which form n+ source and drain regions of memory cells MC 1  to MC 16 . 
     FIG. 4 a  is a graphical representation of the four states of a memory cell as a function of Id (cell current) and gate voltage V g  where the source voltage V s  and the P well back bias (V pw )is zero and external bias on the bit line BL is constant. 
     FIG. 5 is a circuit diagram that senses multi levels of NAND memory cells according to the present invention. The circuit includes a sense node  501 , that senses variations in NAND memory cell current, a flip-flop latch  502  with gates  503  and  504  preconditioned to states 0 and 1, and a pulse source  505  generates reverse and forward bias voltage. For illustration purposes memory cell MC 3  is sensed at a given time t. 
     A fixed gate voltage (V g ) of 0V is applied to cell MC 3  at time t. V pw  is at zero volts and voltage on external bit line BL is kept constant. V s  is varied and that varies the memory cell current Id, depending upon memory cell MC 3 &#39;s state. If memory cell MC 3  is in an erased state, it is conductive and if memory cell MC 3  is programmed, then it is not conductive. 
     FIG. 4 b  shows the I-V characteristic of memory cell MC 3  when a negative V s  is applied at zero gate voltage, zero V pw  and a fixed external bit line voltage. V s  causes a shift of the IV characteristics in the positive direction along the voltage axis and thus differentiates between states (0,0), (0,1) and states (1,0) (1,1), since a current Id only flows in states (0,0) and (0,1) at zero gate voltage. FIG. 4 c  shows the I-V characteristic of memory cell MC 3  when a positive V s  is applied at zero gate voltage, zero V pw  and fixed external bit line bias voltage. In this case, V s  causes a further shift of the I-V characteristic curves in the positive direction along the voltage axis and differentiates between states (0,0) and (0,1) since current Id flows only in state (0,0). Hence the four states of memory cell MC 3  are differentiated and sensed by applying plural V s . 
     Memory cell MC 3  generates a cell current as a result of its memory cell state and the value of V s . If memory cell MC 3  is conductive, it is in an erased state and pulls down sense node  501 . The flip-flop latch circuit  502  stays in the same condition (0,1). If memory cell MC 3  is programmed, it does not conduct and pulls up sense node  501 , generating a signal that flips the flip-flop  502  to a 1,0 state. An output signal Io 1  from the flip-flop  502  circuit is sent to a logic circuit (not shown). The output signal will vary with the cell current Id that varies with applied V s  and the state of memory cell MC 3 . 
     Based upon the output signals from the flip-flop circuit, particular memory cell state is determined. The process is repeated at pre-determined intervals for every memory cell in the NAND array. 
     Although the present invention has been described with reference to specific embodiments, these embodiments are illustrative only and not limiting. Many other applications and embodiments of the present invention will be apparent in light of this disclosure and the following claims.