Abstract:
A memory circuit and method to improve signal margin is disclosed. The circuit includes a memory array arranged in rows  702, 704, 706  and columns  750, 752  of memory cells. Each row of memory cells is connected to a respective wordline. Each column of memory cells is connected to one of a bitline and a complementary bitline. An active wordline accesses a respective row of memory cells. The memory circuit includes a plurality of precharge circuits  724, 726, 728.  Each precharge circuit is connected to a respective column of memory cells and coupled to receive a precharge signal PRE. An active precharge signal renders a respective precharge circuit conductive. A control and decode circuit  700  changes an inactive wordline signal to an active wordline signal while the precharge signal is active.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention generally relates to electronic circuits, and more specifically to nonvolatile semiconductor integrated circuits.  
       BACKGROUND OF THE INVENTION  
       [0002]     Nonvolatile memory circuits such as electrically erasable programmable read only memories (EEPROM) and Flash EEPROMs have been widely used for several decades in various circuit applications including computer memory, automotive applications, and video games. Many new applications, however, require the access time and packing density of previous generation nonvolatile memories in addition to low power consumption for battery powered circuits. One nonvolatile memory technology that is particularly attractive for these low power applications is the ferroelectric memory cell. A major advantage of these ferroelectric memory cells is that they require approximately three orders of magnitude less energy for write operations than previous generation floating gate memories. Furthermore, they do not require high voltage power supplies for programming and erasing charge stored on a floating gate. Thus, circuit complexity is reduced and reliability increased.  
         [0003]     The term ferroelectric is something of a misnomer, since present ferroelectric capacitors contain no ferrous material. Typical ferroelectric capacitors include a dielectric of ferroelectric material formed between two closely-spaced conducting plates. One well-established family of ferroelectric materials known as perovskites has a general formula ABO 3 . This family includes Lead Zirconate Titanate (PZT) having a formula Pb(Zr x Ti 1-x )O 3 . This material is a dielectric with a desirable characteristic that a suitable electric field will displace a central atom of the lattice. This displaced central atom, either Titanium or Zirconium, remains displaced after the electric field is removed, thereby storing a net charge. Another family of ferroelectric materials is Strontium Bismuth Titanate (SBT) having a formula SbBi 2 Ta 2 O 9 . However, both ferroelectric materials suffer from fatigue and imprint. Fatigue is characterized by a gradual decrease in net stored charge with repeated cycling of a ferroelectric capacitor. Imprint is a tendency to prefer one state over another if the ferroelectric capacitor remains in that state for a long time.  
         [0004]     A typical one-transistor, one-capacitor (ITIC) ferroelectric memory cell of the prior art is illustrated at  FIG. 1 . The ferroelectric memory cell is similar to a 1T1C dynamic random access memory (DRAM) cell except for ferroelectric capacitor  100 . The ferroelectric capacitor (FeCAP)  100  is connected between plateline  110  and storage node  112 . Access transistor  102  has a current path connected between bitline  108  and storage node  112 . A control gate of access transistor  102  is connected to wordline  106  to control reading and writing of data to the ferroelectric memory cell. This data is stored as a polarized charge corresponding to cell voltage V CAP . Parasitic capacitance of bitline BL is represented by capacitor C BL    104 .  
         [0005]     Referring to  FIG. 2 , there is a hysteresis curve corresponding to the ferroelectric capacitor  100 . The hysteresis curve includes net charge Q or polarization along the vertical axis and voltage along the horizontal axis. By convention, the polarity of cell voltage is defined as shown in  FIG. 1 . A stored “0”, therefore, is characterized by a positive voltage at the plateline terminal with respect to the access transistor terminal. A stored “1” is characterized by a negative voltage at the plateline terminal with respect to the access transistor terminal. A “0” is stored in a write operation by applying a voltage Vmax across the ferroelectric capacitor. This stores a saturation charge Qs in the ferroelectric capacitor. The ferroelectric capacitor, however, includes a linear component in parallel with a switching component. When the electric field is removed, therefore, the linear component discharges and only the residual charge Qr remains in the switching component. The stored “0” is rewritten as a “1” by applying −Vmax to the ferroelectric capacitor. This charges the linear and switching components of the ferroelectric capacitor to a saturation charge of −Qs. The stored charge reverts to −Qr when the electric field is removed. Finally, coercive points V C  and −V C  are minimum voltages on the hysteresis curve that will degrade a stored data state. For example, application of V C  across a ferroelectric capacitor will degrade a stored “1” even though it is not sufficient to store a “0”. Thus, it is particularly important to avoid voltages near these coercive points unless the ferroelectric capacitor is being accessed.  
         [0006]     Referring to  FIG. 3 , there is illustrated a typical write sequence for a ferroelectric memory cell as in  FIG. 1 . Initially, the bitline (BL), wordline (WL), and plateline (PL) are all low. The upper row of hysteresis curves illustrates a write “1” and the lower row represents a write “0”. Either a “1” or “0” is initially stored in each exemplary memory cell. The write “1” is performed when the bitline BL and wordline WL are high and the plateline PL is low. This places a negative voltage across the ferroelectric capacitor and charges it to −Qs. When plateline PL goes high, the voltage across the ferroelectric capacitor is 0 V, and the stored charge reverts to −Qr. At the end of the write cycle, both bitline BL and plateline PL go low and stored charge −Qr remains on the ferroelectric capacitor. Alternatively, the write “0” occurs when bitline BL remains low and plateline PL goes high. This places a positive voltage across the ferroelectric capacitor and charges it to Qs representing a stored “1”. When plateline PL goes low, the voltage across the ferroelectric capacitor is 0 V, and the stored charge reverts to Qr representing a stored “0”.  
         [0007]     A step sensing read operation is illustrated at  FIG. 4  for the ferroelectric memory cell at  FIG. 1 . The upper row of hysteresis curves illustrates a read “0”. The lower row of hysteresis curves illustrates a read “1”. Wordline WL and plateline PL are initially low. Bitlines BL are precharged low. At time t 0  precharge signal PRE goes low, permitting the bitlines BL to float. At times t 1  and t 2  wordline WL and plateline PL go high, respectively, thereby permitting each memory cell connected to the active wordline WL and plateline PL to share charge with a respective bitline. A stored “1” will share more charge with parasitic bitline capacitance C BL  and produce a greater bitline voltage than the stored “0” as shown between times t 2  and t 3 . A reference voltage (not shown) is produced at each complementary bitline of an accessed bitline. This reference voltage is between the “1” and “0” voltages between times t 2  and t 3 . A difference voltage between either a “1” or “0” voltage and a corresponding reference voltage is applied to each respective sense amplifier. The sense amplifiers are activated at time t 3  to amplify the difference voltage. When respective bitline voltages are fully amplified after time t 3 , the read “0” curve cell charge has increased from Qr to Qs. By way of comparison, the read “1” data state has changed from a stored “1” to a stored “0”. Thus, the read “0” operation is nondestructive, but the read “1” operation is destructive. At time t 4 , plateline PL goes low and applies −Vmax to the read “1” cell, thereby storing −Qs. At the same time, zero voltage is applied to the read “0” cell and charge Qr is restored. At the end of the read cycle, signal PRE goes high and precharges both bitlines BL return to zero volts or ground. The wordline goes low, thereby isolating the ferroelectric capacitor from the bitline. Thus, zero volts is applied to the read “1” cell and −Qr is restored.  
         [0008]     Referring now to  FIG. 5 , a pulse sensing read operation is illustrated for a ferroelectric memory circuit. The read operation begins at time to when precharge signal PRE goes low, permitting the bitlines BL to float. Wordline WL and plateline PL are initially low, and bitlines BL are precharged low. At time t 1 , wordline WL goes high, thereby coupling a ferroelectric capacitor to a respective bitline. Then plateline PL goes high at time t 2 , thereby permitting each memory cell to share charge with the respective bitline. The ferroelectric memory cells share charge with their respective bitlines BL and develop respective difference voltages. Here, VI represents a data “1” and V 0  represents a data “0”. Plateline PL then goes low prior to time t 3 , and the common mode difference voltage goes to near zero. The difference voltage available for sensing is the difference between one of V 1  and V 0  at time t 3  and a reference voltage (not shown) which lies approximately midway between voltages V 1  and V 0  at time t 3 . The difference voltage is amplified at time t 3  by respective sense amplifiers and full bitline BL voltages are developed while the plateline PL is low. Thus, the data “1” cell is fully restored while plateline PL is low and the data “1” bitline BL is high. Subsequently, the plateline PL goes high while the data “0” bitline BL remains low. Thus, the data “0” cell is restored. The plateline PL goes low at time t 4 , and precharge signal PRE goes high at time t 5 . The high level of precharge signal PRE precharges the bitlines to ground or Vss. The wordline WL goes low at time t 6 , thereby isolating the ferroelectric capacitor from the bitline and completing the pulse sensing cycle.  
         [0009]     Turning now to  FIG. 6 , there is a simplified diagram of an unselected ferroelectric memory cell of the prior art illustrating a problem with both step and pulse sensing schemes. Here, the same reference numerals are used as in the memory cell of FIG. I to show comparable elements of the ferroelectric memory cell. Resistor R GATE    114  represents subthreshold leakage path of access transistor  102 . Diode  116  is a parasitic junction diode between storage node  112  and the memory circuit substrate. The wordline terminal WL  106  is adjacent a selected wordline (not shown) during a read operation. Thus, wordline terminal  106  may develop 200 mV during a low-to-high transition of the adjacent active wordline, as will be explained in detail. Plateline  110  is common to cells on the selected wordline as well as the unselected cell. Ferroelectric capacitor  100  stores a respective data signal and preferably has zero volts until a coercive voltage is developed across the terminals as previously explained. For the following exemplary discussion, ferroelectric capacitor  100  has approximately 30 fF capacitance.  
         [0010]     During a read or write operation a selected wordline adjacent wordline WL  106  is driven high to approximately 2.2 V. This capacitively couples 200 mV to wordline terminal  106  and greatly increases subthreshold conduction of access transistor  102 . Bitline BL  108  is driven low, and plateline PL  110  is driven high to approximately 1.65 V. Due to charge sharing with diode  116  and gate-to-source capacitance of access transistor  102 , the plateline PL transition couples 1.6 V to storage node  112 . Thus, storage node  112  goes from 0 V to 1.6 V. Under these conditions at high temperature, subthreshold leakage current ISUB of access transistor  102  increases from less than 1 nA when there is no coupling to wordline  106  to approximately 100 nA, or about two orders of magnitude, when 200 mV is coupled to wordline  106 . This level of subthreshold leakage current through resistor R GATE    114  lasts for approximately 4 ns until the wordline drive circuit can restore wordline WL  106  to 0 V. The subthreshold current ISUB of 100 nA for 4 ns, however, represents a 13 mV decrease in storage node voltage subject to the previously described conditions. Moreover, this charge loss is cumulative. Minimal current flows from bitline BL  108  through access transistor  102  when plateline PL  110  returns to 0 V due to the small drain-to-source voltage. Subsequent memory accesses to memory cells adjacent wordline WL  106  and resulting charge loss, however, may result in a negative voltage of as much as −200 mV at storage node  112 . Such memory accesses would significantly degrade the data “1” level of the ferroelectric memory cell resulting in read errors. This degradation of the data “1” level introduces a bitline voltage imbalance and may even depolarize the ferroelectric capacitor.  
       SUMMARY OF THE INVENTION  
       [0011]     In accordance with a preferred embodiment of the invention, a memory circuit and method to improve signal margin is disclosed. The circuit includes a memory array arranged in rows and columns of memory cells. Each row of memory cells is connected to a respective wordline. Each column of memory cells is connected to one of a bitline and a complementary bitline. An active wordline accesses a respective row of memory cells. The memory circuit includes a plurality of precharge circuits. Each precharge circuit is connected to a respective column of memory cells and coupled to receive a precharge signal. An active precharge signal renders a respective precharge circuit conductive. A control and decode circuit produces an active wordline signal while the precharge signal is active and before a plateline signal is activated. This active precharge signal eliminates accumulated charge at each memory cell storage node, thereby improving signal margin. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The foregoing features of the present invention may be more fully understood from the following detailed description, read in conjunction with the accompanying drawings, wherein:  
         [0013]      FIG. 1  is a circuit diagram of a ferroelectric memory cell of the prior art;  
         [0014]      FIG. 2  is a hysteresis curve of the ferroelectric capacitor  100  of  FIG. 1 ;  
         [0015]      FIG. 3  is a timing diagram showing a write operation to the ferroelectric memory cell of  FIG. 1 ;  
         [0016]      FIG. 4  is a timing diagram of a step sense read operation from the ferroelectric memory cell of  FIG. 1 ;  
         [0017]      FIG. 5  is a timing diagram of a pulse sense read operation from the ferroelectric memory cell of  FIG. 1 ;  
         [0018]      FIG. 6  is a simplified circuit diagram of the prior art showing charge accumulation at the ferroelectric memory cell storage node due to subthreshold leakage;  
         [0019]      FIG. 7  is a schematic diagram of an embodiment of the memory circuit of the present invention;  
         [0020]      FIG. 8A  is timing diagram of a first embodiment of the memory circuit of the present invention; and  
         [0021]      FIG. 8B  is a timing diagram of a second embodiment of the memory circuit of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0022]     Referring to  FIG. 7 , there is a memory circuit of the present invention. Although the memory circuit includes many similar memory arrays, only a portion of one array is shown for clarity. The memory array includes memory cells arranged in rows corresponding to wordlines  702 ,  704 ,  706  and columns  750 ,  752 . Individual memory cells are indicated by circles at intersections of rows and columns. In an embodiment of the present invention there are preferably 512 rows and 64 columns in the memory array. The memory array also includes 16 platelines  710 - 718 . Each plateline is coupled to receive a respective plateline signal PL 0 -PL 15 . Each plateline, for example plateline  710 , is common to 32 rows of memory cells including rows common to wordlines  702 - 706 . Each row of memory cells is selected by an active wordline signal. For example, row  704  is selected by active wordline signal WL X  on wordline  704 . Each column includes a bitline  708  and a complementary bitline  709  that form a bitline pair. Each bitline pair is coupled to a respective sense amplifier such as sense amplifier  730 . Each sense amplifier has complementary output terminals coupled to local input/output lines LIO  746  and /LIO  748  by column select transistors  742  and  744 , respectively. The column select transistors are selected by an active column select signal, for example, YS Y  on lead  740 . Each column has a respective precharge circuit including first  724 , second  726 , and third  728  precharge transistors. The first and second precharge transistors respectively couple the bitline  708  and complementary bitline  709  to voltage terminal GND via lead  722  in response to an active precharge signal PRE on lead  720 . A third precharge transistor couples the bitline  708  and complementary bitline  709  to each other in response to the active precharge signal PRE on lead  720 .  
         [0023]     In operation, the control and decode circuit  700  receives a chip enable signal CE, an address signal A N  including N address bits, and a read/write signal WR. The control and decode circuit produces an active wordline signal WL, an active column select signal YS, an active plateline signal PL, and a precharge signal PRE, where WL, YS, and PL represent a respective group of wordlines, column select lines, and platelines. A selected memory cell at the intersection of the addressed row and column receives or produces data on a respective bitline in response to a logical state of read/write signal WR. For example, when read signal WR is high, a write operation is performed. Alternatively, when read/write signal is low, a read operation is performed. For either a read or a write operation, when a wordline signal such as wordline signal WL X  goes active high, a small voltage is coupled to adjacent wordlines WL X+1    702  and WL X−1    706  through fringe capacitors CF  770  and  772 , respectively. This capacitive coupling increases the voltage on the adjacent wordlines WL X+1    702  and WL X−1    706  by as much as 200 mV and increases subthreshold leakage by approximately two orders of magnitude. Next, a low-to-high transition of plateline signal PL 0    710  induces subthreshold current to flow from the storage node to the bitline. This charge loss couples as much as −13 mV to the storage node of each memory cell along adjacent wordlines  702  and  706  following a subsequent high-to-low transition of plateline signal PL 0    710 . Moreover, the subthreshold current from bitline to storage node of the memory cells on adjacent wordlines  702  and  706  when plateline signal PL 0  is low is much less than when high as previously explained. This is because the drain-to-source voltage of each access transistor is much less. Thus, repeated access to wordline WL X    704  results in accumulated negative voltage of as much as −200 mV at the storage node of each memory cell on adjacent wordlines  702  and  706 .  
         [0024]     Referring to  FIGS. 7 and 8 A, a step sensing read or write memory cycle will be described in detail. In the following description, a memory cycle is from time t 0  through time t 6 . The memory cycle on an adjacent wordline, for example wordline  702 , is initiated when wordline signal WL X+1  goes active high at time t 0 . This turns on access transistor  102  ( FIG. 6 ) while precharge signal PRE remains high and precharge transistors  724 ,  726 , and  728  are still on. Due to the relatively small negative charge at storage node  112 , a voltage of wordline signal WL X+1  slightly greater than the threshold voltage of access transistor  102  is adequate. Likewise, a voltage of precharge signal PRE slightly greater than the threshold voltage of precharge transistors  724  and  726  is adequate. Storage node  112  is charged through access transistor  102  and complementary bitlines  708  and  709  are equalized through precharge transistor  728 , thereby eliminating accumulated negative voltage. This elimination of accumulated negative voltage is highly advantageous. Complementary bitlines are fully equalized prior to sensing and signal margin is not degraded, therefore, by a bitline voltage imbalance.  
         [0025]     Next, precharge signal PRE goes low at time t 1  and turns off precharge transistors  724 ,  726 , and  728 . Then plateline signal PL 0  goes active from an inactive state at time t 2 . The high level of plateline signal PL 0  exceeds the coercive voltage V C  ( FIG. 2 ) of the ferroelectric capacitor and develops a voltage on bitline  709  representing one of a data “1” or a data “0”. Bitline  708  receives a reference voltage intermediate the data “1” and data “0” levels, thereby producing a difference voltage at the input/output terminals of sense amplifier  730 . At time t 3 , sense amplifier  730  is activated to develop either a full data “1” or data “0” level on bitline  709 . If a data “0” is developed on bitline  709 , the memory cell ferroelectric capacitor is restored while the plateline signal PL 0  is high and bitline  709  is low. Alternatively, if a data “1” is developed on bitline  709 , the memory cell ferroelectric capacitor data is destroyed as previously explained with respect to  FIG. 4 . At time t 5 , plateline signal PL 0  goes low. This low level of plateline signal PL 0  and high level of a data “1” bitline  709  restores the memory cell ferroelectric capacitor data. Precharge signal PRE returns to a high level at time t 4 , thereby turning on precharge transistors  724 ,  726 , and  728  and precharging complementary bitlines  708  and  709  to ground GND through lead  722 . The memory cycle is completed when wordline signal WL X+1  goes low and turns off respective access transistors along wordline  702 .  
         [0026]     Referring now to  FIGS. 7 and 8 B, a read or write memory cycle for a pulse sensing circuit will be described in detail. In the following description, a memory cycle is from time t 0  through time t 8 . Adjacent wordline signal WL X+1    702 , for example, goes active high at the beginning of a memory access cycle at time t 0 . As in the previous discussion, access transistor  102  ( FIG. 6 ) turns on while precharge signal PRE remains high and precharge transistors  724 ,  726 , and  728  are still on. Due to the relatively small negative charge at storage node  112 , a voltage of wordline signal WL X+1  slightly greater than the threshold voltage of access transistor  102  is adequate. Likewise, a voltage of precharge signal PRE slightly greater than the threshold voltage of precharge transistors  724  and  726  is adequate. Storage node  112  ( FIG. 6 ) is charged through access transistor  102  and complementary bitlines  708  and  709  are equalized through precharge transistor  728 , thereby eliminating accumulated negative voltage. Thus, any accumulated negative voltage (or positive voltage for any reason not described above) at the sense node is eliminated and complementary bitlines are fully equalized prior to sensing. Signal margin, therefore, is not degraded due to bitline imbalance.  
         [0027]     Next, precharge signal PRE goes low at time ti and turns off precharge transistors  724 ,  726 , and  728 . Then plateline signal PL 0  goes active from an inactive state at time t 2 . The high level of plateline signal PL 0  exceeds the coercive voltage V C  ( FIG. 2 ) of the ferroelectric capacitor and develops a voltage on bitline  709  representing either a data “1” or a data “0”. Bitline  708  maintains a reference voltage intermediate the data “1” and data “0” levels, thereby producing a difference voltage at the input/output terminals of sense amplifier  730 . At time t 3 , plateline signal PL 0  goes low and returns the common mode bitline voltage to near zero. The difference voltage available for sensing is the difference between one of voltages V 1  and V 0  at time t 3  and a reference voltage (not shown) which is approximately midway between voltages V 1  and V 0  at time t 3 . At time t 4 , sense amplifier  730  is activated to develop either a full data “1” or data “0” level on bitline  709 . If a data “1” is developed on bitline  709 , the memory cell ferroelectric capacitor is restored while the plateline signal PL 0  is low and bitline  709  is high. Alternatively, if a data “0” is developed on bitline  709 , the memory cell ferroelectric capacitor is restored at time t 5  after the plateline signal PL 0  goes high and while bitline  709  is low. At time t 6 , plateline signal PL 0  goes low again. This low level of plateline signal PL 0  and high level of bitline  709  provides additional time to restore a data “1” memory cell ferroelectric capacitor between times t 6  and t 7 . Precharge signal PRE returns to a high level at time t 7 , thereby turning on precharge transistors  724 ,  726 , and  728  and precharging complementary bitlines  708  and  709  to ground GND or Vss through lead  722 . The memory cycle is completed when wordline signal WL X+1  goes low and turns off respective access transistors along wordline  702 .  
         [0028]     The present invention advantageously eliminates accumulated negative or positive voltage at the storage node of ferroelectric memory cells prior to either step sensing or pulse sensing. Bitlines and complementary bitlines are precharged to a predetermined voltage until immediately before sensing. Other forms of array noise, therefore, are reduced. Any accumulated negative or positive voltage at the storage node is not imparted to the bitline difference voltage. Thus, signal margin is improved prior to sensing.  4  While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. For example, referring back to  FIGS. 7, 8A , and  8 B, column select signal YS Y  on lead  740  may be activated any time between times t 2  and t 4  during a write operation. Such timing variations depend on sense amplifier design and individual design preference. Furthermore, a preferred embodiment of the present invention has been described with respect to a one-transistor/one-capacitor (1T/1C) storage cell. The present invention, however, is equally applicable to two-transistor/two-capacitor (2T/2C) memory cells. These 2T/2C cells are complementary 1T/1C memory cells. A wordline (or wordlines) activates the 2T/2C memory cell, thereby coupling the complementary 1T/1C memory cells to their respective complementary bitlines. If the 2T/2C memory cell stores a data “1”, for example, the true and complementary bitline voltages change to produce a total difference voltage. The present invention with the previously described timing of  FIGS. 8A and 8B  would advantageously eliminate bitline imbalance due to storage node voltage accumulation as with the previously described embodiments. It is therefore intended that the appended claims encompass any such modifications or embodiments.