Abstract:
A memory with mechanisms for enhancing storage states without boosting voltages to levels that damage storage cell structures. A storage cell according to the present teachings includes a storage structure capable of switching storage states. A memory according to the present teachings includes means for writing the storage cell by applying a first voltage to a first node of the storage structure and for applying a second voltage to a second node of the storage structure such that the first and second voltages have opposite polarities.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The present invention pertains to the field of random access memories. More particularly, this invention relates to storage states in a memory cell.  
           [0003]    2. Art Background  
           [0004]    A typical random access memory includes an array of storage cells. Each storage cell typically includes a storage structure which is capable of changing storage states. For example, a storage cell in a ferroelectric random access memory (FeRAM) typically includes a ferroelectric capacitor capable of changing stored charge polarities.  
           [0005]    The storage state of a storage cell typically indicates its logic state. A storage cell is usually written by applying programming voltages which alter its storage state. For example, an FeRAM storage cell is usually written applying programming voltages that alter the charge polarities of its ferroelectric capacitor.  
           [0006]    It is usually desirable to provide a storage cell that enhances the likelihood that its storage state will be discernable during a read operation. Unfortunately, structures in a storage cell that are used for storage cell access may reduce the discernability of its storage states. In an FeRAM storage cell, for example, a programming voltage is usually applied to the ferroelectric capacitor through an access transistor. Unfortunately, the access transistor usually degrades the amount of programming voltage that reaches the ferroelectric capacitor, thereby limiting the amount of electrical charge that it accumulates.  
           [0007]    One prior method for avoiding such voltage degradation during programming is to apply boosted voltage levels during programming. In programming a typical prior FeRAM cell, for example, a supply-level voltage (V DD ) is usually applied the access transistor of the storage cell and a boosted supply-level voltage (V PP ) is usually applied to a control gate of the access transistor. Typically, V PP  is greater than V DD  by a threshold voltage (V TH,be ) to enable a maximum amount of charge to pass through the access transistor and to the ferroelectric capacitor. Unfortunately, high levels of V PP  may damage the gate structure of the access transistor, thereby reducing the reliability and service life of the memory. Similar problems may be encountered in other types of memories.  
         SUMMARY OF THE INVENTION  
         [0008]    A memory is disclosed with mechanisms for enhancing storage states without boosting voltages to levels that damage storage cell structures. A storage cell according to the present teachings includes a storage structure capable of switching storage states. A memory according to the present teachings includes means for writing the storage cell by applying a first voltage to a first node of the storage structure and for applying a second voltage to a second node of the storage structure such that the first and second voltages have opposite polarities. The magnitude of the second voltage is selected to enhance the overall voltage applied to the storage structure during programming.  
           [0009]    Other features and advantages of the present invention will be apparent from the detailed description that follows.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The present invention is described with respect to particular exemplary embodiments thereof and reference is accordingly made to the drawings in which:  
         [0011]    [0011]FIG. 1 shows one embodiment of a memory according to the present teachings;  
         [0012]    [0012]FIG. 2 shows a programming operation on the storage cell in one embodiment;  
         [0013]    [0013]FIG. 3 shows a plate line driver in a programming circuit in one embodiment.  
     
    
     DETAILED DESCRIPTION  
       [0014]    [0014]FIG. 1 shows one embodiment of a memory  100  according to the present teachings. The memory  100  shown is a ferroelectric random access memory (FeRAM). The present teachings are nevertheless applicable to other types of memories that employ other types of storage structures. Examples include DRAM structures.  
         [0015]    The memory  100  includes an array of storage cells such as a storage cell  10  shown. The storage cell  10  includes a ferroelectric capacitor  12  as its storage structure and an access transistor M 0 . The remaining storage cells (not shown) in the memory  100  may have a similar arrangement.  
         [0016]    The memory  100  also includes a programming circuit  40 . The programming circuit  40  programs the storage cell  10  to a logic state by applying a voltage V BL  to a bit line  20  coupled to the storage cell  10  and applying a voltage V WL  to a word line  22  coupled to the storage cell  10  and applying a voltage V PL  to a plate line  24  coupled to the storage cell  10 .  
         [0017]    When charging a ferroelectric capacitor, the programming circuit  40  generates the voltages V BL , V WL , and V PL  so that voltages of opposite polarities are applied to opposite ends of the ferroelectric capacitor  12 . In one embodiment, the programming circuit  40  applies a positive voltage to a storage node  30  and a small negative voltage to the plate line  24  during programming. These voltages are selected to enhance the amount of voltage V FE  applied across the ferroelectric capacitor  12  during programming.  
         [0018]    The magnitude of the small negative voltage applied to the plate line  24  may be preselected to compensate for an amount of voltage loss caused by the access transistor M 0 . The voltage loss caused by the access transistor M 0  may be increased by the Body effect and may be determined by measurement or other methods.  
         [0019]    [0019]FIG. 2 shows a programming operation on the storage cell  10  in one embodiment. During a programming operation, i.e. a write operation, the programming circuit  40  generates the voltages V WL  and V BL  and V PL  as shown. Also shown are the voltage V SN  at the storage node  30  and the resulting overall voltage V FE  applied across the ferroelectric capacitor  12 .  
         [0020]    Prior to time t 1 , the programming circuit  40  maintains the voltages V WL  and V BL  and V PL  at a substantially zero level. At time t 1 , the programming circuit  40  raises V WL  to V PP  and raises V BL  to V DD . In one embodiment, V PP  is approximately 2.0 volts and V DD  is approximately 1.5 volts.  
         [0021]    After time t 1 , V SN  and V FE  both rise to V PP −V TH,be . In one embodiment, V TH,be  is approximately 0.65 volts. Thus, after time t 1 , V SN  and V FE  both rise to 2.0−0.65=1.35 volts approximately.  
         [0022]    At time t 2 , the programming circuit  40  raises V PL  to V DD —in one embodiment approximately 1.5 volts. This lowers V FE  to approximately −0.15 volts after time t 2 . The programming circuit  40  raises V PL  to V DD  at time t 2  for programming operations to other storage cells coupled to the plate line  24 .  
         [0023]    At time t 3 , the programming circuit  40  drives V PL  to a small negative voltage V NEG . In one embodiment, the small negative voltage V NEG  is preselected to be approximately −0.15 volts in accordance with the effects of the V TH,be  characteristic of the access transistor M 0 . As a consequence of the negative voltage applied to the plate line  24  at time t 3 , the overall voltage V FE  across the ferroelectric capacitor  12  is, in one embodiment, 1.35+0.15=1.5 volts.  
         [0024]    Thus, V FE  after time t 3  is equivalent to full V DD  charging of the ferroelectric capacitor  12  but with only 2.0 volts applied to the gate of the access transistor M 0 . The relatively low level of 2.0 volts at the gate of the access transistor M 0  causes relatively low stress on its gate oxide layer during programming in comparison to a higher voltage level that would otherwise be needed at the gate to yield a V FE  up to V DD —i.e. in the absence of the negative voltage applied to the plate line  24 .  
         [0025]    At time t 4 , the programming circuit  40  returns V PL  to a substantially zero voltage level which causes V FE  to drop to 1.35 volts. Thereafter, the programming circuit  40  returns V WL  and V BL  and V PL  to substantially zero levels.  
         [0026]    [0026]FIG. 3 shows a plate line driver in the programming circuit  40  in one embodiment. The plate line driver provides three voltage levels—V DD , zero, and V NEG —for driving the plate line  24 . The particular voltage applied to the plate line  24  is determined by states of a set of control lines  50 - 54 . The control lines  50 - 54  may be driven by a programming control circuit (not shown) to accomplish the timing shown above. The transistor M P0  is a p-channel device and the transistors M N0  and M N1  are n-channel devices. The transistors M P0  and M N1  function as a CMOS inverter that provide the V DD  and zero voltage levels to the plate line  24  and the transistor M N1  provides the V NEG  level to the plate line  24 .  
         [0027]    The foregoing detailed description of the present invention is provided for the purposes of illustration and is not intended to be exhaustive or to limit the invention to the precise embodiment disclosed. Accordingly, the scope of the present invention is defined by the appended claims.