Abstract:
By using a resistive film as a shunt, the snapback exhibited when transitioning from the reset state or amorphous phase of a phase change material, may be reduced or avoided. The resistive film may be sufficiently resistive that it heats the phase change material and causes the appropriate phase transitions without requiring a dielectric breakdown of the phase change material.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/810,228, filed on Jun. 4, 2007, which is a divisional of U.S. patent application Ser. No. 10/318,706, filed on Dec. 13, 2002, which issued as U.S. Pat. No. 7,242,019. 
     
    
     BACKGROUND 
       [0002]    This invention relates generally to electronic memories and particularly to electronic memories that use phase change material. 
         [0003]    Phase change materials may exhibit at least two different states. The states may be called the amorphous and crystalline states. Transitions between these states may be selectively initiated. The states may be distinguished because the amorphous state generally exhibits higher resistivity than the crystalline state. The amorphous state involves a more disordered atomic structure. Generally any phase change material may be utilized. In some embodiments, however, thin-film chalcogenide alloy materials may be particularly suitable. 
         [0004]    The phase change may be induced reversibly. Therefore, the memory may change from the amorphous to the crystalline state and may revert back to the amorphous state thereafter, or vice versa, in response to temperature changes. In effect, each memory cell may be thought of as a programmable resistor, which reversibly changes between higher and lower resistance states. The phase change may be induced by resistive heating. 
         [0005]    Existing phase change memories may exhibit unpredictably current/voltage characteristics in transitioning from the amorphous to the crystalline phases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is an enlarged cross-sectional view of one embodiment of the present invention; 
           [0007]      FIG. 2  is an enlarged cross-sectional view of another embodiment of the present invention; 
           [0008]      FIG. 3  is a graph of current versus voltage in accordance with one embodiment of the present invention; 
           [0009]      FIG. 4  is a graph of current versus voltage in accordance with an embodiment from the prior art; and 
           [0010]      FIG. 5  is a schematic depiction of a system in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Referring to  FIG. 1 , a phase change memory  10  may include a phase change material  12 , such as a chalcogenide material, located in a pore  20 . The pore  20  may be defined by a resistive film  14  formed in an aperture through an upper electrode  16  and an insulating layer  18 . In one embodiment, the insulating layer  18  may be an oxide layer. The U-shaped resistive film  14  sits on top of a lower electrode  21  which in turn may be positioned over an insulating film  22 . The film  22  may be located over a substrate  24 . 
         [0012]    An electrical potential may be applied to the lower electrode  20  to cause the current to flow through the resistive film  14  to the upper electrode  16 . As a result of the resistance of the film  14 , the phase change material  12  may be heated. As a result, the material  12  may be transitioned between its amorphous and crystalline states. 
         [0013]    Current phase change memory elements exhibit instability in the set/reset behavior. Metal nitride electrodes tend to be unstable at higher temperatures and higher fields. An electrical pulse either melts or quenches the phase change material into an insulating amorphous state or heats and crystallizes the material into a conductive crystalline state. When the phase change material is in the amorphous phase, a large electric field is needed to force sufficient current to heat the memory and to store a bit. This leads to a switching event where the higher resistance insulating material rapidly becomes conductive. The switching process is very non-uniform and there are large variations in the voltage required in the region where the subsequent current conducts. As a result, unpredictable switching in the phase of the material may occur. 
         [0014]    For example, referring to  FIG. 4 , as the voltage is increased in the reset (amorphous phase) state  46 , the current slowly increases until a snapback point is reached. At the snapback point the voltage begins to fall but the current rises. 
         [0015]    By shunting current around the amorphous phase change material  12  using the resistive film  14 , the snapback may be largely reduced or eliminated. The shunt resistance from the resistive film  14  may be significantly higher than the set resistance of the memory element so that the phase change resistance difference is detectable. The shunt resistance of the resistor film  14  may be low enough so that when voltages approaching the threshold voltage of the memory element are present, the resistive film  14  heats up significantly. In other words, the resistance of the film  14  may be higher than the memory&#39;s set resistance and lower than its reset resistance. 
         [0016]    This heat, generated by the film  14 , changes the conductivity of the amorphous phase change material  12  in close proximity to the resistive film  14 . This heated phase change material  12  becomes more electrically conductive as indicated in  FIG. 3 . As shown in  FIG. 3 , the reset state  40  may not exhibit substantial (or any) snapback, but instead steadily increases the current conducted with increasing voltage. 
         [0017]    If the amorphous phase change material becomes conductive enough, the voltage across the memory element never becomes high enough to cause threshold switching. The instabilities resulting from this threshold switching do not occur in the phase transition from the amorphous phase or the reset state to the crystalline phase or set state so that the state transition occurs in a predictable fashion. 
         [0018]    In one embodiment shown in  FIG. 3 , as the reset voltage increases to about 0.4 to 0.5 volts, the heating of the phase change material  12  becomes sufficient to change the conductivity of the memory element and it asymptotically approaches the same current as the set condition. Here the voltage across the memory element in the reset condition never approaches the threshold voltage (around 0.65 volts), so the switching event or snapback, shown in  FIG. 4 , never takes place. 
         [0019]    While in the embodiment illustrated in  FIG. 1 , the resistive film  14  is shown extending completely from the lower electrode  20  to the upper electrode  16 , in some embodiments it may be sufficient to merely shunt the portion of the phase change material  12  that switches between the amorphous and the crystalline phases. This avoids the need to rely on the dielectric breakdown of the phase change material  12 . 
         [0020]    A variety of materials may be suitable for the resistive film  14 , including silicon carbide and metal nitrides. Suitable metals for the metal nitride include titanium, silicon, titanium aluminum, titanium carbon, tantalum, tantalum aluminum, and tantalum carbon, to mention a few examples. In some cases, it may be desirable to use an adhesion promoter between the resistive film  14  and the insulator  18 . 
         [0021]    Referring to  FIG. 2 , in accordance with another embodiment of the present invention, a lateral pore structure may include a phase change material  28  over two electrodes  30  having a separation  31 . The electrodes  30  and the separation  31  may be positioned over the resistive film  32  which corresponds to the film  14  in the previous embodiment. An insulator  34  may be positioned over a substrate  36 . The resistive film  32  functions like the resistive film  14  in the previous embodiment. 
         [0022]    Referring to  FIG. 5 , a processor-based system  50  may include a processor  52  coupled to a bus  54 . The phase change memory  10  may also be coupled to the bus as may be a wireless interface  56 . The interface  56  may be, for example, a radio frequency interface and may include a transceiver and/or an antenna such as a depole or other antenna. Thus, in some embodiments, the system  50  may be a cellular telephone or other wireless or radio frequency processor-based system. In other embodiments, non-wireless applications may be implemented. However, of course, the present invention is in no way limited to any one particular application of the phase change memory  10 . 
         [0023]    While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.