Abstract:
A multilevel phase change memory may be formed of a chalcogenide material formed between a pair of spaced electrodes. The cross-sectional area of the chalcogenide material may decrease as the material extends from one electrode to another. As a result, the current density decreases from one electrode to the other. This means that a higher current is necessary to convert the material that has the largest cross-sectional area. As a result, different current levels may be utilized to convert different amounts of the chalcogenide material to the amorphous or reset state. A distinguishable resistance may be associated with each of those different amounts of amorphous material, providing the opportunity to engineer a number of different current selectable programmable states.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/122,363, filed on May 5, 2005. 
     
    
     BACKGROUND 
       [0002]    This invention relates generally to phase change memories and, particularly, to multilevel phase change memories. 
         [0003]    Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, for electronic memory application. One type of memory element utilizes a phase change material that may be, in one application, electrically switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. The state of the phase change materials is also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until changed by another programming event, as that value represents a phase or physical state of the material (e.g., crystalline or amorphous). The state is unaffected by removing electrical power. 
         [0004]    While conventional phase change memories are two level memory cells, there is a need for phase change memories with multiple levels including three or more levels or states. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is an enlarged, cross-sectional view through one embodiment of the present invention; 
           [0006]      FIG. 2A  shows a hypothetical current density through the region  16   c  of the memory shown in  FIG. 1  in accordance with one embodiment of the present invention; 
           [0007]      FIG. 2B  shows a hypothetical current density for the region  16   a  in accordance with one embodiment of the present invention; 
           [0008]      FIG. 3  is a hypothetical plot of temperature versus distance between the bottom and top electrodes in accordance with one hypothetical embodiment of the present invention; 
           [0009]      FIG. 4  is a hypothetical plot of the length of the melted region versus device current for the embodiment shown in  FIG. 1 ; 
           [0010]      FIG. 5  is a hypothetical plot of resistance versus current pulse amplitude for the embodiment shown in  FIG. 1  in accordance with one embodiment of the present invention; and 
           [0011]      FIG. 6  is a system depiction for one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Referring to  FIG. 1 , a phase change memory cell  10  may include a bottom electrode  14  and a top electrode  12 . The terms bottom and top are simply arbitrary and what is called for are two spaced electrodes with an intervening chalcogenide material  20 . 
         [0013]    As illustrated in  FIG. 1 , the cross-sectional area of the chalcogenide material  20  (the direction transverse to the direction indicated by the arrow Z) expands as the cell  10  extends from the bottom electrode  14  to the top electrode  12 . In the embodiment illustrated, the cross-sectional area of the chalcogenide material  20  expands in discrete steps. Each step may be represented by cylindrical regions  20   a ,  20   b ,  20   c  that increase in diameter. In other embodiments, the chalcogenide material  20  may expand linearly or pursuant to any other geometric relationship. Thus, in some embodiments, the material  20  may be shaped as the series of increasing diameter cylinders  20   a ,  20   b ,  20   c , illustrated as a cone, or any other desirable increasing cross-sectional area shape. 
         [0014]    In the illustrated embodiment, a series of layers  16   a - 16   c  are associated with and surround each of the phase change regions  20   a - 20   c . In some embodiments, the layer  16   c  may have the highest heat transfer coefficient and the layer  16   a  may have the lowest. 
         [0015]    Potentials may be applied to the electrodes  14  and  12  that cause current flow through the chalcogenide material  20 . This current flow may induce a phase change in some number of the regions  20   a - 20   c.    
         [0016]    Because of the increasing diameter of the chalcogenide material  20 , the current density is lowest in the region  20   c  proximate to the layer  16   c , as indicated in  FIG. 2A , and highest in the region  20   a  proximate to the layer  16   a , as shown in  FIG. 2B . This is because current density is a function of area transverse to the direction of current flow. 
         [0017]    As shown in  FIG. 3 , in an embodiment with three different current levels I 1 -I 3 , the temperature along the direction of the arrow Z steps down from being the highest in the region  20   a  to the lowest in the region  20   c . In some embodiments, this may be a function of the different current densities in each of the regions  20   a - 20   c . In other embodiments, it is a combination of the effect of the current densities of the regions  20   a - 20   c  being different (due to their different sizes in the direction transverse to the direction Z), combined with different coefficients of heat transfer of the layers  16   a - 16   c . In some embodiments, the layer  16   c  transfers the most heat, while the layer  16   a  transfers the least heat. 
         [0018]    Thus, referring to  FIG. 4 , the length of the melted or phase changed region in response to current flow increases with device current in steps a-c corresponding to the stepped regions  20   a - 20   c . The steps a-c may be associated with different measurable resistances, R 1 , R 2 , and R 3 , that may indicate different program states, as indicated in  FIG. 5 . 
         [0019]    In some embodiments of the present invention, by providing increasingly greater current flow, an increasingly greater amount of the phase change material  20  is converted from the set to the reset state. The set state may correspond to the crystalline structure of the chalcogenide  20  and the reset state may correspond to the amorphous phase. Because of the lower current density in the region  20   c , (as well as the greater heat transfer coefficient of the layer  16   c , in some embodiments), a relatively high current density corresponding to the highest current level  13  in the embodiment illustrated may be necessary to convert the region  20   c.    
         [0020]    As shown in  FIG. 5 , the resistance increases in steps to the maximum resistance when the current level  13  is provided, which may substantially completely convert all three regions  20   a ,  20   b , and  20   c , to the reset state. Conversely, in some embodiments, the current level  12  may convert only the regions  20   a  and  20   b , while only the region  20   a  may be converted by the lowest current level I 1 . 
         [0021]    Thus, in the embodiment illustrated with three regions  20   a - 20   c , four different phase change memory levels may be formed. Those levels correspond to the currents levels I 1 -I 3 , which correspond to three different amorphous regions. The three different amorphous regions may be associated with different measurable resistivity levels. In addition, a fourth level corresponds to the completely crystalline state of the chalcogenide material  20 . 
         [0022]    Some designers may prefer to design the phase change memory device to give more resistance range for the available current range. More programmable levels can be obtained by simply providing more electrically distinguishable regions of the material  20  or more steps of the layers  16   a - 16   c  or combinations thereof. 
         [0023]    Thus, to program one level, the current level I 1  is applied, to program a second level, the current level  12  is applied, and to program the third amorphous layer, the current level  13  may be applied. With no current, the cell  10  may remain in a crystalline default state, in one embodiment of the present invention, called the set state. However, other nomenclatures and other arrangements of multilevel cell states may be implemented according to some embodiments of the present invention. 
         [0024]    In order to form the structure shown in  FIG. 1 , the bottom electrode  14  may be formed in a dielectric layer over a semiconductor substrate. Then, the stack of layers  16   a - 16   c  may be formed and a hole formed down the middle, for example, using a directional etch. Then, the stack of layers  16   a - 16   c  can be exposed to anisotropic etching gases. If the layers  16  have increasingly less proclivity to being etched by the particular gas that is used, the conical hole profile shown in  FIG. 1  may result. Each layer  16   a - 16   c  may be etched at a different rate due to the different etch rate of the material forming each of the layers  16  in some embodiments. However, the present invention is not limited to any particular manufacturing technique. The above technique is given only for purposes of illustration. 
         [0025]    The chalcogenide material  20  may be a material having electrical properties (e.g. resistance, capacitance, etc.) that may be changed through the application of energy such as, for example, heat, light, voltage potential, or electrical current. The chalcogenide material  20  may be a material that includes at least one element from column VI of the periodic table or may be a material that includes one or more of the chalcogen elements, e.g., any of the elements of tellurium, sulfur, or selenium. 
         [0026]    Programming of material  20  to alter the state or phase of the material may be accomplished by applying voltage potentials to the electrodes  14  and  12 , thereby generating a voltage potential across the chalcogenide material  20 . When the voltage potential is greater than the threshold voltage of the cell  10 , then an electrical current may flow through chalcogenide material  20  in response to the applied voltage potentials, and may result in heating of Chalcogenide material  20 . 
         [0027]    This heating may alter the memory state or phase of chalcogenide material  20 . Altering the phase or state of chalcogenide material  20  may alter the electrical characteristic of chalcogenide material  20 , e.g., the resistance of the material may be altered by altering the phase of the chalcogenide material  20 . 
         [0028]    In the “reset” state, chalcogenide material  20  may be in an amorphous or semi-amorphous state and in the “set” state, chalcogenide material  20  may be in an a crystalline or semi-crystalline state. The resistance of chalcogenide material  20  in the amorphous or semi-amorphous state may be greater than the resistance of chalcogenide material  20  in the crystalline or semi-crystalline state. It is to be appreciated that the association of reset and set with amorphous and crystalline states, respectively, is a convention and that at least an opposite convention may be adopted. 
         [0029]    Using electrical current, chalcogenide material  20  may be heated to a relatively higher temperature to amorphosize all or part of the chalcogenide material  20  and “reset” chalcogenide material  20  (e.g., program chalcogenide material  20  to a particular logic value). Heating all or part of the volume of chalcogenide material  20  to a relatively lower crystallization temperature may crystallize chalcogenide material  20  and “set” chalcogenide material  20  (e.g., program chalcogenide material  20 ). Various resistances of chalcogenide material  20  may be achieved to store information by varying the amount of current flow and duration through the volume of chalcogenide material  20 . 
         [0030]    Suitable materials for the electrodes  12  and  14  may include a thin film of titanium nitride (TiN), titanium tungsten (TiW), carbon (C), silicon carbide (SiC), titanium aluminum nitride (TiAlN), titanium silicon nitride (TiSiN), polycrystalline silicon, tantalum nitride (TaN), some combination of these films, or other suitable conductors or resistive conductors compatible with switching material  24 . 
         [0031]    A system  500  may be any processor based system including, for example, a laptop computer, a desktop computer, a server, a personal digital assistant, an imaging device, a cellular telephone, a set top box, an Internet appliance, a media player, or an embedded device to mention a few examples. 
         [0032]    System  500  may include a controller  510 , an input/output (I/O) device  520  (e.g. a keypad, display), a memory  530 , and a wireless interface  540  coupled to each other via a bus  550 . It should be noted that the scope of the present invention is not limited to embodiments having any or all of these components. 
         [0033]    Controller  510  may comprise, for example, one or more microprocessors, digital signal processors, microcontrollers, or the like. Memory  530  may be used to store messages transmitted to or by system  500 . Memory  530  may also optionally be used to store instructions that are executed by controller  510  during the operation of system  500 , and may be used to store user data. Memory  530  may be provided by one or more different types of memory. For example, memory  530  may comprise any type of random access memory, a volatile memory, a non-volatile memory such as a flash memory and/or a phase change including the cell  10 . 
         [0034]    I/O device  520  may be used by a user to generate a message. System  500  may use wireless interface  540  to transmit and receive messages to and from a wireless communication network with a radio frequency (RF) signal. Examples of wireless interface  540  may include an antenna or a wireless transceiver, although the scope of the present invention is not limited in this respect. 
         [0035]    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.