Abstract:
A heater for a phase change memory may be thrilled by depositing a first material into a trench such that the material is thicker on the side wall than on the bottom of the trench. In one embodiment, because the trench side walls are of a different material than the bottom, differential deposition occurs. Then a heater material is deposited thereover. The heater material may react with the first material at the bottom of the trench to make Ohmic contact with an underlying metal layer. As a result, a vertical heater may be formed which is capable of making a small area contact with an overlying chalcogenide material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of pending U.S. patent application Ser. No. 12/944,134, filed Nov. 11, 2010, which application is incorporated herein by reference, in its entirety, for any purpose. 
     
    
     BACKGROUND 
       [0002]    This relates generally to phase change memories. Phase change memories use a chalcogenide layer that changes phase between more amorphous and less amorphous or more crystalline phases. Generally, the phase transition is the result of Joule heating of the chalcogenide layer. In sonic cases, the heating of the chalcogenide layer is due to electrical heating through a heating element proximate to the phase change material layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]      FIG. 1  is an enlarged, partial, cross-sectional view alone embodiment of the present invention at an early stage; 
           [0004]      FIG. 2  is an enlarged, cross-sectional view at a subsequent stage according to one embodiment; 
           [0005]      FIG. 3  is an enlarged, cross-sectional view at still a subsequent stage in accordance with one embodiment; 
           [0006]      FIG. 4  is an enlarged, cross-sectional view at a subsequent stage in accordance with one embodiment; 
           [0007]      FIG. 5  is an enlarged, cross-sectional view at a subsequent stage according to one embodiment; and 
           [0008]      FIG. 6  is an enlarged, cross-sectional view at a subsequent stage according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    In accordance with some embodiments, differential deposition can be used to form vertical high aspect ratio heaters for phase change memories. The vertical heaters may be more effective because they have a smaller point of contact with the chalcogenide material, that point of contact determined h the thickness in the horizontal dimension of the phase change material heater, which is deposited as a layer in the vertical direction. 
         [0010]    Therefore, the heater can have a critical dimension thinner than any thickness possible with lithographic techniques. As a result of the thin area of contact between the heater and the phase change material layer, less material in the phase change layer must be required to change phase and, therefore, less energy is needed to make the phase transition. As a result, power consumption may be improved in some embodiments. 
         [0011]    Referring to  FIG. 1 , in some embodiments, a vertical wall dielectric  14  may be formed. In some cases, the dielectric  14  may be formed with a trench, Figure I only showing the left side of a trench. The dielectric  14 , in one embodiment, may be stacked oxide and nitride layers. However, any dielectric may be utilized. The dielectric  14  is formed on top of a metal layer  12  which acts as to conductor lower electrode, or address line. 
         [0012]    In some embodiments, a deposition technique is used to form a layer composed of a thicker vertical portion  16  on the dielectric  14  and a thinner horizontal portion  18  on the metal  12 . The difference in deposition thickness is the result of the differential deposition that occurs on metal, compared to dielectric materials. Specifically, with some deposition processes, such as atomic layer deposition, more material is deposited on dielectrics than on metals. For example, in a flow of boron B 2 H 6 , the boron is well physisorbed on an oxide/nitride layer  14  and less effectively physisorbed on metals, resulting in differential thicknesses. Another technique that can be utilized is SiH 4  deposition, however, the film performance may not be as good as that achieved with boron. 
         [0013]    Next, a heater material  20  is deposited, as indicated in  FIG. 2 , using a deposition technique, such as atomic layer deposition. For example, the atomic layer deposition of tungsten may he used to form the heater material  20  which has a vertical portion and a horizontal portion  22 . However, the horizontal portion  18  on the metal  12  is consumed during the heater material  20  deposition process As a result, physical contact is achieved between the heater material  20  and the metal  12 . Next, to second boron layer may be deposited, again, using B 2 H 6  flow, in one embodiment, to form the capping layer  24 , as shown in  FIG. 3 . Again, atomic layer deposition may be used, for example. 
         [0014]    More specifically, the reason for the consumption of the baron forming the horizontal portion  18  at the bottom of the trench is that tungsten fluoride (WF 6 ) reacts with any boron at the metal surface, achieving Ohmic contact to the metal surface, Good adhesion between the tungsten based layer and the dielectric  14  is guaranteed by the residual of boron deposited on the dielectric and still remaining on the dielectric  14 . The second B 2 H 6  atomic layer caps the in situ heater material  20 . 
         [0015]    Next as shown in  FIG. 4 , a dielectric layer  26  is blanket deposited over the structure and then planarized down to the upper surface  28  of the dielectric  14 , as shown in  FIG. 5 , 
         [0016]    Finally, a chalcogenide layer  30  is deposited, as indicated in  FIG. 6 . The vertical heater material  20  makes a small area or point contact with the chalcogenide layer  30 . Thereafter, the chalcogenide layer may be covered with additional layers, including another electrode or metal layer (not shown). Then, additional layers (also not shown may be deposited, such as the layers forming an ovonic threshold switch, as one example. In some embodiments, the surface area of contact between the vertically oriented heater material  20  and the chalcogenide layer  30  is governed by the thickness of the deposition of the heater material  20 . Using atomic layer deposition, this layer can be made extremely is small, resulting in a very small area of contact between the chalcogenide layer  30  and the heater material  20 . 
         [0017]    While the figures depict the formation of the single cell, a large number of cells may be formed at the same time, for example, by forming a plurality of spaced trenches in a dielectric  14 . Then, in each trench, the layers shown in  FIGS. 1-6  may be deposited and processed in the fashion indicated. As a result, a number of cells may be formed, initially, with a common chalcogenide layer  30 . In some embodiments, the chalcogenide layer may then be singulated so that the chalcogenide layer is no longer continuous across the plurality of cells, but is distinct and constitutes a dot at each cell. Thus, a plurality of cells may be formed along a line, defined by the metal  12  and perpendicular lines may be defined by conductors extending transversely to the length of the metal  12 , contacting the upper surface of the chalcogenide  30  after it has been singulated. 
         [0018]    Programming to alter the state or phase of the material may be accomplished by applying voltage potentials to the address lines, thereby generating a voltage potential across a memory element including a phase change layer  30 . When the voltage potential is greater than the threshold voltages of any select device and memory element, then an electrical current may flow through the phase change layer  30  in response to the applied voltage potentials, and may result in heating of the phase change layer  30  through the action of the heater  20 . 
         [0019]    This heating may alter the memory state or phase of the layer  30 , in one embodiment. Altering the phase or state of the layer  30  may alter the electrical characteristic of memory material, e.g., the resistance or threshold voltage of the material may be altered by altering the phase of the memory material. Memory material may also be referred to as a programmable resistance material. 
         [0020]    In the “reset” state, memory material may be in an amorphous or semi-amorphous state and in the “set” state, memory material may be in a crystalline or semi-crystalline state. The resistance of memory material in the amorphous or semi-amorphous state may be greater than the resistance of memory material in the crystalline or semi-crystalline state. It is to be appreciated that the association of reset and set with amorphous and crystal line states, respectively, is a convention and that at least an opposite convention may be adopted. Using electrical current, memory material may be heated to a relatively higher temperature to melt and then quenched to vitrify and “reset” memory material in an amorphous state (e.g., program memory material to a logic “0” value). Heating the volume of memory material to a relatively lower crystallization temperature may crystallize or devitrify memory material and “set” memory material (e.g., program memory material to a logic “1” value). Various resistances of memory material may be achieved to store information by varying the amount of current flow and duration through the volume of memory material. 
         [0021]    References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application. 
         [0022]    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.