Abstract:
A lower electrode may be covered by a protective film to reduce the exposure of the lower electrode to subsequent processing steps or the open environment. As a result, materials that may have advantageous properties as lower electrodes may be utilized despite the fact that they may be sensitive to subsequent processing steps or the open environment.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/020,757, filed Oct. 30, 2001. 
     
    
     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]     In some embodiments, the cell may have a large number of states. That is, because each state may be distinguished by its resistance, a number of resistance determined states may be possible, allowing the storage of multiple bits of data in a single cell.  
         [0006]     A variety of phase change alloys are known. Generally, chalcogenide alloys contain one or more elements from Column VI of the periodic table. One particularly suitable group of alloys is the GeSbTe alloys.  
         [0007]     A phase change material may be formed within a passage or pore through an insulator. The phase change material may be coupled to upper and lower electrodes on either end of the pore.  
         [0008]     One problem that arises with existing lower electrodes is that some suitable lower electrode materials that have advantageous properties cannot be used because they may be adversely affected by necessary subsequent processing steps or upon exposure to the open environment. Among the advantageous attributes of the lower electrode material is good electrical contact to phase change materials and effective resistive heating to promote more efficient phase change programming.  
         [0009]     Thus, there is a need for better designs for phase change memories that may be manufactured using more advantageous techniques. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is an enlarged, cross-sectional view in accordance with one embodiment of the present invention;  
         [0011]      FIG. 2  is an enlarged, cross-sectional view of the device shown in  FIG. 1  taken transversely to the view shown in  FIG. 1 ;  
         [0012]      FIG. 3  is a top plan view of the embodiment shown in  FIGS. 1 and 2 ;  
         [0013]      FIG. 4  is an enlarged cross-sectional view of the initial processing of the structure of  FIG. 1  in accordance with one embodiment of the present invention;  
         [0014]      FIG. 5  shows subsequent processing on the structure shown in  FIG. 4  in accordance with one embodiment of the present invention;  
         [0015]      FIG. 6  shows subsequent processing of the structure shown in  FIG. 5  in accordance with one embodiment of the present invention;  
         [0016]      FIG. 7  shows subsequent processing of the embodiment shown in  FIG. 6  in accordance with one embodiment of the present invention;  
         [0017]      FIG. 8  shows subsequent processing of the embodiment shown in  FIG. 7  in accordance with one embodiment of the present invention; and  
         [0018]      FIG. 9  shows subsequent processing of the embodiment shown in  FIG. 8  in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]     Referring to  FIG. 1 , a phase change memory cell  10  may be formed on a substrate  12  that in one embodiment may be a silicon substrate. A pair of lower electrodes  14  may be formed over the substrate  12 . The electrodes  14  may be separated by an insulator  16 . Furthermore, the electrodes  14  may be covered by a protective film  40 . In some embodiments, an optional base material  42  may be formed over the substrate so that the electrode  14  is sandwiched between the base material  42  on the bottom and the protective film  40  on the top.  
         [0020]     A pore may be formed above the lower electrode  14  between the lower electrode  14  and the top electrode  28 . The pore may include a tapered, cup-shaped phase change material  18  covered by a similarly shaped barrier layer  20 . A fill insulator  22  may fill the central portion of the barrier  20  and the phase change material  18 . An etch stop layer  24  underlies a barrier layer  26  that in turn underlies the top electrode  28 .  
         [0021]     Referring to  FIG. 1 , the top electrode  28  extends along at least two adjacent pores. The pores may be separated by an insulator  16 . Cells defined by the pores may be distributed in large numbers across the substrate  12  in some embodiments. As viewed from above in  FIG. 3 , each electrode  28  covers a plurality of pores including the elements  14 ,  18 ,  20  and  22 , separated by insulator  16  covered by an etch stop layer  24 .  
         [0022]     A technique for forming the memory cells  10 , according to one embodiment, may involve initially forming the lower electrodes  14  on a substrate  12  using conventional patterning and deposition techniques, as shown in  FIG. 4 .  
         [0023]     Referring to  FIG. 4 , a base layer  42  may be deposited on top of the substrate  12  in some embodiments of the present invention. In other embodiments, the base layer  42  may not be utilized. The base layer  42  may be made of material such as cobalt silicide, titanium tungsten or another conductive material.  
         [0024]     The lower electrode  14  may be formed over the base layer  42  if utilized. Finally, a protective film  40  may be formed over the electrode  14 . The lower electrode  14  may be any of a variety of conductive materials including carbon. The protective film  40  may be chosen from a variety of insulating materials including SiO 2 , Si 3 N 4  or A 1   2 O 3 . In general, the protective material may also be any material in the form Si x N y , where x and y represent the stoichiometry and an advantageous stoichiometry is where x is equal to three and y is equal to four.  
         [0025]     The base layer  42 , lower electrode  14  and a protective film  40  may be formed sequentially. Advantageously, the lower electrode  14  and the protective film  40  are formed in situ, for example in the same deposition chamber without venting back to atmosphere.  
         [0026]     Referring to  FIG. 5 , the structure shown in  FIG. 4  may then be subjected to patterning to form the stacks  46   a  and  46   b.  Alternatively, each of the three layers  14 ,  40  and  42  may be separately patterned.  
         [0027]     Referring to  FIG. 6 , the insulator  16  may then be deposited over the patterned lower electrode stacks  46 . In one embodiment, the insulator  16  is an electrical and thermal insulator. One suitable material is silicon dioxide that may be from about 50 to 1500 Angstroms thick in one embodiment. Next a planarization such as, for example, a chemical mechanical planarization (CMP) is performed to achieve global and local planarity. This may be followed by the deposition, if desired, of a CMP etch stop layer  24 . The layer  24  may be silicon nitride or polysilicon having a thickness from 10 to 1000 Angstroms in one embodiment.  
         [0028]     Referring next to  FIG. 7 , the pore openings  32 , defined through the etch stop layer  24  and protective film  40 , receive a side wall spacer  30 . The side wall spacer  30  may be formed using standard techniques of depositing an insulating layer and selectively anisotropically dry etching that layer down to the lower electrode  14 . The insulating spacer  30  may be made of silicon dioxide or nitride such as Si 3 N 4 . The thickness of the insulating spacer  30  may be in the range of 50 to 2000 Angstroms in one embodiment.  
         [0029]     Turning next to  FIG. 8 , deposited in a sequential fashion over the structure shown in  FIG. 7  may be the phase change layer  18 , barrier layer  20 , and fill insulator  22 , in one embodiment. The phase change material  18  may be a chalcogenide-based material such as Ge 2 Sb 2 Te 5  with a thickness of 50 to 1000 Angstroms in one embodiment. The barrier material  20  may be, for example, titanium, titanium nitride or titanium-tungsten, for example, with a thickness in the range of 10 to 500 Angstroms. The fill insulator  22  may be any insulator with low thermal and electrical conductivity. Examples of suitable fill insulator  22  materials include silicon dioxide or silicon nitride, such a Si 3 N 4  with a thickness of about 500 to 10,000 Angstroms, for example.  
         [0030]     Turning finally to  FIG. 9 , CMP removes the fill insulator  22 , barrier layer  20 , and phase change material  18  in all regions above the etch stop layer  24 . CMP thereby defines the structure of the phase change material  18  while eliminating the need for a dry etch in one embodiment. As mentioned earlier, the use of the dry etch may complicate the process flow and raise issues of undercut and re-entrant profiles. Moreover, because the phase change material  18  is defined within an encapsulated, singulated region, the problem of adhesion between the phase change material  18  and the surrounding materials may be substantially reduced or even eliminated, even after exposure to ensuing thermal stresses.  
         [0031]     The imposition of the insulator  22  over the phase change material  18  reduces upward thermal losses. Thermal losses may result in the need for greater programming currents to obtain the same programming effect.  
         [0032]     As shown in  FIG. 1 , the structure of  FIG. 9  may be covered with a barrier layer  26  and a top electrode  28 . In one embodiment, the barrier layer  26  may be titanium, titanium nitride, or titanium-tungsten at a thickness in the range of 10 to 500 Angstroms. The top electrode  28  may be aluminum copper alloy in one embodiment with a thickness in the range of 200 to 20,000 Angstroms. The use of a barrier layer  26  may reduce the incorporation of species from the top electrode  28  into the phase change material  18  in some embodiments. The top electrode  28  and barrier layer  26  may be patterned using standard photolithographic and dry etching techniques to achieve the structures shown in  FIGS. 1, 2 , and  3 .  
         [0033]     In accordance with some embodiments of the present invention, a wider selection of lower electrode  14  material is made available by providing a technique for limiting the exposure of the lower electrode  14  to other process steps or to the open environment. As a result, a purer, less contaminated lower electrode  14  may be achieved in some embodiments, achieving more consistent, predictable device operation.  
         [0034]     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.