Abstract:
The present invention is related to methods and apparatus to produce a memory cell or resistance variable material with improved data retention characteristics and higher switching speeds. In a memory cell according to an embodiment of the present invention, silver selenide and a chalcogenide glass, such as germanium selenide (Ge x Se (1−x) ) are combined in an active layer, which supports the formation of conductive pathways in the presence of an electric potential applied between electrodes. Advantageously, embodiments of the present invention can be fabricated with relatively wide ranges for the thicknesses of the silver selenide and glass layers.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention is generally related to memory technology. In particular, the present invention relates to memory devices formed using chalcogenide glasses.  
           [0003]    2. Description of the Related Art  
           [0004]    Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM). Many memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices are volatile memories. A volatile memory loses its data when power is removed. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.  
           [0005]    In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), and the like.  
           [0006]    U.S. Pat. No. 6,084,796 to Kozicki, et al., entitled “Programmable metallization cell structure and method of making same,” discloses another type of nonvolatile memory device known as a programmable conductor memory cell or a programmable metallization cell (PMC). U.S. Pat. No. 6,084,796 is herein incorporated by reference in its entirety. Such memory cells can be integrated into a memory device, which is known as a programmable conductor random access memory (PCRAM). Additional applications for a programmable metallization cell include use as a programmable resistance and a programmable capacitance.  
           [0007]    One conventional technique for producing the programmable conductor memory cell applies silver (Ag) photodoping to a chalcogenide glass, such as germanium selenide (Ge x Se (1−x) ). As reported by Mitkova, et al, in “Dual Chemical Role of Ag as an Additive in Chalcogenide Glasses,” Physical Review Letters, Vol. 83, no. 19 (Nov. 8, 1999), pp. 3848-3851, silver (Ag) can only be photodoped into glasses of specific stoichiometries that form silver selenide and a new glass stoichiometry backbone. Further, glasses that can be photodoped with silver (Ag) are “floppy” and switch relatively slowly as compared to a rigid glass. Boolchand, et al., in  Onset of Rigidity in Steps in Chalcogenide Glass , Properties and Applications of Amorphous Materials, pp. 97-132, (2001), observes a floppy to rigid transition in Ge x Se (1−x) ) glasses that occurs when x=0.23, where x corresponds to the germanium molar concentration.  
           [0008]    In addition, Mitkova, et al., found that glasses that fall within the stoichiometry range defined by region II of FIG. 1 of the Mitkova reference do not form silver selenide when doped with silver (Ag). For example, a rigid glass, such as germanium selenide (Ge 40 Se 60 ) will not form silver selenide when photodoped with silver (Ag) and, as a result, does not function as a memory switch.  
           [0009]    The presence of silver selenide in a GexSe 1 -x glass photodoped with silver (Ag) allows the glass to be used as a memory switch. Glasses used for silver (Ag) incorporation via photodoping are floppy and switch more slowly electrically, and with worse memory retention, than glasses that are rigid. Preferred rigid glasses, e.g., Ge 40 Se 60 , do not form silver selenide when photodoped with silver (Ag). However, the relatively fast switching times and relatively good memory retention occur in a system which incorporates silver selenide and a rigid glass such as Ge 40 Se 60 . What is needed is a technique to form this type of memory cell.  
         SUMMARY OF THE INVENTION  
         [0010]    Embodiments of the invention overcome the disadvantages of the prior art. Embodiments of the invention include processes that advantageously allow the production of resistance variable material memory cells at relatively high rates and with relatively high yields. The resistance variable memory cells further advantageously feature improvements in switching speed, improvements in switching consistency, and improvements in data retention and operational temperature ranges relative to conventional programmable conductor memory cells.  
           [0011]    Advantageously, embodiments of the present invention can be fabricated with relatively wide ranges for the thicknesses of silver chalcogenide and glass layers. Thus, memory cells can be fabricated without the relatively precise control of silver (Ag) and glass thicknesses that are necessary in a conventional photodoping process to maintain an appropriate amount of silver (Ag) in the glass without inducing crystallization in the memory cell. In addition, embodiments of the present invention can advantageously form memory cells on rigid glasses, such as Ge 40 Se 60 , which would normally incorporate silver (Ag) into the glass backbone making it unavailable for memory switching. These glasses have an additional advantage of having higher glass transition temperatures.  
           [0012]    In one embodiment, silver (Ag) is not added directly to germanium selenide (Ge x Se (1−x) ). Thus, adherence of a layer of silver (Ag) to a layer of germanium selenide (Ge x Se (1−x) ) is advantageously not a concern.  
           [0013]    One embodiment according to the present invention includes a memory cell with a layer of a silver chalcogenide and a layer of a chalcogenide glass, such as germanium selenide (Ge x Se (1−x) ). The layers of silver chalcogenide and chalcogenide glass are formed between two electrodes, which are also formed. The electrodes can be formed from materials such as tungsten (W), tungsten nitride (WN), titanium (Ti), and the like. The silver chalcogenide can correspond to a variety of materials, such as silver selenide, silver sulfide, silver telluride, and silver oxide. The chalcogenide glass can correspond to a variety of materials, such as germanium selenide (Ge x Se (1−x) , germanium sulfide (Ge x S (1−x) ) and arsenic selenide (As x Se y ).  
           [0014]    Another embodiment according to the present invention includes a memory cell with a layer of silver (Ag), a layer of chalcogenide glass, such as germanium selenide (Ge x Se (1−x) ), and a layer of silver selenide disposed between two electrodes. In one embodiment, the layers are arranged such that the chalcogenide glass is disposed between the layer of silver (Ag) and the layer of silver selenide. The chalcogenide glass can be selected from a variety of glasses such as Ge 40 Se 60  and Ge 25 Se 75 . In one embodiment, the silver selenide is slightly poor in silver (Ag) and the presence of silver (Ag) in the silver (Ag) layer allows the memory cell to function as intended.  
           [0015]    Another embodiment according to the present invention includes a memory cell with co-deposited silver selenide and germanium selenide (Ge x Se (1−x) ). The memory cell can correspond to non-volatile memories or to volatile memories.  
           [0016]    One embodiment according to the present invention is a process of fabricating a memory. The process forms an active layer on a bottom electrode. The process forms the active layer, which includes a silver chalcogenide, such as silver selenide, and a selenium-including glass, such as germanium selenide, substantially in the absence of an ultraviolet (UV) photodoping step. The process also forms a top electrode layer such that a voltage applied between the top electrode layer and the bottom electrode layer creates a conductive pathway between the two electrodes, or disrupts a conductive pathway that had connected the two electrodes.  
           [0017]    Another embodiment according to the present invention includes a physical vapor deposition (PVD) process of fabricating an active layer in a memory cell. The PVD process fabricates the active layer by co-depositing silver chalcogenide, such as silver selenide, and a chalcogenide glass, such as germanium selenide (Ge x Se (1−x) ), on a bottom electrode at substantially the same time. The process forms a top electrode layer on the active layer such that a voltage or difference in electric potential applied between the top electrode layer and the bottom electrode can form or disrupt a conductive pathway within the active layer.  
           [0018]    Another embodiment according to the present invention includes a deposition process to form an active layer in a substrate assembly by forming a layer of a chalcogenide glass and forming a layer of silver selenide. The layers are disposed between a top electrode layer and a bottom electrode layer. In one embodiment, the chalcogenide glass is germanium selenide (Ge x Se (1−x) ), and there are no other sources of silver (Ag) other than silver selenide. In another embodiment, the chalcogenide glass is germanium selenide (Ge x Se (1−x) ) and at least one of the electrodes is silver (Ag).  
           [0019]    Another embodiment according to the present invention includes a process that forms an active layer of a memory cell by forming a layer of both germanium selenide (Ge x Se (1−x) ) and silver (Ag) and forming a layer of silver selenide. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    These and other features of the invention will now be described with reference to the drawings summarized below. These drawings and the associated description are provided to illustrate preferred embodiments of the invention and are not intended to limit the scope of the invention.  
         [0021]    [0021]FIG. 1 illustrates a process according to an embodiment of the invention of forming an active layer by layering silver selenide and a chalcogenide glass.  
         [0022]    [0022]FIG. 2 illustrates a process according to an embodiment of the invention of forming an active layer by co-depositing silver selenide and a chalcogenide glass.  
         [0023]    [0023]FIG. 3 illustrates a process according to an embodiment of the invention of forming an active layer by depositing a layer of germanium selenide (Ge x Se (1−x) ) and silver (Ag) and a layer of silver selenide.  
         [0024]    [0024]FIG. 4 illustrates a memory cell with an active layer formed by layering silver selenide and a chalcogenide glass.  
         [0025]    [0025]FIG. 5 illustrates a memory cell with an active layer formed by co-depositing silver selenide and a chalcogenide glass.  
         [0026]    [0026]FIG. 6 illustrates a memory cell with an active layer formed by layering silver (Ag), layering a chalcogenide glass, and layering silver selenide. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    Although this invention will be described in terms of certain preferred embodiments, other embodiments that are apparent to those of ordinary skill in the art, including embodiments which do not provide all of the benefits and features set forth herein, are also within the scope of this invention. Accordingly, the scope of the invention is defined only by reference to the appended claims.  
         [0028]    While illustrated in the context of silver selenide and germanium selenide, the skilled artisan will appreciate that the principles and advantages described herein are applicable to other types of silver chalcogenides and chalcogenide glasses. For example, other silver chalcogenides include silver sulfide, silver telluride, and silver oxide. Another chalcogenide glass includes arsenic selenide (As x Se y ).  
         [0029]    Applicant has discovered that regions of silver selenide within germanium selenide (Ge x Se (1−x) ) are the source of the memory switching characteristic of silver (Ag) ultraviolet (UV) photodoped germanium selenide glasses in a resistance variable material memory cell. Mitkova, et al., observed with Modulated Differential Scanning Calorimetry (MDSC) experiments that silver (Ag) photodoping of germanium selenide (Ge x Se (1−x) ) glasses of glass forming region I (selenide rich glasses) resulted in phase separation of silver selenide and a new stoichiometry of germanium selenide (Ge y Se (1−y) ) backbone.  
         [0030]    [0030]FIG. 1 illustrates a process  100  according to an embodiment of the invention of forming an active layer for a memory cell by layering silver selenide and a chalcogenide glass. The term “silver selenide,” as used herein, includes stoichiometric silver selenide (Ag 2 Se), silver-rich silver selenide (Ag 2+x Se), and silver-poor silver selenide (Ag 2−x Se). The term “chalcogenide glass,” as used herein, includes glasses with an element from group VIA (or group  16 ) of the periodic table. Group VIA elements include sulfur (S), selenium (Se), tellurium (Te), polonium (Po), and oxygen (O). In one embodiment, the process advantageously eliminates the UV photodoping step.  
         [0031]    The process can be applied to a broad variety of substrate assemblies. Advantageously, many configurations for a resistance variable material cell, such as the “PROM configured MDM” described by Kozicki, et al., in U.S. Pat. No. 6,084,796, do not require local transistors as part the storage element and can thus be formed on a variety of substrates and not just semiconductors. For example, a resistance variable material cell can be formed on other materials such as a plastic substrate. The substrate assembly should be electrically insulating so that a difference in electric potential can be applied between electrodes to form or to disrupt a conductive pathway in the cell. Where the substrate assembly is not intrinsically insulating, the process can also form an insulating layer, such as a layer of silicon oxide (SiO 2 ), to electrically insulate the resistance variable material cell. In one embodiment, the substrate assembly is silicon to facilitate the integration of the fabricated memory cell with electronic devices such as switches or transistors.  
         [0032]    The process forms  110  a conductive film on the substrate assembly to form a first electrode of the memory cell. The material used to form the conductive film can be selected from a variety of conductive materials. In one embodiment, the process deposits tungsten (W) as the first electrode. The process advances from forming  110  the first electrode to forming  120  a silver selenide layer.  
         [0033]    The process forms  120  a film or layer of silver selenide onto the first electrode. In the process illustrated by FIG. 1, the process first forms  120  a silver selenide layer and then forms  130  a germanium selenide (Ge x Se (1−x) ) layer. It will be understood by one of ordinary skill in the art that in another embodiment, the process first forms  130  a germanium selenide layer (Ge x Se (1−x) ) and then forms  120  a silver selenide layer. A variety of processes can be used to form  120  the layer of silver selenide. Preferably, physical vapor deposition (PVD) techniques, such as evaporative deposition and sputtering, are used to form  120  the layer of silver selenide. Other processes, such as chemical vapor deposition (CVD), co-evaporation, and depositing a layer of selenide (Se) above a layer of silver (Ag) to form silver selenide can also be used.  
         [0034]    Advantageously, silver selenide is directly deposited, thereby obviating the need to photodope the substrate with UV radiation. Of course, UV photodoping can still be used. Where UV photodoping is still used, the direct forming of a layer of silver selenide can still advantageously reduce the intensity and/or the duration of the applied UV radiation. Further advantageously, since light does not need to shine on the silver selenide layer, the silver selenide layer can be formed  120  prior to the forming  130  of the chalcogenide layer as shown in FIG. 1. The process advances from forming  120  the silver selenide layer to forming  130  the chalcogenide layer.  
         [0035]    The process forms  130  a layer of a chalcogenide glass. For example, the chalcogenide glass can be germanium selenide (Ge x Se (1−x) ), arsenic selenide (As 2 Se 3 ), and the like. Preferably, the chalcogenide glass formed is germanium selenide (Ge x Se (l−x) ). In one embodiment, x is in a range of about 0.2 to about 0.43. An exemplary chalcogenide glass is Ge 40 Se 60 .  
         [0036]    Preferably, the process forms  120  the silver selenide layer, and the process forms  130  the layer of germanium selenide (Ge x Se (1−x) ) such that the silver selenide layer is between about 300 to 1000 Angstroms (Å) thick, and such that the germanium selenide (Ge x Se (1−x) layer is between about 200 to 1000 Å thick. In one embodiment, the silver selenide layer is about 400 Å thick, and the germanium selenide (Ge x Se (1−x) layer is a layer of Ge 40 Se 60  that is about 250 Å thick.  
         [0037]    The process forms  140  a second electrode of the resistance variable material cell, and the process ends. It will be understood by one of ordinary skill in the art that the first electrode and the second electrode can correspond to, for example, a top electrode and a bottom electrode, respectively, or to side electrodes. The layer of silver selenide formed  120  by the process and the layer of chalcogenide glass formed  130  by the process are disposed between the first electrode and the second electrode. When an electric potential is applied between the first electrode and the second electrode, a conductive pathway is formed or is disrupted in the layer of silver selenide and the layer of chalcogenide glass.  
         [0038]    The formation of the conductive pathway lowers the resistance between the electrodes. The conductive pathway persists after the removal of the applied electric potential. This property can permit some embodiments of a resistance variable material cell to retain information in a nonvolatile manner.  
         [0039]    [0039]FIG. 2 illustrates another process  200  according to an embodiment of the invention of forming an active layer for a memory cell. In the illustrated process, the active layer is formed by depositing silver selenide and a chalcogenide glass substantially in a single step. In one embodiment, the process advantageously eliminates the UV photodoping step.  
         [0040]    The process illustrated in FIG. 2 can also be applied to a broad variety of substrate assemblies as described earlier in connection with FIG. 1. The process forms  210  a conductive film on the substrate assembly to form a first electrode of the memory cell. The material used to form the conductive film can be selected from a variety of conductive materials as described earlier in connection with FIG. 1. The process advances from forming  210  the first electrode to forming  220  an active layer.  
         [0041]    The process forms  220  the active film in which a conductive pathway forms and/or is disrupted. The illustrated process co-deposits  220  silver selenide and a chalcogenide glass to form  220  the active layer. In one embodiment, physical vapor deposition (PVD) techniques such as evaporative deposition, sputtering, and the like, are used to form  220  the active layer. The chalcogenide glass can include materials such as germanium selenide (Ge x Se (1−x) ), arsenic selenide (As 2 Se 3 ), and the like. In one embodiment, the chalcogenide glass is germanium selenide (Ge x Se (1−x) ), where x is between about 0.2 and about 0.43.  
         [0042]    The thickness of the active layer formed by the process can vary in a relatively broad range. Preferably, the process forms  220  the active layer to a thickness between about 500 Å and about 2000 Å. More preferably, the process forms  220  the active layer to a thickness between about 500 Å and about 700 Å. In one example, the process forms  220  the active layer to a thickness of about 500 Angstroms (Å).  
         [0043]    Advantageously, the illustrated process can form an active layer without silver (Ag) photodoping with UV radiation. In another embodiment, UV photodoping is still used. The process advances from forming  220  the active layer to forming  230  a second electrode.  
         [0044]    The process forms  230  a conductive film on the substrate assembly to form a second electrode of the memory cell, and the process ends. The active layer formed  220  is disposed between the first electrode and the second electrode. When an electric potential is applied between the first electrode and the second electrode, a conductive pathway is formed or disrupted depending on the polarity of the applied electric potential. The formation and/or disruption of the conductive pathway is stable and can be detected as a change in impedance.  
         [0045]    [0045]FIG. 3 illustrates a process  300  according to an embodiment of the invention of forming an active layer of a memory cell by depositing a layer of germanium selenide (Ge x Se (1−x) ) and silver (Ag), and a layer of silver selenide. In one embodiment, x is in a range of about 0.2 to about 0.43.  
         [0046]    The process forms  310  a conductive film on a substrate assembly to form a first electrode of the memory cell. The material used to form the conductive film can be selected from a variety of conductive materials. In one embodiment, the process deposits tungsten (W) as the first electrode. The process advances from forming  310  the first electrode to forming  320  a film or layer of germanium selenide (Ge x Se (1−x) ) and silver (Ag).  
         [0047]    The process forms  320  the layer(s) of germanium selenide (Ge x Se (1−x) ) and silver (Ag) onto the first electrode. The process can form the layer(s) of germanium selenide (Ge x Se (1−x) ) and silver (Ag) in one layer or as separate layers. In one embodiment, the process co-deposits germanium selenide (Ge x Se (1−x) ) and silver (Ag) to form  320  the layer. In another embodiment, the process forms  320  the layer(s) of germanium selenide (Ge x Se (1−x) ) and silver (Ag) by depositing separate layers of germanium selenide (Ge x Se (1−x) ) and silver (Ag). One embodiment according to the invention forms a relatively thin layer of silver (Ag), and then forms the layer of germanium selenide (Ge x Se (1−x) ). In one embodiment, the relatively thin layer of silver (Ag) is about 50 Å thick. A layer of silver selenide should not be formed adjacent to the relatively thin layer of silver (Ag). Preferably, the process forms  320  the film or layer of germanium selenide (Ge x Se (1−x) ) and silver (Ag) to a thickness between about 250 Å and 1000 Å.  
         [0048]    In the process illustrated by FIG. 3, the process forms  320  the layer(s) of both germanium selenide (Ge x Se (1−x) ) and silver (Ag) and then forms  330  a layer of silver selenide. Preferably, the process forms  330  the layer of silver selenide to a thickness between about 300 Å and about 1000 Å. It will be understood by one of ordinary skill in the art that in another embodiment, the process first forms  330  the silver selenide layer and then forms  320  the layer(s) of germanium selenide (Ge x Se (1−x) ) and silver (Ag). In addition, the deposition of the relatively thin film of silver (Ag) advantageously allows the silver selenide layer to be formed slightly silver-poor because an extra amount of silver (Ag) is available to the memory cell.  
         [0049]    A variety of processes can be used to form  320  the layer(s) of germanium selenide (Ge x Se (1−x) ) and silver (Ag). Preferably, physical vapor deposition (PVD) techniques, such as evaporative deposition and sputtering, are used to form  320  the layer of germanium selenide (Ge x Se (1−x) ) and silver (Ag). Other processes, such as chemical vapor deposition (CVD) and co-evaporation can also be used. The process advances from forming  320  the layer of germanium selenide (Ge x Se (1−x) ) and silver (Ag) to forming  330  a layer of silver selenide.  
         [0050]    The process forms  330  a layer of a silver selenide. The layer of silver selenide should be formed on the germanium selenide (Ge x Se (1−x) ) layer or on a co-deposited layer of silver (Ag) and germanium selenide (Ge x Se (1−x) ), but not directly on a silver (Ag) layer. Advantageously, silver selenide is directly deposited and a UV photodoping step is not needed.  
         [0051]    The process forms  340  a second electrode of the memory cell, and the process ends. It will be understood by one of ordinary skill in the art that the first electrode and the second electrode can correspond to, for example, a top electrode and a bottom electrode, respectively, or to side electrodes. The layer(s) of germanium selenide (Ge x Se (1−x) ) and silver (Ag) formed  320  by the process and the layer of silver selenide formed  330  by the process are disposed between the first electrode and the second electrode. When an electric potential is applied between the first electrode and the second electrode, a conductive pathway is formed or is disrupted in the layer of silver selenide and the layer(s) of germanium selenide (Ge x Se (1−x) ) and silver (Ag).  
         [0052]    The stored information can correspond to programmable resistances and to binary data storage. In one embodiment, where the memory cell stores binary data, a first state corresponds to a relatively low resistance between the electrodes and a second state corresponds to a relatively high resistance between the electrodes. In addition, the polarity of the electrodes can be reversed to alter the conductive pathway, thereby allowing the memory cell to be erased and reprogrammed.  
         [0053]    [0053]FIG. 4 illustrates one embodiment according to the present invention of a memory cell  400  with an active layer formed by layering silver selenide and a chalcogenide glass. The illustrated memory cell  400  includes a first electrode  402 , a first body layer  404 , a second body layer  406 , an insulator  408 , and a second electrode  410 .  
         [0054]    The first electrode  402  is formed on and in contact with a substrate assembly. In one embodiment, the substrate assembly is silicon, and the first electrode  402  is coupled to a conductor such as a crosspoint so that the memory cell  400  can be programmed and read. The skilled artisan will appreciate that the memory cell  400  can be formed on a variety of substrate materials and not just semiconductors such as silicon. For example, the memory cell  400  can be formed on a plastic substrate. The first electrode  402  can be made from a variety of materials and from combinations of materials. For example, the first electrode  402  can be made from tungsten (W), tungsten nitride (WN), polysilicon, and the like.  
         [0055]    When the memory cell  400  is fabricated, the first body layer  404  and the second body layer  406  form a body of the memory cell  400 . The first body layer  404  is formed on the first electrode  402 , and the second body layer  406  is formed on the first body layer  404 .  
         [0056]    In the illustrated embodiment, the first body layer  404  is a layer of silver selenide and the second body layer  406  is a layer of a chalcogenide glass such as germanium selenide (Ge x Se (1−x) ). In another embodiment, the first body layer  404  is the layer of chalcogenide glass and the second body layer  406  is the layer of silver selenide.  
         [0057]    In the illustrated embodiment, the insulator  408  surrounds the body formed by the first body layer  404  and the second body layer  406 . The insulator  408  insulates the body from the bodies of other memory cells and also prevents the undesired diffusion of active material. The insulator  408  can be formed from a variety of materials such as silicon nitride (Si 3 N 4 ). Of course, the insulator  408  can be formed in multiple steps and can include multiple structures.  
         [0058]    The second electrode  410  is formed on the second body layer  406  and on the insulator  408 . The second electrode  410  can be formed from a variety of materials such as silver (Ag), titanium (Ti), tungsten (W), tungsten nitride (WN), and the like. An electric potential applied between the first electrode  402  and the second electrode  410  causes the formation or alteration of conductive pathways in the body of the memory cell  400 .  
         [0059]    [0059]FIG. 5 illustrates one embodiment according to the present invention of a memory cell  500  with an active layer formed by co-depositing silver selenide and a chalcogenide glass. The illustrated memory cell  500  includes a first electrode  502 , an active layer  506 , an insulator  508 , and a second electrode  510 .  
         [0060]    The first electrode  502  is formed on a substrate assembly. The substrate assembly can correspond to a variety of materials including plastic and silicon. Preferably, the first electrode  502  is coupled to a conductor such as a crosspoint so that the memory cell  500  can be programmed and read. The first electrode  502  can be made from a variety of materials and from combinations of materials.  
         [0061]    The active layer  506  is formed on the first electrode  502 . In the illustrated embodiment, the active layer  506  is a co-deposited layer of silver selenide and a chalcogenide glass such as germanium selenide (Ge x Se (1−x) ).  
         [0062]    In the illustrated embodiment, the insulator  508  surrounds the active layer  506 . The insulator  508  insulates the active layer  506  from other memory cells and also prevents the undesired diffusion of active material. The insulator  508  can be formed from a variety of materials such as silicon nitride (Si 3 N 4 ).  
         [0063]    The second electrode  510  is formed on the active layer  506  and on the insulator  508 . The second electrode  510  can be formed from a variety of materials such as silver (Ag), titanium (Ti), tungsten (W), tungsten nitride (WN), and the like. An electric potential applied between the first electrode  502  and the second electrode  510  causes conductive pathways in the active layer  506  to form or to disrupt in response to the applied electric potential.  
         [0064]    [0064]FIG. 6 illustrates one embodiment according to the present invention of a memory cell  600  with an active layer formed by layering silver (Ag), layering a chalcogenide glass, and layering silver selenide. The illustrated memory cell  600  includes a first electrode  602 , a first body layer  603 , a second body layer  604 , a third body layer  606 , an insulator  608 , and a second electrode  610 .  
         [0065]    The first electrode  602  is formed on and in contact with a substrate assembly. In one embodiment, the substrate assembly is silicon, and the first electrode  602  is coupled to a conductor such as a crosspoint so that the memory cell  600  can be programmed and read. The first electrode  602  can be made from a variety of materials and from combinations of materials such as tungsten (W), tungsten nitride (WN), titanium (Ti), and the like.  
         [0066]    When the memory cell  600  is fabricated, the first body layer  603 , the second body layer  604 , and the third body layer  606  form a body of the memory cell  600 . The first body layer  603  is formed on the first electrode  602 , the second body layer  604  is formed on the first body layer  603 , and the third body layer  606  is formed on the second body layer  604 .  
         [0067]    In the illustrated embodiment, the first body layer  603  is a layer of silver (Ag), the second body layer  604  is a layer of a chalcogenide glass such as germanium selenide (Ge x Se (1−x) ), and the third body layer  606  is a layer of silver selenide. In another embodiment, the first body layer  603  is the layer of silver selenide, the second body layer  604  is the layer of chalcogenide glass, and the third body layer  606  is the layer of silver (Ag).  
         [0068]    In the illustrated embodiment, the insulator  608  surrounds the body formed by the first body layer  603 , the second body layer  604 , and the third body layer  606 . The insulator  608  insulates the body from the bodies of other memory cells and also prevents the undesired diffusion of active material. The insulator  608  can be formed from a variety of materials such as silicon nitride (Si 3 N 4 ). Of course, the insulator  608  can be formed in multiple steps and can include multiple structures.  
         [0069]    The second electrode  610  is formed on the third body layer  606  and on the insulator  608 . The second electrode  610  can be formed from a variety of materials such as tungsten (W). An electric potential applied between the first electrode  602  and the second electrode  610  causes the formation or alteration of conductive pathways in the body of the memory cell  600 .  
         [0070]    Various embodiments of the invention have been described above. Although this invention has been described with reference to these specific embodiments, the descriptions are intended to be illustrative of the invention and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined in the appended claims.