Abstract:
A memory device with improved heat transfer characteristics. The device first includes a dielectric material layer; first and second electrodes, vertically separated and having mutually opposed contact surfaces. A phase change memory element is encased within the dielectric material layer, including a phase-change layer positioned between and in electrical contact with the electrodes, wherein the lateral extent of the phase change layer is less than the lateral extent of the electrodes. An isolation material is positioned between the phase change layer and the dielectric layer, wherein the thermal conductivity of the isolation material is lower than the thermal conductivity of the dielectric material.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a divisional of currently pending U.S. patent application Ser. No. 11/338,284 filed on 24 Jan. 2006, which application claims the benefit of U.S. Provisional Patent Application No. 60/736,721 filed on 15 Nov. 2005, both of which are incorporated by reference for all purposes. 
    
    
     PARTIES TO A JOINT RESEARCH AGREEMENT 
     International Business Machines Corporation, a New York corporation; Macronix International Corporation, Ltd., a Taiwan corporation; and Infineon Technologies A.G., a German corporation, are parties to a Joint Research Agreement. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to high density memory devices based on phase change based memory materials, including chalcogenide based materials and other materials, and to methods for manufacturing such devices, and most particularly to a phase change memory element having lowered current and improved heat control characteristics. 
     2. Description of Related Art 
     Phase change based memory materials are widely used in nonvolatile random access memory cells. Such materials, such as chalcogenides and similar materials, can be caused to change phase between an amorphous state and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher resistivity than the generally crystalline state, which can be readily sensed to indicate data. 
     The change from the amorphous to the crystalline state is generally a low current operation. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process, allowing at least a portion of the phase change structure to stabilize in the amorphous state. It is desirable to minimize the magnitude of the reset current used to cause transition of phase change material from a crystalline state to amorphous state. The magnitude of the reset current needed for reset can be reduced by reducing the size of the phase change material element in the cell and of the contact area between electrodes and the phase change material, so that higher current densities are achieved with small absolute current values through the phase change material element. 
     One direction of development has been toward forming small pores in an integrated circuit structure, and using small quantities of programmable resistive material to fill the small pores. Patents illustrating development toward small pores include: Ovshinsky, “Multibit Single Cell Memory Element Having Tapered Contact,” U.S. Pat. No. 5,687,112, issued 11 Nov. 1997; Zahorik et al., “Method of Making Chalogenide [sic] Memory Device,” U.S. Pat. No. 5,789,277, issued 4 Aug. 1998; Doan et al., “Controllable Ovonic Phase-Change Semiconductor Memory Device and Methods of Fabricating the Same,” U.S. Pat. No. 6,150,253, issued 21 Nov. 2000. 
     A particular problem encountered in the prior art has been controlling the operating current, and the heat generated by that current. The phase change process proceeds by joule heating of the phase change material, which produces two problems. First is controlling the current required by a memory unit that has billions of individual memory cells (as a unit offering storage capacity in the gigabyte range clearly does). Second, the heat generated by that number of cells has the potential at least to degrade a memory unit, if not destroy it altogether. 
     It is desirable therefore to provide a memory cell structure having a reduced heat profile and low reset current, and a method for manufacturing such structure that meets tight process variation specifications needed for large-scale memory devices. It is further desirable to provide a manufacturing process and a structure, which are compatible with manufacturing of peripheral circuits on the same integrated circuit. 
     SUMMARY OF THE INVENTION 
     An aspect of the invention is a memory device with improved heat transfer characteristics. The device first includes a dielectric material layer; first and second electrodes, vertically separated and having mutually opposed contact surfaces. A phase change memory element is encased within the dielectric material layer, including a phase-change layer positioned between and in electrical contact with the electrodes, wherein the lateral extent of the phase change layer is less than the lateral extent of the electrodes. An isolation material is positioned between the phase change layer and the dielectric layer, wherein the thermal conductivity of the isolation material is lower than the thermal conductivity of the dielectric material. The isolation layer isolates the material of the phase change layer from etching processes which may damage or undercut the material. In embodiments described herein, the isolation material also improves thermal isolation of the phase change material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the phase change memory element of the present invention. 
         FIG. 2  illustrates a random access memory cell including a phase change memory element of the present invention. 
         FIG. 3  illustrates an initial step in the fabrication of the random access memory cell of the present invention. 
         FIG. 4  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 5  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 6  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 7  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 8  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 9  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 10  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 11  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 12  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 13  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 14  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 15  illustrates a further step in the fabrication of the random access memory cell of the present invention. 
         FIG. 16  illustrates an alternative embodiment of the random access memory cell of the present invention. 
         FIG. 17  illustrates another alternative embodiment of the random access memory cell of the present invention. 
         FIG. 18  illustrates a further alternative embodiment of the random access memory cell of the present invention. 
         FIG. 19  illustrates an initial step in the fabrication of the random access memory cell shown in  FIG. 18 . 
         FIG. 20  illustrates a further step in the fabrication of the random access memory cell shown in  FIG. 18 . 
         FIG. 21  illustrates a further step in the fabrication of the random access memory cell shown in  FIG. 18 . 
         FIG. 22  illustrates a further step in the fabrication of the random access memory cell shown in  FIG. 18 . 
         FIG. 23  illustrates a further step in the fabrication of the random access memory cell shown in  FIG. 18 . 
         FIG. 24  illustrates a further step in the fabrication of the random access memory cell shown in  FIG. 18 . 
         FIG. 25  illustrates a further step in the fabrication of the random access memory cell shown in  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is made with reference to the figures. Preferred embodiments are described to illustrate the present invention, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a variety of equivalent variations on the description that follows. 
     With regard to directional descriptions herein, the orientation of the drawings establish their respective frames of reference, with “up,” “down,” “left” and “right” referring to directions shown on the respective drawings. Similarly, “thickness” refers to a vertical dimension and “width” to the horizontal. These directions have no application to orientation of the circuits in operation or otherwise, as will be understood by those in the art. 
     There follows a description of the phase change element and memory cell of the present invention, after which the process for fabricating them are discussed. 
       FIG. 1  depicts phase change memory element  10 , an embodiment of the present invention. As seen there, the phase change element extends between two electrodes, a plug element  14  and a top electrode, or bit line,  30 . As is known in the art, memory cells are often controlled via two sets of data lines, arranged orthogonally. Word lines are energized to select a specific data word address, while bit lines select a specific bit within that word. Those lines are conventionally laid out in perpendicular arrays. The memory cell of which the present invention is a part can be configured it that layout, with bit line  30  forming the top level of the phase change element, and it is shown extending laterally across the drawing. Word line  16  lies perpendicular to that element. The word line signal connects the plug  14  to a current path used for reading and writing. The conductive material of the plug, bit line and word line can be tungsten, copper, aluminum or other materials and combinations deemed suitable in the art for plug and lines structures generally. 
     At the upper end of the plug element are formed electrode conductive layer  18  and lower conductive layer  20 . The reason for having two such layers is set out more fully below. These layers require three characteristics: First, excellent adhesion to the phase change material discussed below; second, good electrical conductivity; and finally, diffusion barrier characteristics, particularly against diffusion of phase change material into the electrode metals. These layers preferably are made up of TiN or TaN. Alternatively, the conductive layers may be TiAlN or TaAlN, or may comprise, for further examples, one or more elements selected from the group consisting of Ti, W, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, and Ru and combinations thereof. The conductive layers preferably extend across the width of the plug element. 
     Above the conductive layers  18 ,  20  is a film of phase change material  22 , composed preferably of a chalcogenide. Chalcogens include any of the four elements oxygen (O), sulfur (S), selenium (Se), and tellurium (Te), forming part of group VI of the periodic table. Chalcogenides comprise compounds of a chalcogen with a more electropositive element or radical. Chalcogenide alloys comprise combinations of chalcogenides with other materials such as transition metals. A chalcogenide alloy usually contains one or more elements from group IV of the periodic table of elements, such as germanium (Ge) and tin (Sn). Often, chalcogenide alloys include combinations including one or more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag). Many phase change based memory materials have been described in technical literature, including alloys of: Ga/Sb, In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In the family of Ge/Sb/Te alloys, a wide range of alloy compositions may be workable. The compositions can be characterized as Te a Ge b Sb 100−(a+b) . One researcher has described the most useful alloys as having an average concentration of Te in the deposited materials well below 70%, typically below about 60% and ranged in general from as low as about 23% up to about 58% Te and most preferably about 48% to 58% Te. Concentrations of Ge were above about 5% and ranged from a low of about 8% to about 30% average in the material, remaining generally below 50%. Most preferably, concentrations of Ge ranged from about 8% to about 40%. The remainder of the principal constituent elements in this composition was Sb. These percentages are atomic percentages that total 100% of the atoms of the constituent elements. (Ovshinsky &#39;112 patent, cols. 10-11.) Particular alloys evaluated by another researcher include Ge 2 Sb 2 Te 5 , GeSb 2 Te 4  and GeSb 4 Te 7 . (Noboru Yamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks for High-Data-Rate Recording”,  SPIE  v. 3109, pp. 28-37 (1997).) More generally, a transition metal such as chromium (Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum (Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te to form a phase change alloy that has programmable resistive properties. Specific examples of memory materials that may be useful are given in Ovshinsky &#39;112 at columns 11-13, which examples are hereby incorporated by reference. 
     Phase change alloys are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase, and a second structural state in which the material is in a generally crystalline solid phase in its local order in the active channel region of the cell. These alloys are at least bistable. The term amorphous is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has the detectable characteristics such as higher electrical resistivity than the crystalline phase. The term crystalline is used to refer to a relatively more ordered structure, more ordered than in an amorphous structure, which has detectable characteristics such as lower electrical resistivity than the amorphous phase. Typically, phase change materials may be electrically switched between different detectable states of local order across the spectrum between completely amorphous and completely crystalline states. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material may be switched either into different solid phases or into mixtures of two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states. The electrical properties in the material may vary accordingly. 
     Phase change alloys can be changed from one phase state to another by application of electrical pulses. It has been observed that a shorter, higher amplitude pulse tends to change the phase change material to a generally amorphous state. A longer, lower amplitude pulse tends to change the phase change material to a generally crystalline state. The energy in a shorter, higher amplitude pulse is high enough to allow for bonds of the crystalline structure to be broken and short enough to prevent the atoms from realigning into a crystalline state. Appropriate profiles for pulses can be determined, without undue experimentation, specifically adapted to a particular phase change alloy. In following sections of the disclosure, the phase change material is referred to as GST, and it will be understood that other types of phase change materials can be used. A material useful for implementation of a PCRAM described herein is Ge 2 Sb 2 Te 5 . 
     Other programmable resistive memory materials may be used in other embodiments of the invention, including N 2  doped GST, Ge x Sb y , or other material that uses different crystal phase changes to determine resistance; Pr x Ca y MnO 3 , PrSrMnO, ZrO x , or other material that uses an electrical pulse to change the resistance state; TCNQ, PCBM, TCNQ-PCBM, Cu-TCNQ, Ag-TCNQ, C 60 -TCNQ, TCNQ doped with other metal, or any other polymer material that has bistable or multi-stable resistance state controlled by an electrical pulse. 
     The phase change layer  22  of the embodiment shown in  FIG. 1  is a film, preferably having a thickness of from about 10 nm to about 100 nm, most preferably about 40 nm. 
     Above the phase change layer  22  lies a second conductive layer  24 , at least covering the upper surface of the phase change layer. This conductive layer can also formed of TiN, as discussed above. It is convenient to refer to the phase change layer  22  and second conductive layer  24  jointly, which unit hereafter will be referred to as the phase change core  25 . 
     In one embodiment of the invention, the phase change core does not extend completely across the surface of the conductive material. Rather, the phase change material and conductive layer are encased or surrounded by an isolation material  26 . In preferred embodiment, the isolation material has a low thermal conductivity relative to the surrounding dielectric fill  12 . 
     An upper conductive layer  28 , formed of TiN or similar material, as discussed above, lies atop the second conductive layer/isolation assemblage. An embodiment of the invention constructs the plug and subsequently overlying layers as a generally pillar-like structure. As will be seen below, that design allows for efficient fabrication. Bit line  30  lies above and in contact with the upper conductive layer  28 , and it is formed of a suitable metal, as discussed above. 
     Surrounding and encasing the entire phase change memory element  10  is a dielectric fill material  12 . The material preferably consists of one or more layers of silicon oxide or a well-known alternative thereto, such as a silicon oxynitride, doped silicon oxides, polyimide, silicon nitride or other dielectric fill material. 
     In operation, a current path exists between the plug  14  and bit line  30 . In one embodiment, current flows from the bit line, through the phase change element and out through the plug element, but that direction could be reversed in other embodiments. As the current flows though the phase change material, joule heating causes the temperature of the phase change material to rise, and, as explained above, based on the length and amplitude of the current pulse, the element can be placed in a SET or RESET condition. If it is desired only to read the state of the phase change material, a relatively lower current pulse is employed, sufficient to determine the resistance of the material. The isolation layer in embodiments in which it comprises a material with relatively low thermal conductivity, reduces the heat conduction away from the phase change material, and thus keeps its temperature at a higher level. Thus, greater heating is achieved for each unit of current, which allows for faster response time, lower current usage and lower overall heating of the unit. 
     Control of access to the memory cell is provided by control elements, preferably access transistors (not shown). An alternate circuit configuration could employ diodes or similar devices.  FIG. 2  illustrates a preferred arrangement, in which two memory elements,  10   a  and  10   b  in a column share a common source terminal  17 . As can be seen, each of the memory elements is identical in structure to memory element  10  discussed above, including conductive layers  18   a / 18   b ,  20   a / 20   b  and  24   a / 24   b ; phase change layers  22   a / 22   b ; and isolation elements  26   a / 26   b . Bit line  30  makes contact with the upper conductive layers  28   a  and  28   b  of both elements, and each element has an associated word line  16   a / 16   b  disposed close thereto and connected via appropriate circuitry (not shown). A common source line  17  runs through the cell, providing a current path to the sources of the access transistors (not shown). 
     In order to write to memory element  10   b , for example, appropriate enabling signals would be fed to bit line  30  and word line  16   b . As a result, current would flow in the memory cell  10   b , from bit line  30 , through, through the conductive layers  28   b  and  24   b , and into the phase change layer  22   b . There, the current would produce heating, which in turn would cause the material to assume either the crystalline or amorphous phase, based on the amplitude and duration of the current pulse. Reading the memory element would proceed by passing a low level current pulse though the element, sensing the resistance of the phase change element. Current flow proceeds through conductive layers  20   b  and  18   b , plug element  14   b , through the underlying control circuitry and into common source line  17 . 
     A starting point for fabrication of the device of the present invention is seen in  FIG. 3 , showing a point in the fabrication process following the formation of a substrate  13 , consisting of dielectric material  12  and associated features, primarily the plug elements  14   a  and  14   b , word lines  16   a  and  16   b , and source line  17 . Doped regions in the semiconductor substrate (not shown to simplify the drawing) act as terminals of transistors including the word lines  16   a  and  16   b  as gates for coupling the plugs  14   a ,  14   b  to the common source line  17 . These elements are preferably formed in conventional manner, preferably involving deposition of the oxide layer, patterning and etching of the same, and further deposition of the metal elements. 
     A significant problem in the fabrication of phase change elements is the fact that when a metal plug is formed using conventional methods, which most often is chemical vapor deposition or some variant thereof, the deposition pattern typically results in a low area, or dimple, at the center of the plug. Dimple  19 , shown in  FIG. 3 , illustrates that problem. Because a deposited layer generally follows the contour of the surface on which it is deposited, the next layer will unavoidably exhibit some topology, which could easily lead to adhesion or peeling problems. That problem is particularly acute regarding either a directly-applied phase change layer or a phase change layer applied to an intermediate layer. 
     A solution to that problem is presented in  FIG. 4 , which illustrates the formation of a base element  15 , in which electrode conductive layer  18  is deposited over the substrate  13 . After deposition, the conductive layer is planarized, preferably employing chemical-mechanical polishing (CMP), to provide a perfectly planar surface on which to build the remainder of the structure. 
       FIG. 5  depicts the results of the next process step, in which a lower conductive layer  20  is formed on the electrode conductive layer  18 .  FIG. 6  takes the process to the next steps, the deposition of the phase change material  22  and the second conductive layer  24 . Each of these layers may be formed by chemical vapor CVD, or physical vapor deposition (PVD) or a variant thereof. These layers can be from about 10 to about 300 nm thick. The preferred thickness in one embodiment of the invention is 50 nm. The second conductive layer may be formed of TiN or a variant thereof, as discussed above. It is preferred to employ in-situ deposition techniques in depositing layers  20 ,  22  and  24 . That is, the structure is not removed from the processing chamber, nor is that chamber opened between steps, Maintaining the low-pressure processing atmosphere provides an improved interface and bonding between these layers, likewise improving the quality of the device. For this process it is most preferable to employ a PVD sputtering process. An embodiment of the invention further varies the makeup of this element by employing polysilicon, and another embodiment employs tungsten. Those in the art will understand the use of such conventional materials in this application. It is preferred to employ two TiN layers, as shown. The first such layer, after planarization, offers an excellent flat surface for succeeding layers. In one embodiment, the lower conductive layer and the phase change material layer are deposited using PVD cluster tooling equipment, which provides for improved adhesion and deposition characteristics. 
     Next, the layers of material are converted into separate memory elements by a series of lithographic patterning steps. First, seen in  FIG. 7 , mask elements  25   a  and  25   b  are formed on second conductive layer  24 . That is accomplished by the known process of depositing a photoresist material on the structure, exposing the photoresist using a reticle or mask, and stripping away the unexposed portion of resist material to leave the mask elements. If the dimension of the masks is less than the minimum feature size of the lithographic process in use, the mask elements may be deposited conventionally and then trimmed, by timed etching, preferably dry etching of the resulting masks, employing a reactive ion etching (RIE) tool, based on oxygen plasma chemistry. 
     Next, as seen in  FIG. 8 , the mask element is used to perform an etching process to remove the portions of the second conductive layer and phase change layer not under a mask element. The etching process should be conducted to stop when reaching the lower conductive layer  20 . An etching process employed in one embodiment is a dry anisotropic etch using a RIE, utilizing argon fluorine or oxygen plasma chemistry. An optical emission tool may be used to identify and control the end point of the etch, when the TiN layer is encountered. 
     Having removed the phase change material and excess TiN in the second conductive layer,  FIG. 9  shows a layer of isolation material  26  deposited over the structure of  FIG. 8 . 
     Representative materials for the layer of isolation material include materials that are a combination of the elements silicon Si, carbon C, oxygen O, fluorine F, and hydrogen H. Silicon nitride or other materials having higher thermal conductivity than SiO 2  may be used when thermal conductivity is not critical. Examples of thermally insulating materials which are candidates for use for the thermally insulating isolation layer include SiO 2 , SiCOH, polyimide, polyamide, and fluorocarbon polymers, selected so that the material has a lower thermal conductivity that the dielectric fill layer to be deposited over it. When the overlying material is SiO 2 , the thermally insulating material should have a thermal conductivity less than that of SiO 2 , or less than about 0.014 J/cm*degK*sec. Many low-K materials, where low-K materials have permittivity less that that of SiO 2 , are suitable isolations. Examples of materials which are candidates for use for the thermally insulating isolation layer include fluorinated SiO 2 , silsesquioxane, polyarylene ethers, parylene, fluoro-polymers, fluorinated amorphous carbon, diamond like carbon, porous silica, mesoporous silica, porous silsesquioxane, porous polyimide, and porous polyarylene ethers. A single layer or combination of layers can provide thermal isolation. 
     Alternative embodiments employ a second phase change material for this isolation layer, differentiated from the layer  22  in being highly doped, with dopants such as silicon, oxygen or nitrogen. Given these criteria, those in the art can choose from these and other known materials to create an effective isolation layer. This deposition is continued to a depth exceeding that of the two phase change material/conductive layer structures of  22   a / 24   a  and  22   b / 24   b . That structure is planarized, as shown in  FIG. 10 , preferably employing a CMP process, to expose the upper surfaces of second conductive layers  24   a  and  24   b . Planarization is followed by deposition of upper conductive layer  28 , as reflected in  FIG. 11 . This layer is preferably formed of TiN or other suitable materials, as discussed in connection with other conductive layers. 
     The process of defining and isolating phase change memory elements  10   a  and  10   b  proceeds in  FIGS. 12 and 13 . As seen in  FIG. 12 , a layer of photoresist is deposited on the top surface of the upper conductive layer  28 . The layer is patterned and etched to define photoresist films  29   a  and  29   b . The films are formed to have a width equal to the desired width of the eventual phase change memory elements. The patterning step is followed by etching the layers exposed by the photoresist films, as seen in  FIG. 13 , down to the level of the dielectric fill  12 . This etching leaves two isolated structures, phase change memory elements  10   a  and  10   b , each having an electrode and lower conductive layer  18   a / 18   b  and  20   a / 20   b , in contact with plug element  14   a / 14   b , and phase change material  22   a / 22   b  on the upper surface of the lower conductive layer, with a second conductive layer  24   a / 24   b  above the same, the two of which form phase change core  25   a / 25   b . An upper conductive layer  28   a / 28   b  is in contact with the second conductive layer, and both the upper conductive layer and the lower/electrode conductive layer stack extend laterally beyond the phase change core. Into the space between the upper conductive layer and lower conductive layer there extend layers of isolation material  26   a / 26   b  encasing the phase change core on both sides. That structure is defined by the etching step, which preferably is a dry anisotropic etch, which in one embodiment is preferably accomplished with a reactive ion etching (RIE) tool. It is preferred to employ argon, fluorine or chlorine for this etching. Most preferably, a chlorine chemistry is employed for the TiN layer, followed by a switch to fluorine chemistry. 
     Here it should be noted that a preferred isolation material  26  serves not only to provide reduced heat transfer, as discussed above, but during the etching process it also protects the phase change material from action of the etchant. Otherwise, the etching could produce undercutting of the phase change material. The design shown here precludes that result. 
     After etching the phase change and conductive layers, the photoresist is stripped. It is preferred to strip the photoresist, rather than leaving it in place, as the polymer material of the photoresist can be degraded in subsequent steps, producing organic waste material that can be difficult to deal with. The preferred stripping method employs O 2  plasma, which can be followed by a wet-strip using a suitable solvent, such as EKC265, to assist performance. These processes and their employment are well known in the art. 
     The final steps in the process, generally aimed at producing a finished integrated circuit product, are seen in  FIGS. 14 and 15 . First, a deep layer of dielectric fill material  31  is formed, extending from the upper surface of the former dielectric fill layer  12 , to a level above both phase change memory elements  10   a  and  10   b . It is preferred to employ the same material in this layer as was employed in forming layer  12 , but those in the art will understand that it may be advantageous to employ a different material, and those persons will also understand the range of choices available. 
     Following the dielectric deposition, the dielectric layer  31  is planarized to a level that exposes the upper surfaces of the upper conductive layers  28   a  and  28   b , as seen in  FIG. 15 . A CMP process, as known in the art, is preferred for this operation. Then, as shown in  FIG. 1 , bit line  30  is formed of a suitable metal, as discussed above. 
     An alternate embodiment that offers a reduced structural complexity is shown in  FIG. 16 . There, the phase change memory element  200  is made up of only the phase change layer  222 , located between the plug element  14  and bit line  30 . Isolation layer  226  provides thermal isolation for the phase change material. An intermediate structure, phase change element  300 , is seen in  FIG. 17 , which retains the phase change material layer  322  and the insulator layer  326 , but in which two conductive layers  328  and  318  are provided, above and below the phase change layer. Yet another variation is seen in  FIG. 17   a , in which a second conductive layer  324  is added to the structure of phase change element  300 , in a position and having a function tracking the previous discussion of second conductive element  24 . Preferably, this embodiment does not include the upper phase change element  328  of  FIG. 17 . 
     A further embodiment allows a more streamlined fabrication process. The structure of this embodiment is shown in  FIG. 18 . As sent there, a GST base  121  is formed atop lower conductive layer  20 . The second conductive layer  24  of other embodiments is dispensed with, and phase change element  122  extends from the GST base  121  to the upper conductive layer  28 . Function of this embodiment is similar to that of previous embodiments, with current flowing from control circuitry (not shown), through plug element  14 , through the electrode conductive layer  18 , lower conductive layer  20  and GST base  121 , and then through the phase change element  122 . Current then exits the element via upper conductive layer  28 , to the bit line  30 . 
     Fabrication of the embodiment of  FIG. 18  is shown in  FIGS. 19-25 . The initial stages of this process are identical to those of the previous embodiments, through the formation of the substrate and associated elements and deposition of the electrode conductive layer  18  and lower conductive layer  20 . Picking up the process at  FIG. 19 , the GST layer  119  is deposited on the lower conductive layer. This layer is thicker than previous layers, as will be understood from the discussion below. The thickness of this layer preferably is between about 70 nm and 150 nm thick, and most preferably about 100 nm thick. The GST layer is lithographically patterned, as shown in  FIGS. 20 and 21 , to form a GST base, coextensive with the conductive layers, and phase change elements  122   a  and  122   b , in the form of pillar-shaped structures projecting upward from the GST base. As discussed in connection with other lithographic processes, it is preferred to employ a trimming process to form lithographic mask elements having a smaller size than permitted by the minimum feature size of the given process. Unlike previous etching steps, this process does not completely remove the layer being etched. The remaining GST layer servers as a thermal isolation layer between phase change region and bottom electrode and provide better mechanical strength for the memory. Thickness is between 20 and 70 nm, and preferably 50 nm. 
     The isolation material  26  is deposited in the next step ( FIG. 22 ), preferably to a depth completely covering the phase change elements. That material is then removed, preferably employing a CMP process, shown in  FIG. 23 , to expose the upper surfaces of phase change elements  122   a  and  122   b . As in the previous embodiments, an upper electrode layer  28 , preferably composed of TiN, is deposited on the isolation material ( FIG. 24 ). The individual memory elements are formed lithographically, beginning with the deposition of masks  29   a  and  29   b , seen in  FIG. 25 . From that point, the process proceeds with conventional lithographic etching processes, followed by deposition of bit line  30  and additional dielectric fill material  31 , to result in the structure seen in  FIG. 18 . 
     Dimensions set out in regard to prior embodiments are applicable to this embodiment as well, except for the thickness of GST layer. Discussions of materials above are likewise applicable here. 
     Those in the art will understand that a variety of alternatives and embodiments can be fashioned, all lying within the spirit of the invention disclosed herein. The invention itself, however, is defined solely in the claims appended hereto.