Abstract:
An electrode structure including two parallel electrical paths. A plurality of electrode layers, generally tabular in form is formed in a stack, the outermost layers providing electrical contacts, and defining a first electrical current path through the stack. Two sidewall conductor layers are formed to abut either end of the electrode layer stack, two sidewall conductor layers defining a second electrical current path. The ends of the sidewall conduction layers lie in the same planes as the electrode layer electrical contacts, such that electrode structure electrical contacts are each formed from one set of sidewall layer ends and an electrode layer electrical contact.

Description:
PARTIES TO A JOINT RESEARCH AGREEMENT 
     International Business Machines Corporation, a New York corporation; Macronix International Corporation, Ltd., a Taiwan corporation; and Infineon Technologies AG, a German corporation, are parties to a Joint Research Agreement. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of integrated circuit design and more particularly to the field of the design and fabrication of memory devices. 
     2. Description of Related Art 
     Non-volatile memory applications have gained in popularity as the technology has made smaller, reliable units available for storing mass quantities of data. Large external data storage devices gave way to specialized mass storage drives, which in turn have yielded to so-called “memory sticks” and miniature cards, all capable of storing gigabytes of data. 
     At these storage levels, power consumption becomes important. With literally billions of elements, power consumption for each memory storage element must be held to a very low level. The goal of absolute minimization of power usage is critical to the function and capacity of such devices. 
     An important technology in this field is the various forms of Resistance Random Access Memory (RRAM), described in more detail below. U.S. patent application Ser. No. 11/155,067, entitled “Thin Film Fuse Phase Change Ram And Manufacturing Method” and commonly owned by the assignee hereof, discusses such technology in detail and is hereby incorporated herein for all purposes. 
     As shown therein, an important process in the operation of memory elements incorporating such technology is heating interior element rapidly and efficiently. Various methods have been proposed to assist that process, many of which focus on preventing heat loss within the memory element. 
     Obtaining optimum utilization of the power consumption itself has not heretofore been a key aspect of investigation. Memory elements to date have featured conventional means for coupling electrical current to the memory element itself. The art is thus silent on making increased use of such sources for assisting in the heat production of the memory element. 
     SUMMARY OF THE INVENTION 
     An aspect of the invention is an electrode structure including two parallel electrical paths. A plurality of electrode layers, generally tabular in form is formed in a stack, the outermost layers providing electrical contacts, and defining a first electrical current path through the stack. Two sidewall conductor layers are formed to abut either end of the electrode layer stack, two sidewall conductor layers defining a second electrical current path. The ends of the sidewall conduction layers lie in the same planes as the electrode layer electrical contacts, such that electrode structure electrical contacts are each formed from one set of sidewall layer ends and an electrode layer electrical contact. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a - 1   c  depict an embodiment of the invention claimed herein and variations on that embodiment. 
         FIG. 1   d  illustrates schematically the operation of the embodiments of  FIGS. 1   b  and  1   c.    
         FIGS. 2   a  and  2   b  depict an embodiment of a memory element including an embodiment of the invention claimed herein. 
         FIGS. 3   a - 3   e  depict an embodiment of a process for producing an embodiment of the invention claimed herein. 
         FIGS. 4   a  and  4   b  depict an embodiment of a process for producing a further embodiment of the invention claimed herein. 
         FIGS. 5   a  and  5   b  depict an embodiment of a process for producing another embodiment of the invention claimed herein. 
         FIGS. 6   a - 6   e  depict an embodiment of a further process for producing an embodiment of the invention claimed herein. 
     
    
    
     DETAILED DESCRIPTION 
     A detailed description of an electrode structure that provides enhanced thermal properties is provided here, to enable a person of ordinary skill in the art to practice the invention. Embodiments are illustrated in the drawings, as also will be discussed. Those in the art will understand that a number of alternative approaches will be presented, and other approaches will be known by those in the art. The invention itself is defined solely by the claims, as further discussed below. 
       FIG. 1   a  depicts a basic embodiment  10  of an electrode structure. As shown the electrode is generally tabular in form, with contact areas suitable for establishing electrical contact with circuit elements. The electrode includes two layers, a bottom layer  14  and a top layer  12 . It will be understood that the words “bottom” and “top” as used here are employed for convenience of labeling and do not imply any functional importance. The device as shown could be oriented on its side (i.e., rotated 90 degrees from the illustrated position) or flipped over (i.e., rotated 180 degrees) with no change in function. 
     It has been found that constructing an electrode of multiple layers results in reduced heat transfer across the device. This is true whether the layers are formed of different materials or the same material, so that multiple layers result in a lower thermal conductivity than would be exhibited by an equally thick single layer of the same material. Moreover, this phenomenon also gives a designer high flexibility in creating a design that provides exactly the set of characteristics that are required for a particular situation. Here, it is preferred to form the electrode layers of materials chosen to provide suitable thermal and electrical conductivity and resistance. Preferred materials include TiN, TaN, or Ta. It should be noted that a design criterion here is to produce some non-perfect interface that exhibits high resistance and low thermal conductivity. Thus, materials such as TiN, which has good barrier characteristics, are preferred, while Ti, being more active and thus likely to bind with neighboring materials, is not. TiN can be deposited using Chemical Vapor Deposition (CVD) techniques, and all materials can be deposited by Physical Vapor Deposition (PVD), all as known in the art. It is preferred that the layers be deposited from about 0.3 nm to about 20 nm in thickness, most preferably about 5 nm. Thin films have been found to produce the most effective interfaces in the embodiments described. 
     The result of the structure shown in  FIG. 1   a  is an electrode construction that provides the specified electrical current and voltage, but it does not conduct heat as rapidly as does the typical prior art device. The importance of the characteristic is discussed below. 
     As depicted in  FIG. 1   a , two layers of different material are shown. In fact, the number of layers can be varied as desired. The designer can choose a number of layers based on factors such as the total electrical resistance and thermal conductivity desired, the cost or time involved with multiple deposition processes, and other considerations familiar to those in the art. Hereafter, the multi-layer devices will be shown with two layers, but readers will understand that any number of layers are implied by that structure. 
     Additional control over the electrical and thermal properties of an electrode structure  20  is obtained by adding sidewall conductors  26 , as seen in  FIG. 1   b . There, the top and bottom layers  22  and  24  correspond to the similarly-named layers shown in  FIG. 1 , and sidewall conductors  26  spanning both of those layers have been added. Material for the sidewall conductors is Ti in the embodiment shown. The sidewall conductors  26  are positioned so that their ends lie in the same plane as the conductive elements, so that each sidewall conductor offers a current path extending from one electrode to the other, with no material or element transitions in between. It is preferred that the sidewalls be deposited from about 0.3 nm to about 20 in thickness, most preferably about 5 nm. 
     It should be borne in mind that a design may include side conductors or not, as best suits a given application. In the discussion that follows, sidewall conductors are generally shown as being included, but those in the art will understand that such elements are not an essential or necessary part of the invention claimed herein. 
     A further alternative is the addition of thin layers of conductor material to the structure. As shown in  FIG. 1   c , one may construct an electrode  30 , including not only top and bottom electric elements  32  and  34 , and sidewall conductors  36 , but also additional conductor layers  37  and  38 . These layers are composed of materials associated with metallizing layers. For example, copper metallization can be used. Other types of metallization, including aluminum, titanium nitride, and tungsten based materials can be utilized as well. Also, non-metal conductive material such as doped polysilicon can be used. The electrode material in the illustrated embodiment is preferably TiN or TaN. Alternatively, the electrodes 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 alloys thereof. As noted above, material stability is an important design criterion. Thus, most embodiments employ Ta/TaN/TiN/TaSiN. In one embodiment, these characteristics are enhanced by high doping levels of the material. 
     The stabilizing effect of the sidewall conductors can be visualized by reference to  FIGS. 1   c  and  1   d . The stacked layer elements in the central portion of  FIG. 1   c . (layers  32 ,  34 ,  37  and  38  in  FIG. 1   c ) are clearly arranged in a series connection, so that each element added directly to the total resistance. Thus,
 
 RL=R 32+ R 34+ R 37+ R 38  (Eq. 1)
 
     where RL is the resistance of the central portion of the electrode, with the subscripted values being the individual resistance of each layer. The sidewall conductors, however, are arranged in parallel to the layered elements, producing the equivalent circuit shown in  FIG. 1   d , with the layered elements producing a resistance RL and the sidewall conductors a resistance RS. As the arrangement here is a parallel circuit, the resistance calculation is
 
 RE =( RL )( RS )
 
 RL+RS   (Eq. 2)
 
 RE =( RL )( RS )/( RL+RS )  Eq2
 
     As can be seen, the addition of a parallel element has an immediate stabilizing effect, in that the addition of additional elements reduces the resistance depending on the existing values. Again, these changes result in greater design flexibility for the circuit analyst. 
     An application of the embodiment of  FIG. 1   c  is seen in the memory cell  200  shown in  FIG. 2   a , generally comprising an upper electrode  210 , a lower electrode  220  and a memory element  230  lying between and in contact with the electrodes. the electrodes are constructed identically, and thus only upper electrode  210  is labeled. As previously discussed, the elements of the electrodes are top and bottom electrode elements  202  and  204 , two conductor layers  207  and  208 , and sidewall conductors  206 . 
     Between the electrodes lies a memory element  230 , fabricated from a resistance random access memory (RRAM) material. Several materials have proved useful in fabricating RRAM, as described below. 
     An important class of RRAM material is the chalcogenide group. 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 column six 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). Because chalcogenides achieve their dual-memory capabilities by forming two solid phases, each of which exhibits a characteristic resistance, these materials are referred to as “phase change” materials or alloys. 
     Many phase change based memory materials have been described in technical literature, including alloys of: Ge/Sb, 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 Ge2Sb2Te5, GeSb2Te4 and GeSb4Te7. (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. One such material is a colossal magneto resistance (CMR) material, which dramatically change resistance levels in the presence of a magnetic field. Such materials are generally manganese-based perovskite oxides, and the resistance changes encountered are generally in the range of orders of magnitude. A preferred formulation for RRAM applications is Pr x Ca y MnO 3 , where x:y=0.5:0.5, or other compositions in which x:0˜1; y:0˜1. Other CMR materials including an Mn oxide can also be employed. 
     Another RRAM material is a 2-element compound, such as Ni x O y ; Ti x O y ; Al x O y ; W x O y ; Zn x O y ; Zr x O y ; Cu x O y , where x:y=0.5:0.5. Alternatively, another compound in this group could be employed, in which x: 0˜1; y: 0˜1. Also, polymers employing dopants such as Cu, C 60 , Ag can be employed, including 7,7,8,8-tetracyanoquinodimethane (TCNQ), methanofullerene 6,6-phenyl C61-butyric acid methyl ester (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. 
     As previously noted, the electrode elements of  FIG. 2   a  provide the desired levels of electrical voltage and current, but they do not conduct heat away from the RRAM element  230  as rapidly as is seen in the prior art. The RRAM element thus retains a higher proportion of the heat generated by current flow, which reduces the electrical input required to produce a given heat level in the RRAM, which in turn facilitates the changes of state associated with these devices. 
       FIG. 2   b  illustrates a further alternative embodiment  250 , in which the higher resistivity of a multi-layer element  220 , and the concomitant increase in heat generation, is harnessed to provide heat to two RRAM devices,  230  and  240 . Apart from that change, the sub-elements and composition of the parts are identical to those discussed above. 
     An embodiment of a process for fabricating an electrode element as described above is shown in  FIGS. 3   a - 3   e . The process starts, in  FIG. 3   a , with the deposition of layers on a substrate, of the top and bottom electrode material  2  and  4 , as well as the conductive material layers  7  and  8 . Here it should be borne in mind that the designer is free to choose both the specific material and the number of layers to employ here, ranging from a minimal structure of only the top and bottom electrode layers, ranging to multi-level conductive material layers, both with and without sidewall conductors. For exemplary purposes only, the process shown here builds a structure having two conductive material layers, the top and bottom electrodes, and one set of sidewall conductors. 
     The deposition can be accomplished using conventional techniques known in the art, preferably CVD and PVD processes. The specific processes are largely dictated by the nature of the chosen materials, as is known in the art. 
     Following the initial deposition, the process proceeds to establish the dimensions of the layer stack, in  FIG. 3   b , starting with deposition of a mask  9 , positioned and dimensioned to result in a stack having the desired lateral dimensions. This process employs conventional photolithography practices, to produce the stack  1  shown in  FIG. 3   c.    
     The side conductors are added in several steps. First, as seen in  FIG. 3   d , a layer of sidewall conductor material  6  is deposited, producing a layer that covers the entire stack and the surrounding area. As will previous depositions, this process is best accomplished using conventional techniques. 
     Excess material is removed by employing an anisotropic etch to remove material extending laterally from the stack and at least partially from the top, and then a Chemical Mechanical Polish (CMP) process is applied to planarize the upper surface of the completed electrode element, producing the electrode  30 , previously seen in  FIG. 1   c . The planarization must be controlled to expose the upper surface of top electrode  32 , as well as to ensure that both side conductors  36  and the top electrode lie in the same plane. 
     A further variant embodiment  40  is shown in  FIGS. 4   a  and  4   b . There, it is desired to add a dielectric layer  49  over the sidewall conductor  46 . As shown in  FIG. 4   a , this process takes up following deposition of the sidewall conductor material ( FIG. 3   d ), which step is followed by deposition of a dielectric layer  49 . The dielectric material preferably comprises silicon dioxide, a polyimide, silicon nitride or other dielectric fill materials known in the art. 
     There follows a conventional anisotropic etch step to define the lateral dimension of the structure, and a planarization step, employing CMP, the results of which are shown in  FIG. 4   b.    
     The various embodiments described above are all fabricated in what might be best described as a “build-up” process. An alternative approach, based on the same principles, is shown in  FIGS. 5   a  and  5   b . There, an electrode element  50  is produced by a creating and filling a recess in a substrate. As shown, a substrate  51  is provided, as is normally done in chip fabrication, and a recess or depression  51  is formed in the same. Next, a succession of deposition steps, identical to those discussed above, is undertaken to deposit top and bottom electrode layers  52  and  54  and conductive material layers  57  and  58 . These layers fill the recess  53  as shown, creating a “layer cake” effect. The resulting structure is planarized to remove the layered material extending beyond the recess  53 , as seen in  FIG. 5   b.    
     The process of forming such an electrode structure is seen in greater detail in  FIGS. 6   a - 6   d . Here, the electrode structure  60  consists of a substrate  61 , as discussed, having a plug  65  formed therein, seen in  FIG. 6   a . The plug functions as an electrical contact, preferably formed from a refractory metal such as tungsten, formed in the substrate by conventional means. Other refractory metals include Ti, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, and Ru and oxides of the same. 
     A recess  67  is formed in the substrate, shown in  FIG. 6   b , preferably by a preferential etch process, using a process selected to etch the plug at a higher rate than the substrate material. For the preferred structure shown, a suitable embodiment includes a W etching process. 
     The electrode  60  is completed employing the successive deposition steps, followed by CMP, as described above and shown in  FIGS. 6   c  and  6   d . Contact with other circuit elements is accomplished at the plug element  65  and the upper electrode  62 . 
     As is known in the art, deposition into a relatively high-aspect ratio opening, such as the plug element  65 , can result in the formation of deep seams in the deposited material. Deposited material tends to cling conformally to the sides of a receptacle structure, leaving voids or seams instead of a uniformly solid material. A subsequent etching or CMP step can open the seam, but nevertheless a seam can remain in the deposited structure. Because of its high conformity, tungsten is particularly susceptible to that phenomenon, as can be seen in  FIG. 6   e , where seam  69  remains visible in the plug element even after etching to considerable depth. In such an instance, the subsequently-deposited material may not make full contact with the earlier-formed material, resulting in poor inter-layer contact. 
     The present invention alleviates problems associated with that phenomenon because the electrode material and associated structure are active in filling gaps such as seam  69 . This material assures good, continuous contact between the electrode material and the W of the plug  65 . 
     While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.