Patent Publication Number: US-9406878-B2

Title: Resistive memory cells with two discrete layers of programmable material, methods of programming memory cells, and methods of forming memory cells

Description:
RELATED PATENT DATA 
     This patent resulted from a continuation of U.S. patent application Ser. No. 13/956,242, which was filed Jul. 31, 2013, which issued as U.S. Pat. No. 8,811,063, and which is hereby incorporated herein by reference, which resulted from a divisional of U.S. patent application Ser. No. 12/917,361, which was filed Nov. 1, 2010, which issued as U.S. Pat. No. 8,526,213, and which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     Memory cells, methods of programming memory cells, and methods of forming memory cells. 
     BACKGROUND 
     Memory is one type of integrated circuitry, and is used in computer systems for storing data. Integrated memory is usually fabricated in one or more arrays of individual memory cells. The memory cells might be volatile, semi-volatile, or nonvolatile. Nonvolatile memory cells can store data for extended periods of time, and in some instances can store data in the absence of power. Volatile memory dissipates and is therefore refreshed/rewritten to maintain data storage. 
     The memory cells are configured to retain or store memory in at least two different selectable states. In a binary system, the states are considered as either a “0” or a “1”. In other systems, at least some individual memory cells may be configured to store more than two levels or states of information. 
     Integrated circuitry fabrication continues to strive to produce smaller and denser integrated circuits. The smallest and simplest memory cell will likely be comprised of two electrically conductive electrodes having a programmable material received between them. 
     Programmable materials may also be referred to as memory cell materials. Suitable programmable materials have two or more selectable resistive states to enable storing of information by an individual memory cell. The reading of the cell comprises determination of which of the states the programmable material is in, and the writing of information to the cell comprises placing the programmable material in a predetermined resistive state. Some programmable materials retain a resistive state in the absence of refresh, and thus may be incorporated into nonvolatile memory cells. 
     Significant interest is presently being directed toward programmable materials that contain mobile charge carriers larger than electrons and holes. The charge carriers may be ions in some example applications. The programmable materials may be converted from one memory state to another by moving the mobile charge carriers therein to alter a distribution of charge density within the programmable materials. Some example memory devices that utilize ions as mobile charge carriers are resistive RAM (RRAM) cells; which can include classes of memory cells containing multivalent oxides, and which can include memristors in some specific applications. Other example memory devices that utilize ions as charge carriers are programmable metallization cells (PMCs); which may be alternatively referred to as a conductive bridging RAM (CBRAM), nanobridge memory, or electrolyte memory. 
     The RRAM cells may contain programmable material sandwiched between a pair of electrodes. The programming of the RRAM cells may comprise transitioning the programmable material between first a memory state in which charge density is relatively uniformly dispersed throughout the material and a second memory state in which the charge density is concentrated in a specific region of the material (for instance, a region closer to one electrode than the other). A PMC cell may similarly have programmable material sandwiched between a pair of current conductive electrodes. However, programming of the PMC comprises transitioning the PMC between a first state in which there is no conductive bridge between the electrodes and a second state in which mobile ions arrange to form super-ionic clusters or conducting filaments that electrically couple the electrodes to one another. 
     An ideal memory device will stably remain in a memory state after programming, and yet will be easy to program. It is difficult to accomplish both aspects of the ideal memory cell. Specifically, memory cells which are highly stable in their memory states also tend to be difficult to program (since it is difficult to induce the transition between the memory states during the programming operation); and conversely, memory cells which are easy to program tend to be unstable in their memory states. 
     It would be desirable to develop new memory cells which have the desired aspects of stability and relative ease of programming. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are diagrammatic views of a programmable metallization cell in a low resistance state and in a high resistance state, respectively. 
         FIGS. 3 and 4  are diagrammatic views of a RRAM cell in a first memory state and in a second memory state, respectively. 
         FIG. 5  is a diagrammatic cross-sectional view of an example embodiment memory cell. 
         FIG. 6  is a diagrammatic cross-sectional view of another example embodiment memory cell. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     In some embodiments the invention includes novel memory cells incorporating charge carriers larger than electrons or holes as species utilized during the programming of the memory cells. Example charge carriers that may be used in such embodiments are ions. The charge carriers are moved within the memory cells during programming, and in some embodiments it is recognized that it is advantageous for an average charge across the moving charge carriers to have an absolute value greater than 2. Such high average charge can enhance programming of the memory cells without adversely affecting stability of the memory states of the memory cells. 
     The memory cells may be any memory cells that utilize charge carriers larger than electrons or holes, such as, for example, PMC devices and RRAM devices. Prior to discussing specific memory cells of example embodiments, the operation of a PMC device will be described with reference to  FIGS. 1 and 2 , and the operation of a RRAM device will be described with reference to  FIGS. 3 and 4 . 
       FIGS. 1 and 2  diagrammatically illustrate a PMC device  10  in both a low resistance state ( FIG. 1 ) and a high resistance state ( FIG. 2 ). 
       FIG. 1  shows the PMC device  10  to comprise ion conductive material  16  between a pair of electrodes  12  and  14 . The electrode  12  comprises a surface  13  which is electrochemically active, while the electrode  14  comprises a surface  15  which is electrochemically inactive. 
     Electrodes  12  and  14  may comprise any suitable electrically conductive materials, and may be homogenous or non-homogenous. By way of examples only, suitable electrochemically active materials include copper, silver, and alloys including at least one of copper and silver. Example suitable electrochemically inactive materials include titanium nitride, gold, tungsten, platinum, and alloys including at least one of gold, tungsten or platinum. 
     Ion conductive material  16  may be a solid, gel, or any other suitable phase, and may comprise any suitable composition. In some embodiments the material  16  may comprise chalcogenide-type material (for instance, materials comprising one or more of germanium, selenium, antimony, tellurium, sulfur, copper, etc.; with example chalcogenide-type materials being Ge 2 Sb 2 Te 5 , GeS 2 , GeSe 2 , CuS 2 , and CuTe) and/or oxides such as zirconium oxide, hafnium oxide, tungsten oxide, silicon oxide (specifically, silicon dioxide), gadolinium oxide, etc. The ion conductive material  16  may have silver ions or other suitable ions diffused therein for ionic conduction, analogously to structures disclosed in U.S. Pat. No. 7,405,967 and U.S. Patent Publication Number 2010/0193758. 
     Application of electric field (EF+) across the PMC device  10  forms a current conducting filament  20  of ionic particles  21 . The individual ionic particles may be super-ionic clusters, and/or may be individual ions. The filament  20  extends between the electrodes  12  and  14 , and thus provides a low-resistance current conduction path through the ion conductive material  16  within the PMC device  10 . The device  10  having filament  20  therein may be considered to be in a low-resistance state. The conductive path formed by the particles  21  may comprise the particles directly contacting one another (as shown), or may comprise small gaps between some of the particles. 
       FIG. 2  shows device  10  as an electric field (EF−) is applied to the device. The electric field (EF−) is of opposite polarity relative to the field (EF+) of  FIG. 1 , and causes ions to move back to the active surface  13  of electrode  12 —thereby dissipating the filament  20  ( FIG. 1 ). Accordingly, the low-resistance path provided by such filament is removed, and the device  10  is transitioned into a high-resistance state. 
       FIGS. 3 and 4  diagrammatically illustrate an RRAM device  30  in a first memory state ( FIG. 3 ) and a second memory state ( FIG. 4 ). The first and second memory states may differ from one another in resistance or any other suitable electrical property. 
       FIG. 3  shows the RRAM device  30  to have programmable material  36  between a pair of electrodes  32  and  34 . The electrodes  32  and  34  may comprise any suitable composition or combinations of compositions; and in some embodiments may comprise, consist essentially of, or consist of one or more of various metals (for instance, copper, platinum, titanium, tungsten, etc.), metal-containing compositions (for instance, metal nitride, metal silicide, etc.) and conductively-doped semiconductor material (for instance, conductively-doped silicon, conductively-doped germanium, etc.). 
     The programmable material  36  may comprise any suitable composition or combination of compositions containing appropriate mobile charge carriers (with example mobile charge carriers being ions). For instance, material  36  may comprise, consist essentially of, or consist of multivalent oxide containing one or more of barium, ruthenium, strontium, titanium, calcium, manganese, praseodymium, lanthanum and samarium. An example multivalent metal oxide may comprise, consist essentially of, or consist of calcium manganese oxide doped with one or more of Pr, La, Sr and Sm. Although the programmable material  36  is shown as a single homogeneous layer, in some embodiments the programmable material may comprise two or more discrete layers. For instance, the programmable material may comprise a pair of oxide layers, such as the dual-layer oxides described by Meyer et. al. (R. Meyer et. al. “Oxide Dual-Layer Memory Element for Scalable Non-Volatile Cross-Point Memory Technology” (2008), available from IEEE Xplore). 
       FIG. 3  shows a memory state in which mobile charge carriers are relatively uniformly dispersed throughout programmable material  36 , and thus the charge is relatively uniformly dispersed throughout the programmable material. In contrast,  FIG. 4  shows a memory state in which mobile charge carriers are concentrated near one of the electrodes  32  and  34  to form a more positively charged region adjacent the upper electrode  32 . The concentration of charge is illustrated with a charge gradient going from δ −  to δ +  along a direction from the bottom electrode  34  toward the top electrode  32 . The mobile charge carriers may be either positive or negative, and thus may migrate toward the top electrode to form the shown gradient in some embodiments, or may migrate toward the bottom electrode to create the shown gradient in other embodiments. The illustrated gradient is one example configuration that may result from movement of charge carriers within programmable material  36 , and numerous other configurations may be achieved in other embodiments. For instance, in some embodiments the illustrated gradient may be reversed so that the gradient goes from δ +  to δ −  along the direction from the bottom electrode  34  toward the top electrode  32 . 
     The memory states of  FIGS. 3 and 4  may be interchanged with one another through application of electric fields to the device  30 , analogous to the electric field applications discussed above with reference to  FIGS. 1 and 2 . 
     As discussed previously, some embodiments utilize relatively highly charged mobile charge carriers (for instance, a population of charge carriers having an average charge across the population with an absolute value of greater than 2). A possible theoretical basis for the benefit of such highly charged mobile charge carriers is as follows. 
     Ionic transport may be described as an ionic drift (ν D ). The ionic drift may be expressed in terms of an attempt frequency (ν), a jump distance (d), a potential barrier (E a ), a Boltzmann constant (k B ), the absolute temperature (T), the charge number of a mobile charge carrier (z), the elementary charge (q), and the electric field (E) as shown in Equation 1. 
     
       
         
           
             
               
                 
                   
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     Equation 1 may be considered to comprise one term corresponding to a thermal component, and another term corresponding to a field component. Specifically, the thermal component may be expressed as shown in Equation 2, and the field component may be expressed as shown in Equation 3. 
     
       
         
           
             
               
                 
                   
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     The thermal component is related to diffusion of ions, and relates to the stability of a memory state within a memory device. The field component represents the effect of a local electric field on drift, and relates to the ease of programmability of a memory device. Notably, the thermal component is not affected by the charge of the mobile charge carriers (the term “z” of Equation 1), whereas the field component is strongly affected by such charge (since the charge is in a numerator of a sin h function). Accordingly, increasing the charge of mobile charge carriers utilized in a memory device can increase the ease of programming of the device without detrimentally affecting the stability of the various memory states of the device. 
     It is noted that multiple charge states of a charge carrier species will frequently be in equilibrium with one another. For instance, a single charge carrier may exist in the charge states of 0 (i.e., neutral), +1, +2, +3, +4, etc. Thus, it is an overall charge state of a population of charge carriers that is of interest in some embodiments. It is also noted that it is the absolute value of the charge (“z” in Equation 1) that matters, and thus a negative charge can provide equal benefit as a positive charge of the same magnitude. 
     The theoretical basis is provided to assist the reader in understanding the various embodiments presented herein and is not to limit the claims that follow except to the extent, if any, that it is expressly recited in such claims. 
     Example embodiment memory cells utilizing mobile charge carriers with an average value of |z| (i.e., the absolute value of charge) greater than 2 are described with reference to  FIGS. 5 and 6 . 
     Referring to  FIG. 5 , such shows an example embodiment memory cell  50  having a programmable material  56  between a pair of electrodes  52  and  54 . 
     The memory cell may be supported by a semiconductor base (not shown), and thus may be incorporated into integrated circuitry. If the memory cell is over a semiconductor base, such semiconductor base may comprise any suitable composition or combination of compositions, and in some embodiments may comprise, consist essentially of, or consist of monocrystalline silicon lightly background doped with appropriate p-type dopant. The terms “semiconductive substrate,” “semiconductor construction” and “semiconductor substrate” mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above. 
     The electrodes  52  and  54  may comprise any of the materials discussed relative to electrodes  12 ,  14 ,  32  and  34  of  FIGS. 1-4 . In some embodiments the bottom electrode  54  may be part of a conductive line that extends along a first direction (for instance into and out of the page relative to the cross-section of  FIG. 5 ) and the top electrode  52  may be part of a conductive line that extends along a direction orthogonal to the direction of the bottom electrode line (for instance, the top electrode line may extend along a plane of the cross-section of  FIG. 5 ). Accordingly, the programmable material  56  may be formed at a cross-point where the top and bottom electrode conductive lines overlap. The bottom electrode conductive line may be a representative example of a first series of conductive lines, the top electrode conductive line may be a representative example of a second series of conductive lines, and the memory cell  50  may be representative example of a plurality of memory cells formed at cross-points where lines from the second series overlap lines of the first series. The memory cells may be arranged as a memory array, and each memory cell of such array may be uniquely addressed through the combination of a line from the second series and a line from the first series. 
     The programmable material  56  may be suitable for utilization in any of numerous memory cells that utilize mobile charge carriers larger than electrons and holes; and may, for example, be suitable for utilization in PMC devices of the type described with reference to  FIGS. 1 and 2 , or for utilization in RRAM devices of the type described with reference to  FIGS. 3 and 4 . 
     The memory cell  50  is programmed by moving a plurality mobile charge carriers within programmable material  56 . An average charge across the entire population of mobile charge carriers (or in some embodiments across the subset of the population corresponding to moving mobile charge carriers) may have an absolute value of greater than 2, and in some embodiments may be at least about 3. The high average charge can enable rapid programming of the memory cell without adversely affecting memory state stability of the memory cell, as discussed above with reference to Equations 1-3. 
     The programmable material  56  may comprise any suitable charge carrying species, including, for example, one or more of aluminum, chromium, iron, manganese, titanium, tungsten and vanadium. Each of aluminum, chromium, iron, manganese, titanium, tungsten and vanadium has a stable charged species in which an absolute value of the charge exceeds 3. 
     In some embodiments the programmable material  56  may comprise, consist essentially of, or consist of the mobile charge carriers dispersed throughout an aluminum nitride matrix. In such embodiments the mobile charge carriers may correspond to one or more species described by Stampfl and Van de Walle (C. Stampfl and C. G. Van de Walle “Theoretical investigation of native defects, impurities and complexes in aluminum nitride” Physical Review B, volume 65, pages 155212-1 through 155212-10 (2002)). Accordingly, the mobile charge carriers may comprise one or more of interstitial nitrogen, interstitial aluminum (Al 3+ ), nitrogen vacancies (V N   3+ ), and aluminum vacancies. Alternatively, or additionally, the mobile charge carriers may include one or more interstitial atoms other than nitrogen and aluminum; such as, for example, any of various non-metals (such as, for example, oxygen), semimetals (such as, for example, silicon) and metals (such as, for example, magnesium). 
     If memory cell  50  is a PMC-type device, the memory cell may be programmed by moving mobile charges to either form a filament analogous to the filament  20  described above with reference to  FIG. 1 , or to dissolve such filament. If memory cell  50  is an RRAM-type device, the memory cell may be programmed by moving mobile charges to reversibly alter charge distribution within the programmable material  56  analogous to the programming described above with reference to  FIGS. 3 and 4 . 
     The memory cell  50  may be formed with any suitable processing. In some embodiments the bottom electrode  54  may be formed over a semiconductor base. The bottom electrode may be formed as one of a series of spaced apart electrically conductive lines. The programmable material  56  may then be formed over the bottom electrode, and then the top electrode may be formed over the programmable material. The top electrode may be formed as one of a second series of electrically conductive lines, with the lines of the second series extending substantially orthogonally relative to the lines of the first series. The programmable material may be formed only at memory cell locations (i.e., only at cross-points where top electrodes overlap bottom electrodes) in some embodiments, and may extend beyond the memory cell locations in other embodiments. In some embodiments it can be desired that the programmable material of the individual memory cells of a memory array be contained only within the individual memory cells, and that dielectric material be formed between adjacent memory cells of the memory array, to alleviate or prevent cross-talk between the adjacent memory cells. 
       FIG. 5  shows a memory cell comprising programmable material  56  as a single homogeneous layer sandwiched between electrodes  52  and  54 . In other embodiments the programmable material may comprise two or more discrete layers which share a common mobile carrier species.  FIG. 6  shows a memory cell  50   a  comprising the top and bottom electrodes  52  and  54  described above with reference to  FIG. 5 , and comprising a programmable material  56   a  containing two discrete layers  60  and  62 . In other embodiments the programmable material  56   a  may comprise more than two discrete layers. 
     In the shown embodiment the top electrode  52  is directly against one of the discrete layers, and the bottom electrode  54  is directly against the other of the discrete layers. 
     The programmable material  56   a  may comprise any suitable layers. In some embodiments one of the layers  60  and  62  may be referred to as a first layer and the other may be referred to as a second layer. The first layer may comprise an aluminum nitride matrix containing mobile charge carriers, and the second layer may comprise a composition having a mobile charge carrier in common with the first layer. For instance, if interstitial aluminum is a mobile charge carrier within the first layer, the second layer may comprise, consist of, or consist of aluminum oxide. The aluminum oxide may be formed to have excess aluminum (i.e., an aluminum concentration exceeding the stoichiometric concentration of aluminum in Al 2 O 3 ) so that the aluminum oxide second layer has aluminum ions as mobile charge carriers therein. In such examples, the common mobile charge carrier between the first and second layers may be an ion species corresponding to Al 3+ . As another example, the mobile charge carriers within the aluminum nitride matrix of the first layer may include an interstitial metal other than aluminum, and the second layer may comprise a composition containing such metal. As another example, the mobile charge carriers within the aluminum nitride matrix of the first layer may include interstitial semimetals and/or non-metals, and the second layer may comprise a composition containing such semimetals and/or non-metals. 
     The memory cells discussed above may be incorporated into electronic systems. Such electronic systems may be used in, for example, memory modules, device drivers, power modules, communication modems, processor modules, and application-specific modules, and may include multilayer, multichip modules. The electronic systems may be any of a broad range of systems, such as, for example, clocks, televisions, cell phones, personal computers, automobiles, industrial control systems, aircraft, etc. 
     The particular orientation of the various embodiments in the drawings is for illustrative purposes only, and the embodiments may be rotated relative to the shown orientations in some applications. The description provided herein, and the claims that follow, pertain to any structures that have the described relationships between various features, regardless of whether the structures are in the particular orientation of the drawings, or are rotated relative to such orientation. 
     The cross-sectional views of the accompanying illustrations only show features within the planes of the cross-sections, and do not show materials behind the planes of the cross-sections in order to simplify the drawings. 
     When a structure is referred to above as being “on” or “against” another structure, it can be directly on the other structure or intervening structures may also be present. In contrast, when a structure is referred to as being “directly on” or “directly against” another structure, there are no intervening structures present. When a structure is referred to as being “connected” or “coupled” to another structure, it can be directly connected or coupled to the other structure, or intervening structures may be present. In contrast, when a structure is referred to as being “directly connected” or “directly coupled” to another structure, there are no intervening structures present. 
     In compliance with the statute, the subject matter disclosed herein has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the claims are not limited to the specific features shown and described, since the means herein disclosed comprise example embodiments. The claims are thus to be afforded full scope as literally worded, and to be appropriately interpreted in accordance with the doctrine of equivalents.