WO2010118380A2 - Resistive-switching memory elements having improved switching characteristics - Google Patents

Resistive-switching memory elements having improved switching characteristics Download PDF

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Publication number
WO2010118380A2
WO2010118380A2 PCT/US2010/030619 US2010030619W WO2010118380A2 WO 2010118380 A2 WO2010118380 A2 WO 2010118380A2 US 2010030619 W US2010030619 W US 2010030619W WO 2010118380 A2 WO2010118380 A2 WO 2010118380A2
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layer
electrode
oxide
switching
memory element
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PCT/US2010/030619
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French (fr)
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WO2010118380A3 (en
Inventor
Ronald John Kuse
Prashant Phatak
Tony Chiang
Michael Miller
Jinhong Tong
Wen Wu
Imran Hashim
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Intermolecular, Inc.
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Priority to US16853409P priority Critical
Priority to US61/168,534 priority
Priority to US12/608,934 priority patent/US8183553B2/en
Priority to US12/608,934 priority
Priority to US12/610,236 priority
Priority to US12/610,236 priority patent/US8975613B1/en
Priority to US12/705,474 priority patent/US8343813B2/en
Priority to US12/705,474 priority
Application filed by Intermolecular, Inc. filed Critical Intermolecular, Inc.
Publication of WO2010118380A2 publication Critical patent/WO2010118380A2/en
Publication of WO2010118380A3 publication Critical patent/WO2010118380A3/en

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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L45/00Solid state devices adapted for rectifying, amplifying, oscillating or switching without a potential-jump barrier or surface barrier, e.g. dielectric triodes; Ovshinsky-effect devices; Processes or apparatus peculiar to the manufacture or treatment thereof or of parts thereof
    • H01L45/04Bistable or multistable switching devices, e.g. for resistance switching non-volatile memory
    • H01L45/14Selection of switching materials
    • H01L45/145Oxides or nitrides
    • H01L45/146Binary metal oxides, e.g. TaOx
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L45/00Solid state devices adapted for rectifying, amplifying, oscillating or switching without a potential-jump barrier or surface barrier, e.g. dielectric triodes; Ovshinsky-effect devices; Processes or apparatus peculiar to the manufacture or treatment thereof or of parts thereof
    • H01L45/04Bistable or multistable switching devices, e.g. for resistance switching non-volatile memory
    • H01L45/10Bistable or multistable switching devices, e.g. for resistance switching non-volatile memory based on bulk electronic defects, e.g. trapping of electrons
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L45/00Solid state devices adapted for rectifying, amplifying, oscillating or switching without a potential-jump barrier or surface barrier, e.g. dielectric triodes; Ovshinsky-effect devices; Processes or apparatus peculiar to the manufacture or treatment thereof or of parts thereof
    • H01L45/04Bistable or multistable switching devices, e.g. for resistance switching non-volatile memory
    • H01L45/12Details
    • H01L45/122Device geometry
    • H01L45/1233Device geometry adapted for essentially vertical current flow, e.g. sandwich or pillar type devices
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L45/00Solid state devices adapted for rectifying, amplifying, oscillating or switching without a potential-jump barrier or surface barrier, e.g. dielectric triodes; Ovshinsky-effect devices; Processes or apparatus peculiar to the manufacture or treatment thereof or of parts thereof
    • H01L45/04Bistable or multistable switching devices, e.g. for resistance switching non-volatile memory
    • H01L45/16Manufacturing
    • H01L45/1608Formation of the switching material, e.g. layer deposition
    • H01L45/1616Formation of the switching material, e.g. layer deposition by chemical vapor deposition, e.g. MOCVD, ALD
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L45/00Solid state devices adapted for rectifying, amplifying, oscillating or switching without a potential-jump barrier or surface barrier, e.g. dielectric triodes; Ovshinsky-effect devices; Processes or apparatus peculiar to the manufacture or treatment thereof or of parts thereof
    • H01L45/04Bistable or multistable switching devices, e.g. for resistance switching non-volatile memory
    • H01L45/16Manufacturing
    • H01L45/1641Modification of the switching material, e.g. post-treatment, doping
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L27/00Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/24Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including solid state components for rectifying, amplifying or switching without a potential-jump barrier or surface barrier, e.g. resistance switching non-volatile memory structures
    • H01L27/2409Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including solid state components for rectifying, amplifying or switching without a potential-jump barrier or surface barrier, e.g. resistance switching non-volatile memory structures comprising two-terminal selection components, e.g. diodes
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L27/00Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/24Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including solid state components for rectifying, amplifying or switching without a potential-jump barrier or surface barrier, e.g. resistance switching non-volatile memory structures
    • H01L27/2463Arrangements comprising multiple bistable or multistable switching components of the same type on a plane parallel to the substrate, e.g. cross-point arrays, details of the horizontal layout
    • H01L27/2481Arrangements comprising multiple bistable or multistable switching components of the same type on a plane parallel to the substrate, e.g. cross-point arrays, details of the horizontal layout arranged in a direction perpendicular to the substrate, e.g. 3D cell arrays, details of the vertical layout

Abstract

Resistive-switching memory elements having improved switching characteristics are described, including a memory element having a first electrode and a second electrode, a switching layer between the first electrode and the second electrode, the switching layer comprising a first metal oxide having a first bandgap greater than 4 electron volts (eV), the switching layer having a first thickness, and a coupling layer between the switching layer and the second electrode, the coupling layer comprising a second metal oxide having a second bandgap greater the first bandgap, the coupling layer having a second thickness that is less than 25 percent of the first thickness.

Description

RESISTIVE-SWITCHING MEMORY ELEMENTS HAVING IMPROVED SWITCHING CHARACTERISTICS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 61/168,534 entitled "RESISTIVE SWITCHING MEMORY ELEMENT INCLUDING DOPED SILICON ELECTRODE" and filed on April 10, 2009, to U.S. Utility Application No. 12/608,934 entitled "RESISTIVE SWITCHING MEMORY ELEMENT INCLUDING DOPED SILICON ELECTRODE" and filed on October 29, 2009, to U.S. Utility Application No. 12/610,236 entitled "RESISTIVE- SWITCHING MEMORY ELEMENTS HAVING IMPROVED SWITCHING CHARACTERISTICS" and filed on October 30, 2009, and to U.S. Utility Application No. 12/705,474 entitled "RESISTIVE-SWITCHING MEMORY ELEMENTS HAVING IMPROVED SWITCHING CHARACTERISTICS" and filed on February 12, 2010, all of which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to semiconductor memories. More specifically, resistive-switching memory elements having improved switching characteristics are described.

BACKGROUND OF THE INVENTION

[0003] Non- volatile memories are semiconductor memories that retain their contents when unpowered. Non-volatile memories are used for storage in electronic devices such as digital cameras, cellular telephones, and music players, as well as in general computer systems, embedded systems and other electronic devices that require persistent storage. Non- volatile semiconductor memories can take the form of removable and portable memory cards or other memory modules, can be integrated into other types of circuits or devices, or can take any other desired form. Nonvolatile semiconductor memories are becoming more prevalent because of their advantages of having small size and persistence, having no moving parts, and requiring little power to operate.

[0004] Flash memory is a common type of non- volatile memory used in a variety of devices. Flash memory uses an architecture that can result in long access, erase, and write times. The operational speeds of electronic devices and storage demands of users are rapidly increasing. Flash memory is proving, in many instances, to be inadequate for non- volatile memory needs. Additionally, volatile memories (such as random access memory (RAM)) can potentially be replaced by non-volatile memories if the speeds of non-volatile memories are increased to meet the requirements for RAM and other applications currently using volatile memories. [0005] Resistive-switching memories are memories that include a resistive- switching material (e.g. a metal oxide) that changes from a first resistivity to a second resistivity upon the application of a set voltage, and from the second resistivity back to the first resistivity upon the application of a reset voltage. Existing resistive- switching memories have switching characteristics (e.g. set, reset, and forming voltages, retention) that are unsuitable for some applications. [0006] Thus, what is needed is a resistive-switching memory element with improved switching characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings:

[0008] Figure 1 illustrates a memory array of resistive switching memory elements;

[0009] Figure 2A illustrates a memory element including a resistive-switching material and a select element;

[0010] Figure 2B is an enlarged view of a portion of the memory element according to one embodiment.

[0011] Figures 3 and 4 are band diagrams and that illustrate energy levels in a memory element with (Figure 3) and without (Figure 4) an interface layer;

[0012] Figure 5 is a graph illustrating the dependency of forming voltage on the presence of an interface layer;

[0013] Figure 6 illustrates a memory element that shares an electrode with a diode that is used as a select element;

[0014] Figure 7 illustrates a portion of a three-dimensional memory array using memory elements described herein;

[0015] Figures 8 A and 8B illustrate the memory element and the creation and manipulation of oxygen vacancies (defects) within the memory element using an interface layer;

[0016] Figure 9 is a logarithm of current (I) versus voltage (V) plot for a memory element;

[0017] Figure 10 is a current (I) versus voltage (V) plot for a memory element that demonstrates a resistance state change;

[0018] Figures 11 and 12 are graphs showing the relationship between thicknesses of a metal oxide layer and resulting set voltages, reset voltages, and on/off current ratios for several materials (metal oxides) used in memory elements described herein;

[0019] Figures 13 and 14 are flowcharts describing processes and for controlling interface layers;

[0020] Figure 15 is a flowchart describing a process for forming a memory element according to various embodiments; [0021] Figure 16A is a graph showing high cycling yield when the low work function electrode of the resistive-switching memory element receives a negative voltage set pulse and positive voltage reset pulse; and

[0022] Figure 16B is a graph showing high cycling yield when two electrodes of a resistive-switching memory element have work functions that are different by greater than 0.1 eV but less than 1.0 eV.

DETAILED DESCRIPTION

[0023] A detailed description of one or more embodiments is provided below along with accompanying figures. The detailed description is provided in connection with such embodiments, but is not limited to any particular example. The scope is limited only by the claims and numerous alternatives, modifications, and equivalents are encompassed. Numerous specific details are set forth in the following description in order to provide a thorough understanding. These details are provided for the purpose of example and the described techniques may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the embodiments has not been described in detail to avoid unnecessarily obscuring the description.

[0024] According to various embodiments, resistive-switching memory elements are described herein. The memory elements generally have a metal-insulator-metal (MIM) structure in which at least one insulating layer is surrounded by two conductive electrodes. Some embodiments described herein are memory elements that include electrodes of different materials (e.g. one electrode is doped silicon and one is titanium nitride) surrounding a switching layer of a switching material with a bandgap greater than 4.0 eV (e.g. hafnium oxide (HfO2), bandgap = 5.7 eV, thickness -20-1 OOA) and a coupling layer that can be one of several materials, e.g. ZrO2, AI2O3, TiO2, Ti/TiO2, etc. In some embodiments the coupling layer has a higher bandgap than the switching layer (e.g. zirconium oxide (ZrO2), bandgap = 5.8 eV; or aluminum oxide (AI2O3), bandgap = 8.4 eV). The coupling layer has a thickness that is less than 25 percent the thickness of the switching layer, and memory elements including the coupling layer have exhibited improved switching characteristics (e.g. lower set, reset, and forming voltages, and better retention).

[0025] In other embodiments, a metal-rich metal oxide switching layer and techniques for forming the metal-rich switching layer are described. The metal-rich switching layer includes increased numbers of defects (e.g. oxygen vacancies), which can be manipulated to improve switching characteristics. The metal-rich switching layer can be deposited, for example, by reducing the amount of oxidant that is introduced during an atomic layer deposition (ALD) process. In further embodiments, techniques for removing or controlling the size of an interface layer between an electrode and a switching layer deposited thereon are described.

/. Switching Operation

[0026] It is believed that the resistive switching of the memory elements described herein is caused by defects in a metal oxide switching layer of the memory element. Generally, defects are formed in or already exist in the deposited metal oxide, and existing defects can be enhanced by additional processes. For example, physical vapor deposition (PVD) processes and atomic layer deposition (ALD) processes deposit layers that can have some imperfections or flaws. Defects may take the form of variances in charge in the structure of the metal oxide: some charge carriers may be absent from the structure (i.e. vacancies), additional charge carriers may be present (i.e. interstitials), or one element can substitute for another (i.e. substitutional).

[0027] The defects are thought to be electrically active defects (also known as traps) in the bulk of the metal oxide and/or at the interface of the metal oxide and adjoining layers. It is believed that the traps can be filled by the application of a set voltage (to switch from a high to a low resistance state), and emptied by applying a reset voltage (to switch from the low to the high resistance state). Traps can be inherent in the as-deposited metal oxide (i.e., existing from formation of the metal oxide) or created and/or enhanced by doping and other processes. Doping can be performed using adjacent "doping" layers that interdiffuse with the switching layer, using implantation, or using other techniques.

[0028] It is believed that the defects in the switching layer form conductive percolation paths upon the application of the set voltage. It is further believed that the percolation paths are removed upon the application of a reset voltage. For example, a hafnium oxide layer may include oxygen or hafnium vacancies or oxygen or hafnium interstitials that may form traps which can be used to create percolation paths and alter the conductivity of the hafnium oxide layer.

[0029] The switching characteristics of the resistive-switching memory elements can be tailored by controlling the defects within the metal oxides. Switching characteristics include operating voltages (e.g. set, reset, and forming voltages), operating currents (e.g. on and off currents), and data retention. Defect control is achieved by type, density, energy level, and spatial distribution within the switching layer. These defects then modulate the current flow based on whether they are filled (passivated/compensated) or unfilled (uncompensated). Adding different layers, controlling the formation of the switching layer, implanting, controlling stress, certain thermal treatments are all used to control the defect characteristics. In addition, the defects need not be mobile. For example, a coupling layer 212 (see Figure 2A) and an interface layer 214 (see Figures 2A and 8A-8B) can be used to control locations, depths, densities, and/or type of defects, and techniques can be used to form a switching layer having an increased number of defects.

[0030] Additionally, the metal oxide switching layer can have any phase (e.g., crystalline and amorphous) or mixtures of multiple phases. Amorphous-phase metal oxides may have increased resistivity, which in some embodiments can lower the operational currents of the device to reduce potential damage to the memory element.

//. Memory Structure

A. Memory Array

[0031] Figure 1 illustrates a memory array 100 of resistive switching memory elements 102. Memory array 100 may be part of a memory device or other integrated circuit. Memory array 100 is an example of potential memory configurations; it is understood that several other configurations are possible.

[0032] Read and write circuitry may be connected to memory elements 102 using signal lines 104 and orthogonal signal lines 106. Signal lines such as signal lines 104 and signal lines 106 are sometimes referred to as word lines and bit lines and are used to read and write data into the elements 102 of array 100. Individual memory elements 102 or groups of memory elements 102 can be addressed using appropriate sets of signal lines 104 and 106. Memory element 102 may be formed from one or more layers 108 of materials, as is described in further detail below. In addition, the memory elements 102 shown can be stacked in a vertical fashion to make multi-layer 3-D memory arrays (see Figure 7).

[0033] Any suitable read and write circuitry and array layout scheme may be used to construct a non-volatile memory device from resistive switching memory elements such as element 102. For example, horizontal and vertical lines 104 and 106 may be connected directly to the terminals of resistive switching memory elements 102. This is merely illustrative.

[0034] If desired, other electrical devices may be associated (i.e., be one or more of the layers 108) with each memory element 102 (see, e.g., Figure 2A). These devices, which are sometimes referred to as select elements, may include, for example, diodes, p-i-n diodes, silicon diodes, silicon p-i-n diodes, transistors, Schottky diodes, etc. Select elements may be connected in series in any suitable locations in memory element 102.

B. Memory Element

1. MIM Structure

[0035] Figures 2A and 2B illustrate a memory element 102 including a resistive- switching material and a select element (a diode 202). Figure 2B is an enlarged view of a portion of the memory element 102 according to one embodiment. The memory element 102 includes a metal-insulator-metal (MIM)-style stack 204 (in some embodiments, one or more of the metal layers can be a conductive semiconductor material such as doped silicon). The stack 204 includes two electrodes 206 and 208 and a resistive-switching layer 210 (e.g. an insulator or metal oxide). The electrodes 206 and 208 can be metals, metal carbides, metal oxides, or metal nitrides (e.g. Pt, Ru, RuO2, Ir, IrO2, TiN, W, TaN, MoN, MoOx, WN, TiAl alloys, Ni, WOx, Al, doped Al, Cu, HfC, HfN, NbN, TaC, TaSiN, Ti, VC, VN, ZrN, etc.), or can be doped silicon, for example p- or n-type doped polysilicon. The resistive-switching layer 210 can be a metal oxide or other switching material. In some embodiments, the resistive- switching layer 210 is a high bandgap (i.e. bandgap greater than four electron volts (eVs)) material such as HfO2, Ta2O5, Al2O3, Y2O3, CeO2, La2O3, and ZrO2. CeO2 may be advantageous for some embodiments because it may include ions that have higher mobility. In further embodiments, a semiconductive metal oxide (p-type or n- type) such as ZnOx, CuOx, and their nonstoichiometric and doped variants can be used because it is believed that these oxides may have advantageous switching characteristics. In other embodiments, ternary metal oxides, such as HfxMyO2, where M=Al, Zr, Ti, La, or Sr can also be used for the switching layer 210. a. Switching Layer

[0036] The switching layer 210 can have any desired thickness, but in some embodiments can be between 10 and 100A, between 20 and 6θA, or approximately 50 A. The switching layer 210 can be deposited using any desired technique, but in some embodiments described herein is deposited using ALD, or a combination of ALD and PVD. In other embodiments, the switching layer 210 can be deposited using low pressure CVD (LPCVD), plasma enhanced CVD (PECVD), plasma enhanced ALD (PEALD), liquid deposition processes, and epitaxy processes. It is believed that PEALD processes can be used to control defects and improve switching and forming voltages in some embodiments.

[0037] The switching layer 210 additionally can be metal-rich (e.g. HfOu vs. HfO2) such that the elemental composition of the switching layer 210 is less than stoichiometric (e.g. less than HfO2). The switching layer 210 can have a deficit of oxygen, which manifests as oxygen vacancy defects. The additional defects can lead to reduced and more predictable switching and forming voltages of the memory element 102. Techniques for depositing a metal-rich switching layer 210 are described in Figure 13.

b. Coupling Layer

[0038] The stack 204 can also include a coupling layer 212, which may be another metal oxide such as ZrO2, TiO2, or Al2θ3, or a metallic layer, such as metallic Ti, or a layer that includes Ti and TiO2. In other embodiments, the coupling layer 212 can be deposited as a metal layer that will partially or fully oxidize upon the deposition of the adjacent electrode 208 or upon annealing. The coupling layer 212 can, for example, facilitate switching at the electrode 208 by creating defects near the electrode 208. For example, if the coupling layer is a metal such as Ti, during an anneal (such as rapid thermal oxidation or a vacuum anneal), the coupling layer 212 can attract oxygen from the adjacent metal oxide switching layer 210, thereby creating oxygen vacancies in the switching layer 210 while oxidizing the coupling layer 212. [0039] For example, as shown in Figure 2B, a memory element 102 includes a coupling layer 212 that is substantially metallic titanium (or another metal) as deposited, and a switching layer that is a metal oxide (e.g. HfO2). After the memory element 102 is annealed or as a result of the deposition of the electrode 208, the metallic coupling layer 212 can attract oxygen from the switching layer 210, resulting in the coupling layer 212 and switching layer 210 having multiple portions having different characteristics (e.g. different oxygen concentrations). For example, a first portion 212a of the coupling layer 212 is substantially metallic (e.g. substantially metallic titanium), while a second portion 212b that is in contact with the switching layer 210 is substantially metal oxide (e.g. titanium oxide). At the same time, a first portion 210a of the switching layer 210 that is in contact with the coupling layer 210 is oxygen deficient because the oxygen in the metal oxide has migrated to the second portion 212b of the coupling layer 212, leaving oxygen vacancies which can be used to tailor switching characteristics. A second portion 210b of the switching layer 210 has an oxygen concentration that is greater than that of the first portion 210b. In other words, the first portion 210a is more metal rich (e.g. HfO1-2-L5) than the second portion (e.g. HfOi.6-2.o). It is understood that the portions 210a, 210b, 212a, and 212b are merely illustrative, and that each of the switching layer 210 and the coupling layer 212 may have several portions having different concentrations of oxygen, or that the oxygen concentration may be gradated through the thickness of the layers 210 and 212. Further, the amount of oxidation of the coupling layer 212 and the reduction of oxygen of the switching layer 210 can be tailored by changing the parameters (e.g. duration, oxygen concentration for RTO) of the anneal.

[0040] The coupling layer 212 can be thinner than the resistive-switching layer 210, for example the coupling layer 212 can have a thickness that is less than 25% of the thickness of the resistive-switching layer 210, or a thickness that is less than 10% of the thickness of the resistive-switching layer 210. For example, the resistive- switching layer 210 can be a 20-60 A layer, and the interface layer 212 can be a 5-lθA layer.

[0041] The coupling layer 212 in some embodiments has a bandgap that is approximately equal to or greater than a bandgap of the switching layer 210. The higher bandgap of the coupling layer 212 can help improve retention of the memory element 102 by reducing leakage from the switching layer 210. Additionally, the coupling layer 212 can create defects near the electrode 208 (including in near or at the interface between the coupling layer and the switching layer 210 and/or near or at the interface between the electrode 208 and the coupling layer 212), which can assist in switching. The coupling layer 212 is thin enough to provide access to defects in the switching layer 210.

[0042] In some embodiments, the coupling layer 212 is metallic titanium as deposited, which in some embodiments can become at least partially titanium oxide during an anneal. A titanium coupling layer 212 can change the effective work function of the adjacent electrode 208, which can be used to modify switching characteristics (e.g. by reducing leakage).

c. Interface Layer

[0043] The stack 204 further may include an interface layer 214 between the electrode 206 and the switching layer 210. The interface layer 214 can be an oxide of the material of the electrode 206 that is formed as a result of and during the deposition of the switching layer 210, for example as a result of thermal oxidation during processing. The interface layer 214 can, in some embodiments, alter defects in the switching layer 210 (see, e.g. Figures 8A-8B). In other embodiments, it may be desirable to eliminate the interface layer 214 to reduce forming voltage or to enable switching. It is believed that in some embodiments, the interface layer 214 can hinder effective electron injection into the switching layer 210 that enables traps to be filled, which thereby increases forming voltage or causes excessive potential drop across it, producing high electric fields in the switching layer 210 and preventing switching. Techniques for controlling the size of or eliminating the interface layer 214 are described in Figures 13 and 14.

[0044] Figures 3 and 4 are band diagrams 300 and 400 that illustrate energy levels in a memory element with (Figure 3) and without (Figure 4) an interface layer 214. For each of the band diagrams 300 and 400, there are corresponding electric field diagrams 320 and 340 that illustrate the strength of the electric field within a certain region of the memory element 102.

[0045] In the band diagram 300, a memory element has a titanium nitride electrode 302, a zirconium oxide coupling layer 304, a hafnium oxide switching layer 306, a silicon oxide interface layer 308, and an n-type polysilicon electrode 310. The materials used here are illustrations; it is understood that other materials can also be used (e.g. metallic titanium, or TiZTiO2 instead of zirconium oxide). In the band diagram 400, a memory element has a titanium nitride electrode 402, a zirconium oxide coupling layer 404, a hafnium oxide switching layer 406, and an n-type polysilicon electrode 408. As is shown in the electric field diagram 320, the electric field is reduced by a large amount 322 in the interface layer 314. Increased switching voltages may be necessary to overcome the electric field reduction in the interface layer 214. If the interface layer 214 is thick enough, the entire electric field may be lost to the interface layer 214, which may prevent switching altogether. Alternatively, as is shown in the electric field diagram 420, in the memory element without the interface layer 214 the electric field is reduced evenly 422 throughout the memory element 102, including in the switching layer 210, which can reduce switching voltages and lead to more predictable switching. However, as is described regarding Figures 8A and 8B, it may be desirable to retain a controlled-thickness interface layer 214 to increase the number of defects in the switching layer 210. [0046] Figure 5 is a graph 500 illustrating the dependency of forming voltage on the presence of an interface layer 214. Three sets of memory elements were prepared: [0047] A first set of memory elements represented by diamonds 502 includes a titanium nitride electrode 206, a PVD-deposited hafnium oxide switching layer 210, and a platinum electrode 208 without a coupling layer 212. [0048] A second set of memory elements represented by squares 504 includes an n-type polysilicon electrode 206, an ALD-deposited hafnium oxide switching layer 210, and a titanium nitride electrode 208 without a coupling layer 212.

[0049] A third set of memory elements represented by a circle 506 includes an n-type polysilicon electrode 206, a PVD-deposited hafnium oxide switching layer 210, and a platinum electrode 208 without a coupling layer 212.

[0050] The graph 500 shows the median forming voltage of the memory elements as a function of the thickness of the switching layer in the memory elements. As can be seen, for a switching layer having the same thickness, the elements 502 including PVD hafnium oxide on titanium nitride have the lowest forming voltage, elements 506 including PVD hafnium oxide on polysilicon have the next lowest forming voltage, and elements 504 having ALD hafnium oxide on polysilicon have the highest forming voltage. It is believed that ALD processes are more likely to form a thicker interface layer 214 at least partly because of potentially higher processing temperatures (2000C or greater versus room temperature for some instances of PVD), which leads to increased forming voltages. Additionally, the silicon oxide interface layer 214 created on polysilicon electrodes (e.g. the elements 502 and 506) is less conductive than an oxide created on a metal-containing electrode such as titanium nitride. Therefore, techniques for reducing and/or controlling the interface layer 214, especially for silicon-based electrodes, can be used to improve forming voltages.

[0051] Although ALD process may be more likely to form thicker interface layers 214 and result in memory elements having increased forming voltages, it may be desirable to use ALD processing over PVD processing for other reasons (e.g. to form more conformal layers), and Figure 13 describes a process for reducing or eliminating the interface layer 214 using ALD processing. Additionally, as is described regarding Figures 8A and 8B, it may be desirable to retain a controlled- thickness interface layer 214 (e.g. less than or equal to 10A) to increase the number of defects in the switching layer 210, which can also be formed using the process of Figure 13.

[0052] If it is desirable to have an interface layer 214, the order of deposition of the layers of the MIM stack 204 may be important. Since the interface layer 214 is formed during the deposition of the switching layer 210, the switching layer 210 can be formed on the electrode that the interface layer 214 is to be formed from (e.g. formed on the polysilicon layer if a silicon oxide interface layer 214 is desired). As an example, and as is discussed further in Figure 7, when forming a three- dimensional memory array, it may be necessary to always form the memory element in the same orientation (e.g. one electrode always on the bottom), even when the orientation of other elements is to be reversed. In other embodiments however, the interface layer 214 can be created when the memory element 102 is deposited in reverse order by using a post deposition anneal of the memory element 102. d. Electrodes

[0053] The electrodes 206 and 208 can be different materials. In some embodiments, the electrodes have a work function that differs by between 0.1 and 1 electron volt (eV), or by between 0.4 and 0.6 eV, etc.. For example, the electrode 208 can be TiN, which has a work function of 4.5-4.6 eV, while the electrode 206 can be n-type polysilicon, which has a work function of approximately 4.1-4.15 eV. Other electrode materials include p-type polysilicon (4.9-5.3 eV), transition metals, transition metal alloys, transition metal nitrides, transition metal carbides, platinum (5.7 eV), tungsten (4.5-4.6 eV), tantalum nitride (4.7-4.8 eV), molybdenum oxide (approximately 5.1 eV), molybdenum nitride (4.0-5.OeV), iridium (4.6-5.3 eV), iridium oxide (approximately 4.2 eV), ruthenium (approximately 4.7 eV), and ruthenium oxide (approximately 5.0 eV). Other potential electrodes include a titanium/aluminum alloy (4.1-4.3 eV), nickel (approximately 5.0 eV), tungsten nitride (approximately 4.3-5.0 eV), tungsten oxide (5.5-5.7 eV), aluminum (4.2-4.3 eV), copper or silicon-doped aluminum (4.1-4.4 eV), copper (approximately 4.5 eV), hafnium carbide (4.8-4.9 eV), hafnium nitride (4.7-4.8 eV), niobium nitride (approximately 4.95 eV), tantalum carbide (approximately 5.1 eV), tantalum silicon nitride (approximately 4.4 eV), titanium (4.1-4.4 eV), vanadium carbide (approximately 5.15 eV), vanadium nitride (approximately 5.15 eV), and zirconium nitride (approximately 4.6 eV). For some embodiments described herein, the higher work function electrode receives a positive pulse (as measured compared to a common reference potential) during a reset operation, although other configurations are possible. In other embodiments, the higher work function electrode receives a negative pulse during a reset operation. In some embodiments, the memory elements 102 use bipolar switching where the set and reset voltages have opposite polarities relative to a common electrical reference, and in some embodiments the memory elements 102 use unipolar switching where the set and reset voltages have the same polarity. In other embodiments, it is desirable to generally increase the work function of the electrodes in order to reduce leakage and create a more stable metal oxide/electrode interface. 2. Select Elements

[0054] The diode 202 is a select element that can be used to select a memory element for access from amongst several memory elements such as the several memory elements 102 of the memory array 100 (see Figure 1). The diode 202 controls the flow of current so that current only flows one way through the memory elements 102.

[0055] The diode 202 may include two or more layers of semiconductor material. A diode is generally a p-n junction, and doped silicon layers 216 and 218 can form the p-n junction. For example, doped silicon layer 216 can be a p-type layer and doped silicon layer 218 can be an n-type layer, so that a node 220 of the diode 202 is an anode and is connected to the first electrode 206. In this example, a node 222 of the diode 202 is a cathode and is connected to the signal line 106, which may be, for example, a bit line or word line, or connected to a bit line or word line. The nodes 220 and 222 are not necessarily physical features in the memory element 102, for example the electrode 206 may be in direct contact with the doped silicon layer 216. In other embodiments, one or more additional layers such as a low resistivity film are added between the electrode 206 and the doped silicon layer 216. [0056] In some embodiments, doped silicon layer 216 is an n-type layer and doped silicon layer 218 is a p-type layer, and the node 220 is a cathode of the diode 202 and the node 222 is an anode of the diode 202. An optional insulating layer 224 can be between the doped silicon layers 216 and 218 to create a p-i-n or n-i-p diode 202. In some embodiments the insulating layer 224 and one of the doped silicon layers 216 and 218 are formed from the same layer. For example, a silicon layer can be deposited, and a portion of the layer can be doped to form the doped silicon layer 216 or 218. The remaining portion of the layer is then the insulating layer 224. [0057] In other embodiments, one electrode of the memory element 102 can be doped silicon (e.g. p-type or n-type polysilicon), which can also act as a portion of the diode 202. Figure 6 illustrates a memory element 102 that shares an electrode with a diode 202 that is used as a select element. Since the diode 202 is made up of two layers of doped silicon, and since a layer of doped silicon can be used as an electrode of the memory element 202, a single layer of doped silicon (e.g. a layer of n-type polysilicon) can serve as an electrode of the memory element 102 and as a layer of the diode 202. By sharing a doped silicon layer between the diode 202 and the memory element 102, two layers, one doped silicon layer and a coupling layer between the diode 202 and the memory element 102, and their associated processing steps, can be eliminated.

3. Switching Polarity

[0058] A signal line (e.g. the signal line 104) is connected to the "second" electrode 208, and the signal line is configured to provide switching voltages to the second electrode 208. In some embodiments, the second electrode 208 has a higher work function than the first electrode 206, and the signal line 104 is configured to provide a negative set voltage relative to a common electrical reference, and a positive reset voltage relative to the common electrical reference. The embodiments may include those using a lower work function first electrode 206 (e.g. titanium nitride) and a higher work function second electrode such as platinum or ruthenium. For example, the common electrical reference may be ground (i.e. OV), the set voltage would then be a negative voltage (e.g. - 2V), and the reset voltage would be a positive voltage (e.g. 2V). The common electrical reference can be any voltage, however, such as +2 V or -2V.

[0059] In other embodiments, the second electrode 208 also has a higher work function than the first electrode 206, and the signal line 104 is configured to provide a positive set voltage and a negative reset voltage relative to a common electrical reference. For example, in a memory element having a doped silicon first electrode 206 (e.g. n-type polysilicon) and a higher work function second electrode 208 (e.g. titanium nitride), the reset voltage can be negative at the second electrode 208. See for example, Figures 16A and 16B.

[0060] In some embodiments, one switching voltage (e.g. the reset voltage) of the memory element can have a first polarity (e.g. a positive polarity) relative to the common electrical reference, and the other switching voltage (e.g. the set voltage) can have a negative polarity relative to the common electrical reference so that the memory element uses bipolar switching. In other embodiments, the switching voltages have the same polarity relative to a common reference and are referred to as unipolar switching. In some embodiments, it may be desirable to use unipolar switching to provide compatibility with certain configurations of other elements of the memory array (e.g. select elements). Additionally, the switching voltages can be voltage pulses (e.g. square wave pulses) having a limited duration, for example less than 1 ms, less than 50 μs, less than 1 μs, less than 50 ns, etc.

4. Polarity of Forming Voltage

[0061] Lower operating voltages are desirable for resistive switching memory elements to protect associated devices (e.g. diodes) in the memory array. Forming voltage is often the highest magnitude operating voltage, and reduction of the forming voltage is therefore an important goal to improve device operation and reliability. Forming voltage polarity has been shown to affect forming voltage magnitude in some embodiments.

[0062] In one example, memory elements were prepared with a higher work function electrode connected to ground and a lower work function electrode receiving the forming voltage pulse. A first example included an n-type polysilicon electrode, a hafnium oxide switching layer and a titanium nitride electrode. In this example, the titanium nitride electrode (i.e. the higher work function electrode) was grounded and positive and negative forming voltage pulses were applied to the n-type polysilicon electrode. The negative pulses had a median forming voltage of approximately -8V, while the positive pulses had a median forming voltage of approximately +13 V. In a second example, a memory element was prepared having a titanium nitride electrode, a hafnium oxide switching layer, and a platinum electrode. The higher work function electrode (here, the platinum electrode) was grounded, and the lower work function electrode (the titanium nitride electrode) received forming voltage pulses. In this example, the median forming voltage of negative pulses was -4.4V, while the median forming voltage using positive pulses was 6.4V.

[0063] The examples above demonstrate that a negative forming voltage applied at the lower- work function electrode can reduce the magnitude of forming voltage in some embodiments. It is believed that electron injection from the lower-work function electrode can reduce the magnitude of a negative polarity forming voltage compared to a positive polarity forming voltage. [0064] Additionally, it has been demonstrated for some embodiments that the greater the difference between the work function of the electrodes in a memory element, the smaller the magnitude of the forming voltage. For example, in a memory element with an n-type polysilicon electrode, a hafnium oxide switching layer and a titanium nitride electrode, the difference in work function is approximately 0.5 eV and a median forming voltage is -7V. Another memory element having an n-type polysilicon electrode, a hafnium oxide switching layer, and a platinum electrode has a work function difference of 1.6 eV and a median forming voltage of -5.5V. Therefore, in some embodiments it may be desirable to increase the work function difference between the electrodes to reduce the forming voltage, although it may not be necessary to increase the work function difference to 1.6 eV as in the example.

5. Other Characteristics

[0065] It may be desirable to have a low-leakage material as the resistive- switching layer 210 in order to aid memory retention. For example, the layer 210 may be a material that has a leakage current density less than 40 amps per square centimeter (A/cm2) measured at 0.5 volts (V) per twenty angstroms of the thickness of the metal oxide in an off state (e.g. a high resistance state) of the memory element.

6. 3-D Memory Structure

[0066] Figure 7 illustrates a portion of a three-dimensional memory 700 array using memory elements 102 described herein. The array 700 includes two word lines 702a and 702b, and a shared bit line 704. Two MIM stacks 204a and 204b and diodes 202a and 204b are shown in the array 700; a memory cell 706a includes an MIM stack 204a and a diode 202a, and a memory cell 706b includes an MIM stack 204b and a diode 202b.

[0067] The memory array 700 is configured so that the two memory cells 706a and 706b can use the same shared bit line 704. As shown here, the MIM stacks 204a and 204b both have their individual layers (i.e. electrodes 206 and 208 and switching layer 210) built in the same order. In other words, for both MIM stacks 204a and 204b, the electrode 206 is formed first, the switching layer 210 is formed on top of the electrode 206, and the electrode 208 is formed on top of the switching layer 210. As mentioned above, the order of deposition of the layers of the MIM stacks 204 may need to be the same in order to create an interface layer 214. However, in some embodiments the order of deposition can be reversed and the interface layer 214 created as a result of subsequent processes such as electrode deposition or annealing. [0068] The diodes 202a and 202b, on the other hand, are mirrors of each other. In other words the diode 202a has the layer 216 on the bottom, and the diode 202b has the layer 218 on the bottom. For example, the layer 216 may be the n-type layer and the layer 218 may be the p-type layer. Using this configuration, the diodes 202a and 202b are biased in opposite directions, which allows the memory cells 706 to both use the same shared bit line 704. As is shown in circuit diagrams 708a and 708b, the diodes can have any desired orientation, and the orientation may differ based on the configuration of the three-dimensional memory array.

7. Interface Layer and Oxygen Vacancies

[0069] Figures 8A and 8B illustrate the memory element 102 and the creation and manipulation of oxygen vacancies (defects) within the memory element 102 using an interface layer 214. The interface layer 214 is an oxide layer that can be created during the processing of other layers in the memory element 102. For example, the deposition of the switching layer 210 may include processing at a temperature (e.g. 2000C or greater) to create the interface layer 214. If, for example, the electrode 206 is doped silicon (e.g. polysilicon), the deposition of the switching layer 210 (using, for example, PVD or ALD) may include temperatures that can create a silicon oxide interface layer 214. The interface layer 214 can be eliminated in some embodiments, but in other embodiments, the interface layer 214 can be retained to improve retention of the switching layer 210 by improving leakage characteristics and to modulate defects (e.g. oxygen vacancies) in the switching layer 210. In some embodiments where the interface layer 214 is retained, the interface layer 214 may be relatively thin (e.g. less than or equal to 10A) to make the defects in the switching layer 210 visible to the electrode 206 (i.e. the interface layer 214 provides access to the defects of the switching layer 210) and to reduce the effect of the interface layer 214 on switching voltages. [0070] In one example, the bottom electrode 206 is polysilicon. Silicon, particularly, is known for attracting oxygen when heated and can draw oxygen from the metal oxide switching layer 210, leaving oxygen vacancies 802 in the switching layer 210 nearby creating a metal-rich metal oxide switching layer. Without being bound by theory, these oxygen vacancies 802 can serve as traps which modulate the current flow with the application of programming voltages to fill and empty such traps. The oxygen vacancies 802 need not be mobile. In some embodiments, the existence of an interface layer 214 can be combined with the existence of a metallic or partly metallic coupling layer 212 that also attracts oxygen, which can, depending on the processing conditions, create additional oxygen vacancies on both sides of the switching layer 210 or throughout the switching layer 210.

[0071] A thin or zero interlay er thickness interface layer 214 can be used to modulate the density of oxygen vacancies 802 in the switching layer 210. For example, a thinner interface layer 214 (e.g. 5 A vs. 10A) can increase the oxygen vacancy 802 density. Additionally, the thickness of the switching layer 210 can be optimized such that traps (e.g. oxygen vacancies 802) are more spatially equalized throughout the switching layer 210. For example, Figure 8 A shows a thicker switching layer 210, which has oxygen vacancies 802 concentrated near the interface layer 214, while Figure 8B shows a thinner switching layer 210 that has a more even distribution of oxygen vacancies 802. For example, in two memory elements using the same materials, the switching layer 210 of Figure 8 A may be 5θA while the thickness of the switching layer 210 in Figure 8B is 25 A. The distribution of oxygen vacancies 802 within the switching layer 210 can depend on several factors, including the materials used, the thickness of the interface layer 214, the processes used (e.g. temperatures of anneals used), etc. Figures 8A and 8B are only two examples of oxygen vacancy distribution, and it is understood that various other configurations are possible.

///. Memory Operation

[0072] During a read operation, the state of a memory element 102 can be sensed by applying a sensing voltage (i.e., a "read" voltage VREAD) to an appropriate set of signal lines 104 and 106. Depending on its history, a memory element that is addressed in this way may be in either a high resistance state or a low resistance state. The resistance of the memory element therefore determines what digital data is being stored by the memory element. If the memory element has a low resistance, for example, the memory element may be said to contain a logic one (i.e., a "1" bit). If, on the other hand, the memory element has a high resistance, the memory element may be said to contain a logic zero (i.e., a "0" bit). During a write operation, the state of a memory element can be changed by application of suitable write signals to an appropriate set of signal lines 104 and 106.

[0073] Figure 9 is a logarithm of current (I) versus voltage (V) plot 900 for a memory element 102. Figure 9 illustrates the set and reset operations to change the contents of the memory element 102. Initially, memory element 102 may be in a high resistance state ("HRS", e.g., storing a logic zero). In this state, the current versus voltage characteristic of memory element 102 is represented by solid line HRS 902. The high resistance state of memory element 102 can be sensed by read and write circuitry using signal lines 104 and 106. For example, read and write circuitry may apply a read voltage VREAD to memory element 102 and can sense the resulting "off" current IOFF that flows through memory element 102. When it is desired to store a logic one in memory element 102, memory element 102 can be placed into its low- resistance state. This may be accomplished by using read and write circuitry to apply a set voltage VSET across signal lines 104 and 106. Applying VSET to memory element 102 causes memory element 102 to switch to its low resistance state, as indicated by dashed line 906. In this region, the memory element 102 is changed so that, following removal of the set voltage VSET, memory element 102 is characterized by low resistance curve LRS 904. As is described further below, the change in the resistive state of memory element 102 may be because of the filling of traps (i.e., a may be "trap-mediated") in a metal oxide material. VSET and VRESET can be generally referred to as "switching voltages."

[0074] The low resistance state of memory element 102 can be sensed using read and write circuitry. When a read voltage VREAD is applied to resistive switching memory element 102, read and write circuitry will sense the relatively high "on" current value ION, indicating that memory element 102 is in its low resistance state. When it is desired to store a logic zero in memory element 102, the memory element can once again be placed in its high resistance state by applying a reset voltage VRESET to memory element 102. When read and write circuitry applies VRESET to memory element 102, memory element 102 enters its high resistance state HRS, as indicated by dashed line 908. When the reset voltage VRESET is removed from memory element 102, memory element 102 will once again be characterized by high resistance line HRS 904. Voltage pulses can be used in the programming of the memory element 102. For example, a 1 ms, lOμs, 5μs, 500ns, etc. square pulse can be used to switch the memory element 102; in some embodiments, it may be desirable to adjust the length of the pulse depending on the amount of time needed to switch the memory element 102.

[0075] A forming voltage VFORM is a voltage applied to the memory element 102 to ready the memory element 102 for use. Some memory elements described herein may need a forming event that includes the application of a voltage greater than or equal to the set voltage or reset voltage. Once the memory element 102 initially switches the set and reset voltages can be used to change the resistance state of the memory element 102.

[0076] The bistable resistance of resistive switching memory element 102 makes memory element 102 suitable for storing digital data. Because no changes take place in the stored data in the absence of application of the voltages VSET and VRESET, memory formed from elements such as element 102 is non- volatile. [0077] Figure 10 is a current (I) versus voltage (V) plot 1000 for a memory element 102 that demonstrates a resistance state change. The plot 1000 shows a voltage ramp applied to the memory element 102 along the x-axis and the resulting current along a y-axis. The line 1002 represents the response of an Ohmic material when the ramped voltage is applied. An Ohmic response is undesirable, since there is no discrete voltage at which the set or reset occurs.

[0078] Generally, a more abrupt response like graph 1004 is desired. The graph 1004 begins with an Ohmic response 1004a, and then curves sharply upward 1004b. The graph 1004 may represent a set operation, where the memory element 102 switches from the HRS 902 to the LRS 904.

[0079] Without being bound by theory, non-metallic percolation paths are formed during a set operation and broken during a reset operation. For example, during a set operation, the memory element 102 switches to a low resistance state. The percolation paths that are formed by filling traps increase the conductivity of the metal oxide, thereby reducing (i.e., changing) the resistivity. The voltage represented by 404b is the set voltage. At the set voltage, the traps are filled and there is a large jump in current as the resistivity of the metal oxide decreases.

IV. Materials

[0080] A variety of metal oxides can be used for the switching layer 210 of the memory elements 102 described herein. In some embodiments, the memory elements 102 exhibit bulk-switching properties and are scalable. In other words, it is believed that defects are distributed throughout the bulk of the switching layer 210, and that the switching voltages (i.e. VSET and VRESET) increase or decrease with increases or decreases in thickness of the metal oxide. In other embodiments, the memory elements 102 exhibit interface-mediated switching activity. Other embodiments may exhibit a combination of bulk- and interface-mediated switching properties, which may be scalable while still exhibiting defect activity at layer interfaces.

A. Higher-Bandgap Materials for Switching Layer

[0081] Figures 11 and 12 are graphs showing the relationship between thicknesses of a metal oxide layer and resulting set voltages, reset voltages, and on/off current ratios for several materials (metal oxides) used in memory elements described herein. These graphs illustrate the characteristics of a memory element that includes two electrodes and a single layer of metal oxide disposed in between (i.e. without a coupling layer 212) and indicate that certain materials exhibit bulk- switching properties. As can be seen in Figure 11, for memory elements including hafnium oxide 1102, aluminum oxide 1104, or tantalum oxide 1106, set voltage increases with (i.e. is dependent on) thickness, and in some embodiments and for these materials the set voltage is at least one volt (V) per one hundred angstroms (A) of the thickness of a metal oxide layer in the memory element. In some embodiments, an increase in the thickness of the metal oxide layer of IOOA increases the set voltage by at least IV. Similarly, as shown in Figure 12, reset voltage for hafnium oxide 1202, aluminum oxide 1204, or tantalum oxide 1206 also depends on thickness. These data therefore support a bulk-controlled set/reset mechanism for these materials, since a linear relationship indicates the formation of percolation paths throughout the bulk of the metal oxide. In other words, for a thicker material, more voltage is needed to fill the traps.

[0082] Hafnium oxide (HfO2, 5.7 electron volts (eV)), aluminum oxide (Al2O3, 8.4 eV) and tantalum oxide (Ta2Os, 4.6 eV) all have a bandgap greater than 4 eV, while titanium oxide (TiO2, 3.0 eV) and niobium oxide (Nb2Os, 3.4 eV) have bandgaps less than 4 eV. Other higher bandgap metal oxides that can be used with various embodiments described herein include yttrium oxide (Y2O3, 6.0 eV), zirconium oxide (ZrO2, 5.8 eV), lanthanum oxide (La2O3, 4.3 eV) and cerium oxide (CeO2, 5.5 eV) (also see Table 1). As shown in Figures 11 and 12, set voltages for titanium oxide 1108 and niobium oxide 1110 and reset voltages for titanium oxide 1208 and niobium oxide 1210 do not increase with thickness. Therefore, a higher bandgap (i.e., bandgap greater than 4 eV) metal oxide exhibits bulk mediated switching and scalable set and reset voltages. Table 1 summarizes the higher-bandgap materials that can be used for switching layers 210.

Figure imgf000025_0001

Table 1

B. Coupling Layer Materials

[0083] The coupling layer 212 can be a metal, metal oxide, or metal/metal oxide material that is chosen to complement the material of the switching layer 210. For example, the coupling layer 212 may be chosen to complement a bandgap of the switching layer 210. In some embodiments, the coupling layer 212 has a bandgap that is approximately equal to or greater than the bandgap of the switching layer 210. Without being bound by theory, this can improve retention of the memory element 102 by improving leakage characteristics. As is shown in the band diagrams 300 and 400 in Figures 3 and 4, a zirconium oxide coupling layer 212 has a bandgap that is greater than the bandgap of the switching layer 210. The higher bandgap coupling layer 212 can promote retention by reducing leakage from the switching layer 210 into the electrode 208.

[0084] In other examples, the coupling layer 212 can have a lower bandgap that the switching layer 210 (for example, the coupling layer 212 can be titanium oxide (bandgap = 3.5 eV)). In another example, the coupling layer 212 can be a titanium/titanium oxide layer (see Figure 2B).

[0085] In some examples, a switching layer 210 is hafnium oxide (bandgap = 5.7 eV) and has a first thickness (e.g. 20-100 A). The coupling layer can then be either a zirconium oxide (ZrO2, bandgap = 5.8 eV), aluminum oxide (AI2O3, bandgap = 8.7 eV), titanium oxide, titanium/titanium oxide, etc. (see Table 3) layer having a second thickness that is less than 25 percent of the first thickness. For example, the coupling layer can be between 1 and IOA thick, or 5 A or 8 A.

[0086] It has been shown (see Table 3) for some materials systems that switching performance is better when the coupling material is in a discrete coupling layer 212 rather than dispersed throughout the switching layer 210 (e.g. using a hafnium oxide switching layer 210 and an aluminum oxide coupling layer 212 rather than a HfAlOx layer). It is believed that the defects created at the interface between the coupling layer 212 and the switching layer 210 can improve switching characteristics. [0087] In some embodiments, the coupling layer 212 can be used to dope into the switching layer 210. The doping can be either aliovalent or isovalent. In aliovalent doping, the doping species has a different valency than that of the layer being doped. For example, the switching layer 210 can be hafnium oxide and the coupling layer 212 can be aluminum oxide. A typical defect species of hafnium oxide is Hf+4, and a typical defect species of aluminum oxide is Al+3. Al+3 ions displace Hf+4 ions in the hafnium oxide layer, thereby creating defects and traps. In some embodiments, the doping is isovalent, and a coupling layer 212 (e.g., zirconium oxide) may have a metal having the same most common oxidation state (e.g., +4) as a metal of the switching layer 210. In these cases, aliovalent doping may still occur when other species having different oxidation states (e.g., Zr+3) diffuse into the switching layer 210. C. Electrodes

[0088] Various electrodes can be used for the memory elements 102. Some embodiments describe memory elements 102 that use electrodes 206 and 208 that are made of different materials. For example, the electrodes 206 and 208 can have materials that are chosen to have different work functions (e.g. between 0.4 eV and 0.6 eV different, or between 0.1 eV and 1.0 eV different), which it is believed may facilitate bipolar switching in some systems.

[0089] Materials that can be used for electrodes 206 and 208 include doped silicon (e.g. p-type or n-type silicon), titanium nitride, tantalum nitride, tungsten, tungsten nitride, molybdenum nitride, molybdenum oxide, platinum, ruthenium, ruthenium oxide, iridium, and iridium oxide. Other materials are described above and in Table 2. Electrode "pairs" may include n-type polysilicon and titanium nitride; titanium nitride, tungsten nitride, or tantalum nitride and platinum, ruthenium, ruthenium oxide, iridium, iridium oxide, molybdenum nitride, or molybdenum oxide, although other pairings are possible. Other electrodes include metal suicides (see Figure 14) and electrolessly deposited electrodes (e.g. electroless nickel). These electrodes can be used to eliminate the silicon oxide interface layer 214 and therefore reduce forming voltages.

[0090] In some embodiments, the electrodes 206 and 208 can be chosen to dope isovalently into the switching layer 210. In other words, at least one of the electrodes 206 and 208 has a most common oxidation state or valency that is the same as the most common oxidation state or valency of the switching layer 210. In some memory elements 102, it is believed that isovalent doping can create deep traps in the switching layer 210. For example, the electrode 206 can be doped silicon (+4 valency) and the switching layer 210 can be hafnium (+4 valency) oxide. In other embodiments, the electrodes 206 or 208 can contain titanium nitride (titanium has +4 valency), platinum (+4 valency), etc. The silicon isovalently dopes into the hafnium oxide, creating deep traps that can be used to create a greater resistance change and a higher on/off current ratio. Aliovalent doping may in some instances create donors and acceptors, which are shallow traps and may result in a resistive-changing memory that does not exhibit as great a difference in resistance states. D. Material Systems

[0091] Table 2 includes a list of possible materials systems for memory elements 102 described herein. Although certain combinations are described in Table 2, various other configurations are possible within the bounds of the memory elements 102 described herein. For example, other electrode materials (e.g. molybdenum nitride or molybdenum nitride) or switching materials can be used.

Figure imgf000028_0001

Table 2

V. Processing

[0092] Figures 13 and 14 are flowcharts describing processes 1300 and 1400 for controlled deposition of interface layers 214. The process 1300 describes the deposition of a switching layer 210 using an ALD process that reduces the amount of oxygen introduced to create a metal-rich switching layer 210 and increase the amount of defects in the switching layer 210. Additionally, the process 1300 can be used to tailor the size of the interface layer 214 by selecting processing parameters to obtain a desired thickness of the interface layer 214. The process 1400 describes the deposition of a suicide electrode 206 that significantly reduces or eliminates the interface layer 214.

[0093] Atomic layer deposition (ALD) is a process used to deposit conformal layers with atomic scale thickness control during various semiconductor processing operations. For depositing a metal oxide, ALD is a multi-step self-limiting process that includes the use of two reagents: a metal precursor and an oxygen source (e.g. an oxidant). Generally, a first reagent is introduced into a processing chamber containing a substrate and adsorbs on the surface of the substrate. Excess first reagent is purged and/or pumped away. A second reagent is then introduced into the chamber and reacts with the adsorbed layer to form a deposited layer via a deposition reaction. The deposition reaction is self-limiting in that the reaction terminates once the initially adsorbed layer is consumed by reaction with the second reagent. Excess second reagent is purged and/or pumped away. The aforementioned steps constitute one deposition or ALD "cycle." The process is repeated to form the next layer, with the number of cycles determining the total deposited film thickness. [0094] Returning to Figure 13, the process 1300 begins with depositing a bottom electrode on a substrate in operation 1302. The bottom electrode (e.g. the electrode 206) may be one of the electrode materials described above; however, in one embodiment, the bottom electrode is a polysilicon electrode that may form a silicon dioxide interface layer 214 during the deposition of the switching layer 210. In other embodiments, the bottom electrode is a metal electrode that can also oxidize during the deposition of the switching layer 210.

[0095] In operation 1304, a thin PVD metal oxide layer is optionally deposited on the substrate. The thin PVD metal oxide layer can be used to eliminate the interface layer 214 since the PVD deposition process has been shown to not promote the growth of the interface layer 214 (see e.g. Figure 5). Once the thin (e.g. <lθA) PVD metal oxide layer is deposited, the ALD process in operation 1306 can be performed. [0096] In operation 1306, a switching layer 210 is deposited using ALD. The operation 1304 includes several component operations 1308-1320 that describe several cycles of the ALD process. Some of these operations are optional, or may be completed in a different order.

[0097] In operation 1308, the deposition temperature of the ALD process is optionally lowered. The deposition temperature may be lowered by lowering the temperature of a heated substrate pedestal (i.e. the pedestal temperature), for example. In some examples, the deposition temperature or pedestal temperature may be 2500C or less, 2000C or less, 175°C or less, etc. Lower temperatures may change the equilibrium conversion of surface species during oxidation, altering the concentration of electrical defects in the switching layer 210. Additionally, the reduced deposition temperature can reduce or eliminate the interface layer 214 by reducing the rate of thermal oxidation. For example, when using a silicon electrode 206, reducing the ALD deposition temperature to below 2000C may substantially reduce any interface layer 214.

[0098] In operation 1310, the precursor source is maintained at a desired pressure. The desired vapor pressure can be achieved by controlling the temperature of the precursor source. The precursor source is external to the ALD deposition chamber, and therefore can be maintained at a temperature different than the temperature of the deposition chamber. The desired temperature and pressure depends on the precursor used. For example, when using tetrakis(dimethlyamino)hafnium (TDMAH) to deposit hafnium oxide, the precursor source can be maintained at 30-1000C, or 40- 500C. In some embodiments, the temperature of the precursor source can be increased to increase the partial pressure of the precursor, which can also create a more metal-rich switching layer by increasing the concentration of metal precursor in the chamber. In operation 1312, the precursor is introduced to the substrate including the bottom electrode to begin the ALD process.

[0099] Operations 1314 and 1316 describe the treatment of the oxygen source used to form the metal oxide. Depending on the characteristics of the memory element 102, either or both of operations 1314 and 1316 can be used to control the thickness of the interface layer 214. The oxygen source can be ozone, oxygen, water vapor, isopropyl alcohol (IPA), ethanol or another alcohol, or other ALD oxygen sources. For some configurations, using water vapor as an oxygen source in an ALD process has resulted in memory elements having improved switching characteristics. [00100] In operation 1314, when the oxygen source is a liquid or solid such as water or isopropyl alcohol, it is maintained at a lower vapor pressure than is typical to create a switching layer 210 having less oxygen. The partial pressure of the oxidant can be reduced by reducing the source temperature which reduces its vapor pressure. The partial pressure can also be reduced by diluting the oxidant with an inert gas such as argon. Some oxidants such as oxygen and ozone are gases at their sources and therefore their partial pressure cannot be manipulated by changing the source temperature. A third method of reducing the degree of oxidation is through selection of the oxidant itself. For example, ozone and oxygen tend to be more oxidizing (i.e. more quickly create a layer having more oxygen), while water vapor is less oxidizing, and IPA and ethanol are less oxidizing still. Restricting the amount of the oxygen source in the chamber still allows the film to be self-limiting, while reducing the amount of oxygen in the film. Unreacted ligands attached to metal atoms can be partially or nearly fully removed through post-deposition treatments. The oxygen- deficient film will then have oxygen vacancies, which are defects that can be used to control the switching of the memory element 102.

[00101] To deposit a metal-rich hafnium oxide switching layer 210, for example, water vapor can be used as the oxygen source, and the water vapor source can be held at a reduced temperature such as 0 to 100C. The reduced temperature reduces the vapor pressure of the oxygen source, effectively reducing the amount of oxidation per ALD cycle. Hafnium oxide films formed using this technique can result in elemental compositions of HfOi 2 to Hfθi.9, or Hfθi.7. Generally, oxygen concentrations can be reduced to 60-95% of stoichiometric compositions (i.e. the amount of oxygen is between 60 and 95% of a stoichiometric metal oxide, e.g. Hfθi.2 to Hfθi.9). IPA or ethanol can be used to provide oxygen, but at the same temperature will provide less oxygen than water vapor or the other oxygen sources described above. IPA or ethanol may therefore be able to deposit metal-rich films using a room temperature source, although a similar temperature reduction can also be used with IPA and ethanol to reduce the amount of oxygen in the switching layer 210.

[00102] In operation 1318, the oxygen source is introduced to the substrate to create an ALD layer of metal oxide. A single ALD cycle may deposit a film having a thickness of 0.5 A, for example, and multiple cycles are typically needed to build a switching layer 210 of the desired thickness. In operation 1320, if more cycles are needed, the process 1300 returns to operation 1308. If no more cycles are needed, the process 1300 continues to operation 1322.

[00103] In operation 1322, a coupling layer is deposited. The coupling layer 212 can be a thin layer, for example less than 25 percent the thickness of the switching layer. The coupling layer 212 can be deposited using any deposition method, such as

ALD, PVD, etc. In operation 1324, the top electrode (e.g. the electrode 208) is deposited.

[00104] In operation 1326, the memory element is annealed. The annealing can remove unreacted precursor ligands that may exist in the film because of the low deposition temperature of the ALD process. In one example, the element is annealed using a hydrogen/argon mixture (e.g. 2-10% hydrogen, 90-98% argon), although other anneals such as vacuum anneals, oxidizing anneals, etc. can be used.

[00105] Returning to Figure 14, the process 1400 describes the formation of a bottom electrode 206 for use in the memory elements 102. The process 1400 describes the deposition of a suicide electrode that can be used to remove the interface layer 214 if so desired. In some embodiments, a suicide electrode does not form an interface layer 214 during the deposition of the switching layer 210. The process

1400 can in some embodiments, be used in conjunction with the process 1300. For example, the operations 1402-1412 of the process 1400 can be substituted into the operation 1302 of the process 1300.

[00106] In operation 1402, a bottom electrode (e.g. the electrode 206) is deposited on a substrate. The bottom electrode is a metal suicide, for example a titanium, cobalt, nickel, palladium, or platinum suicide, that is deposited according to the operations 1404-1412.

[00107] In operation 1404, silicon is deposited on the substrate. In operation 1406, a metal such as titanium, cobalt, nickel, molybdenum, palladium, or platinum is deposited on the silicon. In operation 1408 a thermal treatment is performed to form the suicide layer by interdiffusing the silicon into the metal. In operation 1410, any unreacted metal is stripped from the electrode, and in operation 1412, the electrode can be optionally annealed to lower the resistivity of the electrode. [00108] After the suicide electrode is deposited, in operation 1414, a switching layer is deposited on the electrode (for example using techniques described in the process 1300), and in operation 1416 a top electrode (e.g. the electrode 208) is deposited over the switching layer. The suicide electrode resists the formation of oxide layers, and therefore does not form the interface layer 214. Although in some embodiments it may be desirable to retain an interface layer 214, in others it is more desirable to eliminate the interface layer 214, and the process 1400 is an alternative technique for doing so.

[00109] Figure 15 is a flowchart describing a process 1500 for forming a memory element according to various embodiments. The process 1500 can be used in conjunction with the process described in operation 1300 if so desired. [00110] In operation 1502, a bottom electrode is deposited on a substrate. The bottom electrode can be, for example, n-type polysilicon or other materials described herein. In operation 1504, a switching layer 210 is deposited on the bottom electrode. The switching layer 210 can be any of the materials described above, for example, HfOx, CuOx, ZnOx, CeOx, etc. The switching layer 210 can be deposited using appropriate deposition techniques such as ALD or PEALD.

[00111] In operation 1506, the switching layer 210 is optionally doped. For example, the switching layer 210 can be doped to create a p-type switching layer (e.g. a p-type ZnOx or CuOx layer). In operation 1508 a coupling layer, such as a metallic or partially metallic (e.g. Ti or Ti/TiOx) or metal oxide (e.g. ZrO2, AI2O3) is deposited on the switching layer.

[00112] In operation 1210, the memory element is annealed, for example using a rapid thermal oxidation (RTO), a hydrogen/argon mixture, a vacuum anneal, etc. The annealing can cause oxygen to migrate from the metal oxide switching layer 210 to the coupling layer 212 in some embodiments (see e.g. Figure 2B).

VI. Representative Data

A. Switching Characteristics

[00113] Table 3 contains various switching metrics for memory elements formed using embodiments described herein, and other memory elements as a comparison: [00114] HfOxZTiO2 refers to a memory element including an n-type polysilicon electrode 206, a 5θA thick hafnium oxide switching layer 210 deposited at 2500C, an 8A titanium oxide coupling layer 212 deposited at

2500C, and a titanium nitride electrode 208.

[00115] HfOxZAl2Os refers to a memory element including an n-type polysilicon electrode 206, a 5θA thick hafnium oxide switching layer 210 deposited at 2500C, an 8A aluminum oxide coupling layer 212 deposited at

2500C, and a titanium nitride electrode 208.

[00116] HfOx/Zrθ2 refers to a memory element including an n-type polysilicon electrode 206, a 5θA thick hafnium oxide switching layer 210 deposited at 2500C, an 8A zirconium oxide coupling layer 212 deposited at

2500C, and a titanium nitride electrode 208.

[00117] HfAlxOy refers to a memory element including an n-type polysilicon electrode 206, a 58A aluminum-doped hafnium oxide switching layer 210 deposited at 2500C, and a titanium nitride electrode 208.

Figure imgf000034_0001

Table 3

[00118] All data are for bipolar switching, and yield refers to the percentage in a given sample of memory elements that reliably switch. The electrode 208 is grounded and the pulses are applied at the electrode 206. As can be seen, the higher bandgap coupling layers in the HfOxZAl2Os and HfOxZZrO2 memory elements show improved forming or reset voltages and improved cycling yields.

[00119] The HfAlxOy and the HfOxZAl2Os memory elements have the same thickness and the same material components. However, the HfAlxOy memory element is aluminum-doped, and has the aluminum dispersed throughout the hafnium oxide layer, while the HfOxZAl2Os memory element has a bulk hafnium oxide layer and a small aluminum oxide coupling layer. The switching characteristics for the HfOx/Al2θ3 are better, suggesting that the improved switching may be due to defects formed at the interface between the coupling layer 212 and the switching layer 210.

B. Interface Layer

[00120] Techniques described in the process 1300 were used to deposit a memory element 102 that substantially eliminated the interface layer 214. Aluminum oxide was deposited using trimethylaluminum and water vapor. The amount of water vapor in the gas phase was restricted by lowering the temperature of the water vapor source to 1-5°. Using this technique, the thickness of the interface layer 214 was reduced from 1.1 nm (when the water source was held at room temperature) to approximately zero. In some embodiments, elimination of the interface layer 214 may reduce forming voltage.

C. Polarity and Switching Yield

[00121] Figure 16A is a graph 1600 showing high cycling yield when the low work function electrode receives a negative voltage set pulse and positive voltage reset pulse. The memory element configuration includes an n-type polysilicon electrode 206, an atomic layer deposition (ALD)-deposited hafnium oxide switching layer 210, an ALD-deposited titanium oxide coupling layer 212 and a physical vapor deposition (PVD)-deposited titanium nitride electrode 208. The plot 1602 reflects memory elements in which the lower work function electrode (here, the n-type polysilicon electrode) receives a negative voltage set pulse and a positive voltage reset pulse relative to a common electrical reference. The common electrical reference can be any voltage, such as ground, +2V, etc. An electrode is said to receive a voltage if that voltage is visible at the electrode; for example the polysilicon electrode 206 may see a positive reset voltage even if the voltage pulse originates elsewhere. [00122] As can be seen in the plot 1602, approximately 85% of the memory elements in which the lower work function electrode receives a negative voltage set pulse last to 100 cycles. In contrast, the plot 1604 reflects memory elements in which the lower work function electrode receives a positive voltage set pulse and a negative voltage reset pulse. As can be seen, none of these memory elements survive past 60 cycles, and very few survive to 10 cycles. It can be surmised then that when using the memory elements described herein, the lower work function electrode should receive a negative voltage set pulse and a positive voltage reset pulse. [00123] Figure 16B is a graph 1620 showing high cycling yield when two electrodes have work functions that differ by between 0.1 eV and 1.0 eV. Memory elements that include electrodes 206 and 208 having work functions that differ by more than 0.1 eV but less than 1.0 eV show high cycling yield compared to memory elements that include electrodes having work functions that differ by more than 1.0 eV. The graph 1620 shows cycling yields for two memory element configurations represented by plots 1622 and 1624. The configuration represented by plot 1622 includes an n-type polysilicon electrode 206, a hafnium oxide switching layer 210, a titanium oxide coupling layer 212, and a titanium nitride electrode 208. N-type polysilicon has a work function of approximately 4.1 eV while titanium nitride has a work function of approximately 4.5-4.6 eV, giving a difference of approximately 0.4- 0.5 eV. The configuration represented by plot 524 includes an n-type polysilicon electrode 402, a hafnium oxide switching layer 406, a titanium oxide layer 418, and a platinum electrode 404. N-type polysilicon has a work function of approximately 4.1 eV while platinum has a work function of approximately 5.7 eV, giving a difference of approximately 1.6eV. As can be seen, the plot 522 shows high cycling yield, while the plot 524 shows low cycling yield, indicating electrodes used in memory elements described herein are more effective when they have work function differences of less than 1.0 eV.

D, Thickness of Coupling Layer and Switching Yield

[00124] Cycling yield has been shown to improve with increased thickness of the higher-bandgap material (i.e. materials with a bandgap greater than 4 eV) and a coupling layer. Table 4 lists the cycling yield for several memory elements that were evaluated using bipolar switching where the lower work function electrode is positive during the set pulse. For each configuration, 18 elements were tested, and each memory element includes an n-type polysilicon electrode beneath the metal oxide switching layer and a titanium nitride electrode above the coupling layer. As can be seen, the highest cycling yield is for those memory elements that include a 5θA thick hafnium oxide layer and a 5 A thick titanium oxide layer, although other combinations (e.g. the 7θΑ hafnium oxide layer) also show acceptable results.

Figure imgf000037_0001

Table 4

[00125] Memory elements in which the thickness of the metal oxide layer is more than 80% of the total thickness of the metal oxide layer and the coupling layer can show improved cycling yield. For example, the memory elements that includes a 5OA hafnium oxide layer and a 5 A titanium oxide layer includes more than 80% hafnium oxide by thickness and shows high cycling yield. By contrast, the memory elements that includes a 3OA hafnium oxide layer and a 5 A titanium oxide layer includes less than 80% hafnium oxide by thickness and show low switching yield. Therefore, the presence of higher bandgap (i.e. greater than 4 eV) materials leads to improved switching characteristics.

[00126] Additionally, another memory element design includes an MIS-style stack having a doped polysilicon bottom electrode, a 5 A titanium oxide layer adjacent to the polysilicon electrode, and a 5θA hafnium oxide layer between the titanium oxide layer and a titanium nitride top electrode. This design showed a cycling yield of 33.3% (6 of 18 devices surviving 100 cycles), illustrating that the arrangement of metal oxide layers within the memory element can affect device survival. [00127] Other higher bandgap materials (e.g. TaOx, AlOx, LaOx, ZrOx, YOx) can substitute or supplement the hafnium oxide in the table above and show good results. For example, the hafnium oxide layer can be replaced with an aluminum oxide layer, a lanthanum oxide layer, a hafnium aluminum oxide layer, or a hafnium lanthanum oxide layer.

VIL Representative Embodiments [00128] In accordance with an embodiment, a resistive-switching memory element is provided that includes a first electrode and a second electrode, a switching layer between the first electrode and the second electrode comprising hafnium oxide and having a first thickness, and a coupling layer between the switching layer and the second electrode, the coupling layer comprising a material selected from the group consisting of aluminum oxide and zirconium oxide, the coupling layer having a second thickness that is less than 25 percent of the first thickness.

[00129] In accordance with a further embodiment, the first electrode of the memory element is doped silicon and the memory element is configured to receive a negative reset voltage relative to a common electrical reference and a positive set voltage relative to the common electrical reference at the second electrode.

[00130] In accordance with a further embodiment, the first electrode of the memory element comprises a first material and the second electrode comprises a second material, and the first material is different from the second material.

[00131] In accordance with a further embodiment, the first material of the first electrode is doped silicon and the second material of the second electrode is titanium nitride.

[00132] In accordance with a further embodiment, the first thickness of the memory element is between 20 and 100 angstroms.

[00133] In accordance with a further embodiment, the switching layer of the memory element comprises a hafnium oxide material having an elemental composition of between Hfθi.2 and Hfθi.7.

[00134] In accordance with a further embodiment, the first material of the first electrode is n-type polysilicon.

[00135] In accordance with a further embodiment, at least one of the first electrode and the second electrode of the memory element has a same most common oxidation state as the switching layer.

[00136] In accordance with a further embodiment, the memory element further includes an interface layer between the first electrode and the switching layer, the interface layer having a thickness less than IOA.

[00137] In accordance with a further embodiment, the interface layer of the memory element comprises silicon oxide. [00138] In accordance with a further embodiment, a work function of the second electrode of the memory element is greater than a work function of the first electrode, and wherein the first electrode is configured to receive a forming voltage pulse having a negative voltage relative to a common electrical reference.

[00139] In accordance with another embodiment, a resistive-switching memory element is provided, including a first electrode and a second electrode, a switching layer between the first electrode and the second electrode, the switching layer comprising a first metal oxide having a first bandgap greater than 4 electron volts

(eV), the switching layer having a first thickness, and a coupling layer between the switching layer and the second electrode, the coupling layer comprising a second metal oxide having a second bandgap greater than or equal to the first bandgap, the coupling layer having a second thickness that is less than 25 percent of the first thickness.

[00140] In accordance with a further embodiment, the first metal oxide of the memory element has an oxygen concentration that is between 60 and 95% of stoichiometric.

[00141] In accordance with a further embodiment, the first electrode of the memory element is selected from the group consisting of doped silicon and titanium nitride, and the second electrode is selected from the group consisting of molybdenum nitride, molybdenum oxide, titanium nitride, tungsten, tantalum nitride, molybdenum nitride, molybdenum oxide, platinum, ruthenium, nickel, iridium, iridium oxide, and ruthenium oxide.

[00142] In accordance with a further embodiment, the first thickness of the switching layer is between 20 and 100A.

[00143] In accordance with a further embodiment, a first metal of the first metal oxide has a first most common oxidation state that is different from a second most common oxidation state of the second metal of the second metal oxide.

[00144] In accordance with a further embodiment, a first metal of the first metal oxide and a second metal of the second metal oxide have a same most common oxidation state. [00145] In accordance with a further embodiment, a second metal of the second metal oxide has a second most common oxidation state that is than less than or equal to a first most common oxidation state of a first metal of the first metal oxide. [00146] In accordance with a further embodiment, the first metal oxide is hafnium oxide and the second metal oxide is selected from the group consisting of zirconium oxide and aluminum oxide.

[00147] In accordance with a further embodiment, the first metal oxide is selected from the group consisting of hafnium oxide, tantalum oxide, aluminum oxide, zirconium oxide, and yttrium oxide, and the second metal oxide is selected from the group consisting of zirconium oxide and aluminum oxide. [00148] In accordance with a further embodiment, the first electrode comprises doped silicon and further comprising an interface layer between the first electrode and the switching layer comprising silicon oxide and having a thickness of less than 10A. [00149] In accordance with a further embodiment, the first electrode comprises a suicide chosen from the group consisting of titanium suicide, cobalt suicide, nickel suicide, palladium suicide, and platinum suicide.

[00150] In accordance with a further embodiment, the memory element is part of a three-dimensional memory array.

[00151] In accordance with another embodiment, a method for forming a resistive-switching memory element is provided, including depositing a first electrode on a substrate, depositing a switching layer comprising a metal oxide over the first electrode using atomic layer deposition (ALD), the depositing the switching layer further comprising maintaining a precursor at greater than 40 degrees Celsius, introducing the precursor to the substrate, maintaining an oxygen source at less than 10 degrees Celsius, introducing the oxygen source to substrate, and depositing a second electrode over the switching layer.

[00152] In accordance with a further embodiment, the oxygen source is at least one of water vapor, isopropyl alcohol (IPA), and ethanol.

[00153] In accordance with a further embodiment, the metal oxide is chosen from the group consisting of hafnium oxide, tantalum oxide, aluminum oxide, yttrium oxide, and zirconium oxide. [00154] In accordance with a further embodiment, the metal oxide has an oxygen concentration that is between 60 and 95 percent of stoichiometric.

[00155] In accordance with a further embodiment, the metal oxide is hafnium oxide and has an elemental composition that is between HfOi 2 and HfOi 7.

[00156] In accordance with a further embodiment, a deposition temperature for the ALD is less than 250 degrees Celsius.

[00157] In accordance with a further embodiment, a deposition temperature for the ALD is approximately 200 degrees Celsius.

[00158] In accordance with a further embodiment, the method includes annealing the memory element after depositing the second electrode.

[00159] In accordance with a further embodiment, the method further includes depositing a physical vapor deposition (PVD) layer over the first electrode using physical vapor deposition, wherein the switching layer is deposited over the PVD layer, and wherein the PVD layer comprises a same material as the switching layer.

[00160] In accordance with a further embodiment, the method further includes depositing a coupling layer over the switching layer, the coupling layer having a thickness that is less than 25 percent of a thickness of the switching layer.

[00161] In accordance with a further embodiment, a bandgap of the coupling layer is greater than a bandgap of the switching layer.

[00162] In accordance with a further embodiment, the coupling layer is selected from the group consisting of aluminum oxide and zirconium oxide.

[00163] In accordance with another embodiment, a method for forming a resistive-switching memory element is provided, including depositing a first electrode on a substrate, depositing a switching layer over the first electrode, the switching layer having a first thickness and a first bandgap that is greater than 4 electron volts (eV), depositing a coupling layer over the switching layer, the coupling layer having a second thickness that is less than 25 percent of the first thickness and a second bandgap that is greater than or equal to the first bandgap, and depositing a second electrode over the coupling layer.

[00164] In accordance with a further embodiment, the first electrode is doped silicon and depositing a switching layer comprises forming an interface layer comprising silicon oxide between the first electrode and the switching layer, the interface layer having a thickness of less than 10A.

[00165] In accordance with a further embodiment, the switching layer is chosen from the group consisting of hafnium oxide, tantalum oxide, aluminum oxide, yttrium oxide, and zirconium oxide.

[00166] In accordance with a further embodiment, the coupling layer is chosen from the group consisting of zirconium oxide and aluminum oxide.

[00167] In accordance with a further embodiment, depositing the switching layer comprises using atomic layer deposition (ALD), including maintaining a precursor at greater than 40 degrees Celsius, introducing the precursor to the substrate, maintaining an oxygen source at less than 10 degrees Celsius, and introducing the oxygen source to the substrate.

[00168] In accordance with a further embodiment, depositing the switching layer comprises depositing a metal oxide having an oxygen concentration that is between 60 and 95 percent of stoichiometric.

[00169] In accordance with a further embodiment, the method includes annealing the memory element.

[00170] In accordance with a further embodiment, the oxygen source is at least one of water vapor, isopropyl alcohol, and ethanol.

[00171] In accordance with another embodiment, a resistive-switching memory element is provided, including a first electrode and a second electrode, a switching layer between the first electrode and the second electrode comprising hafnium oxide and having a first thickness, and a coupling layer between the switching layer and the second electrode, the coupling layer comprising a material including partially metal titanium and partially titanium oxide and having a second thickness that is less than 25 percent of the first thickness.

[00172] In accordance with a further embodiment, the first electrode is doped silicon and the memory element is configured to receive a negative reset voltage relative to a common electrical reference and a positive set voltage relative to the common electrical reference at the second electrode. [00173] In accordance with a further embodiment, first thickness is between 20 and 100 angstroms.

[00174] In accordance with a further embodiment, the switching layer comprises a hafnium oxide material having an elemental composition of between

Figure imgf000043_0001

[00175] In accordance with a further embodiment, the first material is n-type polysilicon.

[00176] In accordance with a further embodiment, the memory element includes an interface layer between the first electrode and the switching layer, the interface layer having a thickness less than 10A.

[00177] In accordance with a further embodiment, the first metal oxide has an oxygen concentration that is between 60 and 95% of stoichiometric.

[00178] In accordance with a further embodiment, the first electrode is selected from the group consisting of doped silicon and titanium nitride, and the second electrode is selected from the group consisting of molybdenum nitride, molybdenum oxide, titanium nitride, tungsten, tantalum nitride, molybdenum nitride, molybdenum oxide, platinum, ruthenium, nickel, iridium, iridium oxide, a titanium/aluminum alloy, and ruthenium oxide.

[00179] In accordance with a further embodiment, the first electrode comprises doped silicon and further comprising an interface layer between the first electrode and the switching layer comprising silicon oxide and having a thickness of less than 10A.

[00180] In accordance with another embodiment, a resistive-switching memory element is provided, including a first electrode comprising doped silicon, a second electrode comprising a material chosen from the group consisting of a transition metal, a transition metal alloy, a transition metal nitride, and a transition metal carbide, a switching layer between the first electrode and the second electrode and comprising a metal oxide selected from the group consisting of hafnium oxide, zirconium oxide, titanium oxide, cerium oxide, zinc oxide, lanthanum oxide, and copper oxide, and a coupling layer between the resistive-switching metal oxide and the second electrode, wherein the coupling layer at least partially comprises titanium metal. [00181] In accordance with a further embodiment, the coupling layer comprises a first portion comprising substantially the titanium metal, and a second portion comprising substantially titanium oxide.

[00182] In accordance with a further embodiment, the second portion of the coupling layer is in contact with the switching layer, and wherein the switching layer comprises a first portion in contact with the second portion of the coupling layer comprising an oxygen-deficient metal oxide and a second portion comprising a metal oxide having a greater oxygen concentration than the first portion of the switching layer.

[00183] In accordance with another embodiment, a method for forming a resistive-switching memory element is provided, including depositing a first electrode on a substrate, depositing a switching layer comprising a metal oxide on the first electrode, depositing a coupling layer comprising titanium metal on the switching layer, depositing a second electrode on the coupling layer, annealing the memory element and causing at least a portion of the coupling layer to form titanium oxide and at least a portion of the switching layer to have an oxygen deficiency. [00184] In accordance with a further embodiment, the switching layer is a metal oxide chosen from the group consisting of cerium oxide, hafnium oxide, aluminum oxide, tantalum oxide, yttrium oxide, and zirconium oxide. [00185] In accordance with a further embodiment, the switching layer is a metal oxide chosen from the group consisting of p-type zinc oxide and p-type copper oxide.

[00186] In accordance with a further embodiment, the method further includes depositing the switching layer on the first electrode, and doping the switching layer to form a p-type metal oxide switching layer.

[00187] In accordance with a further embodiment, depositing the switching layer comprises using at least one of atomic layer deposition (ALD) and plasma- enhanced ALD (PEALD).

[00188] In accordance with a further embodiment, depositing the switching layer comprises using an oxygen source that is at least one of water vapor, isopropyl alcohol (IPA), and ethanol. [00189] In accordance with a further embodiment, a deposition temperature for the ALD is less than 250 degrees Celsius.

[00190] In accordance with a further embodiment, the method further includes depositing the coupling layer having a thickness that is less than 25 percent of a thickness of the switching layer.

[00191] Although the foregoing examples have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed examples are illustrative and not restrictive.

Claims

What is claimed:
1. A resistive-switching memory element comprising: a first electrode and a second electrode; a switching layer between the first electrode and the second electrode comprising [0001] 5 hafnium oxide and having a first thickness; and a coupling layer between the switching layer and the second electrode, the coupling layer comprising a material selected from the group consisting of aluminum oxide and zirconium oxide, the coupling layer having a second thickness that is less than 25 percent of the first thickness. [0001] 10
2. The memory element of claim 1 , wherein the first electrode is doped silicon and the memory element is configured to receive a negative reset voltage relative to a common electrical reference and a positive set voltage relative to the common electrical reference at the second electrode.
[0001] 15
3. The memory element of claim 1, wherein the first thickness is between 20 and 100 angstroms.
4. The memory element of claim 1 , wherein the switching layer comprises a hafnium [0001] 20 oxide material having an elemental composition of between HfO 1.2 and HfO 1.7.
5. The memory element of claim 4, wherein the first electrode is n-type polysilicon.
6. The memory element of claim 1, wherein at least one of the first electrode and the [0001] 25 second electrode has a same most common oxidation state as the switching layer.
7. The memory element of claim 1 , further comprising an interface layer between the first electrode and the switching layer, the interface layer having a thickness less than
10A.
[0001] 30
8. The memory element of claim 3, wherein a work function of the second electrode is greater than a work function of the first electrode, and wherein the first electrode is configured to receive a forming voltage pulse having a negative voltage relative to a common electrical reference.
9. A resistive-switching memory element comprising: [0001] 5 a first electrode and a second electrode; a switching layer between the first electrode and the second electrode, the switching layer comprising a first metal oxide having a first bandgap greater than 4 electron volts (eV), the switching layer having a first thickness; and a coupling layer between the switching layer and the second electrode, the
[0001] 10 coupling layer comprising a second metal oxide having a second bandgap greater than or equal to the first bandgap, the coupling layer having a second thickness that is less than 25 percent of the first thickness.
10. The memory element of claim 9, wherein the first metal oxide has an oxygen [0001] 15 concentration that is between 60 and 95% of stoichiometric.
11. The memory element of claim 9, wherein: the first electrode is selected from the group consisting of doped silicon and titanium nitride; and [0001] 20 the second electrode is selected from the group consisting of molybdenum nitride, molybdenum oxide, titanium nitride, tungsten, tantalum nitride, molybdenum nitride, molybdenum oxide, platinum, ruthenium, nickel, iridium, iridium oxide, and ruthenium oxide.
[0001] 25 12. The memory element of claim 9, wherein a first metal of the first metal oxide has a first most common oxidation state that is different from a second most common oxidation state of the second metal of the second metal oxide.
13. The memory element of claim 9, wherein a first metal of the first metal oxide [0001] 30 and a second metal of the second metal oxide have a same most common oxidation state.
14. The memory element of claim 9, wherein a second metal of the second metal oxide has a second most common oxidation state that is than less than or equal to a first most common oxidation state of a first metal of the first metal oxide.
[0001] 5 15. The memory element of claim 9, wherein: the first metal oxide is selected from the group consisting of hafnium oxide, tantalum oxide, aluminum oxide, zirconium oxide, lanthanum oxide, cerium oxide, and yttrium oxide; and the second metal oxide is selected from the group consisting of zirconium [0001] 10 oxide and aluminum oxide.
16. The memory element of claim 9, wherein the first electrode comprises doped silicon and further comprising an interface layer between the first electrode and the switching layer comprising silicon oxide and having a thickness of less than 10A.
[0001] 15
17. The memory element of claim 9, wherein the first electrode comprises a suicide chosen from the group consisting of titanium suicide, cobalt suicide, nickel suicide, palladium suicide, and platinum suicide.
18. A method for forming a resistive-switching memory element comprising: depositing a first electrode on a substrate; depositing a switching layer comprising a metal oxide over the first electrode using atomic layer deposition (ALD), the depositing the switching layer further [0001] 5 comprising: maintaining a precursor at greater than 40 degrees Celsius; introducing the precursor to the substrate; maintaining an oxygen source at less than 10 degrees Celsius; introducing the oxygen source to substrate; and [0001] 10 depositing a second electrode over the switching layer.
19. The method of claim 18, wherein the oxygen source is at least one of water vapor, isopropyl alcohol (IPA), and ethanol.
[0001] 15 20. The method of claim 18, wherein the metal oxide is chosen from the group consisting of hafnium oxide, tantalum oxide, aluminum oxide, yttrium oxide, and zirconium oxide.
21. The method of claim 18, wherein the metal oxide has an oxygen concentration [0001] 20 that is between 60 and 95 percent of stoichiometric.
22. The method of claim 18, wherein a deposition temperature for the ALD is less than 250 degrees Celsius.
[0001] 25
23. The method of claim 18, further comprising depositing a physical vapor deposition (PVD) layer over the first electrode using physical vapor deposition, wherein the switching layer is deposited over the PVD layer, and wherein the PVD layer comprises a same material as the switching layer.
[0001] 30
24. The method of claim 18, further comprising depositing a coupling layer over the switching layer, the coupling layer having a thickness that is less than 25 percent of a thickness of the switching layer.
PCT/US2010/030619 2007-05-09 2010-04-09 Resistive-switching memory elements having improved switching characteristics WO2010118380A2 (en)

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US12/608,934 2009-10-29
US12/610,236 US8975613B1 (en) 2007-05-09 2009-10-30 Resistive-switching memory elements having improved switching characteristics
US12/610,236 2009-10-30
US12/705,474 US8343813B2 (en) 2009-04-10 2010-02-12 Resistive-switching memory elements having improved switching characteristics
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