Patent Publication Number: US-2013228734-A1

Title: Programmable resistive memory cell with sacrificial metal

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/348,255, filed Jan. 11, 2012, which is a continuation of U.S. application Ser. No. 12/500,899 filed Jul. 10, 2009, now U.S. Pat. No. 8,097,874, which claims the benefit of U.S. Provisional Application No. 61/109,583 filed Oct. 30, 2008, the contents of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Memory devices are common in electronic systems and computers to store data. These memory devices may be volatile memory, where the stored data is lost if the power source is disconnected or removed, or non-volatile, where the stored data is retained even during power interruption. An example of a non-volatile memory device is a programmable metallization cell (PMC). 
     A PMC utilizes a fast ion conductor such as a chalcogenide-type or an oxide-type (e.g., NiO) and at least two electrodes (e.g., an anode and a cathode) with the fast ion conductor between the electrodes. When a voltage is applied across the electrodes, superionic clusters or conducting filaments rapidly grow from the cathode through the fast ion conductor towards the anode. When the clusters or filaments are present, the cell is in a low resistance state. When an electric field of opposite polarity is applied across the electrodes, the conducting filaments dissolve and the conducing paths are disrupted, providing the cell with a high resistance state. The two resistance states are switchable by the application of the appropriate electric field and are used to store the memory data bit of “1” or “0”. 
     While a high ionic conductive solid electrolyte (e.g., chalcogenide) provides a high speed switch between the two resistance states of the PMC, this material can suffer from poor data state retention. Another lower ionic conductive solid electrolyte (e.g., oxide electrolyte) provides for good data state retention, but this material can suffer from slow switching between the two resistance states of the PMC. Thus, there is a tradeoff between switching speed and data retention in a PMC cell depending on what solid electrolyte (in regards to the material property differences) is provided in the PMC cell. There is a need for a PMC cell that can provide both fast switching speeds and extended data retention. 
     BRIEF SUMMARY 
     The present disclosure relates to programmable metallization memory cells having sacrificial metal that has a more negative standard electrode potential than the filament forming metal. The sacrificial metal can donate electrons to the filament forming metal in the low resistance state of the programmable metallization memory cell to stabilize the low resistance state of the programmable metallization memory cell and improve the data retention of the programmable metallization memory cell. 
     In one illustrative embodiment, a programmable metallization memory cell includes an electrochemically active electrode and an inert electrode and an ion conductor solid electrolyte material between the electrochemically active electrode and the inert electrode. A sacrificial metal is disposed between the electrochemically active electrode and the inert electrode. The sacrificial metal has a more negative standard electrode potential than the filament forming metal. 
     These and various other features and advantages will be apparent from a reading of the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be more completely understood in consideration of the following detailed description of various embodiments of the disclosure in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic side view diagram of an illustrative programmable metallization memory cell having a sacrificial metal layer; 
         FIG. 2  is a schematic side view diagram of an illustrative programmable metallization memory cell having a sacrificial metal particles; 
         FIG. 3A  is a schematic side view diagram of an illustrative programmable metallization memory cell in a low resistance state; 
         FIG. 3B  is schematic side view diagram of the illustrative programmable metallization memory cell in a high resistance state; 
         FIG. 4  is a schematic diagram of an illustrative programmable metallization memory unit including a semiconductor transistor; 
         FIG. 5  is a schematic diagram of an illustrative programmable metallization memory array; 
         FIG. 6  is a flow diagram of an illustrative method of forming a programmable metallization memory cell with sacrificial metal; 
         FIGS. 7A-7C  are schematic cross-section views of another programmable metallization memory cell with oxide layer at various stages of manufacture. 
         FIG. 8  is a flow diagram of another illustrative method of forming a programmable metallization memory cell with sacrificial metal; and 
         FIGS. 9A-9B  are schematic cross-section views of another programmable metallization memory cell with oxide layer at various stages of manufacture. 
     
    
    
     The figures are not necessarily to scale. Like numbers used in the figures refer to like components. However, it will be understood that the use of a number to refer to a component in a given figure is not intended to limit the component in another figure labeled with the same number. 
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying set of drawings that form a part hereof and in which are shown by way of illustration several specific embodiments. It is to be understood that other embodiments are contemplated and may be made without departing from the scope or spirit of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense. 
     Unless otherwise indicated, all numbers expressing feature sizes, amounts, and physical properties used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the foregoing specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by those skilled in the art utilizing the teachings disclosed herein. 
     As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” encompass embodiments having plural referents, unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
     Spatially related terms, including but not limited to, “lower”, “upper”, “beneath”, “below”, “above”, and “on top”, if used herein, are utilized for ease of description to describe spatial relationships of an element(s) to another. Such spatially related terms encompass different orientations of the device in use or operation in addition to the particular orientations depicted in the figures and described herein. For example, if a cell depicted in the figures is turned over or flipped over, portions previously described as below or beneath other elements would then be above those other elements. 
     As used herein, when an element, component or layer for example is described as being “on” “connected to”, “coupled with” or “in contact with” another element, component or layer, it can be directly on, directly connected to, directly coupled with, in direct contact with, or intervening elements, components or layers may be on, connected, coupled or in contact with the particular element, component or layer, for example. When an element, component or layer for example is referred to as begin “directly on”, “directly connected to”, “directly coupled with”, or “directly in contact with” another element, there are no intervening elements, components or layers for example. 
     The present disclosure relates to programmable metallization memory cells having sacrificial metal that has a more negative standard electrode potential than the filament forming metal. The sacrificial metal can donate electrons to the filament forming metal in the low resistance state of the programmable metallization memory cell to stabilize the low resistance state of the programmable metallization memory cell and improve the data retention of the programmable metallization memory cell. While the present disclosure is not so limited, an appreciation of various aspects of the disclosure will be gained through a discussion of the examples provided below. 
       FIG. 1  is a schematic side view diagram of an illustrative programmable metallization memory cell  10  having a sacrificial metal layer  15 . Memory cell  10  includes an electrochemically inert electrode  12 , an electrochemically active electrode  14 , and an ion conductor solid electrolyte material  16 . The ion conductor solid electrolyte material  16  is between the electrochemically inert electrode  12  and the electrochemically active electrode  14 . A sacrificial metal  15  is disposed between the electrochemically active electrode  14  and the inert electrode  12 . The sacrificial metal  15  has a more negative standard electrode potential than the filament forming metal forming the electrochemically active electrode  14 . 
     In many embodiments, the programmable metallization memory cell  10  is constructed with a sacrificial metal layer  15  disposed on either the electrochemically active electrode  14  and the inert electrode  12 . The sacrificial metal  15  can have a smaller atomic radius than the filament forming metal forming the electrochemically active electrode  14 . In many embodiments, the filament forming metal  14  is silver and the sacrificial metal  15  is nickel, chromium or zinc, for example. 
     As described below, the sacrificial metal  15  donates electrons to the filament forming metal  14  to stabilize filaments formed by the filament forming metal  14  when the programmable metallization memory cell  10  is in the low resistance state. The sacrificial metal layer  15  is deposited thin enough so it does not participate in the formation of the filaments formed by the filament forming metal  14  when the programmable metallization memory cell  10  is in the low resistance state. In many embodiments the sacrificial metal layer  15  has a thickness of less than 50 nanometers, or less than 40 nanometers, or less than 30 nanometers. 
     The electrochemically active electrode  14  can be formed of any useful electrochemically active material such as, silver (Ag) or copper (Cu). The active electrode  14  can have any useful thickness, for example, from 50 Angstroms to 5000 Angstroms. In many embodiments the active electrode  14  has a greater thickness than the sacrificial metal layer  15 . A top electrode (not shown) can be disposed on the electrochemically active electrode  14 . The top electrode can be formed of any useful electrochemically inert metallic material, as described below. 
     The inert electrode  12  can be formed of any useful electrochemically inert metallic material. In many embodiments, the inert electrode  12  is formed of electrochemically inert metal such as, tungsten (W), nickel (Ni), molybdenum (Mo), platinum (Pt), gold (Au), palladium (Pd), and rhodium (Rh) for example. In some embodiments the inert electrode  12  has two or more metal layers, where the metal layer closest to the ion conductor solid electrolyte material  16  is electrochemically inert while additional layers can be electrochemically active. The inert electrode  12  can also be referred to as a bottom electrode. The inert electrode  12  can be, but need not be formed on a substrate. The substrate, if utilized, can include silicon, a mixture of silicon and germanium, and other similar materials.  FIG. 1  and  FIG. 2  does not depict an optional substrate. 
     The ion conductor solid electrolyte material  16  can be formed of any useful material that provides for the formation of conducting filaments  18  within the ion conductor solid electrolyte material and extend between the electrochemically active electrode  14  and the inert metal contact  12  upon application of an electric field EF+. In many embodiments the ion conductor solid electrolyte material  16  is a chalcogenide-type material such as, for example, GeS 2 , GeSe 2 , CuS 2 , CuTe, and the like. In other embodiments the ion conductor solid electrolyte material  16  is an oxide-type material such as, for example, WO 3 , SiO 2 , Gd 2 O 3 and the like. 
       FIG. 2  is a schematic side view diagram of an illustrative programmable metallization memory cell  10  having a sacrificial metal particles  15 . Memory cell  10  includes an electrochemically inert electrode  12 , an electrochemically active electrode  14 , and an ion conductor solid electrolyte material  16 , as described above. Sacrificial metal  15  particles are dispersed within the ion conductor solid electrolyte material  16 . The sacrificial metal  15  particles have a more negative standard electrode potential than the filament forming metal forming the electrochemically active electrode  14 . The sacrificial metal  15  particles can have a smaller atomic radius than the filament forming metal forming the electrochemically active electrode  14 . In many embodiments, the filament forming metal  14  is silver and the sacrificial metal  15  is nickel, chromium or zinc, for example. 
     As described below, the sacrificial metal  15  particles donate electrons to the filament forming metal  14  to stabilize filaments formed by the filament forming metal  14  when the programmable metallization memory cell  10  is in the low resistance state. The sacrificial metal  15  particles are co-deposited with the ion conductor solid electrolyte material  16  at a concentration that is low enough so it does not participate in the formation of the filaments formed by the filament forming metal  14  when the programmable metallization memory cell  10  is in the low resistance state. 
       FIGS. 3A and 3B  are cross-sectional schematic diagrams of an illustrative programmable metallization memory cell  10 . In  FIG. 3A , memory cell  10  is in the low resistance state. In  FIG. 3B , cell  10  is in the high resistance state. Programmable metallization cell (PMC) memory is based on the physical re-location of superionic regions and forming conducting filaments  18  within an ion conductor solid electrolyte material  16 . 
     Application of an electric field EF+ across the electrochemically active electrode  14  and the inert metal contact  12  allow metal cations (i.e., silver ions) to migrate toward the inert metal contact  12 , electrically connecting the inert metal contact  12  to the electrochemically active electrode  14 . This electrical connection gives rise to the low resistance state of the programmable metallization memory cell  10 . 
     Reading the PMC  10  simply requires a small voltage applied across the cell. If the conducting filaments  18  electrically connect the inert metal contact  12  to the electrochemically active electrode  14 , the resistance will be low, leading to higher current, which can be read as a “1”. If conducting filaments  18  do not electrically connect the inert metal contact  12  to the electrochemically active electrode  18 , the resistance is higher, leading to low current, which can be read as a “0” as illustrated in  FIG. 3B . 
     When the external bias or electric field EF+ is removed, the conducting filaments  18  tend to disintegrate into ions (e.g., silver ions) and start to retreat back to the anode or disperse into the ion conductor solid electrolyte material  16 . The sacrificial metal  15  has a more negative standard potential than the metal forming the conducting filaments  18 , thus electrons will flow from the sacrificial metal  15  to the conducting filaments  18  to stabilize the conducting filaments  18  and thereby improving the low resistance data state retention. In this low resistance state, after donating the electrons, the sacrificial metal is in the ionic state  15 A in ion conductor solid electrolyte material  16 . 
       FIG. 3B  is schematic diagram of an illustrative programmable metallization memory cell  10  in a high resistance state. Application of an electric field of opposite polarity FE− ionizes the conducting filaments  18  and dissolves ions from the electrically conducting filaments  18  back to the electrochemically active electrode  14 , breaking the electrical connection between the inert metal contact  12  to the electrochemically active electrode  14  and gives rise to the high resistance state of the programmable metallization memory cell  10 . The sacrificial metal ions  15 A move toward the negative charged anode and reduce into the metallic state. The low resistance state and the high resistance state are switchable with an applied electric field and are used to store the memory bit “1” and “0”. 
       FIG. 4  is a schematic diagram of an illustrative programmable metallization memory unit  20  including a semiconductor transistor  22 . Memory unit  20  includes a programmable metallization memory cell  10 , as described herein, electrically coupled to semiconductor transistor  22  via an electrically conducting element  24 . Transistor  22  includes a semiconductor substrate  21  having doped regions (e.g., illustrated as n-doped regions) and a channel region (e.g., illustrated as a p-doped channel region) between the doped regions. Transistor  22  includes a gate  26  that is electrically coupled to a word line WL to allow selection and current to flow from a bit line BL to memory cell  10 . An array of programmable metallization memory units  20  can be formed on a semiconductor substrate utilizing semiconductor fabrication techniques. 
       FIG. 5  is a schematic diagram of an illustrative programmable metallization memory array  30 . Memory array  30  includes a plurality of word lines WL and a plurality of bit lines BL forming a cross-point array. At each cross-point a programmable metallization memory cell  10 , as described herein, is electrically coupled to word line WL and bit line BL. A select device (not shown) can be at each cross-point or at each word line WL and bit line BL. 
       FIG. 6  is a flow diagram of an illustrative method of forming a programmable metallization memory cell with an oxide layer.  FIGS. 5A-5C  are schematic cross-section views of a programmable metallization memory cell with an oxide layer at various stages of manufacture. 
     At  FIG. 7A  an ion conductor solid electrolyte layer  16  is deposited on an inert electrode  12  at block  110  of  FIG. 6 . Both the ion conductor solid electrolyte layer  16  and the inert electrode  12  can be formed using known deposition methods such as physical vapor deposition, chemical vapor deposition, electrochemical deposition, molecular beam epitaxy and atomic layer deposition. While not illustrated, the inert electrode  12  can be deposited on a substrate. The substrate includes, but is not limited to silicon, a mixture of silicon and germanium, and other similar material. 
     At  FIG. 7B  a sacrificial metal layer  15  is deposited on the ion conductor solid electrolyte layer  16  at block  120  of  FIG. 6 . The sacrificial metal layer  15  can be formed using known deposition methods, as described above. The sacrificial metal  15  has a thickness in a range from 0.5 to 50 nanometers or from 1 to 25 nanometers. The sacrificial metal  15  can be formed of any useful metal that has a more negative standard electrode potential than the filament forming metal forming the electrochemically active electrode, described above. The sacrificial metal  15  has a smaller atomic radius than the filament forming metal forming the electrochemically active electrode. In many embodiments, the filament forming metal is silver and the sacrificial metal  15  is nickel, chromium or zinc, for example. The sacrificial metal  15  donates electrons to the filament forming metal to stabilize filaments formed by the filament forming metal when the programmable metallization memory cell is in the low resistance state. The sacrificial metal layer  15  is deposited thin enough so it does not participate in the formation of the filaments formed by the filament forming metal when the programmable metallization memory cell  10  is in the low resistance state. 
     At  FIG. 7C  an electrochemically active electrode  14  is deposited on the sacrificial metal layer  15  at block  130  of  FIG. 6 . The electrochemically active electrode  14  can be formed using known deposition methods, as described above. Additional metal contact layer(s) can be formed on the electrochemically active electrode  14 . In many embodiments, at least one inert metal contact layer is deposited on the electrochemically active electrode  14  (not shown). 
       FIG. 8  is a flow diagram of another illustrative method of forming a programmable metallization memory cell with an oxide layer.  FIGS. 9A-9B  are schematic cross-section views of another programmable metallization memory cell with oxide layer at various stages of manufacture. 
     At  FIG. 9A  an ion conductor solid electrolyte layer  16  is co-deposited with sacrificial metal particles  15  on an inert electrode  12  at block  210  of  FIG. 8 . The ion conductor solid electrolyte layer  16  and sacrificial metal particles  15  and the inert electrode  12  can be formed using known deposition methods such as physical vapor deposition, chemical vapor deposition, electrochemical deposition, molecular beam epitaxy and atomic layer deposition. While not illustrated, the inert electrode  12  can be deposited on a substrate. The substrate includes, but is not limited to silicon, a mixture of silicon and germanium, and other similar material. 
     At  FIG. 9B  illustrates an electrochemically active electrode  14  deposited on the co-deposited ion conductor solid electrolyte  16  and sacrificial metal particle  15  layer at block  220  of  FIG. 8 . The electrochemically active electrode  14  can be formed using known deposition methods, as described above. Additional metal contact layer(s) can be formed on the electrochemically active electrode  14 . In many embodiments, at least one inert metal contact layer is deposited on the electrochemically active electrode  14  (not shown). 
     Thus, embodiments of the PROGRAMMABLE RESISTIVE MEMORY CELL WITH 
     SACRIFICIAL METAL are disclosed. The implementations described above and other implementations are within the scope of the following claims. One skilled in the art will appreciate that the present disclosure can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.