Patent Publication Number: US-2013252395-A1

Title: Resistive random access memory and method of manufacturing the same

Description:
PRIORITY STATEMENT 
     This non-provisional application is a Divisional under 35 U.S.C. 121 of U.S. application Ser. No. 11/987,150, filed Nov. 28, 2007, which claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2006-0118560, filed on Nov. 28, 2006, in the Korean Intellectual Property Office (KIPO), the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     Example embodiments relate to a resistive random access memory (RRAM) and a method of manufacturing the RRAM. 
     2. Description of the Related Art 
     A semiconductor memory array structure may include a plurality of interconnected memory cells. For example, a unit memory cell of a dynamic random access memory (DRAM) may include a switch and a capacitor. A DRAM may have higher integration and faster operating speeds. However, when the power is turned off, all of the stored data may be erased from the DRAM. 
     On the other hand, a flash memory may be representative of a non-volatile memory capable of preserving stored data when the power is turned off (unlike a volatile memory) but may have lower integration and slower operating speeds than a DRAM. Examples of a non-volatile memory may include a magnetic RAM (MRAM), a ferroelectric RAM (FRAM), and a phase-change RAM (PRAM). A MRAM may store data using a change in the polarization direction of a tunnel junction. A FRAM may store data using the polarization characteristic of a ferroelectric. The MRAM and the FRAM may have higher integrations, faster operating speeds, and improved data retention characteristics and may be driven at a lower power. A PRAM may store data using a change in a resistance value depending on the phase change of a specific material and may include a resistor and a switch (transistor). However, where a conventional DRAM process is used to manufacture the PRAM, etching may be more difficult and may require a longer period of time. Thus, productivity may be lowered, and the unit cost of products may be increased. Consequently, market competitiveness may be decreased. 
     A resistive RAM (RRAM) may store data using the variable resistance characteristic (e.g., a resistance value varying with voltage) of a transition metal oxide.  FIG. 1A  is a cross-sectional view of a conventional RRAM using a resistive transformation material. Referring to  FIG. 1A , the conventional memory device may include a resistive layer  12  and an upper electrode  13  sequentially formed on a lower electrode  11 . The resistive layer  12  may be formed of a transition metal oxide, and the lower and upper electrodes  11  and  13  may be formed of a conductive material, e.g., a metal or a metal oxide. 
       FIG. 1B  is a graph illustrating a relationship between a voltage applied through the lower and upper electrodes  11  and  13  and a current flowing in the resistive layer  12  of the conventional RRAM of  FIG. 1A . The voltage may be gradually increased from 0 V to measure variations in the current flowing in the resistive layer  12 . The current flowing in the resistive layer  12  may be gradually increased at an applied voltage between about 0 V and about 1 V. The current may decrease with an increase in resistance at a voltage of about 1 V and may increase again with a further increase in voltage. A higher resistance state may refer to a reset state, and a lower resistance state may refer to a set state. Variations in the intensity of a voltage at which the set state is changed into the reset state (e.g., a reset voltage RV) and variations in the intensity of a set resistance SR may be relatively large. Also, the set voltage SV at which the reset state is changed into the set state may have a relatively large variation. 
       FIGS. 2A through 2C  illustrate a current path formed in the resistive layer  12  because of a voltage applied through the lower and upper electrodes  11  and  13  of the conventional RRAM. Referring to  FIG. 2A , when a voltage is applied through the lower and upper electrodes  11  and  13 , a current may flow in the resistive layer  12 . Poles formed on a flat board of  FIG. 2A  may denote the current path. Referring to  FIG. 2B , a current path may not be formed in an off state where voltage is not applied. Referring to  FIG. 2C , when a voltage is applied through the lower and upper electrodes  11  and  13 , a current may flow in the resistive layer  12 . The current path of  FIG. 2A  may be different from the current path of  FIG. 2C . For example, while the current may flow in a circular area of  FIG. 2A , the current path does not appear to flow in a circular area of  FIG. 2C . Thus, a current path formed in the resistive layer  12  may vary with the application of voltage. Accordingly, voltage and resistance variations, as shown in  FIG. 1B , may occur. 
     Because the current path in the resistive layer  12  may not be uniform with the driving of the conventional RRAM (and thus the resistance level may be relatively unstable), the conventional RRAM may have larger voltage and resistance variations. Consequently, the set and reset voltages of the conventional RRAM may be relatively unstable, thus lowering reliability. 
     SUMMARY 
     Example embodiments provide a resistive random access memory (RRAM) having a structure capable of stabilizing a reset current and a set voltage using a resistance varying material and a method of manufacturing the RRAM. The RRAM may include a resistive layer having a metal dopant so as to form a filament as a current path in the resistive layer, thus stabilizing voltage and resistance variations. 
     A RRAM according to example embodiments may include a lower electrode; a resistive layer on the lower electrode, wherein the resistive layer may include a transition metal dopant; and/or an upper electrode on the resistive layer. The lower and upper electrodes may be formed of a metal or a metal oxide, wherein the metal may include at least one element selected from the group consisting of Al, Hf, Zr, Zn, W, Co, Au, Pt, Ru, Ir, and Ti. The resistive layer may be formed of a transition metal oxide, wherein the transition metal oxide may include at least one compound selected from the group consisting of NiO, TiO 2 , HfO, ZrO, ZnO, WO 3 , CoO, CuO, or Nb 2 O 5 . The transition metal dopant may include at least one element selected from the group consisting of Ni, Ti, Hf, Zr, Zn, Cu, W, Co, and Nb. For example, the resistive layer may be formed of NiO and the transition metal dopant may be Ni. Accordingly, the transition metal dopant may be concentrated in a grain boundary in the resistive layer so as to form a filament that may function as a current path. 
     A method of manufacturing a RRAM according to example embodiments may include forming a lower electrode; forming a resistive layer on the lower electrode; doping the resistive layer with a transition metal using an implantation process; and/or forming an upper electrode on the resistive layer. The oxygen partial pressure may be adjusted during the formation of the resistive layer. For example, the resistive layer may be formed of NiO at an oxygen partial pressure of about 25% or more, and the transition metal dopant may be Ni. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of example embodiments may become more apparent upon consideration of the attached drawings, in which: 
         FIG. 1A  is a cross-sectional view of a conventional resistive random access memory (RRAM) having a resistance transformation material; 
         FIG. 1B  is a graph illustrating variations of a current with respect to a voltage applied to the conventional RRAM of  FIG. 1A ; 
         FIGS. 2A through 2C  illustrate a current path formed in a resistive layer of the conventional RRAM of  FIG. 1A  upon application of a voltage; 
         FIG. 3A  is a cross-sectional view of a RRAM according to example embodiments; 
         FIG. 3B  is a cross-sectional view of the RRAM of  FIG. 3A  formed on a transistor structure; 
         FIGS. 4A through 4C  are cross-sectional views illustrating a method of manufacturing an RRAM according to example embodiments; 
         FIG. 5  illustrates a metal diffusion path formed in a resistive layer during an implantation process of a method of manufacturing an RRAM according to example embodiments; 
         FIG. 6  is a graph illustrating a resistance characteristic with respect to variations of the oxygen partial pressure of Ni oxide; 
         FIGS. 7A and 7B  are graphs comparing the resistance variation of a conventional RRAM with the resistance variation of a RRAM according to example embodiments; and 
         FIG. 8  is a graph illustrating a simplified electrical characteristic of an RRAM having a resistance varying material according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     A resistive random access memory (RRAM) and a method of manufacturing the RRAM according to example embodiments will now be described with reference to the attached drawings. In the drawings, the thicknesses of layers and regions may have been exaggerated for clarity. 
     It will be understood that when an element or layer is referred to as being “on”, “connected to”, “coupled to”, or “covering” another element or layer, it may be directly on, connected to, coupled to, or covering the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of example embodiments. 
     Spatially relative terms, e.g., “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” may encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing various embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Example embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of example embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of example embodiments. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, including those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 3A  is a cross-sectional view of a RRAM according to example embodiments. Referring to  FIG. 3A , a resistive layer  22  having a dopant may be formed on a lower electrode  21 , and an upper electrode  23  may be formed on the resistive layer  22 . Materials for forming the lower and upper electrodes  21  and  23  and the resistive layer  22  will now be described. The lower and upper electrodes  21  and  23  may be formed of a suitable electrode material used in semiconductor devices, e.g., Al, Hf, Zr, Zn, W, Co, Au, Pt, Ru, Ir, Ti, and/or a conductive metal oxide. The resistive layer  22  may be formed of a suitable material for an RRAM, e.g., transition metal oxide. For example, the resistive layer  22  may be formed of NiO, TiO 2 , HfO, ZrO, ZnO, WO 3 , CoO, CuO, Nb 2 O 5 , or a combination thereof. The resistive layer  22  may include a transition metal oxide and a dopant which may be implanted using an implantation process. The dopant implanted into the resistive layer  22  may be a metallic material, e.g., Ni, Ti, Hf, Zr, Zn, Cu, W, Co, and/or Nb. 
     The RRAM may be driven in a 1S (Switch)-1R (Resistance) pattern connected to a switch structure, e.g., a transistor or a diode.  FIG. 3B  is a cross-sectional view of a 1T (Transistor)-1R (Resistance) structure in which the RRAM of  FIG. 3A  is connected to a transistor. Referring to  FIG. 3B , a gate structure may be formed on a substrate  201  so as to contact the first and second dopant areas  202  and  203 . The gate structure may include a gate insulating layer  204  and a gate electrode layer  205 . An interlayer insulating layer  206  may be formed on the substrate  201  and the gate structure, and a contact plug  207  may be formed to penetrate the interlayer insulating layer  206  on the first or second dopant area  202  or  203 . The RRAM of  FIG. 3A  may be formed on the contact plug  207 . Alternatively, as mentioned above, the RRAM of  FIG. 3A  may be connected to a diode including p-type and n-type semiconductor layers instead of a transistor. 
     A method of manufacturing a RRAM according to example embodiments will now be described. The RRAM may be formed using sputtering, atomic layer deposition (ALD), or chemical vapor deposition. Referring to  FIG. 4A , a lower electrode  21  may be formed on a lower structure (not shown), e.g., a substrate. The lower electrode  21  may be formed of a conductive material, e.g., metal or metal oxide. For example, the lower electrode  21  may be formed of Al, Hf, Zr, Zn, W, Co, Au, Pt, Ru, Ir, and/or Ti. A resistive layer  22  may be formed on the lower electrode  21 . The resistive layer  22  may be formed of transition metal oxide, e.g., NiO, TiO 2 , HfO, ZrO, ZnO, WO 3 , CoO, CuO, and/or Nb 2 O 5 . 
     Referring to  FIG. 4B , a dopant  20  may be implanted into the resistive layer  22  using an implantation process. The dopant  20  may be metal, e.g., transition metal. For example, the dopant  20  may include Ni, Ti, Hf, Zr, Zn, Cu, W, Co, and/or Nb. When the dopant  20  is implanted into the resistive layer  22 , the dopant  20  may diffuse into an empty area, e.g., a grain boundary of a material constituting the resistive layer or a defective area. The diffused dopant  20  may form a filament so as to operate as a current path  24  (e.g., a current transfer path) in the resistive layer  22 , as shown in  FIG. 5 . 
     Example embodiments having a resistive layer  22  formed of NiO will now be described. When the oxygen partial pressure of a process chamber is adjusted, a transition metal oxide may exhibit the characteristics shown in  FIG. 6 . Atmospheric gas and oxygen may be used to form the resistive layer  22 . When the partial pressure of oxygen is relatively low (area A), the transition metal oxide may have a metallic characteristic similar to Ni. When the partial pressure of oxygen is increased (area B), the resistance of the transition metal oxide may also increase. Thus, the transition metal oxide may have a memory switching characteristic. Additionally, when the partial pressure of oxygen is further increased (area C), the resistance of the transition metal oxide may decrease. Thus, the transition metal oxide may also have a threshold switching characteristic. For example, the switching characteristic may disappear when the oxygen partial pressure is about 25%. When the resistive layer  22  is formed of Ni oxide, the oxygen partial pressure may be adjusted to be greater than that of area C, and Ni may be implanted into the resistive layer  22  formed of Ni oxide using an implantation process. Consequently, interstitially implanting a Ni dopant into the Ni oxide may be more difficult because of its dense structure. Thus, the Ni dopant may shift into an empty position (e.g., a grain boundary) so as to form a filament which may operate as a current path. 
     Referring to  FIG. 4C , an upper electrode  23  may be formed on the resistive layer  22 . The upper electrode  23  may be formed of the same material as the lower electrode  21  (e.g., metal or conductive metal oxide). For example, the upper electrode  23  may be formed of Al, Hf, Zr, Zn, W, Co, Au, Pt, Ru, Ir, and/or Ti. After manufacturing a conventional RRAM, a forming process may be performed to apply a higher voltage than an operating voltage to the conventional RRAM. Thus, impact may be applied to the conventional RRAM, and the operating voltage may be increased. In contrast, the RRAM according to example embodiments may operate without requiring a forming process. 
       FIGS. 7A and 7B  are graphs illustrating variations in the resistance value of a resistive layer according to a conventional RRAM compared with variations in the resistance value of a resistive layer of an RRAM according to example embodiments. Variations in resistance may be measured by the application of a current through the upper and lower electrodes. The horizontal axis may denote the number of times the resistance of the resistive layer is measured, and the vertical axis may denote the resistance value.  FIG. 7A  is a graph illustrating variations in the resistance value of a conventional RRAM, and  FIG. 7B  is a graph illustrating variations in the resistance value of a RRAM according to example embodiments. Referring to  FIG. 7A , the resistance value may have relatively large variations in the on and off states. Referring to  FIG. 7B , the resistance value may be relatively constant in the on and off states. Thus, resistance variation may be relatively low in a RRAM according to example embodiments. 
       FIG. 8  is a graph illustrating a simplified operation characteristic of a RRAM according to example embodiments. The horizontal axis may denote an applied voltage, and the vertical axis may denote variations of a current with respect to the applied voltage. When a voltage applied through the lower and upper electrodes is increased from 0 V, the current may also be increased in proportion to the voltage, as shown by G 1 . However, when a voltage greater than a voltage V 1  is applied, the resistance of the resistive layer may increase, thus decreasing the current. When a voltage between voltages V 1  and V 2  is applied, the current flowing in the resistive layer may vary as shown by G 2 . When a voltage greater the voltage V 2  (V 2  &gt;V 1 ) is applied, the resistance may decrease. Thus, the current may increase as shown by G 1 . 
     Applying a voltage greater than the voltage V 1  may affect the electrical characteristics (e.g., current levels) of the RRAM when a voltage smaller than the voltage V 1  is subsequently applied. This electrical characteristic will now be described in further detail. For example, when a voltage between the voltages V 1  and V 2  is applied to the RRAM followed by a voltage smaller than the voltage V 1 , the resulting current may be as shown by G 2 . On the other hand, when a voltage V 3  (which is greater than the voltage V 2 ) is applied to the RRAM followed by a voltage smaller than the voltage V 1 , the resulting current may be as shown by G 1 . Thus, the intensity of the voltage applied in the range greater than the voltage V 1  (e.g., between the voltages V 1  and V 2  or greater than the voltage V 2 ) may affect the electrical characteristic of the RRAM. 
     Accordingly, a transition metal oxide may be used in a RRAM to provide a multilayer film having a resistance gradient for a non-volatile memory device. For example, if the state of the RRAM supplied with the voltage between the voltages V 1  and V 2  is designated as “0,” then the state of the RRAM supplied with the voltage greater than the voltage V 2  may be designated as “1” to record data. When data is to be reproduced, a voltage smaller than the voltage V 1  may be applied to the RRAM to measure the drain current I d  so as to check whether stored data is in a state of “0” or “1.” The states “1” and “0” may be selectively designated. A set voltage V 2 , a reset voltage V 1 , and a resistance may be constant according to the number of operations of an RRAM having an implant. Thus, the RRAM may have a more stable characteristic. 
     As described above, in a RRAM according to example embodiments and a method of manufacturing the same, the current path of the RRAM may be maintained relatively uniformly. Thus, variations in the resistance of a resistive layer may be reduced. Consequently, the RRAM may have a more stable characteristic. Also, the RRAM may be manufactured without a forming process. Thus, the impact applied to the RRAM may be relieved, and the operating voltage may be lowered. Additionally, information may be stored and reproduced using a simpler method. Accordingly, the RRAM may have a faster operation. 
     While example embodiments have been disclosed herein, it should be understood that other variations may be possible. Such variations are not to be regarded as a departure from the spirit and scope of example embodiments of the present disclosure, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.