Patent Publication Number: US-2009230391-A1

Title: Resistance Storage Element and Method for Manufacturing the Same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2008-061137, filed on Mar. 11, 2008, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The present invention relates to a resistance storage element that stores a plurality of resistance states showing different resistance values, and a method for manufacturing the same. 
     BACKGROUND 
     As a new memory element, a non-volatile semiconductor storage device called a resistance random access memory (ReRAM) has received attention in recent years. An example of the ReRAM is a resistance storage element that has a plurality of resistance states showing different resistance values and changes its resistance state when externally receiving an electric stimulus. A ReRAM is used as a memory element by relating a high-resistance state and a low-resistance state of a resistance storage element, for example, to informational statuses “0” and “1”. A ReRAM characterized by high speed, large capacity, low power consumption, and other properties is a promising element. 
     A resistance storage element is formed by sandwiching a resistance storage material whose resistance state is changed by voltage application between a pair of electrodes: A resistance storage material that has been proposed is an oxide containing a transition metal. 
       FIG. 17  is a graph illustrating the current versus voltage characteristic of a proposed resistance storage element. As illustrated in  FIG. 17 , when the voltage applied to the resistance storage element in a high-resistance state is gradually increased, and the voltage becomes greater than a certain value (set voltage Vset), the resistance abruptly decreases and the resistance storage element transits to a low-resistance state. Such an action is typically called “set.” In a ReRAM, to prevent the resistance storage element and peripheral circuits from being damaged when a large current flows therethrough at the time of the set action, a selection transistor or any other suitable component is used to limit the current. 
     On the other hand, when the voltage applied to the resistance storage element in the low-resistance state is gradually increased to gradually increase the current flowing through the resistance storage element, and the current becomes greater than a certain value (reset current Ireset), the resistance abruptly increases and the resistance storage element transits to the high-resistance state. Such an action is typically called “reset.” 
     As described above, the resistance storage element in the high-resistance state transits to the low-resistance state when a voltage greater than or equal to the set voltage is applied, whereas the resistance storage element in the low-resistance state transits to the high-resistance state when a current greater than or equal to the reset current flows. The resistance of the resistance storage element in the low-resistance state is approximately several kΩ, whereas the resistance of the resistance storage element in the high-resistance state approximately ranges from several tens of kΩ to 1000 kΩ. Such actions can be used to control the resistance state of the resistance storage element. 
     Data can be read by measuring the magnitude of the current flowing through the resistance storage element when a given readout current is conducted through the resistance storage element. 
     The following are examples of related art of the present invention: Japanese Patent Laid-Open No. 2004-363604, Japanese Patent Laid-Open No. 2007-84935, Japanese Patent Laid-Open No. 2007-53125, Japanese Patent Laid-Open No. 10-149797, Japanese Patent Laid-Open No. 2005-191354, S. Seo et al., “Reproducible resistance switching in polycrystalline NiO films”, Applied Physics Letters, Volume 85, Number 23, p. 5655-5657 (2004), and S. Seo et al., “Conductivity switching characteristics and reset currents in NiO films”; Applied Physics Letters, 86, 093509 (2005). 
     There is a need to miniaturize a memory device using a resistance storage element as the packing density increases. There is also a need to reduce the voltage and current levels to operate the memory device. 
     SUMMARY 
     Accordingly, it is an object in one aspect of the invention to provide a method for manufacturing a resistance storage element including forming a lower electrode layer over a semiconductor substrate, forming a transition metal film over the lower electrode layer, forming an upper electrode layer over the transition metal film, and supplying oxygen contained in the lower electrode layer or the upper electrode layer to oxidize the transition metal film. 
     The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are cross-sectional views illustrating a non-volatile semiconductor storage device according to a first embodiment of the present invention; 
         FIGS. 2A to 2L  are process cross-sectional views illustrating a method for manufacturing the non-volatile semiconductor storage device according to the first embodiment of the present invention; 
         FIGS. 3A to 3C  illustrate graphs illustrating the current versus voltage characteristic of the resistance storage element according to the first embodiment of the present embodiment; 
         FIGS. 4A and 4B  illustrate cross-sectional views of a resistance storage element using a sputtered NiO film as a resistance storage layer and graphs illustrating the current versus voltage characteristic of the resistance storage element; 
         FIG. 5  is an electron micrograph of a cross-sectional structure of the resistance storage element using the sputtered NiO film as the resistance storage layer; 
         FIGS. 6A and 6B  are cross-sectional views illustrating a non-volatile semiconductor storage device according to a second embodiment of the present invention; 
         FIGS. 7A to 7G  are process cross-sectional views illustrating a method for manufacturing the non-volatile semiconductor storage device according to the second embodiment of the present invention; 
         FIGS. 8A and 8B  are cross-sectional views illustrating a non-volatile semiconductor storage device according to a third embodiment of the present invention; 
         FIGS. 9A to 9G  are process cross-sectional views illustrating a method for manufacturing the non-volatile semiconductor storage device according to the third embodiment of the present invention; 
         FIGS. 10A to 10D  are process cross-sectional views illustrating a method for manufacturing a non-volatile semiconductor storage device according to a fourth embodiment of the present invention; 
         FIG. 11  illustrates graphs illustrating the current versus voltage characteristic of a resistance storage element according to the fourth embodiment of the present embodiment; 
         FIGS. 12A and 12B  are cross-sectional views illustrating a non-volatile semiconductor storage device according to a fifth embodiment of the present invention; 
         FIGS. 13A to 13D  are process cross-sectional views illustrating a method for manufacturing the non-volatile semiconductor storage device according to the fifth embodiment of the present invention; 
         FIGS. 14A and 14B  are process cross-sectional views illustrating a method for manufacturing a non-volatile semiconductor storage device according to a sixth embodiment of the present invention; 
         FIGS. 15A to 15C  are process cross-sectional views illustrating a method for manufacturing a non-volatile semiconductor storage device according to a seventh embodiment of the present invention; 
         FIGS. 16A to 16D  are process cross-sectional views illustrating a method for manufacturing a non-volatile semiconductor storage device according to an eighth embodiment of the present invention; and 
         FIG. 17  is a graph illustrating the current versus voltage characteristic of a resistance storage element. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Miniaturization of a resistance storage element desirably involves reducing the film thickness of an oxide material as a resistance storage material sandwiched between electrodes. Further, a large film thickness of the oxide material disadvantageously increases the voltage and current levels for operation. It is therefore desirable to reduce the film thickness of the oxide material also from the viewpoint of reducing the voltage and current levels for operation. 
     Simply forming a thin film of oxide material by sputtering or any other suitable method, however, results in degraded uniformity of the film thickness in some cases. The degraded uniformity of the film thickness leads to insufficient insulating performance between the electrodes, and it is therefore difficult to ensure characteristics for a resistance storage element. 
     First Embodiment 
     A description will be made of a resistance storage element, a non-volatile semiconductor storage device using the resistance storage element, and a method for manufacturing the resistance storage element according to a first embodiment of the present invention. 
     The resistance storage element and the non-volatile semiconductor storage device according to the present embodiment will be first described with reference to  FIGS. 1A and 1B .  FIG. 1A  is a cross-sectional view illustrating the non-volatile semiconductor storage device according to the present embodiment.  FIG. 1B  is an enlarged view of only the resistance storage element. 
     As illustrated in  FIGS. 1A and 1B , an element isolation area  12  that defines an element area is formed in a semiconductor substrate  10 . 
     Gate electrodes  14  are formed on the semiconductor substrate  10  in which the element area has been defined, a gate insulating film interposed between each of the gate electrodes  14  and the semiconductor substrate  10 . The gate electrodes  14  also function as word lines. The word lines  14  extend in the direction perpendicular to the plane of view in  FIG. 1A . 
     Source/drain diffusion layers  16  and  18  are formed in the semiconductor substrate  10  on opposite sides of each of the gate electrodes  14 . 
     One gate electrode  14  and source/drain diffusion layers  16  and  18  form a selection transistor  20 . Two selection transistors  20  that share the source/drain diffusion layer  16  are formed in a single active area. 
     An interlayer insulating film  22  is formed on the semiconductor substrate  10  in which the selection transistors  20  have been formed. 
     A contact plug  28  connected to the source/drain diffusion layer  16  and contact plugs  30  connected to the source/drain diffusion layers  18  are embedded in the interlayer insulating film  22 . 
     On the interlayer insulating film  22  in which the contact plugs  28  and  30  have been embedded are formed a source line (ground line)  32  electrically connected to the source/drain diffusion layer  16  (source terminal) via the contact plug  28  and relay wiring lines  34  electrically connected to the source/drain diffusion layers  18  (drain terminals) via the contact plugs  30 . The source line  32  is formed parallel to the word lines  14  and extends in the direction perpendicular to the plane of view in  FIG. 1A . 
     An interlayer insulating film  36  is formed on the interlayer insulating film  22  on which the source line  32  and the relay wiring lines  34  have been formed. Contact plugs  40  connected to the relay wiring lines  34  are embedded in the interlayer insulating film  36 . 
     Resistance storage elements  42  are formed on the interlayer insulating film  36  in which the contact plugs  40  have been embedded. Each of the resistance storage elements  42  includes a lower electrode layer  44  electrically connected to the corresponding source/drain diffusion layer  18  via the corresponding contact plug  40 , relay wiring line  34  and contact plug  30 , a resistance storage layer  48  formed on the lower electrode layer  44 , and an upper electrode layer  50  formed on the resistance storage layer  48 . 
     The lower electrode layer  44  is a film obtained by stacking a close contact layer  52  and a noble metal film  54 . The close contact layer  52  is made of, for example, titanium (Ti), and the noble metal film  54  is made of, for example, platinum (Pt). 
     The resistance storage layer  48  includes a transition metal oxide film made of nickel oxide (NiO x ). The transition metal oxide film  48  is formed, as will be described later, by forming a noble metal oxide film  58 , which forms the upper electrode layer  50 , on a transition metal film  46  made of nickel (Ni) and then carrying out heat treatment to supply oxygen contained in the noble metal oxide film  58  to the transition metal film  46  so as to oxidize the transition metal film  46 . 
     Since the transition metal oxide film  48  is formed by oxidation using oxygen contained in the noble metal oxide film  58 , which forms the upper electrode layer  50 , the oxygen concentration in the transition metal oxide film  48  has a gradient distribution. That is, the oxygen concentration in the transition metal oxide film  48  decreases in the direction from the upper electrode layer  50  toward the lower electrode layer  44 . 
     The transition metal oxide film  48  is not directly deposited by sputtering or any other suitable method, but formed by oxidizing the transition metal film  46  using oxygen contained in the noble metal oxide film  58 . The composition ratio of the oxygen of the transition metal oxide film  48  is thus lower than that in the stoichiometric composition. When an NiO x  film is directly deposited by sputtering in a conventional manner, an NiO x  film with a composition ratio of X=1, that is, the stoichiometric composition of Ni:O=1:1 (NiO film) is formed. It is therefore difficult to form an NiO x  film having a composition different from the stoichiometric composition. In contrast, the present embodiment allows an NiO x  film, for example, with a composition ratio of X=0.8 to 0.9, that is, a composition of Ni:O=1:0.8 to 0.9, to be formed as the transition metal oxide film  48 , which forms the resistance storage layer. 
     The film thickness of the transition metal oxide film  48  is set to a relatively small value, for example, 10 nm or smaller, specifically, 1 to 10 nm. 
     The upper electrode layer  50  includes the noble metal oxide film  58  made of platinum oxide (PtO x ) and a noble metal film  56  formed between the noble metal oxide film  58  and the transition metal oxide film  48 , the noble metal film  56  made of Pt, which is the same type of noble metal that forms the noble metal oxide film  58 . The noble metal film  56 , as will be described later, is formed when the transition metal oxide film  48  is formed by supplying oxygen contained in the noble metal oxide film  58  to the transition metal film  46  to oxidize the transition metal film  46 . 
     In the present embodiment, the transition metal oxide film  48  is formed by supplying oxygen contained in the noble metal oxide film  58  to the transition metal film  46  to oxidize the transition metal film  46 . The present embodiment thus allows the formed transition metal oxide film  48  to be relatively thin, for example, 10 nm or thinner, and have good film thickness uniformity. A resistance storage element operable at low voltage and current levels can be thus provided. 
     An interlayer insulating film  60  is formed on the interlayer insulating film  36  on which the resistance storage elements  42  have been formed. Contact plugs  64  connected to the upper electrode layers  50  of the resistance storage elements  42  are embedded in the interlayer insulating film  60 . 
     A bit line  66  electrically connected to the upper electrodes  50  of the resistance storage elements  42  via the contact plugs  64  is formed on the interlayer insulating film  60  in which the contact plugs  64  are embedded. The bit line  66  extends horizontally in the plane of view in  FIG. 1A . 
     A method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment will be described below.  FIGS. 2A to 2L  are process cross-sectional views illustrating the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment. 
     First, the shallow trench isolation (STI) or any other suitable method is used to form the element isolation area  12  that defines an element area in the semiconductor substrate  10 . The semiconductor substrate  10  is, for example, a silicon substrate. 
     On the semiconductor substrate  10  are then formed the selection transistors  20 , each of which including the corresponding gate electrode  14  and source/drain diffusion layers  16 ,  18 , as in a method for manufacturing a typical MOS transistor (see  FIG. 2A ). 
     CVD (chemical vapor deposition) or any other suitable method is then used to deposit a silicon oxide film on the semiconductor substrate  10  on which the selection transistors  20  have been formed. Thereafter, the CMP or any other suitable method is used to polish the surface of the silicon oxide film to form the interlayer insulating film  22  including the silicon oxide film with a planarized surface. 
     Photolithography and dry etching are then used to form contact holes  24  and  26  in the interlayer insulating film  22  in such a way that they reach the source/drain diffusion layers  16  and  18 . 
     CVD or any other suitable method is then used to deposit barrier metal and tungsten films. Thereafter, the conductive films are etched back to form the contact plugs  28  and  30  in the contact holes  24  and  26 , the contact plugs  28  and  30  electrically connected to the source/drain diffusion layers  16  and  18  (see  FIG. 2B ). 
     CVD or any other suitable method is then used to deposit a conductive film on the interlayer insulating films  22  in which the contact plugs  28  and  30  have been embedded. Thereafter, photolithography and dry etching are used to pattern the conductive film to form the source line  32  electrically connected to the source/drain diffusion layer  16  via the contact plug  28  and the relay wiring lines  34  electrically connected to the source/drain diffusion layers  18  via the contact plugs  30  (see  FIG. 2C ). 
     CVD or any other suitable method is then used to deposit a silicon oxide film on the interlayer insulating film  22  on which the source line  32  and the relay wiring lines  34  have been formed. Thereafter, the CMP or any other suitable method is used to polish the surface of the silicon oxide film to form the interlayer insulating film  36  including the silicon oxide film with a planarized surface. 
     Photolithography and dry etching are then used to form contact holes  38  in the interlayer insulating film  36  in such a way that they reach the relay wiring lines  34 . 
     CVD or any other suitable method is then used to deposit barrier metal and tungsten films. Thereafter, the conductive films are etched back to form the contact plugs  40  in the contact holes  38 , the contact plugs  40  electrically connected to the source/drain diffusion layers  18  via the relay wiring lines  34  and the contact plugs  30  (see  FIG. 2D ). 
     Sputtering or any other suitable method is then used to deposit a Ti film having a film thickness of, for example, 10 nm on the interlayer insulating film  36  in which the contact plugs  40  have been embedded so as to form the close contact layer  52  including the Ti film. A titanium nitride (TiN) film may be deposited as the close contact layer  52  as well as the Ti film. The close contact layer  52  is provided to enhance the close contact property between the noble metal film  54  in the lower electrode layer  44  and the interlayer insulating film  36  including the silicon oxide film. 
     Sputtering or any other suitable method is then used to deposit a Pt film having a film thickness of, for example, 50 nm on the close contact layer  52  to form the noble metal film  54  including the Pt film. 
     Sputtering or any other suitable method is then used to deposit an Ni film having a film thickness of, for example, 8 nm on the noble metal film  54  to form the transition metal film  46  including the Ni film. The transition metal film  46  is formed in an atmosphere without oxygen and other oxidizing gases. 
     Sputtering or any other suitable method is then used to deposit a PtO x  film having a film thickness of, for example, 10 nm on the transition metal film  46  to form the noble metal oxide film  58  formed of the PtO x  film (see  FIG. 2E ). The noble metal oxide film  58  may be amorphous or crystalline. A case where the noble metal oxide film formed on the transition metal film  46  is crystallized will be described in a sixth embodiment. 
     Heat treatment is then carried out at a temperature within a range from 200 to 750° C., more preferably from 300 to 500° C., for example, in an inert gas atmosphere or a mixed gas atmosphere containing an inert gas and an oxidizing gas. The heat treatment causes oxygen contained in the noble metal oxide film  58  to be supplied to the transition metal film  46  so as to oxidize the entire transition metal film  46 . 
     Specifically, for example, the following first to third heat treatment conditions can be used. That is, under a first heat treatment condition, a resistance heating electric furnace is used as the heat treatment apparatus at a heat treatment temperature of 400° C. for a heat treatment period of 30 minutes in an argon atmosphere as the heat treatment atmosphere at atmospheric pressure as the heat treatment pressure. 
     Under a second heat treatment condition, a low-pressure heating furnace is used as the heat treatment apparatus at a heat treatment temperature of 400° C. for a heat treatment period of 3 minutes in an argon atmosphere as the heat treatment atmosphere at 1 Pa as the heat treatment pressure. Under a third heat treatment condition, a rapid lamp heating apparatus (RTA apparatus) is used as the heat treatment apparatus at a heat treatment temperature of 400° C. for a heat treatment period of 1 minute in an argon/oxygen mixed gas atmosphere containing 5% of oxygen as the heat treatment atmosphere at atmospheric pressure as the heat treatment pressure. A heat treatment atmosphere containing oxygen or other oxidizing gases as in the case of the third heat treatment condition increases the rate at which the transition metal film  46  is oxidized, whereby the heat treatment period can be shortened. 
     Instead of oxidizing the entire transition metal film  46 , part of the transition metal film  46  may be left between the transition metal oxide film  48  and the noble metal film  54 . In this case, the heat treatment period may be shorter than those in the above cases. A case where part of the transition metal film  46  is left will be described in a second embodiment. 
       FIGS. 2F to 2H  are enlarged views illustrating how the transition metal film  46  is oxidized in the present embodiment. 
     When heat treatment is carried out after the noble metal oxide film  58  has been formed on the transition metal film  46  as illustrated in  FIG. 2F , oxygen contained in the noble metal oxide film  58  dissociates therefrom and is supplied to the transition metal film  46 , as illustrated in  FIG. 2G . The oxygen supplied from the noble metal oxide film  58  oxidizes the transition metal film  46  gradually from its surface to form the noble metal oxide film  48  in the transition metal film  46 . As the noble metal oxide film  58  is reduced, the noble metal film  56  made of the noble metal that forms the noble metal oxide film  58  is formed in the lower portion of the noble metal oxide film  58 . 
     As the heat treatment continues, the oxidation of the transition metal film  46  by the oxygen supplied from the noble metal oxide film  58  proceeds, and the entire transition metal film  46  is oxidized and the transition metal oxide film  48  is formed, as illustrated in  FIG. 2H . The film thickness of the thus formed transition metal oxide film  48  is, for example, 10 nm or smaller, specifically, ranges from 1 to 10 nm. The noble metal film  56  formed in the lower portion of the noble metal oxide film  58  by the dissociation of the oxygen contained in the noble metal oxide film  58  has a film thickness of, for example, approximately 5 nm. 
     On the interlayer insulating film  36  are thus formed a stacked film including the close contact layer  52 , the noble metal film  54 , the transition metal oxide film  48 , the noble metal film  56 , and the noble metal oxide film  58  (see  FIG. 2I ). 
     As described above, in the present embodiment, the transition metal oxide film  48 , which becomes the resistance storage layer of the resistance storage element  42 , is formed by supplying oxygen contained in the noble metal oxide film  58  to the transition metal film  46  to oxidize the transition metal film  46 . The present embodiment therefore allows the formation of the transition metal oxide film  48  having a relatively thin film thickness of, for example, 10 nm or smaller and good film thickness uniformity, whereby a resistance storage element operable at low voltage and current levels can be provided. 
     Further, in the transition metal oxide film  48  formed by the oxidation described above, the composition ratio of the oxygen thereof is lower than that in the stoichiometric composition. The thus composed transition metal oxide film  48  allows reactions in a forming process, the set action, and the reset action to more readily proceed than a transition metal oxide film having the stoichiometric composition does, whereby it is expected that the voltage and current levels for the operation of the resistance storage element can be reduced. 
     Photolithography and dry etching are then used to pattern the noble metal oxide film  58 , the noble metal film  56 , the transition metal oxide film  48 , the noble metal film  54 , and the close contact layer  52  to form the resistance storage elements  42 , each of which having the lower electrode layer  44  including the stacked layer comprised of the close contact layer  52  and the noble metal film  54 , the resistance storage layer  48  including the transition metal oxide film, and the upper electrode layer  50  including the noble metal film  56  and the noble metal oxide film  58  (see  FIG. 2J ). 
     CVD or any other suitable method is then used to deposit a silicon oxide film on the interlayer insulating film  36  on which the resistance storage elements  42  have been formed. Thereafter, the CMP or any other suitable method is used to polish the surface of the silicon oxide film to form the interlayer insulating film  60  formed of the silicon oxide film with a planarized surface. 
     Photolithography and dry etching are then used to form contact holes  62  in the interlayer insulating film  60  in such a way that they reach the upper electrode layers  50  of the resistance storage elements  42 . 
     CVD or any other suitable method is then used to deposit barrier metal and tungsten films. Thereafter, the conductive films are etched back to form the contact plugs  64  in the contact holes  62 , the contact plugs  64  connected to the upper electrode layers  50  of the resistance storage elements  42  (see  FIG. 2K ). 
     A conductive film is then deposited on the interlayer insulating film  60  in which the contact plugs  64  have been embedded. Thereafter, photolithography and dry etching are used to pattern the conductive film so as to form the bit line  66  electrically connected to the upper electrode layers  50  of the resistance storage elements  42  via the contact plugs  64  (see  FIG. 2L ). 
     An overlying wiring line layer and other layers are then formed. The non-volatile semiconductor storage device is thus completed. 
     A description will be made of results obtained by evaluating the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment.  FIGS. 3A to 3C  are graphs illustrating the current versus voltage characteristic of the resistance storage element according to the present embodiment.  FIG. 4A  is a cross-sectional view illustrating a resistance storage element according to a comparative example using a sputtered NiO film as the resistance storage layer, and  FIG. 4B  shows graphs illustrating the current versus voltage characteristic of the resistance storage element according to the comparative example.  FIG. 5  is an electron micrograph of a cross-sectional structure of the resistance storage element using the sputtered NiO film as the resistance storage layer. 
     The dotted lines in  FIGS. 3A to 3C  and  4 B represent the current versus voltage characteristic in a forming process. The forming process is carried out to give a resistance storage element a resistance storage characteristic that allows a high-resistance state and a low-resistance state to be changed reversibly. The forming process involves, for example, applying a voltage equivalent to a dielectric breakdown voltage to the resistance storage layer. It is believed that applying the voltage to the resistance storage element to cause soft breakdown in the resistance storage layer allows filament-like current paths to be formed in the resistance storage layer and the current paths develop the resistance storage characteristic. The forming process only needs to be carried out once in an initial stage, but not any more in later stages. The voltage for the forming process is called a forming voltage. 
     The solid lines in  FIGS. 3A to 3C  and  4 B represent the current versus voltage characteristic when the set and reset actions are repeated three times in the resistance storage element. Gradually increasing the voltage applied to the resistance storage element in the high-resistance state produces a phenomenon in which the resistance storage element transits from the high-resistance state to the low-resistance state at a certain voltage magnitude and the current abruptly increases (set action). The voltage at which the set action occurs is called a set voltage. On the other hand, gradually increasing the voltage applied to the resistance storage element in the low-resistance state to gradually increase the current flowing through the resistance storage element produces a phenomenon in which the resistance storage element transits from the low-resistance state to the high-resistance state at a certain current magnitude and the current decreases (reset action). The current at which the reset action occurs is called a reset current. 
       FIG. 3A  illustrates the current versus voltage characteristic of the resistance storage element according to an experimental example 1 in which the resistance storage layer is the transition metal oxide film  48  formed by the heat treatment under the first heat treatment condition described above. In the experimental example 1, the forming voltage was 1.32 V. The set voltage when the set and reset actions were repeated three times was 1.32 V, 1.30 V, and 0.80 V in the order of occurrence, and the reset current was 0.91 mA, 0.71 mA, and 0.64 mA in the order of occurrence. 
       FIG. 3B  illustrates the current versus voltage characteristic of the resistance storage element according to an experimental example 2 in which the resistance storage layer is the transition metal oxide film  48  formed by the heat treatment under the second heat treatment condition described above. In the experimental example 2, the forming voltage was 0 V. The set voltage when the set and reset actions were repeated three times was 1.04 V, 1.06 V, and 1.08 V in the order of occurrence, and the reset current was 1.01 mA, 0.75 mA, and 0.88 mA in the order of occurrence. 
       FIG. 3C  illustrates the current versus voltage characteristic of the resistance storage element according to an experimental example 3 in which the resistance storage layer is the transition metal oxide film  48  formed by the heat treatment under the third heat treatment condition described above. In the experimental example 3, the forming voltage was 0 V. The set voltage when the set and reset actions were repeated three times was 1.26 V, 1.40 V, and 1.46 V in the order of occurrence, and the reset current was 0.75 mA, 1.05 mA, and 1.15 mA in the order of occurrence. 
     The set and reset actions occurred in the resistance storage elements according to the experimental examples 2 and 3 that had undergone no forming process. When a resistance storage element in an initial state has a low resistance, no forming occurs in some cases. For example, when the entire Ni is not converted into NiO but part of the Ni is left, the resistance of the entire element decreases and the element functions without undergoing the forming process. When the degree of oxidation of the NiO is low, the resistance of the entire element also decreases and the element functions in some cases without undergoing the forming process. 
     On the other hand,  FIG. 4B  illustrates the current versus voltage characteristic of the resistance storage element according to the comparative example in which the resistance storage element includes a lower electrode layer  68  made of Pt, a resistance storage layer  70  formed of a sputtered NiO film and having a film thickness of 20 nm, and an upper electrode layer  72  made of Pt. In the comparative example, the forming voltage was 5 V. The set voltage when the set and reset actions were repeated three times was 1.20 V, 1.40 V, and 1.60 V in the order of occurrence, and the reset current was 10 mA, 20 mA, and 20 mA in the order of occurrence. 
     As seen from the results described above, the voltage and current levels for the operation in the experimental examples 1 to 3, in particular, the forming voltage and reset current levels are smaller than those in the comparative example. It is seen that the resistance storage element according to the present embodiment can develop the resistance storage characteristic by applying a forming voltage of 3.3 V or lower, even 1.5 V or lower to the transition metal oxide film  48 . 
     The evaluation results described above show that the present embodiment allows a resistance storage element operable at low voltage and current levels to be provided. 
     When the transition metal oxide film used as the resistance storage layer is formed by sputtering as in the comparative example, and the thus formed transition metal oxide film has a relatively small thickness, a step-shaped disconnected portion may occur in the transition metal oxide film and hence the electrodes may be shorted. 
       FIG. 5  is an electron micrograph of a cross-sectional structure of a resistance storage element using a sputtered NiO film and having a film thickness of 10 nm as the resistance storage layer. As illustrated in  FIG. 5 , on an interlayer insulating film  76  in which a tungsten plug  74  is embedded are formed a resistance storage element including a lower electrode layer  78  formed of a Pt film, a resistance storage layer  80  formed of an NiO film having a film thickness of 10 nm, and an upper electrode layer  82  formed of a Pt film. An aluminum wiring line  84  is connected to the upper electrode layer  82 . 
     As seen from  FIG. 5 , the NiO film  80  forming the resistance storage layer, when having the small film thickness of 10 nm, has corrugated undulations reflecting irregularities of the underlying interlayer insulating film  76  and tungsten plug  74 . Therefore, when the NiO film  80  is simply formed by sputtering to have a small thickness, a step-shaped disconnected portion may occur in the NiO film  80  and hence the lower electrode layer  78  and the upper electrode layer  82  may be shorted. 
     In contrast, in the present embodiment, oxygen contained in the noble metal oxide film  58  in the upper electrode layer  50  is supplied to the transition metal film  46  to oxidize the transition metal film  46  into the transition metal oxide film  48 , which becomes the resistance storage layer. In this case, even when the thus formed transition metal oxide film  48  is relatively thin, for example, has a film thickness of 10 nm or smaller, no step-shaped disconnected portion will be produced in the transition metal oxide film  48 . No defect due to short circuit will therefore occur, and a resistance storage element operable at low voltage and current levels can be provided. 
     Second Embodiment 
     A description will be made of a resistance storage element, a non-volatile semiconductor storage device using the resistance storage element, and a method for manufacturing the resistance storage element according to a second embodiment of the present invention. The same components as those of the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment have the same reference characters and description thereof will be omitted or simplified. 
     The resistance storage element and the non-volatile semiconductor storage device according to the present embodiment will be described with reference to  FIGS. 6A and 6B .  FIG. 6A  is a cross-sectional view illustrating the non-volatile semiconductor storage device according to the present embodiment.  FIG. 6B  is an enlarged view of only the resistance storage element. 
     In the resistance storage element according to the present embodiment, when the transition metal film  46  is oxidized in the method for manufacturing the resistance storage element according to the first embodiment, the entire transition metal film  46  is not oxidized, but part of the transition metal film  46  is left, and the remaining transition metal film  46  is present between the noble metal film  54  and the transition metal oxide film  48 . 
     As illustrated in  FIG. 6A , resistance storage elements  42   a  are formed on the interlayer insulating film  36  in which the contact plugs  40  have been embedded. Each of the resistance storage elements  42   a  includes a lower electrode layer  44   a  electrically connected to the corresponding source/drain diffusion layer  18  via the corresponding contact plug  40 , relay wiring line  34 , and contact plug  30 , the resistance storage layer  48  formed on the lower electrode layer  44   a , and the upper electrode layer  50  formed on the resistance storage layer  48 . 
     The lower electrode layer  44   a  is a film obtained by stacking the close contact layer  52 , the noble metal film  54 , and the transition metal film  46  containing Ni. As will be described later, the transition metal film  46  is what is left when the entire transition metal film  46  is not oxidized in the process in which the transition metal film  46  is oxidized to form the transition metal oxide film  48 . 
     The resistance storage layer  48  has a transition metal oxide film containing NiO x . The transition metal oxide film  48  is formed, as will be described later, by forming the noble metal oxide film  58 , which forms the upper electrode layer  50 , on the transition metal film  46  and carrying out heat treatment to supply oxygen contained in the noble metal oxide film  58  to the transition metal film  46  so as to oxidize part of the transition metal film  46 . 
     The upper electrode layer  50  includes the noble metal oxide film  58  containing PtO x  and the noble metal film  56  containing Pt, which is the noble metal contained in the noble metal oxide film  58 , the noble metal film  56  formed between the noble metal oxide film  58  and the transition metal oxide film  48 . 
     It has been found that the transition metal film  46  formed between the noble metal film  54  and the transition metal oxide film  48  in the present embodiment can further reduce the reset current. Although the detail of the reset current reduction mechanism has not been clearly understood, the thus formed transition metal film  46  seems to prevent diffusion of the element from the noble metal film  54  to the transition metal oxide film  48  and diffusion of oxygen from the transition metal oxide film  48  to the noble metal film  54 . 
     A method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment will be described below.  FIGS. 7A to 7G  are process cross-sectional views illustrating the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment. 
     The contact plugs  40  and other components formed before the contact plugs  40  are first formed in the same method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment illustrated in  FIGS. 2A to 2D . 
     Sputtering or any other suitable method is then used to sequentially form the close contact layer  52  including a Ti film, the noble metal film  54  including a Pt film, the transition metal film  46  including an Ni film, and the noble metal oxide film  58  including a PtO x  film on the interlayer insulating film  36  in which the contact plugs  40  have been embedded, as in the first embodiment (see  FIG. 7A ). 
     Heat treatment is then carried out at a temperature within a range from 200 to 750° C., more preferably from 300 to 500° C., for example, in an inert gas atmosphere or a mixed gas atmosphere containing an inert gas and an oxidizing gas. The heat treatment causes oxygen contained in the noble metal oxide film  58  to be supplied to the transition metal film  46  so as to oxidize part of the transition metal film  46 . In the present embodiment, the heat treatment condition is adjusted as appropriate, for example, the heat treatment period is shortened as compared to that in the first embodiment, not to oxidize the entire transition metal film  46  but to leave part of the transition metal film  46 . 
       FIGS. 7C to 7E  are enlarged views illustrating how part of the transition metal film  46  is oxidized in the present embodiment. 
     When the heat treatment is carried out after the noble metal oxide film  58  has been formed on the transition metal film  46  as illustrated in  FIG. 7C , oxygen contained in the noble metal oxide film  58  dissociates therefrom and is supplied to the transition metal film  46 , as illustrated in  FIG. 7D . The oxygen supplied from the noble metal oxide film  58  oxidizes the transition metal film  46  gradually from its surface to form the transition metal oxide film  48  in the transition metal film  46 . As the noble metal oxide film  58  is reduced, the noble metal film  56  containing the noble metal contained in the noble metal oxide film  58  is formed in the lower portion of the noble metal oxide film  58 . 
     In the present embodiment, adjusting the heat treatment condition as appropriate allows part of the transition metal film  46  to be oxidized to form the transition metal oxide film  48  and the remaining transition metal film  46  to be left under the transition metal oxide film  48 , as illustrated in  FIG. 7E . 
     On the interlayer insulating film  36  are thus formed a stacked film including the close contact layer  52 , the noble metal film  54 , the transition metal film  46 , the transition metal oxide film  48 , the noble metal film  56 , and the noble metal oxide film  58  (see  FIG. 7B ). 
     Photolithography and dry etching are then used to pattern the noble metal oxide film  58 , the noble metal film  56 , the transition metal oxide film  48 , the transition metal film  46 , the noble metal film  54 , and the close contact layer  52  to form the resistance storage elements  42   a , each of which having the lower electrode layer  44   a  including a stacked film comprised of the close contact layer  52 , the noble metal film  54 , and the transition metal film  46 , the resistance storage layer  48  including the transition metal oxide film, and the upper electrode layer  50  including the noble metal film  56  and the noble metal oxide film  58  (see  FIG. 7F ). 
     The contact plugs  64 , the bit line  66 , and other components are then formed in the same method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment illustrated in  FIGS. 2K and 2L . The non-volatile semiconductor storage device is thus completed (see  FIG. 7G ). 
     As described in the present embodiment, part of the transition metal film  46  may be oxidized to form the transition metal oxide film  48 , and the remaining transition metal film  46  may be left between the noble metal film  54  and the transition metal oxide film  48 . Leaving part of the transition metal film  46  can further reduce the reset current in the resistance storage element. 
     Third Embodiment 
     A description will be made of a resistance storage element, a non-volatile semiconductor storage device using the resistance storage element, and a method for manufacturing the resistance storage element according to a third embodiment of the present invention. The same components as those of the resistance storage elements and the non-volatile semiconductor storage devices according to the first and second embodiments have the same reference characters and description thereof will be omitted or simplified. 
     The resistance storage element and the non-volatile semiconductor storage device according to the present embodiment will be first described with reference to  FIGS. 8A and 8B .  FIG. 8A  is a cross-sectional view illustrating the non-volatile semiconductor storage device according to the present embodiment.  FIG. 8B  is an enlarged view of only the resistance storage element. 
     In the resistance storage element according to the present embodiment, when the transition metal film  46  is oxidized in the method for manufacturing the resistance storage element according to the first embodiment, the entire oxygen contained in the noble metal oxide film  58  is allowed to dissociate to convert the noble metal oxide film  58  into the noble metal film  56 . The resistance storage element therefore has an upper electrode layer  50   a  including the noble metal film  56 . 
     As illustrated in  FIG. 8A , resistance storage elements  42   b  are formed on the interlayer insulating film  36  in which the contact plugs  40  have been embedded. Each of the resistance storage elements  42   b  includes the lower electrode layer  44  electrically connected to the corresponding source/drain diffusion layer  18  via the corresponding contact plugs  40 , relay wiring line  34 , and the contact plug  30 , the resistance storage layer  48  formed on the lower electrode layer  44 , and the upper electrode layer  50   a  formed on the resistance storage layer  48 . 
     The lower electrode layer  44  has a stacked film comprised of the close contact layer  52  and the noble metal film  54 . 
     The resistance storage layer  48  has a transition metal oxide film containing NiO x . The transition metal oxide film  48  is formed, as will be described later, by forming the noble metal oxide film  58  on the transition metal film  46  and carrying out heat treatment to supply oxygen contained in the noble metal oxide film  58  to the transition metal film  46  so as to oxidize the transition metal film  46 . 
     The upper electrode layer  50   a  has the noble metal film  56  containing Pt. The noble metal film  56  is formed from the noble metal oxide film  58 , as will be described later, by causing all of the oxygen contained in the noble metal oxide film  58  containing PtO x  to dissociate therefrom when the transition metal oxide film  48  is formed. 
     A method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment will be described with reference to  FIGS. 9A to 9G .  FIGS. 9A to 9G  are process cross-sectional views illustrating the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment. 
     The contact plugs  40  and other components formed before the contact plugs  40  are first formed in the same method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment illustrated in  FIGS. 2A to 2D . 
     Sputtering or any other suitable method is then used to sequentially form the close contact layer  52  including a Ti film, the noble metal film  54  including a Pt film, the transition metal film  46  including an Ni film, and the noble metal oxide film  58  including a PtO x  film on the interlayer insulating film  36  in which the contact plugs  40  have been embedded, as in the first embodiment (see  FIG. 9A ). 
     Heat treatment is then carried out at a temperature within a range from 200 to 750° C., more preferably from 300 to 500° C., for example, in an inert gas atmosphere or a mixed gas atmosphere containing an inert gas and an oxidizing gas. The heat treatment causes oxygen contained in the noble metal oxide film  58  to be supplied to the transition metal film  46  so as to oxidize the transition metal film  46 . In the present embodiment, the heat treatment condition is adjusted as appropriate to cause the entire oxygen contained in the noble metal oxide film  58  to dissociate therefrom. 
       FIGS. 9C to 9E  are enlarged views illustrating how the transition metal film  46  is oxidized in the present embodiment. 
     When the heat treatment is carried out after the noble metal oxide film  58  has been formed on the transition metal film  46  as illustrated in  FIG. 9C , oxygen contained in the noble metal oxide film  58  dissociates therefrom and is supplied to the transition metal film  46 , as illustrated in  FIG. 9D . The oxygen supplied from the noble metal oxide film  58  oxidizes the transition metal film  46  gradually from its surface to form the transition metal oxide film  48  in the transition metal film  46 . As the noble metal oxide film  58  is reduced, the noble metal film  56  made of the noble metal that forms the noble metal oxide film  58  is formed in the lower portion of the noble metal oxide film  58 . 
     In the present embodiment, adjusting the heat treatment condition as appropriate allows the entire oxygen contained in the noble metal oxide film  58  to dissociate therefrom. The transition metal film  46  is thus oxidized into the transition metal oxide film  48 , and the noble metal oxide film  58  is reduced into the noble metal film  56 , as illustrated in  FIG. 9E . 
     On the interlayer insulating film  36  are thus formed a stacked film including the close contact layer  52 , the noble metal film  54 , the transition metal oxide film  48 , and the noble metal film  56  (see  FIG. 9B ). 
     Photolithography and dry etching are then used to pattern the noble metal film  56 , the transition metal oxide film  48 , the noble metal film  54 , and the close contact layer  52  to form the resistance storage elements  42   b , each of which having the lower electrode layer  44  including the stacked film comprised of the close contact layer  52  and the noble metal film  54 , the resistance storage layer  48  including the transition metal oxide film, and the upper electrode layer  50   a  including the noble metal film  56  (see  FIG. 9F ). 
     The contact plugs  64 , the bit line  66 , and other components are then formed in the same method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment illustrated in  FIGS. 2K and 2L . The non-volatile semiconductor storage device is thus completed (see  FIG. 9G ). 
     As described in the present embodiment, when the transition metal film  46  is oxidized into the transition metal oxide film  48 , the entire oxygen contained in the noble metal oxide film  58  may be dissociated therefrom to convert the noble metal oxide film  58  into the noble metal film  56 , which then forms the upper electrode layer  50   a.    
     Fourth Embodiment 
     A method for manufacturing a resistance storage element and a non-volatile semiconductor storage device according to a fourth embodiment of the present invention will be described with reference to  FIGS. 10A to 10D .  FIGS. 10A to 10D  are process cross-sectional views illustrating the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment. The same components as those of the resistance storage elements and the non-volatile semiconductor storage devices according to the first to third embodiments have the same reference characters and description thereof will be omitted or simplified. 
     The method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment involves forming a stacked film comprised of the close contact layer  52 , the noble metal film  54 , the transition metal film  46 , and the noble metal oxide film  58 , patterning the stacked film into the shape of the resistance storage element, and carrying out heat treatment for forming the transition metal oxide film  48 . 
     The contact plugs  40  and other components formed before the contact plugs  40  are formed in the same method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment illustrated in  FIGS. 2A to 2D . 
     Sputtering or any other suitable method is then used to sequentially form the close contact layer  52  including a Ti film, the noble metal film  54  including a Pt film, the transition metal film  46  including an Ni film, and the noble metal oxide film  58  including a PtO x  film on the interlayer insulating film  36  in which the contact plugs  40  have been embedded, as in the first embodiment (see  FIG. 10A ). 
     Photolithography and dry etching are then used to pattern the stacked film including the noble metal oxide film  58 , the transition metal film  46 , the noble metal film  54 , and the close contact layer  52  into the shape of the resistance storage element (see  FIG. 10B ). 
     Heat treatment is then carried out at a temperature within a range from 200 to 750° C., more preferably from 300 to 500° C., for example, in an inert gas atmosphere or a mixed gas atmosphere containing an inert gas and an oxidizing gas. The heat treatment causes oxygen contained in the noble metal oxide film  58  to be supplied to the transition metal film  46  so as to oxidize the transition metal film  46  into the transition metal oxide film  48 . As the noble metal oxide film  58  is reduced, the noble metal film  56  containing the noble metal that forms the noble metal oxide film  58  is formed in the lower portion of the noble metal oxide film  58 . 
     In the present embodiment, the heat treatment is thus carried out after the stacked film has been patterned into the shape of the resistance storage element. Therefore, when the heat treatment atmosphere contains an oxidizing gas, the close contact layer  52  is oxidized, which may degrade the electric connection between the lower electrode layer  44  and the contact plug  40 . Therefore, when the heat treatment is carried out in an atmosphere containing an oxidizing gas, it is desirable to adjust the heat treatment condition as appropriate, for example, by setting the concentration of the oxidizing gas to a low level. 
     Resistance storage elements  42   c  are thus formed, each of the resistance storage elements  42   c  including the lower electrode layer  44  including a stacked film comprised of the close contact layer  52  and the noble metal film  54 , the resistance storage layer  48  including the transition metal oxide film, and the upper electrode layer  50  including the noble metal film  56  and the noble metal oxide film  58  (see  FIG. 10C ). 
     The contact plugs  64 , the bit line  66 , and other components are then formed in the same method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment illustrated in  FIGS. 2K and 2L . The non-volatile semiconductor storage device is thus completed (see  FIG. 10D ). 
     As described in the present embodiment, the heat treatment for forming the transition metal oxide film  48  may be carried out after the stacked film including the close contact layer  52 , the noble metal film  54 , the transition metal film  46 , and the noble metal oxide film  58  is formed and the stacked film is patterned into the shape of the resistance storage element. 
     A description will be made of results obtained by evaluating the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment.  FIG. 11  illustrates graphs illustrating the current versus voltage characteristic of the resistance storage element according to the present embodiment. The dotted line in  FIG. 11  represents the current versus voltage characteristic in the forming process. The solid lines in  FIG. 11  represent the current versus voltage characteristic when the set and reset actions are repeated three times in the resistance storage element. 
       FIG. 11  illustrates the current versus voltage characteristic of the resistance storage element according to an experimental example 4 in which the resistance storage layer is the transition metal oxide film  48  formed by carrying out heat treatment after the stacked film has been patterned into the shape of the resistance storage element as described above. In the experimental example 4, the forming voltage was 1.20 V. The set voltage when the set and reset actions were repeated three times was 0.88 V, 1.20 V, and 1.42 V in the order of occurrence, and the reset current was 1.01 mA, 0.37 mA, and 0.57 mA in the order of occurrence. 
     As seen from  FIGS. 11 and 4B , in the experimental example 4 as well, the voltage and current levels for the operation, in particular, the forming voltage and reset current levels are smaller than those in the comparative example. 
     The evaluation results described above illustrate that the present embodiment allows a resistance storage element operable at low voltage and current levels to be provided. 
     Fifth Embodiment 
     A description will be made of a resistance storage element, a non-volatile semiconductor storage device using the resistance storage element, and a method for manufacturing the resistance storage element according to a fifth embodiment of the present invention. The same components as those of the resistance storage elements and the non-volatile semiconductor storage devices according to the first to fourth embodiments have the same reference characters and description thereof will be omitted or simplified. 
     The resistance storage element and the non-volatile semiconductor storage device according to the present embodiment will be first described with reference to  FIGS. 12A and 12B .  FIG. 12A  is a cross-sectional view illustrating the non-volatile semiconductor storage device according to the present embodiment.  FIG. 12B  is an enlarged view of only the resistance storage element. 
     The resistance storage element according to the present embodiment is based on the resistance storage element according to the first embodiment but differs therefrom in that a noble metal film  86  is formed on the noble metal oxide film  58  to prevent upward diffusion of oxygen from the noble metal oxide film  58  and the resistance storage element has an upper electrode layer  50   b  including the noble metal film  56 , the noble metal oxide film  58 , and the noble metal film  86 . 
     As illustrated in  FIG. 12A , resistance storage elements  42   d  are formed on the interlayer insulating film  36  in which the contact plugs  40  have been embedded. Each of the resistance storage elements  42   d  includes the lower electrode layer  44  electrically connected to the corresponding source/drain diffusion layer  18  via the corresponding contact plug  40 , relay wiring line  34 , and contact plug  30 , the resistance storage layer  48  formed on the lower electrode layer  44 , and the upper electrode layer  50   b  formed on the resistance storage layer  48 . 
     The lower electrode layer  44  has a stacked film comprised of the close contact layer  52  and the noble metal film  54 . 
     The resistance storage layer  48  has a transition metal oxide film containing NiO x . The transition metal oxide film  48  is formed, as will be described later, by forming the noble metal oxide film  58  on the transition metal film  46  and carrying out heat treatment to supply oxygen contained in the noble metal oxide film  58  to the transition metal film  46  so as to oxidize the transition metal film  46 . 
     The upper electrode layer  50   b  includes the noble metal oxide film  58  containing PtO x ; the noble metal film  56  formed between the noble metal oxide film  58  and the transition metal oxide film  48 , the noble metal film  56  containing Pt, which is a noble metal; and the noble metal film  86  formed on the noble metal oxide film  58 , the noble metal film  86  made of Pt. 
     The noble metal film  86 , as will be described later, functions as a diffusion prevention layer that prevents upward diffusion of oxygen from the noble metal oxide film  58  when oxygen contained in the noble metal oxide film  58  is supplied to the transition metal film  46  to oxidize the transition metal film  46 . The noble metal film  86  may be made of iridium (Ir) or ruthenium (Ru) instead of Pt. Instead of the noble metal film  86 , the diffusion prevention layer can be made of a material less oxidizable than the material of the transition metal film  46 . That is, when the transition metal film  46  is made of Ni, the diffusion prevention layer can be made of a material less oxidizable than Ni, such as TiN. The reason why a material less oxidizable than the material of the transition metal film  46  is used as the material of the diffusion prevention layer is because a material more oxidizable than Ni, such as aluminum (Al) and Ti, is disadvantageously oxidized before the transition metal film  46  containing Ni is oxidized when the transition metal film  46  containing Ni is oxidized. 
     A method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment will be described with reference to  FIGS. 13A to 13D .  FIGS. 13A to 13D  are process cross-sectional views illustrating the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment. 
     The contact plugs  40  and other components formed before the contact plugs  40  are first formed in the same method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment illustrated in  FIGS. 2A to 2D . 
     Sputtering or any other suitable method is then used to sequentially form the close contact layer  52  including a Ti film, the noble metal film  54  including a Pt film, the transition metal film  46  including an Ni film, and the noble metal oxide film  58  including a PtO x  film on the interlayer insulating film  36  in which the contact plugs  40  have been embedded, as in the first embodiment. 
     Sputtering or any other suitable method is then used to deposit a Pt film on the noble metal oxide film  58  to form the noble metal film  86  including a Pt film (see  FIG. 13A ). 
     Heat treatment is then carried out at a temperature within a range from 200 to 750° C., more preferably from 300 to 500° C., for example, in an inert gas atmosphere or a mixed gas atmosphere containing an inert gas and an oxidizing gas. The heat treatment causes oxygen contained in the noble metal oxide film  58  to be supplied to the transition metal film  46  so as to oxidize the transition metal film  46  into the transition metal oxide film  48 . As the noble metal oxide film  58  is reduced, the noble metal film  56  made of the noble metal that forms the noble metal oxide film  58  is formed in the lower portion of the noble metal oxide film  58 . 
     In the present embodiment, the noble metal film  86  is formed on the noble metal oxide film  58 , the noble metal film  86  functioning as the diffusion prevention layer that prevents upward diffusion of oxygen from the noble metal oxide film  58 . The present embodiment thus allows oxygen contained in the noble metal oxide film  58  to be efficiently supplied to the transition metal film  46 , whereby the period for the heat treatment for oxidizing the transition metal film  46  can be shortened. 
     On the interlayer insulating film  36  are thus formed a stacked film comprised of the close contact layer  52 , the noble metal film  54 , the transition metal oxide film  48 , the noble metal film  56 , the noble metal oxide film  58 , and the noble metal film  86  (see  FIG. 13B ). 
     Photolithography and dry etching are then used to pattern the noble metal film  86 , the noble metal oxide film  58 , the noble metal film  56 , the transition metal oxide film  48 , the noble metal film  54 , and the close contact layer  52  to form the resistance storage elements  42   d , each of which having the lower electrode layer  44  including the stacked film comprised of the close contact layer  52  and the noble metal film  54 , the resistance storage layer  48  including the transition metal oxide film, and the upper electrode layer  50   b  including the noble metal film  56 , the noble metal oxide film  58 , and the noble metal film  86  (see  FIG. 13C ). 
     The contact plugs  64 , the bit line  66 , and other components are formed in the same method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment illustrated in  FIGS. 2K and 2L . The non-volatile semiconductor storage device is thus completed (see  FIG. 13D ). 
     As described in the present embodiment, the noble metal film  86  that functions as the diffusion prevention layer for preventing upward diffusion of oxygen from the noble metal oxide film  58  may be formed on the noble metal oxide film  58 . Forming the noble metal film  86  allows oxygen contained in the noble metal oxide film  58  to be efficiently supplied to the transition metal film  46 , whereby the period for the heat treatment for oxidizing the transition metal film  46  can be shortened. 
     Sixth Embodiment 
     A method for manufacturing a resistance storage element and a non-volatile semiconductor storage device according to a sixth embodiment of the present invention will be described with reference to  FIGS. 14A and 14B .  FIGS. 14A and 14B  are process cross-sectional views illustrating the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment. The same components as those of the resistance storage elements and the non-volatile semiconductor storage devices according to the first to fifth embodiments have the same reference characters and description thereof will be omitted or simplified. 
     The method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment is based on the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment but involves forming the noble metal oxide film  58  on the transition metal film  46  while heating the entire structure to form a crystallized noble metal oxide film  58   c.    
     The contact plugs  40  and other components formed before the contact plugs  40  are formed in the same method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment illustrated in  FIGS. 2A to 2D . 
     Sputtering or any other suitable method is then used to sequentially form the close contact layer  52  including a Ti film, the noble metal film  54  including a Pt film, and the transition metal film  46  including an Ni film on the interlayer insulating film  36  in which the contact plugs  40  have been embedded, as in the first embodiment. 
     Sputtering or any other suitable method is then used to deposit a PtO x  film on the transition metal film  46  while the entire structure is heated, for example, at a temperature of 350° C. or lower to form a noble metal oxide film  58   c  including a PtO x  film (see  FIG. 14A ). The noble metal oxide film  58   c  is crystallized due to the heat during the formation. 
     Heat treatment is then carried out at a temperature within a range from 200 to 750° C., more preferably from 300 to 500° C., for example, in an inert gas atmosphere or a mixed gas atmosphere containing an inert gas and an oxidizing gas. The heat treatment causes oxygen contained in the crystallized noble metal oxide film  58   c  to be supplied to the transition metal film  46  so as to oxidize the transition metal film  46  into the transition metal oxide film  48 . As the noble metal oxide film  58   c  is reduced, the noble metal film  56  made of the noble metal that forms the noble metal oxide film  58   c  is formed in the lower portion of the noble metal oxide film  58   c.    
     In the present embodiment, the crystallized noble metal oxide film  58   c  is formed on the transition metal film  46 . The oxygen that has dissociated from the crystallized noble metal oxide film  58   c  is more activated than the oxygen that has dissociated from the amorphous noble metal oxide film  58 . The present embodiment therefore allows the transition metal film  46  to be more efficiently oxidized, whereby the period for the heat treatment for oxidizing the transition metal film  46  can be shortened. 
     On the interlayer insulating film  36  are thus formed a stacked film including the close contact layer  52 , the noble metal film  54 , the transition metal oxide film  48 , the noble metal film  56 , and the noble metal oxide film  58   c  (see  FIG. 14B ). 
     The following processes are the same as those in the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the first embodiment illustrated in  FIGS. 2J  to  2 L, and the description of those processes will be omitted. 
     As described in the present embodiment, the crystallized noble metal oxide film  58   c  may be formed on the transition metal film  46 . Forming the crystallized noble metal oxide film  58   c  allows the transition metal film  46  to be more efficiently oxidized, whereby the period for the heat treatment for oxidizing the transition metal film  46  can be shortened. 
     Seventh Embodiment 
     A method for manufacturing a resistance storage element and a non-volatile semiconductor storage device according to a seventh embodiment of the present invention will be described with reference to  FIGS. 15A to 15C .  FIGS. 15A to 15C  are process cross-sectional views illustrating the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment. The same components as those of the resistance storage elements and the non-volatile semiconductor storage devices according to the first to sixth embodiments have the same reference characters and description thereof will be omitted or simplified. 
     While in the above embodiments, the description has been made of the case where oxygen contained in the noble metal oxide film  58  is supplied to the transition metal film  46  to oxidize the transition metal film  46 , the surface of the transition metal film  46  may be oxidized before the above oxidation process. In the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment, when the noble metal oxide film  58  is formed on the transition metal film  46 , the noble metal oxide film  58  is formed while heated in an atmosphere containing oxygen or any other suitable oxidizing gas so as to oxidize the surface of the transition metal film  46  and form a thin transition metal oxide film  48   a  on the surface of the transition metal film  46 . 
     As illustrated in  FIG. 15A , sputtering or any other suitable method is used to sequentially form the close contact layer  52  including a Ti film, the noble metal film  54  including a Pt film, and the transition metal film  46  including an Ni film, as in the first embodiment. 
     As illustrated in  FIG. 15B , sputtering or any other suitable method is then used to deposit a PtO x  film on the transition metal film  46  while heating the entire structure, for example, at a temperature of 350° C. or lower in an atmosphere containing oxygen or any other suitable oxidizing gas so as to form the noble metal oxide film  58  including the PtO x  film. In this process, the surface of the transition metal film  46  is oxidized, and a thin transition metal oxide film  48   a  is formed on the surface of the transition metal film  46 . 
     Heat treatment is then carried out at a temperature within a range from 200 to 750° C., more preferably from 300 to 500° C., for example, in an inert gas atmosphere or a mixed gas atmosphere containing an inert gas and an oxidizing gas. The heat treatment causes oxygen contained in the noble metal oxide film  58  to be supplied to the transition metal film  46  so as to further oxidize the transition metal film  46  into the transition metal oxide film  48 , which becomes the resistance storage layer, as illustrated in  FIG. 15C . As the noble metal oxide film  58  is reduced, the noble metal film  56  containing the noble metal contained in the noble metal oxide film  58  is formed in the lower portion of the noble metal oxide film  58 . 
     As described in the present embodiment, when the noble metal oxide film  58  is formed on the transition metal film  46 , the noble metal oxide film  58  may be formed while heated in an atmosphere containing an oxidizing gas so as to oxidize the surface of the transition metal film  46  and form the thin transition metal oxide film  48   a  on the surface of the transition metal film  46 . 
     Eighth Embodiment 
     A method for manufacturing a resistance storage element and a non-volatile semiconductor storage device according to an eighth embodiment of the present invention will be described with reference to  FIGS. 16A to 16D .  FIGS. 16A to 16D  are process cross-sectional views illustrating the method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment. The same components as those of the resistance storage elements and the non-volatile semiconductor storage devices according to the first to seventh embodiments have the same reference characters and description thereof will be omitted or simplified. 
     While in the above embodiments, the description has been made of the case where the noble metal oxide film  58  is formed on the transition metal film  46  and oxygen contained in the noble metal oxide film  58  is supplied to the transition metal film  46  to oxidize the transition metal film  46 , oxygen contained in another noble metal oxide film may be supplied to the transition metal film  46  to oxidize the transition metal film  46 . The method for manufacturing the resistance storage element and the non-volatile semiconductor storage device according to the present embodiment involves forming the transition metal film  46  on a noble metal oxide film  88 , carrying out heat treatment to supply oxygen contained in the noble metal oxide film  88  to the transition metal film  46  so as to oxidize the transition metal film  46  into the transition metal oxide film  48 , which becomes the resistance storage layer. 
     Sputtering or any other suitable method is used to sequentially form the close contact layer  52  including a Ti film and the noble metal film  54  including a Pt film, as in the first embodiment. 
     As illustrated in  FIG. 16A , the noble metal oxide film  88  comprised of a PtO x  film is then formed on the noble metal film  54 . The noble metal oxide film  88  is included in the lower electrode layer of the resistance storage element. The noble metal oxide film  88  may be an iridium oxide (IrO x ) film or a ruthenium oxide (RuO x ) film instead of a PtO x  film. 
     As illustrated in  FIG. 16B , sputtering or any other suitable method is then used to form the transition metal film  46  including an Ni film on the noble metal oxide film  88 . In this process, by carrying out the deposition of the film while heating the entire structure, a layer deposited in an initial stage of the deposition of the transition metal film  46  including an Ni film is oxidized, and a transition metal oxide film  48   b  including an NiO x  film is formed at the interface between the transition metal film  46  and the noble metal oxide film  88 . The heated film deposition, however, may degrade the flatness of the transition metal film  46 . To avoid the problem, it is desirable to carry out the film deposition at room temperature. When the film deposition is carried out at room temperature, a significantly thin transition metal oxide film  48   b  is formed at the interface between the transition metal film  46  and the noble metal oxide film  88 . The transition metal film  46  is formed in an atmosphere without oxygen or any other oxidizing gases, as in the first embodiment. 
     Heat treatment is then carried out at a temperature within a range from 200 to 750° C., more preferably from 300 to 500° C., for example, in an inert gas atmosphere or a mixed gas atmosphere containing an inert gas and an oxidizing gas. A specific heat treatment condition can be the same as any of the first to third heat treatment conditions in the first embodiment. The heat treatment causes oxygen contained in the noble metal oxide film  88  to be supplied to the transition metal film  46  so as to oxidize the transition metal film  46  into the transition metal oxide film  48  in the resistance storage layer, as illustrated in  FIG. 16C . As the noble metal oxide film  88  is reduced, a noble metal film  90  containing the noble metal contained in the noble metal oxide film  88  is formed in the upper portion of the noble metal oxide film  88 . While  FIG. 16C  illustrates a case where part of the transition metal film  46  is left on the transition metal oxide film  48 , the heat treatment condition may be adjusted as appropriate to oxidize the entire transition metal film  46 . 
     As described above, oxygen contained in the noble metal oxide film  88  that forms the lower electrode layer of the resistance storage element may be supplied to the transition metal film  46  to oxidize the transition metal film  46  into the transition metal oxide film  48  that forms the resistance storage layer. In this case as well, the thus formed transition metal oxide film  48  has a relatively small film thickness of, for example, 10 nm or smaller and good film thickness uniformity, whereby a resistance storage element operable at low voltage and current levels can be provided. 
     The composition ratio of the oxygen of the thus formed transition metal oxide film  48  is lower than that in the stoichiometric composition, and the oxygen concentration decreases in the direction from the lower electrode layer toward the upper electrode layer. 
     A conductive film  92  is then formed on the remaining transition metal film  46 , as illustrated in  FIG. 16D . Alternatively, the noble metal oxide film  58  may be formed on the transition metal film  46 , and then oxygen contained in the noble metal oxide film  58  may be supplied to the transition metal film  46  to further oxidize the remaining transition metal film  46 , as in the first embodiment. 
     As described in the present embodiment, oxygen contained on the noble metal oxide film  88  may be supplied to the transition metal film  46  to oxidize the transition metal film  46  into the transition metal oxide film  48 . 
     [Variation] 
     The present invention is not limited to the above embodiments, but a variety of changes can be made thereto. 
     For example, while the description has been made of the case where the transition metal film  46  is made of Ni and the transition metal oxide film  48  containing NiO x  is formed in the above embodiments, the transition metal film  46  is not necessarily made of Ni. A variety of transition metals can be used as the material of the transition metal film  46  as appropriate to form the transition metal oxide film  48  made of the oxides of the transition metals. For example, the transition metal film  46  may be made of Ti, and the transition metal oxide film  48  made of TiO x  may be formed. 
     While the description has been made of the case where the noble metal oxide film  58  formed on the transition metal film  46  is made of PtO x  in the above embodiments, the noble metal oxide film  58  is not necessarily made of PtO x , but a variety of noble metal oxides can be used as appropriate. For example, the noble metal oxide film  58  can be made of IrO x , RuO x , or other noble metal oxides. 
     While the description has been made of the case where the noble metal film  54  in the lower electrode layer  44  is made of Pt in the above embodiments, the noble metal film  54  is not necessarily made of Pt, but a variety of noble metals can be used as appropriate. For example, the noble metal film  54  can be made of Ir, Ru, or any other suitable noble metal. Further, the lower electrode layer  44  is not necessarily made of a noble metal, but a variety of conductive materials can be used as appropriate. For example, the lower electrode layer  44  can be made of a transition metal, a transition metal nitride, or any other suitable metal or compound. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.