Patent Publication Number: US-8530876-B2

Title: Semiconductor memory device and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2010-026867, filed Feb. 9, 2010; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a semiconductor memory device including a filament-operation-type variable resistance film and a method of manufacturing the same. 
     BACKGROUND 
     A resistance-change memory (resistive random access memory [ReRAM]) has been proposed as a memory element that uses the resistance switching phenomenon of a variable resistance film formed between upper and lower electrodes in information recording. An example of the ReRAM is a filament-operation-type memory using a transition metal oxide as the variable resistance film. In this filament-operation-type ReRAM, a thin conduction path (filament) is formed in order to perform switching with a low current. To form the filament, however, it is necessary to divisionally apply a voltage little by little a plurality of number of times in initialization (forming) of the element. Therefore, the forming before the shipment of the product takes a long time, and this interferes with the shipment of the product. To prevent this, attempts have been made to shorten the forming time by using a variable resistance film that facilitates forming. 
     “IEDM Tech. Dig., p. 297, 2008” describes the technique of lowering the forming voltage by thinning the variable resistance film. More specifically, a thin transition metal oxide deposited by chemical vapor deposition (CVD) method and having a thickness of about 5 nm or less is used as the variable resistance film. In this method, however, as the film thickness decreases, it becomes difficult to form a metal-rich film (a film in which the composition ratio [O]/[Me] of oxygen to a metal is lower than the stoichiometric ratio) necessary for a stable switching operation. That is, “IEDM Tech. Dig., p. 297, 2008” can lower the forming voltage, but cannot obtain stable switching characteristics (switching yield and durability [endurance characteristics]). 
     Also, “Symposium on VLSI Technology, p. 30, 2009” describes the method of forming a variable resistance film by metal sputtering and thermal oxidation following the sputtering. More specifically, a metal film is formed by metal sputtering and oxidized by thermal oxidation. A thin transition metal oxide thus formed and having a thickness of about 5 nm or less is used as the variable resistance film. When forming the variable resistance film (metal oxide film) by thermal oxidation, however, the lower portion (near the interface with a lower electrode) of the variable resistance film is readily oxidized. This degrades the adhesion between the variable resistance film and lower electrode. That is, in “Symposium on VLSI Technology, p. 30, 2009”, a metal-rich film is obtained by controlling thermal oxidation, but film peeling readily occurs in the interface between the variable resistance film and lower electrode. In addition, in the method using thermal oxidation, the oxygen amount in the direction of film thickness is difficult to control. Therefore, a film is relatively uniformly oxidized toward the depth of the film. This makes a low-current switching operation difficult. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A ,  1 B, and  1 C are views showing the arrangement of a memory element according to the first embodiment; 
         FIGS. 2A ,  2 B,  2 C,  2 D,  2 E, and  2 F are sectional views showing the steps of manufacturing a semiconductor memory device according to the first embodiment; 
         FIGS. 3A and 3B  are graphs showing the composition ratio of a variable resistance film when using oxidation by oxygen plasma, in the memory element according to the first embodiment; 
         FIG. 4  is a view showing an application example of the memory element according to the first embodiment; 
         FIGS. 5A ,  5 B, and  5 C are views showing the arrangement of a memory element according to the second embodiment; 
         FIG. 6  is a view for explaining the effect of a semiconductor memory device according to the second embodiment; 
         FIGS. 7A ,  7 B, and  7 C are views showing the arrangement of a memory element according to the third embodiment; 
         FIGS. 8A and 8B  are sectional views showing the steps of manufacturing a semiconductor memory device according to the third embodiment; 
         FIGS. 9A ,  9 B, and  9 C are views showing the arrangement of a memory element according to the fourth embodiment; 
         FIGS. 10A and 10B  are sectional views showing the steps of manufacturing a semiconductor memory device according to the fourth embodiment; and 
         FIG. 11  is a view for explaining the effect of the semiconductor memory device according to the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a semiconductor memory device comprises a substrate, a lower electrode, a variable resistance film, and an upper electrode. The lower electrode is on the substrate. The variable resistance film is on the lower electrode and stores data in the form of an electrically programmable resistance. The upper electrode is on the variable resistance film. The variable resistance film comprises a first film, and a second film. The first film is on a side of at least one of the upper electrode and the lower electrode and contains a metal. The second film is between the first film and the other electrode and contains the metal and oxygen. A composition ratio [O]/[Me] of oxygen to the metal in the second film is lower than a stoichiometric ratio and higher than the composition ratio [O]/[Me] in the first film. The composition ratio [O]/[Me] changes between the first film and the second film. 
     Embodiments will be explained below with reference to the accompanying drawing. In the drawing, the same reference numbers denote the same parts. 
     Note that in the following embodiments, an explanation of the same features will be omitted, and different points will be explained in detail. 
     &lt;First Embodiment&gt; 
     The first embodiment is an example in which a variable resistance film is a multilayered film (MeO x /Me multilayered film) including a metal oxide film and metal film. 
     [Structure] 
     First, the structure of a semiconductor memory device according to the first embodiment will be explained below with reference to  FIGS. 1A to 1C .  FIGS. 1A to 1C  illustrate the arrangement of a memory element and the composition ratio of the variable resistance film according to the first embodiment. 
     As shown in  FIG. 1A , the memory element according to this embodiment includes a lower electrode  101 , variable resistance film  102 , and upper electrode  103 . 
     The lower electrode  101  is formed on a substrate (not shown). The variable resistance film  102  is formed on the lower electrode  101 . The resistance of the variable resistance film  102  changes when a current flows. The variable resistance film  102  stores data in the form of this electrically programmable resistance. The upper electrode  103  is formed on the variable resistance film  102 . 
     The variable resistance film  102  of this embodiment includes a metal film  102   a  and metal oxide film  102   b . The metal film  102   a  is formed on the lower electrode  101 . The metal oxide film  102   b  is formed on the metal film  102   a . The metal film  102   a  and metal oxide film  102   b  are made of the same metal. Also, as shown in  FIG. 10 , the composition ratio [O]/[Me] of oxygen to the metal in the metal oxide film  102   b  is set lower than the stoichiometric ratio (metal rich) and higher than the composition ratio [O]/[Me] of oxygen to the metal in the metal film  102   a , in order to improve the switching characteristics. 
     As shown in  FIG. 1B , the oxygen concentration in the variable resistance film  102  abruptly changes in the interfaces with the metal film  102   a  and metal oxide film  102   b . More specifically, in the vicinity of the interface between the metal film  102   a  and metal oxide film  102   b , the oxygen concentration may have a profile that abruptly curvedly increases from the metal film  102   a  toward the metal oxide film  102   b  as indicated by reference number  104  in  FIG. 1B , or a step-function-like profile that changes step by step as indicated by reference number  105  in  FIG. 1B . That is, a region which has a certain film thickness and in which the oxygen concentration is almost zero exists in the metal film  102   a , and a region which has a certain film thickness and in which the oxygen concentration is higher than that in the metal film  102   a  exists in the metal oxide film  102   b . The range within which the composition ratio of oxygen to the metal is constant is the same as the oxygen concentration. 
     Note that as the lower electrode  101  and upper electrode  103 , it is desirable to use, e.g., a metal such as W, Ta, or Cu, a nitride or carbide such as TiN, TaN, or WC, or heavily doped silicon (silicon having a high impurity concentration). Also, a transition metal oxide can be used as the variable resistance film  102  (metal oxide film  102   b ). As the transition metal oxide, it is possible to use, e.g., a binary metal oxide represented by M x O y  (where M represents a metal and O represents oxygen), or a ternary metal oxide represented by A α M β O γ  (where A and M represent metals and O represents oxygen). However, this embodiment is not limited to this, and it is also possible to use a quaternary metal oxide or a higher-order metal oxide. As the transition metal element, it is possible to use a divalent metal element for which the oxide is represented by, e.g., NiO or CoO, a trivalent metal element for which the oxide is represented by, e.g., Al 2 O 3 , a tetravalent metal element for which the oxide is represented by, e.g., TiO 2 , HfO 2 , or ZrO 2 , or a pentavalent metal element for which the oxide is represented by, e.g., Ta 2 O 5 . 
     [Manufacturing Method] 
     A method of manufacturing the semiconductor memory device according to the first embodiment will be explained below with reference to  FIGS. 2A to 2F .  FIGS. 2A to 2F  illustrate the steps of manufacture of the memory element according to the first embodiment. 
     First, as shown in  FIG. 2A , a lower electrode  101  is formed on a substrate  100 . The substrate  100  is, e.g., a semiconductor substrate on which various control circuits are formed. When stacking a plurality of memory elements, the substrate  100  is a memory element below a memory element of interest. The lower electrode  101  is formed by depositing the above-mentioned material by using, e.g., sputtering. However, this embodiment is not limited to this, and it is also possible to use CVD method or atomic layer deposition (ALD) method. 
     Then, as shown in  FIG. 2B , a transition metal film  102 ′ serving as a variable resistance film  102  later is formed on the lower electrode  101 . The transition metal film  102 ′ is formed by, e.g., metal sputtering, metal deposition, metal CVD method, or a metal plating process. 
     Subsequently, as shown in  FIG. 2C , the surface of the transition metal film  102 ′ is oxidized to form a variable resistance film  102  including a metal film  102   a  and metal-rich metal oxide film  102   b . In this step, radical oxidation by oxygen plasma is used. Consequently, a profile in which the oxygen concentration abruptly changes in the interface between the metal film  102   a  and metal oxide film  102   b  as shown in  FIG. 1B  is obtained. 
     Note that the film thickness of the metal oxide film  102   b  is desirably, e.g., about 5 nm or less. This is so because a filament is formed in the metal oxide film  102   b  by forming. However, this embodiment is not limited to this, and it is also possible to appropriately set the film thickness in accordance with the scale of peripheral circuits (not shown) formed on the substrate  100 . 
     As shown in  FIG. 2D , an upper electrode  103  is formed on the variable resistance film  102 . Similar to the lower electrode  101 , the upper electrode  103  is formed by using sputtering, CVD method, or ALD method. 
     As shown in  FIG. 2E , a mask  110  is formed on the upper electrode  103 . The mask  110  is processed into a desired pattern by exposure and development. 
     As shown in  FIG. 2F , the mask  110  is used to process the memory element into the desired pattern by reactive ion etching (RIE). After that, the mask  110  is removed. In this manner, the memory element according to this embodiment is completed. 
     The memory element manufacturing method according to this embodiment is characterized in that the variable resistance film  102  including the metal film  102   a  and metal oxide film  102   b  is formed by oxidizing the surface of the transition metal film  102 ′ by oxygen plasma. 
       FIGS. 3A and 3B  illustrate the comparison of the composition ratios of the variable resistance film  102  when using oxidation by oxygen plasma and thermal oxidation in the method of forming the variable resistance film  102  according to this embodiment. As shown in  FIG. 3A , thermal oxidation diffuses oxygen to the lower portion of the variable resistance film  102 . In addition, the composition ratio ([O]/[Me]) of oxygen to the metal gradually decreases from the upper surface to the lower surface. On the other hand, plasma oxidation can control the diffusion of oxygen more easily than in thermal oxidation. Therefore, the oxygen diffusion portion can be restricted to the upper portion. This makes it possible to obtain a profile in which the composition ratio [O]/[Me] abruptly decreases in the interface between the metal film  102   a  and metal oxide film  102   b . That is, as shown in  FIG. 3B , a profile in which the oxygen concentration changes more abruptly than in thermal oxidation can be obtained by plasma oxidation. 
     [Effects] 
     In the first embodiment described above, the variable resistance film  102  includes the metal film  102   a  and metal oxide film  102   b , and the composition ratio of oxygen to the metal abruptly changes in the interface between the metal film  102   a  and metal oxide film  102   b . More specifically, the oxygen concentration abruptly decreases from the metal oxide film  102   b  toward the metal film  102   a . Also, a region where the oxygen concentration is uniform in the film thickness direction exists in the metal oxide film  102   b . That is, a high-oxygen-concentration region having a certain film thickness exists. Accordingly, a highly insulating region spreads in a portion of the metal oxide film  102   b  except for the filament. In a switching operation, therefore, it is possible to concentrate a current to the filament portion, and perform a low-current operation. Note that even when the high-oxygen-concentration region exists in the metal oxide film  102   b , if the film thickness of the region is too small, no sufficient insulation properties can be obtained in the portion except for the filament. This makes a low-current operation difficult. Accordingly, the film thickness of the high-oxygen-concentration region is desirably, e.g., 2 nm or more with respect to 5 nm as the film thickness of the metal oxide film  102   b.    
     Also, the metal film  102   a  requires no forming because the resistance is sufficiently low. Therefore, the metal oxide film  102   b  is the only portion requiring forming. In this embodiment, the diffusion of oxygen can be restricted to the upper portion of the variable resistance film  102 . This makes it possible to decrease the film thickness of the metal oxide film  102   b  requiring forming. Consequently, it is possible to lower the forming voltage and shorten the forming time. 
     Generally, stable switching characteristics are presumably obtained when a metal oxide film having a composition richer in metal than the stoichiometric ratio exists in the anode interface. In this embodiment, the metal oxide film  102   b  richer in metal than the stoichiometric ratio is formed in, e.g., the anode interface as the upper electrode. Therefore, stable switching characteristics are obtained. 
     Furthermore, in the manufacturing method of this embodiment, the variable resistance film  102  including the metal film  102   a  and metal oxide film  102   b  is formed by oxidizing the surface of the transition metal film  102 ′ by oxygen plasma. Oxidation by oxygen plasma can readily control the oxygen diffusion region in the transition metal film  102 ′. This makes it possible to suppress oxygen from diffusing to the interface between the metal film  102   a  and lower electrode  101 . Accordingly, degradation of the adhesion between the metal film  102   a  and lower electrode  101  can be reduced more than that when performing thermal oxidation. This increases the film peeling margin against the diffusion of oxygen in a later heating step or the like, thereby suppressing film peeling during the process. 
     In addition, oxidation by oxygen plasma can be performed at a relatively low temperature of, e.g., about 100° C. Therefore, the influence of heat on the device can be reduced, and as a consequence the device characteristics can be improved. 
     Note that  FIG. 4  shows an example in which the memory element according to this embodiment is applied to a cross-point cell. As shown in  FIG. 4 , when using the variable resistance film  102  in an ReRAM element using a PIN diode, it is desirable to form the metal film  102   a  on the anode side and the metal oxide film  102   b  on the cathode side in order to improve the switching characteristics. That is, the resistance element is desirably set such that a current flows from the metal film  102   a  to the metal oxide film  102   b . As shown in  FIG. 4 , therefore, the N layer of the diode is formed in contact with the anode. In the cross-point cell, the directions of the PIN diodes (the directions of currents) in layers are alternately opposite, in order to reduce the number of steps. Accordingly, the structures of the variable resistance films  102  in layers are desirably alternately formed in opposite directions. 
     &lt;Second Embodiment&gt; 
     In the first embodiment, the variable resistance film is the multilayered film including the metal-rich metal oxide film and metal film. On the other hand, the second embodiment is an example in which a variable resistance film is a multilayered film (MeO x 1/MeO x 2 multilayered film) including a metal-rich metal oxide film and a metal oxide film richer in metal than the former metal oxide film. 
     [Structure] 
     First, the structure of a semiconductor memory device according to the second embodiment will be explained below with reference to  FIGS. 5A to 5C .  FIGS. 5A to 5C  illustrate the arrangement of a metal element and the composition ratio of the variable resistance film according to the second embodiment. 
     As shown in  FIG. 5A , the memory element according to this embodiment includes a lower electrode  201 , variable resistance film  202 , and upper electrode  203 . This embodiment differs from the first embodiment in that the variable resistance film  202  includes a first metal oxide film  202   a  and second metal oxide film  202   b.    
     The first metal oxide film  202   a  is formed on the lower electrode  201 . The second metal oxide film  202   b  is formed on the first metal oxide film  202   a . The first and second metal oxide films  202   a  and  202   b  are made of the same metal. Also, as shown in  FIG. 5C , the composition ratio [O]/[Me] of oxygen to the metal in the second metal oxide film  202   b  is lower than the stoichiometric ratio (metal rich). In addition, the composition ratio [O]/[Me] of oxygen to the metal in the first metal oxide film  202   a  is much lower than that in the second metal oxide film  202   b . That is, the composition ratio [O]/[Me] of oxygen to the metal in the first metal oxide film  202   a  is low to such an extent that no film peeling occurs in the interface with the lower electrode  201 . 
     As shown in  FIG. 5B , the oxygen concentration in the variable resistance film  202  abruptly changes in the interface between the first and second metal oxide films  202   a  and  202   b . That is, the oxygen concentration abruptly increases from the first metal oxide film  202   a  toward the second metal oxide film  202   b . Also, a low-oxygen-concentration region having a certain film thickness exists in the first metal oxide film  202   a , and a region which has a certain film thickness and in which the oxygen concentration is higher than that of the first metal oxide film  202   a  exists in the second metal oxide film  202   b.    
     Note that the same transition metal oxide as that of the first embodiment can be used as the variable resistance film  202  (first and second metal oxide films  202   a  and  202   b ). 
     [Manufacturing Method] 
     A method of manufacturing the semiconductor memory device according to the second embodiment will be explained below. 
     First, a transition metal film is formed on a lower electrode  201  by performing the steps shown in  FIGS. 2A and 2B  of the first embodiment. After that, the surface of the transition metal film is oxidized to form a variable resistance film  202  including a first metal oxide film  202   a  and second metal oxide film  202   b . In this step, the variable resistance film  202  having the above-mentioned multilayered structure is formed by performing radical oxidation at a temperature and pressure higher than those of the first embodiment. 
     Alternatively, the steps shown in  FIGS. 2A to 2C  are performed, and annealing is performed at, e.g., about 500° C. or more after that. Consequently, oxygen diffuses to the lower portion of the variable resistance film  202 , thereby forming the variable resistance film  202  having the above-mentioned multilayered structure. 
     After that, the same steps as those of the first embodiment are performed. That is, an upper electrode  203  is formed on the variable resistance film  202 , and the memory element is processed into a desired pattern. 
     [Effects] 
     The second embodiment described above can achieve the same effects as those of the first embodiment. 
     In addition, the variable resistance film  202  according to this embodiment improves the data retention characteristics. The principle of this effect will be explained below. 
     As shown in  FIG. 6 , a conduction path (filament)  210  is formed in the second metal oxide film  202   b  of the variable resistance film  202  by forming. Electric charge is trapped in the filament  210  during forming. If this electric charge leaks from the filament  210  toward the lower electrode  201 , the resistance of the filament  210  changes. This may degrade the data retention characteristics. In this embodiment, the first metal oxide film  202   a  containing oxygen and having a bandgap is formed between the lower electrode  201  and filament  210 . This makes it possible to suppress leakage of electric charge from the filament  210 , and improve the data retention characteristics. 
     &lt;Third Embodiment&gt; 
     The third embodiment is an example in which a variable resistance film is a multilayered film (Me/MeO x /Me multilayered film) including a metal film, metal-rich metal oxide film, and metal film. 
     [Structure] 
     First, the structure of a semiconductor memory device according to the third embodiment will be explained below with reference to  FIGS. 7A to 7C .  FIGS. 7A to 7C  illustrate the arrangement of a memory element and the composition ratio of the variable resistance film according to the third embodiment. 
     As shown in  FIG. 7A , the memory element according to this embodiment includes a lower electrode  301 , variable resistance film  302 , and upper electrode  303 . This embodiment differs from each of the above-mentioned embodiments in that the variable resistance film  302  includes a first metal film  302   a , metal oxide film  302   b , and second metal film  302   c.    
     The first metal film  302   a  is formed on the lower electrode  301 . The metal oxide film  302   b  is formed on the first metal film  302   a . The second metal film  302   c  is formed on the metal oxide film  302   b . The first metal film  302   a , metal oxide film  302   b , and second metal film  302   c  are made of the same metal. Also, as shown in  FIG. 7C , the composition ratio [O]/[Me] of oxygen to the metal in the metal oxide film  302   b  is lower than the stoichiometric ratio. That is, in the variable resistance film  302 , the first metal film  302   a  and second metal film  302   c  in which no oxygen is diffused are respectively formed on the side of the lower electrode  301  and on the side of the upper electrode  303 , and the metal-rich metal oxide film  302   b  is formed between them. In other words, in the variable resistance film  302 , oxygen locally exists in the metal oxide film  302   b , and no oxygen exists in the first and second metal films  302   a  and  302   c  below and above the metal oxide film  302   b.    
     As shown in  FIG. 7B , the oxygen concentration in the variable resistance film  302  abruptly changes in the interface between the first metal film  302   a  and metal oxide film  302   b , and in the interface between the metal oxide film  302   b  and second metal film  302   c . More specifically, the oxygen concentration may have a profile  304   a  that abruptly curvedly changes near the interface between the first metal film  302   a  and metal oxide film  302   b , and a profile  304   b  that abruptly curvedly changes near the interface between the metal oxide film  302   b  and second metal film  302   c , or step-function-like profiles  305   a  and  305   b  that change step by step. Also, a region which has a certain film thickness and in which the oxygen concentration is almost zero exists in each of the first and second metal films  302   a  and  302   c , and a region which has a certain film thickness and in which the oxygen concentration is higher than those in the first and second metal films  302   a  and  302   c  exists in the metal oxide film  302   b.    
     [Manufacturing Method] 
     A method of manufacturing the semiconductor memory device according to the third embodiment will be explained below with reference to  FIGS. 8A and 8B .  FIGS. 8A and 8B  illustrate the steps of manufacturing the memory element according to the third embodiment. 
     First, the steps shown in  FIGS. 2A to 2C  of the first embodiment are performed to form a first metal film  302   a  and metal oxide film  302   b  on a lower electrode  301 . 
     Then, as shown in  FIG. 8A , a transition metal film  302   c ′ serving as a second metal film  302   c  is formed on the metal oxide film  302   b . The transition metal film  302   c ′ is formed by, e.g., metal sputtering, metal deposition, metal CVD method, or a metal plating process. 
     Subsequently, as shown in  FIG. 8B , a second metal film  302   c  is formed by performing annealing at, e.g., about 500° C. In this manner, a variable resistance film  302  having the desired oxygen profiles as described above is formed. 
     After that, the same steps as those of the first embodiment are performed. That is, an upper electrode  303  is formed on the variable resistance film  302 , and the memory element is processed into a desire pattern. 
     [Effects] 
     The third embodiment described above can achieve the same effects as those of the first embodiment. 
     In addition, the variable resistance film  302  according to this embodiment includes the second metal film  302   c  between the metal oxide film  302   b  and upper electrode  303 . This makes it possible to reduce degradation of the adhesion in the interface between the variable resistance film  302  and upper electrode  303 , and suppress film peeling. 
     &lt;Fourth Embodiment&gt; 
     The fourth embodiment is an example in which a variable resistance film is a multilayered film (MeO x 1/MeO x 2/MeO x 3 multilayered film) including a highly metal-rich metal oxide film, metal-rich metal oxide film, and highly metal-rich metal oxide film. 
     [Structure] 
     First, the structure of a semiconductor memory device according to the fourth embodiment will be explained below with reference to  FIGS. 9A to 9C .  FIGS. 9A to 9C  illustrate the arrangement of a memory element and the composition ratio of the variable resistance film according to the fourth embodiment. 
     As shown in  FIG. 9A , the memory element according to this embodiment includes a lower electrode  401 , variable resistance film  402 , and upper electrode  403 . This embodiment differs from each of the above-mentioned embodiments in that the variable resistance film  402  includes a first metal oxide film  402   a , second metal oxide film  402   b , and third metal oxide film  402   c.    
     The first metal oxide film  402   a  is formed on the lower electrode  401 . The second metal oxide film  402   b  is formed on the first metal oxide film  402   a . The third metal oxide film  402   c  is formed on the second metal oxide film  402   b . The first, second, and third metal oxide films  402   a ,  402   b , and  402   c  are made of the same metal. Also, as shown in  FIG. 9C , the composition ratio [O]/[Me] of oxygen to the metal in the second metal oxide film  402   b  is lower than the stoichiometric ratio. In addition, the composition ratio [O]/[Me] of oxygen to the metal in each of the first and third metal oxide films  402   a  and  402   c  is much lower than that in the second metal oxide film  402   b . That is, the composition ratio [O]/[Me] of oxygen to the metal in each of the first and third metal oxide films  402   a  and  402   c  is low to such an extent that no film peeling occurs in the interface with the lower electrode. More specifically, in the variable resistance film  402 , the highly metal-rich first and third metal oxide films  402   a  and  402   c  are respectively formed on the lower electrode side and upper electrode side, and the metal-rich second metal oxide film  402   b  is formed between them. 
     As shown in  FIG. 9B , the oxygen concentration in the variable resistance film  402  abruptly changes in the interface between the first and second metal oxide films  402   a  and  402   b , and in the interface between the second and third metal oxide films  402   b  and  402   c . Also, a low-oxygen-concentration region having a certain film thickness exists in each of the first and third metal oxide films  402   a  and  402   c , and a region which has a certain film thickness and in which the oxygen concentration is higher than those in the first and third metal oxide films  402   a  and  402   c  exists in the second metal oxide film  402   b.    
     [Manufacturing Method] 
     A method of manufacturing the semiconductor memory device according to the fourth embodiment will be explained below with reference to  FIGS. 10A and 10B .  FIGS. 10A and 10B  illustrate the steps of manufacturing the memory element according to the fourth embodiment. 
     First, the steps shown in  FIGS. 2A to 2C  of the first embodiment are performed to form a first metal oxide film  402   a  and second metal oxide film  402   b  on a lower electrode  401 . After that, annealing is performed at, e.g., about 500° C. or more in the same manner as in the second embodiment. 
     Then, as shown in  FIG. 10A , an upper electrode  403  is formed on the second metal oxide film  402   b.    
     Subsequently, as shown in  FIG. 10B , the second metal oxide film  402   b  is annealed via the upper electrode  403 . This removes oxygen from the upper portion of the second metal oxide film  402   b  via the upper electrode  403 . Consequently, a third metal oxide film  402   c  having an oxygen concentration lower than that of the second metal oxide film  402   b  is formed on it, thereby forming a variable resistance film  402  having the above-mentioned multilayered structure. 
     After that, the memory element is processed into a desired pattern in the same manner as in the first embodiment. 
     [Effects] 
     The fourth embodiment described above can achieve the same effects as those of the above-mentioned embodiments. 
     In addition, the variable resistance film  402  according to this embodiment can further improve the data retention characteristics. That is, as shown in  FIG. 11 , the first metal oxide film  402   a  containing oxygen and having a bandgap is formed between the lower electrode  401  and a conduction path (filament)  410  formed in the second metal oxide film  402   b . Furthermore, the third metal oxide film  402   c  is formed between the filament  410  and upper electrode  403 . This makes it possible to suppress leakage of electric charge from the filament  410  not only to the lower electrode  401 , but also to the upper electrode  403 . 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.