Patent Publication Number: US-9412937-B2

Title: Memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 14/446,419 filed Jul. 30, 2014, and is based upon and claims the benefit of priority from Japanese Patent Application No.2013-192386, filed on Sep. 17, 2013; the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a memory device. 
     BACKGROUND 
     Two-terminal resistance random access memory is actively being developed as large-capacity nonvolatile memory to replace conventional floating-gate type NAND flash memory. Low voltage/low current operations, high speed switching, and downscaling/higher integration of the memory cells are possible for this type of memory. Various materials are being proposed for the variable resistance layer of resistance random access memory, among which ionic memory is promising because ionic memory includes a silicon layer or a silicon oxide layer, which are compatible with existing LSI processes, as the variable resistance layer. Ionic memory includes silver (Ag) as the ion source electrode. Ionic memory has the advantages such as and low current operations, and rectifying operations. In ionic memory, the on/off operations of the memory cell are realized by causing a filament of silver to precipitate in and disappear from the variable resistance layer. 
     Generally, a trade-off relationship exists between the data retention characteristics and the ease of resetting data for nonvolatile memory. For ionic memory as well, in the case of a variable resistance layer configured to have good retention characteristics, the filament that is formed inside the variable resistance layer in the set state is stable; and therefore, a high voltage must be applied to perform the reset operation, that is, to cause the filament to disappear. The memory operations are harmed by applying the high voltage because the rewriting durability degrades. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a memory device according to a first embodiment; 
         FIGS. 2A and 2B  are cross-sectional views showing a method for manufacturing the memory device according to the first embodiment; 
         FIG. 3  is a graph showing conditions at which an amorphous silicon layer having a thickness of 5 nm crystallizes, where the horizontal axis is a heating temperature, the vertical axis is a heating time; 
         FIGS. 4A to 4F  are schematic cross-sectional views showing an operations of the memory device according to the first embodiment; 
         FIG. 5  is a cross-sectional view showing a memory device according to a second embodiment; 
         FIGS. 6A and 6B  are cross-sectional views showing a method for manufacturing the memory device according to the second embodiment; 
         FIG. 7  is a cross-sectional view showing a memory device according to a third embodiment; 
         FIGS. 8A and 8B  are cross-sectional views showing a method for manufacturing the memory device according to the third embodiment; 
         FIG. 9  is a cross-sectional view showing a memory device according to a fourth embodiment; 
         FIG. 10A  to  FIG. 12B  are cross-sectional views showing a method for manufacturing the memory device according to the fourth embodiment; 
         FIG. 13  is a cross-sectional view showing a memory device according to a fifth embodiment; 
         FIGS. 14A to 14C  are cross-sectional views showing a method for manufacturing the memory device according to the fifth embodiment; 
         FIG. 15  is a cross-sectional view showing a memory device according to a sixth embodiment; 
         FIG. 16  is a cross-sectional view showing a memory device according to a seventh embodiment; and 
         FIG. 17A  is a plane TEM (transmission electron microscopy) photograph of a polycrystalline silicon layer; and 
         FIG. 17B  is a plane TEM photograph of an amorphous silicon layer. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a memory device includes a first electrode, a second electrode and a variable resistance layer. The second electrode includes a metal. The metal is more easily ionizable than a material of the first electrode. The variable resistance layer is disposed between the first electrode and the second electrode. The variable resistance layer includes a first layer and a second layer. The first layer has a relatively high crystallization rate. The second layer contacts the first layer. The second layer has a relatively low crystallization rate. The first layer and the second layer are stacked along a direction connecting the first electrode and the second electrode. 
     Embodiments of the invention will now be described with reference to the drawings. 
     (First Embodiment) 
       FIG. 1  is a cross-sectional view showing a memory device according to the embodiment. 
     As shown in  FIG. 1 , an opposing electrode  11 , a current-limiting layer  12 , a crystallization-inducing metal layer  13 , a variable resistance layer  14 , a diffusion prevention layer  15 , and an ion source electrode  16  are stacked in this order in the memory device  1  according to the embodiment. The crystallization rate of a lower portion  14   a  of the variable resistance layer  14  is higher than the crystallization rate of an upper portion  14   b  of the variable resistance layer  14 . In the embodiment, the lower portion  14   a  and the upper portion  14   b  of the variable resistance layer  14  are formed as one body. Therefore, the boundary between the lower portion  14   a  and the upper portion  14   b  does not always appear to be distinct. 
     In the specification, the “crystallization rate” refers to the proportion of the entire surface area that is a crystal when the layer of interest is observed. The crystallization rate is measurable by, for example, cross section TEM microscopy, electron diffraction, etc. The crystallization rate of the lower portion  14   a  is, for example, at least 10% higher than the crystallization rate of the upper portion  14   b . For example, the lower portion  14   a  is formed of a polycrystalline body; and the upper portion  14   b  is formed of an amorphous body. 
     The opposing electrode  11  is, for example, a conductive layer made of tungsten (W), titanium (Ti), tantalum (Ta), platinum (Pt), or a nitride of these metals, or a multilayered film in which such a conductive layer is multiply stacked. The current-limiting layer  12  is formed of, for example, a high resistance metal such as tantalum silicon nitride (TaSiN), etc. The crystallization-inducing metal layer  13  is a layer that induces the crystallization of the variable resistance layer  14  and is formed of, for example, nickel (Ni). The crystallization-inducing metal layer  13  may be formed of aluminum (Al) or palladium (Pd). 
     The variable resistance layer  14  is formed of, for example, silicon (Si). The variable resistance layer  14  may be formed of silicon-germanium (SiGe), germanium (Ge), or a compound semiconductor. The thickness of the variable resistance layer  14  is, for example, 2 to 50 nm. The diffusion prevention layer  15  is a layer that suppresses the diffusion of the metal of the ion source electrode  16  into the variable resistance layer  14 , is formed of, for example, an oxide of the material of the variable resistance layer  14 , and is formed of, for example, silicon oxide (SiO). The thickness of the diffusion prevention layer  15  is, for example, 1 to 5 nm. 
     The ion source electrode  16  is formed of a metal that is more easily ionizable than the material of the opposing electrode  11 , the material of the current-limiting layer  12 , and the material of the crystallization-inducing metal layer  13  and is formed of, for example, silver (Ag). According to the material of the opposing electrode  11 , the material of the current-limiting layer  12 , and the material of the crystallization-inducing metal layer  13 , the ion source electrode  16  may be formed of, for example, nickel (Ni), cobalt (Co), aluminum (Al), or copper (Cu). In the embodiment, for example, the crystallization-inducing metal layer  13  is formed of nickel; the variable resistance layer  14  is formed of silicon; the diffusion prevention layer  15  is formed of silicon oxide; and the ion source electrode  16  is formed of silver. 
     In the memory device  1 , one memory cell includes a stacked body made of the opposing electrode  11 , the current-limiting layer  12 , the crystallization-inducing metal layer  13 , the variable resistance layer  14 , the diffusion prevention layer  15 , and the ion source electrode  16 . 
     A method for manufacturing the memory device according to the embodiment will now be described. 
       FIGS. 2A and 2B  are cross-sectional views showing the method for manufacturing the memory device according to the embodiment. 
     First, as shown in  FIG. 2A , a stacked body is formed in which the opposing electrode  11 , the current-limiting layer  12 , the crystallization-inducing metal layer  13 , an amorphous silicon layer  19 , the diffusion prevention layer  15 , and the ion source electrode  16  are stacked in this order. 
     Then, as shown in  FIG. 2B , heat treatment of the stacked body is performed. At this time, because the nickel included in the crystallization-inducing metal layer  13  is used as the seed of the crystallization of the amorphous silicon layer  19 , the amorphous silicon layer  19  crystallizes from the lower surface toward the upper surface. This method is called metal-induced crystallization. The arrows of  FIG. 2B  illustrate the direction in which the crystallization progresses. This is similar for other drawings described below. 
     By appropriately controlling the time and temperature of the heat treatment, the crystallization rate of the lower portion of the amorphous silicon layer  19  can be set to be higher than the crystallization rate of the upper portion of the amorphous silicon layer  19 , e.g., the crystallization rate of the lower portion can be set to be at least 10% higher than the crystallization rate of the upper portion. For example, only the lower portion of the amorphous silicon layer  19  may be crystallized; and the amorphous upper portion of the amorphous silicon layer  19  may be maintained as-is. Thereby, the amorphous silicon layer  19  is used as the variable resistance layer  14  described above. 
     Hereinbelow, the case where the thickness of the amorphous silicon layer  19  is 10 nm is described as an example of the relationship between the temperature and time of the heat treatment process recited above. 
       FIG. 3  is a graph showing the conditions at which an amorphous silicon layer having a thickness of 5 nm crystallizes, where the horizontal axis is the heating temperature, the vertical axis is the heating time, the black circle plots (●) illustrate the case where metal-induced crystallization is used, and the white rectangle plots (□) illustrate the case where solid phase epitaxy is used. 
     As shown in  FIG. 3 , in solid phase epitaxy, a typical crystallization temperature is high, i.e., 600 to 700° C. On the other hand, in metal-induced crystallization, the crystallization temperature for a crystallization time on the order of minutes can be as low as 350° C. to 400° C. For example, an amorphous silicon layer having a thickness of 5 nm can be crystallized by heat treatment in which heating is performed for 10 minutes at a temperature of 400° C. 
     Operations of the memory device according to the embodiment will now be described. 
       FIGS. 4A to 4F  are schematic cross-sectional views showing the operations of the memory device according to the embodiment. 
     As shown in  FIG. 4A , a filament is not formed inside the variable resistance layer  14  and inside the diffusion prevention layer  15  in the memory cell in the initial state. At this time, the variable resistance layer  14  is in a high resistance state (an off-state). 
     Then, in the memory cell as shown in  FIG. 4B , when a prescribed set voltage is applied to cause the opposing electrode  11  to be negative and the ion source electrode  16  to be positive, the silver atoms that are included in the ion source electrode  16  are ionized, penetrate into the diffusion prevention layer  15  and into the variable resistance layer  14 , and precipitate by combining with electrons supplied from the opposing electrode  11 . Thereby, a filament F that is made of silver is formed inside the diffusion prevention layer  15  and inside the variable resistance layer  14 . As a result, the variable resistance layer  14  is switched to a low resistance state (an on-state). This operation is called “set.” 
     Continuing as shown in  FIG. 4C , when the set voltage is stopped and a constant amount of time has elapsed, the filament F formed inside the variable resistance layer  14  has a portion that disappears from the upper portion  14   b  which has a relatively low crystallinity and a portion that remains inside the lower portion  14   a  which has a relatively high crystallinity. It is considered that this is because, in the case where the crystallization rate of the main material is high, the silicon inside the crystal regions (grains) forms dense bonds; the filament is formed concentratively in the boundary regions between the grains; the filament that is formed in such limited regions has a high density and is tough; and therefore, the filament is stable and does not disappear easily even after time has elapsed. 
     As shown in  FIG. 4D , when a read-out voltage that is lower than the set voltage is applied to the memory cell in the low resistance state to cause the opposing electrode  11  to be negative and the ion source electrode  16  to be positive, the filament F is reproduced inside the upper portion  14   b  of the variable resistance layer  14  by using the filament F remaining inside the lower portion  14   a  of the variable resistance layer  14  as a starting point; and a read-out current that corresponds to the low resistance state flows inside the variable resistance layer  14 . 
     On the other hand, as shown in  FIG. 4E , when a prescribed reset voltage is applied to the memory cell in the low resistance state to cause the opposing electrode  11  to be positive and the ion source electrode  16  to be negative, an electric field is applied concentratively to a tip portion Ft of the filament F, i.e., the end portion on the ion source electrode  16  side, that remains inside the lower portion  14   a  of the variable resistance layer  14 . Therefore, the silver atoms of the tip portion Ft are ionized and move toward the ion source electrode  16 . As a result, the filament F at this portion disappears; the tip portion Ft recedes slightly toward the opposing electrode  11  side; and the electric field is applied concentratively to the tip portion Ft that has receded. Thus, the tip portion Ft of the filament F continues to recede toward the opposing electrode  11 . Thereby, the filament F that is inside the lower portion  14   a  disappears progressively from the end portion on the ion source electrode  16  side. 
     As a result, as shown in  FIG. 4F , the filament F finally disappears from the variable resistance layer  14 ; and the variable resistance layer  14  is switched to the high resistance state (the off-state). This operation is called “reset.” Even when the read-out voltage is applied in this state, the filament F substantially is not reproduced; and only a read-out current that corresponds to the high resistance state flows inside the variable resistance layer  14 . 
     Then, when the set voltage is applied to the memory cell in the high resistance state, the variable resistance layer  14  is set again, transitions to the low resistance state shown in  FIG. 4B , and subsequently stabilizes in the state shown in  FIG. 4C . Thus, the state of the memory cell can be arbitrarily set to the low resistance state or the high resistance state by repeating set and reset. 
     Effects of the embodiment will now be described. 
     In the embodiment, after the variable resistance layer  14  is set, even when the set voltage is stopped, the filament remains inside the lower portion  14   a  of the variable resistance layer  14  as shown in  FIG. 4C . Because the crystallinity of the lower portion  14   a  is high, the filament F that is formed inside the lower portion  14   a  is stable for, for example, 10 years or more. Therefore, the data retention characteristics of the memory device  1  are good. 
     Also, as shown in  FIG. 4E , when the reset voltage is applied to the variable resistance layer  14  in the low resistance state, the filament F has disappeared inside the upper portion  14   b ; therefore, the electric field concentrates in the tip portion Ft of the filament F remaining inside the lower portion  14   a ; and the filament F disappears efficiently. Therefore, the reset of the memory device  1  is easy. 
     Thus, because the filament F that is formed inside the lower portion  14   a  is exceedingly stable over time, excellent data retention characteristics are realized. On the other hand, because the filament F that is formed inside the upper portion  14   b  disappears quickly, the tip portion Ft is formed in the filament F that is inside the lower portion  14   a ; and the responsiveness to the reset voltage is high because the electric field concentrates in the tip portion Ft. 
     The thickness of the variable resistance layer  14  is thin because the variable resistance layer  14  is a single silicon layer. Therefore, the set voltage can be low. Also, the patterning of the variable resistance layer  14  is easy. Although it is favorable for the thicknesses of the variable resistance layer  14  and the diffusion prevention layer  15  to be thin to reduce the set voltage, it is favorable for the thicknesses to be thick to reduce the leakage current and the fluctuation of the thicknesses. To balance such characteristics, it is favorable for the thickness of the variable resistance layer  14  to be about 2 to 50 nm; and it is favorable for the thickness of the diffusion prevention layer  15  to be about 1 to 5 nm. 
     In the embodiment, because the lower portion  14   a  of the variable resistance layer  14  can be crystallized at a low temperature of, for example, 350 to 400° C., the metal members, i.e., the opposing electrode  11 , the crystallization-inducing metal layer  13 , and the ion source electrode  16 , do not degrade even when performing the heat treatment for crystallization after stacking the layers from the opposing electrode  11  to the ion source electrode  16 . In particular, in the case where the ion source electrode  16  is formed of silver, it is highly advantageous to be able to implement the crystallization heat treatment at a low temperature because silver diffuses easily. 
     Thus, according to the embodiment, an ionic memory-type memory device having both good retention characteristics and reset characteristics can be realized without increasing the set voltage. 
     Conversely, in the case where the entire variable resistance layer  14  is formed of polycrystalline silicon, the data retention characteristics are good because the entire filament is exceedingly stable; but resetting is difficult. 
     Also, in the case where the entire variable resistance layer  14  is formed of amorphous silicon, resetting is easy because the filament is unstable; but the data retention characteristics are poor. 
     Although it may be considered to form the variable resistance layer  14  as a stacked body including a layer in which a stable filament forms and a layer in which an unstable filament forms, the variable resistance layer  14  is undesirably thick in such a case; and a high set voltage is necessary. Further, the manufacturing cost increases because the number of manufacturing processes increases and the patterning of the variable resistance layer  14  is difficult. 
     Although an example is illustrated in the embodiment in which the crystallization rate of the lower portion  14   a  of the variable resistance layer  14  is relatively high and the crystallization rate of the upper portion  14   b  of the variable resistance layer  14  is relatively low, this may be reversed. In other words, the crystallization rate of the lower portion  14   a  may be relatively low; and the crystallization rate of the upper portion  14   b  may be relatively high. In such a case as well, effects similar to the effects described above are obtained. 
     It is sufficient for at least one high crystallization-rate layer having a relatively high crystallization rate and at least one low crystallization-rate layer having a relatively low crystallization rate to be provided in the variable resistance layer  14  and for these layers to be stacked along the direction connecting the opposing electrode  11  and the ion source electrode  16 . Thereby, when the set voltage is applied, the filament is formed to pierce both the high crystallization-rate layer and the low crystallization-rate layer; and when the set voltage is stopped, the filament disappears inside the low crystallization-rate layer while the filament remains inside the high crystallization-rate layer to guarantee the retention characteristics. Then, when the reset voltage is applied, the electric field concentrates at the end portion of the filament on the low crystallization-rate layer side inside the high crystallization-rate layer; and the filament disappears efficiently from this end portion side. Therefore, the reset voltage can be set to be low. 
     (Second Embodiment) 
     The embodiment differs from the first embodiment described above in that an interface oxide layer is included inside the variable resistance layer. Otherwise, the embodiment is similar to the first embodiment; and a description of the common portions is omitted. 
       FIG. 5  is a cross-sectional view showing the memory device according to the embodiment. 
     In the memory device  2  according to the embodiment as shown in  FIG. 5 , the opposing electrode  11 , the current-limiting layer  12 , the crystallization-inducing metal layer  13 , a silicon layer  21 , an interface oxide layer  22 , a silicon layer  23 , the diffusion prevention layer  15 , and the ion source electrode  16  are stacked in this order. The crystallization rate of the silicon layer  21  is higher than the crystallization rate of the silicon layer  23 , e.g., at least 10% higher. For example, the silicon layer  21  is made of polycrystalline silicon; and the silicon layer  23  is made of amorphous silicon. A variable resistance layer  24  is formed of the silicon layer  21 , the interface oxide layer  22 , and the silicon layer  23 . The thicknesses of the silicon layer  21  and the silicon layer  23  are, for example, about 1 to 25 nm. It is desirable for the thickness of the interface oxide layer  22  to be, for example, about 0.1 to 2 nm because the set voltage may increase if the interface oxide layer  22  is too thick. 
     A method for manufacturing the memory device according to the embodiment will now be described. 
       FIGS. 6A and 6B  are cross-sectional views showing the method for manufacturing the memory device according to the embodiment. 
     First, as shown in  FIG. 6A , a stacked body is formed in which the opposing electrode  11 , the current-limiting layer  12 , the crystallization-inducing metal layer  13 , an amorphous silicon layer  26 , the interface oxide layer  22 , an amorphous silicon layer  27 , the diffusion prevention layer  15 , and the ion source electrode  16  are stacked in this order. At this time, the amorphous silicon layer  26 , the interface oxide layer  22 , and the amorphous silicon layer  27  may be formed continuously by, for example, LPCVD (low pressure chemical vapor deposition) using monosilane gas. Specifically, it is sufficient to supply only monosilane gas when forming the amorphous silicon layer  26 , supply an oxidizing agent in addition to monosilane gas when forming the interface oxide layer  22 , and supply only monosilane gas again when forming the amorphous silicon layer  27 . 
     Then, as shown in  FIG. 6B , heat treatment of the stacked body is performed. Thereby, the amorphous silicon layer  26  crystallizes from the lower surface toward the upper surface because the nickel included in the crystallization-inducing metal layer  13  induces crystallization of the amorphous silicon layer  26 . However, the crystallization of the amorphous silicon layer  27  is suppressed because the interface oxide layer  22  impedes the crystallization from the amorphous silicon layer  26 . Thereby, the amorphous silicon layer  26  is used as the silicon layer  21  having a relatively high crystallization rate; and the amorphous silicon layer  27  is used as the silicon layer  23  having a relatively low crystallization rate. Thus, the memory device  2  according to the embodiment is manufactured. 
     Effects of the embodiment will now be described. 
     According to the embodiment, similarly to the first embodiment described above, the trade-off between the memory retention characteristics and the memory reset characteristics can be eliminated. Also, in the embodiment, because the interface oxide layer  22  is formed between the amorphous silicon layer  26  and the amorphous silicon layer  27 , the progression of the crystallization from the amorphous silicon layer  26  toward the amorphous silicon layer  27  can be impeded; and a large difference between the crystallization rate of the silicon layer  21  and the crystallization rate of the silicon layer  23  can be provided easily. 
     According to the embodiment, because the amorphous silicon layer  26 , the interface oxide layer  22 , and the amorphous silicon layer  27  can be formed continuously by the same LPCVD process, there is substantially no increase of the manufacturing cost due to the interface oxide layer  22  being provided. Although the variable resistance layer  24  of the embodiment is thicker than the variable resistance layer  14  of the first embodiment (referring to  FIG. 1 ) by the thickness of the interface oxide layer  22 , the increase of the set voltage can be suppressed by making the diffusion prevention layer  15  thin. Otherwise, the configuration, the manufacturing method, the operations, and the effects of the embodiment are similar to those of the first embodiment described above. 
     (Third Embodiment) 
     The embodiment differs from the first embodiment described above in that the concentration of oxygen (O), the concentration of nitrogen (N), or the total concentration of oxygen and nitrogen is higher in the upper portion of the variable resistance layer than in the lower portion of the variable resistance layer. Otherwise, the embodiment is similar to the first embodiment; and a description of the common portions is omitted. 
       FIG. 7  is a cross-sectional view showing the memory device according to the embodiment. 
     As shown in  FIG. 7 , the memory device  3  according to the embodiment differs from the memory device  1  (referring to  FIG. 1 ) according to the first embodiment described above in that a variable resistance layer  34  is provided instead of the variable resistance layer  14 . The variable resistance layer  34  is formed as, for example, one body of silicon. The oxygen concentration, the nitrogen concentration, or the total concentration of oxygen and nitrogen in an upper portion  34   b  of the variable resistance layer  34  is higher than that of a lower portion  34   a  of the variable resistance layer  34 . For example, the oxygen concentration, the nitrogen concentration, or the total concentration of oxygen and nitrogen in the upper portion  34   b  is higher than 1×10 20  cm −3 ; and the oxygen concentration, the nitrogen concentration, or the total concentration of oxygen and nitrogen in the lower portion  34   a  is lower than 1×10 20  cm −3 . The crystallization rate of the lower portion  34   a  is higher than the crystallization rate of the upper portion  34   b , e.g., at least 10% higher. 
     A method for manufacturing the memory device according to the embodiment will now be described. 
       FIGS. 8A and 8B  are cross-sectional views showing the method for manufacturing the memory device according to the embodiment. 
     First, as shown in  FIG. 8A , a stacked body is formed in which the opposing electrode  11 , the current-limiting layer  12 , the crystallization-inducing metal layer  13 , an amorphous silicon layer  39 , the diffusion prevention layer  15 , and the ion source electrode  16  are stacked in this order. At this time, for example, although the amorphous silicon layer  39  is formed by LPCVD using monosilane gas as the source material, a lower portion  39   a  that is made of amorphous silicon substantially not including oxygen and nitrogen is formed by supplying only monosilane gas for the former half of the film formation; and for the latter half of the film formation, an upper portion  39   b  that is made of amorphous silicon including oxygen and nitrogen is formed by supplying N 2 O gas in addition to monosilane gas. 
     Then, as shown in  FIG. 8B , heat treatment of the stacked body is performed. At this time, although the crystallization of the amorphous silicon layer  39  progresses from the lower surface toward the upper surface by being induced by the crystallization-inducing metal layer  13 , the crystallization speed is greatly reduced at the upper portion  39   b  of the amorphous silicon layer  39  because the oxygen and the nitrogen suppress the crystallization of the silicon. As a result, in the variable resistance layer  34 , the crystallization rate of the upper portion  34   b  can be maintained to be lower than the crystallization rate of the lower portion  34   a . Thus, the memory device  3  according to the embodiment is manufactured. 
     Effects of the embodiment will now be described. 
     In the embodiment, compared to the first embodiment, it is easier to set the crystallization rate to be different between the lower portion  34   a  and the upper portion  34   b  of the variable resistance layer  34  because oxygen and nitrogen are introduced to only the upper portion  39   b  of the amorphous silicon layer  39 . Also, compared to the second embodiment, the increase of the set voltage can be avoided reliably because it is unnecessary to form the oxide layer inside the variable resistance layer. Otherwise, the configuration, the manufacturing method, the operations, and the effects of the embodiment are similar to those of the first embodiment described above. 
     The crystallization-inducing metal layer  13  may not be provided in the embodiment. In such a case as well, the crystallization rate of the lower portion  34   a  of the variable resistance layer  34  can be set to be higher than the crystallization rate of the upper portion  34   b  by providing the concentration difference between the nitrogen and the oxygen. In the case where the crystallization-inducing metal layer  13  is not provided, the crystallization rate of the lower portion  34   a  may be set to be lower than the crystallization rate of the upper portion  34   b  by introducing at least one selected from nitrogen and oxygen to the lower portion  34   a  of the variable resistance layer  34 . 
     (Fourth Embodiment) 
     The embodiment is an example in which a transfer gate transistor is formed in addition to the memory cells of the first embodiment in the same layer as the memory cells. Otherwise, the embodiment is similar to the first embodiment; and a description of the common portions is omitted. 
       FIG. 9  is a cross-sectional view showing the memory device according to the embodiment. 
     In the memory device  4  according to the embodiment as shown in  FIG. 9 , a silicon substrate  40  is provided; an inter-layer insulating film  41  is provided on the silicon substrate  40 ; and the opposing electrode  11  having an interconnect configuration extending in one direction is provided inside the inter-layer insulating film  41 . A stacked body  48  in which the current-limiting layer  12 , the crystallization-inducing metal layer  13 , the variable resistance layer  14 , the diffusion prevention layer  15 , and the ion source electrode  16  are stacked in this order is provided on the opposing electrode  11 ; and an interconnect  49  is provided on the stacked body  48 . The interconnect  49  extends in a direction intersecting, e.g., orthogonal to, the direction in which the opposing electrode  11  extends, i.e., the direction perpendicular to the page surface of  FIG. 9 . A memory cell  42  includes the opposing electrode  11 , the stacked body  48 , and the interconnect  49 . The configurations of the opposing electrode  11  and the stacked body  48  are similar to those of the memory device  1  (referring to  FIG. 1 ) according to the first embodiment described above. In the memory device  4 , multiple memory cells  42  are provided on a common opposing electrode  11 . 
     A break  11   a  is made in the opposing electrode  11 ; and a portion of the inter-layer insulating film  41  is disposed inside the break  11   a . Thereby, the opposing electrode  11  is divided by the break  11   a . The pair of portions of the opposing electrode  11  opposing the break  11   a  on two sides of the break  11   a  are used as a source electrode  11   s  and a drain electrode  11   d . In other words, the opposing electrode  11  includes two electrodes separated from each other with the break  11   a  interposed. A crystallization-inducing metal layer  43   s  is provided on the source electrode  11   s ; and a crystallization-inducing metal layer  43   d  is provided on the drain electrode  11   d . A single channel layer  44  is provided on the crystallization-inducing metal layers  43   s  and  43   d  to straddle the break  11   a . A gate insulator film  45  is provided on the channel layer  44 ; and a gate electrode  46  is provided on the gate insulator film  45 . A transfer gate transistor  47  is formed of the source electrode  11   s , the drain electrode  11   d , the crystallization-inducing metal layers  43   s  and  43   d , the channel layer  44 , the gate insulator film  45 , and the gate electrode  46 . 
     The channel layer  44  is connected to the source electrode  11   s  via the crystallization-inducing metal layer  43   s  and connected to the drain electrode  11   d  via the crystallization-inducing metal layer  43   d . The channel layer  44  is made of a semiconductor material, e.g., polycrystalline silicon; and the crystallization rate of the channel layer  44  is higher than the crystallization rate of the upper portion  14   b  of the variable resistance layer  14 . The difference between the crystallization rate of the lower portion of the channel layer  44  and the crystallization rate of the upper portion of the channel layer  44  is less than 10%. The channel layer  44  contains fluorine. Therefore, the fluorine concentration of the channel layer  44  is higher than the fluorine concentration of the variable resistance layer  14 . 
     As described below, the crystallization-inducing metal layers  43   s  and  43   d  of the transfer gate transistor  47  are formed simultaneously with the crystallization-inducing metal layers  13  of the memory cells  42 ; the channel layer  44  is formed simultaneously with the variable resistance layers  14 ; and the gate insulator film  45  is formed simultaneously with the diffusion prevention layers  15 . 
     A method for manufacturing the memory device according to the embodiment will now be described. 
       FIG. 10A  to  FIG. 12B  are cross-sectional views showing the method for manufacturing the memory device according to the embodiment. 
     First, as shown in  FIG. 10A , the inter-layer insulating film  41  is formed on the silicon substrate  40  (referring to  FIG. 9 ); and the opposing electrode  11  having an interconnect configuration is formed at the upper portion of the inter-layer insulating film  41 . The break  11   a  is made in a portion of the opposing electrode  11  in the region (hereinbelow, called the “transfer gate transistor region”) where the transfer gate transistor  47  is to be formed. A pair of portions of the opposing electrode  11  opposing the break  11   a  on two sides of the break  11   a  is used as the source electrode  11   s  and the drain electrode  11   d.    
     Then, as shown in  FIG. 10B , the current-limiting layer  12  is formed; and the current-limiting layer  12  is removed from the transfer gate transistor region by patterning. 
     Continuing as shown in  FIG. 10C , the crystallization-inducing metal layer  13  is formed; and the crystallization-inducing metal layer  13  is removed from the region directly above the break  11   a  by patterning. Then, the inter-layer insulating film  41  is deposited further; and the upper surface is planarized. 
     Then, as shown in  FIG. 11A , the amorphous silicon layer  19 , the diffusion prevention layer  15 , and the ion source electrode  16  are formed in this order; and the ion source electrode  16  is removed from the transfer gate transistor region by patterning the ion source electrode  16 . 
     Continuing as shown in  FIG. 11B , fluorine ions are implanted into only the transfer gate transistor region. Thereby, fluorine is introduced to the portion of the amorphous silicon layer  19  disposed in the transfer gate transistor region. At this time, it is desirable for the implantation amount to be, for example, not less than 10 14  cm −2 . 
     Then, the gate electrode  46  is formed on the entire surface as shown in  FIG. 11C . 
     Continuing as shown in  FIG. 12A , the gate electrode  46 , the ion source electrode  16 , the diffusion prevention layer  15 , and the amorphous silicon layer  19  are patterned collectively such that the gate electrode  46 , the ion source electrode  16 , the diffusion prevention layer  15 , and the amorphous silicon layer  19  remain only in the transfer gate transistor region and in the region where the memory cells  42  are to be formed (hereinbelow, the “memory cell region”). Hereinbelow, the diffusion prevention layer  15  remaining in the transfer gate transistor region is called the gate insulator film  45 ; the crystallization-inducing metal layer  13  remaining in the transfer gate transistor region is called the crystallization-inducing metal layers  43   s  and  43   d ; and the gate electrode  46  remaining in the memory cell region is called the interconnect  49 . 
     Then, as shown in  FIG. 12B  and  FIG. 9 , heat treatment is performed; and the crystallization of the amorphous silicon layer  19  is performed using the crystallization-inducing metal layers  13 ,  43   s , and  43   d  as seeds of the crystallization. At this time, similarly to the first embodiment described above, because the crystallization of the amorphous silicon layer  19  progresses in the memory cell region by using the portion contacting the crystallization-inducing metal layer  13  as a starting point, the crystallization rate of the lower portion of the amorphous silicon layer  19  is higher than the crystallization rate of the upper portion of the amorphous silicon layer  19  by appropriately adjusting the temperature and time of the heat treatment. Thereby, the variable resistance film  14  described above is formed. 
     On the other hand, in the transfer gate transistor region, although the crystallization of the amorphous silicon layer  19  progresses by using portions at two locations contacting the crystallization-inducing metal layers  43   s  and  43   d  as starting points, the crystallization speed is high because much fluorine is included in the amorphous silicon layer  19 . Therefore, when the heat treatment is performed in the memory cell region at conditions such that only the lower portion of the amorphous silicon layer  19  is crystallized, the entire amorphous silicon layer  19  is crystallized in the transfer gate transistor region. Thereby, the channel layer  44  is formed to contain fluorine and have a crystallization rate throughout the entirety that is higher than the crystallization rate of the upper portion  14   b  of the variable resistance film  14 . Thus, the memory device  4  according to the embodiment is manufactured. 
     Operations of the memory device according to the embodiment will now be described. 
     In the memory device  4  according to the embodiment, the opposing electrodes  11  of the memory cells  42  are electrically connected to the source electrode  11   s  of the transfer gate transistor  47 . Then, when programming and reading data to and from the memory cells  42 , the voltage necessary for the programming operation or the read-out operation is supplied to the opposing electrodes  11  of the memory cells  42  by setting the transfer gate transistor  47  to the on-state. 
     Effects of the embodiment will now be described. 
     In the memory cell  42 , similarly to the first embodiment described above, because the crystallization rate of the upper portion  14   b  of the variable resistance layer  14  is lower than the crystallization rate of the lower portion  14   a  of the variable resistance layer  14 , the trade-off between the data retention characteristics and the data reset characteristics can be eliminated. On the other hand, in the transfer gate transistor  47 , because the crystallization rate of the channel layer  44  is uniformly high and is high at the upper portion of the channel layer  44  in which most of the transistor current flows, the mobility of the electrons flowing through the channel layer  44  can be high; and a high current driving ability can be obtained. Therefore, the read-out speed and programming speed of the memory cells are high. 
     According to the embodiment, the crystallization-inducing metal layers  13  of the memory cells  42  and the crystallization-inducing metal layers  43   s  and  43   d  of the transfer gate transistor  47  are formed simultaneously; the variable resistance layers  14  of the memory cells  42  and the channel layer  44  of the transfer gate transistor  47  are formed simultaneously; the diffusion prevention layers  15  of the memory cells  42  and the gate insulator film  45  of the transfer gate transistor  47  are formed simultaneously; the interconnects  49  of the memory cells  42  and the gate electrode  46  of the transfer gate transistor  47  are formed simultaneously; and therefore, the processes are simple and the manufacturing cost is low. 
     Because fluorine is implanted selectively into only the portion of the amorphous silicon layer  19  formed in the transfer gate transistor region in the process shown in  FIG. 11B , the crystallization in the channel layer  44  can be promoted more than in the variable resistance layer  14 . Thereby, the crystallization rate of the upper portion of the channel layer  44  can be increased while limiting the crystallization rate of the upper portion  14   b  of the variable resistance layer  14  to be low. Thus, by providing different crystallization rate distributions of silicon between the transfer gate transistor  47  and the memory cells  42 , both the memory performance and the transistor performance can be improved. Otherwise, the configuration, the manufacturing method, the operations, and the effects of the embodiment are similar to those of the first embodiment described above. 
     It is also possible to set the gate insulator film  45  of the transfer gate transistor  47  to be thicker than the diffusion prevention layers  15  of the memory cells  42  by additionally forming a silicon oxide layer only in the transfer gate transistor region. Thereby, the breakdown voltage of the transfer gate transistor  47  can be increased. 
     (Fifth Embodiment) 
     The embodiment differs from the fourth embodiment described above in that the crystallization-inducing metal layer exists on the source electrode of the transfer gate transistor, but the crystallization-inducing metal layer does not exist on the drain electrode. Otherwise, the embodiment is similar to the fourth embodiment; and common descriptions are omitted. 
       FIG. 13  is a cross-sectional view showing the memory device according to the embodiment. 
     In the memory device  5  according to the embodiment as shown in  FIG. 13 , the crystallization-inducing metal layer  43   d  (referring to  FIG. 9 ) is not provided on the drain electrode  11   d ; and instead, a metal electrode  53  is provided. The metal electrode  53  does not have the property of inducing the crystallization of silicon. On the other hand, the crystallization-inducing metal layer  43   s  is provided in the region directly above the source electrode  11   s . The crystal grains (not shown) of the channel layer  44  extend along a direction from the source electrode  11   s  side toward the drain electrode  11   d  side. 
     A method for manufacturing the memory device according to the embodiment will now be described. 
       FIGS. 14A to 14C  are cross-sectional views showing the method for manufacturing the memory device according to the embodiment. 
     First, the processes shown in  FIGS. 10A and 10B  are implemented. 
     Then, as shown in  FIG. 14A , the crystallization-inducing metal layer  13  is formed; and the crystallization-inducing metal layer  13  is removed from the region of the transfer gate transistor region other than the region directly above the source electrode  11   s  by patterning. In other words, the crystallization-inducing metal layer  13  remains in the memory cell region and in the region directly above the source electrode  11   s.    
     Continuing as shown in  FIG. 14B , the metal electrode  53  is deposited; and the metal electrode  53  is removed from the regions other than the region directly above the drain electrode  11   d  by patterning. In other words, the metal electrode  53  remains in the region directly above the drain electrode  11   d . Then, the inter-layer insulating film  41  is deposited further; and the upper surface is planarized. 
     Continuing, the processes shown in  FIG. 11A  to  FIG. 12A  are implemented. 
     Then, as shown in  FIG. 14C , heat treatment is performed; and the amorphous silicon layer  19  is crystallized by using the crystallization-inducing metal layers  13  and  43   s  as seeds of the crystallization. At this time, in the transfer gate transistor region, the crystallization of the amorphous silicon layer  19  progresses by using only the portion contacting the crystallization-inducing metal layer  43   s  as a starting point. Therefore, the crystallization of the channel layer  44  grows in one direction from the source electrode  11   s  side toward the drain electrode  11   d  side. Thus, the memory device  5  according to the embodiment is manufactured. 
     Effects of the embodiment will now be described. 
     In the process shown in  FIG. 14A  of the embodiment, because the crystallization-inducing metal layer  43   s  is provided only in the region directly above the source electrode  11   s , the crystals of the channel layer  44  grow in one direction from the source electrode  11   s  side toward the drain electrode  11   d  side in the heat treatment process shown in  FIG. 14C . Thereby, in the interior of the channel layer  44 , the crystal grain boundaries do not occur easily in orientations that obstruct the current flowing between the source electrode  11   s  and the drain electrode  11   d.    
     Conversely, in the fourth embodiment described above, because the crystallization progresses from both the source electrode  11   s  side and the drain electrode  11   d  side, the efficiency of the crystallization is high; but crystal grain boundaries orthogonal to the current direction occur easily at the central portion of the channel layer  44 . The crystal grain boundaries reduce the mobility of the electrons. Accordingly, according to the embodiment, compared to the fourth embodiment, higher mobility and a larger drive current can be obtained; and the read-out speed and programming speed of the data are high. Otherwise, the configuration, the manufacturing method, the operations, and the effects of the embodiment are similar to those of the fourth embodiment described above. 
     (Sixth Embodiment) 
       FIG. 15  is a cross-sectional view showing the memory device according to the embodiment. 
     In the memory device  6  according to the embodiment as shown in  FIG. 15 , two transfer gate transistors  47   a  and  47   b  that are connected to each other in series are provided; and one channel layer  44  is shared between the transfer gate transistors  47   a  and  47   b . Then, the crystallization-inducing metal layer  43   s  is provided only in the region directly above the source electrode  115  of the transfer gate transistor  47   a  disposed on the memory cell  42  side; and in the region directly above the drain electrode  11   d  of the transfer gate transistor  47   a , in the region directly above the source electrode  11   s  of the transfer gate transistor  47   b , and in the region directly above the drain electrode  11   d  of the transfer gate transistor  47   b , the crystallization-inducing metal layer is not provided and the metal electrode  53  is provided. 
     According to the embodiment, the crystallization of the channel layer  44  progresses in one direction from the portion of the transfer gate transistor  47   a  contacting the crystallization-inducing metal layer  43   s  toward the drain electrode  11   d  of the transfer gate transistor  47   b . Therefore, crystal grain boundaries that obstruct the current inside the channel layer  44  shared by the transfer gate transistors  47   a  and  47   b  do not occur easily. In other words, continuous crystal grains are formed along the current direction in the channel layers  44  of the two transfer gate transistors  47   a  and  47   b . As a result, the difference (the fluctuation) of the mobility characteristics, etc., between the two transistors is small. 
     Thereby, when forming an amplifier circuit (e.g., a sense amplifier for the memory read-out) using, for example, a current mirror made of the two transistors, high amplification performance is obtained because the difference of the characteristics between the transistors is small. Otherwise, the configuration, the manufacturing method, the operations, and the effects of the embodiment are similar to those of the fifth embodiment described above. 
     (Seventh Embodiment) 
     The embodiment is an example in which the transfer gate transistor and the memory cells of the fourth embodiment described above are stacked. 
       FIG. 16  is a cross-sectional view showing the memory device according to the embodiment. 
     As shown in  FIG. 16 , multiple layers of interconnect layers  70  are stacked in the memory device  7  according to the embodiment. The configuration of the memory device  4  according to the fourth embodiment described above is realized inside each of the interconnect layers  70 . The opposing electrodes  11  that have interconnect configurations are multiply provided inside each of the interconnect layers  70 . The opposing electrodes  11  are arranged parallel to each other along a direction perpendicular to the page surface of  FIG. 16 . Then, the memory cells  42  and the transfer gate transistor  47  are provided on each of the opposing electrodes  11 . 
     In the embodiment, the bit density of the memory cells per unit surface area can be increased by stacking multiple layers of the interconnect layers  70 . The amorphous silicon layers  19  of the interconnect layers  70  can be crystallized collectively by not performing heat treatment when forming each of the interconnect layers  70  but by performing heat treatment only once after all of the interconnect layers  70  are stacked. Thereby, the manufacturing cost can be greatly reduced because the number of manufacturing processes of the memory device  7  can be drastically reduced. In such a case, the metal portions included in each of the interconnect layers  70  do not degrade because the crystallization heat treatment can be performed at a low temperature of about 350 to 400° C. In particular, the diffusion of the silver of the ion source electrode  16  can be suppressed. Otherwise, the configuration, the manufacturing method, the operations, and the effects of the embodiment are similar to those of the fourth embodiment described above. 
     (Test Example) 
     A test example will now be described. 
       FIG. 17A  is a plane TEM photograph of a polycrystalline silicon layer; and  FIG. 17B  is a plane TEM photograph of an amorphous silicon layer. 
     A memory device having a configuration similar to that of the memory device  1  according to the first embodiment described above in which the polycrystalline silicon layer shown in  FIG. 17A  is used as the variable resistance layer was prototyped; a memory device having a configuration similar to that of the memory device  1  in which the amorphous silicon layer shown in  FIG. 17B  is used as the variable resistance layer was prototyped; and the data retention characteristics were evaluated. As a result, the retention characteristics of the low resistance state were better for the memory device in which the polycrystalline silicon layer was used as the variable resistance layer than for the memory device in which the amorphous silicon layer was used as the variable resistance layer. 
     According to the embodiments described above, a memory device that provides both good data retention characteristics and ease of resetting data can be realized. 
     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 invention. Additionally, the embodiments described above can be combined mutually.