Patent Publication Number: US-2013248803-A1

Title: Molecular memory and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2012-065752, filed on Mar. 22, 2012 and the prior Japanese Patent Application No. 2012-068434, filed on Mar. 23, 2012; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a molecular memory and a method of manufacturing the same. 
     BACKGROUND 
     In a non-volatile memory device, such as a NAND flash memory, a memory cell has been miniaturized to improve recording density. However, the miniaturization of the memory cell has reached its limits due to, for example, restrictions in lithography technique. Therefore, a study on a molecular memory using a resistance-change molecular chain as a storage element has been conducted. The resistance-change molecular chain is a molecule whose electrical resistance value is changed when an electric signal, such as a voltage or a current, is input. Since the size of the resistance-change molecular chain is small, it is possible to significantly reduce the size of the memory cell. In order to manufacture the molecular memory as a product, it is important to ensure reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view illustrating a molecular memory according to a first embodiment; 
         FIG. 2  is a cross-sectional view illustrating the molecular memory according to the first embodiment; 
         FIG. 3  is a diagram illustrating a resistance-change molecular chain according to the first embodiment; 
         FIGS. 4A to 4D  and  FIGS. 5A to 5D  are cross-sectional views illustrating processes of the method of manufacturing the molecular memory according to the first embodiment; 
         FIGS. 6A to 6C ,  FIGS. 7A to 7C ,  FIGS. 8A to 8C ,  FIGS. 9A to 9C ,  FIGS. 10A to 10C ,  FIGS. 11A to 11C , and  FIGS. 12A to 12C  are diagrams illustrating processes of the method of manufacturing the molecular memory according to the first embodiment; 
         FIG. 13  is a cross-sectional view illustrating a molecular memory according to a first comparative example; 
         FIG. 14  is a perspective view illustrating a molecular memory according to a second embodiment; 
         FIG. 15  is a cross-sectional view illustrating the molecular memory according to the second embodiment; 
         FIG. 16  is a perspective view illustrating a molecular memory according to a third embodiment; 
         FIG. 17  is a cross-sectional view illustrating the molecular memory according to the third embodiment; 
         FIG. 18  is a diagram illustrating a resistance-change molecular chain according to the third embodiment; 
         FIGS. 19A to 19C ,  FIGS. 20A to 20C ,  FIGS. 21A to 21C ,  FIGS. 22A to 22C ,  FIGS. 23A to 23C ,  FIGS. 24A to 24C ,  FIGS. 25A to 25C , and  FIGS. 26A to 26C  are diagrams illustrating processes of the method of manufacturing the molecular memory according to the third embodiment; 
         FIG. 27  is a cross-sectional view illustrating a molecular memory according to a second comparative example; 
         FIG. 28  is a perspective view illustrating a molecular memory according to a forth embodiment; 
         FIG. 29  is a cross-sectional view illustrating the molecular memory according to the forth embodiment; 
         FIG. 30  is a cross-sectional view illustrating a molecular memory according to a fifth embodiment; 
         FIG. 31  is a circuit diagram illustrating the molecular memory according to the fifth embodiment; 
         FIG. 32  is a perspective view illustrating a molecular memory according to a sixth embodiment; 
         FIG. 33  is a diagram illustrating a general formula of a resistance-change molecular chain according to a modification; and 
         FIGS. 34A to 34F  are diagrams illustrating molecular units capable of forming a molecule in which a π-conjugated system extends in a one-dimensional direction. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, a molecular memory includes a first electrode, a second electrode, and a resistance-change molecular chain provided between the first electrode and the second electrode. The first electrode includes a core made of a first conductive material, and a side wall made of a second conductive material different from the first conductive material. The side wall is formed on a side surface of the core. The second electrode is made of a third conductive material different from the first conductive material. The resistance-change molecular chain is bonded to the first conductive material. 
     In general, according to one embodiment, a molecular memory includes a first wiring, a second wiring, and a resistance-change molecular chain. The first wiring is made of a first conductive material and extends in a first direction. The second wiring is made of a second conductive material different from the first conductive material and extends in a second direction intersecting the first direction. The resistance-change molecular chain is provided between the first wiring and the second wiring. A surface of the first wiring located at the second wiring side has a first region and a second region. The first region faces a center of the second wiring in a width direction. The second region faces an end of the second wiring in the width direction. The first region is closer to the second wiring than the second region. 
     In general, according to one embodiment, a method of manufacturing a molecular memory includes stacking a first conductive film made of a first conductive material, a sacrificial film, and a second conductive film made of a second conductive material different from the first conductive material in this order. The method includes selectively removing an upper portion of the first conductive film, the sacrificial film, and the second conductive film to form a plurality of first stacked bodies extending in a first direction, and performing side etching on the upper portion of the first conductive film such that the width of the upper portion is less than that of the second conductive film. The method includes embedding a first insulating film between the first stacked bodies. The method includes selectively removing the first insulating film, the second conductive film, the sacrificial film, and the first conductive film to form a plurality of second stacked bodies extending in a second direction intersecting the first direction. The method includes removing the sacrificial film to form a gap. The method includes providing a resistance-change molecular chain in the gap. The method includes embedding a second insulating film between the second stacked bodies in which the resistance-change molecular chain is provided. And, the method includes forming a third conductive film extending in the first direction so as to be commonly connected to parts of the second conductive film arranged in the first direction. 
     Hereinafter, embodiments of the invention will be described with reference to the drawings. 
     First, a first embodiment will be described. 
       FIG. 1  is a perspective view illustrating a molecular memory according to the embodiment.  FIG. 2  is a cross-sectional view illustrating the molecular memory according to the embodiment.  FIG. 3  is a diagram illustrating a resistance-change molecular chain according to the embodiment. 
     For ease of illustration,  FIG. 1  shows only a conductive portion and does not show an insulating portion. 
     As illustrated in  FIGS. 1 and 2 , in a molecular memory  1  according to the embodiment, an interlayer insulating film  10  is provided on a silicon substrate (not illustrated) and a wiring layer  11 , a memory layer  12 , and a wiring layer  13  are stacked on the interlayer insulating film  10  in this order. Hereinafter, the stacked direction is referred to as a “Z direction”. In the wiring layer  11 , a plurality of wirings  21  extending in one direction (hereinafter, referred to as an “X direction”) are periodically arranged. In the wiring layer  13 , a plurality of wirings  22  extending in a direction (hereinafter, referred to as a “Y direction”) intersecting the X direction, for example, in a direction perpendicular to the X direction are periodically arranged. The X direction, the Y direction, and the Z direction are perpendicular to each other. 
     The wiring  21  includes a core  24  that extends in the X direction and a pair of side walls  25  which are formed on both sides of the core  24  in the width direction, that is, both side surfaces facing the Y direction. The core  24  and the side walls  25  come into contact with each other. The wiring  22  is integrally formed without being divided into a core and side walls. The core  24  is made of, for example, tungsten (W). The side wall  25  and the wiring  22  are made of, for example, molybdenum (Mo). A convex portion  22   p  is formed in a region of the lower surface of the wiring  22  facing the wiring  21 . In  FIG. 1 , the convex portion  22   p  is not illustrated. A gap  30  is formed between the closest portions of the wiring  21  and the wiring  22 , that is, directly below the convex portion  22   p.    
     In the memory layer  12 , an organic molecular layer  32  including a plurality of resistance-change molecular chains  31  is provided between the closest portions of the core  24  and the wiring  22 . That is, the organic molecular layer  32  is arranged directly below the core  24  in the gap  30 . The resistance-change molecular chain  31  is a molecule whose electrical resistance value is changed when an electric signal, such as a voltage or a current, is input. Each organic molecular layer  32  includes, for example, tens to hundreds of resistance-change molecular chains  31 . In addition, the molecular memory  1  includes an interwiring insulating film  35  that is provided so as to embed the wiring  21 , the wiring  22 , and the organic molecular layer  32 . The interlayer insulating film  10  and the interwiring insulating film  35  are made of an insulating material, such as a silicon oxide, alumina, or a silicon nitride. 
     As illustrated in  FIG. 3 , the resistance-change molecular chain  31  is, for example, 4-[2-amino-5-nitro-4-(phenylethynyl)phenylethynyl]benzenethiol and has a thiol group (R—SH) at one end thereof. It is easy for a sulfur atom (S) of the thiol group to be bonded to a tungsten atom (W). The resistance-change molecular chain  31  does not include a group which is likely to be bonded to a molybdenum atom (Mo). Therefore, the resistance-change molecular chain  31  is more likely to be bonded to tungsten than to molybdenum. 
     Therefore, the resistance-change molecular chain  31  is bonded to the core  24  including tungsten, but is not bonded to the side wall  25  and the wiring  22 . As a result, one end of each resistance-change molecular chain  31  is bonded to the surface of the core  24  facing the wiring  22  and each resistance-change molecular chain  31  extends from the one end in a direction (Z direction) from the core  24  to the wiring  22 . The length of the resistance-change molecular chain  31  is, for example, about 2 nm. However, the other end of the resistance-change molecular chain  31  does not reach the wiring  22 , but is separated from the wiring  22  with a gap of, for example, about 1 nm therebetween. In addition, the resistance-change molecular chain  31  is not bonded to the side wall  25  made of molybdenum. Therefore, the resistance-change molecular chain  31  is not provided between the side wall  25  and the wiring  22 . 
     Next, a method of manufacturing the molecular memory  1  according to the embodiment will be described. 
       FIGS. 4A to 4D  and  FIGS. 5A to 5D  are cross-sectional views illustrating processes of the method of manufacturing the molecular memory according to the embodiment.  FIGS. 6A to 6C ,  FIGS. 7A to 7C ,  FIGS. 8A to 8C ,  FIGS. 9A to 9C ,  FIGS. 10A to 10C ,  FIGS. 11A to 11C , and  FIGS. 12A to 12C  are diagrams illustrating processes of the method of manufacturing the molecular memory according to the embodiment. 
       FIGS. 4A to 5D  are diagrams illustrating different processes arranged in time series.  FIGS. 6A to 6C  show the same process.  FIG. 6A  is a plan view,  FIG. 6B  is a cross-sectional view taken along the line A-A′ of  FIG. 6A , and  FIG. 6C  is a cross-sectional view taken along the line B-B′ of  FIG. 6A . This holds for  FIGS. 7A to 12C . 
     First, as illustrated in  FIG. 4A , the interlayer insulating film  10  made of an insulating material, such as a silicon oxide or alumina, is formed on the silicon substrate (not illustrated). Then, a conductive material, for example, tungsten is deposited to form a conductive film  24   a  on the interlayer insulating film  10 . 
     Then, as illustrated in  FIG. 4B , the conductive film  24   a  is processed into lines by a lithography technique. In this way, a plurality of cores  24  extending in the X direction are formed. 
     Then, as illustrated in  FIG. 4C , a conductive material different from tungsten, for example, molybdenum is deposited to form a conductive film  25   a  such that the conductive film  25   a  covers the core  24 . 
     Then, as illustrated in  FIG. 4D , anisotropic etching is performed to remove portions of the conductive film  25   a  which are arranged on the upper surface of the interlayer insulating film  10  and the upper surface of the core  24 . In this case, a portion of the conductive film  25   a  which is arranged on the side surface of the core  24  remains. In this way, the side walls  25  are formed on both side surfaces of the core  24 . In this case, an upper portion of the side wall  25  is thinner than an intermediate portion and a lower portion thereof. 
     Then, as illustrated in  FIG. 5A , an insulating material is deposited to form an insulating film  35   a  on the interlayer insulating film  10  such that the insulating film  35   a  embeds the core  24  and the side wall  25 . 
     Then, as illustrated in  FIG. 5B , a planarizing process, such as chemical mechanical polishing (CMP), is performed using the core  24  as a stopper to planarize the upper surface of the insulating film  35   a . In this case, after the core  24  is exposed, the planarizing process is performed for a predetermined period of time to remove the upper portion of the core  24  and the upper portion of the side wall  25 , that is, a relatively thin portion. In this way, the side wall  25  is reliably exposed. The core  24  and the pair of side walls  25  formed on both side surfaces of the core  24  form the wiring  21 . In addition, a plurality of wirings  21  and the insulating film  35   a  remaining therebetween form the wiring layer  11 . 
     Then, as illustrated in  FIG. 5C , a material different from the materials forming the core  24 , the side wall  25 , and the insulating film  35   a , for example, a silicon oxide, alumina, or a silicon nitride is deposited to form a sacrificial film  40  on the wiring layer  11 . 
     Then, as illustrated in  FIG. 5D , a conductive material different from tungsten, for example, molybdenum is deposited to form a conductive film  22   m  on the sacrificial film  40 . 
     Then, as illustrated in  FIGS. 6A to 6C , a lithography technique is used to pattern the conductive film  22   m  and the sacrificial film  40 . In this way, the conductive film  22   m  and the sacrificial film  40  are processed into a linear stacked body extending in the Y direction. 
     Then, as illustrated in  FIGS. 7A to 7C , an insulating material different from the material forming the sacrificial film  40 , for example, a silicon oxide, alumina, or a silicon nitride is deposited to form an insulating film  35   b . Then, the planarizing process, such as CMP, is performed using the conductive film  22   m  as a stopper to planarize the upper surface of the insulating film  35   b . In this way, the insulating film  35   b  is removed from the upper surface of the conductive film  22   m  and the insulating film  35   b  is embedded between the stacked bodies of the sacrificial film  40  and the conductive film  22   m.    
     Then, as illustrated in  FIGS. 8A to 8C , the lithography technique is used to process the conductive film  22   m , the sacrificial film  40 , and the insulating film  35   b  into lines extending in the X direction. In this way, the stacked bodies of the sacrificial film  40  and the conductive film  22   m  are divided in the X direction and the Y direction and become a plurality of island-shaped portions which are arranged in a matrix. In addition, the insulating film  35   b  is divided in the X direction and the Y direction and is arranged between the stacked bodies of the sacrificial film  40  and the conductive film  22   m  which are adjacent to each other in the X direction. That is, the stacked bodies and the insulating films  35   b  are alternately arranged directly below the wiring  21 . 
     Then, as illustrated in  FIGS. 9A to 9C , for example, wet etching is performed to remove the sacrificial film  40  (see  FIGS. 8A and 8B ). In this way, after the sacrificial film  40  is removed, the gap  30  is formed. The wiring  21  is arranged below the gap  30  and the conductive film  22   m  is arranged above the gap  30 . The insulating films  35   b  are arranged on both sides of the gap  30  in the X direction and both sides of the gap  30  in the Y direction are opened. 
     Then, a chemical including the resistance-change molecular chain  31  (see  FIG. 3 ) is infiltrated into the gap  30 . In this way, the thiol group of the resistance-change molecular chain  31  is bonded to a tungsten atom (W) included in the core  24  of the wiring  21  and one end of the resistance-change molecular chain  31  is bonded to the core  24 . The resistance-change molecular chain  31  is not bonded to the side wall  25  and the conductive film  22   m  made of molybdenum. Then, for example, a drying process is performed to remove a liquid in the chemical from the gap  30 . As a result, the organic molecular layer  32  is formed between the closest portions of the core  24  and the conductive film  22   m . Each organic molecular layer  32  includes, for example, tens to hundreds of the resistance-change molecular chains  31 . In this case, since the resistance-change molecular chain  31  is not bonded to the side wall  25  and the conductive film  22   m , the organic molecular layer  32  is not provided between the side wall  25  and the conductive film  22   m.    
     Then, as illustrated in  FIGS. 10A to 10C , an insulating material, such as a silicon oxide, alumina, or a silicon nitride, is deposited to form an insulating film  35   c . Then, the planarizing process, such as CMP, is performed using the conductive film  22   m  as a stopper to planarize the upper surface of the insulating film  35   c . In this way, the insulating film  35   c  is embedded between the stacked bodies including the gap  30 , the organic molecular layer  32 , and the conductive film  22   m . In this case, the insulating material is hardly infiltrated into the gap  30  and the gap  30  remains. Therefore, the insulating material is not infiltrated between the resistance-change molecular chains  31  formed in the gap  30 . As a result, the insulating film  35   c  is arranged directly above the insulating film  35   a  and the insulating film  35   b  is arranged directly above the wiring  21  between the gaps  30 . 
     Then, as illustrated in  FIGS. 11A to 11C , a conductive material different from tungsten, for example, molybdenum is deposited to form a conductive film  22   n . The conductive film  22   n  comes into contact with the conductive film  22   m.    
     Then, as illustrated in  FIGS. 12A to 12C , the lithography technique is used to process the conductive film  22   n  into a plurality of lines extending in the Y direction. In this case, the conductive film  22   n  remains so as to pass through a region directly above the conductive film  22   m . Then, an insulating material (not illustrated) is deposited so as to embed the conductive film  22   n . In this way, the molecular memory  1  according to the embodiment is manufactured. 
     In the molecular memory  1 , the conductive film  22   m  and the conductive film  22   n  form the wiring  22 . The conductive film  22   m  corresponds to the convex portion  22   p  of the wiring  22 . The insulating films  35   a  to  35   c  and the insulating material which is deposited after the insulating films  35   a  to  35   c  are formed are a portion of the interwiring insulating film  35 . In the Z direction, a region in which the wiring  22  is arranged is the wiring layer  13  and a region between the wiring layer  11  and the wiring layer  13 , that is, a region in which the gap  30  and the organic molecular layer  32  are formed in the memory layer  12 . 
     Each memory cell including one organic molecular layer  32  is formed in a space between the closest portions of the wiring  21  and the wiring  22 . In this way, the memory cells are arranged in a matrix in the X direction and the Y direction. When a predetermined voltage is applied between one wiring  21  and one wiring  22 , the state of electrons of the resistance-change molecular chain  31  in the organic molecular layer  32  between the wirings  21  and  22  is changed and an electrical resistance value is changed. In this way, it is possible to write information to each memory cell. In addition, the electrical resistance value between the wiring  21  and the wiring  22  is detected to read the written information. 
     Next, the operation and effect of the embodiment will be described. 
     As illustrated in  FIG. 2 , in the molecular memory  1  according to the embodiment, the wiring  21  includes the core  24  and the side walls  25  and the core  24  comes into contact with the side walls  25 . Therefore, as a wiring for transmitting an electric signal, the core  24  and the side walls  25  integrally function as the wiring  21 . When potential is applied to the wiring  21 , the electric field is concentrated on the corners of the wiring  21 , that is, the upper portion of the side wall  25 . The core  24  and the side wall  25  are made of different conductive materials. The resistance-change molecular chain  31  is more likely to be bonded to the core  24  than to the side wall  25 . Therefore, the resistance-change molecular chain  31  is arranged between the core  24  and the wiring  22 , but is not arranged between the side wall  25  and the wiring  22 . As such, since the resistance-change molecular chain  31  is not bonded to the side wall  25  on which the electric field is concentrated, it is possible to prevent the deterioration of the resistance-change molecular chain  31  due to the concentration of a current. As a result, it is possible to achieve a molecular memory with high reliability. 
     In addition, since the side wall  25  is made of a material which is less likely to be bonded to the resistance-change molecular chain  31 , it is possible to form the above-mentioned structure in a self-aligned manner. 
     Next, a first comparative example will be described. 
       FIG. 13  is a cross-sectional view illustrating a molecular memory according to the comparative example. 
     As illustrated in  FIG. 13 , in a molecular memory  101  according to the comparative example, a wiring  121  is not divided into a core and a side wall, but is integrally formed of tungsten. A wiring  22  is integrally formed of molybdenum. Therefore, as viewed from the Z direction, resistance-change molecular chains  31  are arranged in the entire overlap region between the wiring  121  and the wiring  22 . 
     When potential is applied to the wiring  121 , the electric field applied to the edge E of the wiring  121 , that is, both ends of the wirings  121  in the width direction is stronger than that applied to the center thereof in the width direction. Therefore, even when the resistance-change molecular chains  31  are uniformly formed in the width direction of the wiring  121 , a current is concentrated on the resistance-change molecular chain  31  bonded to the edge E and the resistance-change molecular chain  31  is likely to deteriorate. When the resistance-change molecular chain  31  deteriorates, a defect, such as an increase in leakage current, is likely to occur. Therefore, the reliability of the molecular memory  101  is reduced. 
     Next, a second embodiment will be described. 
       FIG. 14  is a perspective view illustrating a molecular memory according to the embodiment.  FIG. 15  is a cross-sectional view illustrating the molecular memory according to the embodiment. 
     For ease of illustration,  FIG. 14  shows only a conductive portion and does not show an insulating portion. In  FIGS. 14 and 15 , a convex portion  22   p  (see  FIG. 2 ) of a wiring  22  is not illustrated. 
     As illustrated in  FIGS. 14 and 15 , a molecular memory  2  according to the embodiment includes a plurality of wiring layers  11 , a plurality of memory layers  12 , and a plurality of wiring layers  13 . The wiring layers  11  and the wiring layers  13  are alternately stacked in the Z direction, with the memory layers  12  interposed between. That is, the layers are stacked in the order of the wiring layer  11 , the memory layer  12 , the wiring layer  13 , the memory layer  12 , the wiring layer  11 , the memory layer  12 , the wiring layer  13 , . . . . The processes illustrated in  FIGS. 4A to 12C  may be repeatedly performed plural times to manufacture the molecular memory  2 . 
     According to the embodiment, a plurality of wiring layers  11 , a plurality of memory layers  12 , and a plurality of wiring layers  13  are stacked to arrange memory cells in the Z direction. That is, the memory cells can be arranged in a three-dimensional matrix along the X direction, the Y direction, and the Z direction. As a result, it is possible to improve the degree of integration of the memory cells and increase the recording density of the molecular memory. The configurations other than the above, the operation and effect, and a manufacturing method of the embodiment are similar to those of the first embodiment. 
     Next, a third embodiment will be described. 
       FIG. 16  is a perspective view illustrating a molecular memory according to the embodiment.  FIG. 17  is a cross-sectional view illustrating the molecular memory according to the embodiment.  FIG. 18  is a diagram illustrating a resistance-change molecular chain according to the embodiment. 
     For ease of illustrating,  FIG. 16  shows only a conductive portion and does not show an insulating portion. 
     As illustrated in  FIGS. 16 and 17 , in a molecular memory  3  according to the embodiment, an interlayer insulating film  10  is provided on a silicon substrate (not illustrated) and a wiring layer  11 , a memory layer  12 , and a wiring layer  13  are stacked on the interlayer insulating film  10  in this order. Hereinafter, the stacked direction is referred to as a “Z direction”. In the wiring layer  11 , a plurality of wirings  21  extending in one direction (hereinafter, referred to as a “Y direction”) are periodically arranged. In the wiring layer  13 , a plurality of wirings  22  extending in a direction (hereinafter, referred to as an “X direction”) intersecting the Y direction, for example, in a direction perpendicular to the Y direction are periodically arranged. The X direction, the Y direction, and the Z direction are perpendicular to each other. The wiring  21  and the wiring  22  are made of different conductive materials. The wiring  21  is made of, for example, molybdenum (Mo) and the wiring  22  is made of, for example, tungsten (W). 
     In an upper surface  21   a  of the wiring  21 , that is, a surface of the wiring  21  which faces the wiring  22 , a region  21   b  facing the center of the wiring  22  in the width direction (Y direction) is closer to the wiring  22  than a region  21   c  which faces both ends of the wiring  22  in the width direction. The region  21   c  also faces a space between the wirings  22 . In this way, a convex portion  21   d  which protrudes toward the center of the wiring  22  in the width direction is formed on the upper surface  21   a  of the wiring  21 . The convex portions  21   d  are periodically arranged at the same interval as that at which the wirings  22  are arranged in the direction (Y direction) in which the wiring  21  extends. In addition, the convex portion  21   d  is formed over the total length of the wiring  21  in the width direction. 
     In a lower surface  22   a  of the wiring  22 , that is, a surface of the wiring  22  facing the wiring  21 , a region  22   b  facing the wiring  21  is closer to the wiring  21  than a region  22   c  facing a space between the wirings  21 . In this way, a convex portion  22   d  which protrudes toward the wiring  21  is formed on the lower surface  22   a  of the wiring  22 . The convex portions  22   d  are periodically arranged at the same interval as that at which the wirings  21  are arranged in the direction (X direction) in which the wiring  22  extends. In addition, the convex portion  22   d  is formed over the total length of the wiring  22  in the width direction. 
     A gap  30  is formed between the closest portions of the wiring  21  and the wiring  22 , that is, directly below the convex portion  22   d . In this way, in the memory layer  12 , a plurality of gaps  30  are arranged in a matrix in the X direction and the Y direction. An organic molecular layer  32  including a plurality of resistance-change molecular chains  31  is formed in each gap  30 . The resistance-change molecular chain  31  is a molecule whose electrical resistance value is changed when an electric signal, such as a voltage or a current, is input. Each organic molecular layer  32  includes, for example, tens to hundreds of resistance-change molecular chains  31 . 
     As illustrated in  FIG. 18 , the resistance-change molecular chain  31  is, for example, 4-[2-amino-5-nitro-4-(phenylethynyl)phenylethynyl]benzenethiol and has a thiol group (R—SH) at one end. It is easy for a sulfur atom (S) of the thiol group to be bonded to a tungsten atom (W). The resistance-change molecular chain  31  does not include a group which is likely to be bonded to a molybdenum atom (Mo). Therefore, the resistance-change molecular chain  31  is more likely to be bonded to tungsten than to molybdenum. 
     Therefore, the resistance-change molecular chain  31  is bonded to the wiring  22  made of tungsten, but is not bonded to the wiring  21  made of molybdenum. As a result, one end of each resistance-change molecular chain  31  is bonded to the lower surface of the convex portion  22   d  of the wiring  22 , that is, the region  22   b  and each resistance-change molecular chain  31  extends from the one end in a direction (Z direction) from the wiring  22  to the wiring  21 . The length of the resistance-change molecular chain  31  is, for example, about 2 nm. However, the other end of the resistance-change molecular chain  31  does not reach the wiring  21 , but is separated from the wiring  21  with a gap of, for example, about 1 nm therebetween. 
     In addition, the molecular memory  3  includes an interwiring insulating film  35  that is provided so as to embed the wiring  21 , the wiring  22 , and the organic molecular layer  32 . The interlayer insulating film  10  and the interwiring insulating film  35  are made of, an insulating material, such as a silicon oxide, alumina, or a silicon nitride. 
     For example, the end of the wiring  22  in the width direction means about 10% to 30% of the width of the wiring  22 . Therefore, the length of the convex portion  21   d  in the Y direction is about 40% to 80% of the width of the wiring  22 . For example, the width of the wirings  21  and  22  is 10 nm, the length of the convex portion  21   d  in the Y direction is 6 nm, and the height of the convex portion  21   d  is in the range of 4 nm to 5 nm. 
     Next, a method of manufacturing the molecular memory  3  according to the embodiment will be described. 
       FIGS. 19A to 19C ,  FIGS. 20A to 20C ,  FIGS. 21A to 21C ,  FIGS. 22A to 22C ,  FIGS. 23A to 23C ,  FIGS. 24A to 24C ,  FIGS. 25A to 25C , and  FIGS. 26A to 26C  are diagrams illustrating processes of the method of manufacturing the molecular memory according to the embodiment. 
       FIGS. 19A to 19C  show the same process.  FIG. 19A  is a plan view,  FIG. 19B  is a cross-sectional view taken along the line A-A′ of  FIG. 19A , and  FIG. 19C  is a cross-sectional view taken along the line B-B′ of  FIG. 19A . This holds for  FIGS. 20A to 26C . 
     First, as illustrated in  FIGS. 19A to 19C , the interlayer insulating film  10  made of an insulating material, such as a silicon oxide or alumina, is formed on the silicon substrate (not illustrated). Then, a conductive material, for example, molybdenum is deposited to form a conductive film  21   m  on the interlayer insulating film  10 . Then, a material which will be removed by wet etching in the subsequent process, for example, a silicon oxide, aluminum oxide, or a silicon nitride is deposited to form a sacrificial film  40 . Then, a conductive material different from molybdenum, for example, tungsten is deposited to form a conductive film  22   m . In this way, the interlayer insulating film  10 , the conductive film  21   m , the sacrificial film  40 , and the conductive film  22   m  are stacked on the silicon substrate in this order from the lower side, thereby forming a stacked body. 
     Then, as illustrated in  FIGS. 20A to 20C , a lithography technique and an anisotropic etching technique are used to selectively remove the conductive film  22   m  and the sacrificial film  40 , thereby forming lines extending in the X direction. Then, an upper portion  21   u  of the conductive film  21   m  is selectively removed to form lines extending in the X direction. In this way, the upper portion  21   u  of the conductive film  21   m , the sacrificial film  40 , and the conductive film  22   m  are stacked in this order to form a plurality of stacked bodies  41  extending in the X direction. A portion of the conductive film  21   m  other than the upper portion  21   u , that is, a portion which is not processed into lines, but remains flat is a planer portion  21   p.    
     Then, for example, isotropic etching is performed to etch the side of the upper portion  21   u . In this way, the width of the upper portion  21   u  is less than that of the conductive film  22   m  and the sacrificial film  40 . In this case, in some cases, the end of the conductive film  22   m  is etched a little and is damaged. For example, in some cases, the lower surface of both ends of the conductive film  22  in the width direction (Y direction) is inclined. However, the damage is not illustrated. 
     Then, as illustrated in  FIGS. 21A to 21C , an insulating material different from the material forming the sacrificial film  40 , for example, a silicon oxide, an aluminum oxide, or a silicon nitride is deposited to form an insulating film  35   a  on the planer portion  21   p  of the conductive film  21   m  such that the insulating film  35   a  embeds the stacked body  41 . Then, a planarizing process, such as chemical mechanical polishing (CMP), is performed using the conductive film  22   m  as a stopper to planarize the upper surface of the insulating film  35   a.    
     Then, as illustrated in  FIGS. 22A to 22C , the lithography technique and the anisotropic etching technique are used to selectively remove the conductive film  22   m , the sacrificial film  40 , and the conductive film  21   m . In this way, a plurality of stacked bodies  42  each of which includes the conductive film  21   m , the sacrificial film  40 , the conductive film  22   m , and the insulating film  35   a  and extends in the Y direction are formed. In this case, the stacked bodies  41  each including the upper portion  21   u  of the conductive film  21   m , the sacrificial film  40 , and the conductive film  22  are divided in the X direction and the Y direction and become a plurality of island-shaped portions which are arranged in a matrix. In addition, the insulating film  35   a  is divided in the X direction and the Y direction and is arranged between the stacked bodies  41  which are adjacent to each other in the Y direction. The planer portion  21   p  of the conductive film  21   m  is divided into a plurality of lines extending in the Y direction. In this way, the conductive film  21   m  is divided into a plurality of wirings  21 . That is, in each stacked body  42 , the wiring  21  is provided at the lower part of the stacked body  42  and the stacked bodies  41  and the insulating films  35   a  are alternately arranged on the wiring  21  along the Y direction. 
     Then, as illustrated in  FIGS. 23A to 23C , for example, wet etching is performed to remove the sacrificial film  40  (see  FIGS. 22B and 22C ). In this way, the gap  30  is formed in a space from which the sacrificial film  40  is removed. The wiring  21  is arranged below the gap  30  and the conductive film  22   m  is arranged above the gap  30 . The insulating films  35   a  are arranged on both sides of the gap  30  in the Y direction and both sides of the gap in the X direction are opened. 
     Then, a chemical including the resistance-change molecular chain  31  (see  FIG. 18 ) is infiltrated into the gap  30 . In this way, the resistance-change molecular chain  31  is arranged in the gap  30 . Since the thiol group of the resistance-change molecular chain  31  is bonded to a tungsten atom (W) included in the conductive film  22   m , one end of the resistance-change molecular chain  31  is bonded to the lower surface of the conductive film  22   m . The resistance-change molecular chain  31  is not bonded to the wiring  21  made of molybdenum. Then, for example, a drying process is performed to remove a liquid in the chemical from the gap  30 . As a result, the organic molecular layer  32  is formed between the closest portions of each wiring  21  and each conductive film  22   m . Each organic molecular layer  32  includes, for example, tens to hundreds of the resistance-change molecular chains  31 . 
     Then, as illustrated in  FIGS. 24A to 24C , an insulating material, such as a silicon oxide, alumina, or a silicon nitride, is deposited to form an insulating film  35   b . Then, the planarizing process, such as CMP, is performed using the conductive film  22   m  as a stopper to planarize the upper surface of the insulating film  35   b . In this way, the insulating film  35   b  is removed from the upper surface of the conductive film  22   m  and is embedded between the stacked bodies  42 . In this case, the insulating material is hardly infiltrated into the gap  30  and the gap  30  remains. Therefore, the insulating material is not infiltrated between the resistance-change molecular chains  31  formed in the gap  30 . 
     Then, as illustrated in  FIGS. 25A to 25C , for example, molybdenum is deposited to form a conductive film  22   n  on the entire surface. The conductive film  22   n  comes into contact with the conductive film  22   m.    
     Then, as illustrated in  FIGS. 26A to 26C , the lithography technique and the etching technique are used to selectively remove the conductive film  22   n . In this way, the conductive film  22   n  is processed into a plurality of lines extending in the X direction. In this case, the conductive film  22   n  remains so as to pass through a region which is directly above the conductive film  22   m . In this way, the conductive film  22   n  is commonly connected to the conductive films  22   m  which are arranged in a line in the X direction. Then, an insulating material (not illustrated) is deposited so as to embed the conductive film  22   n  which is processed into lines. In this way, the molecular memory  3  according to the embodiment is manufactured. 
     In the molecular memory  3 , the conductive film  22   m  and the conductive film  22   n  form the wiring  22  extending in the X direction. In this case, the conductive film  22   m  is the convex portion  22   d  of the wiring  22 . The upper portion  21   u  of the conductive film  21   m  is the convex portion  21   d  of the wiring  21 . The insulating films  35   a  and  35   b  and the insulating material which is deposited thereafter are a portion of the interwiring insulating film  35 . In the Z direction, a region in which the wiring  21  is arranged is the wiring layer  11 , a region in which the wiring  22  is arranged is the wiring layer  13 , and a region between the wiring layer  11  and the wiring layer  13 , that is, a region in which the gap  30  and the organic molecular layer  32  are formed is the memory layer  12 . 
     Each memory cell including one organic molecular layer  32  is formed in a space between the closest portions of the wiring  21  and the wiring  22 . In this way, the memory cells are arranged in a matrix in the X direction and the Y direction. When a predetermined voltage is applied between one wiring  21  and one wiring  22 , the state of electrons of the resistance-change molecular chain  31  in the organic molecular layer  32  between the wirings  21  and  22  is changed and an electrical resistance value is changed. In this way, it is possible to write information to each memory cell. In addition, the electrical resistance value between the wiring  21  and the wiring  22  is detected to read the written information. 
     Next, the operation and effect of the embodiment will be described. 
     As illustrated in  FIG. 17 , in the molecular memory  3  according to the embodiment, in the upper surface  21   a  of the wiring  21 , the region  21   b  which faces the center of the wiring  22  in the width direction (Y direction) is closer to the wiring  22  than the region  22   c  which faces both ends of the wiring  22  in the width direction. In this way, the distance between the wiring  21  and both ends of the wiring  22  in the width direction is more than that between the wiring  21  and the center of the wiring  22  in the width direction. Therefore, among the resistance-change molecular chains  31  bonded to the lower surface  22   a  of the wiring  22 , only the resistance-change molecular chain  31  bonded to the center of the wiring  22  in the width direction effectively functions as a storage element and the resistance-change molecular chains  31  bonded to both ends of the wiring  22  in the width direction do not function as the storage element. That is, since the distance of both ends of the wiring  22  in the width direction from the wiring  21  is long, the resistance-change molecular chains  31  bonded to both ends of the wiring  22  do not electrically interact with the wiring  21  and do not contribute to the operation of the memory cell. As a result, for example, even when there is a variation in the shape of the end of the wiring  22  in the width direction due process factors, the characteristics of the memory cell are less likely to be affected by the variation. For example, even when the edge E of the wiring  22  is damaged and the lower surface of the wiring  22  is inclined with respect to the XY plane as illustrated in  FIG. 17 , the switching characteristics of the memory cell are less likely to vary due to the damage of the edge E. 
     Next, a second comparative example will be described. 
       FIG. 27  is a cross-sectional view illustrating a molecular memory according to the comparative example. 
     As illustrated in  FIG. 27 , in a molecular memory  102  according to the comparative example, an upper surface  121   a  of a wiring  121  is flat. Therefore, the distance between the wiring  22  and a region of the upper surface  121   a  which faces the center of the wiring  22  in the width direction is substantially equal to the distance between the wiring  22  and a region of the upper surface  121   a  which faces both ends of the wiring  22  in the width direction. Therefore, for example, when a variation in the shape of the end of the wiring  22  in the width direction occurs due to process factors, a variation in the operation of the resistance-change molecular chain  31  occurs due to the variation in the shape, which results in a variation in the switching characteristics of the memory cell. 
     For example, when the edge E of the wiring  22  is damaged and the lower surface of the wiring  22  is inclined, the gap between the wiring  121  and the resistance-change molecular chain  31  bonded to the lower surface increases and the operation characteristics of the resistance-change molecular chain  31  are different from the operation characteristics of another resistance-change molecular chain  31 . Since the variation in the shape of the end of the wiring  22  in the width direction is different for each memory cell, the switching characteristics of the memory cell vary. In particular, when the size of the memory cell is reduced, the percentage of the end in the wiring  22  increases. Therefore, a variation in the switching characteristics increases. 
     Next, a fourth embodiment will be described. 
       FIG. 28  is a perspective view illustrating a molecular memory according to the embodiment.  FIG. 29  is a cross-sectional view illustrating the molecular memory according to the embodiment. 
     For ease of illustration,  FIG. 28  shows only a conductive portion and does not show an insulating portion. 
     As illustrated in  FIGS. 28 and 29 , in a molecular memory  4  according to the embodiment, a plurality of wiring layers  11 , a plurality of memory layers  12 , and a plurality of wiring layers  13  are provided. The wiring layers  11  and the wiring layers  13  are alternately stacked in the Z direction, with the memory layers  12  interposed between. That is, the layers are stacked in the order of the wiring layer  11 , the memory layer  12 , the wiring layer  13 , the memory layer  12 , the wiring layer  11 , the memory layer  12 , the wiring layer  13 , . . . . Convex portions  21   d  are formed on both an upper surface  21   a  and a lower surface  21   e  of the wiring  21 . In this way, in the lower surface  21   e  of the wiring  21 , a region which faces the center of the wiring  22  in the width direction (X direction) is lower than a region which faces both ends of the wiring  22  in the width direction. 
     The convex portion  21   d  on the lower surface  21   e  of the wiring  21  may be formed by the same method as that used to form the convex portion  22   d  on the lower surface  22   a  of the wiring  22 . However, when the wiring  21  is formed, the width of the conductive film  22   m  which is processed into lines in a process corresponding to the process illustrated in  FIGS. 20A to 20C  is less than that of the conductive film  22   n  which is processed into lines in a process corresponding to the process illustrated in  FIGS. 26A to 26C . In this way, it is possible to form the convex portion  21   d  with a length less than the width of the wiring  22  in the X direction. 
     According to the embodiment, since a plurality of wiring layers  11 , a plurality of memory layers  12 , and a plurality of wiring layers  13  are stacked, it is possible to arrange the memory cells in the Z direction. That is, the memory cells can be arranged in a three-dimensional matrix along the X direction, the Y direction, and the Z direction. As a result, it is possible to improve the degree of integration of the memory cells and increase the recording density of the molecular memory. The configurations other than the above, a manufacturing method, and the operation and effect of the embodiment are similar to those according to the third embodiment. 
     Next, a fifth embodiment will be described. 
       FIG. 30  is a cross-sectional view illustrating a molecular memory according to the embodiment.  FIG. 31  is a circuit diagram illustrating the molecular memory according to the embodiment. 
     As illustrated in  FIG. 30 , in a molecular memory  5  according to the embodiment, an element isolation insulator  62  is selectively formed in an upper portion of a silicon substrate  61 , and a source region  63  and a drain region  64  are separately formed in regions partitioned by the element isolation insulator  62 . A gate insulating film  66  is provided immediately above a channel region  65  which is provided between the source region  63  and the drain region  64  on the silicon substrate  61 , and a gate electrode  67  is provided on the gate insulating film  66 . Side walls  68  are provided on the sides of the gate electrode  67 . In this way, a field effect transistor  69  is formed. 
     An interlayer insulating film  50  is provided on the silicon substrate  61 . A contact  51 , a contact  52 , a contact  53 , a word line  54 , and a bit line  55  are provided in the interlayer insulating film  50 . The contact  52  is made of molybdenum and the contact  53  is mode of tungsten. A gap  56  is formed between the contact  52  and the contact  53  in the element separation insulating film  50 . 
     The contact  51  is connected between the source region  63  and the word line  64 . The lower end of the contact  52  is connected to the drain region  64  and the upper end thereof is exposed to the gap  56 . A convex portion  52   d  is formed at the center of the upper end surface of the contact  52 . The contact  53  is disposed immediately above the contact  52  and is separated from the contact  52  with the gap  56  interposed between. The lower end of the contact  53  is exposed to the gap  56  and the upper end thereof is connected to the bit line  55 . A resistance-change molecular chain  31  is provided in the gap and is bonded to the contact  53 . A plurality of resistance-change molecular chains  31  form an organic molecular layer  32 . 
     In this way, as illustrated in  FIG. 31 , in the molecular memory  5 , a one-resistor-one-transistor (1R1T) memory cell in which the organic molecular layer  32  serving as a storage element is connected in series to the field effect transistor  69  serving as a selection element is formed between the word line  54  and the bit line  55 . The operation and effect of the embodiment are the same as those of the third embodiment. 
     Next, a sixth embodiment will be described. 
       FIG. 32  is a perspective view illustrating a molecular memory according to the embodiment. 
     For ease of illustration,  FIG. 32  shows only a conductive portion, but does not show an insulating portion. 
     As illustrated in  FIG. 32 , in a molecular memory  6  according to the embodiment, a convex portion  22   d  (see  FIG. 16 ) is not formed in a wiring  22 . Therefore, a lower surface  22   a  of the wiring  22  is flat. 
     The configurations other than the above and the operation and effect of the embodiment are the same as those of the third embodiment. 
     Next, modifications of the materials in each of the above-described embodiments will be described. 
       FIG. 33  is a diagram illustrating a general formula of a resistance-change molecular chain according to a modification.  FIGS. 34A to 34F  are diagrams illustrating molecular units capable of forming a molecule in which a π-conjugated system extends in a one-dimensional direction. 
     In each of the above-described embodiments, the resistance-change molecular chain  31  is 4-[2-amino-5-nitro-4-(phenylethynyl)phenylethynyl]benzenethiol illustrated in  FIG. 18 , but the invention is not limited thereto. For example, the resistance-change molecular chain  31  may be a molecule with variable resistance. For example, the resistance-change molecular chain  31  may be a derivative of 4-[2-amino-5-nitro-4-(phenylethynyl)phenylethynyl]benzenethiol, which is represented by a general formula illustrated in  FIG. 33 . 
     In the general formula illustrated in  FIG. 33 , a combination of X and Y is a combination of two of fluorine (F), chlorine (Cl), bromine (Br), iodine (I), a cyano group (CN), a nitro group (NO 2 ), an amino group (NH 2 ), a hydroxyl group (OH), a carbonyl group (CO), and a carboxyl group (COOH). In addition, Rn (n=1 to 8) is an arbitrary atom except for an atom in which a peripheral electron is a d electron or an f electron or a characteristic group, for example, any one of hydrogen (H), fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and a methyl group (CH 3 ). 
     The resistance-change molecular chain  31  may be a molecule in which the π-conjugated system extends in a one-dimensional direction and which has a structure other than the molecular structure represented by the general formula illustrated in  FIG. 33 . For example, a paraphenylene derivative, an oligothiophene derivative, an oligopyrrole derivative, an oligofuran derivative, or a paraphenylene vinylene derivative may be used. 
     The molecular unit capable of forming the molecule in which the π-conjugated system extends in a one-dimensional direction may be paraphenylene illustrated in  FIG. 34A , thiophene illustrated in  FIG. 34B , pyrrole illustrated in  FIG. 34C , furan illustrated in  FIG. 34D , vinylene illustrated in  FIG. 34E , or alkyne illustrated in  FIG. 34F . In addition, a six-membered heterocyclic compound, such as pyridine, may be used. 
     When the length of the π-conjugated system is short, an electron injected from the electrode passes without remaining on the molecule. Therefore, it is preferable that the length of the π-conjugated system be greater than a predetermined value in order to store charge. It is desirable that the length of the π-conjugated system be equal to or greater than 5 in the unit of —CH═CH— in one-dimensional direction. In the case of a benzene ring (paraphenylene), this corresponds to 3 or more. The diameter of the benzene ring is about two times more than the width of polaron, which is a carrier of the π-conjugated system. On the other hand, when the length of the π-conjugated system is long, for example, a voltage drop occurs due to charge conduction in the molecule. Therefore, it is preferable that the length of the π-conjugated system be equal to or less than 20 in the unit of —CH═CH— in one-dimensional direction. In the case of a benzene ring, this corresponds to 10 or less. 
     The materials forming each wiring, the core, and the side wall are not limited to those according to each of the above-described embodiments. Preferred conductive materials forming each wiring, the core, and the side wall vary depending on the molecular structure of one end of the resistance-change molecular chain  31 . 
     For example, as illustrated in  FIGS. 3 and 18 , when one end of the resistance-change molecular chain  31  is a thiol group, it is preferable that a material forming a portion which is desired to be chemically bonded to the resistance-change molecular chain  31  be gold (Au), silver (Ag), copper (Cu), tungsten nitride (WN), tantalum nitride (TaN), or titanium nitride (TiN), in addition to tungsten (W). Among them, in particular, it is preferable that the material be tungsten (W), gold (Au), or silver (Ag) which is likely to form chemical bonding. On the other hand, it is preferable that a material forming a portion which is not desired to be chemically bonded to the resistance-change molecular chain  31  be tantalum (Ta), molybdenum nitride (MoN), or silicon (Si), in addition to molybdenum (Mo). The side wall  25  and the wiring  22  illustrated in  FIG. 3  may be made of different materials. 
     For example, when one end of the resistance-change molecular chain  31  is an alcohol group or a carboxyl group, it is preferable that the material forming the portion which is desired to be chemically bonded to the resistance-change molecular chain  31  be tungsten (W), tungsten nitride (WN), tantalum (Ta), tantalum nitride (TaN), molybdenum (Mo), molybdenum nitride (MoN), or titanium nitride (TiN). Among them, in particular, it is preferable that the material be tantalum (Ta), tantalum nitride (TaN), molybdenum nitride (MoN), or titanium nitride (TiN) which is likely to form chemical bonding. On the other hand, it is preferable that the material forming the portion which is not desired to be chemically bonded to the resistance-change molecular chain  31  be gold (Au), silver (Ag), copper (Cu), or silicon (Si). 
     For example, when one end of the resistance-change molecular chain  31  is a silanol group, it is preferable that the material forming the portion which is desired to be chemically bonded to the resistance-change molecular chain  31  be silicon (Si) or metal oxide. On the other hand, it is preferable that the material forming the portion which is not desired to be chemically bonded to the resistance-change molecular chain  31  be gold (Au), silver (Ag), copper (Cu), tungsten (W), tungsten nitride (WN), tantalum (Ta), tantalum nitride (TaN), molybdenum (Mo), molybdenum nitride (MoN), or titanium nitride (TiN). When the material forming the wiring is compound, the composition of the compound may be appropriately selected. In addition, the wiring may be made of, for example, graphene or carbon nanotube. 
     According to the above-described embodiments, it is possible to achieve a molecular memory with high reliability and a method of manufacturing the molecular memory. 
     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.