Patent Publication Number: US-10312257-B2

Title: Semiconductor device and method for manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 15/422,313, filed on Feb. 1, 2017, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2016-176672, filed on Sep. 9, 2016, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a semiconductor device and a method for manufacturing the same. 
     BACKGROUND 
     There is known a technique to form a three-dimensional memory cell array by extending a hole inwardly of a stacked body of electrode layers, each of which functions as a control gate in a memory device, spaced apart by insulating layers provided between the electrode layers, forming a charge storage layer on an inner wall of the hole, and then providing silicon within the hole. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a memory cell portion of a semiconductor device according to first and second embodiments hereof. 
         FIGS. 2A and 2B  are cross-sectional views illustrating the memory cell portion of the semiconductor device according to the first embodiment. 
         FIGS. 3A and 3B  are diagrams illustrating the result of performing a step of a method for manufacturing the semiconductor device according to the first embodiment. 
         FIGS. 4A, 4B, and 4C  are diagrams illustrating the result of performing a further step of method for manufacturing the semiconductor device according to the first embodiment. 
         FIGS. 5A, 5B, and 5C  are diagrams illustrating the result of performing a yet another step of method for manufacturing the semiconductor device according to the first embodiment. 
         FIGS. 6A and 6B  are diagrams illustrating the result of further steps in the method for manufacturing the semiconductor device according to the first embodiment. 
         FIGS. 7A and 7B  are diagrams illustrating the result of further steps in the method for manufacturing the semiconductor device according to the first embodiment. 
         FIGS. 8A and 8B  are diagrams illustrating the result of further steps in the method for manufacturing the semiconductor device according to the first embodiment. 
         FIGS. 9A and 9B  are diagrams illustrating the result of further steps in the method for manufacturing the semiconductor device according to the first embodiment. 
         FIGS. 10A and 10B  are diagrams illustrating the result of a step of the method for manufacturing a semiconductor device according to a modification example. 
         FIG. 11  is a cross-sectional view illustrating the memory cell portion of the semiconductor device according to the second embodiment. 
         FIGS. 12A, 12B, and 12C  are diagrams illustrating a step in the method for manufacturing the semiconductor device according to the second embodiment. 
         FIGS. 13A and 13B  are diagrams illustrating a further step of the method for manufacturing the semiconductor device according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, there is provided a semiconductor device having a channel region more efficiently crystallized and a method for manufacturing the same. 
     In general, according to one embodiment, a method for manufacturing a semiconductor device includes forming a first semiconductor layer on a first underlying layer, forming a stacked body including alternately formed first layers and second layers on the first semiconductor layer, forming a hole, having an inner wall and a base, from an upper surface of the stacked body to the first semiconductor layer to expose the first semiconductor layer therein. A first insulating layer is formed on the inner wall of the hole, and a second semiconductor layer is formed on the first insulating layer within the hole, wherein the second semiconductor layer is electrically connected to the first semiconductor layer. A metal layer is provided in contact with at least one of the first semiconductor layer and the second semiconductor layer. The stacked body, first and second semiconductor layers, insulating layer and metal layer are exposed to an annealing temperature sufficient to cause migration of metal in the metal layer into one of the first and second semiconductor layers. 
     Hereinafter, embodiments for implementing the disclosure will be described. 
     (First Embodiment) 
     A semiconductor device according to a first embodiment is described with reference to  FIG. 1  to  FIGS. 10A and 10B . 
     Furthermore, in the following illustrations of the drawings, the same portions of the device shown in different drawing figures are denoted by the same reference numerals or characters. Additionally, the drawings are schematic, in which, for example, the relationships and ratios between the thickness and the planar size of features may be different from those of an actual device. 
     The semiconductor device according to the present embodiment is, for example, a NAND-type non-volatile memory device, and includes a memory cell portion MCP including three-dimensionally arranged memory cells.  FIG. 1  is a perspective view illustrating a memory cell portion MCP of the semiconductor device  1 . Furthermore, in  FIG. 1 , an insulating layer that is provided between bit lines BL and a stacked body  100  in an actual device is omitted from the figure. 
     In the present specification, for ease of explanation, an XYZ Cartesian coordinate system is used. In this coordinate system, two directions that are parallel to the principal surface of an insulating layer  10  and are orthogonal to each other are referred to as an X-direction and a Y-direction, and a direction that is orthogonal to both the X-direction and the Y-direction is referred to as a Z-direction. A plurality of electrode layers  50  are stacked in the Z-direction and extend in the X-direction. 
     The memory cell portion MCP illustrated in  FIG. 1  includes, on a substrate (not illustrated), an insulating layer  10 , stacked bodies  100 , which are located over a source line  20  and a source layer  30 , and columnar portions CL, which penetrate through the stacked bodies  100  in the Z-direction as shown in  FIGS. 2A and 2B . The stacked bodies  100  include a plurality of electrode layers  50  stacked in the Z-direction, with an insulating layer  40  provided between each adjacent electrode layer  50  in the Z-direction. Each electrode layer  50  serves as a control gate for a memory cell, i.e., as a word line. Furthermore, in each stacked body  100 , the uppermost electrode layer  50  is referred to as an “electrode layer  50   a ”, and the lowermost electrode layer  50  is referred to as an “electrode layer  50   b”.    
     As illustrated in  FIG. 1 , individual stacked bodies  100  are arranged side by side in the Y-direction on the source layer  30 . An insulating layer  60  is provided between adjacent stacked bodies  100 . The columnar portions CL include a semiconductor layer  70 , and the semiconductor layer  70  is electrically connected to a bit line BL through an overlying contact plug Cb. Moreover, the semiconductor layer  70  is electrically connected to the source layer  30  ( FIG. 2A ). 
     Next, a structure of the memory cell portion MCP is described in detail with reference to  FIGS. 2A and 2B .  FIG. 2A  is a schematic sectional view taken in the Y-Z plane of the memory cell portion MCP.  FIG. 2B  is a schematic sectional view illustrating a portion of a memory cell MC indicated by a dashed line portion illustrated in  FIG. 2A . 
       FIG. 2A  shows only a small number of individual electrode layers  50 , smaller in number than those of  FIG. 1 , for ease of understanding. 
     As illustrated in  FIG. 2A , the source line  20  is provided on the insulating layer  10 , and the source layer  30  is provided on the source line  20 . The source line  20  is in contact with the insulating layer  10 , and includes therein a high melting point metal, such as tungsten or tantalum. Moreover, the source line  20  can be, for example, a metal compound, such as tungsten nitride (WN) or tungsten silicide (WSi). Furthermore, the position of the source line  20  is not specifically limited, and can be on the source layer  30  rather than on the insulating layer  10 . 
     The source layer  30  is, for example, an N-type semiconductor layer, and includes a first portion  31  and a second portion  33 . The first portion  31  is located between the source line  20  and the second portion  33 . The first portion  31  is, for example, a doped crystallized silicon layer with an impurity concentration of 1e18/cm 3  to 1e21/cm 3 . The second portion  33  is, for example, a doped crystallized silicon layer with an impurity concentration of 1e15/cm 3  to 1e19/cm 3 , which contains a lower concentration of N-type impurities than those in the first portion  31 . Furthermore, the crystallized silicon layer is one of a single crystal structure, or polysilicon having a large crystal grain size, and the same also applies to the following description. 
     Furthermore, the source layer  30  is in not limited to the above example. The source layer  30  can be, for example, one or more semiconductor layers that have the same, uniform, N-type impurity concentration. The source layer  30  can be, for example, a P-type semiconductor layer. In that case, the second portion  33  contains P-type impurities lower in concentration than those in the first portion  31 . 
     As illustrated in  FIG. 2A , the columnar portion CL extends in the Z-direction, and penetrates through the stack of insulating layers  40  and electrode layers  50 . An insulating layer  60  is provided between adjacent stacked bodies  100 . Furthermore, an electrode layer  65 , which serves as a contact plug to the source layer  30 , can be provided inside the insulating layer  60 . 
     The columnar portions CL include a semiconductor layer  70 , a core  75 , and an insulating layer  80 . The core  75  is, for example, silicon oxide, and it extends in the Z-direction inside the columnar portion CL. The semiconductor layer  70  includes a first semiconductor layer  71  and a second semiconductor layer  73 , and these each extend in the Z-direction. The semiconductor layer  70  is, for example, a crystalline silicon layer formed by crystallizing amorphous silicon, and it surrounds the core  75 . The insulating layer  80  surrounds the outer perimeter of the semiconductor layer  70 . In other words, the semiconductor layer  70  is located between the core  75  and the insulating layer  80 . The first semiconductor layer  71  is in contact with the insulating layer  80 , and the second semiconductor layer  73  is in contact with the core  75 . 
     As illustrated in  FIG. 2A , where the columnar portion CL penetrates through the electrode layer  50   b  which is the lowermost electrode layer in a plurality of electrode layers included in the stacked body  100 , a source-side selection transistor STS is formed. The semiconductor layer  70  serves as the channel, and the electrode layer  50   b  serves as the source-side selection gate, of the selection transistor STS. The portion of the insulating layer  80  located between the electrode layer  50   b  and the semiconductor layer  70  serves as a gate insulating layer of the selection transistor STS. 
     A drain-side selection transistor STD is formed where the columnar portion CL penetrates through the electrode layer  50   a  (referring to  FIG. 1 ), which is the uppermost electrode layer in the plurality of electrode layers  50 . The electrode layer  50   a  serves as a drain-side selection gate of the drain-side selection transistor STD. A plurality of memory cells MC are provided by the electrode layers  50  located between the electrode layer  50   a  and the electrode layer  50   b  of each stacked body  100 . The insulating layer  80 , which is in contact with the electrode layers  50 , serves as a gate insulating layer of each memory cell MC, and the semiconductor layer  70  serves as a channel for each memory cell MC. 
     As illustrated in  FIG. 2B , the insulating layer  80  is formed as a multi-layered structure, including, for example, a first layer  81 , a second layer  83 , and a third layer  85 . Each of the first layer  81 , the second layer  83 , and the third layer  85  extend in the Z-direction alongside, and around, the semiconductor layer  70 . The second layer  83  is located between the first layer  81  and the third layer  85 . The first layer  81  is located between the stack of electrode layers  50  and insulating layers  40  and the second layer  83 . The third layer  85  is located between the semiconductor layer  70  and the second layer  83 . Each of the first layer  81  and the third layer  85  is, for example, a silicon oxide layer, and the second layer  83  is, for example, a silicon nitride layer. 
     The insulating layer  80  provides a charge storage layer at the portions thereof located between the electrode layers  50  and the semiconductor layer  70 . For example, electric charge is injected from the semiconductor layer  70  to the insulating layer  80  by a bias applied between the electrode layer  50  and the semiconductor layer  70 . Then, the injected electric charge is trapped in the second layer  83 . Moreover, the electric charge trapped in the second layer  83  is released to the semiconductor layer  70  if a reverse bias is applied between the electrode layer  50  and the semiconductor layer  70 . With this structure and voltage application, data writing to a memory cell MC, and data erasure from the memory cell MC, are carried out. Furthermore, in addition to the above example, the insulating layer  80  can contain a conductive body, which serves as a floating gate, at a portion thereof located between the electrode layer  50  and the semiconductor layer  70 . 
     In the present embodiment, the amorphous silicon initially comprising each of the source layer  30  and the semiconductor layer  70  is crystallized by a metal induced lateral crystallization (MILC) method. Although details are described below, the MILC method is a method of forming polysilicon having a large crystal grain size or a single crystal by performing annealing of amorphous silicon using a metal catalyst. Hereinafter, in the present specification, it is considered that a single crystal is formed of each of the source layer  30  and the semiconductor layer  70  by crystallization. However, it is also possible that polysilicon having a large crystal grain size is formed. 
     By crystallizing the source layer  30  and the semiconductor layer  70 , the resistances of the source layer  30  and the semiconductor layer  70  are decreased. As a result, an issue where an increase in the channel length when the number of stacked electrode layers  50  is increased in the three-dimensionally arrayed memory cells results in increased resistance which lowers the reading and writing speeds of the memory cells MC can be avoided. 
     A method for manufacturing the semiconductor device  1  according to the present embodiment is now described. 
     Hereinafter, the method for manufacturing the semiconductor device  1  according to the present embodiment is described with reference to  FIGS. 3A and 3B  to  FIGS. 9A and 9B . 
     As illustrated in  FIG. 3A , the source line  20  which includes a metal such as tungsten is formed on the insulating layer  10 , and a metal layer  25  is formed on the source line  20 . The insulating layer  10  is, for example, a silicon oxide layer formed using tetraethyl orthosilicate (TEOS)-chemical vapor deposition (CVD). The metal layer  25  includes, for example, at least one of nickel, cobalt, palladium, gold, and copper. 
     Next, the non crystallized source layer  30   a  is formed. Herein, the use of the sub notation of a, for example  31   a ,  33   a , denote that the silicon is in the amorphous state and has not yet been crystallized. The source layer includes a first source sub-layer  31   a  and a second source sub-layer  33   a . The first source sub-layer  31   a  is, for example, an amorphous silicon layer formed using CVD. Ions of phosphorus (P), which is an N-type impurity, are implanted into the upper surface of the first source sub-layer  31   a , so that phosphorus, which is an N-type impurity, is doped thereinto. The second source sub-layer  33   a  is located on the upper surface side of the first source sub-layer  31   a , and contains N-type impurities lower in concentration than those of the first source sub-layer  31   a . Furthermore, the doping method for doping the second source sub-layer  33   a  with N-type impurities is not specifically limited to ion implanting. 
     As illustrated in  FIG. 3B , a stacked body  100   a  is formed on the source layer  30   a . The stacked body  100   a  includes, for example, a plurality of insulating layers  40  and  45  that are alternately stacked one over the other in the Z-direction. The insulating layers  40  are, for example, silicon oxide layers formed using CVD (chemical vapor deposition). The insulating layers  45  are, for example, silicon nitride layers formed using CVD. The insulating layers  45  are formed from a material that can be selectively removed with respect to the insulating layers  40  according to a predetermined etching condition, i.e., they may be selectively etched away without significant etching or degrading of the insulating layer  40  during the etching. 
     As illustrated in  FIG. 4A , a hole MH is formed which extends from the upper surface of the stacked body  100   a  to the source layer  30   a . The hole MH is formed by selectively removing portions of the plurality of insulating layers  40  and  45  in the shape of the hole MH using, for example, reactive ion etching (RIE). During the etching, the etching conditions are selected such that the etching rate of the source layer  30   a  below the stack of insulating layers  40 ,  45  is lower than the etching rate of the insulating layers  40  and  45 . Thus, the source layer  30   a  functions as an etch stop layer and the hole terminates on or slightly inwardly of the source layer  30   a.    
     As illustrated in  FIG. 4B , after the hole MH is formed, an insulating layer  80  and a first semiconductor layer  71   a  are formed in that sequence on the exposed surfaces of the hole MH. The insulating layer  80  is formed on the exposed surface of the hole MH using, for example, CVD. The insulating layer  80  has a structure in which a silicon oxide layer, a silicon nitride layer, and a silicon oxide layer are sequentially formed in this order. The first semiconductor layer  71   a  is, for example, an amorphous silicon layer, and is formed on the insulating layer  80  using CVD. The first semiconductor layer  71   a  is formed having a thickness allowing space (referring to  FIG. 2A ) for forming a second semiconductor layer  73   a  and the core  75  inside the hole MH. 
     As illustrated in  FIG. 4C , the portion of the first semiconductor layer  71   a  and the portion of the insulating layer  80  which were formed on the bottom surface of the hole MH have been removed to expose the underlying source layer  30   a  at the base of the hole MH (bottom surface). These portions of the first semiconductor layer  71   a  and portion of the insulating layer  80  are removed using, for example, anisotropic RIE. 
     As illustrated in  FIG. 5A , the second semiconductor layer  73   a  is formed, and it covers the first semiconductor layer  71   a  and the exposed portions of the insulating layer  80  and source layer  30   a  at the base of the hole MH. The second semiconductor layer  73   a  is, for example, an amorphous silicon layer, and is formed using CVD. The second semiconductor layer  73   a  covers the first semiconductor layer  71   a  formed on the inner wall of the hole MH, and is electrically connected to the source layer  30   a  at the bottom surface of the hole MH. The core  75  infills the gap inside of the hole MH surrounded by the second semiconductor layer  73 . The core  75  includes, for example, silicon oxide formed using CVD. In the above-described way, the hole MH is formed and filled. Herein, the first semiconductor layer  71   a  and the second semiconductor layer  73   a  are collectively referred to as a “semiconductor layer  70   a”.    
     Next, an insulating film  40   a  composed of, for example, a silicon oxide, is formed on the upper surface of the filled hole MH and the top surface of the stacked body  100   a  ( FIG. 5B ) to form a capping layer. 
     The resulting structure is annealed at, for example, 500° C. to 750° C. This initiates and progresses MILC crystallization of the amorphous silicon layers of the source layer  30   a  and the semiconductor layer  70   a  as illustrated by the solid arrows in  FIG. 5C , using the metal of the metal layer  25  as a catalyst. As the metal of the metal layer  25  diffuses into and through the source layer  30   a  and the semiconductor layer  70   a , the Si—Si bonds of the amorphous silicon of the source layer  30   a  and the semiconductor layer  70   a  are subjected to surface rearrangement, so that a region of the amorphous silicon through which the metal has passed becomes crystallized. Furthermore, since the metal of the metal layer does not diffuse to the metal source line, the diffusion of the metal progresses from the lowermost amorphous silicon layer toward the uppermost amorphous layer, in the Z-direction. In other words, crystallization of the first portion  31   a  occurs first, and then, crystallization of the second portion  33   a  occurs. As a result, the first portion  31   a  becomes a crystallized silicon layer (first sub-layer  31 ) changed from an amorphous silicon layer, and the second portion  33   a  becomes a crystallized silicon layer (second sub-layer  31 ) changed from an amorphous silicon layer. Furthermore, in each of these crystallized silicon layers, a single crystal of silicon, or polysilicon having a large crystal grain size, is formed as a result of crystallization. 
     As the upward diffusion of the metal of the metal layer  25  progresses, the semiconductor layer  70   a  is next crystallized. The semiconductor layer  70   a  is crystallized with the crystal orientation thereof aligned with that of the source layer  30   a  having had the amorphous silicon layer thereof crystallized. Thus, the semiconductor layer  70  becomes crystallized into semiconductor layer  70 . 
     Once crystallized, the semiconductor layer  70  may be a single crystal as mentioned above, but can be polycrystalline. In other words, in the semiconductor layer  70 , each of the channels (the semiconductor layer  70 ) of at least two memory cells MC electrically connected to each other (for example, memory cells adjacent to the uppermost and lowermost electrode layers  50 ) become a single crystal. However, the memory cells are not limited to the uppermost and lowermost memory cells. 
     Furthermore, in the present embodiment, since the metal of the metal layer  25  diffuses from the lowermost layer to the uppermost layer in the Z-direction, the crystal grain size in the lower layer is larger and is likely to be more uniform, as compared with a case where metal is diffused from the upper side of the layers. Furthermore, the term“crystal grain size” refers to the maximum vertical dimension of the crystal grain with respect to the Z-direction. For example, the semiconductor layer  70  has a crystal grain size equal to or greater than the film thickness thereof in the z direction (the dimension between the insulating layer  80  and the core  75 ). Thus, in the semiconductor layer  70 , the average of crystal grain sizes becomes larger in the lower layer as compared with that in the upper layer. Furthermore, the term “average of crystal grain sizes” refers to, for example, an average value of grain sizes of crystals included in a unit area on the Z-Y plane of the semiconductor layer  70 . Moreover, the unit area is set to an area including at least a plurality of crystals. 
     When annealing is further continued at 500° C. to 750° C., the metal from the metal which has diffused through the silicon layers segregates from the semiconductor layer  70  and accumulates in between the top-surface insulating film  40   a  and the stacked body  100   a  as illustrated in  FIG. 6A . More specifically, the metal segregates from the semiconductor layer  70  at the boundary between the second semiconductor layer  73  and the insulating film  40   a  as shown by reference numeral  25  in  FIG. 6A . 
     Next, as illustrated in  FIG. 6B , the insulating film  40   a  and the segregated metal are removed by, for example, chemical mechanical polishing (CMP) or dry etching. Furthermore, impurities may be injected into the crystallized silicon in the upper portion of the hole MH. 
     As illustrated in  FIG. 7A , a trench ST is formed into the stacked body  100   a , which divides the stacked body  100   a  into a plurality of individual stacked body  100  portions. The trench ST is formed so as to have a depth extending from the upper surface of the stacked body  100   a  to the source layer  30   a  using, for example, anisotropic RIE. The trench ST extends in the X-direction, and divides the stacked body  100   a  into a plurality of portions each of which serves as an individual stacked body  100 . 
     As illustrated in  FIG. 7B , the insulating layers  45  are selectively removed. The insulating layers  45  are selectively subjected to etching, for example, by an etchant supplied thereto through the trench ST. In a case where the insulating layer  45  is a silicon nitride layer and the insulating layer  40  is a silicon oxide layer, using thermal phosphoric acid as an etchant enables selectively removing the insulating layers  45  while leaving the insulating layers  40  substantially un-etched. Furthermore, the source layer  30  is formed of a material that is resistant to the etchant for the insulating layers  45 . 
     As illustrated in  FIG. 8A , metal layers  55  are formed in the spaces  45   s  which were formed by removing the insulating layers  45  by the etchant. The metal layer  55  is, for example, a tungsten layer formed using CVD. A source gas for CVD is supplied to the spaces  45   s  through the trench ST. 
     As illustrated in  FIG. 8B , the portions of the metal layer  55  formed on the inner surface of the trench ST are removed, to form individual electrode layers  50 . With this, a stacked body  100  including a plurality of electrode layers  50  (referring to  FIG. 1 ) is completed. The adjacent electrode layers  50  in the Z-direction are electrically insulated from each other by the insulating layer  40 . 
     As illustrated in  FIG. 9A , the insulating layer  60  is then formed in the trench ST. The insulating layer  60  is, for example, a silicon oxide layer formed using CVD, and it electrically insulates adjacent stacked bodies  100  from each other (referring to  FIG. 1 ). The bit lines BL are then formed over the stacked body  100  on an interlayer insulating layer (not illustrated), thus completing the memory cell portion MCP. Furthermore, as illustrated in  FIG. 2A , an electrode layer  65  may be formed, which includes, for example, a tungsten layer which is used as a contact plug, inside the insulating layer  60 . 
     In the above-described way, the memory cell portion MCP of the semiconductor device  1  according to the present embodiment is completed. 
     In the above-described method for formation, a metal layer  25  is formed prior to the forming of the stacked body  100 , annealing is performed to cause the metal of the metal layer  25  to migrate into the amorphous silicon layers thereover in the Z-direction, and crystallization of the amorphous silicon source layer  30   a  and semiconductor layer  70  is performed using the metal as a catalyst, such that a source layer  30  and a semiconductor layer  70 , each of which is a crystallized silicon layer composed of a single crystal or polysilicon having a large crystal grain size, can be formed. Crystallizing the silicon layer decreases the resistance of the silicon layer, and hence of the channel. Accordingly, in a case where a large number of electrode layers are stacked in layers in, for example, a three-dimensional array semiconductor device, increased resistance and resulting lower reading and writing speeds of the memory cells can be avoided. 
     Moreover, since the metal layer  25  is formed prior to forming the layers of the source and semiconductor layer  70   a , for example, between the source layer  30   a  and the source line  20 , and not on the upper layer of the semiconductor device, the metal of the metal layer  25  migrates in the direction of the layers thereover and eventually segregates from the semiconductor layer  70  at the interface thereof with the insulating layer  40  located at the uppermost surface (the uppermost layer) of the structure. When the insulating layer  40  is removed, the metal layer is removed, and no metal remains in the source layer and the semiconductor layer as compared with a case where the metal layer is formed at the upper layer of the semiconductor device and migrated therefrom inwardly of the structure in the direction of the source layer  30 . Accordingly, the possibility that metal remains in the semiconductor layer to cause the deterioration of cell transistor characteristics, such as unevenness of parasitic resistances of memory cells, increase of power during power-off, and degradation of reliability of a tunnel film caused by metal diffusion in the subsequent process, can be reduced. 
     Furthermore, in the semiconductor device according to the present embodiment, as illustrated in  FIG. 9B , portions of the metal layer  25  may remain where its presence does not cause concern about the above-mentioned deterioration of transistor characteristics, for example in one or more locations of between the source line  20  and the source layer  30  (A), an SiO 2 /Si interface, such as between the second portion  33  of the source layer  30  and the insulating layer  40  (B), and inside the hole MH (C). In this case, the concentration of the remaining metal at the lower levels of the structure is higher than at the upper level in the Z-direction. 
     Hereinafter, a modification example of the method for manufacturing the semiconductor device according to the first embodiment is described with reference to  FIGS. 10A and 10B . 
     The method for manufacturing the semiconductor device according to the modification example differs from that of the first embodiment in the location of the metal layer before annealing. 
     As illustrated in  FIG. 10A , the metal layer  25  is provided, for example, between the first sub-layer  31   a  and the second sub-layer  33   a  of the source layer  30   a . In this case, during annealing the metal of the metal layer  25  migrates into the layers thereover and thereunder in the Z-direction, and the first sublayers  31   a , second sub-layers  33   a  and the semiconductor layer  70   a  are crystallized. Furthermore, the temperature and other conditions during annealing are similar to those in the first embodiment. 
     On the other hand, as illustrated in  FIG. 10B , the metal layer  25  can be formed between the source layer  30   a  and the lowermost insulating layer  40  of the stacked body. In this case, during annealing the metal of the metal layer  25  migrates in the + and −X directions, and the source layer  30   a  and the semiconductor layer  70   a  are crystallized. 
     In the above-described way, the position of formation of the metal layer  25  can be varied. In all cases, the possibility that the metal remains especially in the semiconductor layer can be reduced. 
     Furthermore, except the position in which to form the metal layer  25 , the method for formation and the advantages in the modification example are similar to those of the first embodiment. 
     (Second Embodiment) 
     Hereinafter, a second embodiment is described with reference to  FIG. 11  to  FIGS. 13A and 13B . 
     The semiconductor device  1  according to the second embodiment differs from that of the first embodiment in that a third semiconductor layer  90  is provided in a lower region of the hole MH. In other words, the third semiconductor layer  90  is provided between the semiconductor layer  70  and the source layer  30 . Elements of the structure similar to those of the first embodiment are omitted from the following description, where appropriate. 
       FIG. 11  is a schematic sectional view illustrating the memory cell portion MCP of the semiconductor device  1  according to the second embodiment. As illustrated in  FIG. 11 , the memory cell portion MCP in the present embodiment includes the crystallized third semiconductor layer  90  in the lower portion of the hole MH. The third semiconductor layer  90  is, for example, a single crystal, but can be polycrystalline. The third semiconductor layer  90  is formed so as to, for example, contact the source layer  30  in the Z-direction and extend across electrode layer  50   b . Insulating layer  87  is provided between the third semiconductor layer  90  and the electrode  50   b . Furthermore, the other structures are similar to those of the first embodiment. 
     The third semiconductor layer  90  is, for example, a crystallized silicon layer, and contains, for example, the same impurities as those in the source layer  30 . The impurity concentration of the third semiconductor layer  90  is not specifically limited. Insulating layer  87  is, for example, silicon oxide. A transistor is formed in electrode layer  50  through which the third semiconductor layer  90  is formed (for example, the electrode layer  50   b ). 
     In the present embodiment, crystallized silicon layer, i.e., the third semiconductor layer  90 , is formed in the lower portion of the hole MH, so that electric current flows easily to the source layer  30  and the source line  20 . 
     Next, a method for manufacturing the semiconductor device  1  according to the present embodiment is described with reference to  FIGS. 12A, 12B, and 12C  and  FIGS. 13A and 13B . 
     First, a stacked body  100   a  is formed on the source layer  30   a , and then a hole MH is formed by, for example, etching (referring to  FIG. 3B  and  FIG. 4A ). 
     As illustrated in  FIG. 12A , a third semiconductor layer  90   a  is formed inside the hole MH. At this time, the third semiconductor layer  90   a  is, for example, an amorphous silicon layer. 
     Next, as illustrated in  FIG. 12B , the third semiconductor layer  90   a  is removed, except for the portion thereof located in the lower portion of the hole MH and across lower electrode layer  50   b  using, for example, RIE. 
     As illustrated in  FIG. 12C , an insulating layer  80 , a semiconductor layer  70   a , and a core  75  are formed on the third semiconductor layer  90   a  in the hole MH in a manner similar to that in the first embodiment, and then an insulating film  40   a  is formed on the uppermost surface (referring to  FIG. 4B  to  FIG. 5B ). 
     Next, as illustrated in  FIG. 13A , annealing is performed at, for example, 500° C. to 750° C. The annealing causes the metal layer  25  to move toward the upper layer side of the structure in the Z-direction and to crystallize the amorphous silicon layer, with the metal used as a catalyst. Accordingly, the source layer  30   a , the third semiconductor layer  90   a , and the semiconductor layer  70   a  are crystallized in this order, and each of the crystalline source layer  30 , the third semiconductor layer  90 , and the semiconductor layer  70  are formed. The crystallized silicon layer becomes a single crystal or polysilicon having a large crystal grain size. 
     When annealing is further continued at the above-mentioned temperature, as illustrated in  FIG. 13B , the metal layer  25  segregates, and accumulates at the boundary between the top-surface insulating film  40   a  and the stacked body  100   a . The metal layer  25  is removed by, for example, dry etching or polishing in a similar way to that of the first embodiment, and, finally, a trench ST is formed, insulating layer  87  is formed by oxidation of the third semiconductor layer  90 , and ST filled, to complete the semiconductor device  1  according to the present embodiment. 
     In the semiconductor device according to the present embodiment, which has advantages similar to those of the first embodiment, since a semiconductor layer is provided in a lower portion of the hole, an insulating layer (a charge storage layer) does not need to be provided in the hole that is in contact with the lower electrode layers of the stacked body. Therefore, the electric current flows easily to the source layer  30  and the source line  20 , and a semiconductor device more improved in reliability can be manufactured. 
     Furthermore, the modification example described in the first embodiment can also be applied to the second embodiment. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.