Abstract:
A nonvolatile semiconductor memory device according to the present invention includes a memory cell array layer including a first line; a plurality of second and third lines that are formed below or above the first line and cross each other; and a plurality of memory cells arranged at each intersection of the second and third lines, the memory cell including a variable resistor and a transistor, which are connected to each other in series between the first line and the third line, the variable resistor being electrically rewritable and storing a resistance value as data in a nonvolatile manner, and the transistor being a columnar transistor having the second line arranged at its side face as a gate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2009-36056, filed on Feb. 19, 2009, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a nonvolatile semiconductor memory device, and more particularly to a nonvolatile semiconductor memory device employing a variable resistor that stores an electrically rewritable resistance value as data in a nonvolatile manner, and a production method thereof. 
         [0004]    2. Description of the Related Art 
         [0005]    There has conventionally been known a flash memory, as an electrically rewritable nonvolatile memory, which includes a memory cell array of NAND-connected or NOR-connected memory cells having a floating gate structure. A ferroelectric memory is also known as a nonvolatile fast random access memory. 
         [0006]    On the other hand, technologies of pattering memory cells much finer include a resistance variable memory, which uses a variable resistor in a memory cell as proposed. Known examples of the variable resistor include a phase change memory element that varies the resistance in accordance with the variation in crystal/amorphous states of a chalcogenide compound; an MRAM element that uses a variation in resistance due to the tunnel magneto-resistance effect; a polymer ferroelectric RAM (PFRAM) memory element including resistors formed of a conductive polymer; and a ReRAM element that causes a variation in resistance upon electrical pulse application (Patent Document 1: Japanese Patent Application Laid-Open No. 2006-344349, paragraph 0021). 
         [0007]    It has been known that there are two operation modes in the ReRAM variable resistor. One of them is such that the high-resistance state and the low-resistance state are changed by the control of the voltage value and the application time without changing the polarity of the applied voltage. This mode is referred to as a unipolar type. The other one is such that the high-resistance state and the low-resistance state are changed by changing the polarity of the applied voltage. This mode is referred to as a bipolar type. 
         [0008]    Since the variable resistor and the rectifying element such as a diode are connected in series in the unipolar memory cell, the memory cells are easy to be stacked. Further, when the nonvolatile memory is configured three-dimensionally, high integration of the memory cell can be achieved (Patent Document 2: Japanese Patent Application Laid-Open No. 2005-522045). In the memory cell array using the unipolar memory cell employing a diode, all word lines have to rise when the memory cell is accessed, so that the first access is slow. Since many memory cells are inversely biased, the power consumption increases, which limits the through-put. 
         [0009]    On the other hand, in the memory cell employing a transistor instead of a diode, only the selected word line and bit line may be moved upon the access to the memory cell, whereby the first access can be accelerated. Further, the power consumption can be reduced, and high through-put can be obtained. Moreover, the memory cell using a transistor can be operated as a bipolar memory cell by changing the polarity of the applied voltage with respect to the source voltage to the bit line and the word line. However, the integration is poorer than that of the memory cell using a diode, which is a problem. 
       SUMMARY OF THE INVENTION 
       [0010]    An aspect of the present invention provides a nonvolatile semiconductor memory device including: a memory cell array layer including a first line; a plurality of second and third lines that are formed below or above the first line and that cross each other; and a plurality of memory cells arranged at each intersection of the second and third lines, the memory cell including a variable resistor and a transistor, which are connected to each other in series between the first line and the third line, the variable resistor being electrically rewritable and storing a resistance value as data in a nonvolatile manner, and the transistor being a columnar transistor having the second line arranged at its side face as a gate. 
         [0011]    Another aspect of the present invention provides a nonvolatile semiconductor memory device including: a first memory section including a first memory cell array layer having a first line, a plurality of second and third lines that are formed above or below the first line and cross each other, and a plurality of first memory cells arranged at each intersection of the second and third lines; a second memory section including a second memory cell array layer having a plurality of fourth and fifth lines that cross each other, and a plurality of second memory cells arranged at each intersection of the fourth and fifth lines; and a control section that controls a memory operation of the first and second memory sections, the first memory cell including a variable resistor and a transistor connected in series between the first and third lines, the variable resistor being electrically rewritable and storing a resistance value as data in a nonvolatile manner, and the transistor being a columnar transistor having the second line arranged at its side face as a gate, and the second memory cell including a variable resistor and a diode that are connected in series between the fourth and fifth lines, are electrically rewritable, and store the resistance value as data in a nonvolatile manner. 
         [0012]    Another aspect of the present invention provides a production method of a nonvolatile semiconductor memory device, including: sequentially forming a first line layer, a first insulating layer, and a second line layer; forming a plurality of second lines extending in a predetermined direction through a selective etching of the second line layer; forming a second insulating layer on the second line; forming a memory-cell forming hole so as to penetrate the second insulating layer, the second line, and the first insulating layer; forming a gate insulating layer at an inner peripheral surface of the memory-cell forming hole; and sequentially stacking a silicon layer and a variable resistor material, which constitute a transistor, at the memory-cell forming hole having the insulating layer formed on its inner peripheral surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a block diagram of a nonvolatile semiconductor memory device according to a first embodiment of the present invention; 
           [0014]      FIG. 2  is a perspective view of part of a memory cell array in the nonvolatile semiconductor memory device according to the first embodiment; 
           [0015]      FIG. 3  is a schematic cross-sectional view illustrating one example of a variable resistor according to the first embodiment; 
           [0016]      FIG. 4  is an equivalent circuit diagram of the memory cell array in the nonvolatile semiconductor memory device according to the first embodiment; 
           [0017]      FIG. 5  is a graph illustrating pulse voltages for realizing a writing operation in the nonvolatile semiconductor memory device according to the first embodiment; 
           [0018]      FIG. 6A  is a cross-sectional view of one memory cell array taken along line I-I′ and seen from the direction of the arrow in  FIG. 2 ; 
           [0019]      FIG. 6B  is a cross-sectional view of one memory cell array taken along line II-II′ and seen from the direction of the arrow in  FIG. 2 ; 
           [0020]      FIG. 6C  is a view seen from the top surface of the nonvolatile semiconductor memory device in the stacking direction in  FIG. 2 ; 
           [0021]      FIGS. 7A to 16A  are cross-sectional views, taken along line I-I′ and seen from the direction of the arrow in  FIG. 2 , for explaining a production method of the nonvolatile semiconductor memory device according to the first embodiment; 
           [0022]      FIGS. 7B to 16B  are cross-sectional views, taken along line II-II′ and seen from the direction of the arrow in  FIG. 2 , for explaining a production method of the nonvolatile semiconductor memory device according to the first embodiment; 
           [0023]      FIG. 17  is a cross-sectional view showing a part of a memory cell array in a nonvolatile semiconductor memory device according to a second embodiment of the present invention; 
           [0024]      FIG. 18  is a cross-sectional view of a part of the memory cell array in the nonvolatile semiconductor memory device according to the second embodiment; 
           [0025]      FIG. 19  is a block diagram illustrating a memory system according to a third embodiment of the present invention; 
           [0026]      FIG. 20  is a block diagram illustrating a memory chip according to the third embodiment; 
           [0027]      FIG. 21  is an equivalent circuit diagram of a memory cell array in a nonvolatile semiconductor memory device according to a comparative example; 
           [0028]      FIG. 22  is a graph illustrating pulse voltages for realizing a writing operation in the nonvolatile semiconductor memory device according to the comparative example; and 
           [0029]      FIG. 23  is a cross-sectional view illustrating one memory cell in a nonvolatile semiconductor memory device according to another comparative example. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0030]    Embodiments of the nonvolatile semiconductor memory device according to the present invention will now be described with reference to the drawings. 
       First Embodiment 
     Overall System 
       [0031]      FIG. 1  is a block diagram of a nonvolatile semiconductor memory device according to a first embodiment of the present invention. 
         [0032]    The nonvolatile semiconductor memory device includes a memory cell array  1  of memory cells arranged in matrix, each memory cell including a later-described ReRAM (variable resistor). A column control circuit  2  is provided on a position adjacent to the memory cell array  1  in a bit line BL direction. The column control circuit  2  controls the bit line BL in the memory cell array  1  to erase data from the memory cell, write data in the memory cell, and read data from the memory cell. A row control circuit  3  is provided on a position adjacent to the memory cell array  1  in a word line WL direction. The row control circuit  3  selects the word line WL in the memory cell array  1  and applies voltages required to erase data from the memory cell, write data in the memory cell, and read data from the memory cell. 
         [0033]    A data I/O buffer  4  is connected to an external host, not shown, via an I/O line to receive write data, receive erase instructions, output read data, and receive address data and command data. The data I/O buffer  4  sends received write data to the column control circuit  2 , receives read data from the column control circuit  2  and outputs it to outside. An address fed from outside to the data I/O buffer  4  is sent via an address register  5  to the column control circuit  2  and the row control circuit  3 . A command fed from the host to the data I/O buffer  4  is sent to a command interface  6 . The command interface  6  receives an external control signal from the host and determines whether the data fed to the data I/O buffer  4  is write data, a command or an address. If it is a command, then the command interface  6  transfers it as a received command signal to a state machine  7 . The state machine  7  manages the entire nonvolatile semiconductor memory device to receive commands from the host, read, write, erase, and execute data I/O management. The external host can also receive status information managed by the state machine  7  and determine the operation result. The status information is also utilized for control of write and erase. 
         [0034]    The state machine  7  controls a pulse generator  8 . Under this control, the pulse generator  8  is allowed to output a pulse of any voltage at any timing. Specifically, the state machine  7  receives an externally-fed address via the address register  5  to determine which memory layer is the access target, and uses a parameter corresponding to that memory layer to control the height and width of the pulse from the pulse generator  8 . This parameter is a value derived by grasping characteristics of write and the like in the memory layers to even write characteristics in the memory layers, and saved in the memory cells. The pulse formed herein can be transferred to any line selected by the column control circuit  2  and the row control circuit  3 . 
         [0035]    Peripheral circuit elements other than the memory cell array  1  can be formed in a silicon substrate immediately beneath the memory cell array  1 . Thus, the chip area of the semiconductor memory device can be made almost equal to the area of the memory cell array  1 . 
       Outline of Memory Cell Array 
       [0036]      FIG. 2  is a perspective view showing a part of a memory cell array  100 , which is one example of the memory cell array  1 . 
         [0037]    The memory cell array  100  includes a source line SL, which is a plane first line formed on a silicon substrate, not shown. The memory cell array  100  also includes a plurality of word lines WL 0  to WL 3 , which are the second lines, extending in the direction of II-II′ line (hereinafter referred to as “row direction”) in the figure, which is the second direction, above the source line SL, and a plurality of bit lines BL 0  to BL 3 , which is the third lines, extending in the direction of I-I′ line (hereinafter referred to as “column direction”) in the figure, which is the third direction, so as to cross the word lines WL. Desirably, the word lines WL, the bit lines BL, and the source line SL are composed of heat-resistive low-resistance material such as W, WSi, NiSi, and CoSi. A memory cell MC is arranged at each intersection of the plurality of word lines WL and bit lines BL. 
         [0038]    Each memory cell MC is formed into a cylindrical column in which the word line WL extends between the source line SL and the bit line BL. The portion above the word line WL is defined as a variable resistor VR, and the other portion is defined as polysilicon (p-Si) doped with p + -type impurity. A gate insulating film GIS is interposed between the polysilicon and the word lines WL. The gate insulating film GIS and the polysilicon form a transistor. Further, the transistor TR and the variable resistor VR form the memory cell MC. 
         [0039]      FIG. 3  is a view showing an example of the variable resistor VR. The variable resistor VR shown in  FIG. 3  includes a recording layer  12  formed between electrode layers  9 ,  11 . The recording layer  12  is made of a composite compound having at least two types of cation elements. At least one of the cation elements is a transition element having the d-orbit incompletely filled with electrons, and the shortest distance between adjacent cation elements is 0.32 nm or less. Specifically, it is represented by a chemical formula A x M y X z  (A and M are different elements) and may be formed of material having a crystal structure such as a spinel structure (AM 2 O 4 ), an ilmenite structure (AMO 3 ), a delafossite structure (AMO 2 ), a LiMoN 2  structure (AMN 2 ), a wolframite structure (AMO 4 ), an olivine structure (A 2 MO 4 ), a hollandite structure (A x MO 2 ), a ramsdellite structure (A x MO 2 ), and a perovskite structure (AMO 3 ). 
         [0040]    In the example of  FIG. 3 , A is Zn, M is Mn, and X is O. In the recording layer  12 , a small white circle represents a diffused ion (Zn), a large white circle represents an anion (O), and a small black circle represents a transition element ion (Mn). The initial state of the recording layer  12  is the high-resistance state. When the electrode layer  11  is kept at a fixed potential and a negative voltage is applied to the electrode layer  13 , some diffused ions in the recording layer  12  migrate toward the electrode layer  13  to reduce diffused ions in the recording layer  12  relative to anions. The diffused ions having arrived at the electrode layer  13  accept electrons from the electrode layer  13  and precipitate as a metal, thereby forming a metal layer  14 . Inside the recording layer  12 , anions become excessive and consequently increase the valence of the transition element ion in the recording layer  12 . As a result, the carrier injection brings the recording layer  12  into electron conduction and thus completes setting. Upon regeneration, a current may be allowed to flow, of which value is very small so that the material configuring the recording layer  12  causes no resistance variation. The programmed state (low-resistance state) may be reset to the initial state (high-resistance state) by supplying a large current flow in the recording layer  12  for a sufficient time, which causes Joule heating to facilitate the oxidation reduction reaction in the recording layer  12 . Application of an electric field in the opposite direction from that at the time of setting may also allow resetting. 
         [0041]    The setting/resetting operation to the memory cell MC using the transistor according to the present embodiment will be described below. 
         [0042]    Before describing the present embodiment, a setting/resetting operation to the memory cell using a diode will firstly be described. 
         [0043]      FIG. 21  is an equivalent circuit diagram showing a part of the memory cell array according to a comparative example. The memory cell array includes a plurality of word lines WL and bit lines BL, which cross each other, and a memory cell MC arranged at each intersection of the plurality of word lines WL and bit lines BL. The memory cell MC includes a diode Di and a variable resistor VR that are connected in series. 
         [0044]      FIG. 22  is a waveform chart during the setting operation to the memory cell MC 1  connected at the intersection between the word line WL 1  and the bit line BL 1  shown in  FIG. 21 . 
         [0045]    The word lines WL and the bit lines BL of the memory cell array are usually at “L”. 
         [0046]    During the setting operation, the non-selected word line WL 2  and the selected bit line BL 1  are set to have a set voltage VSET, which is required for the setting operation of the variable resistor VR, for a predetermined period t-SET (e.g., 10 ns to 100 ns). The set voltage VSET is, for example, 1.5 V. Considering the forward voltage of the diode Di that is 0.6 V, the set voltage VSET is 2.1 V. As a result, the diode Di in the memory cell MC 1  is forward biased and accordingly current flows in the direction of the arrow in  FIG. 21 . The variable resistor VR in the memory cell MC 1  changes from the high-resistance state to the low-resistance state. On the other hand, the diodes Di in the other memory cells MC are reverse biased, so that voltage is not applied to the variable resistors VR in the memory cells MC. Accordingly, the resistance state is not changed. 
         [0047]    During the resetting operation, the non-selected word line WL 1  and the selected bit line BL 1  are set to have a reset voltage VRESET, which is required for the resetting operation of the variable resistor VR and which is lower than the set voltage, for a predetermined period t-RESET (e.g., 500 ns to 2 μs) that is longer than the period t-SET. The reset voltage VRESET is, for example, 0.6 V. Considering the forward voltage of the diode Di that is 1.0 V, the reset voltage VRESET is 1.6 V. As a result, Joule heat is generated in the variable resistor VR, so that the variable resistor VR in the memory cell MC 1  changes from the low-resistance state to the high-resistance state. 
         [0048]    Next, the setting/resetting operation of the memory cell according to the present embodiment will be described. 
         [0049]      FIG. 4  is an equivalent circuit diagram showing a part of the memory cell array  100  in the nonvolatile semiconductor memory device according to the present embodiment. The memory cell MC, which includes a transistor TR and a variable resistor VR connected in series, is arranged at the intersection of the word line WL and the bit line BL as shown in  FIG. 4 . 
         [0050]      FIG. 5  is a waveform chart during the setting operation to the memory cell MC 1  connected at the intersection of the word line WL 1  and the bit line BL 1  in  FIG. 4 . 
         [0051]    The word lines WL and the bit lines BL of the memory cell array are usually at “L”. 
         [0052]    During the setting operation, the selected word line WL 1  is set at “H”, and the selected bit line BL 1  is set to have the set voltage VSET for the period t-SET. As a result, the transistor TR in the memory cell MC 1  is turned on, whereby current flows in the direction indicated by the arrow in  FIG. 4 . Therefore, the variable resistor VR in the memory cell MC 1  changes from the high-resistance state to the low-resistance state. On the other hand, the transistors TR in the other memory cells MC are kept to be off, so that the resistance state of the variable resistor VR is not changed. 
         [0053]    During the resetting operation, the selected word line WL 1  may be set at “H”, and the reset voltage VRESET may be applied to the selected bit line BL 1  during the period t-RESET, like the setting operation. 
         [0054]    As is understood from the above, in the comparative example, the voltages of all non-selected word lines WL except for the selected word line WL 1  have to rise to the set voltage VSET or reset voltage VRESET during the setting/resetting operation. Therefore, the power consumption increases, and further, a first access to the memory cell MC 1  is delayed. 
         [0055]    On the other hand, according to the present embodiment, only the voltage of the selected bit line BL 1  is set to the set voltage VSET or the reset voltage VRESET as shown in  FIG. 4 . Therefore, the power consumption is reduced, and the first access can be accelerated, compared to the comparative example. 
         [0056]    In the description above, a CMOS transistor is used for the transistor TR in the memory cell MC. However, a bipolar transistor can be used for the transistor TR. 
       Structure of Memory Cell Array 
       [0057]    Next, a structure of the memory cell array using the memory cell described above will be described. 
         [0058]    Before describing the structure of the memory cell array according to the present embodiment, a structure of a memory cell array  600  according to a comparative example will be described with reference to  FIG. 23 . 
         [0059]    The memory cell array  600  according to the comparative example includes a word line WL extending in the row direction, a source line SL extending in the row direction, and a bit line BL extending in the column direction. These lines are formed on a silicon substrate  601 , and stacked in a different layer. The source line SL is connected to an impurity-diffusion layer  602  in the silicon substrate  601  through a conductor  604 . The bit line BL is connected to an impurity-diffusion layer  603  in the silicon substrate  601  through the variable resistor VR and a conductor  605 . The word line WL is connected to the position between the impurity-diffusion layers  602  and  603  in the silicon substrate  601  through a gate insulating film  606 . In this arrangement, the direction of the gate length of the transistor in the memory cell is the column direction. In order to arrange one memory cell, an arrangement area having a width equal to the total of the widths in the column direction of the variable resistors VR, connected to the source line SL, the word line WL, and the bit line BL, is needed. 
         [0060]    Subsequently, the structure of the memory cell array  100  according to the present embodiment will be described. 
         [0061]      FIGS. 6A and 6B  are cross-sectional views taken along a line I-I′ and a line II-II′ in  FIG. 2 .  FIG. 6C  is a view seen from the top surface of the memory cell array in the stacking direction that is the first direction. 
         [0062]    A peripheral circuit  103  such as a sense amplifier circuit is formed on a silicon substrate  101 . A first insulating film  102  is formed to cover the peripheral circuit  103 . Since the peripheral circuit is arranged at the lower portion of the memory cell array  100 , the chip area of the nonvolatile semiconductor memory device can be made substantially equal to the area of the memory cell array  100 . 
         [0063]    A plane first metal  104  serving as the source line SL, which is the first line, is formed from a low-resistance metal such as W on the first insulating film  102 . A second metal  107  is formed on the first metal  104  through a protection film  105  and a second insulating film  106 . The second metal  107  is divided by a plurality of trenches  108  extending in the row direction that is the second direction. Each of the divided portions is the word line WL that is the second line. A third insulating film  109  is formed on the second insulating film  106  formed at the bottom of the trench  108 , the trench  108 , and the second metal  107 . A third metal  114  is formed on the third insulating film  109 . The third metal  114  is divided by a plurality of trenches  115  extending in the column direction that is the third direction. Each of the divided portions is the bit line BL that is the third line. A transistor hole  110  having a cylindrical column shape and formed between the source line SL and the bit line BL is formed at the intersection of the word line WL and the bit line BL. A hollow cylindrical gate insulating film  111  having a predetermined thickness is formed at the inner wall of the transistor hole  110 . A polysilicon  112  composing the transistor TR is embedded into the inner wall of the gate insulating film  111  to the height equal to the top surface of the word line WL from the top surface of the source line SL. A variable resistor  113  is embedded from the top of the polysilicon  112  to the lower surface of the bit line BL. The gate insulating film  111 , the polysilicon  112 , and the variable resistor  113  form the memory cell MC. 
         [0064]    Further, a fourth insulating film  116  is formed on the third insulating film  109 , the trenches  115 , and the third metal  114 . 
         [0065]    When the memory cell is configured two-dimensionally as in the comparative example shown in  FIG. 23 , a sufficient channel length cannot be secured, if the arrangement area of the memory cell is reduced. Therefore, a short channel effect is increased. Accordingly, it is difficult to reduce the arrangement area. Since the source line SL has a linear shape, the wiring resistance is great, so that the power consumption increases, and the property of the memory cell varies. 
         [0066]    According to the present embodiment, the memory cell MC can be arranged at the intersection of the word line WL land the bit line BL. Accordingly, a highly integrated memory cell array can be realized, compared to the comparative example. By virtue of this, the increase in the chip area can be suppressed, whereby reduced cost can be realized. Since the direction of the gate length of the transistor TR is the stacking direction of the memory cell array  100 , the short channel effect can be eased. Further, since the source line SL has a flat plane shape, the wiring resistance can be reduced, whereby the increase in the power consumption and variation in the property of the memory cell MC can be reduced. 
       Production Method of Memory Cell Array 
       [0067]    The production method of the memory cell array  100  according to the present embodiment will be described with reference to  FIGS. 7 to 16 .  FIGS. 7A to 16A  are sectional views taken along the line I-I′ in  FIG. 2 , while  FIGS. 7B to 16B  are sectional views taken along the line II-II′ in  FIG. 2 . 
         [0068]    First, a metal layer  154 , serving as the first line layer and made of tungsten (W), a protection layer  155  made of silicon nitride (SiN), a first insulating layer  156  made of silicon oxide (SiO 2 ), and a metal layer  157 , serving as the second line layer and made of tungsten (W), are sequentially stacked on the top surface of the layer, which is to be the first insulating layer in  FIG. 2 , as shown in  FIGS. 7A and 7B . These become the first metal  104  serving as the source line SL, the protection film  105 , the second insulating film  106 , and the second metal  107 . 
         [0069]    Next, in  FIGS. 8A and 8B , a hard mask of TEOS, not shown, is formed on the top surface of the metal layer  157 . A first anisotropy etching is executed with this hard mask used as a mask. By this process, a trench  158  penetrating the metal layer  157  and extending in the row direction with a predetermined pitch is formed. The metal layer  157  becomes the word line WL serving as the second line. 
         [0070]    Then, in  FIGS. 9A and 9B , a second insulating layer  159  made of silicon oxide (SiO 2 ) is formed on the top surfaces of the insulating layer  156  and the metal layer  157 . The insulating layer  159  becomes the third insulating film  109 . 
         [0071]    Subsequently, in  FIGS. 10A and 10B , a transistor hole  160  (corresponding to the transistor hole  110  in  FIGS. 6A and 6B ), which has a cylindrical column shape, penetrates the protection layer  155 , the insulating layer  156 , the metal layer  157 , and the insulating layer  159 , and serves as a memory-cell forming hole, is formed. The transistor hole  160  is formed in a matrix, as viewed from the top surface of the memory cell array  100  in the stacking direction, at the position matching the metal layer  157 . 
         [0072]    Next, in  FIGS. 11A and 11B , a gate insulating layer  161  made of silicon oxide (SiO 2 ) is stacked so as to cover the inner wall of the transistor hole  160  and the top surface of the insulating layer  159  with a CVD (Chemical Vapor Deposition). 
         [0073]    Then, in  FIGS. 12A and 12B , the bottom portion of the transistor hole  160  and the gate insulating layer  161  on the top surface of the insulating layer  159  are selectively etched in order to remove them. According to this process, the gate insulating film  111  is formed. 
         [0074]    Next, in  FIGS. 13A and 13B , a polysilicon  162  (corresponding to the polysilicon  112  in  FIG. 2 ) made of p+ polysilicon (p-Si) and a variable resistor material  163  that becomes the variable resistor VR are sequentially stacked so as to fill the transistor hole  160 . The polysilicon  162  is stacked to have a height equal to the top surface of the metal layer  157 . The polysilicon  162  and the variable resistor material  163  form the memory cell MC. Thereafter, the top surfaces of the insulating layer  159 , the gate insulating layer  161 , and the variable resistor material  163  are flattened by CMP. 
         [0075]    Subsequently, in  FIGS. 14A and 14B , a metal layer  164 , which is made of tungsten (W) and serves as the third line layer, is stacked onto the top surfaces of the flattened insulating layer  159 , the gate insulating layer  161 , and the variable resistor material  163 . 
         [0076]    Then, in  FIGS. 15A and 15B , a hard mask of TEOS, not shown, is formed on the top surface of the metal layer  164 . A second anisotropy etching is executed with this hard mask used as a mask. By virtue of this process, a trench  165  (corresponding to the trench  115  in  FIG. 2 ) penetrating the metal layer  164  and extending in the column direction with a predetermined pitch is formed. According to this process, the metal layer  164  becomes the bit line BL serving as the third line. 
         [0077]    Finally, in  FIGS. 16A and 16B , a third insulating layer  166  made of silicon oxide (SiO 2 ) is stacked on the top surfaces of the insulating layer  159  and the metal layer  164 . This insulating layer  166  becomes the fourth insulating film  116 . 
         [0078]    As described above, the present embodiment can provide a nonvolatile semiconductor memory device that can realize high integration and reduced cost, while keeping high-speed property and low power consumption, which are characteristics of a bipolar memory cell, and a production method thereof. 
       Second Embodiment 
       [0079]    A nonvolatile semiconductor memory device according to the second embodiment of the present invention is a nonvolatile semiconductor memory device having the stacked plural memory cell arrays according to the first embodiment. 
         [0080]    A memory cell array  200  shown in  FIG. 17  is one example of a double-layer structure. The memory cell array  200  has a structure in which layers  204  to  206 , having the same configuration as the layers  104  to  116 , are stacked on the top surface of the layer  116  of the memory cell array  100 . 
         [0081]    When the memory cell array  200  is produced, the processes in  FIGS. 7 to 16  may be repeated after the third insulating layer  166  is stacked in the production process of the memory cell array  100 . 
         [0082]    The memory cell array  200  can realize the large-capacity nonvolatile semiconductor memory device that keeps high integration of the memory cell array  100 . 
         [0083]    A memory cell array  300  shown in  FIG. 18  is also an example of the double-layer structure. Unlike the memory cell array  200 , the bit line  114  serving as the third line is shared by the upper and lower layers in the memory cell array  300 . A source line  304 , a variable resistor  313 , and the polysilicon  112 , which are obtained by turning the source line  104 , the variable resistor  113 , and the polysilicon  112  upside down, are formed on the bit line  114  of the memory cell array  100 . 
         [0084]    When the memory cell array  300  is produced, the insulating layer  166  may be etched back until the top surface of the bit line  114  appears, and then, the layer that is to become the memory cell MC and the metal layer  304  that is to become the source line SL may be sequentially formed, after the process shown in  FIGS. 16A and 16B . 
         [0085]    The memory cell array  300  can provide large capacity like the memory cell array  200 , and further, provide much higher integration, since the line layer is reduced due to the sharing of the bit line  114 . 
         [0086]    The memory cell arrays  200  and  300  have the two-layer structure. However, a multi-layer structure having three or more layers can be formed by repeatedly stacking the layers. 
       Third Embodiment 
       [0087]    As shown in  FIGS. 19 and 20 , a nonvolatile semiconductor memory device according to the third embodiment of the present invention is a semiconductor memory device having both of a memory cell array block including the above-mentioned memory cell using a diode, and a memory cell array block including a memory cell using a transistor. 
         [0088]    In this case, a high integration property, which is the characteristic of the memory cell that employs a diode, and a high-speed property and reduced power consumption, which are the characteristics of the memory cell that employs a transistor, can both be achieved. 
         [0089]      FIG. 19  shows a memory system  400 , such as a memory card, including a memory chip  401  that includes a memory cell using a diode and serves as a second memory section, a memory chip  402  that includes a memory cell using a transistor and serves as a first memory section, and a controller  403  that controls the access to the memory chips  401  and  402 . 
         [0090]      FIG. 20  shows a memory chip  500  including a memory block  501  that includes a memory cell using a diode and serves as a second memory section, a memory block  502  that includes a memory cell using a transistor and serves as a first memory section, and a control circuit  503  that controls the access to the memory blocks  501  and  502 . 
         [0091]    The example of use of the memory system and the memory chip described above will be described below. 
         [0092]    As a first example of use, the memory chip  401  or the memory block  501  is defined as a data memory area, and the memory ship  402  or the memory block  502  is used as a code memory area. 
         [0093]    In this case, the memory system  400  or the memory chip  500  can be provided in which a large capacity data storage utilizing high integration, which is the characteristic of the memory cell employing a diode, can be realized, and further, a code execution utilizing the high-speed property, which is the characteristic of the memory cell employing a transistor, can be realized. 
         [0094]    As a second example of use, the memory chip  401  or the memory block  501  is defined as a main data storage area, and the memory chip  402  or the memory block  502  is used as a cache area. 
         [0095]    In this case, data is temporarily stored in the memory chip  402  or the memory block  502  by utilizing the high-speed property that is the characteristic of the memory cell employing a transistor, and then, the data can be transferred to the large-capacity memory chip  401  or the memory block  501  utilizing the high integration that is the characteristic of the memory cell employing a diode. 
         [0096]    As a third example of use, the memory cell using a transistor is used as a bipolar memory cell. The polarity of the voltage applied to the bit line and the word line is reversed with respect to the common source voltage VSS, whereby the memory cell using a transistor can function as a bipolar memory cell. The bipolar memory cell can be operated at lower voltage than a unipolar memory cell. Therefore, in the third example, the memory chip  401  or the memory block  501  is used as an operation area with high voltage, and the memory chip  402  or the memory block  502  is used as an operation area with low voltage. 
         [0097]    In this case, the memory system or the memory chip can be provided in which large-capacity data can be hand-speed by the memory chip  401  or the memory block  501  under the environment where high voltage can be secured, and under the environment where power saving is needed, high-speed property can be secured and reduced power consumption can be realized by the memory chip  402  or the memory block  502 .